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The coronal mass ejections from the Sun, with velocities up to 2000 km/s, give raise to shock-waves in the solar wind. The related pressure pulses, when impinging the Earth's magnetosphere, both compress it and increase the magnetopause current. This leads into a few tens of nT intensifications in the low-latitude ground-based magnetic field intensity, lasting typically for some tens of minutes. These signatures are called Storm Sudden Commencements (SSC) or Sudden Impulses (SI), depending whether a magnetospheric storm is initiated or not. As one could expect, the 11-year solar cycle is evident in the yearly number of SSCs. The role of SI/SSC in substorm triggering is not that clear.

For list of SSCs from 1968, see the NGSC ftp server.

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