RECONNECTION IN PLANETARY MAGNETOSPHERES
C. T. Russell
Department of Earth and Space Sciences and Institute of Geophysics and Space Physics
University of California Los Angeles
ABSTRACT
Reconnection plays an important role in the energetics of the magnetospheres of both the magnetized and
unmagnetized planets, but it seems to play the most dominant role in the dynamics of the magnetized planets. Most
of our observations of magnetized planets have been obtained at Jupiter and the Earth. The magnetospheres of
Earth and Jupiter both undergo substorm-like cycles as part of an unsteady magnetosphere-wide circulation despite
the fact that they are driven by quite different processes: the solar wind interaction in the case of the Earth and mass
loading by the moon, Io, in the case of Jupiter. Each magnetosphere provides lessons for the other and together they
give a clearer insight as to how reconnection works at a magnetized planet. Most importantly the Alfven velocity
controls the flow velocity of the accelerated plasma so the reconnection rate is very sensitive to the density. In
regions in which the plasma density decreases away from a field reversing neutral sheet, the reconnection rate can
increase with time as the neutral point moves from the high density to the low-density region. When regions of
very low density are reached, reconnection appears to occur explosively. While most treatments of planetary
magnetotails cover the behavior of a single neutral point, two or more neutral points may exist. In the Earth’s
magnetosphere these two neutral points can interact to control a variety of otherwise puzzling phenomena such as
double onset substorms, pseudo breakups, and triggering of substorms by northward-turning IMF.
INTRODUCTION
Dungey’s [1961, 1963] sketches of the circulation induced in the magnetospheric plasma by reconnection with
northward and southward IMF provided the beginning for our understanding of the dynamic magnetosphere even
though these models were not explicitly time varying. As illustrated in Figure 1 when the IMF is due southward
reconnection occurs at the subsolar point and plasma is accelerated by the bent magnetic field. The flow goes over
the poles and drags the field and its plasma over the polar cap. Along the magnetopause in the tail, the
interconnected field slows the solar wind flow and Poynting flux crosses from the solar wind into the tail. This
energy is stored in the magnetic field of the tail and is derived from the mechanical energy flux of the solar wind.
There is, of course, Poynting flux in the solar wind incident on the Earth, but it is small compared to the mechanical
energy flux, and does not appear to play a role in the energization of the magnetosphere. When the IMF turns
northward, reconnection occurs behind the cusp with the magnetotail field lines, adding solar wind plasma to closed
magnetospheric field lines and generating a circulation pattern that is opposite that for the southward case.
Interplanetary Field Southward
Interplanetary Field Northward
North
N
North
Solar
Wind
N
N
Solar
Wind
N
Figure 1. Dungey’s models of the reconnecting magnetosphere for southward (left) and northward
interplanetary magnetic field (IMF) [Dungey, 1961; 1963].
The reconnection mechanism itself depends on processes that take place on the smallest scales conceivable in
a plasma, on the electron inertial length, but everything we need to know to understand the dynamics of the
magnetosphere can be determined from the macroscale. This occurs because the system evolves to the
microphysical state required to enable reconnection. This is not to say that reconnection is independent of plasma
conditions but that a planetary magnetosphere can create reconnection when it is required to complete its plasma
circulation patterns. Locations and rates may be controlled by the plasma and field configurations but reconnection
happens at least at some rate quite readily.
We find the reconnection process to be quite unsteady whether it be turbulence on the microscale, flux transfer
events on the mesoscale, or substorms on the global scale. Part of this variability is intrinsic to reconnection, part
of it is due to the 3D geometry in which the reconnection occurs, and part is due to the time variability of the
boundary conditions.
One phenomenon that is puzzling is the approximately 8-minute repetition pattern of flux transfer events at the
dayside magnetopause [Russell and Elphic, 1978; Kuo et al., 1995]. The original interpretation of these structures
is shown in Figure 2. A flux rope becomes interconnected with the magnetospheric field due to patchy, time
varying reconnection producing a pair of ropes that go up and over the two polar caps. This model does not answer
the question as to why reconnection is initiated every 8 minutes. A more recent interpretation of what appears to be
the same phenomenon in MHD simulations has been offered by Fedder et al. [2000]. They find that the rope is
created by the successive operation of a neutral line first on one side of the rope and then the other. A similar
interpretation has arisen from the study of low altitude plasma dispersion signatures [Boudouridis et al., 2000].
The rate of reconnection is most notably controlled by the direction of the magnetic fields and there is much
evidence that the magnetic fields must have an anti-parallel component for reconnection and there is evidence that
the reconnection process occurs most effectively for anti-parallel fields. The claim that reconnection can occur for
parallel fields [Fuselier et al., 2000] flies in the face of too much data to the contrary and can be explained in more
classical terms [Russell et al., 2000a].
