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 ( A obs ) and fitted them as a function of the median primary rigidity ( R m ) of each detector and observational method. The result A obs = ( 10.9 % ± 0.9 % ) × ( R m / 10 GV ) − 0.55 ± 0.07 exhibits the hard nature of this event. Our nondetection in the Any2 rate decrease constrains the amplitude with a 2 σ 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.