The 1 June 2025 Forbush Decrease Measured over a Range of Primary Cosmic Ray Energies

The 1 June 2025 Forbush decrease in the terrestrial ground-level flux of cosmic ray secondaries was recorded by many cosmic ray systems. This was the deepest such decrease from the quiescent value of the flux, which has been observed over the past two decades. It resulted from a complex series of solar events, none of which on its own reached the most extreme level. The extreme depth of this decrease has enabled measurements of the flux reduction to be made, which would normally be severely limited by particle counting statistics. In particular, here we examine the decrease phenomenon over a primary cosmic ray energy range, which is rarely accessible due to the low flux of high energy cosmic rays. This work considers data mainly from a muon telescope system, which can respond to both unaccompanied muons and small cosmic ray air showers, providing data from GeV to mid-TeV energies, where the Forbush decrease ceases to be statistically observable. This paper examines the depth of the flux decrease, as a fraction below its quiescent value, over that primary energy range. The progressive development of internal time structure in the flux through the seven days of the phenomenon is also demonstrated.