SH41D-2398
Interstellar Flow Longitude from Pickup Ion Cut-off Observations at 1 AU
Thursday, 17 December 2015
Poster Hall (Moscone South)
Eberhard Moebius, University of New Hampshire, Institute for the Study of Earth, Oceans and Space, and Department of Physics, Durham, NH, United States, Martin A Lee, Univ of New Hampshire, Durham, NH, United States, George Gloeckler, University of Michigan Ann Arbor, Ann Arbor, MI, United States and Christian Drews, University of Kiel, Kiel, Germany
Abstract:
The precise determination of the interstellar neutral (ISN) flow direction is important in several different ways. As a cardinal axis of the heliosphere it has strong leverage on the plane subtended by the ISN velocity and interstellar magnetic field vector, which controls the shape of the heliosphere and its interaction with the interstellar medium. The observation of the ISN flow through the heliosphere for several decades has initiated a discussion about potential temporal variations in the ISN flow. To tackle these questions, a precision measurement of the ISN flow velocity vector is needed over a long time period. Recent efforts to obtain a consistent ISN vector and temperature with Ulysses and IBEX point to remaining uncertainties and potential systematic effects. In particular, IBEX measurements provide a very precise relation between ISN flow longitude and speed via the hyperbolic trajectory equation, but they contain larger uncertainties separately for longitude and speed. Pickup ion (PUI) observations of the ISN flow pattern at 1 AU can provide a complementary determination of the flow longitude with high precision. The interstellar PUI cut-off speed is a function of the ratio of the radial ISN flow component and the solar wind speed at the observer location [Möbius et al., 1999, GRL, 26, 3181]. We have compared STEREO A PLASTIC observations with a simple analytic model of the cut-off and performed a Pearson correlation analysis of the cut-off as a function of ecliptic longitude with its mirrored function. The two complementary approaches demonstrate that the ISN flow longitude can be obtained with a precision on the order of 0.1o. The cut-off speed is much less sensitive to systematic effects on PUIs, such as variations in the solar wind parameters, ionization, and transport. ACE SWICS, STEREO PLASTIC, and SOHO CTOF data are available that span almost two decades, which will allow long term studies with high precision. The availability of O and Ne PUI data with high counting statistics from PLASTIC and CTOF will also allow the determination of the deflection of secondary O, in comparison with He and Ne.
