A magnetic cloud (MC) is defined as either a special type of coronal mass ejection (CME) that occurs from the surface of the Sun or an event that immediately precedes or follows a CME and its solar wind stream of charged particles. It envelops the Earth as it spreads out in a toroidal, or donut-like, shape, with one side of the torus being centered on the Sun and the other encompassing a large region of space immediately inside of the orbit of the Earth. The space within a magnetic cloud is a region of magnetic flux where a large scale rotation of the magnetic field of the Earth takes place. The field rotation of a magnetic cloud has been projected as of 1981 to be at least 0.25 astronomical units (AUs) up to to 1 AU in size, with the Earth itself 1 AU in distance from the Sun.
The presence of space weather-like magnetic clouds can have significant impacts on the Earth both because they can cause storms in the Earth's magnetosphere and because they occur fairly frequently. Examples of the magnetic cloud event have been charted at least 106 times between February 1995 and November 2007, with 16 instances occurring in 1997 alone. Typically, each event lasts for a period of less than one day and is oriented in a southward-to-northward magnetic direction.
While a magnetic cloud exhibits a relatively low proton temperature, it can cause disturbances in both the magnetosphere and ionosphere of the Earth. These disturbances can change the nature of aurora displays, as well as disrupt the functioning of satellites, communications systems based upon them, and electrical power grids. While the solar wind from the Sun is fairly constant, a CME is a special event in the solar wind where massive amounts of plasma and magnetic energy are ejected from the Sun at speeds that can reach up to 2,236,936 miles per hour (1,000 kilometers per second). Such events are often followed by magnetic clouds, which are also known as magnetic flux ropes due to their shape and behavior.
A magnetic cloud can precede or follow a CME by several hours or more in observations. One difference between the two events, however, is that the magnetic field lines in a magnetic cloud remain connected to the Sun while CME particle streams do not. The field line strength of the cloud weakens with its increasing distance from the Sun, though the lines of magnetic force become more twisted in the region of space near Earth. The magnetic shock wave that the Earth passes through can endure for as little as 10 to 20 hours or a few days. Because of this long duration and deviations in the orientation of the solar wind caused by the magnetic cloud and CME events, the activity can be predicted in advance with up to a day's notice before it becomes prominent.
Since almost one-third of all CME events are linked to a magnetic cloud occurrence, scientific research into the phenomenon has been ongoing for many decades. Several spacecraft launched by the National Aeronautics and Space Administration (NASA) in the US have been engaged in the detection of magnetic clouds and other solar activity, including the Advanced Composition Explorer (ACE) launched in 1997 and the Comprehensive Solar Wind Laboratory (WIND) launched in 1994. Older spacecraft have also been used to monitor magnetic cloud events such as the Interplanetary Monitoring Platform 8 (IMP 8) launched in 1973, and the International Sun-Earth Explorer 3 (ISEE 3), a group of three satellites used for studying the magnetosphere that were launched between 1977 and 1982. The third of the ISEE satellites was retired in 1985, however, when it was used to fly through the tail of the incoming comet, P/Giacobini-Zinner for close-up observation.