Encyclopedia of the Solar System 2nd ed

102 Encyclopedia of the Solar System

Here r,φ, andθare radial distance, longitude, and lati-
tude in a Sun-centered spherical coordinate system,Br,Bφ,
andBθare the magnetic field components,ωis the Sun’s
angular velocity (2. 9 × 10 −^6 radians sec−^1 ),Vswis the flow
speed (assumed constant with distance from the Sun), and
φ 0 is an initial longitude at a reference distancer 0 from Sun
center. This model is in reasonably good agreement with
suitable averages of theheliospheric magnetic fieldmea-
sured over a wide range of heliocentric distances and lati-
tudes. However, the instantaneous orientation of the field
usually deviates substantially from that of the model field
at all distances and latitudes. Moreover, there is evidence
that the magnetic field lines wander in latitude as they ex-
tend out into the heliosphere. This appears to be a result
of field line foot point motions associated with differential
solar rotation (the surface of the Sun rotates at different
rates at different latitudes) and convective motions in the
solar atmosphere.

4. Coronal and Solar Wind Stream Structure

The solar corona is highly nonuniform, being structured by
the complex solar magnetic field into arcades, rays, holes
(regions relatively devoid of material), and streamers. [See
TheSun.] The strength of the Sun’s magnetic field falls off
sufficiently rapidly with height above the solar surface that
it is incapable of containing the coronal expansion at alti-
tudes above∼0.5–1.0 solar radii. The resulting solar wind
outflow produces the “combed-out” appearance of coronal
structures above those heights in eclipse photographs.
The solar wind is also highly nonuniform. In the ecliptic
plane, it tends to be organized into alternating streams of
high- and low-speed flows. Figure 2, which shows solar wind
flow speed, flow azimuth, the radial component of the he-
liospheric magnetic field, and the field strength at 1 AU for
a 50-day interval in 2004, illustrates certain characteristic
aspects of thisstream structure. Four high-speed streams
with flows exceeding 700 km/s are clearly evident in the
figure. The third and fourth streams were actually reen-
counters with the first and second streams, respectively, on
the following solar rotation. Each high-speed stream was
asymmetric with the speed rising more rapidly than it fell,
and each stream was essentially unipolar in the sense that
Brwas either positive or negative throughout the stream.
Reversals in field polarity occurred in the low-speed flows
between the streams. Those polarity reversals correspond
to crossings of theheliospheric current sheet(discussed
in more detail in the following section) that separates solar
wind regions of opposite magnetic polarity. The magnetic
field and plasma density (not shown) peaked on the leading
edges of the streams, and the solar wind flow there was de-
flected first westward (positive flow azimuth) and then east-
ward. This pattern of variability is highly repeatable from

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FIGURE 2 1-hr average solar wind speed, flow azimuth, radial

component of the heliospheric magnetic field, and the field

magnitude at 1 AU for a 50-day interval in 2004.

one stream to the next and is the inevitable consequence of

the evolution of the streams as they progress outward from

the Sun (see Section 6).

Recurrent high-speed streams originate primarily in

coronal holes, which are large, nearly unipolar regions in the

solar atmosphere having relatively low density. Low-speed

flows, on the other hand, tend to originate in the coronal

streamers that straddle regions of magnetic field polarity

reversals in the solar atmosphere. Both coronal and solar

wind stream structure evolve considerably from one solar

rotation to the next as the solar magnetic field, which con-

trols that structure, continuously evolves. It is now clear

that the mysterious M-regions, hypothesized long before

the era of satellite X-ray observations of the Sun, are to

be identified with coronal holes, and the long-lived particle

streams responsible for recurrent geomagnetic activity are