Abstract
Abstract. To validate the usage of global indices in studies of geomagnetic activity, we have examined the latitude dependence of geomagnetic variations in Fennoscandia and Svalbard from 1994 to 2010. Daily standard deviation (SD) values of the horizontal magnetic field have been used as a measure of the ground magnetic disturbance level. We found that the timing of the geomagnetic minimum depends on the latitude region: corresponding to the minimum of sunspot cycle 22 (in 1996), the geomagnetic minimum occurred between the geomagnetic latitudes 57–61° in 1996 and at the latitudes 64–67° in 1997, which are the average auroral oval latitudes. During sunspot cycle 23, all latitude regions experienced the minimum in 2009, a year after the sunspot minimum. These timing differences are due to the latitude dependence of the 10 s daily SD on the different solar wind drivers. In the latitude region of 64–67°, the impact of the high-speed solar wind streams (HSSs) on the geomagnetic activity is the most pronounced compared to the other latitude groups, while in the latitude region of 57–61°, the importance of the coronal mass ejections (CMEs) dominates. The geomagnetic activity maxima during ascending solar cycle phases are typically caused by CME activity and occur especially in the oval and sub-auroral regions. The strongest geomagnetic activity occurs during the descending solar cycle phases due to a mixture of CME and HSS activity. Closer to the solar minimum, less severe geomagnetic activity is driven by HSSs and mainly visible in the poleward part of the auroral region. According to our study, however, the timing of the geomagnetic activity minima (and maxima) in different latitude bands is different, due to the relative importance of different solar wind drivers at different latitudes.
Highlights
Solar activity varies in about 11-year cycles (Schove, 1955) and is the driving force for long-term geomagnetic variations
We found that the timing of the geomagnetic minimum depends on the latitude region: corresponding to the minimum of sunspot cycle 22, the geomagnetic minimum occurred between the geomagnetic latitudes 57–61◦ in 1996 and at the latitudes 64–67◦ in 1997, which are the average auroral oval latitudes
The first geomagnetic activity peak during the sunspot maximum is often related to an enhanced occurrence rate of coronal mass ejections (CMEs), while the second peak mostly relates to the increased high-speed stream frequency later in the same spot cycle (Gonzalez et al, 1990; Richardson et al, 2001)
Summary
Solar activity varies in about 11-year cycles (Schove, 1955) and is the driving force for long-term geomagnetic variations. The solar cycle evolution of geomagnetic activity often consists of two peaks during the solar maximum and in the declining phase (Gonzalez et al, 1990; Echer et al, 2004). The coronal mass ejections (CMEs) and high-speed solar wind streams (HSSs) drive the most severe geomagnetic variations. The occurrence rate of CMEs increases with the enhanced solar activity (Gopalswamy et al, 2004), while the number of HSSs peaks during the declining phase of the sunspot cycle (Tanskanen et al, 2005). The first geomagnetic activity peak during the sunspot maximum is often related to an enhanced occurrence rate of CMEs, while the second peak mostly relates to the increased high-speed stream frequency later in the same spot cycle (Gonzalez et al, 1990; Richardson et al, 2001)
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