Observations of the Lyman-a emissions from Interplanetary Hydrogen (IPH) atoms are made from Mars’ orbit using a high spectral resolution instrument in echelle configuration. The measurements can uniquely be used to resolve IPH from planetary H emissions and to subsequently determine the brightness, velocity, and thermal broadening of the IPH flow along the instrument line of sight. Planned as well as serendipitous observations, both upwind and downwind of the flow, are analyzed to determine these IPH properties and to examine the variability of IPH brightness with solar activity through the declining phase of Solar Cycle 24. A heliospheric interface model was used to simulate and interpret the derived IPH properties. The results show that the IPH brightness trends with solar irradiance, the flow is fainter downwind than upwind, the IPH brightness is variable and non-negligible compared with planetary emissions, and that deriving thermal properties of IPH requires higher spectral resolution than is presently available. These results can improve the theoretical understanding of solar system dynamics by providing empirical constraints to simulations from the inner boundary of the heliosphere and can guide the development of future interplanetary missions.

SIHLA (Spatial/Spectral Imaging of Heliospheric Lyman Alpha pronounced as ‘Scylla’ [e.g. Homer, Odyssey, ~675-725 BCE] investigates fundamental physical processes that determine the interaction of the Sun with the interstellar medium (ISM); the Sun with the Earth; and the Sun with comets and their subsequent evolution. To accomplish these goals, SIHLA studies the shape of the heliosphere and maps the solar wind in 3D; characterizes changes in Earth’s extended upper atmosphere (the hydrogen ‘geocorona’); discovers new comets and tracks the composition changes of new and known ones as they pass near the Sun. SIHLA is a NASA Mission of Opportunity that has just completed its Phase A study (the Concept Study Report or CSR). At the time of the writing of this abstract NASA has not decided whether to fly this small satellite mission or its competitor (GLIDE: PI Prof. Lara Waldrop). SIHLA observes the ion-neutral interactions of hydrogen, the universe’s most abundant element, from the edge of the solar system to the Earth, to understand the fundamental properties that shaped our own home planet Earth and the heliosphere. From its L1 vantage point, well outside the Earth’s obscuring geocoronal hydrogen cloud, SIHLA maps the entire sky using a flight-proven, compact, far ultraviolet (FUV) hyperspectral imager with a Hydrogen Absorption Cell (HAC). The hyperspectral scanning imaging spectrograph (SIS) in combination with the spacecraft roll, creates 4 maps >87% of the sky each day, at essentially monochromatic lines over the entire FUV band (115 to 180nm) at every point in the scan. During half of these daily sky maps, the hydrogen absorption cell (HAC) provides a 0.001nm notch rejection filter for the H Lyman a. Using the HAC, SIHLA builds up the lineshape profile of the H Lyman a emissions over the course of a year. SIHLA’s SIS/HAC combination enables us to image the result of the ion-neutral interactions in the heliosheath, 100 AU away, in the lowest energy, highest density, part of the neutral atom spectrum – H atoms with energies below 10eV. The novel aspects of SIHLA are the scope of the science done within a MoO budget. The SIHLA projected costs were below the $75M cap with a 31.3% reserve for Phase B-D. The re-purposing of a spectrographic that was part of the DMSP SSUSI line (a copy was flown and NASA TIMED/GUVI and as NASA NEAR/NIS). Risk is extremely low in this Class-D mission with all major elements at least at TRL6 at this time. SIHLA has a high potential for discovery. We expect that we will 1) First detection of the hot H atoms produced directly from the ion-neutral interactions at the heliopause; 2) First detection of structures in Interplanetary Medium H emission, 3) First detection of response of the Earth’s extended (out to lunar orbit) geocorona to solar/geomagnetic drivers, 4) New UV-bright comets as they enter the inner solar system. SIHLA is a hyperspectral imager; at every point in the sky SIHLA obtains the entire FUV spectrum.

The Earth’s magnetosheath and cusps emit soft X-rays due to the interaction between highly charged solar wind ions and exospheric hydrogen atoms. The LEXI and SMILE missions are scheduled to image the Earth’s dayside magnetosphere system in soft X-rays and thus to investigate global-scale magnetopause reconnection modes under varying solar wind conditions. The exospheric neutral hydrogen density distribution is an important consideration in the calculation of X-ray emissivities. The value of this density at the subsolar magnetopause is of particular interest for understanding X-ray emissions near this boundary, and is used as a comparison between competing models of hydrogen distribution. This paper estimates the exospheric density during solar minimum by using X-ray Multimirror Mission (XMM) astrophysics observations. We searched 11 years of XMM soft X-ray data and provided a list of 193 events with a possible detection of X-rays of magnetospheric origin. These events occurred during relatively constant solar wind and interplanetary magnetic field conditions. During these events the location of the magnetopause was measured in-situ by heliospheric missions. Thus the location of the solar wind ions responsible for the magnetospheric emission are well constrained by observation. We detected one particular event on 12-Nov-2008 and and estimated an exospheric density using the Open Geospace Global Circulation Model and a spherically symmetric exosphere model. The OpenGGCM magnetosheath parameters were used to disentangle soft X-rays of exospheric origin from the XMM signal. The lower limit of the exospheric density of this solar minimum event is 36.8 cm$^{-3}$ at 10 $R_E$ subsolar location.