Cosmic ray Ground Level Enhancements (GLEs) of october 28, 2003 and january 20, 2005: a simple comparison

Abstract

The evolutions of the Ground Level Enhancements (GLEs) of Oct. 28, 2003 (the famous Halloween event) and the subsequent Jan. 20, 2005 (a verylarge event in the declining phase of cycle 23) were examined. It was noticed that the Oct. 28, 2003 GLE was a very small one, in contrast to the large CR Forbush decreases and geomagnetic Dst storms that occurred the next day, on Oct. 29, 2003. These may not have the same origins. Hence, three more events were also studied, namely the largest GLE 5 of Feb. 23, 1956 (meager data), the second largest GLE 42 of Sep. 29, 1989, and the fourth largest GLE 45 of Oct. 24, 1989 (comparable to GLE 69, the third largest GLE of Jan. 20, 2005). For each, the plots of few-minute and/or hourly values as also the latitude-longitude distributions of the GLE magnitudes (percentage increases) were examined. It was noticed that at similar mid-latitudes, locations at different longitudes showed different latitude distributions of the magnitudes, indicating that events had longitudinal anisotropies, more in some events, less in others. Thus, the present paper illustrates a simple way of detecting anisotropies qualitatively. Only the maximum enhancement magnitudes were used, irrespective of the phases (maxima occurring at different times at different locations). If simultaneous magnitudes are used as these occurred at specific UTs in succession, more details could be studied such as changes in the characteristics (spectra etc., or multiple populations) of the incoming particles, as is done in sophisticated analyses. The present approach may be considered as a first look at a complex phenomenon.