| Summary: | This thesis was made in collaboration with the Sodankylä Geophysical Observatory (SGO) and the Ionospheric Physics group in Oulu University. In my work I analysed data from the SGO riometer network. The data I used is from years 1997–2013, and seven stations located at different latitudes. The stations are located in Hornsund, Abisko, Ivalo, Sodankylä, Rovaniemi, Oulu and Jyväskylä. In the analysis I concentrated on comparing the effect of different quiet-day curve (QDC) methods to the statistics of absorption events. In my thesis I study the yearly number of absorption events and their diurnal variation in magnetic local time (MLT). In my work I used absorption data calculated by two different QDC methods. The first method is based on automatic determination of the QDC and the second is based on manual determination. We decided to concentrate on using the data calculated by the automatic method, but I also compare our results to the manual data. In the beginning of this project I determined a simple method for finding absorption events from the riometer data. In collaboration with my supervisors and SGO staff we chose the criteria for an absorption event. I wrote a Matlab-script that was used to find the times and magnitudes of events from the data using the criteria. With this information I made the yearly and MLT-distributions to the automatic and manual data. We found a problem in the atomatic QDC method during the winter months of the maximum years of the solar cycle. We decided to manually remove the problematic periods. The automatic data set where the interfered periods are removed from forms the third data set used in this study, called the corrected automatic data. The yearly distributions of absorption events in the corrected automatic and manual data set have similarities with the yearly average Kp index, which is the index of geomagnetic activity. The Kp maximum year, 2003, and the minimum year, 2009, are visible at most of the stations. Correlation with the yearly average sunspot number is less ...
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