216 Encyclopedia of the Solar System
FIGURE 4 Atmospheric depths affected by various wavelengths
in the solar spectrum. (Courtesy of Windows to the Universe,
http://www.windows.ucar.edu.)) The longer wavelength radiation
affects mainly heating, while the shorter wavelengths can
produce ionization and associated chemistry changes. The letters
F1, F2, E, and D are used to designate different ionospheric
layers.
The solar wind is the outflowing, ionized gas or plasma
of the solar upper atmosphere. [SeeThe Solar Wind.]
This outermost extension of the corona fills a space up to
at least∼80 AU in radial extent, defining theheliosphere,
the region surrounding the Sun. The mainly hydrogen solar
wind flows primarily from places in the corona that are mag-
netically “open” to interplanetary space. These open field
regions are often called coronal holes because of their dark
appearance in soft X-ray and EUV images. [SeeThe Sun.]
The solar wind also carries with it the stretched out coronal
magnetic field that takes on an average outward or inward
orientation depending on the magnetic field direction or
polarity at its photospheric base. There is also a component
of the quiet solar wind that comes from the edges of coronal
closed magnetic field regions, producing the equivalent of a
boundary layer between outflows from different open field
regions. On the average, the solar wind speed is slowest in
these boundary layers and fastest where it flows from the
center of large open field regions. Typical solar wind speeds
range from∼300 km/s to∼800 km/s and are roughly con-
stant with radial distance. Undisturbed solar wind magnetic
field strengths at 1 AU range from∼5 to 10 nT, and densities
range from∼5 to 15 particles/cm^3. Because the open and
closed field regions change with the distribution of active
regions on the Sun, the solar wind stream structure and field
polarity pattern evolve with the solar activity cycle. They are
simplest at the quietest times of the cycle, during which the
corona usually exhibits two main solar wind sources near
the Sun’s polar regions, one with positive (outward) and
one with negative (inward) magnetic polarity.
A critical aspect of the solar wind stream structure for
the Sun–Earth connection is Earth’s location near the solar
rotational equator. This region is often dominated by the
presence of the slow wind and the related heliospheric cur-
rent sheet that separates the solar wind from open coronal
field sources with outward and inward magnetic field polar-
ities. This circumstance, together with the rotation of the
Sun, produces local interplanetary conditions that at low
solar activity exhibit repeating or corotating 27-day varia-
tions in solar wind speed and density, and interplanetary
magnetic field polarity. E. Parker, who first proposed the
existence of the solar wind, also recognized that the Sun’s
rotation would wind up the interplanetary magnetic field
into a spiral shape. Figure 5 illustrates this “Parker Spiral”FIGURE 5 Illustration of the Parker Spiral interplanetary
magnetic field carried in the outflowing solar wind, and wound
up by solar rotation. At 1 AU the typical angle the field makes
with respect to the Sun–Earth line is∼ 45 ◦.(SeeSOLAR WIND
for further information.)