Further sounding rocket observations of structured whistler mode auroral emissions

Abstract

Two recent sounding rockets were launched into active auroral substorms from Poker Flat, Alaska: CHARM on 27 February 2007 and SIERRA on 14 January 2002. Both payloads included HF wave receivers. As previously reported, the SIERRA HF receiver recorded short‐lived, periodic, time‐dispersed features at 200–600 kHz (“stripes”), in which high‐frequency components precede low frequencies, leading to a nearly linear frequency‐time signature. These features coincided with a region identified as Alfvénic and dominated by suprathermal electron bursts and 10–110 eV downward going ions poleward of the main inverted V auroral arc. The CHARM experiment, using different electric field sensors, confirmed the occurrence of the stripe features with characteristics similar to those observed with SIERRA. The most promising mechanism to explain stripes is a cyclotron resonance between downgoing Z mode waves and upgoing electron conics in the topside ionosphere at altitudes of 3000–5000 km. These electron conics resonate with successively lower‐frequency waves as they ascend, leading to emissions in which lower frequencies are delayed relative to higher frequencies; detailed modeling reproduces even fairly subtle nonlinear features of the stripes’ frequency‐time signature. The resulting structured Z mode waves then convert to whistler mode waves, which propagate to rocket altitudes. Previous simulations have shown that electron conics can be generated by Alfvénic acceleration and retain the periodicity of the driving Alfvén wave. If this acceleration could be driven by Alfvén waves at the frequency of the periodicity of the stripes, it could explain this periodicity and would be consistent with their correlation with an Alfvénic region.