Browsing by Author "Hynönen, Reko"

Geomagnetic storms and auroral substorms are manifestations of space weather. They are disturbances in the geomagnetic field caused by solar activity that consists of flare eruptions, coronal mass ejections, high-speed streams, corotating interaction regions and other disturbances within the solar wind. The occurrence rate and properties of these events vary greatly within the solar cycle some maximizing during the solar maximum and others during the declining phase of the solar cycle. Any solar activity measure can be used to define the solar cycle though traditionally the sunspot number has been used. In addition to the sunspot number we have examined e.g. sunspot area, solar radio flux and solar X-ray flux. The solar cycle itself can be divided into four distinct phases: ascending, maximum, declining and minimum phases. Their properties depend on the solar activity measure they are based on. Occurrence rates of geomagnetic storms, substorms and events of solar origin along with geomagnetic indices show that the most recent solar cycle, number 23, had its most disturbed time interval in 2003 in its declining phase. Though solar flares and CMEs were found to maximize in the solar maximum as expected, the slow CMEs and coronal hole originated structures like high-speed streams were found to maximize during the declining phase of the solar cycle. The same conclusion was confirmed studying the geomagnetic storm indices and the ultra-low frequency (ULF) fluctuations within solar wind and the magnetosphere, identified with the method of power spectra. Ground-based Pc5 pulsations from three magnetic stations (KEV, OUJ and KIL) were identified and two maxima were found: the largest one in the declining phase of the solar cycle and the other one during the solar maximum. The ground Pc5 pulsations during the solar cycle 23 follow nicely the ULFs identified based on the ACE satellite measurements at the L1. Records of storm indices show that the declining phase has been the most disturbed time interval in majority of the solar cycles during the modern solar maximum, not only during solar cycle 23.