Dayside ELF electromagnetic wave survey: A Polar statistical study of chorus and hiss

Abstract

Statistical analyses of Polar plasma wave data are performed to determine the occurrence frequency, intensity, and Poynting direction of ∼360 Hz to ∼1.8 kHz extremely low-frequency (ELF) electromagnetic waves (chorus, magnetosonic mode, and hiss) in the dayside sector of the magnetosphere. The study is limited to an L* range of 2 to 9 and a magnetic local time (MLT) range of 0900 to 1500, a region infrequently covered in past statistical surveys. The study was performed on 1996–1997 data, an interval near solar minimum. It is determined that in the outer region of the magnetosphere, from L* = 6 to 9, the ∼360 to ∼800 Hz waves at Polar altitudes are typically characterized by downward (toward Earth) propagation. The downgoing waves have been previously identified as chorus in Tsurutani et al. (2011). The downgoing chorus have intensities of ∼10−2 nT2, are right-hand circularly polarized and are propagating close to parallel to the ambient magnetic field B0. The high rate of occurrence of these downward propagating waves narrows to a smaller region of L* = 6 to 7 for ∼1.2 kHz waves. In the inner region of the magnetosphere, L* = 3 to 6, the ∼360 to 800 Hz waves are characteristically oblique (to B0) and upwards propagating, away from the Earth. The upcoming waves are most likely plasmaspheric hiss and low altitude magnetospherically reflected waves. These waves are an order of magnitude less intense and less coherent than the downward propagating chorus waves. At low frequencies, ∼360 Hz, there is a region near L* = 4 to 5, where obliquely propagating waves are detected. These are most probably a mixture of obliquely propagating plasmaspheric hiss and magnetosonic waves. A detailed (case study) examination of upward propagating waves is made for one Polar pass to add context to the statistical results. The upward propagating waves are quasicoherent and slightly elliptically polarized at Polar altitudes. From this and the statistical results, we ascribe to the scenario that ∼360 to 800 Hz chorus enters the plasmasphere at low altitude entry points and propagates through the plasmasphere as semicoherent hiss, in basic agreement with the Bortnik et al. (2008, 2009a) hypothesis for the origin of some plasmaspheric hiss. However, for ∼1.2 and 1.8 kHz waves inside the nominal location of the plasmasphere, downward propagating waves have higher intensities than upward propagating or oblique waves, perhaps indicating effects associated with different source locations, different entry points and different reflection regions and/or damping/amplification. At ∼800 Hz and ∼1.2 kHz upward propagating waves with weak intensities are common in the range L* = 8–9. These may be chorus waves magnetospherically reflected to larger L*. For frequencies above ∼1.5 kHz, most wave events are low intensity and upward propagating. It is possible that sferics and power line harmonics are contributors to these signals.