Reinhart Leitinger

Geomagnetic Activity Indices

The predominant part of the geomagnetic field, as observed at the Earth’s surface, originates from sources in the Earth’s core and, to a lesser degree, in the Earth’s crust. Spatial distribution and secular variation of this internal part were described, e.g., by Schmucker (1985). Superimposed, there is a small external part due to large-scale current systems in the ionosphere and magnetosphere finally resulting from the motion of charged particles in the Earth’s magnetic field. Short-term time variations of the external part are governed by solar wave (W) and particle (P) radiation and thus may serve to yield information on solar-terrestrial relationships that otherwise cannot so easily and continuously be achieved.

Chemistry in the Atmosphere

Trace gases play a major role in controlling radiation exchanges in the atmosphere and consequently in determining the atmospheric circulation and, to a large extent, the climatic conditions at the Earth’s surface. Coupling between the lower, the middle and the upper atmosphere, as well as between the atmosphere, the oceans and the biosphere is largely achieved by fluxes of chemical compounds. The elucidation of chemical and photochemical processes in the atmosphere is a prerequisite for assessing global changes in the ecosystems, for realistically predicting future climate, and for understanding the causes and consequences of global, regional and urban air pollution.

Chemistry and Physico-Chemistry

This chapter deals mainly with the methods and techniques developed and used over the last decades, in order to characterize the composition of the upper atmosphere.

Computer Network Communications

There are several wide-area computer networks around the world that are used extensively by the space and Earth science community. These networks can be grouped by the computer communications protocol that they implement such as TCP/IP, NJE, X.25 and DECnet. The Internet (or TCP/IP Internet) uses TCP/IP, BITNET uses NJE, the international public packet switch network use X.25, and the DECnet Internet uses DECnet. Figure 1 illustrates the current computer connectivity in the world for the TCP/IP Internet, BITnet, other smaller networks and regions where no international computer connectivity exists. The DECnet Internet connectivity covers much of the same area as the TCP/IP connectivity map.

The Thermosphere: Selected Features

The thermosphere is the region above the mesosphere and exhibits the strongest height variation of temperature of all atmospheric layers, from the absolute minimum at the mesopause to the absolute maximum, the exospheric temperature. It is also the region where the most drastic changes in atmospheric composition occur and where the atmosphere becomes partly ionized and acquires the properties of a magneto-plasma. The solar EUV radiation is absorbed in the thermosphere and here we localize most of the electric current systems which are responsible for the so-called variations of the geomagnetic field.

Solar Activity, Solar Cycle, Coordinates

The structure and the dynamics of the upper atmosphere depend on the energy input from the Sun via extreme ultraviolet and X-ray radiation (XUV) and on the properties of the solar wind. Contrary to the good temporal stability of the Sun’s radiation intensity at longer wavelengths (from infrared to near ultraviolet) the energy output of the Sun is highly variable in the XUV range and, roughly, the variability increases with decreasing wavelength (see e.g. the review book edited by OR White, 1977). The general level of XUV intensity is indicated by the “solar activity”, which changes in the longtime average with the “solar cycle”. Geophysicists think in 11-year solar cycles, but one should be aware that the configuration of astronomically measured properties of the sun (e.g. the magnetic field configuration) show rough similarities in cycles of about 22 yr. (The longtime average number of sunspots changes in an 11-year cycle but their magnetic polarity in a 22-year cycle.) The true length of a solar cycle is variable and the beginning of a new cycle can be determined only some time after the fact. The (11-year) cycles are numbered: presently we are in cycle 22, which began in September 1986. The numbering starts with the cycle which began in 1755 (see e.g. the reviews and commented excerpts of older literature in the book edited by DJ Schove, 1983). The start of a cycle coincides approximately with “solar minimum” (minimum of average XUV radiation, minimum of average sunspot number), “solar maximum” occurs about 3–5 yr after the start of a cycle, the earlier the higher the maximum.

Selected Features of the Middle Atmosphere

The region between the tropopause and 100 km, which is known as the middle atmosphere (MA), has become increasingly well observed in recent decades due to the use of the new technologies associated with satellites, lidar, and radar. Twenty years ago it would still have been possible to give quite a full account of the mean structure of the MA in the space allotted to this chapter, but now one can describe only selected, essential features in the same space. Further information can be obtained from the works listed in the references at the end of the chapter.

Electromagnetic Waves in the Upper Atmosphere

The refractive index, n, is the ratio of the phase speed of an electromagnetic wave in vacuum to its speed in a material medium, and refractivity is n − 1. The refractive index of atmospheric gases is described from the near ultraviolet to radio wavelengths. Nitrogen, oxygen, argon, water, carbon dioxide, and their mixture known as air, are the important refracting gases. Some gases present in trace concentrations, such as methane and nitrogen oxides, have significant absorption at certain wavelengths; but they are not significant for refraction at any wavelength longer than ultraviolet wavelengths. The origin of refraction from atomic and molecular resonances is discussed. The method of calculation of refractive index at any wavelength is given. The absorption windows offer the opportunity for simple and accurate refraction formulae valid over a range of wavelengths. The radio-, infrared-, and visible-wavelength windows are examples.

Transatmospheric Propagation of Radio Waves: The Role of the Atmosphere in Communication, Navigation, Geodesy etc.

The propagation of radio waves through the atmosphere of the Earth finds a wide range of applications for science and engineering. Among the scientific application areas we name ionospheric physics, radio astronomy, space geodesy and oceanography. The engineering applications span such diverse areas as communication, navigation and time dissemination via satellites. (Terrestrial communication links, e.g. HF-radio, are not considered.) The atmosphere influences all applications because it changes the properties of the radiowaves. All measurable signal parameters are affected. The most important ones are the amplitude, the phase and the polarization of the wave components.

The Antarctic Ozone Hole

The ozone hole over Antarctica reflects the as yet largest observed perturbation of the stratospheric ozone layer. During this event the ozone layer in an altitude range 14–22 km is significantly depleted with local losses up to 90% and changes in the total ozone column of up to 60%. The antarctic ozone hole appears annually during the months of September/October. Although it was first discovered in 1985, its first appearance dates back to the mid-1970s. Since this time, it has grown in both depth and size. In October 1991 total column amounts were as low as 110 DU; the total area with low ozone covered approximately 7% of the total area of the southern hemisphere.