Introductory and Background Material - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 19

Supersonic
solar wind Solar wind

Shock
wave

o

Magnetopause

Magnetosphere

Viscous
interaction
region

Ecliptic plane

Magnetospheric
flow

Supersonic
flow

(a) (b)

Figure 1.13 (a) Schematic diagram of a closed magnetosphere. The bow shock wave
and internal flow pattern are sketched in (b).

access to particles from the solar wind and all of the magnetic field lines in the
magnetosphere have two ends on the earth. The sketch in Fig. 1.13b shows the
complication that arises when the solar wind is supersonic. A shock wave must
form, across which the solar wind density and temperature rise abruptly. The
solar wind velocity decreases, allowing for subsonic flow around the obstacle.
The sketch in Fig. 1.13b also shows that a “closed” magnetosphere need not be
static even though particles cannot readily enter. A “viscous” interaction may
occur (Axford and Hines, 1961) in which plasma inside and near the flanks
of the magnetosphere is weakly coupled to the solar wind flow, possibly by
waves that propagate across the boundary. Some magnetospheric plasma is then
dragged along in the antisunward direction, and because of the resulting pres-
sure buildup in the nightside of the magnetosphere, there must be a flow back
toward the sun in the center of the magnetosphere. The circulation that results
(sketched in Fig. 1.13b) is in good qualitative agreement with observations but
is now thought to be less important than the magnetospheric flow driven by a
process termed “reconnection,” which involves the interplanetary magnetic field
and results in a partially open magnetosphere. This process is described next.
Before discussing reconnection, we must point out that both the single-particle
dynamic and magnetofluid dynamic viewpoints discussed in Chapter 2 are such
that the relationships

B 2 V

⊥ =

E × B / (1.2a)

and

E

⊥ =−

V × B (1.2b)