Electron-cyclotron maser emission by power-law electrons in coronal loops

Abstract

Context. The electron-cyclotron maser (ECM) instability is an important mechanism that amplifies electromagnetic radiation directly by nonthermal electrons trapped in magnetic fields. The nonthermal electrons frequently have a negative power-law distribution with a lower energy cutoff (Ec), which will depress the instability. Aims. In this paper, it is shown that the lower energy cutoff behavior of power-law electrons trapped in coronal loops can drive the ECM instability efficiently. Methods. Based on the dispersive relation for high-frequency waves and distribution function for power-law electrons with a lower energy cutoff in a coronal loop, the growth rates of the O and X mode waves at fundamental and harmonic frequencies are calculated. Results. The results show that the instability is driven when δ>α because of a population inversion below the cutoff energy Ec, where δ is the steepness index describing the cutoff behavior and α the power-law spectrum index. The growth rates increase with δ and Ec, but decrease with α, σ ,a ndΩ ,w hereσ is the magnetic mirror ratio of the loop and Ω the ratio frequency in the loop. Conclusions. This novel driving mechanism for the ECM emission can be expected to have a potential importance for understanding the microphysics of radio bursts from the Sun and others.