Impact line linear polarization as a diagnostic of 100 keV proton acceleration in solar flares

Abstract

Hard X-ray emission is generally interpreted as bremsstrahlung of 10-100 keV electron beams. This requires a high number of electrons to explain the intense X-ray flux observed. An interesting suggestion was made recently that 100 keV proton beams bombarding the atmosphere would create a hot thermal plasma at the origin of the observed X-ray emission. Such low-energy protons have never been detected. We propose to use impact linear polarization of chromospheric lines as a diagnostic of 100 keV protons. Recent observations of Ha linear polarization in solar flares are presented. A polarization degree as high as 2.5% is observed in some Ha-bright patches. The electric vector is radial. This polarization is interpreted as impact polarization by low-energy protons. From existing observations we cannot discriminate between local and nonlocal origins of the detected population of energetic particles. However, we show that nonlocal proton acceleration leading to chromospheric bombardment is a likely explanation of the observed polarization. Whatever the initial distribution, transport effects in the chromosphere generate an anisotropic velocity distribution for protons.

Properties of Energetic Ions in the Solar Atmosphere from γ-Ray and Neutron Observations

“…Other possible diagnostics for lower energy ions include Doppler-shifted Lyman-α line emission (Canfield and Chang 1985) and line impact polarization of Hα, Hβ and other lines (Henoux et al 1990;Vogt and Hénoux 1996). Observations of Hα and Hβ linear polarization in flares seem increasingly well-established (Xu et al 2005(Xu et al , 2006Firstova et al 2008) although there remains an ambiguity in its interpretation between ion beams and streaming electron distributions (e.g., return currents).…”

First Detection of Chromospheric Magnetic Field Changes During an X1-Flare

“…But no observation to date employed chromospheric polarimetry to unambiguously trace the magnetic field changes. Former chromospheric polarization measurements mostly focused on linear polarization and the search for "impact" polarization (e.g., Henoux et al 1990;Bianda et al 2005;Judge et al 2015). In this paper, we report the first direct observations of chromospheric magnetic field changes that were observed through spectropolarimetry of the Ca II 8542Å line during an X-class flare and we compare them to the photospheric changes of the same flare.…”

Solar Flares: The Impulsive Phase

“…In this model neutral beams are generated by direct electric field acceleration making protons with typical energies of 200 keV the main carriers of the beam energy. Henoux et al (1988) have recently presented observations of Ha linear polarization in a flare as evidence for the existence of the atmospheric bombardment by deka-keV protons. Their suggestion that impact linear polarization of chromospheric lines can be used as a diagnostic of deka-keV protons opens up the possibility for detecting these enigmatic protons and determining if they do in fact play a dominant role during impulsive flares.…”