Encyclopedia of the Solar System 2nd ed

The Solar Wind 105

FIGURE 5 Snapshots of solar wind flow speed (above) and
pressure (below) as functions of heliocentric distance at different
times during the evolution of a large-amplitude, high-speed solar
wind stream as calculated from a simple one-dimensional
numerical model. (Adapted from A. J. Hundhausen, 1973,
J. Geophys. Res. 78 , 1528.)

stream evolution occur discontinuously at the shocks, giving
a stream speed profile the appearance of a double saw-tooth
wave. The stream amplitude decreases and the compression
region expands with increasing heliocentric distance as the
shocks propagate. Observations indicate that the shocks typ-
ically do not form until the streams are well beyond 1 AU.
Nevertheless, becauseCfgenerally decreases with increas-
ing heliocentric distance, virtually all large-amplitude solar
wind streams steepen into shock wave structures at helio-
centric distances beyond∼3 AU. At heliocentric distances
beyond the orbit of Jupiter (∼5.4 AU) a large fraction of the
mass in the solar wind is found within compression regions
bounded by shock waves on the rising portions of damped
high-speed streams. The basic structure of the solar wind
in the solar equatorial plane in the distant heliosphere thus
differs considerably from that observed at 1 AU. Stream am-
plitudes are severely reduced, and short wavelength struc-

ture is damped out. The dominant structure in the solar

equatorial plane in the outer heliosphere is the expanding

compression region where most of the plasma and magnetic

field are concentrated.

6.3 Stream Evolution in Two and Three Dimensions

When the coronal expansion is spatially variable but time-

stationary, a steady flow pattern such as that sketched in

Fig. 6 develops in the equatorial plane. This entire pattern

corotates with the Sun, and the compression regions are

known as corotating interaction regions (CIRs); however,

only the pattern rotates—each parcel of solar wind plasma

moves outward nearly radially as indicated by the black ar-

rows. The region of high pressure associated with a CIR

is nearly aligned with the magnetic field line spirals in the

equatorial plane, and the pressure gradients are thus nearly

perpendicular to those spirals. Consequently, at 1 AU, the

pressure gradients that form on the rising speed portions of

high-speed streams have transverse as well as radial compo-

nents. In particular, not only is the low-speed plasma ahead

of a high-speed stream accelerated to a higher speed, but

it is also deflected in the direction of solar rotation. In con-

trast, the high-speed plasma near the crest of the stream

is both decelerated and deflected in the direction opposite

FIGURE 6 Schematic illustrating two-dimensional,

quasi-stationary stream structure in the ecliptic plane in the

inner heliosphere. The compression region on the leading edge

of a stream is nearly aligned with the spiral magnetic field, and

the forces associated with the pressure gradients have transverse

as well as radial components. (From V. J. Pizzo, 1978,

J. Geophys. Res. 83 , 5563.)