Magnetic Storms
In addition to the relatively predictable and smoothly varying diurnal variations, there can be transient, large amplitude (up to 1000 nT) variations in the field that are referred to as Magnetic storms. The frequency of these storms correlates with sunspot activity. Based on this, some prediction of magnetic storm activity is possible. The most intense storms can be observed globally and may last for several days.
The figure to the right shows the magnetic field recorded at a single location during such a transient event. Although the magnetic storm associated with this event is not particularly long-lived, notice that the size of the magnetic field during this event varies by almost 100 nT in a time period shorter than 10 minutes!!
Exploration magnetic surveys should not be conducted during times of magnetic storms. This is simply because the variations in the field that they can produce are large, rapid, and spatially varying. Therefore, it is difficult to correct for them in acquired data.