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 of the rotational pattern associated with an eastward current is clearly seen in the dayside equatorial region at geocentric distances around 2.5 Re but not on the nightside. The current density is approximately 0.05μ A/m2, and does not show any clear dependence on geomagnetic disturbance, but shows a solar activity dependence which possibly suggests that a substantial part of the closure of the eastward current is through the dayside mid-latitude ionosphere. (2) The azimuthal component of the magnetic field associated with field-aligned currents is seen in the low-latitude magnetosphere. The intensity of this component is about 5 to 10 nT, and has a dependence on geomagnetic activity. (3) Compared with the currently available magnetospheric models, the observed inner magnetospheric magnetic fields have much more complex structures than the models indicate. The necessity of including the field-aligned current system and an eastward current in the models is evident. (4) A noon-midnight asymmetry of the magnetic structure is clearly seen, particularly in the southward component associated with the ring current. The asymmetry is much larger in this meridian than that in the dawn-dusk meridian. This is in contrast with the asymmetry observed on the ground or by low-altitude satellites over the ionosphere.
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