Science
Spatial Domain Concepts of the Solar-Terrestrial System
Following are brief descriptions of various domains of the solar-terrestrial system. They continue to be important concepts, although CAWSES will seek a new paradigm - that of a system in which the study of scientific "problems" from end-to-end supersedes focused research within these somewhat arbitrary spatial distinctions.
Sun
The Sun is a rotating variable star whose energy source is nuclear fusion energy generated from within. The principal agent of solar activity is the magnetic field, which structures the inner solar atmosphere and corona (Figure 2: SOHO EIT image of the sun on July 14, 2000). Convection and turbulence favor magnetic reconnection and energy release in the corona in the form of electromagnetic and particle radiation. Solar activity is characterized by the 11-year solar cycle, implying changing numbers of sunspots, irradiance, flares and coronal mass ejections (CMEs).
Heliosphere
The heliosphere is that region of the solar terrestrial system that extends from the Sun's outer corona into space. Continuous solar plasma emissions produce the "solar wind". High-energy particles ejected from active regions travel through the heliosphere, reaching the Earth within minutes to hours, while large-scale solar wind structures (e.g. "plasma clouds" that CMEs produce) take days to reach Earth. Co-rotating sector boundaries of reversing magnetic field orientation and streams of fast solar wind from "coronal holes" sweep past the Earth with a 27-day recurrence pattern.
SOHO LASCO image sequence of CME of 6 November 1997
The LASCO sensor on SOHO creates an artificial blocking of the bright disk of the Sun so that materials extending into the diffuse corona may be imaged. In this sequence on 6 November 1997, the imager captures a large CME erupting from the Sun's west limb. The arched prominence seen crossing the solar equator at 11:50 UT explosively breaks apart as it expands into space. Even though active regions on the solar limb are not typically directed Earthward, the speckled effect produced by arriving high-energy ions are prominent about 1.5 hours after the first image of the eruption.
Magnetosphere
Cartoon illustrations of the Sun-Earth-System with an expanding cloud of plasma from a CME that has not yet reached the magnetosphere (upper panel) and the disturbed conditions with the magnetosphere distorted by arrival of the plasma cloud. This represents conditions during the disturbance of 18-20 October 1995. The geomagnetic storm associated with this event had the maximum 24-hour disturbance index Ap* = 54. This is a significant storm, but was not a particularly large one.
The magnetosphere is the local region of interplanetary space where the Earth's magnetic field largely excludes the solar wind. The magnetosphere buffers the Earth's atmosphere from energetic charged particles and solar wind plasma. Regions of trapped high-energy charged particles surrounding the Earth are called radiation (or "Van Allen") belts (inner zone and outer zone). Some events are so powerful as to produce a new intermediate zone radiation belt. Intense intervals of fast solar wind and variable interplanetary magnetic field structure can result in dramatic increases in the inner magnetosphere's particle populations and produce geomagnetic "storm" activity. These geomagnetic storms result in large amounts of energy being transmitted to the atmosphere through energetic particle precipitation and electric currents, which are concentrated in Polar Regions where they cause auroral substorms.
Thermosphere/Ionosphere
The thermosphere is the Earth's atmosphere that extends outward from about 100 km, comprising primarily oxygen and nitrogen. The density, composition, and dynamics of this region are highly responsive to variations in direct energy inputs from above - the Sun and the magnetosphere (at high latitudes) - and the middle atmosphere below. Changes in the neutral atmosphere scale height due to heating of the lower thermosphere can result in the premature de-orbiting of low-altitude satellite systems due to the increased drag of the expanded atmosphere on the satellite.
Created by the interaction of solar extreme ultraviolet (EUV) radiation with the upper atmosphere, the ionosphere extends from about 60 km above the Earth's surface to several 1000 km (Figure 4: Ionosphere). At high latitudes, the precipitation of energetic electrons into the lower ionosphere from the magnetosphere creates localized regions of additional, highly variable ionization. These organized streams of charged particles cause auroral magnetic substorms, which can affect regional technology. Such events often are associated with aurorae visible from above by satellites and from below by people on the ground. The ionized and neutral gases are tightly coupled via collisions and dynamical motions, and respond together to energy and momentum drivers from both the magnetosphere above and the atmosphere below. Fluctuations in ionosphere electron densities affect radio communication and navigation since they alter radio wave paths.
Middle Atmosphere
Noctilucent clouds are Middle Atmosphere phenomena. They are the highest altitude clouds known to exist, and occur during the polar summer. Observers on the Earth's surface see them after night has fallen there, but while sunlight still illuminates the tenuous clouds. They are caused by ice particles produced in the intensely cold polar summer Mesopause region (~85 Km).
The mesosphere and stratosphere are found between 10 and 100 km. These regions interface the radiation-dominated thermosphere above with the troposphere, where life exists, below. Ozone residing in the stratosphere protects biological species from exposure to harmful solar UV radiation. The middle atmosphere is the site of the most significant anthropogenic effect yet identified in the solar-terrestrial system - namely the Antarctic Ozone Hole. This, and the apparent global depletion of ozone, prompted policy making that led to the Montreal Protocol. In Figure 5, the setting sun illuminates noctilucent clouds, the highest altitude form of clouds, seen overhead after the ground beneath is dark. Increased sightings of noctilucent clouds may be related to climate change.
Lower Atmosphere and Climate
Most of the Earth's atmospheric mass resides below about 15 km, in the troposphere, where weather and climate occur, and life exists. Solar irradiance is the ultimate source of the energy that powers the climate system and enables the biosphere. Climate varies in response to both anthropogenic and natural forcings, as well as from internal oscillations. Specifying and understanding climate forcings and response has major societal benefits, and is a high priority of current research. Figure 6 is a plot of changing global temperatures since the 19th century based upon observations compiled in the "Global Historical Climatology Network - GHCN). It shows temperature trends as produced by three different, widely recognized research programs. All used the same data, but applied different reconstruction techniques. All three graphs agree that the global surface temperature has been increasing significantly for the last 100 years. CAWSES participants will join with others involved in paleoclimate and climate studies to evaluate what part of global temperature rise may be due to solar forcing.
Limits/ Extreme Cases of Variability
Each planet has its own environment, which is characterized by the results of physical and chemical processes in different parameter regimes unrealized in the Earth's system. Knowledge obtained from other planetary systems therefore offers a broader understanding of the physical and chemical processes that may occur in the solar-terrestrial environment, under conditions different from those of the present time.
Comparison of the magnetospheres of the outer planets, such as Jupiter with that of the Earth, will contribute new understanding of solar wind-magnetosphere coupling. For example, in the Jovian ionosphere-magnetosphere system, auroral emissions as well as radio emissions are significantly controlled by the fast rotation period of Jupiter, and there is less dependence on solar wind parameters. On the other hand, Mercury's magnetosphere is very small and is probably dominated by the strong solar wind flow near to the sun. Mercury does not have an ionosphere. The upper atmospheres of Venus and Mars have no magnetosphere to provide a buffer to the solar wind. On Earth, the Little Ice Age is a recent example of anomalously lower solar activity concurrent with possible terrestrial consequences. This is an example of an extreme set of conditions in the Sun-Earth system.
CONTACT US:
CAWSES Office, Center for Space Physics, Boston University, 725 Commonwealth Ave. Boston, MA 02215 USA;
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