The role of solar wind pressure jumps in the initiation and control processes of magnetospheric substorms

By the example of commencement and development of two successive substorms on August 1, 1998, the mechanism resulting in the initiation of auroral processes (substorms) is discussed. These events were chosen due to the favorable position of satellites in the solar wind, magnetosphere, and plasma sheet, as well as due to the availability of satellite and ground-based observation data. The isolated spontaneous substorm was caused by the accumulation of excess energy in the magnetic tail and by energetic electron injection from the plasma sheet. Thirty minutes after the end of the spontaneous substorm, solar wind pressure jumps against the background of the strong induced dawn-dusk electric field resulted in significant variations with a high degree of correlation between the magnetic field and energetic particle fluxes at geosynchronous orbit and ground-based stations, and caused new auroral activations. The second substorm occurred as an abrupt intensification of auroral electrojets in the premidnight sector and four successive activations of auroras observed by the Polar satellite. As assumed from analysis of the development of geophysical phenomena caused by variations in the solar wind pressure, this event was characterized by the mechanism of direct energy transmission from the solar wind into the auroral zone through two channels simultaneously: the induced electric field and compressive intensification of magnetospheric currents. From analysis of the development of two substorms caused by different mechanisms, such as reconnection in the first case and direct energy transmission in the second, it can be assumed that both mechanisms can result in the same substorm phenomena. The statistical analysis with the use of the method of superimposed epochs includes additional arguments in favour of formulated suggestions.