110 Encyclopedia of the Solar System
a bc dFIGURE 10 Sketches of successive steps in
three-dimensional reconnection in the corona
beneath a departing CME. The sketches are not to
scale and are intended only to illustrate successive
changes in CME magnetic topologies resulting
from reconnection. (From J. T. Gosling et al.,
1995,Geophys. Res. Lett. 22 , 869.)ICME that drives it. As a result, spacecraft often encounter
ICME-driven shocks without also encountering the ICMEs
that drive them.
8. Variation with Distance from the Sun
For a structureless solar wind, the speed remains nearly
constant beyond the orbit of Earth, the density falls off
with heliocentric distance (r,asr–^2 ), and the magnetic
field decreases with distance as described by the equations
in Section 2. The temperature also decreases with increas-
ing heliocentric distance due to the spherical expansion of
the plasma; however, the precise nature of the decrease de-
pends upon particle species and the relative importance of
such things as collisions and heat conduction (e.g., protons
and electrons have different temperatures and evolve dif-
ferently with increasing heliocentric distance). For an adia-
batic expansion of an isotropic plasma, the temperature falls
off asr−^4 /^3 ; for a plasma dominated by heat conduction, the
temperature falls asr−^2 /^7.
Of course, the solar wind is not structureless. The contin-
ual interaction of high- and low-speed flows with increasing
heliocentric distance produces a radial variation of speed
that differs considerably from that predicted for a structure-
less wind. High-speed flows decelerate and low-speed flows
accelerate with increasing heliocentric distance as a result of
momentum transfer (see Sections 6 and 7). Consequently,
near the solar equatorial plane far from the Sun (beyond
∼15 AU) the solar wind flows at 400 to 500 km/s most of
the time (Fig. 11). Only rarely are substantial speed pertur-
bations observed at these distances; these rare events usu-
ally are associated with disturbances driven by very large
and fast ICMEs that require a greater-than-usual distance
to share their momentum with the low-speed wind.
With increasing heliocentric distance an ever-greater
fraction of the plasma and magnetic field in the wind be-
comes concentrated within the compression regions on the
rising speed portions of high-speed flows; extended rarefac-
tion regions relatively devoid of plasma and field follow
these compressions. Thus, at low heliographic latitudes, so-
lar wind density and magnetic field strength tend to vary
over a wider range in the outer heliosphere than near the
orbit of the Earth, although the average density falls roughly
asr−^2 , and the average magnetic field falls off roughly
as predicted by the equations in Section 2. On the other
hand, plasma heating associated with the compression re-
gions causes the solar wind temperature to fall off with in-
creasing heliocentric distance more slowly than it otherwise
would. Observations reveal that both proton and electron
temperatures decrease with distance somewhere between
the adiabatic and conduction-dominated extremes.9. Termination of the Solar Wind
Interstellar space is filled with a dilute gas of neutral and
ionized particles and is threaded by a weak magnetic field.
In the absence of the solar wind, the interstellar plasma
would penetrate deep into the solar system. However, the
interstellar and solar wind plasmas cannot interpenetrate
one another because of the magnetic fields embedded in
both. The result is that the solar wind creates a cavity in the
interstellar plasma.
The details of the solar wind’s interaction with the in-
terstellar plasma are still somewhat speculative largely be-
cause, until recently, we lacked direct observations of this
interaction. Figure 12 shows what are believed to be the ma-
jor elements of the interaction. The Sun and heliosphere