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5—
Solar Variations and the Earth's
Near-Space Environment
Background
The geospace environment is the region of transition between the
Earth's protective atmosphere and the onrushing solar coronal
plasma (the solar wind). It is a region of even greater variability
than the Earth's upper atmosphere, transmitting and redepositing
the fluxes of mass, momentum, and energy received from both the Sun
and the Earth. The near-space environment responds dramatically to
changing solar energy inputs (e.g., Gorney, 1990), but this solar
forcing appears to have little direct impact on the Earth's
climate.
The Earth's near-space environment does provide a critical
buffer between the highly dynamic space environment and the
relatively placid lower and middle atmospheres. To a large extent,
it determines the penetration of these layers by energetic
particles accelerated on the Sun, from outside of the solar system,
and within the magnetosphere. As discussed in Chapter 3, both
energetic solar protons and relativistic electrons can destroy
ozone and affect the middle atmosphere. Also, large ejections of
mass and magnetic fields from the Sun, whose influence is
transmitted to the Earth through the near-space environment, can
significantly affect certain complex technological systems,
including electrical power grids, Earth-orbiting spacecraft, and
communication links (Joselyn, 1990).
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The Solar Wind and the Earth's
Magnetosphere
Flowing from the Sun is the solar wind, whhch continuously
carries magnetized plasma and energetic solar particles into the
vicinity of the Earth. The Earth and its atmosphere are shielded
from the direct impact of these particles and plasmas by the
magnetosphere, a relatively self-contained region in space whose
global topology is organized by the intrinsic magnetic field of the
Earth. This field, which may be represented to a reasonable
approximation by a dipole originating in the Earth's molten metal
core, extends far into space and serves to deflect the onrushing
solar wind. The stand-off distance (the magnetopause), commonly
about 10 Earth radii (RE ) at the
subsolar point, depends on the solar wind pressure and is highly
variable. In the outer reaches of the Earth's near-space
environment, tangential stresses applied by the solar wind set up a
system of boundary region currents that effectively constrain the
outer geomagnetic field to a comet-shaped form with a long tail
extending downstream from the Sun (Figure 5.1). Thus, the Earth's
magnetosphere extends from the upper atmosphere/ionosphere to
altitudes of about 10 RE on the
sunlit dayside and to more than 1000 R
E on the nightside.
Mass, momentum, and energy are imparted to the magnetosphere
with great variability by the continuously flowing solar wind. The
primary form of plasma energy available at 1 astronomical unit (AU)
is kinetic, as a result of the motion of the solar wind relative to
the Earth. Solar wind plasma interacts with the projected
cross-section of the entire magnetosphere (a disk of radius about
20 RE ), so that the total power
intercepted due to the solar wind kinetic energy is about one
thousandth of the radiant energy intercepted by the disk of the
Earth. This energy transfer occurs with much greater variability
than the radiant heating variations associated with the 0.1 percent
solar cycle change in total solar irradiance. However, it is not
the solar wind kinetic energy flux per se that seems to
control geomagnetic activity, but rather the embedded solar wind
magnetic field.
The major processes that extract, store, and dissipate energy
from the solar wind flowing past the Earth, subsequently disturbing
the geospace environment, involve the generation of plasma and
energetic particles from stored magnetic fields. Three primary
forms of energy dissipation detectable in the Earth's atmosphere
are auroral particle precipitation,
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Figure 5.1 The Earth and its atmosphere are
surrounded by the near-space environment. The solar wind carries
magnetized plasma and energetic solar particles into the vicinity
of the Earth, which is shielded from their direct impact by the
magnetosphere, a relatively self-contained region in space whose
global topology is organized by the magnetic field associated with
the Earth. Courtesy of T. Potemra, NASA publication.
auroral Joule heating, and energetic neutral atoms produced from
extraterrestrial ring current flows. Eventually, plasma particles
convert part of their energy to radiation modes such as auroral
displays and kilometric radiation.
Solar Eruptive Events and Geomagnetic
Storms
Explosive outbursts from the Sun release energy primarily in the
form of X-rays, UV radiation, energetic particles, magnetized
plasma, and shock waves. Large injections into the magnetosphere of
magnetized plasma from the Sun generate major disturbances called
geomagnetic
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storms. Moderate magnetic storms may occur relatively frequently
(every month or so), but really large storms due to major solar
disturbances usually occur at intervals of many years.
