ANGEO Annales Geophysicae ANGEO Ann. Geophys. 1432-0576 Copernicus Publications Göttingen, Germany 10.5194/angeo-27-1019-2009 The burst of solar and geomagnetic activity in August–September 2005 Papaioannou A. 1 Mavromichalaki H. 1 Eroshenko E. 2 Belov A. 2 Oleneva V. 2 Nuclear and Particle Physics Section, Department of Physics, University of Athens, 15771 Athens, Greece Institute of Terrestrial Magnetism Ionosphere and Radio wave Propagation (IZMIRAN) 142092, Troitsk, Moscow Region, Russia 02 03 2009 27 3 1019 1026 Copyright: © 2009 A. Papaioannou et al. 2009 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://angeo.copernicus.org/articles/27/1019/2009/angeo-27-1019-2009.html The full text article is available as a PDF file from https://angeo.copernicus.org/articles/27/1019/2009/angeo-27-1019-2009.pdf

During the August–September 2005 burst of solar activity, close to the current solar cycle minimum, a significant number of powerful X-ray flares were recorded, among which was the outstanding X17.0 flare of 7 September 2005. Within a relatively short period (from 22 August to 17 September) two severe magnetic storms were also recorded as well as several Forbush effects. These events are studied in this work, using hourly mean variations of cosmic ray density and anisotropy, derived from data of the neutron monitor network. During these Forbush effects the behavior of high energy cosmic ray characteristics (density and anisotropy) is analyzed together with interplanetary disturbances and their solar sources, and is compared to the variations observed in geomagnetic activity. A big and long lasting (~6 h) cosmic ray pre-decrease (~2%) is defined before the shock arrival on 15 September 2005. The calculated cosmic ray gradients for September 2005 are also discussed.

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