Measuring Magnetic Fields of Coronal Mass Ejection in Corona and Inner Heliosphere using Wide Field of View Spectro-polarimetric Radio Imaging

Coronal mass ejections (CMEs) are the strongest drivers of space weather. Measurements of the plasma parameters of CMEs, particularly magnetic fields entrained in the CME plasma, are crucial to understand their propagation, evolution, and geo-effectiveness. Spectral modeling of gyrosynchrotron (GS) emission from CME plasma has long been regarded as one of the most promising remote observation techniques for estimating spatially resolved CME plasma parameters. However, imaging the very low flux density CME GS emission in close proximity to the Sun with orders of magnitude higher flux density, has proven to be rather challenging. This challenge has only recently been met using the combination of data from the Murchison Widefield Array (MWA) and the recently developed spectropolarimetric snapshot imaging pipeline optimized for this data (P-AIRCARS). This has now brought routine detection of GS within reach, and the next challenge to be overcome is that of constraining the large number of free parameters in GS models. A few of these parameters are degenerate, and need to be constrained using the limited number of spectral measurements typically available. We present studies of spectropolarimetric modeling GS emissions from two different CMEs, which establish that these degeneracies can be broken using polarimetric imaging. 

However, this methodology is only useful to measure CME magnetic fields up to ~10 R๏. At  higher coronal heights and inner heliosphere CME magnetic fields can be estimated by measuring Faraday rotation of linearly polarized galactic/extragalactic radio sources. This method has been used using small field of view (FoV) instruments at high frequency (e.g., VLA) to measure magnetic fields along a single line of sight (LoS). We have recently started exploring the FR measurements due to CME using the MWA. The  advantage of using the MWA is its wide FoV and lower observing frequency. Lower observing frequency provides sensitivity to smaller magnetic fields. At the same time, wide FoV will provide simultaneous measurements along multiple LoSs and enable estimation of vector magnetic fields by constraining empirical flux-rope models of CMEs.  We present the challenges which need to be overcome to achieve these goals and some initial results.