Modeling Differential Faraday Rotation in the Solar Corona
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Modeling Differential Faraday Rotation in the Solar Corona Jason E. Kooi1
· Molly E. Kaplan2
Received: 28 February 2020 / Accepted: 5 August 2020 / Published online: 14 August 2020 © This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply 2020
Abstract For decades, radio remote-sensing techniques have been used to probe the plasma structure of the solar corona at distances of 2 – 20 R . Measurement of Faraday rotation, the change in the polarization position angle of linearly polarized radiation as it propagates through a magnetized plasma, has proven to be one of the best methods for determining the coronal magnetic-field strength and structure. Faraday-rotation observations of spatially extended radio sources provide the unique opportunity to measure differential Faraday rotation [RM] the difference in the Faraday-rotation measure between two closely spaced lines of sight (LOS) through the corona. RM is proportional to the electric current within an Ampèrian loop formed, in part, by the two closely spaced LOS. We report the expected RM for two sets of models for the corona: one set of models for the corona employs a spherically symmetric plasma density, while the other breaks this symmetry by assuming that the heliospheric current sheet (HCS) is a finite-width streamer-belt region containing a high-density plasma. For each plasma-density model, we evaluate the RM for three model coronal magnetic fields: a radial dipole and interplanetary magnetic field (DIMF), a dipole + current sheet (DCS), and a dipole + quadrupole + current sheet (DQCS). These models predict values of 0.01 RM 120 rad m−2 over the range of parameter space accessible by modern instruments such as the Karl G. Jansky Very Large Array. We conclude that the HCS contribution to RM is not negligible at moderate heliocentric distances (< 8 R ) and may account for 20 % of previous observations of RM (e.g. made by Spangler, Astrophys. J. 670, 841, 2007). Keywords Corona · Corona, models · Electric currents and current sheets · Magnetic fields, corona · Plasma physics · Polarization, radio · Others, Faraday rotation
B J.E. Kooi
[email protected]
1
U.S. Naval Research Laboratory, Code 7213, 4555 Overlook Ave. SW, Washington, DC 20375, USA
2
Department of Physics, University of California, Broida Hall, Santa Barbara, CA 93106, USA
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J.E. Kooi, M.E. Kaplan
1. Introduction Electric currents play a significant role in the solar corona. Parker (1972) proposed that magnetic-field topologies more complicated than simple twists extending uniformly along magnetic-field lines would quickly lead to reconnection and the generation of turbulent current sheets that would be rapidly dissipated. Due to the complexity of the solar magnetic field, currents are found everywhere from reversals in the photospheric magnetic field (see, e.g., Vaiana, Krieger, and Timothy, 1973; Habbal and Withbroe, 1981; Büchner, 2006) to the heliospheric current sheet (HCS, see, e.g.,
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