The Earth’s Magnetosphere and its
Coupling with the Solar Wind
Stefan Eriksson
Contact: eriksson@lasp.colorado.edu
Magnetism is familiar to all of us, usually
from permanent magnets and compasses.
How are Magnetic Fields Generated?
The Danish professor Hans Christian
Oersted first discovered (1820) that an
electric current in a wire could deflect a
compass needle ….
Hans Christian Oersted
(1777-1851)
How are Magnetic Fields Generated?
After hearing about Oersted’s findings,
the Frenchman Ampere firmly
established (1820) the relationship
between electricity and magnetism.
André-Marie Ampère
(1775-1836)
How are Magnetic Fields Generated?
After hearing about Oersted’s findings,
the Frenchman Ampere firmly
established (1820) the relationship
between electricity and magnetism.
An electric current was found to
generate a magnetic field and this is
the basis of Ampere’s Original Law:


  B  0 J
André-Marie Ampère
(1775-1836)
The Active Sun
A Plasma Object with Dynamic Magnetic Fields
SOHO: LASCO C2
Plasma: the 4th State of Matter
solid (ice)
liquid (water)
gaseous (steam)
plasma
(positively charged hydrogen & oxygen nuclei and electrons)
Plasma: the 4th State of Matter
solid (ice)
liquid (water)
gaseous (steam)
plasma
The charged particles (electrons and ions) of the plasma are glued to the
magnetic field and move around it in circular orbits. Lorentz stated this
force of nature as ….
V
B
F=qV x B
(positive charge q)
… where F is the force acting on a particle with charge q and velocity V in a
magnetic field B.
The Active Sun
A Plasma Object with Dynamic Magnetic Fields
SOHO: LASCO C2
The Active Sun
11 February 2000
SOHO: LASCO C2 (enhanced)
The Solar Wind carries the solar
magnetic field and the plasma tied
to it into space.
We refer to this magnetic field as
the Interplanetary Magnetic Field (IMF).
IMF
SOHO: LASCO C2 (enhanced)
The Earth’s Magnetic Field
Density (cm-3)
Magnetic field
strength (nT)
Convection
speed (km/s)
Solar Wind
5
5
450
Magnetosheath
10
10
200
Magnetotail
lobe
0.01
25
4
Magnetopause current and tail current
via Ampere’s Law:
  B  0 J
Forces on plasma due to magnetic field
dv
m
 q( E  v  B )
dt
du

 j  B  p
dt
  ni mi  ne me
Equation of motion
Momentum fluid equation
u  ni mi ui  ne meue  
j  ni qi ui  ne qeue
The jxB force of the magnetopause current deflects
the solar wind plasma around the magnetosphere.
Density (cm-3)
Magnetic field
strength(nT)
Convection
speed (km/s)
Plasma mantle 1
25
5
Plasma sheet
0.01
10
4
Plasmasphere
1000
>100
<1
Earth magnetic field strength at the poles (equator): 62000 (31000) nT
Charged particles gyrate around an axial magnetic field.
Is this the only motion of space plasma particles?
Plasma Drift Velocity due to Electric Field
dv
m
 q( E  v  B )
dt
v  u  vc
du
0
dt
0  E  u  B
u  E  B B 2
u=ExB/B2
The total perpendicular motion in a homogeneous magnetic
and electric field consists of a gyration about the magnetic
field and a drift that we call the plasma convection.
Plasma Drift Velocity due to General Force F
(example gradient magnetic field)
u  E  B B2
E F q
u  F  B qB 2
F   B
u  B  B qB 2
u=ExB/B2
F=mg (gravity)
R=mv/qB
Coronal Mass Ejections
The Earth’s Magnetic Field
The Earth’s Magnetic Field
The Earth’s Magnetic Field
Breaking of the magnetic field: “magnetic reconnection”
Magnetic Reconnection
A process that:
• changes the field topology
by “breaking” and “mending”
individual field lines in a local
region.
• converts magnetic energy to
a jetting plasma
The original sketch by Dungey (1961) of a global circulation
driven by dayside and nightside reconnection.
SUN
Aurora and the Auroral Oval
Aurora and the Auroral Oval
Aurora and the Auroral Oval
Aurora and the Auroral Oval
Aurora and the Auroral Oval
Magnetopause Reconnection
Lobe Reconnection Event
Cluster C1
Cluster C3
The Sun
IMF
Plasma Jets at Earth’s Magnetopause
Observed by Two Satellites
magnetosphere
cusp
magnetosheath
IMF
Northward IMF MHD Simulation
XZ GSM Plane at 1900 UT
Solar wind
magnetosheath
magnetosphere
Bow Shock
Northward IMF MHD Simulation
XZ GSM Plane at 1900 UT
Solar wind
magnetosheath
magnetosphere
Bow Shock
High-latitude Electrodynamics
TIMED/GUVI
TIMED/GUVI
TIMED/GUVI
Summary
Invisible magnetic fields occur throughout the universe. At stars, such as
our own Sun, at many planets, and in the space in between.
These magnetic fields are not static, as the field around a bar magnet, but
they are very dynamic (observations of the Sun, measurements of the
Earth’s magnetic field).
The dynamics is generated by interactions of the fields and a charged gas
(plasma) which is tied to it by the Lorentz force.
Charged particles gyrate around the magnetic field and they drift
perpendicular to it when there is an electric field. This drift is chargedependent for other forces F and such drifts generates a current.
Summary
The magnetosphere is separated into regions of different plasma
characteristics by the magnetic field (plasma mantle, tail lobes, plasma
sheet, plasmasphere).
Major current systems are generated at these plasma boundaries:
magnetopause current, cross-tail current, ring current.
The outer magnetosphere currents couple to the ionosphere by fieldaligned currents. The current circuits are closed there by electric fieldaligned Pedersen currents.
When magnetic fields of different direction collide in space, we know
from satellite observations that they may break and re-connect. We call
this process “magnetic reconnection”. This process allows the plasma to
gain access to “forbidden” regions, such as the immediate environment
around our own planet.
Summary
The magnetic reconnection between the IMF and the Earth’s magnetic
field drives a Dungey circulation of plasmas and magnetic field in the
magnetosphere via the ExB drift. It also drives the auroral region fieldaligned current system and the aurora itself.
The aurora occurs at any one time in an eccentric oval centered at the
two magnetic poles. This is the auroral oval.
It is believed that reconnection in the near-Earth magnetotail generates
major auroral activity that spreads from the midnight region of the
auroral oval toward the dayside. This is called an auroral substorm.
The magnetopause reconnection observations by e.g. Cluster confirm
that reconnection occurs also for northward IMF. The reconnection site
is located poleward of the cusps. The magnetosphere is not closed for
these conditions and polar high-latitude electrodynamics (ExB drift,
aurora, field-aligned currents) is driven by northward IMF.