Suprathermal ions from WIND/STICS and comparison with theory

The velocity distribution function (VDF) of solar wind protons (as well as other ion populations) is comprised of a thermal Maxwellian core and an accelerated suprathermal tail, beginning at around 400 km/s in the frame co-moving with solar wind bulk velocity.

We fit suprathermal tail VDF data from the WIND spacecraft, specifically the SupraThermal Ion Composition Spectrometer (STICS) with an empirical form. The form of the suprathermal tail is a power law in phase space density, f, vs. speed, v, such that f is proportional to v^-γe^-v/v0 where γ is the power law index, with an exponential rollover parameter, v0.

The results are that, for the years 2005 and 2006, the mean, median, and mode of γ are all about 4.8, and the mode of v0 is about 900 km/s and a median of 1300 km/s. The standard deviation of γ is about 0.25, making the result consistent with previous studies that have found a consistent value of 5. For v0, there is a large amount of variation, and the variations are very asymmetric, with a long large-v0 tail to the distribution.

One prominent theory, that of Fisk & Gloeckler, makes a prediction for v0 based on the variation of solar wind bulk speed. Other theories make other predictions for v0 as well as γ based on parameters, such as magnetic field spectra. We analyze bulk solar wind data and perform a detailed comparison between models and the data.