Geoscience Reference
In-Depth Information
Fig. 1.8
The deflection of
solarwindbythe
geomagnetic field shown in
an ecliptic plane.
1
—solar
wind,
2
—bow shock,
3
—magnetopause,
4
—magnetopause current,
5
—the Earth's magnetic field,
6
—secondary magnetic field
originated from the
magnetopause current. The
symbols e
and p
C
denote
electrons and protons/positive
ions;
z
axis and the unit
vectors
n
are both
perpendicular to the bow
shock, and A is the bow
shock point nearest the Sun
The solar wind particles of charge q moving at velocity
V
are subject to the
magnetic force
F
D
q
V
B
:
The Earth's magnetic field will therefore deflect the positive-charged and negative-
charged particles to the different sides of the magnetosphere. The deflected particles
will spiral around the fields lines and drift perpendicularly to the meridian around
the Earth.
Owing to the fact that the solar wind is a supersonic flow at high speed the Mach
number of Alfvén and magnetosonic waves of the supersonic solar wind reach a
value of about 7 at the distance equal to the Earth orbit radius. Therefore, a shock
wave must form when the supersonic and super-Alfvén solar wind fall on the Earth
magnetic field. At the front of the so-called collisionless bow shock the velocities
of the solar wind particles change their direction and the solar wind flow around the
Earth magnetic field forming the Earth's magnetosphere as shown in Fig.
1.8
.The
outer boundary of the magnetosphere is termed the magnetopause.
To study the electrodynamics of this region in a little more detail we note that
the magnetoplasma itself is so tenuous that it can be treated to first order as a
collisionless magnetized plasma with an MHD approach. The plasma conductivity
is so high that the plasma is assumed to be a single fluid having infinite conductivity.
This implies that in a reference frame moving at the plasma velocity
V
the electrical
field
E
0
D
E
C
V
B
vanishes both parallel and perpendicular to
B
. In a reference
framefixedtotheEarth
E
D
B
V
:
(1.35)
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