Joachim M. Schmidt, Nat Gopalswamy
NASA Goddard Space Flight Center, Code 695, Solar System
Exploration, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
When Coronal Mass Ejections (CMEs) propagate within a few solar radii heliocentric distance from the Sun, they are illuminated oftentimes in radio light from behind by the Sun as a source of radio emission. This radio emission can propagate only, when its frequency is above the local plasma frequency. Also, the solar corona close to the solar surface is a highly dispersive medium for radio waves. This can lead to a significant refraction of radio emission.
We show in a three-dimensional simulation, that radio waves in the regime of 164 MHz can in fact propagate from locations close to the solar surface through the solar corona, disturbed by a moving CME, into the rear of such a CME. We find that the magnetic cloud of the CME, as a density rarefaction region, can attract such radio waves due to strong refraction effects. Within the cloud, and behind the shock driven by the CME, which is a density enhancement, most of these radio waves get reabsorbed in the plasma of the solar wind. The overall effect is an eclipse of the radio signal at the location of the magnetic cloud of the CME. This eclipse can be used as a means for detecting magnetic clouds in solar radio observations.
CMEs are launched very often into the channels of coronal streamers. We show that radio waves from behind can overtake such CMEs in the channel's border region to form radio beams tangential to the CME eruption.
The shock driven by the CME is a local source for plasma emission moving with the shock. We present three-dimensional simulations of this plasma emission due to shock drift-accelerated electrons and amplification due to stochastically grown Langmuir wave turbulence. These regions of plasma emission can be used to detect CME-driven shocks in radio observations of the Sun.