Author(s)
Olson, Darren Montgomery
Title
Radar observations of field-aligned plasma propagations associated with NASA's PMG
experiment
Publisher
Monterey, California. Naval Postgraduate School
Issue Date
1994-09
URL
http://hdl.handle.net/10945/43046
This document was downloaded on May 04, 2015 at 22:45:35
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for public release; distribution
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Radar Observations of Field-Aligned Plasma Propagations
Associated with
NASA's
PMG Experiment
by
Darren M. Olson
Navy
Naval Academy, 1988
Lieutenant, United/States
B.S.E., United States
Submitted
in partial fulfillment
of the requirements for the degree of
MASTER OF SCIENCE IN SYSTEMS TECHNOLOGY
(SPACE SYSTEMS OPERATIONS)
from the
NAVAL POSTGRADUATE SCHOOL
September 1994
6./
DUDLEY KNOX LIBRARY
NAVAL POSTGRADUATE SCHOOL
MONTEREY CA 93943-5101
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September 1994
Master's Thesis
TITLE AND SUBTITLE RADAR OBSERVATIONS OF FIELDALIGNED PLASMA PROPAGATIONS ASSOCIATED WITH NASA'S PMG
EXPERIMENT
4.
6.
AUTHOR(S)
7.
PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
5.
Darren Montgomery Olson
PERFORMING
ORGANIZATION
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Naval Postgraduate School
Monterey CA 93943-5000
SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)
9.
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1
SUPPLEMENTARY NOTES
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The views expressed
in this thesis are
those of the author and do not reflect
the official policy or position of the Department of Defense or the U.S. Government.
DISTRIBUTION/AVAILABILITY STATEMENT
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13. ABSTRACT (maximum 200 words)
NASA's Plasma Motor Generator (PMG)
into the
ambient ionospheric plasma.
DISTRIBUTION
*A
CODE
was launched in June 1993 to
currents from an electrodynamic
tethered satellite mission
plasma sources
the ability of hollow cathode
12b.
unlimited.
to
couple electric
verify
tether
This large-scale coupling process resulted in turbulent plasma
signatures associated with the orbiting plasma generator, which propagated over great distances along
the earth's geomagnetic field lines.
VHF
radars in Hilo, Hawaii and Jicamarca, Peru recorded
observations of these field-aligned disturbances as part of the experiment.
radar observations and tracking data of
traveling
PMG's
plasma waveforms was calculated
these disturbances, associated with
to
PMG's
orbit, the effective
Based on analysis of these
propagation velocity of these
be of the order of 1000 meters per second. Detection of
passage overhead, supports the existence of a phantom
current loop allowing current flow along the magnetic field lines of the earth and into the lower
ionosphere from either end of an electrodynamic tether.
14.
SUBJECT TERMS
Motor/Generator,
NASA
Electrodynamic Tether, Hollow Cathode Plasma Source, Plasma
15.
16.
17.
SECURITY CLASSIFICATION OF REPORT
Unclassified
NSN
7540-01-280-5500
SECURITY CLASSIFICATION OF THIS PAGE
I
Unclassified
NUMBER OF
PAGES 97
Experiment
SECURITY CLASSIFICATION OF ABSTRACT
19.
20.
PRICE
CODE
LIMITATION
OF ABSTRACT
UL
Unclassified
Standard
Form 298
Prescribed by
(Rev. 2-89)
ANSI
Std. 239-
li
ABSTRACT
NASA's Plasma Motor
in
June 1993 to verify the
Generator
ability of
(PMG)
tethered satellite mission
was launched
hollow cathode plasma sources to couple
currents from an electrodynamic tether into the ambient ionospheric plasma.
scale coupling process resulted in turbulent
electric
This large-
plasma signatures associated with the orbiting
plasma generator, which propagated over great distances along the earth's geomagnetic
field lines.
VHF
radars in Hilo, Hawaii and Jicamarca, Peru recorded observations of
these field-aligned disturbances as part of the experiment.
radar observations and tracking data of
these traveling plasma waveforms
second.
PMG's
Based on analysis of these
orbit, the effective
was calculated
to
propagation velocity of
be of the order of 1000 meters per
Detection of these disturbances, associated with
PMG's
passage overhead,
supports the existence of a phantom current loop allowing current flow along the
magnetic
field lines of the earth
electrodynamic
tether.
and into the lower ionosphere from either end of an
VI
TABLE OF CONTENTS
I.
INTRODUCTION
II.
BACKGROUND ON TETHERS
III.
IV.
V.
1
5
A.
HISTORY
5
B.
GRAVITY-GRADIENT STABILIZATION
7
C.
ELECTRODYNAMIC TETHERS
10
THE SPACE ENVIRONMENT
17
A.
EARTH'S GEOMAGNETIC FIELD
17
B.
IONOSPHERIC PLASMA
19
THE PMG EXPERIMENT
21
A.
MISSION OBJECTIVES
21
B.
PHYSICAL DESCRIPTION
22
C.
FLIGHT PROFILE
24
D.
RADAR OBSERVATION SITES
24
EXPERIMENTAL OBSERVATIONS AND ANALYSIS
29
A.
HAWAII PASSES
29
B.
JICAMARCA PASSES
34
1.
PASS
2
35
2.
PASS
3
37
3.
PASS 4
38
VI.
SUMMARY AND DISCUSSION
41
VII.
CONCLUSIONS
47
LIST OF REFERENCES
79
INITIAL DISTRIBUTION LIST
81
Vll
Vlll
LIST OF FIGURES
FIGURE
1
:
AND TABLES
Early Gravity-Gradient Concepts
49
FIGURE
2:
Forces Acting on Tethered Satellites
50
FIGURE
3:
Restoring Forces Acting on Tethered Satellites
51
FIGURE 4:
Current Paths for Electrodynamic Tethers
52
FIGURE
5:
The Phantom Current Loop
53
FIGURE
6:
The Generator
54
FIGURE
7:
The Motor
FIGURE
8:
The Eccentric-Dipole Model of the Earth's Magnetic
FIGURE
9:
Geometry
FIGURE
10:
Geometric Latitude and Equatorial Crossing of a Field Line
58
FIGURE
11:
Ionospheric Electron Density Layers
59
FIGURE
12:
PMG Hollow Cathode Assembly Schematic
60
FIGURE
13:
PMG Mission Flight Profile
61
FIGURE
14:
PMG Ground track and Observing Radar Sites
62
FIGURE
15:
Hawaii Radar
FIGURE
16:
PMG Hawaii
FIGURE
17:
Hawaii Pass
1
Plasma Returns
-
Range Gate
FIGURE
18:
Hawaii Pass
1
Plasma Returns
-
Range Gates 4 and
5
66
FIGURE
19:
Hawaii Pass
1
Plasma Returns
-
Range Gates 6 and 7
67
FIGURE
20:
Geometry
for
FIGURE
21:
Geometry
for Jicamarca Pass 2 Calculations
FIGURE
22:
Flux-Tube Height Variation Relative
Principle
55
Principle
Field
for a Static Dipolar Field
Beam Geometry
Pass
1
Echo
Hawaii Pass
57
for Pass
63
1
64
Activity
1
to
65
3
Calculations
IX
56
Jicamarca
68
69
70
FIGURE
23:
FIGURE
24: Formation of Alfven
FIGURE
25: Formation of Whistler
FIGURE
24: Whistler
TABLE
1:
Geometry
for Jicamarca Pass 3 and
Mode
4 Calculations
Wings
72
Wings
73
Phase Velocity/Frequency Relationship
PMG Meridian Crossing for Hawaii Pass
TABLE 2: PMG Meridian
TABLE 3: PMG
71
1
74
75
Crossing for Jicamarca Pass 2
76
Meridian Crossing for Jicamarca Pass 3
77
TABLE 4: PMG Meridian
Crossing for Jicamarca Pass 4
XI
78
Xll
ACKNOWLEDGMENTS
I
would
and guidance.
like to
acknowledge
His expertise
has proven invaluable to
my
thesis advisor, Dr. Olsen, for his superb support
in the fields
me
of plasma physics and
throughout the writing of
Greg McCaskill
am
I
to
my
answer
which ranged from providing experimental
for their help,
numerous
orchestrating the successful
In addition,
I
would
like to thank
and constant encouragement, and
Most
importantly,
during these
involvement
last
in
I
thank
two
questions.
PMG experiment
my
years,
my
wife,
typewritten pages.
The
fruits
is
my
my
-
final
who remain
the pride
to
form
make
is
this
and joy of
my
life.
and patience
thesis a reality.
Her
impossible to describe on these
of her labor are most appreciated.
Xlll
in
parents, for their reassurance
for her never-ending understanding
its
were always
also deserving of special note.
family
children
whom
McCoy's involvement
Jim
Dr.
whose love helped
bringing this work to
in this
deeply indebted to Dr.
data and technical support to holding theoretical discussions, both of
willing
environment
and resulted
this thesis,
research being conducted in a most satisfying environment.
Jerry Jost and
the space
INTRODUCTION
I.
GMT
At approximately 13:27
roared off the launch pad
Satellite
(GPS)
at
on June 26, 1993, an Air Force Delta-II rocket
Cape Canaveral,
as the primary
its
third stage carrying a Global Positioning
pay load. Attached to the second stage of the Delta-II was
NASA's Plasma Motor Generator (PMG)
tethered satellite mission. Following third stage
separation and second stage fuel depletion burn, the
orbit,
approximately 207 x 922 kilometers,
2000 seconds mission elapsed
time, a Far
Delta-II second stage, at a rate of
conducting wire from the
PMG
two
at a
PMG
system was
an
left in
elliptical
27.5 degree inclination. At approximately
End Package (FEP) was
spring-ejected from the
500-meter
to three meters per second, trailing a
tether deployer assembly.
Within minutes, the two-body
tethered satellite system stabilized in a gravity-gradient configuration with the
FEP above
the Delta-II second stage.
