Electron cyclotron maser instability (ECMI) in strong magnetic guide field reconnection

Abstract

Abstract. The ECMI model of electromagnetic radiation from electron holes is shown to be applicable to spontaneous magnetic reconnection. We apply it to reconnection in strong current-aligned magnetic guide fields. Such guide fields participate only passively in reconnection, which occurs in the antiparallel components to both sides of the guidefield-aligned current sheets with current carried by kinetic Alfvén waves. Reconnection generates long (the order of hundreds of electron inertial scales) electron exhaust regions at the reconnection site X point, which are extended perpendicular to the current and the guide fields. Exhausts contain a strongly density-depleted hot electron component and have properties similar to electron holes. Exhaust electron momentum space distributions are highly deformed, exhibiting steep gradients transverse to both the reconnecting and guide fields. Such properties suggest application of the ECMI mechanism with the fundamental ECMI X-mode emission beneath the nonrelativistic guide field cyclotron frequency in localized source regions. An outline of the mechanism and its prospects is given. Potential applications are the kilometric radiation (AKR) in auroral physics, solar radio emissions during flares, planetary emissions and astrophysical scenarios (radiation from stars and compact objects) involving the presence of strong magnetic fields and field-aligned currents. Drift of the exhausts along the guide field maps the local field and plasma properties. Escape of radiation from the exhaust and radiation source region still poses a problem. The mechanism can be studied in 2-D particle simulations of strong guide field reconnection which favours 2-D, mapping the deformation of the electron distribution perpendicular to the guide field, and using it in the numerical calculation of the ECMI growth rate. The mechanism suggests also that reconnection in general may become a source of the ECMI with or without guide fields. This is of particular interest in extended turbulent plasmas where reconnection serves as an integral dissipation mechanism of turbulent energy in myriads of small-scale current filaments.

The effects of density inhomogeneities on the radio wave emission in electron beam plasmas

“…There are several possible solutions to this escape problem of radio waves. This could be appropriate mode conversion processes (Treumann & Baumjohann 2017) or the existence of solar coronal flux tubes. According to them, the cutoff frequency (i.e.…”

“…Note that our study focuses on a local generation mechanism of type III SRBs due to magnetic reconnection generating superthermal electrons in solar flares. While some aspects of this process have already been investigated previously (Wu et al 2014; Treumann & Baumjohann 2017; Zhou et al 2020), we analysed here the generation mechanisms of fundamental Langmuir and electron cyclotron waves as well as their harmonics when the electron beams propagate through small-scale density gradients in the ambient plasma. We show that density inhomogeneities fundamentally change the properties of the resulting waves.…”

“…In addition, numerical simulations have demonstrated that density cavities form along one pair of the separatrices of guide-field magnetic reconnection (Pritchett & Coroniti 2004; Ricci et al 2004; Muñoz & Büchner 2016). It has recently been theoretically proposed that the electron cyclotron maser instability (ECMI) can be operative in those separatrix regions of magnetic reconnection (Treumann & Baumjohann 2017).…”

“…In the latter case they were formed via surfatron and shock acceleration (Bingham et al 2003). Magnetic reconnection in strong magnetic fields has also been proposed to be able to generate perpendicular gradients in the EVDFs along the separatrix regions (Treumann & Baumjohann 2017). A linear stability analysis of ring-beam distributions was carried out, for example, by Vandas & Hellinger (2015).…”

Topside Reconnection

“…In a recent paper [1] we suggested that strong-guide field reconnection may play a role in the generation of radiation in the topside auroral ionosphere by the electron cyclotron maser instability mechanism [2][3][4]. This idea was based on the assumption that at the boundary between the upward and downward current regions the magnetic fields of kinetic Alfvén waves might undergo reconnection, causing electron exhausts which are similar to electron holes while being of larger scale.…”