Observations of field‐aligned currents, particles, and plasma drift in the polar cusps near solstice

Abstract

Observations of particles, magnetic fields, and convective flow in the southern polar region by the AE‐C spacecraft in the spinning mode have provided a unique opportunity to study the polar cusp. Magnetic perturbations observed by TRIAD within 2 hours of the AE‐C pass provide field‐aligned current data from the same local time in the northern hemisphere. The spinning mode of AE‐C has enabled the utilization of the Z axis magnetometer for Birkeland current observations in conjunction with particle and drift measurements. This pass, AE‐C orbit 16723, occurred on January 15, 1977, at ∼0145 UT, 25 days after solstice. The 3‐hour Kp = 4‐ and the interplanetary magnetic field By was negative during this time. The average Bz = 1.9 nT and −1.1 nT during the first two hourly intervals on January 15, 1977. At this UT, the geomagnetic axis was at a near maximum inclination of ∼35° with the southern polar cusp directed toward the sun. Measurements from the low energy electron experiment (LEE) revealed intense fluxes of soft (100 eV) ‘cusp‐like’ (Maxwellian) electrons throughout the prenoon polar cap. AE‐C magnetometer and drift meter measurements detected three alternately directed current sheets associated with the ‘throat’ of the ion convection pattern and consistent with gradients in the ion drift velocities. The upward directed current can be identified as the dominant cusp current appropriate for By < 0. The downward directed current has the appropriate sign of a dayside region 1 current but is observed to lie entirely within westerly antisunward convecting plasma. TRIAD measurements revealed the presence of similar ‘cusp currents’ that persist in the northern hemisphere at ∼3° lower latitude than those detected by AE‐C.