Local Interstellar Cloud Temperatures as Observed with IBEX
Monday, 15 December 2014
The Interstellar Boundary Explorer (IBEX) observes the interstellar neutral gas flow trajectories at their perihelion in Earth’s orbit every year from December through early April, when the Earth’s orbital motion is into the oncoming flow. These observations have defined a narrow region of possible, but very tightly coupled interstellar neutral flow parameters, with inflow speed, latitude, and temperature as well-defined functions of inflow longitude for the local interstellar cloud (LIC) at the location of the Sun. The temperature of each observed neutral gas species is determined from the angular distribution of the incoming flow, which represents a Mach cone that is related to the locally observed bulk flow speed. With the large collecting power of the IBEX-Lo sensor, the statistical uncertainties of the observed angular distributions are very small (typically a few %). For He and the combination of O and Ne, the observed Mach cones scale inversely with the mass of these species, thus indicating isothermal conditions for them in the LIC. After eliminating all potential sensor related contributions, the main source of uncertainty is attributed to a degeneracy along the aforementioned, coupled parameter region This degeneracy is due to the limited observation range along the Earth orbit each year. Making use of variations in the spin axis pointing of IBEX, the possible range of LIC parameters along the coupled parameter region has been refined. One of the key results is a substantially higher temperature than reported previously based on Ulysses GAS observations. Such local temperature measurements provide an independent and likely the most detailed determination of LIC temperatures, which will then be placed into context with astronomical measurements that represent values averaged over long lines-of-sight.