Cyclotron maser emission from power-law electrons with strong pitch-angle anisotropy

TL;DR

This paper explores how electron pitch-angle anisotropy influences electron cyclotron maser emission (ECME) properties, revealing that different wave modes and anisotropy types significantly affect growth rates and emission characteristics.

Contribution

It introduces a new electron distribution function modeling various pitch-angle anisotropies and demonstrates their impact on ECME properties, extending previous research focused on lower-energy cutoffs.

Findings

Growth rates vary with pitch-angle anisotropy and wave mode.

O and X modes can both be the fastest growth modes depending on conditions.

Maximum growth rate of X2 mode decreases rapidly with electron pitch-angle cosine.

Abstract

Energetic electrons with power-law spectrum are most commonly observed in astrophysics. This paper investigates electron cyclotron maser emission (ECME) from the power-law electrons, in which strong pitch-angle anisotropy is emphasized. The electron distribution function proposed in this paper can describe various types of pitch-angle anisotropy. Results show that the emission properties of ECME, including radiation growth, propagation, and frequency properties, depend considerably on the types of electron pitch-angle anisotropy, and different wave modes show different dependences on the pitch angle of electrons. In particular, the maximum growth rate of X2 mode rapidly decreases with respect to the electron pitch-angle cosine $\mu_0$ at which the electron distribution peaks, while the growth rates for other modes (X1, O1, O2) initially increase before decreasing as $\mu_0$ increases. Moreover, the O mode as well as the X mode can be the fastest growth mode, in terms of not only the plasma parameter but also the type of electron pitch-angle distribution. This result presents a significant extension of the recent researches on ECME driven by the lower-energy cutoff of power-law electrons, in which the X mode is generally the fastest growth mode.