Differential behaviors of suprathermal $^4$He and Fe populations in the interplanetary medium during solar cycle 24

Abstract

Investigations on the solar cycle variation of the properties of suprathermal populations (H and other heavy ions like $^4$He, $^3$He, C, O and Fe) in the solar wind are sparse and hence, poorly understood. In the present investigation, solar cycle variations of "quiet" time suprathermal elements are investigated using $<$ $\sim$ 1 MeV/n particle flux data obtained from Ultra-Low Energy Isotope Spectrometer on board Advanced Composition Explorer satellite during the solar cycle 23 and 24. The analysis reveals that helium ($^4$He) shows zero or positive lags with respect to sunspot numbers in solar cycle 23 while it shows zero or negative lag in solar cycle 24. On the contrary, although iron (Fe) shows zero or positive lag in cycle 23 similar to $^4$He, it shows only zero lag in cycle 24 and no negative lag is seen. Further, significant differences in the spectral indices are seen between $^4$He and Fe in cycle 24 compared to the cycle 23. These results suggest that generation mechanisms responsible for suprathermal $^4$He and Fe underwent changes in cycle 24 and these mechanisms are probably dependent on the first ionization potential and mass to charge ratio. This proposition gets credence from the fact that changes in the lag and spectral slopes for C and O are not significantly different in cycle 23 and 24.