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LOCAL GEOMAGNETIC ACTIVITY RECORDED ON THE BULGARIAN TERRITORY FOR THE SOLAR CYCLE 24
Abstract
Records of the magnetic variations are different at every observatory. They depend mainly on the latitude of the observatory and the local time. To give a quantitative measure of the amount of magnetic disturbance at any time, we need to rely on magnetic indices. Usually, local K-index is preferred as indicator of the geomagnetic activity in a specific area. Each observatory assigns a digit ranging from 0 to 9 to both H and D, for every three-hour interval starting at midnight, universal time (UT). Recently, in December 2019 finished the last Solar Cycle - one of the main periodic variations of the solar activity. This inspired the topic of the present research to investigate the dependence of the local magnetic activity recorded at Panagjurishte (PAG) observatory and the records of the sunspot numbers during the 11 years long cycle. PAG observatory is situated at middle latitude and does not experience the polar electrojet which causes intense 'negative bays' or polar magnetic substorms in the high latitudes and the terrestrial ring current centered at the equatorial plane and responsible also for global decreases in the Earth's surface magnetic field. The Solar Cycle 24 was announced as one of the weakest in the last century of observations. Similar is the statistics of the recorded geomagnetic event at the PAG observatory. Plot of the days with registered geomagnetic storms for the whole 11-year period clearly shows two maxima of the annual numbers - one in 2012 and another (larger) in 2015 when 67 days have had K-index ?5. In June that year happened also the largest event with K-index=8. Examining the monthly distribution of magnetic storms, it was found that the periods in which there are largest recorded events are also two: February - March and September- October with more than 30 stormy days in total. As a conclusion it was found the for Solar Cycle 24, 75% of the time has had calm geomagnetic condition, 16 % has been disturbed and only in 9% of the time (less than a year), the Sun has caused geomagnetic storms.
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