Did the 28 October 2003 solar flare accelerate protons to $\gtrsim 20\mathrm{GeV}$? A study of the subsequent Forbush decrease with the GRAPES-3 tracking muon telescope

Solar flares accelerate charged particles through a variety of mechanisms, which may be constrained through observations at high energies ($>10\mathrm{GeV}$). We report here a search for direct emission of protons of energy $\gtrsim 20\mathrm{GeV}$ 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 $1.4\times {10}^{-6}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$. A separate upper limit on the narrow solid angle flux of protons at $4\times {10}^{-6}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$ 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 ($\sim 0.3\mathrm{sr}$) shows a remarkably similar behavior, with an evolution on a time scale of $\sim 1\mathrm{h}$. 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 “$\gamma =0.53\pm 0.04$,” in agreement with earlier measurements.

Figure 1

A schematic layout for the 721 detector (open circles) GRAPES-3 shower array of which 217 detectors (filled circles) used at present are shown. Each of the 16 squares represents a $35{\mathrm{m}}^2$ area muon tracking detector with $E_{\mu }\ge 1\mathrm{GeV}$.

Figure 2

A view of the GRAPES-3 experiment showing the four halls housing the four supermodules of the muon detector. The scintillator detectors of the GRAPES-3 air shower array, placed on the roof and around the muon halls are also visible.

Figure 3

A schematic display of the 4-layer tracking muon detector module with 58 PRCs per layer. The four layers of the PRCs labeled Layer-0, Layer-1, etc. are embedded in concrete blocks. Inclined lines represent a set of parallel muon tracks.

Figure 4

The percent deviation in the 4-layer muon counting rate of one of the $35{\mathrm{m}}^2$ area module relative to the mean rate of remaining 11 modules as a function of time, on 28 October 2003. The rates are taken for 30 min intervals.

Figure 5

The variation in the total muon rate for the three supermodules, for 28 October 2003 after atmospheric pressure correction. Start of the solar flare is indicated by the first arrow. The second arrow indicates the start of the prompt component detected by NMs.

Figure 6

The percent variation in the total muon rate for the three supermodules, for 28 October 2003 9–13 h UTC; (i) for 1 min interval (top panel), (ii) for 3 min (middle), and (iii) for 6 min (bottom). The left and right arrows in each panel indicate the start of the x-ray flare and that of the prompt component detected by NMs.

Figure 7

A schematic view of muon arrival angle selection based on the PRC triggered in the lower and 15 PRCs directly above, in the upper layer. Triggered PRCs are shown as filled squares.

Figure 8

A schematic of the nine groups of muon arrival directions which combine to form solid angle bins; $3\times 3$ vertical bin V or four $3\times 5$ central bins N, E, W, S or four $5\times 5$ outer bins NE, SE, SW, NW.

Figure 9

A schematic of the nine solid angle bins of muon arrival directions. Also shown are contours of constant geomagnetic cutoff rigidity in the field of view. The cutoff rigidity varies from 12 to 42 GV in the FOV of GRAPES-3 muon telescope.

Figure 10

The observed variation in the muon rate on 28 October 2003 for nine solid angle bins is shown, for 6 min intervals. Successive plots have been shifted down by 1.5% for clarity. The start of solar flare is indicated by the arrow.

Figure 11

Integral flux of protons with energy on 28 October 2003; (a) based on 100–600 MeV GOES-10/11 and sub-GeV balloon data, (b) worldwide NM data, (c) ▪ $\gtrsim 20\mathrm{GeV}$ GRAPES-3 upper limit, (d) ▴ $>10\mathrm{GeV}$ AGASA flux on 4 June 1991, (e) ▾ $>40\mathrm{GeV}$ L3 upper limit on 14 July 2000, (f) ★ $>500\mathrm{GeV}$ Baksan upper limit on 29 September 1989, (g) ● NM data on 15 April 2001, (h) ◆ NM data on 23 February 1956.

Figure 12

The observed variation in the muon rate on 29 October 2003 from the vertical solid angle bin “V.” Muon rates shown are for 6 min intervals. The diameter of the small dot to the right of legend $1\sigma$ equals 1 sigma statistical error in data.

Figure 13

The hourly variation in the observed neutron rate on 29 October 2003 for the Tibet neutron monitor. The change in rate is normalized, relative to the rate at 0 h UTC.

Figure 14

The observed variation in the muon rate on 29 October 2003 from the nine angular bins grouped into three sets, namely, east (NE, E, SE), center (N, V, S), and west (NW, W, SW). Muon rates shown are for 6 min intervals. Successive sets have been shifted downwards by 4% for clarity. Each pointer represents the time of the Fd minimum for the 3 sets, respectively.

Figure 15

Observed variation in the correlation coefficient with time shift (hours) for pairs of angular bins; (a) category-1, W-SW, (b) category-2, N-NE, (c) category-3, E-W for the data of 29 October 2003.

Figure 16

Observed variation in the correlation coefficient with time shift (hours) for pairs of angular bins; (a) category-1, W-SW, (b) category-2, N-NE, (c) category-3, E-W for the data of 27 October 2003, 1 day before the solar flare.

Figure 17

(a) Observed variation in the magnitude of Forbush decrease (%); (b) simulations for the magnitude of Forbush decrease (%). Both as a function of geomagnetic cutoff rigidity (GV) for the three sets of nine solid angle bins, namely, (i) NE, E, SE (filled circles), (ii) N, V, S (filled triangles), (iii) NW, W, SW (filled squares). The middle bins (E, V, W) display sizable deviations $\sim 0.5\%$ from the remaining two bins. Simulations are for a rigidity dependence of $\mathrm{Fd}\mathrm{\text{magnitude}}=12.8\%\times (r/30{)}^{-0.53}$.