224 Encyclopedia of the Solar System
magnetic field, which can be highly variable during geo-
magnetic storms. Changes in the ionosphere disrupt radio
communications by changing ionospheric transmission or
reflection characteristics. Global positioning system (GPS)
navigational signals, which pass through and are altered
by the ionosphere as they are transmitted from very high-
altitude satellites to ground receivers, can degrade during
disturbed conditions, giving inaccurate locations. On the
ground, currents induced in power system transformers and
in oil pipelines by storm-related magnetic field perturba-
tions lead to overload and corrosion.
In the era of human space flight, there is also great con-
cern over space radiation hazards from energetic particles.
Human-occupied vehicle orbits at low latitude and low al-
titude are largely protected from the radiation belts and
the less common but potentially dangerous major solar en-
ergetic particle events. However, the orbit of theInterna-
tional Space Stationis sufficiently inclined with respect to
Earth’s equator that it is occasionally exposed to solar ener-
getic particles at high latitudes, when the magnetosphere is
disturbed as it often is during solar particle events. Astro-
naut radiation exposure is carefully monitored and is limited
by NASA. To minimize it, plans for extravehicular activities
take into account conditions on the Sun and the likelihood of
a major solar event that might affect the Earth. Even com-
mercial and military aircraft on polar routes monitor major
solar events as a precaution. For future space travel outside
of the effective but imperfect magnetospheric shield, pro-
tection from solar particle radiation is a major problem to
be solved.
In response to both international civilian interests and
military needs, the National Oceanic and Atmospheric
Administration (NOAA) runs a Space Environment Cen-
ter (SEC) that collects, analyzes, and distributes informa-
tion on the Earth’s space environment and solar activ-
ity. Space weather reports are regularly issued via the in-
ternet (http://sec.noaa.gov), where one can also find ac-
cess to the archives of solar, heliospheric, magnetospheric,
and upper atmosphere/ionosphere data that are used. Cus-
tomers of these services seek information on subjects rang-
ing from interference to radio transmissions by solar radio
bursts or ionospheric scintillations, to satellite orbit decay
rates based on solar EUV emission intensities. Alerts are
posted when a forecaster interprets behavior in the relevant
data to mean a solar energetic particle event, geomagnetic
storm, or ionospheric disturbance will occur within the next
minutes to days. One of the most useful geomagnetic storm–
forecasting methods takes advantage of theSOHOspace-
craft, which allows the forecaster to identify CMEs and the
location of active regions on the solar disk. When a CME is
headed toward Earth, it sometimes appears in theSOHO
coronagraph images as a ring around the Sun called a halo
CME. These events are known to have an increased proba-
bility of causing a geomagnetic storm. However storm fore-
casts are still extremely difficult, with false alarms, including
halo CMEs actually heading away from the Earth, having
originated on the far side of the Sun, a major problem.
Geomagnetic indices, calculated from ground-based
measurements of magnetic field perturbations, are rou-
tinely used as a measure of the level of space weather
disturbance. Different indices emphasize particular Earth
responses depending on how and from what stations they
are calculated. Several of these were mentioned above. The
auroral electrojet index AE is primarily a measure of au-
roral zone ionospheric currents obtained from high- and
mid-latitude monitors, while the ring current index Dst
is mainly a measure of the ring current obtained at lower
latitudes. The planetary index Kp uses ground stations in
both regions. These indices were developed and have been
recorded since before the space age. They are used both
to parameterize empirical models (e.g., of the auroral zone
ionosphere), and to maintain a continuous long-term his-
torical record of the Sun–Earth connection in concise form.
A major goal of Sun–Earth connection research to-
day is a physics-based model of the coupled heliosphere–
magnetosphere–upper atmosphere/ionosphere system,
including CMEs. Such a model could provide both a fore-
cast tool for space weather events based on solar observa-
tions, as well as a numerical experiment framework to gain
greater insight into the physics of the coupled system and
its extremes. For example, severe space weather events are
occasionally observed, but even larger events have been
inferred from records of the Earth’s historical cosmic ray
exposure present in ice cores. What is the worst that could
happen to our planet and space assets after one or more
of these greatest of solar activity episodes? It is an intrigu-
ing question. As a practical matter, the plan to again send
humans into deep space, to the Moon and to Mars, also
renews the concerns of space radiation hazard issues faced
to a lesser degree on theInternational Space Station.The
unpredictability of CMEs, and the fact that historically the
strongest space weather events have not been at solar max-
imum, helps motivate applied space weather research.
For many decades, there has also been research on and
discussion about the connection of solar activity and Earth’s
climate. It is possible that the very small changes in total so-
lar irradiance, the so-called solar constant, or changes in UV
radiation can have measurable climatic effects. It is also pos-
sible that some Sun–Earth connection–related phenomena
can reach the troposphere. One highly cited example is the
coincidence of the Maunder Minimum in solar activity with
the Little Ice Age in Europe during the 15th and 16th cen-
turies. Ideas on how nonradiative effects might play a role
include mechanisms such as cloud cover alteration by low-
altitude ionization effects of energetic cosmic rays reach-
ing Earth (Tinsley, 2000). The Maunder Minimum climate
change has also inspired speculation about the climatic and
other consequences of geomagnetic field reversals. Analy-
ses of ice age records and geomagnetic field reversals have
as yet produced no definitive results, although it may be