The Sun–Earth Connection 223
FIGURE 13 Plot showing the approximate depth
of penetration into the atmosphere of energetic
electrons and protons. (Kivelson and Russell, 1995,
“Introduction to Space Physics.”)(15–50 km). Increases in nitric oxide affect ozone levels be-
cause chemical reactions involving nitric oxide, ozone, ni-
trogen dioxide, and atomic oxygen form a catalytic cycle that
reduces ozone. For some major solar proton events∼30%
depletions of ozone in the mesosphere and upper strato-
sphere have been detected. Even though the reduction of
mesospheric ozone does not have the biological impact of
reductions in the denser lower stratospheric ozone, it can
modify the temperature and thus the dynamics of the meso-
sphere. These alterations can in turn modify the transmis-
sion of energy from the stratosphere and troposphere to the
upper atmosphere. Whether these effects have significant
consequences for the lower atmosphere, especially over the
long term, is unknown.
As noted above, the solar wind electric field within the
open magnetospheric field regions at high latitudes typi-
cally stirs the polar ionosphere in a twin vortex pattern, as
illustrated in Fig. 10. Ionospheric ions and electrons are
dragged antisunward over the polar caps and then forced
into a return flow at lower latitudes. The differential mo-
tion of the ions and electrons caused by the competition
between atmospheric and ionospheric interparticle colli-
sions and the electric and magnetic fields for control of
their motion leads to an ionospheric current. This auroral
electrojet current has a strength dependent on the com-
bination of the solar wind electric field and the level of
ionization in the auroral ionosphere. Collisional dissipation
or friction within the volume occupied by the electrojet
heats the auroral zone atmosphere. This resistive “joule”
heating results in large density perturbations called auroral
gravity waves in the upper atmosphere. These travel equa-
torward, in some cases depositing significant energy and
modifying upper atmosphere circulation globally. Traveling
ionospheric disturbances are one manifestation of the pas-
sage of these waves. Magnetic field perturbations associated
with the time-varying electrojet current and the ring cur-
rent mentioned earlier are detected on the ground, giving
the geomagnetic storm its name. A particular geomagnetic
index called AE is a widely used measure of the level of
ground magnetic field modifications by the auroral electro-
jet currents.4. Practical Aspects of the Sun–Earth
Connection
The Sun–Earth connection is a complex and fascinating
physical system that also has many practical consequences.
Society is increasingly dependent on space-based telecom-
munications and satellite systems that monitor tropospheric
weather, global resources, and the results of human activ-
ity. The satellite environment is part of the design of these
spacecraft, which can suffer radiation damage to electron-
ics if the extremes they may encounter are not taken into
account. Satellite orbits are affected by drag as they pass
through the upper thermosphere, where changes in den-
sity caused by EUV radiation and geomagnetic storms af-
fect their tracking and lifetime. This is particularly true of
large, relatively low-altitude vehicles such as theHubble
Space Telescopeand theInternational Space Station.In ad-
dition, satellite orientation controls often rely on Earth’s