Scattering Polarization in Solar Flares

Abstract

There is an ongoing debate about the origin and even the very existence of a high degree of linear polarization of some chromospheric spectral lines observed in solar flares. The standard explanation of these measurements is in terms of the impact polarization caused by non-thermal proton and/or electron beams. In this work, we study the possible role of resonance line polarization due to radiation anisotropy in the inhomogeneous medium of the flare ribbons. We consider a simple two-dimensional model of the flaring chromosphere and we solve self-consistently the non-LTE problem taking into account the role of resonant scattering polarization and of the Hanle effect. Our calculations show that the horizontal plasma inhomogeneities at the boundary of the flare ribbons can lead to a significant radiation anisotropy in the line formation region and, consequently, to a fractional linear polarization of the emergent radiation of the order of several percent. Neglecting the effects of impact polarization, our model can provide a clue for resolving some of the common observational findings, namely:(1) why a high degree of polarization appears mainly at the edges of the flare ribbons;(2) why polarization can also be observed during the gradual phase of a flare; (3) why polarization is mostly radial or tangential. We conclude that the radiation transfer in the realistic multi-dimensional models of solar flares needs to be considered as an essential ingredient for understanding the observed spectral line polarization.

Electric Currents in Flare Ribbons: Observations and Three-Dimensional Standard Model

“…The non-thermal particle beams and neutralizing return currents are generally cited as possible sources for an extra polarization signal. However, more recently, Štěpán & Heinzel (2013) showed that this polarization is not due to energetic particles impacting the chromosphere, but to a radiation anisotropy near the chromospheric ribbons. Since this polarization effect is mainly seen for chromospheric lines (e.g.…”

Hard X-ray emitting energetic electrons and photospheric electric currents

“…At energies above 50 keV, the apparition of the X-ray source D' is accompanied with the appearance of cospatial photospheric currents in the same time interval. Note that the observation of the current density evolution has to be carefully examined since polarization measurements during flares can be biaised, and some of the observed features could result from polarization artifacts due to impact of non-thermal particle beams (see Hénoux & Karlický 2013, and references therein) or induced by resonant scattering due to radiation anisotropies at the edges of flare ribbons (Štěpán & Heinzel 2013). Janvier et al (2014) discussed this issue for this specific event.…”

ON HELIUM 1083 nm LINE POLARIZATION DURING THE IMPULSIVE PHASE OF AN X1 FLARE

“…This simply means that the atomic alignment for the 1083nm component is positive, which is entirely consistent with the calculations presented above ( Figure 6). Stěpán & Heinzel (2013) calculated polarization in a stratified atmosphere assuming a two dimensional geometry to study effects of asymmetries in radiation at the edges of flare ribbons. Our slab calculations are consistent with theirs away from the regions of strong horizontal gradients.…”

“…We note that λ is expected to be on the order of a pressure scale height, perhaps 10 2 km, far smaller than the ribbon's width of 3 Mm or more (Figure 1). Our work differs from Štěpán & Heinzel (2013) mainly through our assumption of a slab, and by the fact that we look at all three components of a multiplet sampling very different optical depths. The change in sign of alignment between the 1083 and 1082.9 nm components implied by Figure 6 is an entirely new and unexpected result.…”