The primary
objective of the
PMG
experiment was to verify the
ability
of hollow
cathode plasma sources to couple electric currents from either end of a long wire moving
through the ambient
low-impedance
orbital velocity
Low
electrical
Earth Orbit
(LEO) ionospheric plasma.
By
providing a
connection with the ambient environment, while moving at
through the earth's magnetic
field, significant
currents can be induced in a
single-conductor tether wire, providing the capability for direct conversion of orbital
energy to extractable
electrical
current
(generator principle).
Alternatively,
orbital
"steering" forces can be induced through the tether by driving current in the opposite
direction with applied voltage
(motor
principle).
The
PMG
experiment utilized these
physical principles to investigate the overall practicality of reciprocal conversion of orbital
energy and
electricity,
and
also, to evaluate current
technology for transferring charge
charge between a spacecraft and the
LEO
environment, utilizing hollow cathode plasma
contactors.
The
PMG
electrodynamic tether orbited within the tenuous, electrically charged
layer of the atmosphere
known
as the ionosphere, providing the opportunity to increase
understanding of physical processes
same time exploring
objective
was
to
near-Earth space environment, while at the
to
plasma generator and the ambient ionospheric
artificial
accomplish
this
study utilized sensitive
ionospheric radars to sense field-linked plasma perturbations
plasma source. Near Lima, Peru, the powerful
Radio Observatory was aligned
horizontal magnetic field lines.
at Piura,
to
Two
fifty
at large
VHF
meter-wave
distances from the
megahertz radar
at
Jicamarca
observe field-aligned backscatter from the near
less sensitive fifty
megahertz wind-profilers, located
Peru and Pohnpei, Micronesia, were configured for ionospheric measurements
along the geomagnetic field
radar
key mission
investigate large-scale coupling processes and turbulent signatures
The approach
orbiting
A
the mechanical dynamics of tethered systems.
associated with an orbiting
plasma.
in the
system
to
Hilo,
lines.
Hawaii,
Additionally,
developed
for
NASA/JSC
remote
deployed a transportable
field
study
of
ionospheric
modification effects, spacecraft interactions with the lower ionospheric environment, and
reentering orbital debris.
Electromagnetic scattering from strong ionospheric turbulence generated during
operation of the
PMG
experiment was observed from several of the
radar probing of the geomagnetic flux tubes that
radar's fixed field-of-view.
On
PMG
lines at distances
plasma source. The purpose of
radar sites by
the orbiter through the
several passes, field-aligned backscatter
from plasma structure propagating along the
kilometers from the
mapped from
VHF
up
was observed
to several
hundred
this thesis is to present these
observations,
and
develop
disturbances by the tether.
the
implications
for
the
generation
of
field-aligned
II.
"Come,
let
us build ourselves a city, with a tower that reaches to the heavens..."
--
A.
BACKGROUND ON TETHERS
The Tower of Babel, from
the
Book of Genesis, Chapter
1
HISTORY
The
earliest
concepts for the application of tethers
in
space can be traced back to
the last century. In 1895 Kostantin Tsiolkovsky, the Russian schoolteacher
astronautics, reflected on a
way
and pioneer of
to create an environment devoid of terrestrial gravity
through the construction of an equatorial tower pointing into space, reaching beyond
geostationary altitude.
effects of gravity
he
did
not
He
postulated that upon ascending such a "Heavenly Tower," the
would gradually diminish
realize
it,
until they
had
Tsiolkovsky
disappeared completely.
conceived
the
first
Although
geostationary
satellite.(Tsiolkovsky, 1895)
Sixty-five years later, at the
dawn
of the space age, the Leningrad engineer
Artsutanov conceived an alternate concept, consisting of a platform anchored
pointing towards the earth, with a cable deployed
the earth's surface at the equator.
would carry a
A
down from
in
space,
the satellite to connect
second cable deployed upward beyond the
Yu
it
to
satellite
ballast to maintain the system's center of gravity in the geostationary orbit.
Artsutanov visualized his "Heavenly Funicular" as a means of supporting transportation
and thus was born the idea of a space
into space,
The
early 1960's
as rolling satellites, lunar
the earth,
saw
elevator.( Artsutanov, 1960)
a flurry of tether-related proposals, including concepts such
and asteroid
tethers, space elevators, a space "necklace"
Several early concepts involving
and tether rescues of stranded astronauts.
gravity-gradient stabilized tethers are
shown
in
Figure
around
1
.
Initial
application experiments
for tethered vehicle operations in space began in late 1966 as part of the
The
Gemini program.
effects of firing translational thrusters and the effects of gravity gradient stabilization
on the motion of the tethered system were examined using the Gemini XI and XII
spacecraft with the spent stage of their Atlas-Agena
1967)
D
launch vehicle. (Lang and Nolting,
Further development of the gravity-gradient concept
came
1969 when A.R.
in
Collar and J.W. Flower proposed a very long tether connecting two satellites
beyond geostationary
was located
altitude
and one
in
low earth
-
one
such that the center of gravity
orbit,
geostationary distance. (Collar and Flower, 1969) Additionally, the idea for
at
a space propulsion system was introduced by R.D. Moore, consisting of a conducting
wire with plasma contactors
at
His "geomagnetic thruster" was a true
either end.
forerunner of present day electrodynamic tethers. (Moore, 1966)
It
was
investigate the
complex dynamics of long
transportation.
Mario Grossi's proposal of a shuttle-borne
antenna and radio physics
use of tethers
beginning
in
made
not until the early 1970's, however, that coordinated efforts were
in
facility
space. (Grossi,
1974, Giuseppe
tethers as an innovative
was one of the
1973)
Colombo
Through
his
approach to space
ULF/ELF
tether for use as an
first practical
to
applications study for the
involvement
in
this
program,
quickly recognized the far-reaching potential of
the tether in space transportation. (Colombo et
al.,
1974)
Professor Colombo,
who
is
considered to be the modern pioneer of tethers applications, conceived, introduced or
analyzed virtually
new methods
and
in
all
of the
first
space propulsion schemes involving tethers.
of generating propulsion through
It
momentum exchange between
was these
spacecraft,
generating electric power and thrust through electrodynamic interactions with the
space environment, that established tethers
capability today.
Tether applications
in
in
space as a revolutionary operational
space exist
in a
wide range of
fields,
including
aerodynamics, gravity control, electrodynamics, transportation, science studies, and
planetary exploration.
GRAVITY-GRADIENT STABILIZATION
B.
There are two basic configurations
an orbiting tether system.
A
that provide controlled acceleration fields for
gravity-gradient stabilized tether rotates once per orbit in an
coordinate frame, while a rotating tether configuration consists of multiple
inertial
rotations per orbit.
gradient forces
is
An
understanding of the basic physical principles behind gravity-
fundamental when studying general tether applications.
single satellite in a circular orbit about the earth.
G
satellite,
is
the gravitational constant,
and
r is
must equal the
the radial distance
subject to a gravitational force,
-GMm
_
where
It is
Consider a
M
is
the
mass of
the earth,
from the center of the earth
m
is
the
mass of
to the satellite.
the
This force
centripetal force necessary to cause circular motion,
F = mrco
2
c
where
which
gravity,
satellite
must be
the satellite as
it
zero.
To remain
in
the force of
and the outward centrifugal force, keeps
pulls the satellite towards earth,
the satellite in orbit.
The balance between
angular velocity.
co is the satellite's orbital
a circular orbit, the net vertical force acting on the
Thus, by equating the two forces, the constant angular velocity of
orbits the earth can be solved for,
GM
2
co
3
r
Now
system
is
consider two masses connected by a tether
oriented vertically.
with altitude, thus each body
in
a circular orbit such that the
Gravity, centrifugal force, and atmospheric drag
in a
tethered system
simple "dumbbell" configuration, shown
in
is
Figure
all
vary
subject to separate influences.
2,
will
be
in
A
an equilibrium state
orbiting the earth as long as the tether remains aligned in the vertical and
system have the same orbital angular velocity.
gravitational forces acting over the system
centrifugal forces acting
For
this orientation,
must be equal and opposite
on the system. Neglecting the
parts of the
all
the
sum of
the
sum of
the
to the
tether's weight, the
summation of
force becomes,
— + GMm,
^ = m,r
GMm,
2
this
2
r
2 2
ft)
equation the tethered system's orbital angular velocity can be computed.
gravitational
at
+m
h
n
From
,
ffl
]
and centrifugal forces acting on the tethered system are equal and balanced
only one place
-
the system's center of gravity.
This equilibrium point
condition and experiences no net force in the radial direction.
two end masses
the earth, but the
the
same angular
center of gravity
They
are not.
is
For short tethers,
It is
are constrained
velocity as the center of gravity.
is in
in free fall as
by the tether
The value of
a zero-g
it
orbits
to orbit with
the orbital radius for the
given by,
3
the system.
The
this
GM
value will be nearly equal to the position of the center of mass of
However, since
from the center of the
gravitational acceleration changes non-linearly with distance
system does not
earth, the center of gravity of the tethered
necessarily coincide with the center of mass.
The separation
is
not dramatic for systems
using short tether lengths, but can be significant for long tethered systems. (Arnold, 1987)
Without the connecting
orbital velocity, while the
tether, the higher
mass would tend
lower mass would tend to
move
at
to
move
at a
slower
a greater orbital velocity.
Thus, the presence of the tether speeds up the higher mass and slows
down
the lower
mass. This causes the higher mass to be subject to a larger centrifugal than gravitational
force,
and the lower mass to be subject
to a larger gravitational than centrifugal force.
With
a tether in place, the net effect of these unbalanced forces
The
tether.
resulting
upward acceleration of
to create tension in the
is
mass and downward acceleration
the higher
of the lower mass gives rise to a force couple applied to the system, which forces
vertical orientation.
from the
Should a disturbing torque
vertical position, this force
illustrated in Figure 3,
which
act
into a
on the tethered system and displace
couple produces restoring forces
act to return the
it
system
at
it
each mass,
to a vertical orientation.
