Hard X‐Ray Polarization from Non‐vertical Solar Flare Loops

Abstract

McConnell et al. have reported preliminary results of hard X-ray polarization measured by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in an intense solar flare on 2002 July 23. The magnitude of the reported polarization is broadly consistent with the predictions of existing solar flare models which invoke the precipitation of a nonthermal electron beam into a dense chromospheric target. However, the orientation of the polarization vector lies at a substantial angle to the local solar radial direction. This is inconsistent with model predictions of a polarization vector along the local radial direction, a prediction that is a direct consequence of the assumption of a vertical guiding magnetic field. Smith et al., in a study of the same 2002 July 23 event, have suggested that the magnetic structure in which the flare occurs is tilted relative to the local vertical. Motivated by this observation, and by the preliminary nonradial polarization vector reported by McConnell et al., we explore the effect of tilt of the flaring loop on the magnitude and orientation of the predicted polarization vector. We find that allowing loops tilted from the local solar vertical does indeed permit a much wider range of polarization vector orientations than allowed by purely vertical loop geometries. In particular, adding tilt of the magnitude inferred by Smith et al. for the 2002 July 23 event can in principle account for both the magnitude and direction of the polarization vector reported by McConnell et al. for that event.