Directional muon telescopes not useful for estimating the magnitudes of Forbush decreases and geomagnetic storms

Abstract

When a coronal mass ejection (CME) escapes from the Sun and spreads in interplanetary space as a blob (termed as interplanetary CME, ICME), galactic cosmic rays (CR) passing through the same get modulated and one can detect anisotropies in data of muon directional telescopes located on the Earth. For 15 severe storms (Dst < -200 nT) during solar cycle 23 (1996-2006), the hourly data for Nagoya muon directional telescopes showed that for each one of these storms, there were anisotropies in one or more of the 16 directional telescopes. The maximum magnitudes of the anisotropies were ~1% or less and had reasonably good relationships (correlation +0.75) with the magnitudes of the following Forbush decrease (FD) in the muons in the vertical direction V (muon V) and also in CR neutron monitor (NM) data at Climax, Colorado, USA. But a correlation of ~0.75 implies a variance explained (square of correlation) of only ~55%, leaving ~45% as random component. With geomagnetic Dst(min), the muon anisotropy magnitudes had a still lower correlation (+0.44±0.20), which would imply a variance explained of only ~20%, leaving ~80% as random component, resulting in regression prediction errors exceeding 50%. Thus, the anisotropies were only a rough indicator of there being some anomalous structure out in the interplanetary space, but the magnitudes of the anisotropies could not give any accurate indication of the magnitudes of the Forbush decreases that may follow, much less for the magnitudes of the geomagnetic storm Dst(min) that may follow.