Was the cosmic ray burst detected by the GRAPES-3 on 22 June 2015 caused by transient weakening of geomagnetic field or by an interplanetary anisotropy?

Abstract

The GRAPES-3 muon telescope in Ooty, India had claimed detection of a 2 hour (h) high-energy ($\sim$20 GeV) burst of galactic cosmic-rays (GCRs) through a $>$50$\sigma$ surge in GeV muons, was caused by reconnection of the interplanetary magnetic field (IMF) in the magnetosphere that led to transient weakening of Earth's magnetic shield. This burst had occurred during a G4-class geomagnetic storm (storm) with a delay of $\frac{1}{2}$h relative to the coronal mass ejection (CME) of 22 June 2015 (Mohanty et al., 2016). However, recently a group interpreted the occurrence of the same burst in a subset of 31 neutron monitors (NMs) to have been the result of an anisotropy in interplanetary space (Evenson et al., 2017) in contrast to the claim in (Mohanty et al., 2016). A new analysis of the GRAPES-3 data with a fine 10.6${}^{\circ }$ angular segmentation shows the speculation of interplanetary anisotropy to be incorrect, and offers a possible explanation of the NM observations. The observed 28 minutes (min) delay of the burst relative to the CME can be explained by the movement of the reconnection front from the bow shock to the surface of Earth at an average speed of 35 km/s, much lower than the CME speed of 700 km/s. This measurement may provide a more accurate estimate of the start of the storm.

Accepted for Publication in Physical Review D

References in corpus (4)

• Observations of an extreme storm in interplanetary space caused by successive coronal mass ejections
• A Review of the 0.1 Reconnection Rate Problem
• On the Value of the Reconnection Rate
• Forbush decreases and turbulence levels at CME fronts

Cited by in corpus (5)

• Measurement of the Electrical Properties of a Thundercloud Through Muon Imaging by the GRAPES-3 Experiment
• Cosmic ray short burst observed with the Global Muon Detector Network (GMDN) on June 22, 2015
• Modeling of rigidity dependent CORSIKA simulations for GRAPES-3
• Energy sensitivity of the GRAPES-3 EAS array for primary cosmic ray protons
• Dependence of the estimated electric potential in thunderstorms observed at GRAPES-3 on the hadronic interaction generators used in simulations