Solar Variations and the Earth's
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).
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,
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
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.
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
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).