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.