Humankind lives on Earth in a thin shell of air only 100 miles
thick. Energy from the Sun provides light and heat, without which
life would be impossible. Most of the conditions we experience
on Earth-wind, rain, heat, cold-derive from this solar energy,
together with the fortuitous placement of Earth in the solar system.
We are not too close to the Sun, like Venus with its clouds of
carbon dioxide and surface temperature of 480 degrees Celsius;
nor are we too far away or too small, like Mars.
The space above the atmosphere is not empty. Rather, the Earth is suspended in a rich, active, electrically charged environment known as a plasma. The most obvious manifestation of the active phenomena that occur within this environment is the aurora, or the Northern and Southern Lights. Known to the ancients, these lights in the regions of the Boreal and Austral Poles were first interpreted as being supernatural in origin. Later, attempts were made to ascribe scientific causes. In the Middle Ages, the Italian scientist Galileo, while looking at the Sun through a telescope for the first time, noticed dark spots on the surface that were constantly changing. As the sunspots were studied over the years, it was noted that the number of spots waxed and waned in an eleven-year cycle and that the chance of seeing the aurora rose and fell with the number of sunspots.
Thus began the realization that the Sun communicated with the Earth in ways other than by providing light and heat. This was the first real beginning of modern space physics.
Even before the more recent era of direct space exploration with satellites, scientists had begun to perceive that electrically charged atoms flowing outward from the Sun collided with the magnetic field of the Earth and played a significant role in creating the Earth's environment. Reception of radio waves over long distances showed the presence of a layer of conducting gas above the atmosphere. At the time, explorations were limited to ground-based instruments, and much of the important data necessary to build even a rudimentary understanding were not available.
Launches of Soviet and United States scientific satellites came during and after the International Geophysical Year 1958. These brought increased awareness of the importance of the electromagnetic environment that surrounds the Earth. The premier scientific discovery was of the Earth's radiation belts. This discovery showed that charged atomic particles, electrons, and ions, were trapped within the Earth's magnetic field above the atmosphere. The scientific quest to understand the source of these particles and how they obtained high energies began immediately.
Three decades of probing the environment around the planet with a wide variety of instruments have led to a model of the essential features of the Earth's magnetic field and the plasma it contains-a region known as the magnetosphere. (The Earth and its atmosphere seem insignificant in scale, compared to the size of the magnetosphere.) Close to Earth, the magnetic field looks like the familiar field of a pattern of iron filings around a bar magnet.
Solar-terrestrial physics today is aimed at achieving a better understanding of the Sun as a star, and its effects throughout the solar system. Interior processes within the Sun feed energy to its outer regions; the photosphere, the chromosphere, and the solar corona. Masses of hot plasma, ejected from low density regions of the corona, are streaming away from the Sum, and form the so-called solar wind. Interplanetary shocks are formed between solar wind streams of different velocity and density and a shock is also created when the solar wind encounters the obstacles to the Earth's magnetic field. For example, solar-flare explosions associated with sunspots, can cause strong gusts of solar wind. At increasing distance from the Earth, as the Earth's magnetic field weakens, this hot solar wind is able to push against the magnetic field, compress it on the sunward side and stretch it out downwind to form a long tail or wake. Although the magnetic field at this distance is weak, it is still capable of holding within it masses of highly energetic plasma.
The solar wind-composed of electrified particles-streams toward Earth at a million kilometers per hour, carrying particles and magnetic fields from the Sun outward past the planets. As the solar wind approaches the Earth's magnetic field, a highly supersonic shock wave is created sunward of the Earth somewhat analogous to the shock wave created when a jet plane breaks the sound barrier. This shock wave is called the bow shock. Most of the solar wind particles are heated and slowed at the bow shock and detour around the Earth through a volume of space called the magnetosheath. Some particles are actually reflected back from the bow shock into the solar wind stream in a region of turbulence called the foreshock.
As the solar wind flows around the Earth, it stretches out the magnetosphere out into its long tail, the magnetotail Some of the solar particles leak through the barrier at the boundary of the Earth's magnetic field (the magnetopause), are trapped inside the magnetosphere, and are stored in the plasma sheet and radiation belts. Some particles rush through funnel-like openings at the poles, called the polar cusps. Many of these energetic particles descend Earth's magnetic field lines to enter the upper atmosphere and create the aurorae-the visible signature of the otherwise unseen energy transfer from the Sun to the Earth.
Events on the Sun can also trigger other changes in the electrical and chemical properties of the atmosphere, in the region above the atmosphere known as the ionosphere, and out into the magnetosphere. These changes cause magnetic storms, communications static, power blackouts, and navigation problems for ships and airplanes with magnetic compasses. Also, satellites and spacecraft can be damaged or can re-enter the atmosphere prematurely because solar storms increase the density of the atmosphere, and hence the drag on satellites.
The ability to anticipate particle outbursts and fluctuations in the plasma flowing from the Sun and in the magnetic field due to solar activity will be increasingly useful as more of our science, commerce depend on the operation of vehicles in space.
Alterations in the Earth's environment caused by these solar phenomena happen over a variety of time scales ranging from less than a minute to over a century. Violent changes on the Sun can swiftly affect specific regions on Earth-within 8 minutes, for example, with a solar x-ray burst. Such is the effect of the Sun on Earth that some researchers also think that this coupling may affect long-term climatic changes.
Many then are the connections between the Sun and Earth. Solar-terrestrial physics today is aimed at achieving a better understanding of the Sun as a star, and of its effects throughout the solar system. In the study of this interconnected region, its characteristics and the interplay among its components this complex system requires a similarly complex array of multiple spacecraft, ground observations and theoretical studies in order to achieve understanding. Coordinate of this array of research efforts requires international cooperation between the world's space agencies and scientific organizations. The Inter-Agency Consultative Group (IACG) for Space Science provides the mechanism for this close coordination of the four major space agencies.
Multiple spacecraft from agencies in Europe, Japan, Russia and
the USA will study the Sun and solar wind, and processes they
cause in the Earth's environment.
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