Electron-cylotron maser radiation from electron holes: Upward current region

TL;DR

This paper investigates electron-cyclotron maser radiation from electron holes in auroral regions, showing they can emit band-limited, perpendicular radiation with potential for amplification, but with growth rates too small to explain observed flux without trapping effects.

Contribution

It demonstrates that electron holes in ring-horseshoe distributions can generate band-limited, perpendicular electron-cyclotron maser radiation with detailed theoretical and observational analysis.

Findings

Radiation bandwidth is about 1 kHz under auroral conditions.

Growth rate of a single hole is too small for observed flux without trapping.

Radiation can become oblique or exceed local electron cyclotron frequency.

Abstract

Electron holes are suggested to be an important source for generation of electron-cyclotron maser radiation. We demonstrate that electron holes generated in a ring-horseshoe distribution in the auroral-kilometric radiation source region have the capacity to emit band-limited radiation. The radiation is calculated in the proper frame of a circular model hole and shown to be strictly perpendicular in this frame. Its bandwidth under auroral conditions is of the order of $\sim1$ kHz, which is a reasonable value. {It is also shown that much of the drift of fine structure in the radiation can be interpreted as Doppler shift. Estimates based on data are in good agreement with theory. Growth and absorption rates have been obtained for the emitted radiation. However, the growth rate of a single hole obtained under conservative conditions is small, too small for reproducing the observed fine structure flux. Trapping of radiation inside the hole for the hole's lifetime helps amplifying the radiation additionally but introduces other problems. This entire set of questions is discussed at length and compared to radiation from the global horseshoe distribution.} The interior of the hole produces a weak absorption at slightly higher frequency than emission. The absorptivity is roughly two orders of magnitude below the growth rate of the radiation thus being weak even when the emission and absorption bands overlap. Transforming to the stationary observer's frame it is found that the radiation becomes oblique against the magnetic field. For approaching holes the radiated frequencies may even exceed the local electron cyclotron frequency.