Fig. 2

Left histograms of the solar wind thermal proton temperature distributions observed by Voyager 2 across the HTS measured in the SW and IHS: (black) solar wind distribution, (red) IHS distribution, and (blue) distribution of the solar wind temperature multiplied by 13, the ratio between the upstream solar wind and downstream IHS temperatures. No reflected solar wind protons can be identified from the distribution function (Richardson 2008). Center the IHS constructed proton distribution (black curve) assuming that the transmitted but not reflected PUIs evolve into a Maxwellian distribution. The blue curve shows a \(\kappa\)-distribution with a value of \({-}1.63\). The black curve is the superposition of transmitted solar wind protons, transmitted PUIs, and reflected and then transmitted PUIs. The red curve is a Maxwellian distribution assuming the observed downstream density and temperature. The particle velocity \(v_{x}\) is normalized to the Maxwellian thermal speed \(v_{th} = \sqrt{2kT/m_{p}},\) where k is Boltzmann’s constant, \(m_{p}\) the proton mass, and T the total downstream temperature (Zank et al. 2010). Right one-dimensional cut of the proton velocity distribution function downstream of the HTS. The particle velocities are normalized by the upstream flow speed \(V_1\) in the shock rest frame. The solar wind protons and the PUIs are identified by the solid and dashed curves, respectively, and the thick black curves are their sum (Oka et al. 2011)