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Space Weather: A Research Perspective
Space Weather: A Research Perspective
Earth-Space "Meteorology"
Disturbed Space Weather Conditions
The space surrounding Earth is a highly dynamic environment that responds sensitively to changes in
the electromagnetic radiation, particles, and magnetic fields arriving from the Sun. Depending on the
type of change, the near-Earth space environment responds with time delays of minutes to days. The
sequence and approximate durations of various solar activity effects are illustrated below.
Time sequence of space weather events following a major solar disturbance. Not all elements of this sequence are
detected every time (courtesy of M.A. Shea, Phillips Laboratory).
Sudden Ionospheric Disturbances
The energetic electromagnetic radiation bursts (ultraviolet and x-rays) accompanying flares on the
Sun travel at the speed of light, and so arrive at Earth just eight minutes after leaving the flare site,
well ahead of any particles or coronal material associated with the flare. Moreover, unlike the
electrons and ions of the solar wind plasma and the solar energetic particle populations, the passage
of electromagnetic waves is not affected by the presence of Earth's magnetic field. The direct
response of the upper atmosphere to a burst of solar flare ultraviolet and x-ray emissions is a
temporary increase in ionization in the sunlit hemisphere of minutes to hours duration called a
"sudden ionospheric disturbance." The ionization increase below 100-km altitudes is especially
significant on these occasions.
Solar Energetic Particle Events
Particles are accelerated to "cosmic-ray-like" energies by the interplanetary shocks preceding fast
coronal mass ejections and in the vicinity of solar flare sites. The large extent of the shocks compared
to the flares makes them a more common source of solar energetic particles near Earth. The most
energetic particles arrive at Earth within tens of minutes of the event on the Sun, while the lower-
energy population arrives over the course of a day. These particles temporarily enhance the radiation
in interplanetary space around the magnetosphere. In the polar regions they then penetrate to low
altitudes along the magnetic fields that map into the auroral oval. Sometimes they also find their way
into the deeper magnetosphere by means of other transport processes. As shown by the figure below,
although solar energetic protons are encountered most frequently around solar maximum, they can
occur at any time in the solar cycle.
Diagram showing when major solar proton events occur during the solar cycle (courtesy of Ron Turner, ANSER home
page).
Like the energetic solar electromagnetic emissions in the ultraviolet and x-ray wavelengths, the
energetic solar particles enhance the ionospheric density below 100-km altitudes. In this case,
however, the effect is limited to the polar regions where the particles can travel directly to the
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Space Weather: A Research Perspective
atmosphere. These deeply ionizing events can alter the local atmospheric chemistry in ways that
affect the ozone concentrations in the upper levels of the ozone layer.
Space Weather Storms
In general, disturbances in the solar wind arrive at Earth 1-2 days after leaving the Sun. These
disturbances, especially those caused by coronal mass ejections, trigger global changes in Earth's
magnetic field and particle populations and are called magnetic storms. The fastest coronal mass
ejections or CMEs, traveling at up to 2000 kilometers per second (compared to normal solar wind
speeds of 400 to 800 kilometers per second) cause the most severe magnetic storms.
An artist's conception of the interaction of a coronal mass ejection with Earth's magnetosphere (courtesy of NASA).
The details of the Earth-space response are quite complex, and our understanding of it only partially
complete. When a fast CME passes, its leading shock causes the sudden onset of a variety of
magnetospheric activity. A large disturbance of the geomagnetic field reaching to latitudes near the
equator signals the injection of a newly energized charged-particle population into the magnetosphere
(forming what is called a "ring current"). New, temporary "radiation belts" may also appear.
Intensifications of auroral light are caused by increases in the numbers of electrons and ions raining
or "precipitating" from the magnetosphere into the upper atmosphere in the auroral oval. The
magnetosphere as a whole is energized by a much stronger interaction than usual with the solar wind,
resulting in enhanced ionospheric electric currents and an associated stronger coupling between the
magnetosphere and ionosphere. The latter is particularly true if the disturbed magnetic field in the
ejected coronal material or the piled-up interplanetary field preceding it has a long period (hours to
days) of southward-directed magnetic field. Signs of this closer coupling include a substantial
increase in the numbers of ionospheric ions appearing in the magnetosphere, as well as a greatly
disturbed high-latitude ionosphere. Ionospheric currents, embedded in the region of intense auroral
particle precipitation, intensify and spread equatorward with the expanding oval, an example of which
is pictured in the image below.
Ultraviolet images from the Dynamics Explorer satellite showing the expansion of the auroral oval during a magnetic
storm (courtesy of Dynamics Explorer University of Iowa Imaging Experiment, L.A. Frank, principal investigator).
The upper atmosphere heats and then expands in response to the storm-generated increases in
auroral currents and particle precipitation, which both deposit energy. During a large magnetic storm,
the density of the upper atmosphere at satellite altitudes may reach 100 times its quiet time value.
These episodic increases in the atmospheric density are superimposed upon longer-term trends in
the overall heating and expansion of the atmosphere in response to the 11-year solar activity cycle.
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Space Weather: A Research Perspective
The red areas in this image represent regions with 20% or more increases in the atmospheric density
at the altitude of the space shuttle orbit (about 300 km) during a moderate magnetic storm period
(courtesy of A. Burns, University of Michigan Space Physics Research Laboratory).
The occurrence rates of magnetic storms and all of their effects follow the frequency of CMEs and are
highest during solar maximum periods (see below).
