Propagation and absorption of electron-cyclotron maser radiation during solar flares

Abstract

The electron-cyclotron maser is believed to be the source of microwave spike bursts often observed during solar and stellar flares. Partial absorption of this radiation as it propagates through the corona can produce plasma heating and soft X-ray emission over an extended region. In this paper, the propagation and absorption of the maser radiation during solar flares are examined through linear theory and electromagnetic particle simulations. It is shown using linear theory that strong absorption of the radiation should occur as it propagates towards the second harmonic layer where the magnetic field is half as strong as in the emission region. Only radiation propagating nearly parallel to the magnetic field in a low-temperature plasma may be able to escape under certain, limited conditions. Finite temperature effects can cause radiation propagating nearly perpendicular to the magnetic field to refract, causing enhanced absorption. Particle simulations are then used to evaluate the nonlinear response of the plasma as the maser radiation propagates through the absorption layer. It is shown that some of the maser radiation is able to escape through a process of absorption below the second harmonic of the local gyrofrequency and re-emission above it. The fraction able to escape is much higher than that predicted by linear theory, although the amount of escaping energy is only a small fraction of the incident energy. The bulk of incident energy goes into the perpendicular heating of the ambient electrons, with the rate of energy absorption showing no signs of leveling off during the simulations. This indicates that the absorption layer does not become optically thin after continuous heating by the maser radiation. A few electrons are accelerated to several tens of keVs as a result of the heating.

Two Alternative Mechanisms for Very Bright Radio Emission from Flare Stars

“…A major ongoing difficulty with the interpretation of solar spike bursts in terms of ECME is that simple theory implies that the emission cannot escape from the solar corona, due to very strong absorption at the second harmonic layer, where the wave frequency is equal to twice the local cyclotron frequency (Melrose and Dulk 1982). There are various suggestions as to how this difficulty might be overcome (e.g., McKean et al 1989, Robinson 1989. The predicted degree of polarisation may be influenced strongly by the way this difficulty is resolved.…”

Commission 10: Solar Activity (Activite Solaire)

“…examined the cross field transport of energy associated with the electron cyclotron maser instability. McKean, Winglee and Dulk (1989) used a numerical code to study the propagation and absoption of electron cyclotron maser radiation. They found that the bulk of the cross field energy goes into heating of the ambient plasma and a few electrons are accelerated to several KeV.…”

Harmonic maser emissions from electrons with loss-cone distribution in solar active regions

“…One major problem of ECME when applied to solar radio bursts is about how fundamental emissions (ω ≈ Ω ce ) pass through the second-harmonic layer in the solar corona without being absorbed considerably (Melrose & Dulk 1982). Ideas suggested to circumvent the problem include: propagation through a density-depleted tunnel (e.g., Wu et al 2014;Melrose & Wheatland 2016), escape through transmission windows at θ = 0 • or 90 • (Robinson 1989, where θ is the angle between the directions of wavevector and background magnetic field), and re-emission after being absorbed by the electrons (McKean et al 1989).…”