A STATISTICAL STUDY OF THE MAGNETIC-FIELD STRUCTURE IN THE INNER MAGNETOSPHERE

Three-dimensional magnetic field structure in the inner magnetosphere is statistically analyzed using data obtained by the DE-1 magnetometer . The main results are as follows: (1) The magnetic field. structure o f the rotational pattern associated with an eastward current is clearl y seen in the dayside equatorial region at geocentric distances around 2.5R(e) but not on the nightside, The current density is approximatel y 0.05 mu A/m(2), and does not show any clear dependence on geomagneti c disturbance, but shows a solar activity dependence which possibly su ggests that a substantial part of the closure of the eastward current is through the dayside mid-latitude ionosphere, (2) The azimuthal comp onent of the magnetic field associated with field-aligned currents is seen in the low-latitude magnetosphere, The intensity of this componen t is about 5 to 10 nT, and has a dependence on geomagnetic activity, ( 3) Compared with the currently available magnetospheric models, the ob served inner magnetospheric magnetic fields have much more complex str uctures than the models indicate, The necessity of including the field -aligned current system and an eastward current in the models is evide nt, (4) A noon-midnight asymmetry of tile magnetic structure is clearl y seen, particularly in the southward component associated with the ri ng current, The asymmetry is much larger in this meridian than that in the dawn-dusk meridian, This is in contrast with the asymmetry observ ed on the ground or by low-altitude satellites over the ionosphere.