These
restoring torques act in both in-plane and out-of-plane directions to tend to keep the
system
in its
equilibrium
Although
state.
this vertical orientation is a stable one, there are
which cause the tethered system
forces
to oscillate
weak
but persistent
These forces
about the vertical.
include atmospheric drag, solar heating effects, electrodynamic forces (for conducting
tethers), solar pressure torque,
the gravity-gradient
tether length
due
is
and geomagnetic torques. For short tether lengths, where
relatively constant, the libration frequencies are
to the fact that both the displacement
system vary linearly with tether length.
This
is
independent of
and the restoring forces on the
not the case for very long tethers,
however, since the gravity-gradient can vary significantly over the length of the
The
tether.
natural restoring torque for out-of-plane disturbances tends to be stronger than that
for in-plane disturbances, thus out-of-plane librations
have a higher frequency than
plane librations. The natural frequency for in-plane oscillations
is
in-
given by,
f=2co
For out-of-plane
oscillations,
f=Sco
where
go is
the orbital angular velocity of the center of mass.
Optimum
tether's
design considerations for tethers used
mass while maintaining
its
required strength.
in
space
This
is
call for
minimizing the
accomplished by using
materials such as
weight
aluminum wire and Teflon
insulation
which have high strength
Additionally, aluminum's conductivity per mass
ratios.
is
to
about twice that of
copper, and Teflon provides good resistance to atomic oxygen erosion.
The
tether cannot
be too thin because of the significant risk of being cut by a micrometeorite impact.
Collisions with orbital debris pose a threat to any deployed tether, since any such
encounter with particles or objects of more than a few grams will almost certainly sever
it.
The
tether
must be constructed
expected lifetime.
most impacts so as
to survive
to
have a reasonable
Tethers designed with a constant cross-sectional area have a limited
length, however, tethers tapered to maintain a constant stress per unit cross sectional area
can theoretically have unlimited length. Thus the optimum design for high tether tensions
would be an exponentially tapered
and minima
at the
end masses.
maximum
tether with a
Although there
achieved, the payload that can be supported
is
no
area at the center of gravity
limit to the length that
the ends keeps getting
at
can be
smaller.(NASA
Report, 1989)
ELECTRODYNAMIC TETHERS
C.
One of
may
the
most rewarding applications of
be the electrodynamic tether, a
satellite
system
connected with an insulated conductive wire.
interactions
plasma
between a moving conductor, the
in the
tethers to
in
enhance man's use of space
which two separate payloads are
Electrodynamic tethers make use of the
earth's
magnetic
and the ambient
field,
ionosphere to allow propulsion and power generation. Such a system could
be used with solar arrays to offset drag
in
LEO,
replace batteries for
power
storage, or
provide propulsion for orbital maneuvers.
Consider the ionospheric plasma, co-rotating with the Earth, relative
tether
is
moving with
orbital velocity v.
For most tether applications v
10
is
to
which a
Eastward, while
the earth's magnetic field B, acts Northward.
This produces an induced electric field
acting upward, causing a sizable potential difference across the conducting tether due to
its
motion through the geomagnetic
By
field.
providing contactors (devices capable of
passing sufficient current to and from the plasma with
the
tether,
electrical
with
contact
the
Earth's
little
LEO
voltage drop) at both ends of
plasma environment
made,
is
establishing a current loop through the tether, external plasma, and lower ionosphere.
the undriven state, electrons
the tether and emitted
from the ambient plasma are collected by the upper end of
from the lower end, creating a net positive charge cloud
end and a net negative charge cloud
at the
move along
free charges are constrained to
ends of the tether
In
until they reach the
E
bottom end.
In a quasi-static
the top
at
model, excess
the geomagnetic field lines intercepted
by the
region of the lower atmosphere where their are
sufficient collisions with neutral particles to allow the electrons to migrate across the field
and complete the
lines
that
true
by the time the
Taking
circuit.
satellite
motion into account, however, one finds
circuit "closes", the satellite has traveled
dynamic problem appears
to
be a complex one.
plasma.(Bainum
et al.,
space environment. The
of the tether.
first
The upper conductor
at the
end
in this configuration,
the
how
is
coupled into and through
to
provide electrical closure
consists of passive large area conductors at both ends
collects electrons, while the lower
utilizes its large surface area to collect ions.
the upper
Thus
1986)
There are three basic plasma contactor configurations
to the
kilometers.
Figures 4 and 5 illustrate
current flowing along the wire due to the potential difference
the ambient ionospheric
many
A
plasma contactor
large conducting balloon could be
used
at
with the conductive surface of the spacecraft serving
lower end.
An
alternate
at the positive
method of
end of the
current coupling consists of a passive large area conductor
tether,
and an electron gun
11
at
the negative end.
In this case, the
ejection of a negative electron at a high energy
While
ion current.
this configuration
problem with electron guns,
is
the equivalent to collecting a positive
allows higher currents to be achieved,
power supply
requires an on-board electrical
is
that they tend to
to drive the electron gun.
become
One
it
also
general
current limited, typically by space
charge effects.
The
at
final configuration consists
of a plasma-generating hollow cathode assembly
each tether end. Instead of relying on a passive and physically large conducting surface
hollow cathodes generate an expanding cloud of highly conductive
to collect currents,
plasma.
This plasma provides the necessary thermal electron density to carry the
tether current in either direction
ionospheric plasma currents.
power supply,
from a
Although
tether end, until
this configuration
the bipolar nature of the emitted
been demonstrated
is
it
merged
into the
full
ambient
also requires an on-board
plasma offers greater
flexibility
and has
to work.(01sen, 1981)
Hollow cathode systems
are
considered to be the most desirable contactor
configuration. Their current reversibility allows the tether to function alternately as either
a generator or a motor.
the desired electrical
Additionally, they operate at lower voltages while
power
levels.
They
still
producing
are safer for the spacecraft system because they
establish a vehicle ground reference potential with respect to the local plasma.
with hollow cathode contactors, shorter tethers
may be used
Finally,
for required current levels,
greatly reducing the size requirements for stabilizing end masses, and simplifying tether
deployment and dynamics. (NASA Report, 1989)
Tethered systems
spacecraft orbital
in
LEO
generate electrical power
energy due to drag forces.
Consider a
stabilized, insulated, conducting tether, terminated at both
contactors.
As
this
system orbits the earth,
it
12
at
a rate equal to the loss in
vertical,
gravity-gradient
ends by hollow cathode plasma
cuts across the geomagnetic field lines at
about eight kilometers per second.
orbital velocities of
(emf)
where
is
induced
dO
in
relative to the
potential
an element of the length of the tether, given by the equation:
emf
the induced
is
A Faraday electromotive
geomagnetic
across the tether element length, v
field,
B
is
the tether velocity
is
the magnetic field strength, and dl
is
a differential
emf
element of the tether length pointing
in the direction
potential difference across the tether
by making the upper end of the tether positive with
This
of current flow.
By
respect to the lower end, allowing current to flow through the wire.
using variable
impedance matching techniques, the current passing through the
resistance or
be controlled, providing power for on-board systems, as shown
When
the current generated by the induced
emf
is
in
Figure
creates a
tether can
6.
allowed to flow, a force
is
exerted on the current, and thus on the tether, by the geomagnetic field given by:
dF= iJlxB
where dF
is
the force exerted
tether current, dl
is
on an element of the
tether
by the magnetic
B
is
the magnetic field strength.
In
LEO, where
velocity between the orbiting tether and the rotating geomagnetic field
a drag on the tether.
A
system for on-board use,
maintain
its
I is
the
a differential element of tether length pointing in the direction of
positive current flow, and
is
field,
altitude, this
it
direct result
is
generated
is
that
at the
when
electrical
expense of
power
is
is
orbital energy.
the relative
large, this force
generated by the
If the tether is to
electrodynamic braking must be compensated for by rockets or
other propulsion means.
Alternately,
against the
and
it
when
a current
emf induced by
the
geomagnetic
becomes a propulsive force
principle,
which allows the
from an on-board power supply
field, the direction
acting on the tethered system.
tether can
is
fed into the tether
of the force
is
reversed
Figure 7 illustrates this
be used as an electrodynamic thruster, with
13
applications of orbital maneuvering and drag compensation.
instance
is
that the propulsive force
generated
is
at
The
price to be paid in this
the expense of on-board electrical
power.
Electrodynamic tethers also have applications
in
power
Using solar
storage.
arrays, current could be fed into a tether during periods of array illumination, providing a
Then, during periods of darkness, orbital
propulsive force to boost orbital altitude.
energy could be traded for useful
DC
electrical
power
as the
geomagnetic
field
induced
voltages across the tether, inducing current flow in the opposite direction. This reversible
energy
storage
would complement
system
By
discharging of batteries.
battery
itself,
however,
it
systems
is
employing
charging
the
and
not mass-competitive with conventional
systems.(NASA Report, 1989)
Thus
the electrodynamic tether system presents a
new form
of continuous power
or thrust generation in space, which can in theory produce either an efficient power
supply of a few to hundreds of kilowatts of usable
electricity,
maneuvering forces for the
to orbit altitude
which
satellite.
There are effects due
or provide orbital
and inclination
act to limit the effective operating range of the electrodynamic tether
2000 kilometers and
orbit altitudes of less than
The major cause
for the altitude effect
is
orbit inclinations
the orbit inclination
magnetic
which decreases
is
field vector is
A
smaller cause
is
which
the orbit
as the inverse square root of the orbit radius.
As
increased, the angle between the orbit velocity vector and the
decreased until
this point the vector cross
Thus equatorial and low
tether velocity
below sixty degrees.
the decrease in geomagnetic field strength
goes as the inverse of the third power of the orbit radius.
velocity of the system
system to
at a
polar orbit velocity
product goes to zero and no voltage
inclination orbits generate the highest
and magnetic
field lines are perpendicular.
