Pluto's Interaction With Energetic Heliospheric Ions

Pluto energies of a few kiloelectron volts and suprathermal ions with tens of kiloelectron volts and above. We measure this population using the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument on board the New Horizons spacecraft that flew by Pluto in 2015. Even though the measured ions have gyroradii larger than the size of Pluto and the cross section of its magnetosphere, we find that the boundary of the magnetosphere is depleting the energetic ion intensities by about an order of magnitude close to Pluto. The intensity is increasing exponentially with distance to Pluto and reaches nominal levels of the interplanetary medium at about 190RP distance. Inside the wake of Pluto, we observe oscillations of the ion intensities with a periodicity of about 0.2 hr. We show that these can be quantitatively explained by the electric field of an ultralow‐frequency wave and discuss possible physical drivers for such a field. We find no evidence for the presence of plutogenic ions in the considered energy range. Plain Language Summary Space around Pluto is not entirely empty but filled with solar wind plasma and ions that originate from interstellar space and are pushed outward by the solar wind. All planetary bodies interact with their surrounding medium. In the case of a magnetized body like Earth, this interaction is strong and creates a magnetosphere. Unmagnetized bodies like that of the dwarf planet Pluto have a much weaker interaction. What makes Pluto special is that it is far outside in our solar system and therefore embedded in relatively high intensities of interstellar ions. When New Horizons passed Pluto and measured the distribution of these ions, we found that Pluto is forming a wake in the interstellar ion flow. It is more difficult to deflect the motion of the relatively fast‐moving interstellar ions than it is to deflect the lower‐energy solar wind. Therefore, it was not obvious that we would observe this. Even Pluto's wake is not entirely empty because some interstellar ions do manage to enter. Another finding was that a wave is propagating within the tenuous medium of the wake. This wave modulates the ion intensities resembling a sound wave propagating through air and modulating the gas density. Key Points Pluto forms a wake for energetic heliospheric ions Waves cause ion intensity oscillations in the wake