Impulsive Increase of Galactic Cosmic Ray Flux Observed by IceTop
2017 August 16
On 2017 January 18 scaler rates in the IceTop detectors at the South Pole revealed an impulsive increase in the galactic cosmic ray flux lasting a few hours. In addition to the neutron monitor at Pole the event was detected clearly by the Mawson neutron monitor and faintly at Jang Bogo. No other neutron monitors appear to have seen the increase. The event was in many ways reminiscent of the 2015 June 22 event observed by the GRAPES muon detectors. Both events occurred during the declining phase of a Forbush decrease, at a time of increasing geomagnetic activity, and were observed by a limited number of neutron monitors with similar asymptotic directions. The magnitude of the impulse was in both cases such that the flux returned briefly to approximately the pre-decrease level. Distinctly unlike the 2015 June 22 event, a changing geomagnetic cutoff cannot explain the 2017 January 18 event because the cutoff at South Pole is nearly zero and the detector response is atmosphere limited. We therefore interpret the 2017 January 18 event in terms of the structure of the Forbush decrease and (possibly changing) asymptotic directions. With our interpretation of the January event in mind we also comment on possible alternative interpretations of the GRAPES event.