14
is
parallel to the field.
is
induced
emfs since
At
in the tether.
it
is
here the
Other
critical
issues that affect the performance of an electrodynamic tether
include plasma contactor cloud instabilities which impede the current closure process, as
well as the characterization of the magnetosphere current closure path and
losses.
The
tether's susceptibility to
its
potential
micrometeoroid/debris damage and the effects of
long-term insulator exposure also play a
role.
And
finally,
long tether dynamics and
associated librations will continually affect the performance of the tether.(Bainum et
1986)
15
al.,
16
THE SPACE ENVIRONMENT
III.
EARTH'S GEOMAGNETIC FIELD
A.
The magnetic
uniformly magnetized
field
of the Earth
in the direction
the orientation of this dipole
is,
to a first approximation, that of a sphere
of a centered dipole
undergo slow time
Both the magnitude and
axis.
variations, with a time scale of the order
of months to years, which give rise to gradual changes of the geomagnetic field
components.
error
The
"best
when compared
fit"
for this equivalent dipole model, for the smallest cumulative
to the actual field of the Earth,
from the center of the Earth, displaced towards the
model, shown
in
Figure
measured geomagnetic
8, is
known
field to
is
obtained by an offset of 436
Pacific ocean.
as the Eccentric Dipole
km
This displaced dipole
Model, and describes the
an accuracy of two to three percent.
The geomagnetic
poles of the dipole, the location where the axis of the fictitious dipole magnet intersect the
surface of the Earth, are located about 800 miles from the geographic poles, producing a
tilt
angle with respect to the Earth's rotation axis of
pole
is
1 1
.3
degrees.
located near Thule, Greenland at 78. 3N 291.0E.
located near Volstok Station, Antarctica, at 78. 3S
geomagnetic equator as the great
circle
1 1
The north geomagnetic
The geomagnetic south pole
IE.
One may
90 degrees away from
is
accordingly define a
either pole in
geomagnetic
latitude.(Heinz and Olsen, 1993)
The
relationships
between the geomagnetic
corresponding geographic coordinates (9
sin
X = cos78.3° cos0
cos A =
sin 78. 3°
cos6 -
,<|>
- 291°) + sin
cos(<|>
—-
-
78.3° sin 6
29 1° ) - cos78. 3°
cos A,
17
and longitude
are given by the equations,
)
cos(<|>
latitude
sin
9
(A,,A),
and the
A
static
dipole field can be defined by three components.
and azimuthal components of the dipole
tangential
geomagnetic
field
of the field
given by,
is
vector can be obtained
in spherical
=_
field's tangential
component
is
magnitude and direction of the
The
coordinates.
fi
sine
is,
Figure 9 depicts the coordinate system for the magnetic dipole
distance from the dipole to the fieldpoint,
<J>
is
component
given by,
field in the azimuthal direction
latitude of the fieldpoint,
radial
2B cos0
_
The component of the
radial,
(r/Rj
'
The
field the
With the
RE
is
field,
where
the radius of the Earth,
the azimuthal angle, and
BQ
is
9
is
r
is
the
the co-
the surface field at the
geomagnetic equator. (Tascione, 1988)
An
equation for an individual field line
which has the
B
rdd~
fi
is
equation,
its
latitude, the
r
e
2
sin
6
the radial distance of the equatorial crossing of the magnetic field line.
geometry depicted
in
Figure 10,
may be
rewritten in terms of geomagnetic
- R cos 2 X
inclination of the field line, called the magnetic dip angle,
latitude
This
compliment of co-latitude,
r
The
be obtained by setting,
solution,
r=/?
where Rq
may
dr
by the equation,
/
=arctan(2tan^)
is
related to magnetic
Finally, the
magnitude of the
field at
any given point
is
obtained from solving the
equation,
B=
-
B°
(3sin
,
2
3U.1)K
(r/RE j
B.
IONOSPHERIC PLASMA
More
than 99 percent of matter in the universe exists
of ionized and electrically neutral gases.
at the
same
time, provided there are as
positive charges
on the positive gas
in the
extends upward from about
altitude,
and
it
is
fifty
state, a
mixture
A gas
can be both ionized and electrically neutral
many
free electrons in the gas as there are net
Thus, plasma consists of a homogeneous
ions.
mixture of electrons and positive ions, surging and swirling
This plasma becomes important
plasma
in
electromagnetic bondage.
the Earth's environment in the ionosphere, which
in
kilometers above the ground to about 1000 kilometers
in
sustained by the ionizing action of solar ultraviolet and x-ray radiation on
the neutral atmospheric gas.
Between these
altitudes, ions
sufficient quantities to affect the propagation of radio
and electrons are present
waves
at frequencies
in
from a few
Hertz up to several megahertz. (Heinz and Olsen, 1993)
The
of electron density with altitude led to the subdivision of the
variation
ionosphere into distinctive layers
Ionospheric plasma
particles.
Even
is
at the
-
the
very tenuous, and
most dense
E
D,
is
and F regions shown
not predominantly
is
typically
one
radio waves back to earth, and
it
was through
this
numbers
1
1
more than one
In comparison, the neutral
billion particles
positive ions and electrons are present in sufficient
Figure
composed of charged
layers of the ionosphere, there are rarely
million electron-ion pairs in a cubic centimeter (cc) of space.
gas density for the same region
in
per cc.
in the
Nevertheless, the
ionosphere to reflect
process that ground-based radio wave
observations
detected the region of plasma above the Earth's electrically neutral
first
atmosphere.(NAS A Report, 1993)
On
the surface of the earth and
atmosphere can be considered to be an
currents flowing in that region.
atmosphere across the
ionosphere.
for approximately fifty kilometers, the
However, convective movements of the conducting upper
magnetic
earth's
field
produce
lines
In addition to natural variations in this
of the study of tethers
environment of an
in
in
VHF
tether.
plasma environment,
An
develops around the
the water,
radar
observations
satellite
Traveling
satellite,
trails
and over
the satellite
is
can
of the
space
at
orbital
velocities,
while a wake,
behind.
Due
in
some
it
actively
the satellite affects the density,
A
respects resembling a
to long range interactions
far greater distances than ordinary gases.
predicted to be unstable and to change
identify
system and the plasma.
in size
plasma sheath
wake
left
by a
between charges, a plasma
can sustain cooperative phenomena such as waves or oscillations
densities,
satellites
important scientific objective
temperature, and electrical properties of the surrounding plasma.
in
the
special features are likely to develop in the vicinity of a tether as
perturbs the plasma.
boat
in
These observations can be used to
between the tethered electrodynamic
Some
currents
an effort to characterize the environment before and after the
passes through the radar beam.
interactions
electric
space, involves measuring the plasma and magnetic field
electrodynamic
environment can be made
and therefore there are no
electrical insulator,
space plasma as they pass through their orbits.
alter the
satellite
upward
down
to
much lower
The plasma sheath around
and shape with variations
in
ionospheric density, magnetic field alignment, and the voltage developed across the tether.
These changes
affect the type
and amplitude of waves excited
observable turbulent disturbances
in
in
the wake, creating
the environment around the satellite. (NASA Report,
1993)
20
IV.
A.
THE PMG EXPERIMENT
MISSION OBJECTIVES
NASA's Plasma Motor Generator (PMG)
conducted
in
Low
mission was an active space experiment
Earth Orbit (LEO), designed to
validate theoretical predictions that a
plasma discharge from an onboard hollow cathode assembly (HCA) would provide a low
impedance
between an orbiting spacecraft and the ambient ionosphere
electrical coupling
for bipolar charge
transfer.
addition to
In
the
demonstration of plasma contactor
performance was an investigation of electrodynamic-tether behavior functioning as either
an orbit-boosting electrical motor, or as a generator that converts orbital energy into
electricity. (Jost
A
and Stanley, 1994)
comprehensive
globally-distributed
interactions
fixed
ground
of
stations,
between the operational
and transportable
measurements
was used
set
in
radar,
ground-based
measurements,
was defined
PMG
to
provide
system and the
an
LEO
utilizing
several
evaluation
of the
environment.
Sixteen
magnetometer, and optical systems were used for remote
support of the mission science objectives.
to study large-scale geophysical interactions
This multi-sensor approach
between the orbiter and the ambient
environment, monitor spacecraft/tether dynamics, and provide general orbital tracking
records. Analysis of these remote
performance objectives
•
(Jost
measurements provided
details
on the following mission
and Stanley, 1994):
Far End Package (FEP) deployment to greater than 200 meters
Tether deployment dynamics and damping
•
Space-based hollow cathode plasma production operation
Current flow along geomagnetic field
lines,
and closure through the ionosphere
Tether deflection by IxB drag and thrust forces
21
PHYSICAL DESCRIPTION
B.
The experiment
connected via an
consisted of a tethered system of two identical plasma contactors
18-AWG 500
meter conducting
included four major subsystems; the Far
Experiment
tether.
system was stabilized
away from
earth following
The FEP housed
its
hardware
End Package (FEP), Near End Package (NEP),
an electronics box, and a Plasma Diagnostics Package (PDP).
satellite
flight
The two-body
tethered
FEP
oriented
in a gravity-gradient configuration,
with the
spring-ejection from the second stage of the
DELTA-II.
a hollow cathode-based plasma contactor inside an open metal box of
dimensions 0.3m x 0.3m x 0.3m. The NEP, consisting of the electronics box and another
plasma contactor, remained fixed
attached
to
the
rocket
Electrostatic Analyzer
al.,
to the
second stage of the Delta n. The
PDP was
body, and consisted of two detectors, the Small
(SESA) and
the Ion
also
Electron
Mass and Energy Analyzer (MESA).(Lilley
et
1994)
In addition to the
were used
to bias the
vx B
induced potential difference along the tether, batteries
NEP and FEP
series with the tether,
with respect to each other. Varying resistive loads, in
were cycled through during the experiment. The resulting current
through the tether was the net effect of both contactor plasma clouds, one collecting and
the other emitting electron current to the local
electrons were collected at the FEP, the
When
electrons are collected at the
in the
Generator mode. (Figure 6)
NEP
The
PMG
space plasma environment.
system was
and by the Delta
in the
II
When
Motor mode. (Figure
rocket body, the system
limiting factor of this electrical circuit
is
7)
was
the
ionosphere's ability to carry current. For this reason, plasma contactors must effectively
spread the currents over a large enough area to reduce the current densities to the
necessary levels.
