Estimates of the solar coronal magnetic field using extreme-ultraviolet, radio, and whitelight observations

Authors: Kumari, A., Ramesh, R., Kathiravan, C., Wang, T. J. & Gopalswamy, N.

We report a solar coronal split-band type II radio burst that was observed on 2016 March 16 with the Gauribidanur Radio Spectro-Polarimeter in the frequency range ≈90-50 MHz, and the Gauribidanur RadioheliograPH at two discrete frequencies, viz. 80 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 white-light data suggests 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 cotemporal set of observations in two different regions (radio and white-light) 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 to the Sun, have been used to estimate B(r) in the corresponding distance range. with the spherical approximation. Never the less, the spherical approximation remains insufficiently accurate in modelling the solar wind delay (especially within 20 degrees of angular distance from the Sun), as it leaves timing residuals with rms values that reach the equivalent of 0.3 μs at 1400 MHz. This is because a spherical model ignores the large daily variations in electron density observed in the solar wind. In the short term, broad-band observations or simultaneous observations at low frequencies are the most promising way forward to correct for solar-wind-induced delay variations.

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Recommended citation: Kumari, A., Ramesh, R., Kathiravan, C., Wang, T. J., and Gopalswamy, N., “Direct Estimates of the Solar Coronal Magnetic Field Using Contemporaneous Extreme-ultraviolet, Radio, and White-light Observations”, The Astrophysical Journal, vol. 881, no. 1, 2019. doi:10.3847/1538-4357/ab2adf.