Transients and Mass Ejections Observed in Radio Occultation Measurements of the Solar Corona

A wide variety of radio propagation and scattering phenomena observed when a radio source is occulted by the solar corona has formed the basis for probing the solar corona for over four decades. The purpose of this paper is to review the subject of transients — variations in the radio occultation measurements that appear different from those of the background solar wind. A major surprise in the study of transients has been the realization that they can represent the passage of coronal streamers in addition to the coronal mass ejections (CMES) observed in white-light coronagraphs. Although common radio occultation and white-light measurements of CMES have been few and incomplete, some general features of CMEs are emerging. Two regions of enhanced density and density fluctuations appear common to CMES. The first represents the compressed plasma ahead of the CME, and is present whether or not the CME is traveling fast enough to generate an interplanetary shock, while the second corresponds to the bright core of the CME observed in white-light. Polarity reversals, presumably due to the deflection of the magnetic field by the advancing CME, occur behind the compressed plasma. As in the case of CMES observed by in situ measurements and identified as counterstreaming supertherrnal electrons events, polarity reversals are not always observed in the white-light CMES. Multiple polarity reversals in the body of the CME are suggestive of large internal field rotations, magnetic flux ropes, and magnetic cloud. These coronal features of CMES are consistent with their interplanetary manifestation as observed by in situ measurements of the solar wind at 1 AU.