Earthward directed high‐density Birkeland currents observed by HILAT

An intense (94 µA/m²) earthward directed Birkeland current was detected by the HILAT satellite on July 23, 1983, less than one month after launch. It was located at the equatorward edge of a large‐scale earthward flowing current in the late evening sector. This current is the most intense ever to be reported that is consistent with upward flowing thermal electrons. Simultaneous vacuum ultraviolet images from the auroral imager mapper show that the large‐scale current is embedded within the diffuse aurora. Energetic electron measurements in the range 20 eV to 20 keV indicate an increase in downward flux from ∼5×10 7 to ∼7×10 9 (cm² sec sr) −1 in ∼0.75 s. This increase in flux results in a gradient of ∼2 mhos/km in the height‐integrated Pedersen conductivity (Σ p ). In the presence of a typical dc electric field (∼40 mV/m), this sharp gradient in the ionospheric conductivity is consistent with the inferred high‐density current. Measurement of horizontal ion drift from the ion drift meter (IDM) reveals strong turbulence present for ∼10 km on both sides of the region of intense Birkeland current. The IDM also measured an ion density ( n i ) of ∼2×10 4 cm −3 ; thus, if n e ≃ n i , the parallel electron drift velocity derived from J = n e V d q is ∼30 km/s, resulting in a net electron flux of ∼6×10 10 cm −2 s −1 . A flux of electrons of this magnitude is sufficient to destabilize O + ion cyclotron waves. The velocity of 30 km/s coupled with an F region ion density gradient scale length of ∼3 km is also comparable to criteria required to drive the current convective instability.