Did the 28 October 2003 solar flare accelerate protons to ≳20 GeV? A study of the subsequent Forbush decrease with the GRAPES-3 tracking muon telescope

Abstract

Solar flares accelerate charged particles through a variety of mechanisms, which may be constrained through observations at high energies (>10 GeV). We report here a search for direct emission of protons of energy ≳20 GeV in association with an X17 class solar flare that occurred on 28 October 2003, using a large area tracking muon telescope of the GRAPES-3 experiment at Ooty. Some features of the telescope, including its novel capability of high sensitivity search for the directional enhancement of the solar protons are also described. A 99% C.L. upper limit on the flux of protons due to the solar flare has been placed at 1.4 ×10^-6 cm^-2 s^-1 sr^-1. A separate upper limit on the narrow solid angle flux of protons at 4×10^-6 cm^-2 s^-1 sr^-1 is also placed. Solar flares are also associated with coronal mass ejections, which propagate through the interplanetary space producing geomagnetic storms and Forbush decrease (Fd) events, upon their arrival at the Earth. New information on the structure and time evolution of the large Fd observed on 29 October 2003 by GRAPES-3 is presented. The onset of Fd in nine different solid angle bins (∼ 0.3 sr) shows a remarkably similar behavior, with an evolution on a time scale of ∼1 h. A power law dependence of the magnitude of the Fd on the cutoff rigidity has been derived, using the data from tracking muon telescope, over a narrow range of cutoff rigidity 14.3-24.0 GV, which shows a spectral slope "γ =0.53±0.04," in agreement with earlier measurements.