Encyclopedia of the Solar System 2nd ed

The Solar Wind 101

TABLE 1 Statistical Properties of the Solar Wind at 1 AU

Parameter Mean STD Most Probable Median 5–95% Range

n(cm–^3 ) 8.7 6.6 5.0 6.9 3.0–20.0

Vsw(km/s) 468 116 375 442 320–710

B(nT) 6.2 2.9 5.1 5.6 2.2–9.9

A(He) 0.047 0.019 0.048 0.047 0.017–0.078

Tp(× 105 K) 1.2 0.9 0.5 0.95 0.1–3.0

Te(× 105 K) 1.4 0.4 1.2 1.33 0.9–2.0

Tα(× 105 K) 5.8 5.0 1.2 4.5 0.6–15.5

Te/Tp 1.9 1.6 0.7 1.5 0.37–5.0

Tα/Tp 4.9 1.8 4.8 4.7 2.3–7.5

nVsw(× 108 /cm^2 s) 3.8 2.4 2.6 3.1 1.5–7.8

Cs(km/s) 63 15 59 61 41–91

CA(km/s) 50 24 50 46 30–100

ecliptic plane at 1 AU. The table includes mean values,
standard deviations about the mean values, most proba-
ble values, median values, and the 5–95% range limits for
the proton number density (n), the flow speed (Vsw), the
magnetic field strength (B), the alpha particle abundance
relative to protons [A(He)], the proton temperature (Tp),
the electron temperature (Te), the alpha particle temper-
ature (Ta), the ratio of the electron and proton tempera-
tures (Te/Tp), the ratio of alpha particle and proton temper-
atures (Ta/Tp), the number flux (nVsw), the sound speed (Cs),
and theAlfven speed ́ (CA) (the speed at which small ampli-
tude perturbations in the magnetic field propagate through
the plasma). All solar wind parameters exhibit considerable
variability; moreover, variations in solar wind parameters
are often coupled to one another. Proton temperatures are
considerably more variable than electron temperatures, and
alpha particle temperatures are almost always higher than
electron and proton temperatures. Alpha particles and the
protons tend to have nearly equal thermal speeds and there-
fore temperatures that differ by a factor of∼4. The solar
wind flow is usually both supersonic and super-Alfv ́enic. Fi-
nally, we note that the Sun yearly loses∼ 6. 8 × 1019 g to the
solar wind, a very small fraction of the total solar mass of
∼ 2 × 1033 g.

3. Nature of the Heliospheric Magnetic Field

In addition to being a very good thermal conductor, the
solar wind plasma is an excellent electrical conductor. The
electrical conductivity of the plasma is so high that the so-
lar magnetic field is “frozen” into the solar wind flow as it
expands away from the Sun. Because the Sun rotates, mag-
netic field lines in the equatorial plane of the Sun are bent
into spirals (Fig. 1) whose inclinations relative to the radial
direction depend on heliocentric distance and the speed of

V

sw

B

FIGURE 1 Configuration of the heliospheric magnetic field in

the ecliptic plane for a uniform, radial solar wind flow.

the wind. At 1 AU, the average field line in the equatorial

plane is inclined∼ 45 ◦to the radial direction.

In Parker’s simple model, the magnetic field lines out of

the equatorial plane take the form of helices wrapped about

the rotation axis of the Sun. These helices are ever more

elongated at higher solar latitudes and eventually approach

radial lines over the solar poles. The equations describing

Parker’s model of the magnetic field far from the Sun are

Br(r,φ,θ)=B(r 0 ,φ 0 ,θ)(r 0 /r)^2

Bφ(r,φ,θ)=−B(r 0 ,φ 0 ,θ)(ωr 02 /Vswr) sinθ

Bθ= 0