Heliospheric Hydrogen as a Probe of the Heliopause and Interstellar Magnetic Field

The physical extent of the heliosphere is defined by the interaction between plasma populations in the solar wind and local interstellar medium (LISM), with the size and shape of the boundary region determined locally by the dynamic and magnetic pressures on either side and by the magnitude of the projected relative velocity vectors of both media. Neutral hydrogen in the LISM also plays an important role via resonant charge exchange (RCE) with plasma on both sides of the border that acts to increase pressure upstream and decrease it in the heliosphere. The nearest part of the boundary is ~100 AU in the 'upwind' direction defined by the relative motion of the LISM and solar system. Since these distances are difficult to reach with in situ spacecraft, the heliopause has been probed by observing the neutral LISM after it enters the solar system. This entry affects the primary LISM constituents (H and He) differently, with the H velocity distribution being altered via RCE with decelerated LISM protons. Measurements of the Doppler shift of scattered solar Ly-α emission show that RCE decelerates the interplanetary hydrogen (IPH) relative to He (from Ulysses) and the bulk LISM. They also imply a deflection in the H vs He flow directions and an asymmetry in the brightness distribution of the scattered background, which models tie to the strength and alignment of the interstellar magnetic field (IMF). These same models show a deflection from the expected upwind IPH direction that can be mapped spectrally with ~1 km/sec precision. More recently, the IBEX satellite has revolutionized our understanding of the heliopause via detection and mapping of energetic neutrals, including H and He. They find conditions consistent with shifted upstream LISM flow direction and velocities 10-15% slower than those based on earlier measurements. This result is significant for the structure of the boundary, because the implied 25% reduction in plasma pressure will reduce the Mach number of in the upstream direction, potentially eliminating the need for a fast upstream bowshock. Later theoretical work has suggested that a variety of shock conditions are possible, depending on the LISM velocity and the angle of the interstellar magnetic field with respect to the LISM flow direction. The IBEX results were tested using new IPH spectra obtained with HST-STIS in 2012 and 2013. They returned results consistent with the earlier studies showing higher velocities in the LISM, but they are affected by solar maximum conditions, which act to decelerate the flow and by the spectral resolving power that have limiting velocity precision of 1-2 km/sec. They also but lack the spatial sampling to map the upstream deflection, and hence the IMSF. The proposed research program aims to further refine the extent of the discrepancy between the IBEX LISM velocity and STIS. Both aims will be achieved using a sounding rocket experiment built around an all-reflective spatial heterodyne spectrometer (ARSHS). ARSHS is an inteferometric technique that represents a significant advance for high resolving power measurements of diffuse vacuum ultraviolet emission line targets. The étendue (field of view times collecting area) of ARSHS for wide-field sources is more than three orders of magnitude greater than HST-STIS while providing >4x resolving power of R ~120000. As designed, the ARSHS provides limiting velocity of ~0.3 km/sec for spectral line centroiding in a 30 second exposure. We will fly this experiment in 2018 near solar minimum, when the measured IPH velocity is least affected by inner solar system forcing, to obtain 8 spectra along different lines to sight. These will be used to provide an improved lower limit to the LISM velocity and to identify the deflection of H and He flow vectors. More »