Authors: Nariaki Nitta
Affiliation: Lockheed Martin, Solar and Astrophysics Laboratory
It has been established that coronal mass ejections (CMEs) are the primary driver of major interplanetary disturbances, which sometimes cause severe space weather effects. By definition, CMEs are observed by coronagraph images. However, a question arises as to whether modern coronagraph data (e.g., SOHO/LASCO) are sensitive enough to observe all the Earth-directed and potentially geo-effective CMEs. Moreover, not only the likely low coronal origin of CMEs is masked by the coronagraph occulters, but also it is difficult to use the coronagraph measurement for understanding the geo-effectiveness apart from the kinetic aspect. Here comes the importance of low coronal images, which often show violent motions on different scales. Furthermore, these images may be used to characterize the magnetic field topology around the erupting regions, in combination with extrapolation of field measurements in the lower atmosphere, to understand the origin of the southward magnetic field component, which is an important factor for geomagnetic storms. We give examples of low coronal images showing CME signatures possibly (or controversially) better than coronagraph images do. At this time, we cannot tell for sure from coronal images alone whether a given motion will have heliospheric consequences (e.g., CMEs vs “unsuccessful” eruptions). This situation may change with SDO, which provides regular high cadence of full disk images at a wide and well-sampled temperature range.