About Coronal Mass Ejections
Coronal mass ejection (CME) is the scientific name of “solar eruption”, a large, propagating cloud of plasma (protons, electrons, alpha particles and some heavier ions) and magnetic field that is generated by internal processes at the Sun. CMEs occur on average several times per day and take one to four days to propagate from the Sun to the Earth at speeds ranging from 300 to 3000 km/s. They consist one of the largest energy releases in the solar system, thought to be driven by magnetic instabilities. While they are named “coronal mass ejections”, the magnetic field is the key quantity to investigate CMEs, as most CMEs are magnetically dominated (the magnetic pressure is much larger than the thermal pressure). As such, they represent a unique medium for which we can study plasma physics, including magnetic reconnection, stability of magnetic field, and turbulence. Some CMEs (on average, about once per month) impact Earth’s magnetosphere (and the magnetospheres of other planets) and often cause global effects, such as aurorae, electrical currents in the ground but their interaction with Earth’s magnetic field can also modify the conditions into which satellites orbit. The effect of CMEs (and other solar phenomena) on human and human-made technology is overall referred to as “space weather”. CMEs are the main driver of intense and extreme space weather.
During their propagation from the Sun to Earth and beyond, CMEs interact with the background solar wind as well as other CMEs. They also expand, growing in size from a fraction of a solar radius to ~50 solar radii once they reach Earth. A CME is often characterized by multiple substructures: fast CMEs drive a shock wave, which is able to accelerate particles to near-relativistic energies. Shock physics is its own branch of space science. The core of the CME, i.e. the magnetically dominated region is often referred to as a magnetic cloud or magnetic ejecta. The region composed of dense, heated and turbulent material in front of the ejecta (and behind the shock if there is one) is referred to as the CME sheath.
CMEs can be investigated through a number of techniques: they are imaged remotely, in white-light (like eclipses) with coronagraphs and heliospheric imagers, in ultra-violet and X-ray light. They can be measured in situ (meaning “in place”) by spacecraft carrying magnetometers and plasma instruments. Theories and models are then developed to use these measurements to understand the nature of CMEs. In parallel, large-scale numerical simulations can be performed to determine the effect of the interaction of CMEs with the background solar wind, with fast or dense solar wind streams and with other CMEs. Other simulations can be used to determine the physical causes of solar eruptions and the conditions at the Sun resulting in eruptions. Lastly, other topics related to CMEs include the acceleration of solar energetic particles (SEPs) by CME-driven shocks, their transport in the solar wind and the interaction of CMEs with Earth’s magnetosphere.
Researchers from the CME group at UNH perform investigations using all these methods: remote-sensing, in situ measurements, theory, models and simulations, and also investigate other related phenomena, such as SEPs and magnetospheric effects.
