The Effect of SW Compression Regions on the PUI He+ Cutoff Determined ISN Inflow Direction

We characterize the effect solar wind (SW) compression regions have on the interstellar neutral ( ISN ) inflow direction as determined by the longitudinal variation of the pickup ion ( PUI ) cutoff. PUIs are former neutral atoms that are ionized in the inner heliosphere from the interstellar neutral wind. The interstellar neutral wind has a defined flow direction due to the Sun's movement relative to the partially ionized interstellar medium. Coupled with gravitation lensing , this flow drives a systematic spatial variation in the PUI's radial neutral injection speed which translates to a shift in the maximum PUI "cutoff" speed that is longitudinally symmetric around the ISN flow direction. The PUI cutoff's variation as a function of elliptic longitude can thus be exploited as a statistically precise method of determining the ISN inflow direction, but, because the cutoff speed is still subject to SW related effects, variations in the SW flow still manifest as systematic error. Stream interaction regions, for one, have been shown to be effective at energizing The core PUI distribution. In an effort to understand the effects of SW variation we use STEREO-A PLASTIC He+ PUI observations, between 2007 to 2013, to characterize the impact of stream-stream interaction regions on the PUI cutoff determined ISN flow direction. Due to their stochastic distribution around the spacecraft orbit, we find that removing the compression periods does not affect the measurement error, for the full 7 year-integrated data set. But, for individual orbits, we find that the compression removal does reduce the year to year variability. Over short time periods, and in particular, orbits outside of a peak or well of a solar cycle, compressions are not necessarily stochastically distributed, and need to be accounted for. Additionally, the reduced variation of the annual measurement suggests that the ISN inflow direction is constant over our observation period. To preserve statistics, we use knowledge of the systematic response of the PUI cutoff to compressions of varying strength to develop a heuristic method of correcting for the compression effects without removing the impacted periods. Finally, we discuss implications for our understanding of PUI cutoff evolution and determination of the ISN inflow direction.