Abstract
Abstract. The electron-cyclotron maser emission theory from electron holes is applied to holes generated in the downward current region of the aurora. It is argued that the main background auroral kilometric radiation source may still be located in the upward current region electron-ring (horseshoe) distribution while the fine structure is caused by electron holes predominantly in the downward current region. There the existence of electron holes is well established and electron densities are high enough for substantial maser growth rates. Trapping of radiation by the holes provides strong amplification. Upward motion of holes favours the escape of radiation both, from the holes and from the downward current region, into the upward current region. Since upward and downward current regions always exist simultaneously, they are acting in tandem in generating auroral kilometric radiation and its fine structure by the same mechanism though in different ways. This mechanism solves the long-standing problem of auroral kilometric radiation fine structure.
Highlights
Emission of auroral kilometric radiation has commonly been attributed to the upward current region for reasons of observation of electron distributions with sufficiently steep and positive perpendicular velocity space gradients ∂fe(v,v⊥)/∂v⊥ > 0 on the electron phase-space distribution fe(v,v⊥), viz. loss-cone, ring or horseshoe distributions etc
The electron component observed here is known to consist purely of ionospheric electrons which, by a sufficiently strong downward field-aligned electrostatic potential of not yet completely clarified origin, are sucked up from the ionosphere and become accelerated into an upward magnetic-field aligned and fairly cold electron beam which carries the downward auroral current (Carlson et al, 1998). Such an electron distribution is believed to be inactive with respect to the electron-cyclotron maser; if it causes radiation, at the best via a nonlinear wave-wave interaction process like the one known in generating solar type III bursts and emission from the electron foreshock at low harmonics of the electron plasma frequency ωe
We found that electron holes can be emitters of narrowband X-mode radiation via the electron-cyclotron maser
Summary
Emission of auroral kilometric radiation has commonly been attributed to the upward current region for reasons of observation of electron distributions with sufficiently steep and positive perpendicular velocity space gradients ∂fe(v ,v⊥)/∂v⊥ > 0 on the electron phase-space distribution fe(v ,v⊥), viz. loss-cone, ring or horseshoe distributions etc. (more precisely the relativistic momentum distribution, cf. e.g. Wu and Lee, 1979; Winglee, 1983; Pritchett, 1984a,b,c, 1986; Louarn, 2006, and others). The downward current region has the advantage of hosting much higher electron densities than in the auroral cavity Still, these densities are low enough for igniting the electron cyclotron maser instability if only the electron distribution exhibits a sufficiently steep perpendicular velocity gradient. Any radiation excited by the electron-cyclotron maser inside the holes, being trapped for the lifetime of the hole and afterwards released, can leak out from the downward current region into the upward current region where it may contribute to fine structure and, may escape to free space from the macro-scale auroral cavity. Under these conditions the Buneman instability grows resulting in formation of chains of electron holes
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