High amplitude anisotropic events (HAE) in cosmic ray diurnal variation during Solar cycle 23

Abstract

<smarttagtype namespaceuri="urn:schemas-microsoft-com:office:smarttags" name="City" downloadurl="http://www.5iamas-microsoft-com:office:smarttags"><smarttagtype namespaceuri="urn:schemas-microsoft-com:office:smarttags" name="place" downloadurl="http://www.5iantlavalamp.com/"> Cosmic ray (CR) intensity shows significant diurnal variations of day-to-day most probable amplitudes of ~0.5% (annual vector averages <0.35%), often with wave trains of high amplitude anisotropic events<b style=""> </b>(HAEs) and low amplitude anisotropic events (LAEs). Using the pressure-corrected data for the neutron monitor at Calgary (51^oN, 114^oW, cut-off rigidity 1.09 GV) during Solar cycle 23 (1996-2005), trends were removed by subtracting 24-hour running means from the hourly values. In the residues, days were searched when the maximum value of the diurnal amplitudes exceeded 1.0%. It was noticed that there were several HAE single events (amplitude exceeding 1.0% only on one day, not on the previous or succeeding day), double events (amplitudes exceeding 1.0% on two successive days), triple events (three successive days), and so on, with some having 10 or more days in succession. These events occurred at all levels and phases of CR variations, namely when CR daily values were almost constant for several days, or when daily values decreased slowly or increased slowly (few percent in several days, including 27-day variations) or just before the start of a Forbush decrease (FD) or during the recovery of a FD (During the main phase of sharp, strong FDs, the diurnal variation is highly distorted and difficult to access, hence omitted from the study). The occurrence of HAEs was found to be unrelated to geomagnetic indices or any interplanetary near-Earth parameters and is probably related to features in remote regions of the heliosphere. </smarttagtype></smarttagtype>