Thus, plasma contactors
22
at
each end of the electrodynamic tether
collection and emission by neutralizing space charge and scattering electrons across the
geomagnetic
field lines. (Jost
and Stanley, 1994)
Plasma contactors are a promising technology with applications
tether
in
electrodynamic
systems as well as grounding spacecraft to the space plasma environment.
cathode based plasma contactor
is
A
hollow
a device that emits a dense, low-temperature plasma
cloud through which ions and electrons are emitted and electrons are collected from the
surrounding plasma. The construction of the hollow cathode assembly
PMG
is
relatively
illustrated in Figure
high
12.
temperatures
Functionally, the
(typically
1100°
(barium-oxide impregnated tungsten) cathode
in
HCA
(HCA) used aboard
heats a flow of xenon gas to
C) within
a
hollow
electron-emitter
the presence of a strong voltage gradient
between the cathode and a corresponding anode
plate.
In this condition, the partially
ionized gas establishes a highly-ionized "plasma-discharge," which allows current to flow
freely
between
cathode
element
and
an
anode,
external
in
this
case
the
LEO
ionosphere. (Jost and Stanley, 1994)
In electron emission
mode, the electrons from the plasma contactor carry the
current while the ions neutralize the electron's space charge.
plasma, the electron density
is
In the dense contactor
approximately equal to the ion density.
Higher energy
electrons stream through the slowly expanding ion cloud while the lower energy electrons
are trapped within the cloud by the potential distribution near the cathode.
ion allows a
number of electrons
When
to
the plasma contactor
be emitted,
is
at
roughly a ratio which
Each outgoing
is,
collecting electrons, the contactor ions once again
neutralize the space charge of the ambient electrons.
The contactor plasma
turbulent due to current driven electrostatic instabilities.
23
is
extremely
Incoming electrons are scattered
by the contactor plasma and can be collected across the geomagnetic
effective radius of several meters. (Lilley et
field lines within an
1994)
al.,
FLIGHT PROFILE
C.
The
experiment
duration,
in
terms
plasma
of
contactor
operation
and
consequential active environment interaction, extended six to seven hours (approximately
four orbits) until the
activation
NEP
and
FEP
As
batteries expired.
a result of the successful
and extended plasma contactor operations, the ground
opportunities to collect data during
at least
three passes.
sites
typically
Figures 13 and 14
show
had
the
mission flight profile and the relative positions of the four ionospheric radars used to
view the
PMG geomagnetic flux-tubes. (Jost and Stanley,
1994)
RADAR OBSERVATION SITES
D.
The
VHF
radar technique utilized for
PMG
has been demonstrated during several
NASA CHARGE-H,
spaceborne particle beam and plasma source experiments including
SDIO SPEAR
monostatic
I,
NASA ATLAS,
(MST) doppler
and the Soviet-French
ARAKS
series.
Beam
steering
radars measure the doppler shift of oblique backscattered
echoes resulting from the ubiquitous small-scale turbulence
in the
These
atmosphere.
phase coherent radars allow measurement of the amplitude and doppler velocity
direction of probing of radio
Explaining
namely
scattering
MST
and
beam
is
that are scattered
back
radar observations requires
reflection.
from the variation of the
radar
waves
in the
to the receiving antennas.
two basic echoing mechanisms,
For monostatic radars, backscatter and reflection arise
refractive index, n,
half the radar wavelength.
whose
spatial scale
along the axis of the
For a near vertical beam
at fifty
megahertz,
observed echoes are usually a combination of Bragg scatter (also called turbulent
24
scatter),
The major process causing
Fresnel scatter, and Fresnel reflection.
from
the
irregularities
due
radars
ionosphere
is
the
observable radar echoes from meteor
and electron density fluctuations.
and incoherent
trails
MST
and reflection from refractive index
scattering
to temperature, humidity
the echoes of
scatter
from
In addition,
free electrons in
the ionosphere should frequently occur.(Liu and Edwards, 1989)
When
a radio
aligned disturbance,
reflection.
wave
it
is
pointed such that
The
scattered such that
For the case when the wave
the incident and reflected
maximum
traveling at an angle to the magnetic field encounters a field-
it
waves
is
1
is
its
normal
its
to the disturbance, the angle
angle of
between
80 degrees and the wave backscatters. Thus a radar
has a line of sight that
is
strength backscatter signal from any
refractive bending of radar
angle of incidence equals
waves
paths can be approximated by straight
normal
to the field lines will detect a
waves propagating along the
VHF frequencies
in the
lines. (Liu
is
field lines.
small enough that the ray
and Edwards, 1989)
Observations of the spacecraft/environment interactions were to be obtained from
a ground-based sensor network during the
index
caused by ionospheric plasma
variations
interaction of the
operating
VHF
PMG
mission.
Scattering from refractive
irregularities
was expected.
The
plasma clouds, produced by the HCA's, with the ionosphere was
expected to cause
ground-based
PMG
turbulent
radar.
ambient-plasma electron distributions
Similarly, the refractive index of the
plasma source should have been overdense
detectable
with
plasma surrounding the
relative to
incoming
VHF
electromagnetic waves, causing the apparent vehicle radar cross section to be evident
the collected radar data.
Thus, two primary measurable characteristics of the
in
PMG
plasma clouds were expected:
•
Enhanced
ionization levels and plasma turbulence in the vicinity of the vehicle
during contactor operation.
25
Propagation of plasma turbulence along geomagnetic flux lines over long
•
distances (100's of kilometers). (Jost and Stanley, 1994)
VHF
Four medium and large power-aperture
support of the
PMG
radar systems were utilized in
mission, located in relative positions for favorable viewing of the
geomagnetic flux-tubes for the
first four,
active-orbit
ground tracks of the mission.
All
four systems operated in a pulsed, narrow-band, fixed-pointing configuration with a
center frequency of approximately
Jicamarca Radio Observatory
in
megahertz.
fifty
The powerful ionospheric radar
W) was
Lima, Peru (11.948 S, 76.872
at
activated to
observe spacecraft/environment interactions, and provide data on plasma contactor
operation, plasma cloud size and diffusion properties, and geomagnetic field propagation
NASA/JSC
effects.
W)
151.849
for
deployed a transportable radar
complementary
direct
observations
to Jicamarca.
In addition,
plasma
clouds,
two wind-profilers, the
W)
Facility in Piura, Peru (5.167 S, 80.617
Pohnpei Island, Micronesia (6.96 N,
PMG
of
and the
158.19
E)
Hawaii (19.514 N,
to the island of
providing
NOAA VHF
NOAA VHF
Radar
data
Radar
Facility
on
were configured for ionospheric
measurements, supporting the Hawaii measurements, and providing a measure of
turbulence propagated along the geomagnetic field.
Finally, the
USSPACECOM
Radar
Tracking network and the Kwajalein Missile Range provided precision orbital tracking of
the tethered system during the course of the experiment. (Jost
The
primary
goal
for
all
plasma-perturbation signatures caused by
Based on
the
high
scattering
cross
VHF
the
PMG
sections
turbulence, for electromagnetic radiation in the
radar
and Stanley, 1994)
sites
moving through
of field
VHF bands,
was
of
the lower ionosphere.
aligned
it
observation
ionospheric
was possible
to
plasma
observe the
space-environmental effects of field-aligned disturbances projected great distances along
the geomagnetic field lines.
coupled
electrical
current,
Observation of such disturbances supports the concept of
flowing from within the conducting tether wire into the
26
rarefied ionospheric plasma.
to the
geomagnetic
enough
(at
circuit. (Jost
low
These traveling waves are theorized
field lines until the
to stay tightly confined
ambient density and collision frequencies are high
altitudes) to allow cross-field diffusion
and Stanley, 1994)
27
and closure of the
electrical
28
EXPERIMENTAL OBSERVATIONS AND ANALYSIS
V.
HAWAII PASSES
A.
The Transportable Radar System (TRS) deployed
observations was a
medium power-aperture coherent
several transmitter modules,
Hawaii for ionospheric
to
detection system consisting of
two coherent multi-channel
receivers,
and several phased-
array antenna systems that allowed tailoring for particular radiation pattern configurations.
Electronic
beam
steering
was incorporated
into the design
switch rapidly between direct illumination of the predicted
specular viewing of the projected geomagnetic lines which
this
which enabled the radar to
PMG
orbit,
and
the orbiter.
In
position in
mapped from
its
way, antenna pointing was optimized for each observation period to investigate the
characteristics of field-aligned
plasma turbulence propagating away from the active
PMG
tether-system along the geomagnetic flux tubes.
The monostatic RX/TX phased
array
was capable of generating a peak power of
160 kilowatts with a duty cycle of one percent
megahertz.
An
additional
monopulse
RX
array
at
its
operating frequency of
was configured
fifty
to provide interferometry
measurements. The antenna configuration provided a gain of twenty-six decibels and was
aimed
in
azimuth
at the
geomagnetic north pole, which for the Hawaii location was eleven
degrees east of geographic north.
It
was phased
in
the vertical direction to produce the
required elevation angle for the planned ionospheric observations.
Hawaii,
PMG
was expected
of thirty-seven degrees.
over Hawaii were made
vertical
In
its first
to pass through the radar's field-of-view at an elevation angle
Subsequent observations perpendicular to the magnetic
at
pass over
an elevation angle of fifty-five degrees. (Figure 15)
field lines
The
radar's
and horizontal beam widths were twenty degrees and five degrees respectfully.
29
Electromagnetic energy with a pulse width of
every interpulse period (IPP).
