Annual integral solar proton fluences for 1984–2019

Abstract

Aims. Long-term fluxes or integral fluences of solar energetic particles (SEPs), and their variability within and beyond the 11-year solar cycle, make an important contribution to space physics. However, large uncertainties exist in the evaluation of average SEP fluxes or fluences over the last few decades, as they have been assessed by different methods and from different datasets. Here we revisit the derivation of annual integral SEP fluences from available data based on in situ measurements since 1984. Methods. We reconstructed a full time series of integral SEP fluxes above 10, 30, 60, 100, and 200 MeV for the period from 1984 to 2019 using observations performed by the GOES satellites. Intercalibration of the fluxes was performed via a linear relation between overlapping pairs of observations in order to obtain a uniform dataset. Galactic cosmic ray (GCR) background subtraction and identification of SEP event periods were carefully performed, allowing for a precise calculation of annual SEP fluences. Results. Annual integral fluences of SEPs with energies above 10, 30, 60, 100, and 200 MeV were calculated for the period from 1984 to 2019 (solar cycles 22-24), along with their uncertainties. It is shown that solar cycle 24 was significantly (by a factor of 5-8) weaker in the SEP fluence than the preceding cycles 22 and 23. The cumulative occurrence probability of years with the fluence above a given value is found to be perfectly described by the Weibull distribution. This can be used as a projection for the occurrence of solar extreme eruptive events on the secular timescales.

Consistency of the average flux of solar energetic particles over timescales of years to megayears

“…The average SEP fluxes that were directly measured during the space era are systematically lower than the megayear-averaged lunar data. This implies a significant discrepancy, particularly for the moderate SC 24, which represents the typical long-term solar 1), similar to the green line, including CI-proxy data, but with the direct data for SC 24 alone F * ∞ (blue), directly measured average fluxes for SC 22 -24 (green stars) (Raukunen et al 2022), and the directly measured average fluxes for SC 24 (blue dots) (Raukunen et al 2022). Uncertainties are for the full range for the lunar data and 68% c.i.…”

“…Solar energetic particles have traditionally been quantified via the integral particle flux F(>E) (in units of cm −2 s −1 ) above the given energy E. This flux definition differs from the standard definition, and should strictly speaking be called the omnidirectional intensity (see Chapter 1.6.3 in Grieder 2001), but it has been used in the field historically. Typical energies E for the measured integral flux of SEPs are 10, 30, 60, 100, and 200 MeV (Shea & Smart 2000;Raukunen et al 2022), with the corresponding integral fluxes denoted henceforth as F 10 through F 200 , respectively.…”

“…We used the annually averaged SEP integral fluxes based on the GOES dataset (Raukunen et al 2022) in the energy range from 30 -200 MeV for 1987 -2019 (solar cycles (SCs) 22 -24), shown in Table 1. This dataset is an estimate of the regular SEP flux, including the occurrence of weak-to-strong SEP events, but excluding ESPEs.…”

“…One way (see Section 2.1) is based on direct measurements of SEPs in space (e.g., Shea & Smart 2000;Reedy 2012;Vainio et al 2013) by various spacecraft since the 1960s, but these estimates earlier suffered from large uncertainties related to the difficult intercalibration of various space-borne detectors. Primary SEP detectors on board the Geostationary Operational Environmental Satellite (GOES) family have recently been recalibrated (Raukunen et al 2022), which reduced the uncertainties of the SEP flux measurements since 1984.…”

“…Since the Moon is not protected from energetic particles by the atmosphere and magnetosphere, CIs in lunar rocks can serve as a rough cosmic-ray spectrometer (Miyake et al 2019;Poluianov et al 2018) over the isotope lifetime, which may reach several megayears. The GCR contribu- 22 -24 (1987 -2019) as taken from Table D1 of Raukunen et al (2022); (3) the same but for SC 24 only (2009 -2019); (4) F ∞ (see Eq. 2) for the combined GOES and CI-based data (green curves in Figure 1); (5) F * ∞ -the same as (3) but with GOES data, for SC 24 only; (6) average SEP flux reconstructed from lunar rocks for timescales of ≈10 6 years (Poluianov et al 2018).…”