Mass Ejections Observed in Radio Propagation Measurements Through the Solar Corona

A wide variety of radio propagation and scattering phenomena observed when a radio source is occulted by the solar corona, often referred to as radio occultation measurements, has formed the basis for probing the corona for over four decades. These measurements serve as an important bridge between white-light coronagraph and in situ plasma measurements beyond 0.3 AU. In the past, temporal variations that seemed different from the background solar wind were usually identified as transients and thought to represent propagating interplanetary disturbances such as coronal mass ejections (CMEs) observed in white-light coronagraphs. Recent progress has surprisingly shown that these temporal variations can also represent the rotation of quasi-stationary spatial structures such as coronal streamers across the radio path. This paper summarizes our current understanding of temporal variations, including CMEs, observed in radio occultation measurements. Two regions of enhanced density and density fluctuations appear to be associated with CMEs. The first region represents the compressed plasma ahead of the CME, while the second region appears to be associated with the main body of the CME observed in white-light coronagraph measurements and the CME identified at 1 AU by in situ plasma measurements based on counterstreaming suprathermal electrons. Single magnetic field polarity reversals are generally found ahead of the second region and most likely represent the deflection of the ambient magnetic field ahead of the advancing CME, or the draping of the ambient magnetic field around the front of the CME. Multiple magnetic field polarity reversals, which are sometimes observed within the second region of the CME, are suggestive of large internal field rotation, magnetic ropes, and magnetic clouds.