Coronal Mass Ejections (CMEs) are large scale eruptions of plasma and magnetic field which propagate from the Sun into the Heliosphere. A typical CME has a magnetic field strength of tens of nT, a mass in the range of 1013-1016 g velocity between 10 – 2,000km/s sometimes even reaching 3,500 km/s close to the Sun . At 1AU, CME velocities (300 -1,000km/s) tend to be closer to the solar wind speed. The energies associated with CMEs are of the order of 1024-1025 J making CMEs the most energetic events on the Sun. Although CMEs often exhibit a three part structure which consists of a bright front followed by a dark cavity and bright core they may also exhibit more complex structures.
CMEs are known to be the most important driver of adverse space weather on Earth and in the near-Earth environment as well as on other planets. The most famous phenomena associated with space weather is the Aurora Borealis or Northern Lights. The Aurora is caused by energetic particles traveling along the Earth’s magnetic field lines interacting with atoms (mainly nitrogen and oxygen) in the upper atmosphere producing emission.
One of the most extreme space weather events in recent history occurred on 2 September 1859 (Carrington event). This event was associated with a white light are observed by Carrington. About 18 hours after the flare a severe geomagnetic storm occurred causing widespread sightings of the Aurora down to lower latitudes and the loss of a significant portion of the telegraph service for many hours. On the 13 March 1989 a geomagnetic storm caused large geomagnetically induced currents (GIC) which caused the failure of a transformer that ultimately led to collapse of the Hydro-Quebec network. This left some six million people without power for nine hours causing substantial economic losses upwards of $13.2 million. Other CMEs have knocked-out or caused damage to satellites, most recently on 5 April 2010, when Galaxy 15 or “Zombiesat” (Intelsat) stopped responding to ground commands. The increased radiation due to space weather poses hazards to astronauts and passengers on long distance flights, especially those over the poles. Also, polar flights may be restricted due to communications blackouts caused by space weather. In the modern era, society’s dependence on GPS, communication satellites and inter-connected power grids mean space weather is an increasing concern.
The earliest observation of a CME probably dates back to the eclipse of 1860 in a drawing recorded by G. Temple. It took more than 100 years for the CME to be formally discovered, the first definitive observation being made in 1973 using the coronagraph on-board the seventh Orbiting Solar Observatory (OSO-7). Following this a number of space-borne coronagraphs recorded numerous CMEs. While the first imaging observation of a CME was in 1973, it is now apparent that CMEs and their effects had been observed much earlier, in a number of different types of observations. For example geomagnetic disturbances caused by CMEs had been recorded as early as 1724. CMEs were also observed in radio observations via interplanetary scintillation (IPS) from the 1960s, however it was not until 1980s that the IPS could be directly related to the CMEs. Also fast CMEs produce shock waves and these shock waves then produce radio emissions at the local plasma frequency. These are known as Type II bursts.
At the beginning of 20 century some authors suggested that transient ejections of plasma from the Sun could be the source of geomagnetic storms. At this time there were two main interpretations of these transients: 1) flare induced transient ejections of plasma from formerly closed regions which drag the magnetic field outward with them and 2) and shocks formed in open regions by the rapidly increasing expansion speed of the solar wind caused by the flare which would not drag any magnetic field with them (Parker, 1963)
Entry into the space age allowed direct measurements of the properties of these coronal transients. The in situ signatures of the outwardly propagating shocks were first discovered in Mariner 2 observations, and later in Vela 3 observations. It was found that the plasma driving these shocks had different properties compared to that of the normal solar wind. The material often showed enhanced helium content and low proton and electron temperatures. These early in situ observations indicated the Parker model was incorrect, and that the disturbances were associated with previously closed field regions. Used observations of shock disturbances to estimate the mass and energy associated with a large shock to be 1013 kg and 1025 J.
The magnetic signature of these transient events was first observed by Burlaga et al. (1981) in observations from five different spacecraft. The signature was a smooth rotation of the magnetic field vector following the shock, which they called a “magnetic cloud” (MC) citing earlier theoretical work. After this, another features of MC were discovered: low temperatures and high magnetic field strength.
Appearing the first space-born coronagraphs led to the first remote imaging observation of these coronal transients. Soon after, a direct link between these transients and radio Type II bursts was made. The near continuous monitoring of the solar corona by various coronagraphs in the 1970s provided some unexpected results. Far more ejections were found than would have been expected on the frequency of occurrence of shock wave disturbances, and the ejections were far more commonly associated with eruptive prominences than impulsive flares. They showed the wide range of velocity, morphology and energy. Also, surprisingly, most of the material in the ejection was of coronal origin rather than flare or prominence ejecta. Much of the material contained in CMEs is already present in the corona so it is expected to have coronal temperatures and densities. This is not true of the cool, dense prominence material of the core. The cavity is believed to be of coronal temperature but lower density than the front and core. This is often interpreted as a flux rope seen in 1 AU in situ measurements.
Up until the early 1990s, despite all the unexpected results, all the activity discussed above was primarily attributed to solar flares, and CMEs were believed to be the result of flare-driven shock waves. However CMEs and geomagnetic storms were often not associated with flares, and the energy required to launch the CME was greater than the flare. Further in a number of papers confirmed that the flare occurred sometime after the CME onset.
The general appearance of a CME was shown previously in Figure 2, but not all CMEs share this appearance. CMEs are also associated with on-disk `dimming’ regions. Dimming is the reduction in the intensity of radiation due to physical changes in the plasma (mass motion, density, and temperature) typically observed in X-rays or EUV. The long term observations of LASCO have also discovered a number of unclassified morphologies, flux ropes, prominence-less CMEs and jet-like CME-s, which contain no aspect of the three-part structure.
CMEs have a wide range of speeds, accelerations and widths. The kinematic evolution of CMEs is complex, consisting of several phases. They are classified into two classes: slow gradual CMEs associated with prominences and impulsive fast CMEs associated with flares. Several authors suggested a three phase model of initiation, acceleration and propagation. A strong correlation between the soft X-ray flux and CME acceleration was found. This relationship indicates a strong relationship between magnetic reconnection and acceleration.
While our knowledge of CMEs has greatly expanded with improving observations and theoretical interpretations, new questions have been raised. Some of the most fundamental questions about CMEs still remain unanswered. How is the energy required to launch a CME built up and stored? What leads to the release of this energy and the eruption of a CME? A related question is: are CMEs pre-existing structures, or are they formed during the eruption?
After leaving the Sun they are accelerated by an interaction with the solar wind. Very little is known about this interaction other than that it tends to accelerate most CME towards the solar wind speed while others seem to be unaffected. Also at 1AU why do some CMEs contain MC structures, and others not. This could be an observational effect, but it could also be a manifestation of different underlying structures and mechanisms at play in some CMEs.
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