Unusual Cosmic Ray Variations During the Forbush Decreases of June 2015
Although the current Solar Cycle 24 is characterized by low solar activity, an intense geomagnetic storm (G4) was recorded in June 2015. It was a complex phenomenon that began on 22 June 2015 as the result of intense solar activity, accompanied by several flares and coronal mass ejections that interacted with the Earth’s magnetic field. A Forbush decrease was also recorded at the neutron monitors of the worldwide network, with an amplitude of 8.4%, and in its recovery phase, a second Forbush decrease followed, with an amplitude of 4.0% for cosmic rays of 10 GV obtained with the global survey method. The Dst index reached a minimum value of −204 nT that was detected on 23 June 2015 at 05:00 – 06:00 UT, while the Kp index reached the value eight. For our analysis, we used hourly cosmic-ray intensity data recorded by polar, mid-, and high-latitude neutron monitor stations obtained from the High Resolution Neutron Monitor Database. The cosmic-ray anisotropy variation at the ecliptic plane was also estimated and was found to be highly complex. We study and discuss the unusual and complex cosmic-ray and geomagnetic response to these solar events.
KeywordsSolar activity Cosmic ray intensity Forbush decrease Neutron monitors Geomagnetic activity
Special thanks to the colleagues of the NM stations ( http://www.nmdb.eu ) for kindly providing the cosmic-ray data used in this study in the frame of the high-resolution neutron monitor database NMDB, funded under the European Union’s FP7 Program (contract no. 213007). Thanks are due to the IZMIRAN group of the Russian Academy of Sciences for kindly providing Forbush decrease data. The co-author D. Lingri thanks the General Secretariat for Research and Technology (GSRT) and the Hellenic Foundation for Research and Innovation (HFRI) of the Greek Ministry of Education for supporting her PhD fellowship. Thanks are also due to the anonymous referee for useful suggestions that improved this manuscript significantly.
Conflict of Interest
The authors declare that they have no conflicts of interest.
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