Transient weakening of geomagnetic shield probed by GRAPES-3 experiment

The large area (560 m$^2$) GRAPES-3 tracking muon telescope in Ooty, India recorded a 2 hour (h) muon burst on 22 June 2015 starting at 19:00 UT in the midst of a continuing Forbush decrease. The burst occurred following the arrival of a coronal mass ejection containing a 40 nT south-ward interplanetary magnetic field (IMF) that had triggered a G4 class geomagnetic storm. During the 2 hour, the muon telescope recorded an excess of $\sim$ 10$^{6}$ muons with a significance of 54$\sigma$. Simultaneous occurrence of the burst was observed in 9 directions covering 2.3 sr field of view suggesting its origin close to Earth.Trajectory computation of cosmic ray primary protons followed by atmospheric simulation by superimposing the 2 hour IMF variation on geomagnetic field (GMF) could reproduce the observed profiles with a high degree of correlation (0.89 $\pm$ 0.05). The simulations could explain the burst profiles due to lowering of cutoff rigidity (0.5 - 0.7 GV) for 2 h which allowed an excess near threshold galactic cosmic rays (GCRs) to enter into the atmosphere. It showed that the near cutoff rigidity GCRs were deflected 210$^{\circ}$ in longitude offering a natural explanation of its midnight detection by the GRAPES-3 experiment. This exciting observation has attracted considerable global attention following its publication in Physical Review Letters. Further studies of this event since then have shed more light on this phenomenon which will be presented during the conference.