Propagation of whistler mode chorus to low altitudes: Spacecraft observations of structured ELF hiss

Abstract

We interpret observations of low‐altitude electromagnetic ELF hiss observed on the dayside at subauroral latitudes. A divergent propagation pattern has been reported between 50° and 75° of geomagnetic latitude. The waves propagate with downward directed wave vectors which are slightly equatorward inclined at lower magnetic latitudes and slightly poleward inclined at higher latitudes. Reverse ray tracing using different plasma density models indicates a possible source region near the geomagnetic equator at a radial distance between 5 and 7 Earth radii by a mechanism acting on highly oblique wave vectors near the local Gendrin angle. We analyze waveforms received at altitudes of 700–1200 km by the Freja and DEMETER spacecraft and we find that low‐altitude ELF hiss contains discrete time‐frequency structures resembling wave packets of whistler mode chorus. Emissions of chorus also predominantly occur on the dawnside and dayside and have recently been considered as a possible source of highly accelerated electrons in the outer Van Allen radiation belt. Detailed measurements of the Cluster spacecraft at radial distances of 4–5 Earth radii show chorus propagating downward from the source region localized close to the equator. The time‐frequency structure and frequencies of chorus observed by Cluster along the reverse raypaths of ELF hiss are consistent with the hypothesis that the frequently observed dayside ELF hiss is just the low‐altitude manifestation of natural magnetospheric emissions of whistler mode chorus.