Direct Estimates of the Solar Coronal Magnetic Field Using Contemporaneous Extreme-ultraviolet, Radio, and White-light Observations

Abstract

We report a solar coronal split-band type II radio burst that was observed on 2016 March 16 with the Gauribidanur Radio Spectro-Polarimeter (GRASP) in the frequency range ≈ 90 -50 MHz, and the Gauribidanur RadioheliograPH (GRAPH) at two discrete frequencies, viz. 80 MHz and 53.3 MHz. Observations around the same epoch in extreme-ultraviolet (EUV) and white-light show that the above burst was associated with a flux rope structure and a coronal mass ejection (CME), respectively. The combined height-time plot generated using EUV, radio, and whitelight data suggest that the different observed features (i.e. the flux rope, type II burst and the CME) are all closely associated. We constructed an empirical model for the coronal electron density distribution (N e (r), where r is the heliocentric distance) from the above set of observations themselves and used it to estimate the coronal magnetic field strength (B) over the range of r values in which the respective events were observed. The B values are consistent with each other. They vary as B(r) = 2.61×r −2.21 G in the range r ≈ 1.1 -2.2R . As far as we know, similar 'direct' estimates of B in the near-Sun corona without assuming a model for N e (r), and by combining co-temporal set of observations in two different regions (radio and whitelight) of the electromagnetic spectrum, have rarely been reported. Further, the present work is a novel attempt where the characteristics of a propagating EUV flux rope structure, considered to be the signature of a CME close the Sun, have been used to estimate B(r) in the corresponding distance range.

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

“…For example, Cho et al (2007) used MK4 coronameter data to constrain the electron density and deduced magnetic field of 1.3-0.4 G at heights of 1.6-2.1 R ⊙ . Similarly, Kumari et al (2017) and Kumari et al (2019) derived the electron density from white light space born coronograph images and reported 0.47-0.44 G at heliocentric 2.61-2.74 R ⊙ and 1.21-0.5 G at heliocentric 1.58-2.15 R ⊙ , respectively. Gopalswamy et al (2012), using additional information on the geometry of the shock from SDO/AIA images, determined the coronal magnetic field to be in the range of 1.3-1.5 G at heliocentric 1.2-1.5 R ⊙ .…”

Global maps of the magnetic field in the solar corona

“…Coronal magnetic field strengths can be inferred from observations of waves and oscillations, though previous studies only provided an estimate of the average field strengths in individual oscillating structures (e.g., [6][7][8][9]. Observations of shocks driven by solar eruptions can also be used to infer coronal magnetic field strengths along the shock paths (e.g., 10,11), but such shocks are only occasionally observed. Radio observations have also been used to estimate the coronal magnetic field, but only in localized regions (e.g., 12,13); this method requires accurate identification of the radio emission mechanisms, which are not always clear.…”

Mapping the magnetic field in the solar corona through magnetoseismology

“…Our derived field strengths are consistent with results from some previous measurements using other techniques. For example, shock observations have revealed field strengths of 1.3-1.5 G in the height range of 1.3-1.5 R [20] and 1.7-2.1 G in the range of 1.1-1.2 R [66]. And a spectropolarimetric measurement has yielded a field strength of ∼4 G above an active region at the height of 1.1 R [19].…”