STEREO Embraces the Sun – A First Complete View of a Star

Abstract

It is generally accepted that the fast interplanetary manifestations of coronal mass ejections (ICMEs) are the major solar drivers of many space weather phenomena, including large, non-recurrent geomagnetic storms and solar energetic particle events. High-quality synoptic observations of the solar corona, as carried out by the EIT and LASCO experiments on SOHO, provide near real-time imaging of CMEs from the base of the corona out to a projected distance of 30R⊙ (R⊙ is the solar radius). The average characteristics of CMEs observed by LASCO are similar to those from earlier coronagraphs. Most CMEs travel with approximately constant speed through the LASCO field of view. However, a significant number accelerate as they move outward, and some fast events have been observed to decelerate. LASCO has observed a great deal of internal structure in many CMEs. Concave-outward structures that are interpreted as magnetic flux ropes are observed in approximately one third of all events. Complex structures are often observed in the trailing portions of CMEs. Halo CMEs, many of which are very faint, can be routinely observed with LASCO. Extreme-ultraviolet (EUV) images of the low corona and solar disk, as recorded by EIT, reveal a complex web of CME signatures. These include large-scale waves in the inner corona associated with CMEs, extended regions of depleted EUV intensity, and bright arcades that form following the occurrence of a CME. The combination of LASCO and EIT imagery allows the source regions of halo CMEs to be identified, and can be used to help predict the occurrence of geomagnetic disturbances several days in advance. The most important parameters in determining the geoeffectiveness of an ICME are the magnetic field strength and direction, and the speed of the disturbance. The most intense geomagnetic storms are associated with strong and persistent southward fields, either within the ICME itself or in the compressed sheath of solar wind plasma ahead of it.