
Variation of the observed H+ PUI tail number density fraction (filled blue circle) with the shock compression ratio. The variation of reflected H+ PUI density fraction from the theory of electrostatic CSP (filled red circle) and its possible range based on the 16th–84th percentile value of the magnetic field magnitude over 23.5–37.0 au is also shown (red area around the red circle).
Shrestha et al. performed a detailed analysis of five distant interplanetary shocks observed by the Solar Wind Around Pluto instrument on board New Horizons, which exhibit the signature of a suprathermal H+ pickup ion (PUI) tail in the downstream distribution. These shocks were observed with a PUI data cadence of approximately 24 hr, covering a heliocentric distance range of 23.71–36.75 au. They found that the H+ PUI density and temperature show a gradual increase across the shock, while the H+ solar wind density shows erratic behavior without a distinct downstream compression. The H+ PUI cooling index variation across the shock displays different characteristics in each shock. Their study demonstrated, for the first time, the variation of the number density of downstream H+ PUI tails with the shock compression ratio, revealing an increase in tail density with stronger shocks. Additionally, their theoretical estimates of reflected PUI number densities derived from the electrostatic cross-shock potential agree very well with the observed H+ PUI tail densities for stronger shocks.