Source characteristics of ELF/VLF chorus
Authors: Lauben D. S. , Inan U. S. , Bell T. F. , Gurnett D. A. 
Published: December 2002

Source characteristics of nonducted dawn‐sector magnetospheric ELF/VLF chorus emissions are deduced from in situ chorus wave properties measured with the POLAR/PWI six‐channel wideband wave receiver and applied as constraints to a simple chorus emission and propagation model. The model accounts for wave/particle resonance at the source and employs a multi‐frequency ray tracing technique to estimate wave refraction and dispersion from source to receiver in order to accurately reproduce the observed frequency‐time (f‐t) spectral form and wave normal angle θp at the spacecraft, for some optimal combination of source L shell, magnetic latitude λs, and emission wave normal angle θs determined by iteration. The best‐fit match for numerous chorus emissions observed over a dawn‐sector orbit pass covering 3.5 < L < 6.5 reveals that such chorus is generated consistently near the magnetic equator at latitudes ∣λs∣ ≤ 5°, with chorus risers (df/dt > 0) produced on the opposite side of the equator with respect to the receiver, and chorus fallers (df/dt < 0) produced on the same side, in agreement with theory and simulation. The emission wave normal angle θs deduced at the source is found to be systematically related to the frequency band occupied by a given chorus element in order for waves to arrive at the spacecraft with the observed θp, in accord with Snell's law applied to the frequency‐dependent topology of the respective whistler‐mode refractive index surface. In particular, upper‐band chorus waves (f ≥ 0.5 fHeq, where fHeq is the equatorial gyrofrequency) are emitted with wave normal θs ≃ 0, while lower‐band chorus waves (f ≤ 0.5 fHeq) are emitted with ∣θs∣ ≃ ±θG, where θG is the so‐called Gendrin angle giving minimum value of refractive index parallel to the static magnetic field. For both frequency bands, these respective θs values lead to wave propagation paths which remain naturally parallel to the static magnetic field in the source region over a latitude range of typically 3°–5°, providing ample opportunity for cumulative wave/particle interaction and thus rapid wave growth, notably in the absence of field‐aligned cold plasma density enhancements (i.e., ducts). These results support a theory of nonducted chorus generation akin to that for ducted chorus, whereby each emission is seeded by embryonic noise at a resonance point located within a few degrees of the magnetic equator, wherafter rapid coherent wave growth at some starting frequency favored by the natural wave focusing effects set by prevailing magnetospheric conditions follows from interaction with subsequently phase‐bunched counterstreaming electrons. For the common case of chorus risers, although the embryonic source point is located across the magnetic equator from the receiver as indicated for df/dt > 0, since the spatial wave growth profile increases in the direction towards the receiver and in fact reaches maximum intensity at or just beyond the magnetic equator, the waves appear to emanate from the equator itself. A symmetric ensemble of such chorus emissions thus produces waves which appear to propagate universally away from the magnetic equator, in accord with the observations of LeDocq et al. [1998].