WARMER LOCAL INTERSTELLAR MEDIUM: A POSSIBLE RESOLUTION OF THEULYSSES-IBEXENIGMA

Abstract

Interstellar Boundary Explorer (IBEX) measurements from 2009-2010 identified a set of possible solutions with very tight coupling between the interstellar He inflow longitude, latitude, speed, and temperature. The center of this allowable parameter space suggested that the heliosphere could be moving more slowly and in a slightly different direction with respect to the interstellar medium than indicated by earlier Ulysses observations. In this study we examine data from 2012-2014 and compare results from an analytic analysis and a detailed computer model. For observations where the IBEX spacecraft pointing is near the ecliptic plane, the latest measurements indicate a different portion of IBEX's four-dimensional tube of possible parameters—one that is more consistent with the Ulysses flow direction and speed, but with a much higher temperature. Together, the current combined IBEX/Ulysses values we obtain are V ISM∞ ~ 26 km s–1, λISM∞ ~ 75°, βISM∞ ~ –5°, and T He∞ ~ 7000-9500 K. These indicate that the heliosphere is in a substantially warmer region of the interstellar medium than thought from the earlier Ulysses observations alone, and that this warmer region may be roughly isothermal. However, measurements taken when IBEX was pointing ~5° south of the ecliptic are inconsistent with this solution and suggest a slower speed, lower temperature, and flow direction similar to IBEX's prior central values. IBEX measures much deeper into the tails of the distributions of the inflowing interstellar material than Ulysses did and these observations indicate that the heliosphere's interstellar interaction is likely far more complex and interesting than previously appreciated.