Energy Spectra of low-Flux Protons in the Inner Heliosphere under Quiet Solar Conditions

The energy spectra of protons at energies in the range of about 1–100 MeV are investigated during time periods of low solar activity using data sets from near Earth spacecraft. These populations pose a tough experimental and theoretical problem that remains unsolved up to now. We attempt to provide a consistent definition of low-flux quiet-time periods relevant to low solar activity as well as quasi-stationary periods useful at higher levels of solar activity. Using statistical methods, the possible instrumental contribution to the lowest observed proton fluxes for various detectors is estimated. We suggest and prove that there exists a low-flux population of charged particles in the energy range of about 1–10 MeV, which is present in the inner heliosphere even during the quietest conditions at lowest solar activity. The dynamics of the variations of proton spectra over the solar cycle is investigated. A series of low-flux periods is examined in detail and energy spectra of protons are approximated in the form of J(E)=AE^–+CE. By determining the best fitting parameters to the energy spectra correlations are made among them as well as with monthly sunspot numbers characterizing solar activity. It has been demonstrated that the value of the energy minimum of proton spectrum E_min that `divides' the two populations – `solar/heliospheric' and `galactic' – is shifted towards higher values with increasing solar activity. Protons have been argued to be predominantly of solar origin up to several MeV near the solar cycle minimum and up to 20–30 MeV at maximum. The slope of the lower spectrum branch (parameter ) slightly decreases with increasing solar activity. The minimum fluxes observed during the last 3 minima of solar activity are compared; the lowest fluxes were those during the 1985–1987 period.