On the diamagnetic effect of the plasma sheet near 60RE

The two-dimensional (YZ plane) spatial distribution of magnetic field magnitudes in the geomagnetic tail at the lunar distance is given in both the solar magnetospheric and the neutral-sheet coordinate systems by using 3 years of data from the Ames magnetometer on Explorer 35. The effect of changes in geomagnetic activity is also presented. In the magnetotail near 60 RE a broad region in which the magnetic field intensity is relatively weak in comparison with that in the other region of the tail is located adjacent to the solar magnetospheric equatorial plane and the calculated neutral sheet. This depression of the field due to the diamagnetic effect of the plasma sheet is more evident during times of minimum geomagnetic activity (Kp ≤ 1+). Plasma energy densities of 200–300 ev/cm³ in the plasma sheet are inferred by interpreting the field magnitude decrease as the effect of plasma diamagnetism.