104 Encyclopedia of the Solar System
FIGURE 4 Solar wind speed as a
function of heliographic latitude as
measured by theUlyssesspace
probe during the declining phase
of the solar activity cycle (left) and
near solar activity maximum
(right). The speed data are
color-coded according to the
polarity of the magnetic field and
are superimposed on
representative images of the solar
corona at those phases of the solar
cycle. Smoothed sunspot numbers
are shown at the bottom. The S
and N labels on the latter indicate
the solar hemisphere thatUlysses
was in at those times. (From D. J.
McComas et al., 2003,Geophys.
Res. Lett. 30 ,10
10.1029.2003GL017136.)solar activity maximum, the band of solar wind variability
extends up to the highest latitudes sampled byUlysses.
6. Evolution of Stream Structure
with Heliocentric Distance
6.1 Kinematic Stream Steepening
and the Dynamic Response
Because the coronal expansion is spatially variable, alter-
nately slow and fast plasma is directed outward along any
radial line from the Sun as the Sun rotates (with a pe-
riod of 27 days as seen from Earth). Faster-moving plasma
overtakes slower-moving plasma ahead while outrunning
slower-moving plasma behind. The result is that the lead-
ing edges of high-speed streams steepen with increasing
distance from the Sun, producing the asymmetric stream
profiles obvious in Fig. 2. Material within the streams is
rearranged as the streams steepen; plasma and field on the
leading edge of a stream are compressed, causing an in-
crease in plasma density, temperature, field strength, and
pressure there, while plasma and field on the trailing edge
become increasingly rarefied. The buildup of pressure on
the leading edge of a stream produces forces that accelerate
the low-speed wind ahead and decelerate the high-speed
wind within the stream itself. The net result is a transfer of
momentum and energy from the fast-moving wind to the
slow-moving wind.6.2 Shock Formation
As long as the amplitude of a high-speed solar wind stream is
sufficiently small, it gradually dampens with increasing he-
liocentric distance in the manner just described. However,
when the difference in flow speed between the crest of a
stream and the trough ahead is greater than about twice the
local fast mode speed,Cf[the fast mode speed is the char-
acteristic speed with which small amplitude pressure sig-
nals propagate in a plasma:Cf=(Cs^2 +CA^2 )^0.^5 ], ordinary
pressure signals do not propagate sufficiently fast to move
the slow wind out of the path of the oncoming high-speed
stream. In that case, the pressure eventually increases non-
linearly, andshockwaves form on either side of the high-
pressure region (see Fig. 5). The leading shock, known as a
forward shock, propagates into the low-speed wind ahead,
and the trailing shock, known as a reverse shock, propagates
back through the stream. Both shocks are, however, con-
vected away from the Sun by the high bulk flow of the wind.
The major accelerations and decelerations associated with