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.

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