Radio Occultation Observations of the Solar Corona Over 1.60–1.86 R_⊙: Faraday Rotation and Frequency Shift Analysis

Abstract

The study of coronal energy transport, central to the solar wind acceleration problem, relies upon accurate representation of magnetic fields and plasma electron densities. This information is difficult to obtain in middle‐to‐lower coronal regions that may contain complex magnetic structures. Faraday rotation (FR) solar radio occultation observations, which reveal line‐of‐sight (LOS) integrated product of the coronal magnetic field and electron density, can help characterize the coronal environment and constrain magnetic field strengths. Global magnetohydrodynamic (MHD) models use specified synoptic solar surface magnetograms and may be used to facilitate FR interpretation by estimating detailed magnetic field properties along the radio LOS. We present a hybrid FR analysis incorporating magnetic field solutions from an MHD coronal model, and an electron density radial profile conforming to radio frequency shift observations. The FR modeled by the hybrid method is compared to MErcury Surface, Space ENvironment, GEochemistry and Ranging spacecraft radio FR observations through a coronal region of low heliolatitudes and radial distance 1.60–1.86 R⊙ from the heliocenter, collected during a state of relative solar quiescence. The hybrid model reasonably reproduces the form, polarity, and magnitude of the observed FR. For this specific coronal region, the calculated radial profile of electron concentrations and varied magnetic field strengths indicate Alfvén wave speeds below 50 km/s close to the point of closest approach but near 400 km/s in adjacent regions along the sounding LOS. The new approach of combining MHD models with radio sounding observations supports study of MHD wave processes in the challenging middle‐coronal magneto‐ionic environment.

Radio Measurements of the Magnetic Field in the Solar Chromosphere and the Corona

“…Kooi et al (2017) also used white-light information and deduced fields of ∼ 11 mG for two CMEs located at heliocentric distance of around 10 R ⊙ and 2.4 mG for a jet-like CME at ∼ 8 R ⊙ . Faraday rotation measurements of interplanetary space probe signals, such as Helios (e.g., Pätzold et al, 1987;Efimov et al, 2015) and MESSENGER (e.g., Wexler et al, 2019) can provide information on the magnetic field lower in the corona, but this information is highly dependent on electron density models and variations of the magnetic field in the region of closest solar approach. Pätzold et al (1987) deduced the following relation:…”

“…valid for R between 2 and 9 solar radii. Wexler et al (2019) quote values of 1,000-12,000 nT (10-120 mG) at 1.61 R ⊙ .…”

Coronal Electron Density Fluctuations Inferred from Akatsuki Spacecraft Radio Observations

“…An electron number density model is needed to establish the constant K. A number of coronal electron density models are reviewed in Bird and Edenhofer (1990). Also see Wexler et al (2019a). Streamer regions are known to have higher electron density than coronal hole regions of similar altitude.…”

Tackling the Unique Challenges of Low-frequency Solar Polarimetry with the Square Kilometre Array Low Precursor: The Algorithm

“…By measuring the FR due to the heliospheric and/or CME plasma along the lines of sight to background linearly polarized sources or the diffuse Galactic emission, radio observations provide the only known remote-sensing tool for measuring these magnetic fields. This approach has been successfully implemented by using radio beacons from satellites (e.g., Bird & Edenhofer 1990;Jensen et al 2013;Wexler et al 2019) as background sources, and more interestingly also using astronomical sources (e.g., Mancuso & Spangler 2000;Kooi et al 2017Kooi et al , 2021. These observation were carried out at higher frequencies and using small FOV instruments, which can sample only a small part of the heliosphere at any given time.…”