Abstract
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 ~1 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.
Highlights
It is common view that the electron-cyclotron maser acts on the global anisotropic electron phase-space distribution and that structures which are localised in momentum space, so-called fine-structures, have no or little effect on the generation of radiation.What, if electron holes cause a distinct structure in real space and in the electronmomentum space distribution? Could they contribute to the generation of radiation? Quite generally, electron holes have been suggested to be an important radiation source as well via the electron-cyclotron maser mechanism
Data obtained during the second of these passages will serve as a guide for the present investigation. These observations even led to the further suggestion (Pottelette et al, 2001) that, possibly, all of the auroral kilometric radiation could be made up by electron-cyclotron maser radiation emitted from electron holes drifting at high speed through space, with the frequency drift mapping the variation of the electron-cyclotron frequency along the magnetic flux tubes to which they are confined
1. generate oblique electron-cyclotron maser radiation emitted at a finite angle against the magnetic field; 2. this radiation will for the same reason in general be in a frequency range that is not restricted to the immediate vicinity of the electron cyclotron frequency but can occur at higher frequencies quite above the electroncyclotron frequency
Summary
It is common view (for a review see Treumann, 2006) that the electron-cyclotron maser acts on the global anisotropic (or loss-cone) electron phase-space distribution (more precisely the relativistic momentum distribution, cf., e.g. Wu and Lee, 1979) and that structures which are localised in momentum space, so-called fine-structures, have no or little effect on the generation of radiation (for a review cf., e.g. Louarn, 2006, and references therein). Electron holes have been suggested to be an important radiation source as well via the electron-cyclotron maser mechanism This suggestion was based on observations of auroral kilometricradiation spectra with high time and frequency resolution (Pottelette et al, 2001; Pottelette and Pickett, 2007) and on the analysis of low-frequency electrostatic waves in the auroral upward current region, i.e. the auroral kilometric radiation source (Pottelette and Treumann, 2005). Data obtained during the second of these passages will serve as a guide for the present investigation These observations even led to the further suggestion (Pottelette et al, 2001) that, possibly, all of the auroral kilometric radiation could be made up by electron-cyclotron maser radiation emitted from electron holes drifting at high speed through space, with the frequency drift mapping the variation of the electron-cyclotron frequency along the magnetic flux tubes to which they are confined. Just a brief argument will be given for justifying the model of the electron hole used in the calculation
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