Coronal mass ejection

A coronal mass ejection (CME) is an ejection of material from the solar corona, usually observed with a white-light coronagraph.

The ejected material is a plasma consisting primarily of electrons and protons (in addition to small quantities of heavier elements such as helium, oxygen, and iron), plus the entraining coronal magnetic field.

Past CMEs

The first detection of a CME as such was made on December 14, 1971 by R. Tousey (1973) using the 7th Orbiting Solar Observatory (OSO-7). Earlier observations of coronal transients or even phenomena observed visually during solar eclipses are now understood as essentially the same thing.

The largest geomagnetic perturbation, resulting presumably from a "tobay" CME, coincided with the first-observed solar flare, in 1859. The flare was observed visually by Richard Christopher Carrington and the geomagnetic storm was observed with the recording magnetograph at Kew Gardens. The same instrument recorded a crotchet, an instantaneous perturbation, which could not easily be understood at the time because it predated the discovery of X-rays by Roentgen and the recognition of the ionosphere by Kennelly and Heaviside.

Impact of a CME

When the ejection reaches the Earth as an ICME (Interplanetary CME), it may disrupt the Earth's magnetosphere, compressing it on the day side and extending the night-side tail. When the magnetosphere reconnects on the nightside, it creates trillions of watts of power which is directed back toward the Earth's upper atmosphere. This process can cause particularly strong aurora also known as the Northern Lights, or aurora borealis (in the Northern Hemisphere), and the Southern Lights, or aurora australis (in the Southern Hemisphere). CME events, along with solar flares, can disrupt radio transmissions, cause power outages (blackouts), and cause damage to satellites and electrical transmission lines.

Physical properties

A typical CME has a three part structure consisting of a cavity of low electron density, a dense core (the prominence, which appears as a bright region on coronagraph images) embedded in this cavity, and a bright leading edge. It should be noted, however, that many CMEs are missing one of these elements, or even all three.

Most CMEs originate from active regions (groupings of sunspots associated with frequent flares). These regions have closed magnetic field lines, where the magnetic field strength is large enough to allow the containment of the plasma; the CME must open these field lines at least partially to escape from the sun. However, CMEs can also be initiated in quiet sun regions (although in many cases the quiet region was recently active). During solar minimum, CMEs form primarily in the coronal streamer belt near the solar magnetic equator. During solar maximum, CMEs originate from active regions whose latitudinal distribution is more homogeneous.

Coronal Mass Ejections range in speed from about 20 km/s to 2,700 km/s with an average speed (based on SOHO/LASCO measurements between 1996 and 2003) of 489 km/s. The average mass based on coronagraph images is 1.6 x 10¹⁵ g. Due to the two-dimensional nature of the coronagraph measurements, these values are lower limits. The frequency of ejections depends on the phase of the solar cycle: from about one every other day near solar minimum to 5-6 per day near solar maximum. These values are also lower limits because CMEs propagating away from the Earth ("backside CMEs") can usually not be detected by coronagraphs.

Current knowledge of CME kinematics indicates that the CME starts with an initial pre-acceleration phase characterised by a slow rising motion, followed by a period of rapid acceleration away from the Sun until a near-constant velocity is reached. Some "balloon" CMEs (usually the very slowest ones) lack this three-stage evolution, instead accelerating slowly and continuously throughout their flight. Even for CMEs with a well-defined acceleration stage, the pre-acceleration stage is often absent (or perhaps unobservable).

Association with other solar phenomena

Coronal Mass Ejections are often associated with other forms of solar activity, most notably:

solar flares
eruptive prominence and X-Ray sigmoids
coronal dimming (long-term brightness decrease on the solar surface)
EIT and Moreton waves
coronal waves (bright fronts propagating from the location of the eruption)
post-eruptive arcades.

The association of a CME with some of those phenomena is common but not fully understood. For example, CMEs and flares were at first thought to be directly connected, with the flare driving the CME. However, only 60% of flares (M-class and stronger) are associated with CMEs. Similarly, many CMEs are not associated with flares. It is now thought that CMEs and associated flares are caused by a common event (the CME peak acceleration and the flare peak radiation often coincide). In general, all of these events (including the CME) are thought to be the result of a large-scale restructuring of the magnetic field.

CME models

At first, it was thought that CMEs might be driven by the heat of an explosive flare. However, it soon became apparent that many CMEs were not associated with flares, and that even those that were often began before the flare did. Because CMEs are initiated in the solar corona (which is dominated by magnetic energy), their energy source must be magnetic. Only flares could provide enough heat energy to drive the CME, and flares get their energy from the magnetic field anyway.

Because the energy of CMEs is so high, it is unlikely that their energy could be directly driven by emerging magnetic fields in the photosphere (although this is still a possibility). Therefore, most models of CMEs assume that the energy is stored up in the coronal magnetic field over a long period of time and then suddenly released by some instability or a loss of equilibrium in the field. There is still no consensus on which of these release mechanisms is correct, and observations are not currently able to constrain these models very well.

Interplanetary CMEs

CMEs typically reach Earth one to five days after the eruption from the Sun. During their propagation, CMEs interact with the solar wind and the Interplanetary Magnetic Field (IMF). As a consequence, slow CMEs are accelerated toward the speed of the solar wind and fast CMEs are decelerated toward the speed of the solar wind. Fast CMEs (faster than about 500 km s^-1) eventually drive a shock. This happens when the speed of the CME in the frame moving with the solar wind is faster than the local fast magnetosonic speed. Such shocks have been observed directly by coronagraphs in the corona and are related to type II radio bursts. They are thought to form sometimes as low as 2 R_s (solar radii). They are also closely linked with the acceleration of Solar Energetic Particles.

STEREO mission

On 25th October 2006, NASA launched the Solar TErrestrial RElations Observatory (STEREO), two near-identical spacecraft which form widely separated points in their orbits will produce the first stereoscopic images of CMEs and other solar activity measurements. The spacecraft will orbit the Sun at distances similar to that of the Earth, with one slightly ahead of Earth and the other trailing. Their separation will gradually increase so that after 4 years they will be almost diametrically opposite each other in orbit.

In popular culture

In the novel Congo, by Michael Crichton, a CME disrupts the transmission from the Congo research team's computers to the satellites and back to Houston. Type II radio emissions are the basis of the "Stargate" LP and CD tracks and gallery installations, recorded and exhibited by sound art group Disinformation in 1996 - see Disinformation (art and music project).
A coronal mass ejection also appears in the Stargate Atlantis episode Echoes a particularly massive one threatened to wipe out all life on the planet Lantea. The process happened three times over the last 100,000 years.

References

Natchimuthukonar Gopalswamy, Richard Mewaldt, Jarmo Torsti, Editors, Solar Eruptions and Energetic Particles, Am. Geophys. Union Geophys. Mongraph Series Vol 165, ISBN: 0-87590-430-0, 2006.