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 D_st, 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.