Cosmic ray Forbush decreases after the giant solar flare of 15 February 2011

520-523<span style="mso-ansi-language: EN" lang="EN">A very strong X-ray solar flare occurred at about 0200 hrs UT on 15 February 2011. It was associated with a coronal mass ejection (CME) but the CME was not very strong and its interplanetary CME (ICME) took about 70 h to reach the Earth. Also, the ICME blob had a moderate, short-lived magnetic field B, so the resulting cosmic ray Forbush decrease (FD) was weak. The Bz component was only slightly negative, so the resulting geomagnetic storm magnitude Dst was negligibly small. Thus, this giant solar flare had very poor geo-effectiveness indicating that large flares need not necessarily lead to significant storm effects on Earth, particularly in the early part of a sunspot cycle when overall solar activity would be weak.