Observation of the Forbush Decrease on 2024 May 10;  Using the ALPAQUITA Air-shower Array at the 70–1000 GV Rigidity Range

The Andes Large area PArticle detector for Cosmic ray physics and Astronomy is a new air-shower array experiment under construction in the Bolivian Andes, and its prototype ALPAQUITA surface array has been operating since 2023 April. In addition to the traditional ≥ 3-hit or ≥ 4-hit coincidences to trigger recording air-shower events, ALPAQUITA records the counting rates of the ≥ 1-hit and ≥ 2-hit events (Any1 and Any2, respectively). We report a successful detection of a Forbush decrease (FD) that occurred on 2024 May 10 caused by a passage of an interplanetary shock formed ahead of the interplanetary coronal mass ejection. The amplitude detected in the Any1 rate is 4.26% ± 0.33% at the median primary rigidity of 76 GV, which is consistent with the observations with the worldwide neutron monitor and muon detector networks. Under the assumption of a power-law rigidity spectrum, we renormalized the errors of the observed amplitude (Aobs) and fitted them as a function of the median primary rigidity (Rm) of each detector and observational method. The result exhibits the hard nature of this event. Our nondetection in the Any2 rate decrease constrains the amplitude with a 2σ upper limit to be 0.95% at 960 GV. This marginally suggests an existence of a spectral softening between 100 and 1000 GV, as also suggested by the Misato underground muon detector at 145 GV. Although a strong geomagnetic storm was observed during this period, we conclude it does not impact our results. Our novel technique realizes a unique coverage to study the behavior of the FDs at the highest rigidity.

Observation of the Forbush Decrease on 2024 May 10; Using the ALPAQUITA Air-shower Array at the 70–1000 GV Rigidity Range

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