2015ApJ...815...20M

We provide a proof of concept that the pickup ion (PUI) cutoff speed variation with ecliptic longitude can provide the interstellar neutral (ISN) flow longitude λ_ISN∞ outside the heliosphere with a precision of the order of 0.°1. We compare 2007-2014 STEREO A PLASTIC observations in the solar wind frame with a simple analytical cutoff model and perform a Pearson correlation of the cutoff as a function of longitude with those values mirrored about a location λ_M that slides in longitude. The resulting maximum correlation at λ_M= 255.°21±0.°04 reflects the ISN upwind direction with a purely statistical uncertainty. This result is consistent with recent ISN flow longitude values, but a precision determination requires the evaluation of underlying systematic effects. Obtaining a precision ISN flow longitude is highly complementary to the IBEX ISN observations, which return a narrow tube in the ISN parameter space that couples flow speed and longitude tightly, but leaves a substantially larger error bar along the tube. The ISN flow provides one cardinal axis of the heliosphere, with a strong leverage on the plane subtended by the ISN velocity and the interstellar magnetic field vector, which controls the shape of the heliosphere and its interaction with the interstellar medium. In addition, a discussion about potential temporal variations of the ISN flow has started. Both issues require a precision determination of the ISN flow velocity over decade-long time intervals. ACE SWICS, STEREO PLASTIC, and SOHO CTOF PUI data allow such long-term studies over almost two decades.