Large-magnitude geomagnetic disturbances in the North Sea region: Statistics, causes, and forecasting

Many scientific and commercial activities require an undisturbed geomagnetic field. The Danish Meteorological Institute currently offers real-time magnetometer data as a service to such activities in Denmark and the North Sea region. As a part of an ongoing project aiming at the development of geomagnetic forecast services, we have investigated the statistics of geomagnetic disturbances at the Brorfelde Geomagnetic Observatory in Denmark, with a focus on large-magnitude disturbances that might affect directional-drilling operations in the North Sea. We here report on the distribution of such disturbances as a function of season, time-of-day, and phase of the solar cycle. We demonstrate a close association with strong magnetic storms identified from significant depressions of the ring current index Dst, which have been shown to be caused primarily by coronal mass ejections (CMEs). Very few of the weaker storms, even though much more frequently occurring, generate disturbances exceeding the large-magnitude thresholds, here defined from an actual example of a directional-drilling campaign in the North Sea. Limiting the range of acceptable geomagnetic disturbances would, however, make the weaker storms play a more important role. The prospects of forecasting geomagnetic disturbances are briefly discussed.