Time delay between outer heliosheath crossing and observation of interstellar neutral atoms

Abstract

In situ measurements of the heliospheric particle populations by the Voyager spacecraft can only be put in an appropriate context with remote-sensing observations of energetic and interstellar neutral atoms (ENAs and ISN, respectively) at 1 au when the time delay between the production and the observation times is taken into account. ENA times of flight from the production regions in the heliosheath are relatively easy to estimate because these atoms follow almost constant speed, force-free trajectories. For the ISN populations, dynamical and ballistic selection effects are important, and times of flight are much longer. We estimate these times for ISN He and H atoms observed by IBEX and in the future by IMAP using the WTPM model with synthesis method. We show that for the primary population atoms, the times of flight are on the order of three solar cycle periods, with a spread equivalent to one solar cycle. For the secondary populations, the times of flight are on the order of ten solar cycle periods, and during the past ten years of observations, IBEX has been collecting secondary He atoms produced in the OHS during almost entire 19th century. ISN atoms penetrating the heliopause at the time of Voyager crossing will become gradually visible about 2027, during the planned IMAP observations. Hypothetical variations in the ISN flow in the Local Interstellar Medium are currently not detectable. Nevertheless, we expect steady-state heliosphere models used with appropriately averaged solar wind parameters to be suitable for understanding the ISN observations.