The Solar Wind 111
FIGURE 11 Solar wind speed as a function of time
as measured byVoyager 2during a 1.5-year interval
when the spacecraft was beyond 18 AU from the
Sun. Because stream amplitudes are severely
damped at large distances from the Sun, the solar
wind speed there generally varies within a very
narrow range of values. Compare with the speed
variations evident in Fig. 2 that were obtained at 1
AU during a comparable period of the solar cycle.
[Adapted from A. J. Lazarus and J. Belcher, 1988,
in “Proceedings of the Sixth International Solar
Wind Conference” (V. J. Pizzo, T. E. Holzer, and D.
G. Sime, eds.), National Center for Atmospheric
Research, Boulder, Colorado.]move at a speed of∼23 km/s relative to the interstel-
lar medium. If this relative motion exceeds the fast mode
speed,Cf, in the interstellar plasma, then a bow shock must
stand in the interstellar plasma upstream of the heliosphere
to initiate the slowing and deflection of the interstellar
plasma around the heliosphere. Theheliopause, which
is the outermost boundary of the heliosphere, separates
the interstellar and solar wind plasmas. Sunward of the he-
liopause is atermination shockwhere the solar wind flow
becomes subsonic so that it can be turned to flow roughlyFIGURE 12 Simulated structure
of the solar wind’s interaction with
the interstellar plasma. Color-
coding represents the proton
temperature and arrows indicate
the direction of the solar wind and
interplanetary plasma flows.
(Courtesy of G. P. Zank and H. R.
Mueller.)