For the
first
pass of
microseconds was sent out
fifty
PMG
over Hawaii, an IPP of
five
milliseconds was used. Following each transmission, the receivers were used to detect any
incoming
fifty
megahertz radiation. Sampling every forty microseconds resulted
gate interval of
Due
6000 meters.
waveform would be required
to
in
a range
to the long slant ranges to the satellite, a
coded
negate the effects of range-aliasing, while ensuring
adequate doppler coverage, and sufficient average radiated power. However, a coded
pulse was not used, and thus returns from
two
six
kilometer intervals, separated by 750
kilometers, are actually contained in each sampled gate.
sampled, beginning
at a slant
A
total
of 48 range gates were
range from the radar of 107 kilometers for
PMG's
first
pass
over Hawaii.
At the Hawaii radar
site,
the in-phase and quadrature
components of the received
radar signal from each scattering volume were recorded continuously on analog tape.
vast majority of
what the receivers detected was white noise background. However,
turbulent propagations were present
those caused by meteor
trails,
any
the observed region of the ionosphere, such as
in
then an enhancement of the incoming
radiation could be detected at a specified time delay after the
sight range
if
The
from the radar to the returned
initial
megahertz
fifty
The
pulse.
signal could then be determined
line
of
by the time
delay between the transmitted and received signal.
The Hawaii data underwent
Corporation
-
Center for Space Physics
sampled and analyzed,
be prepared.
the
in
first
phase of processing
to allow plots of signal intensity as a function of time
The power representing
power
Systems Planning
Houston, Texas. The analog tape was
the signal
to the system noise, the signal plus noise
30
digitally
and range to
was determined by summing
of the in-phase and quadrature components of the received signal.
the received
at
the squares
In order to normalize
power was
first
calculated,
then the noise
power was subtracted from
Noise estimates were calculated by
it.
averaging sampled ranges that did not contain radar echoes, while signal plus noise
estimates were calculated by averaging sampled ranges that included returns.
collected during the
first
pass of
PMG over Hawaii are shown in Figure
At the Naval Postgraduate School,
The backscattered
performed.
signals
Hawaii data was
sampled from the forty-eight range gates provided
A
fast
Fourier transform was used to
information for each return echo observed
Interpretation of the collected data
16.
additional analysis of the
both intensity and doppler frequency information.
derive this spectral
Data
was complicated by
in
the
Hawaii pass.
background noise of meteor
the
events and unstable E-layer echoes, as well as the ambiguity resulting from the rangealiased
By comparing
waveform.
the spectral
components of each observed
return,
however, discrimination signatures to unambiguously distinguish natural events from
PMG-induced echoes were determined. This
analysis
as part of his Master's Thesis. (Brewster, 1994)
PMG
The
not observed at thirty-seven degrees elevation during
through the pass,
at
time 14:56:00
(HHMMSS),
was performed by Wayne Brewster,
satellite
its first
the radar
system was apparently
pass over Hawaii.
Halfway
beam was switched
to an
elevation angle of fifty-five degrees to look perpendicular to the magnetic field lines.
Approximately four minutes
after the anticipated
PMG
meridian passage, a cluster of
strong echoes were detected at a slant range consistent with that of the ray-path specular
point for the geomagnetic flux-tube linked to
PMG. The
discrimination signatures for
these signals were consistent with those determined to be plasma.(Brewster,
1994)
Figures 17-19 display the spectral characteristics of these signals, as well as the time and
slant range information
of returns
in
about the echoes. The top panel of Figure 17 shows the intensity
range gates 1-16.
The bottom panel
is
a fast fourier transform versus time
plot of the radar returns for range gate 3, corresponding to slant ranges
31
from 869 to 875
Figures 18 and 19 display similar returns for range gates 4,
kilometers.
each six kilometer interval beginning
A
kilometers respectfully.
First
seen
in
approximately time 14:58:26. Six seconds
is
observed
at
881,
7,
with
and 893
887,
disturbance, interpreted as a traveling plasma
these figures.
identified in
a slant range of 875,
at
and
5, 6,
wave can be
range gates 3 and 4, the event occurs
range gates
later, in
5, 6,
and
at
disturbance
7, a
time 14:58:32.
The Kwajalein Missile Range
radar
was tracking
the orbiting tether system as
it
passed overhead, recording the slant range, azimuth and elevation of the Delta
II in its
PMG
altitude
orbit.
From
these measurements an accurate orbit
and latitude and longitude as
forward
until
it
orbited the earth. Projecting the orbit of
it
it
PMG experiment
reached Hawaii allowed a precise determination of the time
the geomagnetic meridian to be made.
PMG as
was derived, giving the
made
its first
Table
contains a portion of the orbital data for
1
pass over Hawaii. Meridian crossing occurred
angle reached eleven degrees,
orbiting over the Earth
at
at
time 14:54:19
a geographic
PMG crossed
(HHMMSS).
latitude
At
when
this
the azimuth
time
PMG
was
of 25.8 degrees north, geographic
longitude of 206.5 degrees east, altitude of 641.2 kilometers, and a slant range from the
radar site of 982.5 kilometers.
track at this time, and
While no
PMG was approaching its highest inclination on
was decreasing
direct scatter
the exact time
PMG
meridian was established.
radar
beam perpendicular
system was observed
line
beam and crossed
the magnetic
of plasma echoes, subsequently detected with the
magnetic
field,
were a
the field line, then these signals equate to a turbulent
magnetic field
at the anticipated
through reconstruction of tracking radar data,
the center of the
If the cluster
to the
satellite
this pass,
moved through
ground
in altitude.
from the
meridian crossing point during
its
intersected by the
PMG
32
result of
PMG's
passage through
plasma waveform aligned with the
orbiter,
traveling
at
some propagation
velocity.
line,
The
velocity of the plasma waveform, in the direction along the magnetic field
PMG
can be calculated using the approximate flux-tube path length from
to the
radar field-of view perpendicular to the field lines, and the time delay between meridian
crossing and the detection of the plasma events.
Using the geographic coordinates of the Hawaii radar
site, its
magnetic latitude
can be calculated to be 20.5 degrees north. Likewise, using the geographic coordinates of
the nadir point associated with
degrees north
is
PMG's
meridian crossing, a magnetic latitude of 26.3
The magnetic dip angle of
found.
then be calculated using these magnetic latitudes.
the field lines at these locations can
Above Hawaii,
the field lines are
inclined at an angle of approximately thirty-seven degrees, while at the point of meridian
crossing this inclination increases to approximately forty-five degrees. Thus, at the point
in the
ionosphere where the Hawaii radar was pointing perpendicular to the field
value of the inclination
is
is
between these two angles. Since the inclination of the
relatively constant over this region, a straight line approximation will
flux-tube distance between
line, the
field line
be used for the
PMG meridian crossing and the field-of-view perpendicular to
the field line.
Forming a
right triangle
from the pass geometry, shown
in
Figure 20, the
approximate propagation distance of the plasma waveform can be calculated by
multiplying the slant range to
the
two radar beam
positions.
PMG
The
meridian crossing by the sine of the angle between
result is a propagation distance of
The time delay between meridian crossing and
minutes and seven seconds.
Using
this
detection of the
first
303.6 kilometers.
plasma echo was four
time and distance, a propagation speed of
1
.20
kilometers per second can be calculated.
The nominal sources of
error
approximation for the geometry of the
in
this
field
33
calculation
line,
six
include
the
straight
second uncertainty
in
line
time of
observation of the disturbance, as well as single source projection errors for the orbital
position of the Delta
II
rocket based on radar tracking by Kwajalein.
JICAMARCA PASSES
B.
In the foothills of the
also
is
employed
Andes mountains,
measure the
to
Radio Observatorio de Jicamarca was
the
intensities of field-aligned propagations.
Since Jicamarca
located approximately one degree north of the geomagnetic equator, the local magnetic
field lines are near horizontal, presenting an excellent opportunity to
propagations associated with
PMG
charge neutralization.
megawatts and a duty cycle of 0.6 percent,
largest power-aperture radar.
The
this fifty
A
With a peak power of two
megahertz observatory
has a two-way
radar's antenna
degree, and provides forty-five decibels of gain.
observe plasma
waveform with
is
the world's
beam width
of one
a pulse width of 6.67
microseconds and a pulse period of 1100 microseconds was used for the ionospheric
observations.
interval of
Sampling of
return echoes
500 meters, providing a
total
was divided
into
window of 125
250 range
gates,
each with an
kilometers for data collection.
Throughout the data acquisition, the Jicamarca radar was pointed perpendicular
the geomagnetic field lines in the lower ionosphere.
8-mm
the receivers were digitized and recorded to
this data
was not
initial
PMG Mission Report. (Jost and Stanley,
and
and quadrature outputs of
recording tape
at the
radar
site.
While
A-scope detections of propagating plasma
turbulence, similar to those observed in the Hawaii,
2, 3,
real
available to the researchers at the Naval Postgraduate School for further
processing, the times and ranges of
passes
The
to
4,
were recorded and presented
in the
1994) Discrete radar detections were obtained
where strong echo returns from flux-tube heights consistent with
were observed.
34
in
PMG
Pass 2
1.
Pass 2 over Jicamarca was a descending pass north of the observatory
was moving from northwest
the satellite
losing altitude in
meridian
when
The
its orbit.
its
to southeast in
its
groundtrack), with
PMG
tethered satellite system crossed the Jicamarca magnetic
reached an azimuth angle of six degrees east of geographic north.
From
the orbital tracking data in Table 2, this occurred at time 16:57:03
when
PMG
was
(i.e.,
(HHMMSS),
orbiting over the Earth at a geographic latitude of 7.2 degrees south,
geographic longitude of 283.6 degrees
from the radar
site
east, altitude
of 193.2 kilometers, and a slant range
The radar event associated with plasma
of 566.7 kilometers.
turbulence projected along the field line that coupled the
PMG
system and the radar's
field-of-view occurred at a time of approximately 17:03:30 (plus or minus thirty seconds),
at
The geometry
a slant range of 265.0 kilometers.
shown
is
Figure 2
A
different
propagating plasma wave
is
approach
latitude
in
calculating
radar's
beam
altitude of
is
displacement of
PMG
typical-altitude reference.
