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\begin{document}

\title{SIHLA , a Mission of Opportunity to L1 to Map H Lyman Alpha Emissions
from the Heliopause, the Interplanetary Medium, the Earth's Geocorona
and Comets}



\author[1]{Larry Paxton}%
\author[2]{Elena Provornikova}%
\author[1]{ed roelof}%
\author[3]{eric quemerais}%
\author[4]{vlad izmodenov}%
\author[5]{olga katushkina}%
\author[6]{ed mierkiewicz}%
\author[5]{igor baliukin}%
\author[7]{mike gruntman}%
\author[8]{makato taguchi}%
\author[9]{wayne pryor}%
\author[10]{majd mayyasi}%
\author[3]{dimitra Koutroumpa}%
\author[11]{merav opher}%
\author[12]{rosine lallement}%
\author[13]{aroh barjatya}%
\author[14]{ron vervack}%
\author[1]{carey lisse}%
\author[1]{robert schaefer}%
\author[1]{rob barnes}%
\author[15]{brian wood}%
\affil[1]{jhu/apl}%
\affil[2]{JHU/APL}%
\affil[3]{LATMOS}%
\affil[4]{IKI}%
\affil[5]{iki}%
\affil[6]{ERAU}%
\affil[7]{usc}%
\affil[8]{rikkyo university}%
\affil[9]{SET}%
\affil[10]{boston university}%
\affil[11]{Boston University}%
\affil[12]{Service d' Aeronomie du CNRS}%
\affil[13]{Embry Riddle Aeronautical University}%
\affil[14]{jkhu/apl}%
\affil[15]{naval research laboratory}%


\vspace{-1em}



  
  \date{June 13, 2024}


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\begin{abstract}
SIHLA (Spatial/Spectral Imaging of Heliospheric Lyman Alpha pronounced
as `Scylla' {[}e.g. Homer, Odyssey, \textasciitilde{}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 \textgreater{}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.%
\end{abstract}%



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