The annual number of geomagnetically disturbed days compared to the sunspot number. The solar activity cycle
clearly controls the occurrence of disturbed space weather (courtesy of the NOAA National Geophysical Data Center,
Boulder, CO).
Major space weather storms could in principle be predicted from observations of fast coronal mass
ejections coming toward Earth. At the speeds of fast CMEs, this would allow about 1-2 days warning if
the initiation was seen at the Sun. About 1 hour's warning is possible if the interplanetary disturbance
is detected upstream of Earth at the location of most solar wind-monitoring spacecraft.
Illustration of the situation preceding (by the order of a day) a major magnetic storm
(courtesy of the Space Physics and Aeronomy Section slide set, American Geophysical
Union, Washington, D.C.).
Space Weather "Substorms"
"Substorms" are in some ways like small versions of the storms described above. However,
substorms can occur without the passage of a major interplanetary disturbance. They are thought to
build up during periods when the undisturbed interplanetary magnetic field has a southward
component. Researchers believe there is a gradual transfer of solar wind energy into the
magnetosphere under these circumstances because of the greater amount of interconnection
between the interplanetary and Earth magnetic fields in the polar regions. Any small perturbation, like
a solar wind pressure increase, or a change in the interplanetary field orientation, can release that
stored energy in storm-like ways. The substorm may also occur spontaneously if the
magnetosphere's ability to store the transferred energy reaches its limit. Substorms sometimes occur
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Space Weather: A Research Perspective
during the course of the major storms, modulating the longer term and more intense activity
associated with them. The reconfiguration of magnetotail fields thought to accompany a substorm is
shown below. This reconfiguration causes the dumping of some of the magnetosphere's particles into
the auroral zone as the magnetosphere readjusts, producing auroras and ionospheric disturbances as
do small magnetic storms.
Illustration of the behavior of the stretched geomagnetic tail field during a substorm, as inferred from spacecraft
measurements. In a sense the magnetotail is "shedding" some of the magnetic field that accumulates when a
prevailing southward interplanetary field continues to interconnect with Earth's field (from: S.W.H. Cowley, December
19, 1995, EOS, v. 76).
Energetic Electron Increases
Earth's radiation belts are observed to undergo significant quiet time changes even in the absence of
storms and substorms. In particular, it has been found that the population of relativistic electrons
trapped in the magnetosphere increases and decreases with the prevailing solar wind velocity. The
stream structure of the solar wind and solar rotation produce the 27-day modulation shown below in
both solar wind speed measured on the WIND spacecraft upstream of Earth and energetic electrons
measured by the GOES-8 spacecraft in the radiation belts. The reasons for the relationship between
the solar wind speed and electron population are currently unresolved.
Comparison of solar wind speed measured on the WIND spacecraft upstream of Earth and the energetic electron flux
in the radiation belts measured on GOES-8. The 27-day solar rotation influence is clearly seen here (courtesy of H.
Singer and T. Onsager, NOAA Space Environment Center).
Ionospheric Irregularities
The ionosphere exhibits regular daily and seasonal variations, as well as disturbances directly caused
by flares and by the auroral particles and currents during magnetic storms and substorms. In addition,
the ionosphere exhibits irregular variations related to the dynamics of the underlying atmosphere.
These depend upon the combination of traditional "weather" near the ground, which produces waves
in the atmosphere like the waves in the deep ocean, and the winds between the ground and the upper-
atmosphere levels that act like a filter to the passage of those waves. While this aspect of space
weather may appear to have a non-solar origin, its effects are most pronounced when the upper-
atmosphere winds or lower-ionosphere electron density is enhanced by the energy inputs from the
active Sun or magnetosphere.
One striking example is equatorial "spread F," a disturbance of the nighttime low-latitude ionosphere.
Spread F can be thought of as a huge convective storm in the ionosphere, thousands of kilometers
across, as much as 1000 kilometers high, and dwarfing even the largest hurricanes. It derives its
name from the effect of the associated ionospheric density irregularities on over-the-horizon radar and
satellite-to-ground radio communications. The figure below shows regions in the ionosphere where 3
meter wavelength radar signals reflected from a spread F layer during a disturbance over Peru.
Neither the origins nor solar controls of spread F conditions are well understood, although the
interaction of the ionized and neutral gases in the affected layer seems to play an important role. As
developing countries near the equator become more dependent on advanced systems for
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Space Weather: A Research Perspective
communication with the rest of the world, a better understanding of equatorial spread F will become
essential.
A record showing where 3-meter wavelength radar signals reflected from the ionosphere over a site in Jicamarca,
Peru, during an episode of "spread F" (courtesy of W. Swartz, Cornell University).
High-Altitude "Sprites" and "Jets"
Optical phenomena called red sprites and blue jets are observable by sensitive cameras at altitudes
extending from the tops of strong thunderstorms (at around 15-kilometers altitude) to the lower
ionosphere (about 95-km altitude). Possibly related to these optical signatures, short-duration gamma-
ray bursts have been detected over thunderstorm regions on the Compton Gamma Ray Observatory,
as were intense electromagnetic pulses (10,000 times stronger than lightning-related pulses) on the
ALEXIS satellite. This collection of observations suggests that there may be a stronger connection
between global thunderstorm activity and the ionosphere and upper atmosphere than previously
suspected. In particular, they may signify the presence of powerful large-scale discharges in Earth's
"global electrical circuit."
Picture of a
red sprite
and blue jet
over a
thunderstorm
(courtesy of
D. Sentman,
Geophysical
Institute,
University of
Alaska at
Fairbanks).
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