Analysis of suprathermal tails using hourly-averaged proton velocity distributions at 1 AU

Abstract

We obtain hourly values of tail densities and of power law indices, γ, of suprathermal (speeds above 2.48 times the solar wind speed) protons from power law fits to hourly velocity dis-tribution functions in the solar-wind-frame. ACE/SWICS and ULEIS data, which often include very low counting statistics, are used to derive hourly proton phase space densities. We find that during part of the recent deep solar minimum (first 82 days in 2009): (a) the spectrum averaged over the entire 82 day period reveals the bulk and the halo solar wind components, interstellar pickup protons (seldom seen at 1 AU), and the common Fisk and Gloeckler (F&G) suprathermal tail (v−5 in velocity v with an exponential rollover at some higher speed); (b) hourly values of the tail densities range from ~1•10−6 to ~3•10−3 cm−3 and vary by a factor of ~2-10 over periods of hours as well as in a quasi-periodic manner by factors of 20 to 50 over 4 to 10 days; (c) about 95% of the nearly 2000 hourly spectra have complex shapes and that are not power laws; (d) about half of the ~5% of the hourly spectra that are monotonically decreasing with increasing speed (e.g. exponentials or Max-wellians, or F&G) are observed at times of high tail densities (>−5•10−5 cm−3) where the spectra have the common F&G shapes; (e) each of the six sharp (few day long), large (tail density > 5•10−4 cm−3) increases observed during this time period is associated with solar wind compression regions; (f) the eight shocks recorded locally that were not contained in compression regions did not produce signif-icant increases in the tail densities. We conclude that during times of low solar activity the higher energy portions of locally accelerated suprathermal tail spectra are often obscured by significant contributions from remotely accelerated particles whose spectra below (1-3)•108 cm/s are modified (modulated) by propagation from this remote acceleration region. In those instances where strong acceleration occurs locally, the observed tail spectra have the common F&G spectral shapes.