Flow of neutral interstellar helium into the heliosphere as inferred from IBEX-Lo observations and simulations

Previously, a team coordinated through the International Space Science Institute (ISSI) examined direct neutral gas, pickup ion, and UV backscatter observations to produce consensus values for the inflow direction (λ, β), speed v, and temperature T of neutral interstellar helium from the Local Interstellar Cloud (LIC) with relatively small uncertainties. Since then, NASA’s Interstellar Boundary Explorer (IBEX) has started to provide new observations of the interstellar He flow in Earth orbit, with data currently available from the spring seasons of 2009 and 2010. Using a test-particle simulation to compute the spin-phase distributions that IBEX observes during each orbit, we optimize input LIC parameters to best fit the observations. The simulations take into account actual ionization rates as derived from solar EUV observations by SOHO CELIAS SEM, the OMNI solar wind data set, the positions and velocity vectors of the Earth during the actual integration intervals for each orbit, and the IBEX-Lo field-of-view. The simulations were performed on a grid of bulk flow vectors and temperatures, starting with the ISSI team consensus values based most heavily on Ulysses GAS observations (v = 26.4 km/s, T = 6318 K, and λ = 255.4o, β = -5.31o in J2000 coordinates). The Mach number of the flow derived from the IBEX observations is generally lower than derived from Ulysses measurements, which would be consistent with a higher temperature (up to 10 000 K) and/or a lower flow speed (down to ≈22.5 km/s) of the LIC. Based on these findings, the possible LIC parameter sets most probably lie within a narrow range between (λ = 255.4°, β = 5.3°, v = 26.4 km/s, M = 4.5 or T = 10,000 K) and (λ = 261.2°, β = 4.9, v = 23.05 km/s, M = 4.9, or T = 6300 K). At one end of the acceptable range, the parameters agree with the previous values except for the temperature, and at the other end, the temperatures agree, but the direction is different by almost 6° and the velocity is lower by ≈4 km/s. The simulations appear to agree with the IBEX observations slightly better for the different inflow direction. The results obtained separately from the two passes through the He flow (in 2009 and 2010) are identical within observational uncertainties. Potential reasons for the differences from previous results will be discussed. Any modifications in these critical inflow parameters will require modification of current global heliosphere models. In particular, a reduced flow speed and thus reduced ram pressure would require an increase in the total LIC density and/or the magnetic field strength to maintain pressure balance at the heliopause.