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

Abstract

[1] 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 Alfvénic wave-particle acceleration of an ambient ionospheric electron source occurring a few hundred km above the observation point. These observations parameterize the curtain of Alfvénic 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 Alfvénic 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.

Physical Processes of Meso-Scale, Dynamic Auroral Forms

“…This plasma initially showed a broadband energy distribution, associated with Alfvén wave acceleration, which merged with the existing plasmasheet population and evolved into a more mono-energetic population, generally associated with quasi-static electric potential drops. Similar multi-point observations by sounding rockets at lower altitude showed similar results (Mella et al 2011;Lynch et al 2012). Thus, the implication is that the initial formation of the PBI is driven by Alfvénically accelerated electrons, possibly arising from a reconnection site, which subsequently become quasi-statically accelerated electrons.…”

Coexisting structures from high‐ and low‐energy precipitation in fine‐scale aurora

“…Electron data from the Physics of the Auroral Zone Electrons II rocket showed a case of a so-called inverted-V structure with plasma sheet electrons accelerated to an energy of several keV, during which field-aligned bursts of cold electrons with a spread in energy from a few tens of eV up to the inverted-V energy, were seen superimposed [Semeter et al, 2001]. A similar event was observed with instruments on the Auroral Turbulence 2 rocket [Ivchenko et al, 1999], and the Cascades-2 rocket, where Alfvénic processes were found superimposed on a quasi-static acceleration signature [Mella et al, 2011;Lynch et al, 2012]. Time-dispersed field-aligned electron bursts by Alfvén waves have also been seen in an inverted-V structure with instruments on the Reimei satellite [Asamura et al, 2009], where the low-energy populations were found to drive instabilities giving rise to vortical auroral forms.…”

“…A similar event was observed with instruments on the Auroral Turbulence 2 rocket [Ivchenko et al, 1999], and the Cascades-2 rocket, where Alfvénic processes were found superimposed on a quasi-static acceleration signature [Mella et al, 2011;Lynch et al, 2012]. Time-dispersed field-aligned electron bursts by Alfvén waves have also been seen in an inverted-V structure with instruments on the Reimei satellite [Asamura et al, 2009], where the low-energy populations were found to drive instabilities giving rise to vortical auroral forms.…”

Statistical relationships between enhanced polar cap flows and PBIs

“…Two‐dimensional optical observations show that PBIs can appear as north‐south (N‐S), tilted, east‐west (E‐W) structures, beads/swirls, and patches [ Zesta et al ., ]. Along PBIs, there also exist smaller‐scale ray structures which are associated with Alfvén wave activity [ Lynch et al ., ]. PBIs are generally believed to be related to longitudinally localized regions of enhanced reconnection that extend along the polar cap boundary from the ionosphere to the tail.…”