Multiwavelength Analysis of the Impact Polarization of 2001 June 15 Solar Flare

Abstract

We report here on the temporal and spatial evolution of the impact polarization of the Hα and Hβ lines during an M6.3 solar flare observed on 2001 June 15 with the THEMIS telescope in the multiwavelength spectropolarimetric mode. Typical spectral intensity and polarization profiles are presented. Both lines are linearly polarized. The Hαline degree of polarization exceeds 4% at line center and in the near line wings. The Hβ line is also linearly polarized, with a degree of polarization reaching 6%. The directions of polarization are either parallel or perpendicular to the local transverse magnetic field (i.e., either radial or tangential because the transverse magnetic field is directed almost in the flare-to-disk center direction). However, contrary to Hα, the Hβ polarization direction is radial only. The Hα and Hβ polarization islands are located at the edges of flare kernels. Only for radial polarization are these islands cospatial. No Hβ polarization is found at the places where tangential Hα polarization is present. The origin of the observed polarization is discussed. Bombardment by low-energy protons or high-energy electrons associated with return currents can explain the radial polarization observed in the lowest flare kernel. The tangential Hα polarization observed in the surge near the upper flare location is interpreted as due to the electric current at the origin of the electromagnetic force that lifts the surge.