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(sub x) and B(sub y) from large sets of spacecraft data. The data were binned by x and y for -10 greater than x(sub GSM) greater than -35 and absolute value of y(sub GSM) less than or equal to 20 R(sub E) (less than or equal to 30 R(sub E) for x(sub GSM) less than or equal to -25 R(sub E)) and in each bin their distribution in the (B(sub x), B(sub y)) plane was fitted by least squares to a piecewise linear function. That gave average x-y distributions of the flaring angle between B(sub xy) and the x direction, as well as that angle's variation across the thickness of the plasma sheet. Angles 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, and from this dfiference we derived the dawn-dusk profiles of the tail Birkeland currents for several x(sub GSM) intervals. It was found that (1) the Birkeland currents have the sense of region 1 currents, when mapped to the ionosphere; (2) both the linear current density (kiloamperes/R(sub E)) and the net magnitude of the field-aligned currents decrease rapidly down the tail; (3) the total Birkeland current at x approximately equals -10 R(sub E) equals approximately equals 500-700 kA, which is approx. 30% of the net region 1 current observed at ionospheric altitudes, in agreement with model mapping results; and (4) the B(sub z) and B(sub y) components of the interplanetary magnetic field influence the distribution of Birkeland currents in the tail.