Abstract
The importance of measurement and modelling of the coronal magnetic
fields has been appreciated for a long time, and in view of the fact
that magnetic fields of Coronal Mass Ejections (CMEs) play a crucial
role in determining their geo-effectiveness, this also has considerable
societal impact. The coronal magnetic fields are, however, very hard to
measure directly. Unlike those at play in X-ray, EUV and optical
regimes, radio emission mechanisms are sensitive to the local magnetic
fields and hence can potentially lead to their measurements. In
practise, however, this potential has been hard to realize. To the best
of our knowledge, in the past eighteen years, there have been only three
published instances of radio detection of CME like structures. Only for
two of these instances, it was possible to estimate coronal magnetic
fields by fitting the measured spectra with gyrosynchrotron models.
Using data from the Murchison Widefield Array (MWA), for two CMEs we
have detected radio structures resembling the CME morphology. These
structures are cospatial and cotemporal with the white light coronagraph
images of the CME and we can convincingly demonstrate that this is not
plasma emission. The maximum heliocentric distance where we can detect
such emission is 4.74 solar radii, and the flux densities we measure are
among the lowest reported. We note that these detections have been
enabled by the confluence of the availability of data from the modern
arrays like the MWA; and an automated interferometric solar radio
imaging which we have developed. The MWA provides unprecedentedly dense
sampling of Fourier (uv) plane, an essential pre-requisite for high
imaging fidelity; and the imaging pipeline is tuned to extract the best
imaging performance from these data. We present the hypothesis that
gyrosynchrotron emission from CMEs intrinsically is not rare, it only
appeared so because the dynamic range and imaging fidelity of available
solar radio images was typically insufficient to convincingly detect
this emission. If true, this then provides an exciting opportunity for
routine estimation of the CME magnetic fields at coronal heights and a
host of other coronal diagnostics, uniquely associated with
gyrosynchrotron emissions.