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A graphic of a magnetic cloud, of helical field lines, shown propelled from the Sun as part of an event called a coronal mass ejection. The cloud travels through the solar wind and eventually, if moving in the right direction, engulfs the Earth's magnetic field (shown in green). When this happens a magnetic storm usually occurs at Earth. The magnetic field lines in the magnetic cloud (shown in red with a helical geometry) are unusually very strong compared to the field that exists in the surrounding solar wind. That strong field is part of the reason for the storm. Also the dynamics of the cloud is controlled by its strong magnetic field and not by its internal gas pressure, which plays a bigger role in the normal solar wind surrounding the cloud. (Magnetic field acts like a pressure when pushing perpendicular to its own direction.) The cross-section of the magnetic cloud at its biggest part is typically about 200 times the diameter of the Earth's magnetosphere, at its largest cross-section, in the plane which usually contains the Earth itself. |