He+ Pickup Ion Tail Variations with Location across Solar Wind Compressions and Interplanetary Conditions

Even during quiet times, with no simultaneous observation of solar energetic particles, the presence of suprathermal tails of solar wind and pickup ions in interplanetary space has been widely observed. One of the reported characteristics of these tails are power law spectra with a v^-5 dependence in the solar wind reference frame and exponential fall-off at higher energies, but variations in the spectra of various species have also been observed. Attempts to explain the formation of suprathermal tails during quiet times include continuing acceleration by compressive fluctuations of the solar wind or the superposition of power law, exponential, and Gaussian spectra from diffusive shock and stochastic acceleration. While the former may produce tails at all times in the solar wind, the latter may be more or less prominent, depending on spatial or temporal distance from disturbances in the solar wind. Based on a superposed epoch analysis of the evolution of He⁺ pickup ion distributions across compression regions, we study the related spatial variation of He⁺ tails and their dependence on solar wind and interplanetary magnetic field conditions, using STEREO PLASTIC data from 2007 through 2014. We analyze the effect of quiet time selection criteria used in previous studies, such as setting a threshold in solar wind speed, in observed suprathermal tail rates, or in the observed energetic particle rates. We find that the suprathermal tail flux is dependent on the compression strength and varies substantially across the compression region, with the lowest tail fluxes found in the center of rarefaction regions. Strong tails with spectra close to v^-5 occur in the compressed slow solar wind, with spectra consistently softer in the fast wind and substantial variations in fluxes and spectral slope with location across the compression regions and interplanetary conditions. The observed variations of the tails appear consistent with compressions and shocks as their source followed by subsequent transport as an emerging hypothesis that yet needs to be thoroughly tested.