Magnetic clouds are a type of coronal mass ejection from the Sun that can cause disturbances in Earth’s magnetosphere and ionosphere, affecting aurora displays, satellites, communication systems, and power grids. They occur frequently and can be predicted up to one day in advance. NASA spacecraft have been used to monitor magnetic clouds since the 1970s.
A magnetic cloud (MC) is defined as a special type of coronal mass ejection (CME) occurring from the surface of the Sun or an event that immediately precedes or follows a CME and its solar flux of charged particles. It envelops the Earth as it expands into a torus, or donut-like shape, with one side of the torus centered on the Sun and the other enclosing a large region of space immediately within Earth’s orbit. The space within a magnetic cloud is a region of magnetic flux in which a large-scale rotation of the Earth’s magnetic field occurs. The rotation of a magnetic cloud’s field has been projected since 1981 to be at least 0.25 astronomical units (AU) up to 1 AU in size, with the Earth itself 1 AU away from the Sun.
The presence of magnetic clouds similar to space weather can have significant impacts on Earth both because they can cause storms in the Earth’s magnetosphere and because they occur quite frequently. Examples of the magnetic cloud event have been recorded at least 106 times between February 1995 and November 2007, with 16 cases occurring in 1997 alone. Typically, each event lasts for less than a day and is oriented in a magnetic direction from south to north.
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 operation of satellites, satellite-based communication systems, and power grids. While the solar wind from the Sun is fairly constant, a CME is a special event in the solar wind in which enormous 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, also known as magnetic flux strings 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 magnetic field lines in a magnetic cloud remain connected to the Sun while CME particle streams do not. The cloud’s field line strength weakens with its increasing distance from the Sun, although the magnetic lines of force become more convoluted in the near-Earth region of space. The magnetic shock wave that passes through the Earth can last from 10 to 20 hours or a few days. Due to this long duration and the deviations in solar wind orientation caused by magnetic cloud and CME events, activity can be predicted in advance with up to one day’s notice before it becomes apparent.
Since nearly a third of all CME events are related to the occurrence of a magnetic cloud, scientific research into the phenomenon has been ongoing for many decades. Several spacecraft launched by the National Aeronautics and Space Administration (NASA) in the United States 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 Interplanetary Monitoring Platform 8 (IMP 8) launched in 1973 and 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 across the tail of the incoming comet, P/Giacobini- Zinner, for close observation.
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