Interstellar He Parameters in Front of the Heliosphere: View from Ibex and Ulysses

Abstract:

The initial analyses of the Ulysses and IBEX interstellar neutral (ISN) gas observations showed small but significant differences in the ISN flow parameters. In particular, the IBEX observations have defined a narrow and tightly coupled tube in the 4-dimensional ISN parameter space (temperature and inflow speed, longitude, and latitude), which extended to the original Ulysses flow vector but with a somewhat different optimum set of values. However, adopting the Ulysses velocity vector resulted in a significantly higher temperature. Interestingly, the optimum flow vector obtained with IBEX resulted in the same temperature obtained from the Ulysses GAS analysis. These intriguing results were the starting point for a hypothesis that then was tested with a literature study from the past four decades, i.e., that the flow direction of interstellar He may be changing over time.

Here, we analyze the data from both experiments using the Warsaw Test Particle Model, i.e., same simulation program and ionization rate model. For Ulysses GAS, we use the original data set and, in addition, the same set reprocessed with improved information on the instrument pointing in the sky. For IBEX, we model a subtle, previously unaccounted for data throughput issue for the first two observation seasons, and we analyze, for the first time, data from the two most recent seasons, when the data throughput issue was eliminated through a mode change and when the IBEX pointing strategy was changed to improve the sensitivity of the observations to tighten the possible range of the ISN flow parameters. In addition, we have refined the data uncertainty system, improved the analysis of the IBEX pointing information, and we now explicitly take into account the newly discovered Warm Breeze.

In combination, we present the ISN velocity vector relative to the Sun and the He temperature for the entire Ulysses GAS observations and the past two IBEX-Lo observation seasons as a time series from 1994 until 2014.