A study of the Forbush decrease event of September 11, 2005 with GRAND

A Forbush decrease (FD) is an intensity depression in cosmic ray count caused by a coronal mass ejection (CME) [2]. When a CME arrives at the Earth, a sudden storm commencement (SSC), an intensification in the low latitude ground-based magnetic field intensity [9], can be detected and used as the onset time of a geomagnetic storm [4]. On September 11, 2005, the National Geophysical Data Center [7] website reported an SSC at 1:14 UT. The event was ranked by 12 different magnetic observatories as "very remarkable" [7]. In this paper, data are analyzed from Project GRAND for evidence of a FD event associated with this CME/SSC event. These data are compared to data taken from the Nagoya Multidirectional Telescope and the Oulu Neutron Monitor. A directional study of the FD event was undertaken utilizing the angular resolution capabilities of GRAND in order to study precursor anisotropies to SSCs which can result from a "loss cone" effect, during which the detector observes the deficiency of particles which trace to the cosmic-ray-depleted region behind the shock [4].

Project GRAND is an array of 64 proportional wire stations located adjacent to the University of Notre Dame campus at 41. . Each pair contains a horizontal plane of wires running north-south, and another plane of wires running east-west. When a charged particle (such as a secondary muon created by cosmic rays) passes through the chamber, it leaves a trail of ions. These ions are then accelerated toward the closest signal wire; as they are accelerated, they collide with more gas molecules and release more charge in a process known as gas amplification which further increases the charge collected on the signal wire. A small current is formed on this signal wire which is amplified. This electronic signal denotes the position of the charged particle which passed near this wire. By comparing hit wires in planes vertically above each other, the angle of the muon track can be reconstructed to within 0.26°, on average, in each of two projected planes: up/east and up/north. A 50 mm thick steel plate is situated above the bottom two PWC planes allowing for the distinction of muon tracks from the electron tracks which stop, shower, or are deflected by the steel. The array collects data at a rate of ~2000 identified muons per second. Added details are available at: http://www.nd.edu/~grand .

GRAND data from September 9, 2005 at 5.0 UT through September 20, 2005 at 5.0 UT were considered for this study. The muon count for this time interval was examined in one hour bins. In order to ensure experimental accuracy, the r.m.s. deviation was calculated for each station and compared to its expected statistical variation. A histogram of these ratios was compiled for each station for each day, and a cutoff was determined such that if its ratio was higher than this cutoff it was excluded in the analysis. There were 30 stations left in the analysis after this severe cut, ensuring that time changes in the sum-of-huts muon count rate could only be the result of actual muon rate changes.

The muon counts were pressure corrected using data from the National Climactic Data Center [6]. The pressure corrected muon data from Project GRAND are shown in Figure 1a 1b and1c (here the points are plotted at the beginning of each hour). The Nagoya Multidirectional Muon Telescope detected an approximately 5% drop on September 11, 2005 with the greatest decrease in count in one hour occurring between 4.5 UT and 5.5 UT. The Oulu data demonstrate a decrease of approximately 11% on September 11, 2005, with the greatest decrease in count in one hour occurring between 1.5 UT and 2.5 UT. Because Oulu measures secondary neutrons as opposed to muons, its primary energy sensitivity is influenced by different mechanisms producing neutrons rather than muons. In addition, Oulu has a lower geomagnetic cutoff rigidity (0.8 GV) compared to Nagoya (11.5 GV) and GRAND (1.9 GV) As a result of these factors, Oulu is more sensitive to lower energy primaries than muon stations and therefore can account for its larger drop in count rate; the percentage drops of Nagoya and GRAND are similar. Utilizing GRAND’s angular resolution, the directions of the secondary muon tracks were studied. The sky was divided into a three-by-three grid of viewing directions. The grid is defined by two orthogonal projected angles: the angle from zenith going north, and the angle from zenith going east. Divisions between the viewing directions were made at 9.7° and 61° [3] in both the north and east directions such that each viewing direction bin would have roughly the same number of muon counts per hour during a background day (and hence similar statistical precision). Directional data from ten background days were taken as a baseline of comparison. The percentage of counts coming from each viewing direction for each hour was calculated for background days and the day in study. A mean percentage was calculated for each viewing di

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