Spectro-Polarimetric Observations of Interplanetary Hydrogen as a Diagnostic of the Heliopause Plasma Boundary

Neutral interplanetary hydrogen (IPH) does not originate in the solar system, but is dominated by material in the interstellar medium (ISM) that has penetrated into the solar system. This material is not pristine or thermal in its properties, but contains a record of both the ISM and its passage across the broad mixing- plasma boundary formed by the interaction of the Solar wind and the ionized component of the ISM. This record is provided by charge exchange interactions between hydrogen atoms and protons in the boundary region that effectively translate the characteristics of the plasma interaction and boundary region to the non- interacting neutral population. The primary observational technique used to observe IPH is resonance scattering of solar Ly-alpha emission in the optically thick medium. These data show evidence of line of sight dependent line-broadening and deceleration associated with the boundary crossing. Here we describe a new class of instrument, a spatial-heterodyne spectro-polarimeter (SHSPOL) that offers improved diagnostic capabilities for the study of the IPH. SHSPOL combines the high sensitivity and spectral resolving power of SHS with a novel far ultraviolet polarimeter design. Combined, these technologies permit rapid accumulation of IPH line-shape measurements with the added measurement of the offers a new way to explore the radial properties of the IPH along a given line of sight from the scattering angle dependent polarization of the incoming light. We will discuss the principle of SHSPOL and our development of prototype instrument for observing the IPH from a sounding rocket platform.