Characterizing the abundance of atmospheric hydrogen (H) at Mars is critical for determining the current and, subsequently, the primordial water content on the planet. At present, the atmospheric abundance of Martian H is not directly measured but is simulated using proprietary models that are constrained with observations of H Lyman-a emission brightness, as well as with observations of other atmospheric parameters, such as temperature and solar UV irradiance. To make the data needed to model H abundances and escape rates more accessible to the community, this work utilizes over nine years of observations of H Lyman-a emissions made with the Mars Atmosphere and Volatile Evolution (MAVEN) mission. The H brightness in the upper atmosphere of Mars is analyzed for statistical variability across multiple variables and found to be dependent on solar illumination, solar cycle, and season. The resulting data trends are used to derive empirical fits to build a predictive framework for future observations or an extrapolative tool for primordial estimates. Data that was intentionally not included in the empirical derivations are used to validate the predictions and found to reproduce the H Lyman-a brightness to within 18% accuracy, on average. This first of its kind predictive model for H brightness is presented to the community and can be used with atmospheric models to further derive and interpret the abundances and escape rate of H atoms at Mars.

We evaluate for the first time the large-scale influence of the upstream interplanetary magnetic field (IMF) magnitude and orientation on Martian proton aurora. We identify a correlation between proton aurora activity and IMF magnitude that varies seasonally, with highest correlations around Mars southern summer solstice when proton aurora activity reaches an annual high. We identify preferentially increased proton aurora emission enhancements at IMF cone and clock angles consistent with the shape of the Parker spiral at Mars’s heliocentric distance. An increased proton aurora occurrence rate is observed for near-radial IMF orientations, particularly evident when the hydrogen corona and bow shock are annually decreased. Lastly, our results suggest that dayside magnetic reconnection may influence proton aurora, which are observed to have preferentially higher occurrence under strong ±Bz orientations. These findings enhance our understanding of the upstream IMF’s influence on Martian proton aurora as an important driver for auroral brightness and occurrence.

We compare observations of hydrogen (H) and protons associated with Martian proton aurora activity, co-evaluating remote sensing and in situ measurements during these events. Following the currently understood relationship between penetrating protons and H energetic neutral atoms (ENAs) in the formation of proton aurora, we observe an expected correlation between the H Lyman-alpha (Ly-α) emission enhancement (used herein as a proxy for H-ENAs) and penetrating proton flux. However, we observe a notable spread in the trend between these two datasets. We find that this spread is contemporaneous with one of two major impacting events: high dust activity or extreme solar activity. Proton aurora events exhibiting a relative excess in penetrating proton flux compared to Ly-α enhancement tend to correspond with periods of high dust activity. Conversely, proton aurora events exhibiting a relative deficit of penetrating proton flux compared to Ly-α enhancement are qualitatively associated with periods of extreme solar activity. Moreover, we find that the largest proton aurora events occur during concurrent dust storm and solar events, primarily due to the compounding intensified increase in H column density above the bow shock. Finally, we present a simplified empirical estimate for Ly-α emission enhancement during proton aurora events based on observed penetrating proton flux and a knowledge of local dust/solar activity at the time, providing a straightforward method for predicting auroral activity when direct observations are not available. The results of this study advance our understanding of the interconnected relationship between H and protons during Martian proton aurora activity.

Gravity waves in the thermosphere of Mars are complex and variable phenomena capable of causing significant changes to processes in the upper atmosphere of Mars, which can affect atmospheric escape. The objective of this study is to determine how both dust storm activity and variation in Local Solar Time (LST) affect thermospheric gravity wave activity. Analyzing in-situ neutral Argon density data from the Mars Atmospheric and Volatile EvolutioN (MAVEN) satellite’s Neutral Gas and Ion Mass Spectrometer (NGIMS), we measure the strength of the gravity wave activity across five nightside observation datasets in a variety of dust conditions: three outside of the Martian Dust season with nominal, low dust conditions, one during the 2018 Global Dust Storm (GDS), and one during the regional C storm observed in Mars Year (MY) 34. From nominal conditions, we find thermospheric gravity wave activity increases on the nightside, as seen in previous studies, but is twice as high post-midnight as it is pre-midnight. During the 2018 GDS, the thermospheric gravity wave activity observed between 22:00 and 06:00 LST is generally consistent with gravity wave activity observed during nominal dust conditions. Between 18:00 and 22:00, however, gravity wave activity during the 2018 GDS is ~7 times higher than the weak activity seen during these LSTs in nominal dust conditions. A similar effect is observed during the MY 34 regional C storm, during which gravity wave activity increased in step with the global dust loading.

