A novel mechanism for electron-cyclotron maser

Abstract

Context. It has been a long-standing puzzle on how to produce natural radio bursts of various cosmic objects, ranging from remote active galactic nuclei and pulsars to the nearest solar radio bursts and terrestrial auroral kilometer radiations.Aims. An electron-cyclotron maser (ECM) driven by fast electron beams trapped in magnetic fields has been suggested as a dominant mechanism of producing natural high-power radio radiation. However, there have been two serious difficulties: the magnetization condition of requiring the electron gyrofrequency over the plasma frequency and the inversion condition of the perpendicular velocity distribution of the fast electrons, which has held back the popularization of ECM in the astrophysical community. Methods. By including effects of self-generated Alfvén waves (AW) excited by the beam current, this paper proposes a novel, selfconsistent ECM model. Results. The results show that the self-generated AW can effectively make a density-depleted duct, in which the magnetization condition is easily satisfied, and result in the inversion condition of perpendicular velocity distribution of the beam electrons.Conclusions. This self-consistent ECM model can effectively overcome the two difficulties, make ECM very easily occur, and, hence, has greatly interesting implications and general significance in radio astrophysics because of its self-consistency, simplicity, and efficiency.

A self‐consistent mechanism for electron cyclotron maser emission and its application to type III solar radio bursts

“…Following Wu et al [], in the low‐frequency ( ω ≪ ω ci ≪ ω ce ) and long‐wavelength ( k ρ ce ≪ k ρ ci ≪1) limits the current instability of AWs by the beam‐return current system can be dealt with by the hydromagnetic limit of the kinetic theory [ Hasegawa , ; D. J. Wu , ]. For the parallel propagating AW with a wave vector k = (0,0, k z ), the dispersion equation of the beam‐return current system reads as [ Wu et al , ] where the parameters with n Q ≡ n 0 − n e − n b and with are the normalized charge and current density of the beam‐return current system, respectively, and λ i ≡ v A / ω ci is the ion inertial length.…”

“…For the limit case with complete compensation (i.e., n Q =0 and j z =0), one has the parameters which give the well‐known dispersion relation for the ideal AW, ω = v A k z . Another limit case without compensation (i.e., n Q =− n b and j z =− e n b v b ) has the parameters for which the dispersion relation was considered by Wu et al [] and called the self‐generated AW.…”

“…In general, this ECM emission might be more efficient than the plasma emission because the linear instability, in principle, should be a preferential developing process rather than the nonlinear instability when these two simultaneously take place in a plasma. However, the ECM emission mechanism has been not adopted extensively in the solar radiophysical community as expected because there are some heavy difficulties in the application to SRBs [ Wild , ; C. S. Wu , ; D. J. Wu , ; Wu et al , ].…”

“…One of them is the escape difficulty that requires the escape condition of the local electron cyclotron frequency over the plasma frequency, that is, ω ce > ω pe , implying a very high local Alfvén velocity (where m e(i) is the electron (ion) mass, [ Wu et al , ]), which is considered to be satisfied hardly in the corona based on the standard solar atmosphere model [ Wild , ]. The other one is the inversion difficulty of the velocity distribution of FEBs in the direction perpendicular to the ambient magnetic field.…”

“…Wu et al [] renewedly proposed the ECM emission as an alternative model to the plasma emission for type III SRBs, in which type III SRBs are excited by the ECM instability inside a density‐depleted duct and propagate initially confined within and along this duct until they escape from the duct when the exterior plasma frequency becomes lower than their initially emitting frequency. In a recent work, Wu et al [] proposed a novel self‐consistent ECM emission mechanism driven by a FEB along a magnetic field, in which the self‐generated AW excited by the beam current instability plays an important and crucial role. However, a propagating FEB can cause a series of electrodynamic responses of the ambient plasma.…”