Structure and dynamics of the nightside poleward boundary: Sounding rocket and ground‐based observations of auroral electron precipitation in a rayed curtain

The Cascades2 auroral sounding rocket provides a case study for comparing multipoint in situ ionospheric observations of a nightside auroral poleward boundary intensification with ground-based optical observations of the same event. Cascades2 was launched northward from Poker Flat Alaska on 20 March 2009 at 11: 04 UT. The 13 min flight reached an apogee of 564 km over the northern coast of Alaska. The experiment included a five-payload array of in situ instrumentation, ground cameras at three different points under the trajectory, multiple ground magnetometers, the Poker Flat Incoherent Scatter Radar (PFISR) radar, and the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft in the magnetotail. The rays of the poleward boundary intensification (PBI) curtain have along-arc motions of 8.5 km/s and along-arc spacings of 16 km. Modulated maximum energy envelopes and energy fluxes of the associated electron precipitation correspond to this spatial structure of the visible rays. The electron precipitation is additionally modulated at a higher frequency, and velocity dispersion analysis of these 8 Hz signatures implies Alfvenic wave-particle acceleration of an ambient ionospheric electron source occurring a few hundred km above the observation point. These observations parameterize the curtain of Alfvenic activity above the PBI event, both in the dispersive ionosphere and in the magnetotail reconnection region. The along-arc variations in brightness correspond to variations in precipitating electron energy flux interpreted as an along-arc modulation of the maximum energy of the Alfvenic wave-particle acceleration process; this is a new interpretation of the formation of rayed structures in auroral curtains. We consider the various possible magnetospheric and ionospheric drivers for the control of the observed along-arc structuring and motions.