From
Due
traveled
by
the
to the dipole-behavior of
from Jicamarca
as the tethered satellite
this flux-tube height
The height of
is
dependence
the field line centered in the
found by adding the flux-tube height variation from Figure 22 to the
PMG at meridian crossing.
PMG crossed Jicamarca's
'
distance
above the radar of the field-aligned current flow
system crosses the magnetic meridian. Figure 22 graphs
300 kilometer
the
used for the Jicamarca passes.
the flux-tube geometry, the height
for a
is
1
slightly
dependent on the
associated with these observations
magnetic meridian during pass 2
at
a time of 16:57:03.
the geographic position of the nadir point, the difference in latitude between
meridian crossing and radar location was 4.7 degrees.
height variation of 365 kilometers using this value.
35
Figure 22 indicates a flux tube
Subtracting out the 300 kilometer
reference height results in the magnetic field line above Jicamarca being sixty-five
kilometers
higher
in
than
altitude
geomagnetic position of the radar
at
site,
the
point
PMG
of
meridian crossing.
The
located one degree north of the magnetic equator,
allows the dip angle of the field lines above Jicamarca to be calculated
at
two degrees.
beam
is
pointing for
This angle
is
also equal to the angle off vertical that the radar
perpendicular observation of the field lines overhead
From Figure
.
21, the arc length S can be calculated using the latitude difference
between Jicamarca and the point of meridian crossing, and the radius from the center of
the earth to the altitude at
which
PMG crossed the field line,
5 = (4.7")(— )(6378.1km + 193.2 km) = 541.5
km
180
In order to find the
propagation distance D,
length calculated above that
is
we must
first
subtract that part of the arc
swept out due to the inclination angle
using a radius equal to the height
at
which
I
of the radar beam,
PMG crossed the field line,
(2")(— )(193.2km) =
6.7
km
180
The difference between
this
the distance the propagating
magnetic
field line.
two
calculations
is
534.7 kilometers,
plasma wave traveled
By forming
which approximates
in a horizontal direction
along the
a right triangle with this distance as one side, and the
sixty-five kilometers traveled in the vertical direction as the other, the value of the
hypotenuse describes the approximate
total distance traveled
by the wave, calculated to
be 538.7 kilometers.
The
first
radar return signals were detected
and twenty-seven seconds following
PMG
at
approximately 17:03:30, six minutes
meridian crossing.
the distance calculated above, an average propagation
second can be found.
The
thirty
second uncertainty
gives a range from 1.29 to 1.51 kilometers per second.
36
Using
this
time span and
speed of 1.39 kilometers per
in the
reported time of observance
Pass 3
2.
Pass 3 over Jicamarca was also a descending pass, however,
satellite
time the
this
system crossed the magnetic meridian south of the observatory, and was gaining
altitude in
its orbit.
Meridian crossing occurred 180 degrees from the Pass 2 location,
an azimuth angle of 186 degrees east of geographic north.
time 18:38:29
(HHMMSS), when
PMG
was
From Table
3, this
occurred
at
at
orbiting over the Earth at a geographic
latitude of 17.3 degrees south, geographic longitude of 282.5 degrees east, altitude of
215.0 kilometers, and a slant range from the radar
event associated with this pass occurred
minus
at
site
of 644.1 kilometers.
radar
a time of approximately 18:48:30 (plus or
Figure 23 illustrates the
range of 240.0 kilometers.
thirty seconds), at a slant
The
geometry associated with these observations.
For Pass 3 calculations, the same method utilized
that
PMG
in
crossed the magnetic meridian at time 18:38:29.
Pass 2 was used.
Observe
This time, however,
PMG
passed south of Jicamarca, and from the geographical position of the nadir point, the
difference in latitude from the radar site
was
-5.4 degrees.
From Figure
22, this angular
difference equates to a flux-tube height variation of twenty-seven kilometers once the 300
kilometer reference altitude
kilometers from the point
at
is
subtracted.
which
Thus the
PMG crosses
it
field
line
rises
twenty-seven
to the point located at the center of the
radar beam.
With
this
pass geometry,
the horizontal direction
is
shown
Figure 23, the desired propagation distance
in
the combination of the arc length S, and the arc length swept
out by the inclination angle of the radar
(5.4"
in
)(— )(6378.1km
beam from
local vertical,
+ 215.0 km) + (2")(— )(2 15.0 km) = 625.4 km
180
180
37
Once again forming
the right triangle with sides of 625.4 kilometers and twenty-seven
kilometers, the hypotenuse describes the approximate total distance of
wave propagation.
This value calculates to be 626.0 kilometers.
The plasma echoes were
reported at time 18:48:30 during this pass, which
minutes and one seconds following
PMG
meridian crossing.
was
ten
This time delay equates to
an average propagation speed for this pass of 1.04 kilometers per second. The uncertainty
again comes from the reported time of echo activity, resulting in a probable range of 0.99
to 1.10 kilometers per second.
Pass 4
3.
Like Pass
with
PMG gaining
186 degrees
this
time
at
3,
Pass 4 was also a descending pass south of the observatory,
altitude in
time 20:19:11
its
orbit.
From Table
(HHMMSS), marking
PMG was orbiting over the Earth
geographic longitude of 281.9 degrees
from the radar
site
4,
its
reached an azimuth angle of
magnetic meridian crossing.
east, altitude
of 282.8 kilometers, and a slant range
As with
the previous passes, strong radar
returns were observed, occurring at a time of approximately 20:42:30 (plus or
thirty seconds), at a slant range of
At
a geographic latitude of 23.7 degrees south,
at
of 1365.1 kilometers.
PMG
445.0 kilometers.
minus
Figure 23 again illustrates the
geometry associated with these observations.
Pass 4 calculations are very similar to Pass
much
further south in this case.
At time 20:19:1
crossed Jicamarca's magnetic meridian.
3,
1,
however,
PMG crossed the meridian
orbital tracking data
The geographic
-1 1.8
in a flux-tube height variation
kilometers, once the 300 kilometer reference height
38
is
that
PMG
latitude associated with the
nadir point of this crossing differed from the radar position by
Figure 22 with this value results
shows
degrees.
Entering
of approximately 181
subtracted.
This
is
the vertical
distance the plasma
waveform
transited
from the meridian crossing point before passing
through the radar beam.
The
two
horizontal propagation distance
is
calculated as for Pass 3, the combination of
arc lengths,
(11.8")(— X6378.1 km + 282.8 km) + (2")(— )(282.8 km) = 1378.2 km
180
Thus, the right triangle for
180
this
pass geometry has sides of 1378.2 kilometers and 181
kilometers, resulting in a total propagation distance of approximately 1390.0 kilometers.
The
radar event associated with this pass occurred at time 20:42:30, twenty-three
minutes and nineteen seconds following meridian crossing.
propagation speed of 0.99 kilometers per second.
The
detection of the event results in a probable range of 0.97 to
39
This equates to an average
thirty
1
second uncertainty
.02 kilometers per second.
in
40
VI.
The two-body
was
SUMMARY AND DISCUSSION
tethered satellite system that comprised
stabilized in a gravity-gradient configuration, with hollow
either
end of a 500 meter conducting
wire.
NASA's
PMG
experiment
cathode plasma sources
at
These hollow cathode plasma contactors
coupled electric currents from the wire directly into the ambient ionospheric plasma,
providing an
turbulent
excellent
signatures,
opportunity
observe the ionospheric modification effects,
to
and spacecraft-environment interactions of an orbiting plasma
generator.
At any given time, the hollow cathodes
at either
end of the tethered
satellite
system apply potential pulses of opposite polarization to the magnetic flux tubes they
intercept.
The perturbation
waves which
travel
that occurs as a result of this charge transfer generates
away from
between the tether end
the
PMG
plasma
system, transporting the space charge set up
These turbulent plasma waveforms, triggered by the
flux tubes.
conducting tether and plasma contactors traversing the earth's geomagnetic
field lines in
the ionosphere, appear to travel tightly confined along the field lines over very large
distances.
Due
these traveling
to the excellent scattering properties of ionospheric plasma irregularities,
waveforms
are detectable
was observed on
radar.
VHF
radar
disturbances of a propagating plasma structure associated with
PMG
Through range probing of
sites, field-aligned
from ground-based
the geomagnetic flux-tubes
several of the spacecraft's passes.
the predicted flux-tube heights
above several
Strong, persistent echo returns from
were observed, which mapped back to
PMG
at the
time of
meridian crossing with an average velocity corresponding to average ion velocities
lower ionosphere.
The following
experiment, from observations
table
made
in
summarizes the
results obtained during the
Hawaii and Jicamarca,
41
in the
PMG
Site
Radar
Meridian
Pass
Time
Propagation
Effective
Crossing
Event
Delay
Distance
Speed
(HHMMSS)
(HHMMSS)
(SEC)
(KM)
(KM/SEC)
HI
1
14:54:19
14:58:29
247
303.6
1.20
JIC
2
16:57:03
17:03:30
387
538.7
1.39
JIC
3
18:38:29
18:48:30
601
626.0
1.04
JIC
4
20:19:11
20:42:30
1399
1390.0
0.99
The observation of two
distinct signals during the
Hawaii pass remains an unsolved
problem. Perhaps a small component of the wave's velocity
the echoes to be observed at the
eddying motion
occurs
in the radial direction
two specified times. Notice
over the
distances
large
also that
of observation
experiment, seen by the decreasing waveform propagation speeds
some
diffusion or
involved
in
causes
in
this
the successive
Jicamarca passes as the propagation distances increased.
