BIRKELAND CURRENTS IN THE PLASMA SHEET

A search was conducted for the signatures of Birkeland currents in the Earth's magnetic tail, using observed values of B(x) and B(y) from la rge sets of spacecraft data. The data were binned by x and y for - 10 > x(GSM) > -35 and \y(GSM)\ less-than-or-equal-to 20R(E) (less-than-or -equal-to 30R(E) for x(GSM) less-than-or-equal-to -25R(E)) and in each bin their distribution in the (B(z), B(y)) plane was fitted by least squares to a piecewise linear function. That gave average x-y distribu tions of the flaring angle between B(x), and the x direction, as well as that angle's variation across the thickness of the plasma sheet. An gles obtained in the central plasma sheet differed from those derived near the lobe boundary. That is the expected signature if earthward or tailward Birkeland current sheets are embedded in the plasma sheet, a nd from this difference we derived the dawn-dusk profiles of the tail Birkeland currents for several x(GSM) intervals. It was found that (1) the Birkeland currents have the sense of region 1 currents, when mapp ed to the ionosphere; (2) both the linear current density (kiloamperes /R(E)) and the net magnitude of the field-aligned currents decrease ra pidly down the tail; (3) the total Birkeland current at x almost-equal -to - 10R(E) equals almost-equal-to 500-700 kA, which is approximately 30% of the net region 1 current observed at ionospheric altitudes, in agreement with model mapping results; and (4) the B(z) and B(y) compo nents of the interplanetary magnetic field influence the distribution of Birkeland currents in the tail.