The detection of the arrival of the CME is by means of an array of three magnetometers that will detect the disturbances to the Earth's magnetic field caused by the impact of the CME on this field. These magnetometers are arranged so that we can look at the disturbance in three directions, E-W, N-S and the vertical component

Since we know the time of the CME leaving the Sun and when it arrived here, and we know the distance between the Sun and Earth, we can calculate the velocity of the CME - Our record so far is 2.2 million mph, the average is in the region of 750,000 mph.

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