In
comparison
to the
average propagation velocities shown
thermal velocity of the ionosphere electrons
v
where k
is
-
(
is
m
e
is
above, the
given by the equation,
«/
)" 2
23
Boltzmann's constant (1.38x 10~ //" K), Te
electrons (use 1000°K), and
in the table
the electron
is
the average temperature of the
mass (9.1x \0~"kg).
Calculating this
value results in an electron thermal velocity of approximately 123 kilometers per second
much
greater than the observed velocities above.
the ions
is
the
same
as the electrons,
Assuming
-
the thermal temperature of
and substituting the oxygen ion atomic weight
(15.999) into the above equation, the ion thermal velocity can be calculated to be
approximately 0.72 kilometers per second.
42
This value
is
of the same order as the
propagation velocities observed
in the table
sources were emitting xenon gas, this calculation
The
weight of 131.30).
second, which
is
resulting
Since the hollow cathode plasma
above.
is
repeated using xenon ions (atomic
xenon ion thermal velocity
0.25 kilometers per
is
close to an order of magnitude less than the values in the table.
Plasma waves propagating away from an electrodynamic
tether system in low-
earth orbit have been postulated to be Alfven and fast-mode waves.
wave
dispersion can be thought of as representing the
loaded with plasma,
is
plucked
confined to the Earth's magnetic field
occur
at
velocity
travel tightly
and possible reflection of these waves
lines,
may
given by the equation,
is
vA
fi () is
B-
=
V
7
the permeability constant {Any. \0~
and the measured value of the Earth's magnetic
is
Such waves
a field line,
density gradients like the E-layer of the Earth's ionosphere. (Dobrowolny, 1993)
The Alfven
where
when
that propagates
in the transverse direction.
Alfven wave
Ba = 3.13x 10~
5
7\
H / m),
p m =n o m
mass
density,
field at the Earth's surface at the
equator
Repeating the calculation for xenon ions results
velocities
the ion
Using a value for the ionospheric number density of n o = 10
and assuming oxygen ions, the resulting Alfven velocity
per second.
is
l
in
is
n
m~
3
540 kilometers per second.
an Alfven velocity of 189 kilometers
Neither of these results are of the same order as the observed propagation
above Hawaii and Jicamarca.
Following the results of Urrutia
disturbances propagate in a whistler mode.
et
al.,
These
43
1994,
it
may be
results differ
that
these
plasma
fundamentally from the
traditional current
model: instead of Alfven wings and a phantom current loop,
Urrutia
predicts whistler wings and a short diffuse current loop, due to cross-field shunting
electron Hall currents.
At each position
in their orbit, the
plasma contactors on the
electrodynamic tether excite a low-frequency whistler wave packet along the magnetic
The superposition of
field line.
structure propagating
wave packets forms
these
away from
the tether system,
which
a coherent, wing-like current
travel along the field lines at
whistler speeds and disperse by inducing secondary plasma currents.
The
results of this
study suggest that an electrodynamic tether cannot generate a long, filamentary current
loop.
Instead, the current loop
shunted by cross field currents associated with the
is
continuous shedding of whistler waves, well before encountering the boundary of the
plasma.
Such a whistler wing generated by a long space
whistler tone detectable at the ground
In
when
tether could possibly
the tether passes by. (Urrutia et
order to find the phase velocity of a whistler
ionosphere,
it
is
necessary to
This refractive index,
n,
first
wave
al.,
produce a
1994)
traveling though the
solve for the refractive index of the ambient plasma..
can be found from the following equation,
/c°
2
n = \-
2
i-V
/CO
where
co is the
frequency of the wave, the plasma frequency,
2
no e
2
£„me
and the electron gyrofrequency,
co
,
is
given by the equation,
i
44
co
p
,
is
given by the equation,
(oc
qB
—
m
=
e
For these equations, n
(1.6xlO~
q
is
19
C), e
the charge
magnetic
field
is
is
number
the ionosphere
density, e
the permittivity constant (8.85x 10
on the
(use
particle
BJ.
-
in this
F/m),
and
case an electron,
Calculating
plasma
the
gyrofrequency using these numbers results
megahertz, respectfully.
_12
m
B
e
is
is
frequency
values of
in
the electron charge
is
the electron mass,
the strength of the
and
electron
2.84 megahertz and 0.88
For the special case of whistler wings, where
square of the refractive index of the plasma
the
co«co
c
,
the
inversely proportional to the frequency of
is
the wave,
CO
ft)
Finally, the phase velocity of the traveling
CO,
wave can be
CO
calculated using the relationship,
c
n
where
c
is
the speed of light in a
vacuum (3.0x \{fmls).
Considering possible wave
frequencies from 0-1000 Hertz, the corresponding phase velocity was calculated and
plotted in Figure 24.
For frequencies
in
the
ELF/VLF
range, the phase velocity of
whistler wings appear to be in the 100-1000 kilometer per second range, again far greater
than the propagation velocities in the table above.
In conjunction with the
PMG radar measurements,
the University of
provided a highly sensitive ground-fixed magnetometer for observations
45
at
Genoa,
Italy
the Earth's
surface of spontaneous and
man-made
emissions, propagating from
the orbiting tether
through the Earth-ionosphere cavity. This super-conducting quantum interference device
(SQUID) magnetometer was used
magnetic fluctuations during
to investigate perturbations in the
PMG's
Although
pass overhead.
ambient natural
final analysis
has not yet
been completed, preliminary results so far do not exclude the presence of possible
perturbations, such as whistler wings, affecting the
satellite
(Minna
et al.,
The
table
below summarizes the various propagational wave calculations made
Only the oxygen ion thermal velocity appears
electron thermal velocity
Xe+
0+
noise.
1994)
the preceding paragraphs.
0+
ELF background
1
to
in
be of the same
23 km/sec
0.72 km/sec
ion thermal velocity
0.25 km/sec
ion thermal velocity
Alfven velocity
540 km/sec
Xe+ Alfven velocity
ELF/VLF whistler wave
189 km/sec
100- 1000 km/sec
velocity
order of magnitude as the proposed propagation velocity of the plasma disturbances
observed
at
Hawaii and Jicamarca.
associated with
PMG's
This suggests that the traveling disturbances
meridian crossing are ion sound waves, composed of the ambient
plasma of the ionosphere. Alternatively, these signals might be thought of as ambipolar
diffusion of plasma from the cathodes along the field line at approximately the ion sound
speed.(Chen, 1984)
46
VII.
CONCLUSIONS
Multiple observations of the plasma-perturbation signatures associated with
PMG
meridian crossing supports the concept of disturbances, caused by passage of an
electrodynamic tether through the ionospheric plasma, propagating along the magnetic
In each instance, the observed
field lines.
plasma disturbance lasted several seconds and
had a propagation velocity of the order of 1000 meters per second. The large distances of
propagation suggest that the signals would continue their path along the field lines, until
reaching the
this
E
layer of the ionosphere. If these signals represent field-aligned currents,
would allow
The
true
for current loop closure.
dynamics of the phantom current loop
further processing of
more
detail
duration.
all
still
remain a mystery, however,
data collected for each pass during this experiment will provide
regarding signal strength, spectral behavior, spatial diffusion, and echo
This
detail
should be more interpretable
in
terms of field-aligned current flow,
current closure phenomenology, and general electrified-spacecraft interactions with the
ionospheric environment.
moving
in
The complex sheath
structure around a highly charged
a magnetoplasma and the processes occurring there, such as
and plasma expansion phenomena, are
still
largely unexplored.
characterizing the satellite environment, and
wave and
A
particle
large
body
wave generation
wake
phenomena
is
probably
associated
with this wake, and with the expansion of the outside plasma into this evacuated cavity
needs to be investigated.
47
The concepts
However,
the
for use of tethers
continuing
in
space have been with us for some time.
new developments concerning electrodynamic
provide a revolutionary capability for generating propulsion through
and
in
tethers
momentum
exchange,
generating electric power through interactions with the space environment.
48
now
Gravity-gradie nt
Test COMSAT
(1964)
Grossi (1972)
Columbo (1974)
^rs.
Figure
1:
Early Gravity-Gradient Concepts
49
Centrifugal
Force =
Gravitational
r_
MM
Force » r*xm+M
G
,
/
2
r
Ti
M
w
r
1
1^
1
_
Tether Tension
j
Center of
Gravity
^J»h.
Orbit
r"
w„
Gravitational
Force
I
I
Local Vertical
Figure
2:
Forces Acting on Tethered Satellites
50
=GMM,/r
2
2
CentrifugalGravitational Force
Resulatant
V
Restoring
Force
Component
Tether Tension
Resultant
Restoring
/' Force
-
\v*'
X
C.»„.«»1
Local
CentrifugalGravitational Force
\
Vertical
EARTH
Figure
3:
Restoring Forces Acting on Tethered Satellites
51
Figure
4:
Current Paths for Electrodynamic Tethers
52
s
21
u
3
u
E
o
c
a
**
a*
JC
H
53
FAR END PACKAGE
(PLASMA CONTACTOR)
POWER (GENERATOR)
Figure
6:
The Generator
Principle
54
FAR END PACKAGE
(PLASMA CONTACTOR)
THRUST (MOTOR)
Figure
7:
The Motor
Principle
55
*
Figure
8:
The Eccentric-Dipole Model
B
of the Earth's Magnetic Field
56
*-
Figure
9:
Geometry
for a Static Dipole Field
57
y
Figure 10: Geomagnetic Latitude and Equitorial Crossing Distance of a Field Line
58
DAY/NIGHTTIME ELECTRON CONCENTRATIONS
600
NIGHTTIME
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10
ELECTRON CONCENTRATION
Figure 11: Ionospheric Electron Density Layers
59
(cm"
i
10'
3
)
CROSS SECTION
BARIUM
IMPREGNATED
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INSER
9
NEUTRAL ..= •;
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Figure 12:
KEEPER (ANODE)
PMG Hollow Cathode Assembly Schematic
60
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HAWAII 06/93
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Figure 16:
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64
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78
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80
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