Coronal Mass Ejection
Coronal mass ejections are often associated with other forms of solar activity, most notably solar flares, but a causal relationship has not been established. Most ejections originate from active regions on Sun's surface, such as groupings of sunspots associated with frequent flares. CMEs occur during both the solar maxima and the solar minima of sun activity, albeit with decreased frequency during the minima.
Coronal mass ejections release huge quantities of matter and electromagnetic radiation into space above the sun's surface, either near the corona or farther into the planet system or beyond (interplanetary CME). The ejected material is a plasma consisting primarily of electrons and protons, but may contain small quantities of heavier elements such as helium, oxygen, and even iron. It is associated with enormous changes and disturbances in the coronal magnetic field.
Coronal mass ejections are usually observed with a white-light coronagraph.
Recent scientific research has shown that the phenomenon of magnetic reconnection is responsible for CME and solar flares. Magnetic reconnection is the name given to the rearrangement of magnetic field lines when two oppositely directed magnetic fields are brought together. This rearrangement is accompanied with a sudden release of energy stored in the original oppositely directed fields.
On the sun, magnetic reconnection may happen on solar arcades—a series of closely occurring loops of magnetic lines of force. These lines of force quickly reconnect into a low arcade of loops, leaving a helix of magnetic field unconnected to the rest of the arcade. The sudden release of energy in this reconnection causes the solar flare. The unconnected magnetic helical field and the material that it contains may violently expand outwards forming a CME.
This also explains why CMEs and solar flares typically erupt from what are known as the active regions on the sun where magnetic fields are much stronger on average.
Impact on Earth
When the ejection is directed towards the Earth and reaches it as an interplanetary CME (ICME), the shock wave of the traveling mass of Solar Energetic Particles causes a geomagnetic storm that may disrupt the Earth's magnetosphere, compressing it on the day side and extending the night-side magnetic tail. When the magnetosphere reconnects on the nightside, it releases power on the order of terawatt scale, which is directed back toward the Earth's upper atmosphere.
This process can cause particularly strong auroras in large regions around Earth's magnetic poles. These are also known as the Northern Lights (aurora borealis) in the northern hemisphere, and the Southern Lights (aurora australis) in the southern hemisphere. Coronal mass ejections, along with solar flares of other origin, can disrupt radio transmissions and cause damage to satellites and electrical transmission line facilities, resulting in potentially massive and long-lasting power outages.
Humans in space or at high altitudes, for example, in airplanes, risk exposure to intense radiation. Short-term damage might include skin irritation. Long-term consequences might include an increased risk of developing skin cancer.
A typical coronal mass ejection may have any or all of three distinctive features: 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.
Most ejections originate from active regions on the surface, such as groupings of sunspots associated with frequent flares. These regions have closed magnetic field lines, in which the magnetic field strength is large enough to contain the plasma. These field lines must be broken or weakened for the ejection to escape from the sun. However, CMEs may also be initiated in quiet surface 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, they originate from active regions whose latitudinal distribution is more homogeneous.
Coronal mass ejections reach velocities between 20km/s to 3200km/s with an average speed of 489km/s, based on SOHO/LASCO measurements between 1996 and 2003. The average mass is 1.6×1012kg. The values are only lower limits, because coronagraph measurements provide only two-dimensional data analysis. The frequency of ejections depends on the phase of the solar cycle: from about one every other day near the solar minimum to 5–6 per day near the solar maximum. These values are also lower limits because ejections propagating away from Earth (backside CMEs) can usually not be detected by coronagraphs.
Current knowledge of coronal mass ejection kinematics indicates that the ejection starts with an initial pre-acceleration phase characterized 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 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 are normally closely related, but there was confusion about this point caused by the events originating beyond the limb. For such events no flare could be detected. Most weak flares do not have associated CMEs; most powerful ones do. Some CMEs occur without any flare-like manifestation, but these are the weaker and slower ones.It is now thought that CMEs and associated flares are caused by a common event (the CME peak acceleration and the flare impulsive phase generally 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; the presence or absence of a CME during one of these restructures would reflect the coronal environment of the process (i.e., can the eruption be confined by overlying magnetic structure, or will it simply break through and enter the solar wind).
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. 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.
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 Rs (solar radii). They are also closely linked with the acceleration of Solar Energetic Particles.
NASA mission STEREO
On 25 October 2006, NASA launched the Solar TErrestrial RElations Observatory (STEREO), two near-identical spacecraft which from 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 four years they will be almost diametrically opposite each other in orbit.
The largest recorded geomagnetic perturbation, resulting presumably from a CME, coincided with the first-observed solar flare, on 1 September 1859, and now referred to as the solar storm of 1859. The flare was independently observed by R. C. Carrington and R. Hodgson. The geomagnetic storm was observed with the recording magnetograph at Kew Gardens. The same instrument recorded a crotchet, an instantaneous perturbation of the Earth's ionosphere by ionizing soft X-rays. This could not easily be understood at the time because it predated the discovery of X-rays by Röntgen and the recognition of the ionosphere by Kennelly and Heaviside.
First clear detections
The first detection of a CME as such was made on December 14, 1971, by R. Tousey (1973) of the Naval Research Laboratory using the Seventh Orbiting Solar Observatory (OSO-7). The discovery image (256 × 256 pixels) was collected on a Secondary Electron Conduction (SEC) vidicon tube, transferred to the instrument computer after being digitized to 7 bits. Then it was compressed using a simple run-length encoding scheme and sent down to the ground at 200 bps. A full, uncompressed image would take 44 minutes to send down to the ground. The telemetry was sent to ground support equipment (GSE) which built up the image onto Polaroid print. Mr. David Roberts, an electronics technician working for NRL who had been responsible for the testing of the SEC-vidicon camera, was in charge of day to day operations. He thought that his camera had failed because certain areas of the image were much brighter than normal. But on the next image the bright area had moved away from the Sun and he immediately recognized this as being unusual and took it to his immediate superior, Dr. Guenter Brueckner, and then to the solar physics branch head, Dr. Tousey. Earlier observations of coronal transients or even phenomena observed visually during solar eclipses are now understood as essentially the same thing.
On 1 August 2010, during solar cycle 24, scientists at the Harvard-Smithsonian Center for Astrophysics (CfA) observed a series of four large CMEs emanating from the Earth-facing hemisphere. The initial CME was generated by an eruption on August 1 associated with sunspot 1092, a sunspot which was large enough to be seen without the aid of a solar telescope. The event produced significant aurorae on August 4.