Educational
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Magnetic Field

Historical Large Geomagnetic Disturbances

Large geomagnetic disturbances are quite rare events, even near the peak of the solar cycle. However, they have a host of interesting effects on global radio communications, on satellite and spacecraft operations, on geophysical exploration and a host of technological systems.

But how do we compare these disturbances? One way is to use the daily geomagnetic disturbance index, Ap, and to rate disturbances according to its value. This has been done in the table below, which lists the 20 most disturbed days (to 31 December 2014) since the Ap index was introduced in 1932. After the table, we look at some of the common factors amongst the disturbances - such as the influence of season and the importance of solar cycle.

Ranking Date Solar cycle Ap value
113 November 196019280
213 March 198922246
31 April 196019241
415 July 195919236
518 September 194117232
65 July 194117222
728 March 194618213
81 March 194117207
929 October 200323204
1006 October 196019203
118 February 198621202
128 July 195819200
1311 February 195819199
146 September 198221199
1522 September 194618198
165 June 199122196
1725 March 194618195
1831 March 200123192
1930 October 200323191
2030 March 194017190

A number of interesting features can be seen in the table:

  • Cycle 19 (peak sunspot number of 201 in 1958) made the largest contribution to the table with six entries. However, cycle 17 (peak sunspot number of 119 in 1937) contributed four entries in spite of being a relatively modest cycle in amplitude.
  • Most of the disturbances (15 out of the 20) occurred after the time of solar maximum of the cycle.
  • The equinox months (March-April and September-October) were the worst months with 13 entries out of the twenty.

Material prepared by Richard Thompson

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