Spectropolarimetric Insight into Plasma Sheet Dynamics of a Solar Flare

Abstract

We examine spectropolarimetric data from the CoMP instrument, acquired during the evolution of the September 10th 2017 X8.2 solar flare on the western solar limb. CoMP captured linearly polarized light from two emission lines of Fe XIII at 1074.7 and 1079.8 nm, from 1.03 to 1.5 solar radii. We focus here on the hot plasma-sheet lying above the bright flare loops and beneath the ejected CME. The polarization has a striking and coherent spatial structure, with unexpectedly small polarization aligned with the plasma-sheet. By elimination, we find that small-scale magnetic field structure is needed to cause such significant depolarization, and suggest that plasmoid formation during reconnection (associated with the tearing mode instability) creates magnetic structure on scales below instrument resolution of 6 Mm. We conclude that polarization measurements with new coronagraphs, such as the upcoming DKIST, will further enhance our understanding of magnetic reconnection and development of turbulence in the solar corona.

Dynamics of Late-stage Reconnection in the 2017 September 10 Solar Flare

“…Reconnection outflows along the plasma sheet during the post-impulsive phase of this flare are explored in Yu et al (2020). French et al (2019) provided further evidence to suggest the presence of reconnection-induced instabilities, based upon Coronal Multi-channel Polarimeter (CoMP) spectropolarimetric data from coronal forbidden Fe XIII lines. In spite of the strikingly laminar appearance of the plasma sheet in unpolarized measurements (e.g., from AIA), the amplitude of linearly polarized light was found to be anomalously small along the sheet.…”

“…Finally, we include observations from the Mauna Loa Solar Observatory (MLSO) Coronal Multi-channel Polarimeter (CoMP) (Tomczyk et al 2008) and K-coronagraph (K-Cor) instruments. 2017 September 10 data from these instruments have been presented in French et al (2019). Mauna Loa observations continued the next local morning on 2017 September 11, from 17:41 and 17:37 UT for CoMP and K-Cor respectively.…”

“…Using infrared polarization measurements from CoMP, we can examine more closely the base of the plasma sheet structure observed by K-Cor. Linear polarization provides unique and complementary information about source regions, enabling the study of thermal and magnetic conditions of both large-scale (Dove et al 2011;Gibson et al 2017) and small-scale (French et al 2019) structures in the solar corona. For the Fe XIII emission observed by CoMP, linear polarization (normalized by measured intensity, =…”

“…) is proportional to q µ -P 3 cos 1 B 2 (Casini & Judge 1999;Judge 2007;French et al 2019), where θ B is the POS angle between the center of the incident solar radiation and the local magnetic field. Therefore, P varies between a maximum value at θ B =0 (i.e., a radial field), and a minimum of zero at θ B =θ VV =54.74°, where θ VV is the "Van Vleck" angle.…”

“…In a process of elimination, the authors concluded that the magnetic field could not be laminar, and instead must contain unresolved structure on sub-pixel scales. Given theoretical work on plasmoid formation during reconnection, these data were interpreted by French et al (2019) as evidence for the presence of instabilities within the reconnecting plasma sheet. These CoMP measurements took place 4 hr after the flare onset, long beyond the impulsive phase of the event.…”