Visibility of Earth-directed coronal mass ejections

In marked contrast to the high energy particles that sometimes accompany major solar flares, and which tend to follow the Parker spiral, the massive plasma ejecta known as coronal mass ejections (CMEs), travelling at speeds of the order of the solar wind speed, are most likely to impact and Earth's magnetosphere if they originate near the center of the solar disk, and are emitted directly toward Earth. This introduces certain difficulties into the observation of such ejecta, with the result that conventional wisdom in the past has sometimes discounted the possibility of detecting gee-effective plasma emissions with spaceborne coronagraphs. It is noted that regions close to the solar limb are always obscured by the over-occultation required in order to achieve acceptably low instrumental stray light, and that Thomson scattering of photospheric light from free electrons in the corona, the mechanism of K-coronal visibility, is a maximum at right angles to the direction of illumination and falls to zero when the scattering particle, the source of illumination, and the observer are all co-aligned, as would be the case for a solar particle moving straight toward Earth. Despite the truth of these assertions, CMEs, while more readily observed when they occur at the solar limb, are by no means impossible to detect when they originate near disk center. We review several considerations having a bearing on the detectability of solar plasma ejections when they are projected toward (or away from) the observer and, as examples of current interest, present the observations from SOHO of major solar eruptions on 06-07 January, and 07 April, 1997, to illustrate both the potential and the limitations of cinematographic coronal imaging.