The Solar Wind 103
FIGURE 3 Right, schematic
illustrating the configuration of the
heliospheric current sheet when
the solar magnetic dipole is tilted
substantially relative to the rotation
axis of the Sun. The heliospheric
current sheet separates magnetic
fields of opposite magnetic polarity
and is the heliospheric extension of
the solar magnetic equator. Left,
schematic illustrating the changing
tilt of coronal structure and the
solar magnetic dipole relative to
the rotation axis of the Sun as a
function of the phase of the solar
activity cycle. [Adapted from
J. R. Jokipii and B.Thomas, 1981,
Astrophys. J. 243 , 1115, and from
A. J. Hundhausen, 1977, in
“Coronal Holes and High Speed
Wind Streams” (J. Zirker, ed.),
Colorado Associated University
Press, Boulder, Colorado.]to be identified with high-speed solar wind streams. [See
Sun–EarthConnection.]
5. The Heliospheric Current Sheet
and Solar Latitude Effects
5.1 The Sun’s Large-Scale Magnetic Field
and the Ballerina Skirt Model
On the declining phase of the solar activity cycle and near
solar activity minimum, the Sun’s large-scale magnetic field
well above the photosphere appears to be approximately
that of a dipole. The solar magnetic dipole is tilted with
respect to the Sun’s rotation axis; this tilt changes with the
advance of the solar cycle. As illustrated in the left-hand
side of Fig. 3, near the solar activity minimum the solar
magnetic dipole tends to be aligned nearly with the rotation
axis, whereas on the declining phase of the activity cycle it
is generally inclined at a considerable angle relative to the
rotation axis. Near the solar maximum, the Sun’s large-scale
field is probably not well approximated by a dipole.
When the solar magnetic dipole and the solar rotation
axis are closely aligned, the heliospheric current sheet,
which is effectively the extension of the solar magnetic
equator into the solar wind, coincides roughly with the so-
lar equatorial plane. On the other hand, at times when the
dipole is tilted substantially, the heliospheric current sheet
is warped and resembles a ballerina’s twirling skirt, as illus-
trated in the right-hand side of Fig. 3. Successive outward
ridges in the current sheet (folds in the skirt) correspond
to successive solar rotations and are separated radially by
about 4.7 AU when the flow speed at the current sheet is
300 km/s. The maximum solar latitude of the current sheet
in this simple picture is equal to the tilt angle of the mag-
netic dipole axis relative to the rotation axis.5.2 Solar Latitude Effects
On the declining phase of the solar activity cycle and near
the solar activity minimum, stream structure and solar wind
variability are largely confined to a relatively narrow latitude
band centered on the solar equator. This is illustrated in the
upper left portion of Fig. 4, which shows solar wind speed
as a function of solar latitude measured byUlysseson the
declining phase of the most recent solar cycle. (Ulyssesis
in a solar orbit that takes it to solar latitudes of± 80 ◦in its
∼5.5-year journey about the Sun.) At this phase of the so-
lar cycle, the solar wind is dominated by stream structure at
low latitudes, but it flows at a nearly constant speed of∼ 850
km/s at high latitudes. This latitude effect is a consequence
of the following: (1) Solar wind properties change rapidly
with distance from the heliospheric current sheet, with flow
speed being a minimum in the vicinity of the current sheet;
and (2) the heliospheric current sheet is commonly tilted
relative to the solar equator but is usually found within about
± 30 ◦of it during this phase of the solar cycle. The width of
the band of solar wind variability changes as the solar mag-
netic dipole tilt changes. The upper right portion of Fig. 4
demonstrates that, in contrast, in the years surrounding the