Magnetospheric Models and their Checking by Cosmic Rays

Tsyganenko (1989) noted that the region near the inner edge of the plasma sheet in the nightside magnetosphere plays a key role in the dynamics of disturbances. The structure of the geomagnetic field and plasma in this region is extremely variable, since it is just here that the boundary between the “spheres of influence” of the earth's internal field sources and the magnetotail currents, controlled by the solar wind, is located. Several experimental facts concerning this region can be pointed out, which should be taken into account in any quantitative model aimed at an adequate representation of the average magnetic field and current distribution. There are the following results (Tsyganenko, 1989):

• 1. Strong evidence exists that an intense and thin current sheet can approach the earth as close as (3–5)r_E at the nightside. This was suggested by Sugiura (1972) as a direct implication of the observed features of the ▵B distribution in the inner magnetosphere. Hedgecock and Thomas (1975) pointed out that the tail-like configuration is clearly discernible in the HEOS magnetic field data at tailward distances of (6–8)r_E, with the current sheet thickness less than 1r_E. Lin and Barfield (1984) showed in a statistical study that the tail-like fields can often be observed at geosynchronous orbit in the midnight sector, with increasing probability during disturbed periods, and estimated the current sheet thickness to be on the order of several tenths of r_E. Kaufman (1987) also addressed the question of tail-like magnetic configurations observed near synchronous orbit during disturbed periods and showed, by means of a simple wire model that a dramatic increase of the current in the inner nightside magnetosphere must accompany the sub-storm growth phase. A detailed study by Fairfield et al. (1987) based on AMPTE magnetic field measurements also corroborates the concept of a thin intense tail current sheet deeply embedded into the inner nightside magnetosphere.

• 2. Statistical studies of the average shape and position of the tail-neutral sheet (Russell and Brody, 1967; Fairfield, 1980; Gosling et al., 1986), as well as theoretical considerations (Voigt, 1984) have shown that, for non-zero tilt angle ψ between the z_GSM-axis and that of the earth's dipole, the current sheet undergoes a two-dimensional warping. Near the midnight meridian plane, the warping results in a gradual departure of the current sheet from the dipole equatorial plane toward that parallel to the solar-wind stream. This is accompanied by a bending of the sheet in the YZ projection in such a way that, for ψ > 0, the current surface is raised above the GSM equatorial plane in the central tail region, whereas it is depressed below this plane near the tail flanks (and vice versa for ψ < 0).

• 3. Still in early experiments, it has been established that the inner edge of the plasma sheet encircles the earth over a considerable interval of local time (Frank, 1971), and the current flow line pattern in this region should also exhibit an arched configuration, which is manifested in a relatively large value of the B _y -component of the magnetic field observed outside the current sheet in the dawn and dusk sectors (Speiser and Ness, 1967; Fairfield et al., 1987).