Reconnection produces flows at approximately the Alfven velocity and thus the rate of reconnection is
controlled by the local field strength and the plasma density. This expectation from theory was verified by
Paschmann et al. [1979] using the ISEE 1 and 2 plasma and magnetic field data. A controlling factor that seems not
to have been predicted by theory is control by plasma beta. This control was inferred from the behavior of
geomagnetic activity when the solar wind Mach number was high [Scurry and Russell 1991]. Figure 3 shows the
efficiency of reconnection using geomagnetic activity as a proxy for the rate of reconnection. When the Mach
number is high the magnetosheath beta becomes high and the field strength drops. Whether this drop is more
important in controlling reconnection than the drop in plasma density that also must occur near the boundary
because of the high temperature is not certain. Perhaps it has an even different cause. In any event, the behavior of
the reconnection efficiency in the vicinity Mach numbers of 6 to 10 is complicated suggesting possibly several
Bi
20
Magnetosphere
L
B0t
N
1
Bt
M
Magnetosphere
Magnetosheath
∆am adj /∆VB tan [nT/mV-m-1]
Bi
10
0
Bo
Magnetosheath B o
-10
a)
b)
Figure 2. Formation of a flux transfer event (FTE) on the
magnetopause. (Left) FTE is connected through the magnetopause.
(Right) Cross section of FTE and magnetopause.
2
0
2
4
6
8
10
Magnetosonic Mach Number
Figure 3. Dependence of the efficiency of reconnection
on the magnetosonic Mach number [Scurry and Russell,
1991].
competing effects. This decrease in the efficiency of reconnection at high Mach numbers should be significant for
the interaction of the solar wind with the magnetospheres of the outer planets because the Mach number of the solar
wind flow past the outer planets is very high. This may be the reason that the magnetospheres of Saturn, Uranus
and Neptune are not very dynamic [Huddleston et al., 1997].
In the inner solar system the Mach number of the solar wind flow relative to the planets is much smaller and
especially so at Mercury where it is clear that the magnetosphere is very responsive to the IMF direction. However,
whether this is a substorm in which there is stored energy or whether this is due to driven reconnection with little
storage of energy is subject to debate [Siscoe et al., 1975; Luhmann et al., 1998]. Reconnection is also deemed
responsible for phenomena seen in Venus’ magnetotail and those of comets. For a more detailed review of the role
of reconnection in these various magnetospheres the reader is referred to the recent review by Russell [2001]. In
the more limited space presented by the pages of Advances in Space Research, we concentrate on some of the
recent increase in our understanding of the reconnection process in the terrestrial and jovian magnetospheres. These
advances help us understand better the behavior of both magnetospheres and have applications to reconnection in
both planetary and astrophysical settings.
RECONNECTION AT JUPITER
The jovian magnetosphere is powered by the production of ions at the moon Io that are accelerated to near corotational velocities by the coupling to the ionosphere via field aligned currents. The magnetic field at Io is very
strong but the density builds up in the torus until a circulation patterns is established that transports ions outward so
that they can be lost from the magnetosphere. In the Earth’s magnetosphere ions are lost from the dayside
magnetosphere but at Jupiter in part because of the inefficiency of reconnection on the magnetopause, the ion loss
process occurs on the nightside of Jupiter. The result of this rapidly spinning, outwardly convecting magnetosphere
is the jovian magnetosheath sketched in Figure 4. Beyond about 25 RJ a thin current sheet forms with almost
vacuum conditions above and below the field reversing current sheet. This is an ideal situation for the occurrence
of reconnection and indeed the correlated flows and reversed normal component of the magnetic field expected for
reconnection have been seen [Nishida, 1983].
While Jupiter needs to shed ions to maintain a steady state, it does not need to shed magnetic flux. The magnetic
flux leaving the surface of Jupiter is determined by the currents flowing inside the planet. Figure 5 shows
Vasyliunas’ [1983] solution to this problem. As the plasma spirals slowly outward and moves toward the nightside
the current sheet begins to reconnect and produces islands of magnetized ions that go down the tail. The net
magnetic flux carried off by the islands is zero. The now emptied magnetic tube is free to move inward against the
flow by its buoyancy. This appears to occur via narrow rapidly moving flux tubes [Russell et al., 2000] much like
bubbles rising from a leak in the bottom of an otherwise sealed container of water. All the water is moving slowly
downward toward the leak. Small bubbles can replace the volume of the water displaced because they can move
much more rapidly than the sinking water.
Magnetic
X-Line
60
4
Magnetopause
4
40
20
Z [R J ]
Galileo Orbital Plane
Magnetic
0-Line
3
0
3
-20
2
Current Sheet
2
-40
1
-60
-60
-40
-20
0
20
40
1
60
X [R J ]
Figure 4. Cross-section of the jovian magnetosphere
showing magnetic field lines and current sheet.
