Encyclopaedia: Geospace

2015-03-30 Andrew B. Collier

A contribution which I wrote for Antarctica and the Arctic Circle: A Geographic Encyclopedia of the Earth’s Polar Regions. As a side note, I am normally quite pedantic about “Earth” and “Sun” being capitalised but the editor of the Encyclopaedia evidently had different views on this point.

The space immediately around the earth, including the upper atmosphere, ionosphere, and magnetosphere, is known as geospace. This region is of considerable scientific interest since it is the environment inhabited by most artificial (communication, navigation, meteorological, etc.) satellites. In addition to in situ measurements, observations of geospace are commonly made at high latitudes because the earth’s magnetic field focuses phenomena from a large volume of space down onto a relatively small portion of the earth’s surface. Both the Arctic and Antarctic are suitable sites. However, the Antarctic continent provides a better environment for the construction of research facilities. A significant number of countries with bases in Antarctica are engaged in geospace research.

Geospace is filled with plasma, which is a partially or fully ionized gas. As the fourth state of matter, plasma has characteristics that are completely different from those of neutral gas: the motions of charged particles in plasma are strongly influenced by externally imposed magnetic fields, as well as the electric fields from other charged particles and electromagnetic waves. This has important consequences for charged particle dynamics as well as the propagation of electromagnetic waves.

The plasma in geospace is permeated by the earth’s magnetic field and largely controlled by it. The earth’s intrinsic magnetic field is roughly dipolar (similar to that of a bar magnet). Magnetic field lines radiate outward from the surface of the earth near the North Magnetic Pole and extend a great distance into space before converging near the South Magnetic Pole. The magnetic field lines therefore cluster near the poles but are widely separated in the equatorial plane. For example, field lines separated in latitude by 60 miles (100 km) near the magnetic latitude of 60° on the surface of the earth are separated by about 900 miles (1500 km) in the equatorial plane. This means that a relatively small area on the earth’s surface maps to a very large volume of geospace. The magnetic field lines near the magnetic poles therefore act like an enormous funnel, concentrating the influences of most of geospace down onto only a small portion of the surface of the planet. Somewhat surprisingly, the earth’s North Magnetic Pole is currently located in the Southern Hemisphere (64° S 138° E), which is off the coast of Antarctica. The South Magnetic Pole is in the Canadian Arctic territory.

The geospace environment is linked to the sun by the interplanetary magnetic field (IMF) and the solar wind. The solar wind is plasma that flows at supersonic speeds (typically around 400 km/s) from the surface of the sun. The solar wind exerts pressure on the earth’s magnetic field, confining it to a cavity known as the magnetosphere. The orientation of the IMF and the speed and density of the solar wind, which vary with solar activity, have a marked effect on the state of the magnetosphere. The magnetosphere forms an enormous plasma physics laboratory, orders of magnitude larger than any terrestrial laboratory. This means that large-scale plasma physics phenomena, which are impossible to replicate on earth, can be studied.

Charged particles move easily parallel to magnetic field lines. However, the Lorentz force causes them to gyrate in a plane perpendicular to magnetic field lines. As a result, ions and electrons in the magnetosphere move on spiral paths along magnetic field lines. As a charged particle approaches a magnetic pole, the magnetic field strength increases and the speed of the particle along the magnetic field line is reduced. At some point, the particle is reflected back up the magnetic field line into the magnetosphere. This mirror point is at an altitude determined by the particle’s velocity. If the mirror point is within the neutral atmosphere, then there is a high probability that the particle will collide with a neutral atom or molecule, in which case it will lose energy and will not return to the magnetosphere. Such a particle is said to have precipitated. Positive and negative charged particles also drift in opposite directions around the earth, forming a ring current that causes measurable magnetic effects on the earth’s surface.

The aurora australis and aurora borealis (the southern and northern lights) are caused by energetic electron precipitation. Collisions between these electrons and neutral atoms or molecules in the upper atmosphere leave the neutrals in an excited state from which they subsequently decay by emitting light. Typically, the light is either green, red or violet, depending on the altitude of the emission. Green aurora, the most common, is due to the excitation of atomic oxygen.

Cosmic rays are extremely high-energy particles originating from outer space, generally outside the solar system. Most cosmic rays are atomic nuclei (predominantly hydrogen and helium atoms stripped of their electrons). They produce a cascade of secondary particles when they enter the earth’s atmosphere. Some of these secondaries are able to penetrate the earth’s surface. Since cosmic rays are charged particles, they too are able to access the earth’s lower atmosphere more readily near the magnetic poles. Only the most energetic cosmic rays can penetrate to the ground near the equator.

Certain electromagnetic waves are also affected by the earth’s magnetic field. Whistler-mode waves propagate along magnetic field lines through a plasma. Lightning strokes are the main source of whistler-mode waves. Some electromagnetic energy from lightning penetrates through the ionosphere and enters the magnetosphere, where it propagates in the whistler mode. Magnetic field lines guide the waves to the opposite hemisphere where they penetrate down through the ionosphere to reach the ground. The whistler mode is dispersive, which means that distinct frequencies travel at different speeds. The brief electromagnetic pulse generated by lightning is thus transformed as it passes through the magnetosphere into a unique signal that emerges in the opposite hemisphere. This is called a “whistler.” Measurements of whistler dispersion are used to determine plasma densities at great distances from the earth, which otherwise could be measured only by satellites.

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