Determining the Interstellar Wind Longitudinal Inflow Evolution Using Pickup Ions in the Helium Focusing Cone

The size, shape, and composition of the heliosphere are affected by its interaction with the local interstellar medium. A means of quantifying this interaction is by measuring the relative motion of the two media. We determine the flow direction, ${\lambda }_{\mathrm{ISN}}$ , of the interstellar wind through the heliosphere and its trend over an 11 yr solar cycle using Advanced Composition Explorer/Solar Wind Ion Composition Spectrometer He ^+ double-coincidence pickup ion (PUI) measurements from a data set spanning 13 full orbits, with focusing cone crossings occurring between the years 1998 and 2010. We measure the flow direction by fitting a kappa function to the focusing cone signature in the count rates of He ^+ at the PUI cutoff measured as a function of ecliptic longitude. We determine ${\lambda }_{\mathrm{ISN}}$ for each three-orbit boxcar centered on the years 1999 through 2009 and find the trend of the resulting linear fit. We account for solar transients by removing measurements associated with CMEs. However, there still may be effects from compression regions, such as stream–stream and corotating interaction regions. We additionally make considerations to ensure the PUI torus is in view, to account for varying interplanetary magnetic field angles, to ensure that we analyze newly injected interstellar PUIs largely unaffected by transport effects, and to account for general solar activity. We measure a slope in the flow direction to be 0.00° ± 0.51°​​​​ yr ^−1 , indicating that we do not measure a varying trend over this period. We repeat the focusing cone analysis for the combined data set and determine an overall flow direction of 75.37° ± 0.43°.