Estimation of a Coronal Mass Ejection Magnetic Field Strength using Radio Observations of Gyrosynchrotron Radiation

TL;DR

This study demonstrates how radio observations of gyrosynchrotron radiation can be used to estimate the magnetic field strength within a CME, providing insights into the CME's magnetic energy content.

Contribution

It introduces a method combining radio spectral analysis, X-ray diagnostics, and modeling to measure CME magnetic fields directly from observational data.

Findings

CME magnetic field strength estimated at 4.4 G at 1.3 solar radii.

Non-thermal electrons with energies >1 MeV produce observed radio emissions.

The analysis links radio and X-ray electron distributions to plasma properties.

Abstract

Coronal mass ejections (CMEs) are large eruptions of plasma and magnetic field from the low solar corona into interplanetary space. These eruptions are often associated with the acceleration of energetic electrons which produce various sources of high intensity plasma emission. In relatively rare cases, the energetic electrons may also produce gyrosynchrotron emission from within the CME itself, allowing for a diagnostic of the CME magnetic field strength. Such a magnetic field diagnostic is important for evaluating the total magnetic energy content of the CME, which is ultimately what drives the eruption. Here we report on an unusually large source of gyrosynchrotron radiation in the form of a type IV radio burst associated with a CME occurring on 2014-September-01, observed using instrumentation from the Nan\c{c}ay Radio Astronomy Facility. A combination of spectral flux density measurements from the Nan\c{c}ay instruments and the Radio Solar Telescope Network (RSTN) from 300MHz to 5 GHz reveals a gyrosynchrotron spectrum with a peak flux density at $>$1 GHz. Using this radio analysis, a model for gyrosynchrotron radiation, a non-thermal electron density diagnostic using the Fermi Gamma Ray Burst Monitor (GBM) and images of the eruption from the GOES Soft X-ray Imager (SXI), we are able to calculate both the magnetic field strength and the properties of the X-ray and radio emitting energetic electrons within the CME. We find the radio emission is produced by non-thermal electrons of energies >1MeV with a spectral index of $\delta$$\sim$3 in a CME magnetic field of 4.4 G at a height of 1.3 R$_{\odot}$, while the X-ray emission is produced from a similar distribution of electrons but with much lower energies on the order of 10 keV. We conclude by comparing X-ray and radio-emitting electron distributions and how such an analysis can be used to define the plasma properties of a CME.