Solar Phys (2010) 266: 181–193 DOI 10.1007/s11207-010-9601-9 On the Analysis of the Complex Forbush Decreases of January 2005 A. Papaioannou · O. Malandraki · A. Belov · R. Skoug · H. Mavromichalaki · E. Eroshenko · A. Abunin · S. Lepri Received: 12 February 2010 / Accepted: 29 June 2010 / Published online: 5 August 2010 © Springer Science+Business Media B.V. 2010 Abstract In this work an analysis of a series of complex cosmic ray events that occurred between 17 January 2005 and 23 January 2005 using solar, interplanetary and ground based cosmic ray data is being performed. The investigated period was characterized both by sig- nificant galactic cosmic ray (GCR) and solar cosmic ray (SCR) variations with highlighted cases such as the noticeable series of Forbush effects (FEs) from 17 January 2005 to 20 Jan- uary 2005, the Forbush decrease (FD) on 21 January 2005 and the ground level enhancement (GLE) of the cosmic ray counter measurements on 20 January 2005. The analysis is focusing A. Papaioannou ( ) · H. Mavromichalaki Nuclear and Particle Physics Section, Physics Department, National and Kapodistrian University of Athens, Zografos, 15771 Athens, Greece e-mail: atpapaio@phys.uoa.gr H. Mavromichalaki e-mail: emavromi@phys.uoa.gr O. Malandraki Institute of Astronomy and Astrophysics, National Observatory of Athens, Athens, Greece e-mail: omaland@astro.noa.gr A. Belov · E. Eroshenko · A. Abunin Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN) of the Russian Academy of Sciences, after Pushkov, Moscow Region, Russia A. Belov e-mail: abelov@izmiran.ru E. Eroshenko e-mail: erosh@izmiran.ru R. Skoug Space Science and Applications Group ISR-1, MS D466, Los Alamos National Laboratory, Los Alamos, USA e-mail: rskoug@lanl.gov S. Lepri The University of Michigan, Ann Arbor, MI 48109, USA e-mail: slepri@umich.edu