Electron beam induced radio emission from ultracool dwarfs

TL;DR

This study uses numerical simulations to explore electron-beam-driven electron-cyclotron maser emissions in ultracool dwarfs, revealing how magnetic fields and electron beams influence intense, polarized radio emissions across a broad frequency spectrum.

Contribution

It presents the first detailed numerical simulation of ECM in ultracool dwarfs considering various plasma parameters and magnetic fields, explaining observed radio emissions.

Findings

Electromagnetic energy from ECM can rapidly heat plasma and produce high-energy electron tails.

Simulations show significant linear and circular polarization in emitted radio waves.

Harmonics in the spectrum range from 10 to 70 or 600 times the plasma frequency, complicating fundamental frequency identification.

Abstract

We present the numerical simulations for an electron-beam-driven and loss-cone-driven electron-cyclotron maser (ECM) with different plasma parameters and different magnetic field strengths for a relatively small region and short time-scale in an attempt to interpret the recent discovered intense radio emission from ultracool dwarfs. We find that a large amount of electromagnetic field energy can be effectively released from the beam-driven ECM, which rapidly heats the surrounding plasma. A rapidly developed high-energy tail of electrons in velocity space (resulting from the heating process of the ECM) may produce the radio continuum depending on the initial strength of the external magnetic field and the electron beam current. Both significant linear polarization and circular polarization of electromagnetic waves can be obtained from the simulations. The spectral energy distributions of the simulated radio waves show that harmonics may appear from 10 to 70$\nu_{\rm pe}$ ($\nu_{\rm pe}$ is the electron plasma frequency) in the non-relativistic case and from 10 to 600$\nu_{\rm pe}$ in the relativistic case, which makes it difficult to find the fundamental cyclotron frequency in the observed radio frequencies. A wide frequency band should therefore be covered by future radio observations.