Contrasting behaviour of suprathermal 4He and Fe populations in the interplanetary medium during solar cycle 23 and 24

Suprathermal particles (H and other heavy ions like 4He, 3He, C, O and Fe) are, in general, characterized by the so-called `universal' -1.5 spectral index. However, variation in this spectral index with composition has not been tested critically. In the present investigation, solar cycle variation of individual `quiet' time suprathermal elements is investigated using < ~ 1 MeV/n particle flux data obtained from Ultra Low Energy Isotope Spectrometer on board Advanced Composition Explorer during the solar cycle 23 and 24. The analysis reveals that 4He lags the sunspot numbers in solar cycle 24 by almost 700 days and there is no noticeable lag for the 0.07-0.20 MeV/n 4He during solar cycle 23. Fe, on the other hand, lags the sunspot variation by almost 300 days in cycle 23 while negligible lag is observed in cycle 24. Further, significant changes in spectral slopes are seen for 4He and Fe in the minimum of cycle 24-25 and 23-24 respectively. Although 4He and Fe behave differently, all other elements exhibit random lags in both the solar cycles with respect to sunspot numbers and significant changes in spectral slopes are also not observed. These results suggest the contribution of energetic events in the suprathermal ion pool during `quiet' periods. Although no single generation mechanism is found to be adequate to explain these observations, the contrasting behaviour of 4He and Fe indicates towards sensitive dependence of the generation of suprathermal population on the first ionization potential and mass to charge ratio