Figure 5. Circulation and reconnection in the jovian
magnetosphere [Vasyliunas, 1983].
3
20
25.8 R J
Delta B
Delta B
Ba
0
41.4 R J
Ba
10
0
Normal
20
-10
10
Bb
Current
0
20
Bc
0
40
20
Magnitude
Current
Normal
-20
Magnitude
Magnetic Field in Current Sheet Coordinates [nT]
20
Bmag
0
Bb
0
10
0
20
Bc
Bmag
10
0
1400
1500
1600
1700
0900
1000
12
51.9 R J
6
0
1100
June 24, 1996
Delta B
Delta B
Ba
54.6 R J
Ba
6
0
Normal
6
0
12
6
0
0600
Bb
Current
0
6
Bc
Magnitude
Current
Normal
-6
Magnitude
Magnetic Field in Current Sheet Coordinates [nT]
September 5, 1996
Bmag
-6
3
0
Bb
0
-3
Bc
Bmag
6
0
0700
Universal Time
0800
September 2, 1996
2000
2100
Universal Time
2200
September 1, 1996
Figure 6. Magnetic field at four jovian current sheets in current sheet coordinates.
It is instructive to examine the structure of the current sheet as a function of radial distance before the onset of
reconnection [Russell et al., 1999]. The four panels of Figure 6 show the magnetic field at four different radial
distances in a current sheet ordered coordinate system. The Bb coordinate is the direction perpendicular to the
current-sheet. Everywhere there is turbulence near the center of the current sheet but with increasing radial
distance the structure begins to look like tearing islands. By 55 RJ we cannot explain the structure otherwise. This
reconnection, though, is not rapid and does not disrupt the sheet. It seems to be quiescent, slowly evolving in the
low field strength and high density of the current sheet.
When the current sheet with its incipient reconnection points rotates past midnight, explosive reconnection
begins beyond about 50 RJ [Russell et al., 1998]. This is illustrated in Figure 7 that shows the magnetic field for a
21-day period. The frequent reversals in the field direction are due to the rotation of the planet and its tilted dipole
carrying the current sheet over Galileo. The second trace from the top, Bs, is the component crossing the current
sheet. Of note are the occasional, strong, positive and negative normal components. These are not telemetry
glitches. Figure 8 shows one of these events on an expanded scale. The component normal to the sheet has
exceeded the magnetic field in the pre-existing lobe of the magnetodisk by over a factor of three. This could occur
only if the reconnection point moved swiftly into the lobes because of their small densities. An additional feature
of the jovian data is twisting of the magnetic field due to angular momentum conservation as the field lines pull
away from the reconnection site. These transient reconnection events are most certainly responsible for the
disturbances in this region seen in the energetic particle and plasma wave data [Wach et al., 1999; Louarn et al.,
1998; Menietti et al., 1999].
One of the important issues for reconnection at Jupiter is the role of the solar wind in driving the circulation of
the plasma versus the role of mass addition at Io. Certainly any auroral phenomena in the open polar cap region
should be associated with reconnection at the dayside magnetopause but is the rate of reconnection seen in the near
tail driven mainly by the rate of mass addition at Io (remembering that it takes months for plasma to travel from Io
to the tail) versus solar wind IMF or pressure variations. The coordinated observations of Galileo and Cassini
during the Cassini flyby of Jupiter at the end of 2000 and early in 2001 may answer this question. A related
question is the role of tail reconnection in causing particle injections deep in the jovian magnetosphere. Mauk et
al., [1997; 1999] has reported substorm like injections of energetic particles as close as 10 RJ. Could they be
caused by volcanic outbursts at Io that increase the mass loading rate the turbulence in the magnetic fields in the
4
inner magnetosphere, hastening energetic particle diffusion and energization? These events need to be correlated
with the location of Io but they have not been so correlated.
Br
A
Outward
Inward
-8
Magnetic field [nT]
0
Bt
-10
0
8
South
0
North
Azimuthal
Bs
10
Theta
16
4
0
With Rotation
Opposite Rotation
16
Magnitude
Radial
0
0
-10
Southward
Magnetic Field in Corotational Coordinates [nT]
10
Magnitude Tangential
A
Radial
B
B
-10
10
0
June 1
June 6
June 11
June 16
8
0
1200
June 21
1230
1997
1300
Universal Time
Figure 7. The magnetic field measured by Galileo from 50 to
100 RJ in the post midnight sector showing evidence for
reconnection in the component crossing the current sheet.
June 17, 1997
Figure 8. Expanded view of one of the transients seen in the
previous figure.
