Near‐Earth Cosmic Ray Modulation Index ϕ and Neutron Monitor Cutoff Rigidities at Hourly Cadence: Toward an Improved Monitoring of Solar‐Terrestrial Radiation Environment, Space Weather, and Space Climat

Abstract

Cosmic rays (CRs) are energetic particles originating from galactic and extragalactic sources. When arriving into the solar system and the Sun's heliosphere, they are diverted and modulated by the magnetic activity of the Sun. This modulation can be quantified by the modulation parameter determined with the Force‐field approximation (FFA). Even though this approximation is a highly simplified representation of the underlying physics, has been successfully used to describe the long‐term variations in CR modulation. Here we present results of the Near‐Earth modulation (NEM) index in a 1‐hr cadence, which is determined from the measurements of 38 neutron monitors (NMs) for the range 1.1.1951–31.1.2026. The changed naming convention underlines the change from a classical heliospheric modulation potential to a near‐Earth index. To address the effects of geomagnetic activity, we have also determined 1‐hr rigidity cutoffs for this period. We validate the result by comparing particle fluxes modeled using the NEM index to fluxes measured by the AMS‐02 instrument, which show a slightly better match than previous work. Additionally, we model and compare space radiation doses with those measured by CRaTER, revealing that the hourly cadence can effectively estimate the radiation dose background caused by CRs. We also cross‐analyze the NM stations' scaling factors, which reveal that the FFA performs well even at 1‐hr resolution. We also find that during Forbush decrease events caused by coronal mass ejections (CMEs), the measurements and model differ, revealing anisotropy effects during the passage of the CME flux ropes.