[2] The beginning of the space era became a new step in the study of the Earth in a broad sense, from its interiors up to the interface with the heliosphere, by ground-based as well as spaceborne instruments. The integration of observations from the Earth and space within the framework of the International Geophysical Year (1957-1958) gave a stimulus to all science such that its development follows just the path of comparison and consideration of measurements from space and from the ground. The ever increasing rate of space exploration has set a problem of assessment and prediction of the NES environment state and in any given region of the heliosphere.
[3] In this paper we mean for near-Earth space the region beginning from heights 50-60 km above the Earth and extending to distances of tens of Earth radii up to the interface with the heliosphere; in the heliosphere, the material that fills it is still connected to our planet, not with the Sun or any other celestial body. It is clear that in this region we deal mainly with natural plasma.
[4] For "space weather'' we mean the state of the upper Earth's atmosphere (mesosphere, thermosphere), magnetosphere, ionosphere, all layers of near-Earth space in any given time interval. By present it has been understood that the main effect on near-Earth space is exerted by the Sun and by geoeffective phenomena in its atmosphere. However, the events of the last decade have given examples of an appreciable effect of other factors, both heliospheric (asteroid and comet hazards) and galactic: galactic cosmic rays and ultrahard X-ray bursts from nonstationary galactic objects (the event of 27 December 2004).
[5] A vivid example of the asteroid and comet hazards was vividly demonstrated by the fall of fragments of Comet Shoemaker-Levy 9 on Jupiter in November 2001; it showed that such an event, though infrequent, is however quite real. This is confirmed by the discovery of giant craters (ring structures) on the Earth's surface; these craters were formed by impacts of rather large ( < 1 km) asteroids or fragments of comets. The comprehension of this risk has resulted in the creation of Near-Earth Asteroid Tracking Service, which follows significant Solar System objects passing in the immediate proximity from the Earth (asteroids, comets). The list of such potentially hazardous objects is regularly published on the Web page http://www.spaceweather.com/.
[6] Flows of galactic cosmic-ray particles arrive to the Earth from all directions of space. They consist of nuclei of chemical elements from hydrogen to uranium in proportions that approximately match the average element abundances in the Universe. At present the main presumed sources of galactic cosmic rays are supernova explosions. In the heliosphere at the Earth's orbit the energy spectrum of particles in these flows has a sharp peak at energies 0.5-1 GeV; the exact peak position depends on the state of the interplanetary magnetic field of the Solar System; in its turn, the latter is determined by the solar activity level. The flux of galactic cosmic ray particles changes in antiphase with solar activity variations [Panasyuk et al., 2006].
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