Modifications of thick-target model: re-acceleration of electron beams by static and stochastic electric fields

Abstract

Context. The collisional thick-target model (CTTM) of the impulsive phase of solar flares, together with the famous Carmichael, Sturrock, Hirayama, and Kopp-Pneuman (CSHKP) model, presented for many years a "standard" model, which straightforwardly explained many observational aspects of flares. On the other hand, many critical issues appear when the concept is scrutinised theoretically or with the new generation of hard X-ray (HXR) observations. The famous "electron number problem" or problems related to transport of enormous particle fluxes though the corona represent only two of them. To resolve the discrepancies, several modifications of the CTTM appeared. Aims. We study two of them based on the global and local re-acceleration of non-thermal electrons by static and stochastic electric fields during their transport from the coronal acceleration site to the thick-target region in the chromosphere. We concentrate on a comparison of the non-thermal electron distribution functions, chromospheric energy deposits, and HXR spectra obtained for both considered modifications with the CTTM itself. Methods. The results were obtained using a relativistic test-particle approach. We simulated the transport of non-thermal electrons with a power-law spectrum including the influence of scattering, energy losses, magnetic mirroring, and also the effects of the electric fields corresponding to both modifications of the CTTM. Results. We show that both modifications of the CTTM change the outcome of the chromospheric bombardment in several aspects. The modifications lead to an increase in chromospheric energy deposit, change of its spatial distribution, and a substantial increase in the corresponding HXR spectrum intensity. Conclusions. The re-acceleration in both models reduces the demands on the efficiency of the primary coronal accelerator, on the electron fluxes transported from the corona downwards, and on the total number of accelerated coronal electrons during flares.

New Insights into White-Light Flare Emission from Radiative-Hydrodynamic Modeling of a Chromospheric Condensation

“…A possible alternative is to employ lower NT electron fluxes and explore possible re-acceleration mechanisms in the chromosphere in a similar way as done recently by (Varady et al, 2014). Including additional energy deposition mechanisms, such as NT proton energy deposition (e.g., a neutral beam simulation such as in Karlický et al, 2000) or a combination of energy deposition by other means (e.g., Fletcher and Hudson, 2008) may also be important for producing a similar atmospheric response with a smaller flux of NT electrons.…”

Numerical RHD simulations of flaring chromosphere with Flarix

“…The transport, scattering and progressive thermalization of the beam electrons 234 P. Heinzel et al due to Coulomb collisions with particles of ambient plasma in the magnetized flaring atmosphere and the resulting flare heating corresponding to the local energy losses of beam electrons is calculated using an approach proposed by Bai (1982) and Karlický & Hénoux (1992) based on test particles and Monte Carlo method. This approach, fully equivalent to direct solution of the corresponding Fokker-Planck equation (MacKinnon & Craig 1991), provides a flexible way to model many various aspects of the beam electron interactions with the ambient plasma, converging magnetic field in the flare loop or with additional electric fields (Varady et al 2014). These were proposed in various modifications of the standard CSHKP flare model (e.g.…”

White-light continuum in stellar flares

“…Future observations with high cadence echelle flare spectra around the Balmer jump and Hα can provide critical constraints for the formation of such dense CC's in M dwarf flares. If high density CC's are formed in M dwarf flares, then we may need to consider a physical mechanism, such neutral beams (Karlický et al 2000), re-acceleration processes (Varady et al 2014), or in-situ chromospheric acceleration (Fletcher & Hudson 2008), to allow F13 beams to penetrate into the lower atmosphere.…”