Modulation of Galactic Low-Energy Cosmic Rays according to SOHO/EPHIN Data from January 1996 to May 2025

The dynamic background in the SOHO/EPHIN proton channels—soft 4.3–7.8 MeV and hard 25–53 MeV—has been calculated from January 1996 to May 2025 in the past 23rd and 24th, and current 25th cycles of the solar activity (SA). The variations V(t) of the background signal I(t) according to SOHO/EPHIN data relative to the average I1996 flux recorded in the soft 6.2 × 10–5 (cm2 s sr MeV)–1 and in the hard 8.7 × 10–5 (cm2 s sr MeV)–1 channels between days 50 and 110 in 1996: V(t) = 100% × (I1996 – I(t))/I1996. The variations in the 4.3–7.8 MeV channel are positive and in phase with the SA cycles. The amplitude of variations in the SA cycles is equal to: 150% in the 23rd, 55% in the 24th, and 50% in the 25th. The variations in the 25–53 MeV channel are in antiphase with the SA cycles. The maximum variation is delayed relative to the beginning of the odd 23rd and 25th cycles, but coincides with the beginning of the even 24th cycle. They were equal to 20, 15 and 13%, which were reached in August 1997, October 2009 and October 2020, respectively. The minima of the variations of the flux in the 25–53 MeV channel are negative and correspond to approximately 70–80% near the maxima of the 23th–25th cycles. This indicates the solar origin of the most <10 MeV protons recorded in the SA maxima near the Earth. The energy spectra of cosmic ray protons are estimated at the minima and maxima of the SA. The problem of the first and last SEP (Solar Energetic Particles) events in the SA cycles is discussed. The NON index is introduced, which is the number of days without SEP events near the Earth and characterizes the absence of proton activity from the Sun. The maximum values of NON of 943 and 543 days precede the first SEP events of the 24th and 25th cycles. Before the last SEP event of the 24th cycle on April 20–21, 2019, NON = 431 days, which practically equalizes the minimums before the 24th and 25th cycles.