ICMEs

Interplanetary Coronal Mass Ejections, or ICMEs, are large-scale eruptions of magnetized plasma that originate from the Sun and travel through the interplanetary medium. They are the interplanetary manifestation of coronal mass ejections (CMEs) and are driven by solar activity such as flares and filament eruptions. As they propagate, they can travel from roughly 400 to 1500 kilometers per second and may drive shock waves ahead of them when they exceed the ambient solar wind speed.

A typical ICME structure includes a leading sheath of compressed solar wind, followed by the ejecta or magnetic flux rope that contains enhanced magnetic field strength. When the ejecta has a coherent, closely rotating magnetic field, it is referred to as a magnetic cloud. Magnetic clouds often exhibit a smooth rotation of the magnetic field direction, low proton temperatures, and other distinctive plasma signatures.

Common observational signatures of ICMEs in situ (as they pass a spacecraft) include enhanced magnetic field magnitude, reduced plasma beta, lower-than-expected proton temperatures, elevated heavy ion charge states, a higher alpha-to-proton ratio, and bidirectional suprathermal electrons. Remote sensing from coronagraphs and heliospheric imagers can identify CMEs near the Sun and track their propagation outward, complementing in-situ measurements.

ICMEs are a major driver of space weather. When their magnetic field has a southward component, interaction with Earth's magnetosphere can trigger geomagnetic storms, auroral activity, and disturbances in technological systems. Classification into magnetic clouds versus non-magnetic-cloud ICMEs reflects differences in internal magnetic structure and observational signatures.