Encyclopedia of the Solar System 2nd ed

The Solar Wind 113

10 -4

10 -2

100

102

104

106

108

110

F(W) Phase Space Density ACE Quiet 98 55-120

(s

3 /k m

6 )

W Ion Speed/Solar Wind Speed

98.055-120

Quiet periods only

H+

SWICS ACE

He+

He++

(a) (b)

FIGURE 13 (a) A cut through a solar wind ion count spectrum parallel to the magnetic field. The

first two peaks are protons, and the second two peaks are alpha particles. (The velocity scale for the

alpha particles has been increased by a factor of 1.4.) Both the proton and alpha particle spectra

show clear evidence for a secondary beam of particles streaming along the field relative to the main

solar wind beam at about the Alfv ́en speed. Such secondary beams, not always well resolved, are

common in both the low and the high-speed wind. (From J. R. Asbridge et al., 1974,Solar Phys. 37 ,

451.) (b) Solar wind speed distributions of H+,He^2 +, and He+observed in the low-speed solar wind

at 1 AU, averaged over a 65-day period in 1998 and excluding intervals of shocks and other

disturbances. Such extended suprathermal tails appear to be ubiquitous in the low-speed solar wind.

The He+ions are primarily of interstellar origin. (From G. Gloeckler et al., Acceleration and

transport of energetic particles observed in the heliosphere, in Mewaldt et al., 2000.)

(a) (b)

FIGURE 14 (a) One-dimensional

cut through a solar wind electron

distribution showing the thermal

and suprathermal populations. (b)

Suprathermal electron pitch angle

distribution (relative to the

magnetic field) showing the

field-aligned strahl and the nearly

isotropic halo components.