INTERPLANETARY MAGNETIC FIELD SECTOR FROM SOLAR WIND AROUND PLUTO (SWAP) MEASUREMENTS OF HEAVY ION PICKUP NEAR PLUTO

ABSTRACT On 2015 July 14, the New Horizons spacecraft flew by the Pluto system. The Solar Wind Around Pluto (SWAP) instrument on board New Horizons, which detects ions in the energy per charge range ∼0.035 to 7.5 keV/q, measured the unique interaction between the solar wind and Pluto's atmosphere. Immediately after the closest approach, SWAP detected a burst of heavy ion counts when the instrument's field of view (FOV) was aligned north and south of the Sun–Pluto line and approximately normal to the solar wind flow direction, suggesting their origin as heavy neutral atoms from Pluto that were ionized and being picked up by the solar wind. The trajectories of heavy pickup ions depend on the interplanetary magnetic field (IMF). New Horizons is not equipped with a magnetometer, and we cannot directly measure the IMF. However, we can utilize SWAP's measurements and instrument FOV during this brief period of time to determine the most likely sector of the IMF that could reproduce SWAP's observations of heavy ion pickup. We find that the IMF was most likely in an outward sector, or retrograde to the planets’ motion, during the Pluto encounter, and that the heavy ions detected by SWAP are more likely CH 4 + than N 2 + . This supports the existence of a methane exosphere at Pluto.