The cause of magnetic disturbances in the earth's ionosphere.

This thesis studies the phenomenon of the magnetic storm, which is a global disturbance of the Earth's magnetic field and is defined in this thesis as an interval during which the Dst index falls below -50 nT for four consecutive hours. Storms are classified as either storm sudden commencements (SSCs; all storms which are initiated by a sudden impulse recorded by ground magnetometer stations) and storm gradual commencements (SGCs; all other storms). A superposed epoch analysis of solar wind plasma, interplanetary magnetic field (IMF) and magnetic indices AL, Kp and Dst has been undertaken for 538 storms identified between 1963 and 1991. The superposed epoch signature of the interplanetary medium during SSCs is similar to the signature of coronal mass ejections, whereas the corresponding superposed epoch signature of SGCs is similar to the signature of a high speed/ low speed coronal stream interface. A statistical study on the occurrence of the 538 identified storms has also been undertaken. The previously observed semi-annual variation in the occurrence of magnetic storms is shown to apply to SGC events only. Universal time (UT) variations in the onset times of SGCs have been observed. A UT variation has also been observed in the time-of-peak activity in the A m and Dst indices during storms. A case study of the ionospheric convection during the magnetic storm of March 20-21, 1990 has been undertaken. The response time of the ionospheric convection to changes of the IMF at the magnetopause associated with the SSC was a factor of two quicker than previous observations under normal solar wind conditions. A latitudinal dependence in the convection response time was also observed. Reconfigurations of the nightside convection pattern in response to substorm expansion phase onsets were observed implying that the nightside convection pattern can be dominated by substorm activity.