214 Encyclopedia of the Solar System
FIGURE 1 Triptych illustrating
the coupled Sun–Earth system,
showing from left to right an image
of the erupting solar corona from
theSOHOspacecraft, and images
of the Earth’s auroral emissions
from space (center) and from the
ground (right). (See
http://sohowww.nascom.nasa.gov/
hotshots/2003 03 14/.)wind–magnetosphere couplings as described in more detail
in the main text below.
The magnetosphere, the region of near-Earth space
dominated by the magnetic field of the Earth and shaped
by its interaction with the solar wind (see Fig. 1), orga-
nizes geospace. Various particle populations in the mag-
netosphere, including the plasmas originating in the solar
wind and Earth’s ionosphere, and the more energetic par-
ticles trapped in theradiation belts,are constantly modi-
fied by changing interplanetary conditions. The ionosphere
acts as a conducting inner boundary affecting the magneto-
sphere’s response to those conditions, but it is also a source
of ions and electrons for the magnetosphere. Under the
disturbed local interplanetary conditions that occur after an
Earth-directed CME, a collection of major magnetospheric
modifications called a geomagnetic storm occurs. The pop-
ulation of trapped energetic particles in the radiation, or
Van Allen, belts surrounding the Earth undergoes enhance-
ments, losses, and redistribution. Current systems and par-
ticle exchanges couple the magnetosphere and ionosphere
to a greater than normal degree. The result is enhanced solar
wind energy transfer into geospace, causing auroral emis-
sions and related changes in the high-latitude dynamics of
the ionosphere, as well as in the density and composition
in the thermosphere. Evidence of atmospheric influences
of geomagnetic storms and other Sun–Earth connection ef-
fects down to the stratosphere has been reported, although
it remains controversial. On the other hand, induced cur-
rents in conductors on the ground from storm-associated
magnetic field changes are unarguable proof of the depth
of influence of extreme space weather.
Studies of the Sun–Earth connection investigate the
physics that makes the solar wind, magnetosphere, and
upper atmosphere/ionosphere a highly coupled system.
Figure 2 shows an attempt to diagram its various com-
ponents and their relationships. There are also practical
aspects to understanding the connections shown. Specifica-
tions of radiation tolerances for spacecraft electronics com-
ponents, designs of protective astronaut suits and on-orbit
shielding, and definitions of the surge limits for power grids
on the ground can be made with a better understanding of
space weather effects. Forecast models can help predict
the changes in the magnetosphere that alter the radiation
belts and the changes in the ionosphere that disrupt radio
communications and GPS navigation. Sun–Earth connec-
tion knowledge also increases our understanding of other
areas of astronomy and astrophysics such as planet–solar
wind interactions, extra-solar planetary systems, stellar ac-
tivity, and the acceleration of particles in the universe.1. The Solar and Heliospheric Roles in the
Sun–Earth Connection
Solar radiation in the ultraviolet, EUV, and X-ray wave-
lengths are the primary sources of ionization in the Earth’s
atmosphere. Of these, solar EUV fluxes are the most im-
portant source of the ionosphere. Figure 3 illustrates the
relatively large variability of this part of the solar spec-
trum, compared to the visible and infrared wavelengths that
dominate the “solar constant.” As mentioned before, this