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 since the Ap index was started 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.

Data up to Dec 31, 2001

Ranking        Date                Solar Cycle           Ap Value
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   1        13 November, 1960           19                 280
   2        13 March, 1989              22                 246
   3        01 April, 1960              19                 241
   4        15 July, 1959               19                 236
   5        18 September, 1941          17                 232
   6        05 July, 1941               17                 222
   7        28 March, 1946              18                 213
   8        01 March, 1941              17                 207
   9        06 October, 1960            19                 203
  10        08 February, 1986           21                 202
  11        08 July, 1958               19                 200
  12        11 February, 1958           19                 199
  12        06 September, 1982          21                 199
  14        22 September, 1946          18                 198
  15        05 June, 1991               22                 196
  16        25 March, 1946              18                 195
  17        31 March, 2001              23                 192
  18        30 March, 1940              17                 190
  19        24 March, 1940              17                 187
  20        07 October, 1960            19                 186
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A number of interesting features can be seen in this Table:-

1. Cycle 19 (peak sunspot number of 201 in 1975) made the largest contribution to the table with 7 entries. However, cycle 17 (peak sunspot number of 119 in 1937) contributed 5 entries in spite of being a relatively modest cycle in amplitude.

2. Most of the disturbances (16 out of the 20) occurred after the time of solar maximum of the cycle.

3. The equinox months (March-April and September-October) were the worst months with 14 entries out of the twenty.

4. Even though the date 13th November 1960 rates as the single worst disturbed day, the period 24-30 March 1940 must surely rank as the worst week - the period contributed 2 entries to the table (plus a day that almost made it).

Material Prepared by Richard Thompson. © Copyright IPS - Radio and Space Services.