Coronal Mass Ejections at the Sun and in Interplanetary Space

Abstract

This chapter reviews the properties of coronal mass ejections (CMEs) in the solar corona, solar wind and interplanetary space. CMEs are now widely believed to be responsible for the most severe geomagnetic storms and are consequently the major solar driver of space weather. As seen in coronagraphs, CMEs involve an expulsion of solar plasma (and magnetic field) into interplanetary space at speeds that lie anywhere between 100 and 1500 km/s. The largest CMEs are as energetic as a major solar flare, but are not caused by large flares. Instead, it is now clear that CMEs and flares are both phenomenon arising from a large-scale destabilisation of the coronal magnetic field. A common (but not unique) CME scenario involves the eruption into space of a three-part structure comprising of an inflated helmet streamer (which leads), and a prominence cavity and associate prominence (which trail). The prominence cavity is believed to be a large magnetic flux rope. In the interplanetary medium, CMEs are detected at 1 AU 2–3 days after they leave the Sun. They are often preceded by an interplanetary shock, but their speeds are generally within 100 km/s of the ambient solar wind speed, indicating that a significant interaction between CME and solar wind has occurred. A significant fraction (30-40%) of interplanetary CMEs (ICMEs) have a geometry consistent with a magnetic flux rope, and are referred to as magnetic clouds. The interplanetary flux rope is likely to be the same structure as formed the prominence cavity at the Sun. Magnetic clouds are distinguished by the appearance in many cases of prolonged (many hours) periods of Southward interplanetary magnetic field, and are hence responsible for major geomagnetic storms.