The Sun–Earth Connection 215
FIGURE 2 Flow diagram illustrating the
connections in and complexity of the coupled
Sun–Earth system. The solar dynamo (top)
generates the solar magnetic field, which modulates
the solar outputs of extreme ultraviolet and X-ray
emissions, as well as the solar wind plasma. The
solar wind and its gusty counterpart, CMEs (green
box, upper right), directly determine the state of
the local heliosphere, which controls the state of
the magnetosphere (including its energetic particle
or radiation belt populations). In the meantime,
both solar photons (upper right boxes) and solar
energetic particles directly affect the state of the
upper atmosphere. The possible connection to
climate, suggested at the bottom, is currently a
matter of speculation. (Adapted from
http://lws-trt.gsfc.nasa.gov/lika radtg.ppt.)variability is a result of the control of these emissions by the
solar magnetic field, which undergoes significant evolution
during the course of the∼11-year solar activity cycle. [See
The Sun.]The extreme ultraviolet emissions come largely
from bright plage areas seen on the photosphere and from
the chromospheric network, while the X-rays come mainly
from hot plasma-containing coronal loops structured by
the coronal magnetic field. Both of these features can be
seen in the composite solar image in Figure 1. The plages
and X-ray bright loops are related to active regions, areas
with the strongest photospheric magnetic fields, that are
nonuniformly distributed over the solar surface. The chang-
ing numbers of active regions, and their areas, determine
the solar activity cycle. Thus, the solar EUV flux experi-
enced at Earth undergoes variations on both the 27-day
time scale of solar rotation and the near-decadal time scale
of solar activity. [SeeThe Sun.]The transient brighten-
ings in active regions called solar flares occasionally pro-
duce solar EUV and X-ray emission enhancements of up to
several orders of magnitude at photon energies extending
into the gamma ray range. These outbursts affect Earth’s
atmosphere and ionosphere at depths depending on their
wavelengths as indicated in Fig. 4. The magnetosphere re-
sponds to changes in the ionosphere and upper atmosphere,
but its primary solar controller is the magnetized solar wind
plasma.FIGURE 3 Illustration of the solar spectrum,
showing the intensities of various wavelength
emissions and their variation from active (red) to
quiet times. Notice that order of magnitude
variations from solar minimum to maximum occur
at the short (¡ 1000A is equivalent to 0.1 ̊ A) ̊
wavelengths. (1 nanometers.)