Energetic particles produced by solar eruptions (and also
galactic cosmic rays) are excluded from low magnetic latitudes by
the geomagnetic field but, as discussed in Chapter 3, the polar
regions of the Earth are exposed to the full interplanetary flux
(Figure 3.3). During geomagnetic substorms and storms, energized
particles bombard the Earth's upper atmosphere, colliding with
atmospheric constituents, transferring their energy (Table 1.1),
and causing large auroral displays. Substorms and storms have long
been detected by virtue of the intense magnetic disturbances that
they cause in the auroral regions. These magnetic effects are
associated with strong field-aligned (Birkeland) currents that flow
in the auroral zones and dissipate energy resistively in the upper
ionosphere. This Joule heating associated with substorm currents
can be monitored from the Earth (through arrays of magnetometers),
and ionospheric conductivity models can be employed to convert
measured currents to ohmic dissipation.
One of the primary manifestations of a geomagnetic storm is a
large enhancement of the extraterrestrial ring current, composed of
trapped particles drifting in the Earth's inner magnetospheric
region. During such enhancements the ring current can cause large
magnetic disturbances in the low-latitude magnetic field at the
Earth's surface. Accelerated particles and plasma are injected from
the tail of the magnetosphere into the ring current. There, these
partciles gradually lose their energy (over hours or days) due to
precipitation and charge-exchange processes. Hence, the ring
current is a major sink of magnetosperic energy.
A significant part of the energy dissipated during geomagnetic
activity can be assessed by examination of auroral and ring current
terms. Over the years, indices of auroral disturbance (e.g., the AE
index) and ring current disturbances (e.g., the Dst index) have
been formulated. These are basically parameters of levels of
magnetic disturbances as measured on the Earth's surface, and they
calibrate the disturbance level. In turn, it has been possible to
assess magnetospheric energy losses in terms of these indices.
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Terrestrial Impacts
Energetic solar outbursts impact the Earth in a variety of ways,
depending on how the released energy couples into the global system
through the Earth's near-space environment. The impact of solar
outbursts can be severe. For example, associated with the March
1989 eruption were high-frequency communication outages and
disruptions of navigation, anomalous radar echoes at high
latitudes, and electrical power outages including a 9-gigawatt
failure in Quebec that affected 6 million people for half a day and
caused millions of dollars of damages. Heating of the Earth's
atmosphere during the storm increased satellite drag, leading to
uncontrolled tumbling of several satellites and more rapid orbital
decay of others (Allen et al., 1989; Joselyn, 1990). Depletion of
stratospheric ozone over Antarctica may also have occurred
(Stephenson and Scourfield, 1991). Particle events such as these
could create serious radiation hazards to manned space missions at
high orbital inclination or outside the magnetosphere.
Huge disturbances in the polar ionosphere often result from
geomagnetic storms, and intense auroral luminosity can reach over
much of the high-latitude portion of the Earth. Low-latitude
magnetometers on the Earth's surface can register changes up to
about 1 percent of the normal ambient field. Such changes in the
magnetic field can have significant short term effects on
navigation, resource exploration, and other human activities.
Particle events associated with eruptions on the Sun cause failures
in microelectronic circuits, buildup of electric charge on
spacecraft, and potentially harmful radiation doses to crews of
high-altitude aircraft (Joselyn and Whipple, 1990). Clearly, the
highly sporadic and unpredictable nature of geomagnetic activity
makes it very difficult to estimate the importance of its effects
on the terrestrial environment on the time scale of decades to
centuries. In an historical context, knowledge of the role of the
geomagnetic field itself is important because variations in its
strength modulate the exposure of the Earth's atmosphere to
bombardment by galactic cosmic rays, allowing variations in
production of 14C and other
cosmogenic nuclides that could mimic variations in solar
activity.
Longer term terrestrial influences may also arise from the
fluxes of relativistic electrons with energies of several million
electron volts, frequently present within the magnetosphere, that
can reach to depths
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similar to those reached by solar protons. These energetic
electrons are accelerated primarily within the magnetosphere
(rather than on the Sun) and occur much more frequently than major
flare events. The presence or absence of the relativistic electron
fluxes appears to be strongly controlled by the existence of
high-speed solar-wind streams emanating from persistent coronal
holes, which in turn are strongly controlled by the solar activity
cycle. The extent to which precipitating relativistic electrons
actually enter the atmosphere is currently uncertain. As noted in
Chapter 3, if a significant amount of these ionizing particles
reaches the middle atmosphere, the long term effects on ozone
concentrations in the stratosphere could be significant (Callis et
al., 1991).
Representative terms from entire chapter:
ring current