Flare line impact polarization

Abstract

Context. The impact polarization of optical chromospheric lines in solar flares is still being debated. For this reason, additional observations and improved flare atmosphere models are needed still. Aims. The polarization-free telescope THEMIS used in multiline 2 MulTiRaies (MTR) mode allows accurate simultaneous linear polarization measurements in various spectral lines. Methods. In the 2001 June 15 flare, Hα, Hβ, and Mg D2 lines linear impact polarization was reported as present in THEMIS 2 MTR observations. In this paper, THEMIS data analysis was extended to the Na D2 line. Sets of I ± U and I ± Q flare Stokes S 2D-spectra were corrected from dark-current, spectral-line curvature and from transmission differences. Then, we derived the linear polarization degree P and polarization orientation angle α 2D-spectra. No change in relative positioning could be found that would reduce the Stokes parameters U and Q values. No V and I crosstalks could explain our results either. Results. The Na D2 line is linearly polarized with a polarization degree exceeding 5% at some locations. The polarization was found to be radial at outer ribbons edges, and tangential at their inner edges. This orientation change may be due to differences in electron distribution functions on the opposite borders of flare chromospheric ribbons. Electron beams propagating along magnetic field lines, together with return currents, could explain both radial and tangential polarization. At the inner ribbon edges, intensity profile-width enlargements and blueshifts in polarization profiles are observed. This suggests chromospheric evaporation.

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

“…polarization effects Although the current structures that we see appearing after the flare at 02:00 UT have a coherent spatial organization, polarization artifacts should be seriously considered especially since these appear at flare ribbon locations. Indeed, during solar flares, such effects have often been reported at the chromospheric level (see Hénoux & Karlický 2013, and references therein). The non-thermal particle beams and neutralizing return currents are generally cited as possible sources for an extra polarization signal.…”

Scattering Polarization in Solar Flares

“…Interestingly enough, in many spectropolarimetric observations, the orientation of polarization vector is either perpendicular or parallel to the nearest solar limb regardless of the ribbon orientation at the solar surface (Xu et al 2005;Hénoux & Karlický 2013). This fact is usually used for advocating the role of impact polarization due to the particle beams of various energies propagating along the vertical magnetic field lines.…”

“…One of the long-standing issues in solar physics is the problem of the origin and even the very existence of a high degree of linear polarization of some chromospheric spectral lines observed in solar flares. Over the last few decades, several authors have reported the detection of linear polarization signals in solar flares, with fractional polarization amplitudes at the level of several percent (e.g., Hénoux et al 1990;Fang et al 1993;Vogt & Hénoux 1996;Xu et al 2005;Firstova et al 2012;Hénoux & Karlický 2013). The observed spectral lines include Ca ii K, Na D 2 , and the hydrogen lines Hα and Hβ.…”

Infrequent Occurrence of Significant Linear Polarization in Hα Solar Flares

“…The reason Bianda et al (2005) did not find significant linear polarization is thought to be that the binning size of 10 ′′ × 10 ′′ and 40 s was too large to resolve the compact flare kernels (Hénoux & Karlický 2013;Firstova et al 2014). Our results show that the enhancement of the linear polarization extended 8 ′′ and lasted for 330 s. The polarization with such spatial and temporal sizes is still detectable after the binning done by Bianda et al (2005).…”

“…Firstova et al (2014) also performed a statistical investigation by the Large Solar Vacuum Telescope, and they found linear polarization of 2-7% in 13 out of 32 solar flares. Hénoux & Karlický (2013) discussed possible causes why some authors find linear polarization and others do not; the causes may be lack of spatial and temporal resolutions for dynamic solar flares, crosstalk originated from the atmospheric seeing, and too few data to overcome the lack of the resolutions and errors.…”