Observation of the Forbush decrease on 2024 May 10, using the ALPAQUITA air-shower array at the 70-1000 GV rigidity range

TL;DR

The paper addresses how Forbush decreases extend to very high cosmic-ray rigidities, a regime difficult to access with traditional neutron monitors. It introduces ALPAQUITA’s counting-mode approach (Any1/Any2) within an air-shower array to map detector counts to median primary rigidities up to ~960 GV, validated by Monte Carlo simulations. The May 10, 2024 FD is analyzed, yielding a measured Any1 amplitude of $A_{obs} = 4.26\% \pm 0.33\%$ at $R_m \approx 76$ GV and a 2σ upper limit for Any2 of 0.95% at $R_m \approx 960$ GV, with a global fit giving $A_{obs} = (10.9\% \pm 0.9\%) \times (R_m/10\,\mathrm{GV})^{-0.55\pm0.07}$, indicating a hard high-rigidity spectrum. The results are consistent with NM/GMDN observations and Misato's indication of spectral softening around higher rigidities, while magnetospheric effects are found negligible for this analysis. This work demonstrates a novel capability to study high-rigidity FDs and motivates upgrades to enhance sensitivity at the highest rigidities.

Abstract

The Andes Large area PArticle detector for Cosmic ray and Astronomy (ALPACA) 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 $\ge$3-hit or $\ge$4-hit coincidences to trigger recording air-shower events, ALPAQUITA records the counting rates of the $\ge$1-hit and $\ge$2-hit events (Any1 and Any2, respectively). We report a successful detection of a Forbush decrease occurred on 2024 May 10 caused by a passage of an interplanetary shock formed ahead of the Interplanetary Coronal Mass Ejection. The amplitude detected in the Any1 rate is 4.26$\pm$0.33% at the median primary rigidity of 76GV which is consistent with the observations with the worldwide neutron monitor and muon detector networks. Under the assumption of a power-law rigidity spectrum, we renormalized the errors of the observed amplitude ($A_{obs}$) and fitted them as a function of the median primary rigidity ($R_{m}$) of each detector and observational method. The result $A_{obs} = (10.9\% \pm 0.9\%) \times (R_{m}/10\,GV)^{-0.55 \pm 0.07}$ exhibits a hard nature of this event. Our non-detection in the Any2 rate decrease constrains the amplitude with a 2$σ$ upper limit to be 0.95% at 960GV. This marginally suggests an existence of a spectral softening between 100GV and 1000GV as also suggested by the Misato underground muon detector at 145GV. 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 Forbush decreases at the highest rigidity.

Figures (5)

• Figure 1: Pressure corrected time profiles of the NM (black curves) and GMDN (red curves) relative counting rate during the FD started at around 17 UT, May 10.
• Figure 2: Layout of the ALPAQUITA surface array. Each square represents a 1 m$^{2}$ scintillating counter.
• Figure 3: Response functions of the ALPAQUITA Any1 (red), Any2 (blue), Any3 (green) and Any4 (black) counting modes. Rigidity range below 12 GV, which is the vertical cutoff rigidity at the ALPAQUITA site, as indicated by shade is not included in the calculation of the median primary rigidities.
• Figure 4: Time profiles of the ALPAQUITA Any1 (black) and Any2 (red crosses) relative counting rates after the accidental coincidence and pressure corrections.
• Figure 5: Median primary rigidity and amplitude of the FD event on May 10, 2024. Black open circles, blue open squares, green open triangle and red filled circle show the data of NMs, GMDN, Misato muon detector and ALPAQUITA Any1, respectively. The red arrow shows the upper limit determined by the ALPAQUITA Any2. Narrower error bars in each plot are derived from the standard deviation of each station while the wider errors are defined to obtain $\chi^2$/dof=1 in the power law fitting shown with a black line and hatched area indicating 1$\sigma$ error band.