0919
0935
LESSONS FOR THE EARTH
The near Earth neutral point model of substorms [Russell and McPherron, 1973] forms the basis for our
understanding of the role of reconnection in controlling the dynamics of the Earth’s magnetosphere. Figure 9
shows an interpretative sketch of how time delays in the system lead to the phenomena associated with a substorm.
Southward turning of the IMF increases reconnection at the nose of the magnetosphere (rate M) and add flux to the
lobes of the tail removing some from the dayside magnetosphere. Reconnection in the tail is delayed allowing flux
to build up in the tail and then be explosively released later. Our Jupiter results support the notion that the delay in
onset is occasioned by the time required for the neutral point to reach a region in which the reconnection rate is
rapid. This model by itself does not explain many of the types of substorm activity such as pseudo breakups,
double-onset substorms, and northward IMF triggering of substorms.
Φ Lobe
Solar
Wind
MLT*
2232
NOW 69.3
2243
YEK
69.1
0024
BRW 68.9
2053
Φ PS
500 nT
X-Component
Flux Transport Rate
Φ Day
M
C
Φ Day
R
Φ NPS Φ Lobe
Total Magnetic Flux
Φ
70.7
INK
FSP
67.3
2355
FYU
66.9
2158
COL
64.9
2155
TLK
62.2
2158
MEA
62.0
0055
SIT
60.2
2311
AMU
60.9
2156
(*UT=0919)
8
Thin
9
10
11
Universal Time
May 3, 1986
Exp
Figure 10. Stacked plot of the H-component of the Alaska
magnetometer chain showing a multiple onset substorm
[Mishin et al., 2000].
Figure 9. The near Earth neutral point model of substorms in
which tail flux builds up because of a time delay prior to tail
reconnection.
5
An example of a double onset substorm in the Alaska chain of stations is shown in Figure 10 [Mishin et al.,
2000, 2001]. At 0919 UT a substorm begins at College but does not expand to the pole. Later at 0935 UT an
expansion phase onset occurs that moves rapidly poleward and equatorward in the auroral current systems. This
behavior is also seen in statistical studies such as the superimposed epoch study of a set of substorms in Figure 11.
Here the two onsets at different latitudes are clear as is the fact that the second substorm initiated removal of
magnetic flux from the tail but the first one did not.
60
Φ Lobe
Solar
Wind
40
M
RD
Φ Day
ΦPM
Φ NPS
ΦDPS
RN
20
0
0
-600
0
Flux Transport Rate
<Φ> = 68.3o
o
<Φ> = 63.4
RD
C
Φ Day
-300
R
RN
M
-600
60
Figure 11. Superposed epoch study showing the flux leaves
the tail (ψ1 decreases) during a double onset substorm [Mishin
et al., 2000].
Figure 12. The near-Earth neutral point model of substorms
with a second neutral point [Russell, 2000]. The
interplanetary magnetic field turns southward at S and
northward at N. At P the near-Earth neutral point forms and at
L it begins to reconnect open magnetic field lines.
Φ NPS Φ Lobe
0
Minutes
Total Magnetic Flux
-60
Thin
ΦPM
-300
P NL
S
0
ΦDPS
X (nT)
X (nT)
Ψ1 (10 7 Wb)
We can modify the diagram in Figure 9 very simply to explain this behavior and other puzzling behavior of
the magnetosphere by adding the plasmoid and the distant neutral point as sketched in Figure 12 [Russell, 2000].
The distant neutral point produces plasma that surrounds the near Earth neutral point and keeps the rate of
reconnection there low. This allows the near-Earth neutral point remain quiescent for a long time after formation,
only suddenly to reconnect rapidly when the reconnection reaches a region of high Alfven velocity. This behavior
can be controlled by the IMF by its effects on the distant neutral point by increasing and decreasing the rate of
plasma sheet production that controls the near Earth neutral point. This can stop an onset (pseudo breakup) and it
could produce a double onset as in our example as well as the northward turning onsets.
0
Exp
Time
CONCLUSIONS
Reconnection plays a fundamental role in the circulation of the plasma in both the terrestrial and jovian
magnetospheres even though the engines that drive the circulation are quite different. In both magnetospheres time
varying reconnection causes substorm-like behavior. The stratification of the mass density and hence the Alfven
velocity is important in controlling the evolution of the reconnection rate enabling explosive reconnection to occur
when the density becomes very low. While the behavior of the Earth’s magnetotail can often appear very puzzling,
the appreciation that the Earth’s magnetotail can contain two (or more) neutral points helps explain these
phenomena. In particular the reconnection rate associated with the nearest neutral point can be controlled by the
more distant one.
6
ACKNOWLEDGMENTS
This research was supported by the National Aeronautics and Space Administration through grant NAG58064.
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8