Institute of Geophysics, University of Tehran, P.O. Box 14155-6466, Tehran, Iran
One of the transient variations is named geomagnetic bay, which looks like a map of an indentation in a coastline or Gaussian curve. This type of variation (and all variations) induces electric currents in the earth. The induced electric currents give rise in turn to an internal component of the magnetic variations observed at the surface. The ratio of the parts of the magnetic field of internal and external origin is a measure of the response, and is dependent on both the external current system and the distribution of the electrical conductivity within the earth. Using this method, the electrical conductivity of the earth can be determined for various depths according to the period of the magnetic variations. The duration of bay can be from a few minutes to a few hours. The bays generally occur on solar quiet days accompanied with Pi1 and Pi2 micropulsations. The bays are two-stage events; the early stage is the trigger bay and the second is the main bay. In this paper, a statistical study of bays recorded at Tehran geomagnetic observatory for the period of 1971-80 is made according to the time of their occurrence, intensity and direction. We have also taken into consideration the geomagnetic seasons. Maximum numbers of bays occur at midnight with moderate intensity and positive direction. Most of the bays take place during the Equinox and December solstice. Return currents are predominant current system. The delay time between the onsets of the two stages lie the range of 10-30 minutes. The relation between the numbers of the sunspots and the bays is inversely proportional.