PECULIARITIES OF THE GEOMAGNETIC PULSATION BEHAVIOUR DURING THE RECENT SOLAR MINIMUM

The time period from 2007 till 2009 was a period of the deepest and longest solar minimum in the past two centuries. Following the sunspot dip the magnetic activity also dropped to very low values. Nevertheless, solar coronal holes continued to throw off high speed streams (HSS) of the solar wind, thus producing individual isolated magnetic disturbances in the Earth's magnetosphere. This situation provides great scope for experimental research in the solar wind-magnetosphere-ionosphere system dynamics including ULF wave phenomena and wave-particle interaction in their pure form, without a superposition of concurrent events or extraneous turbulence. In this work, we study various manifestations of weak magnetic disturbances in the ULF activity during the sunspot minimum using data from both polar and mid-latitudinal stations, as well as in situ measurements in the solar wind. To illustrate advantages of the extremely quiet solar and magnetic conditions, we analyse a case of direct penetration of 3-4 millihertz MHD waves from the solar wind into the magnetosphere. The lack of an interplanetary shock wave ahead of the HSS in that case made it possible to separate penetrated oscillations from those generated inside the magnetosphere due to dynamic pressure pulses. In spite of low intensity of the penetrated waves they produced a detectable increase in relativistic electron fluxes at the geosynchronous orbit. Statistics of different kinds of geomagnetic pulsations observed during the period under consideration is also given. Characteristics of oscillations were analysed in relation to the magnetic disturbance and the interplanetary parameters.