An auroral arc in the late evening plasma flow reversal

Abstract

A sounding rocket equipped to measure the electric field, the charged particle distribution, and plasma waves was launched on January 27, 1980, into the late evening plasma flow reversal. Chatanika radar provided plasma flow velocities and plasma densities from overhead at the launch site to 500 km north of the launch site. The radar observations before, during, and after the rocket flight together with the rocket measurements give an unusually detailed view of the plasma flow reversal near midnight and of an auroral arc in the flow reversal. A small substorm started about an hour before launch, causing a southward movement of the flow pattern. One half hour before launch the flow reversal line was 300‐km north of the launch site, within range of the rocket trajectory. Shortly before launch the flow pattern abruptly moved north but returned briefly as the rocket was passing through peak altitude. Just after the rocket passed through the reversal, near peak altitude, an auroral arc was encountered. Associated with the auroral arc was an inverted V electron energy spectrum and, on the equatorward edge, an intense burst of auroral hiss and a very thin sheet of field‐aligned electrons. The flow reversal moved north after this brief encounter and returned south again only after the rocket flight.