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.

A NASA sponsored study conducted at John Hopkins University Applied Physics Lab culminated in a community-inspired heliospheric mission concept called the Interstellar Probe (ISP). The ISP’s science goals include understanding our habitable astrosphere by investigating its interactions with the interstellar medium, and determining the structure, composition, and variability of its constituents. A suite of instruments were proposed to achieve these and other science objectives. The instruments include a Lyman-a spectrograph for velocity-resolved measurements of neutral H atoms. The capability to address key components of the ISP’s science objectives by utilizing high spectral resolution Lyman-a measurements are described in this presentation. These findings have been submitted as a community White Paper to the recent Heliophysics decadal survey.

The upper atmosphere of Mars is directly affected by solar activity and the resulting solar irradiance impinging upon it. Variations in solar forcing can affect the rate at which atmospheric species escape from the planetary system. Remotely sensed observations of the upper atmosphere of Mars have been made during solar activity extrema of Solar Cycles 22 and 24. These observations were made of D and H Lyman-a emissions using the Mars Atmosphere and Volatile Evolution (MAVEN) mission and the Hubble Space Telescope (HST) high resolution spectrographs. Data obtained from the two missions are analyzed and used to derive densities and escape rates of D and H from the martian upper atmosphere. The results show that the properties of these two water-spawned atoms vary with solar cycle, and display significant inter-annual variability, mainly due to variations in atmospheric temperature. The findings suggest that cooler atmospheric temperatures due to reduced solar EUV flux may enhance the abundance of H atoms in the upper atmosphere of Mars, yet this does not increase their escape rates.

In situ measurements of ionospheric and thermospheric temperatures are experimentally challenging because orbiting spacecraft typically travel supersonically with respect to the cold gas and plasma. We present O2+ temperatures in Mars’ ionosphere derived from data measured by the SupraThermal And Thermal Ion Composition (STATIC) instrument onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. We focus on data obtained during nine special orbit maneuvers known as Deep Dips, during which MAVEN lowered its periapsis altitude from the nominal 150 km to 120 km for one week in order to sample the ionospheric main peak and approach the homopause. We use two independent techniques to calculate ion temperatures from the measured energy and angular widths of the supersonic ram ion beam. After correcting for background and instrument response, we are able to measure ion temperatures as low as 100 K with associated uncertainties as low as 10%. It is theoretically expected that ion and electron temperatures will converge to the neutral temperature at altitudes below the exobase region (~180-200 km) due to strong collisional coupling; however, no evidence of the expected thermalization is observed. We have eliminated several possible explanations for the observed temperature difference between ions and neutrals, including Coulomb collisions with electrons, Joule heating, and heating caused by interactions with the spacecraft. Our current study leaves one plausible heating mechanism, the release of internal energy from O2+ that becomes vibrationally excited as a result of atmospheric chemistry, but future work is needed to assess its validity.

The Mars Atmosphere and Volatile Evolution (MAVEN) mission instrument suite includes an ultraviolet echelle spectrograph with high-spectral resolution designed to resolve D and H Lyman-α emissions. The high-spectral resolution mode was previously characterized in the lab and in the cruise phase to Mars and had been calibrated using observations and models of interplanetary hydrogen Lyman-α emissions. This work presents improved characterizations of the high-spectral resolution mode using in-orbit observations that allow for more robust detections of the faint D Lyman-α emission line. Additionally, the instrument was re-calibrated using simultaneous and comparable observations made with the Hubble Space Telescope high-spectral resolution instrument. Comparisons to Lyman-α observations made with the low-resolution UV channel on the spectrometer, that had been calibrated with stars, showed consistency in the brightness values for measurements obtained at similar observational conditions. The combined upgrades to the faint-emission fitting and new calibration techniques of the MAVEN echelle channel have resulted in an improved data-reduction pipeline with favorable implications for the science utility of D and H Lyman-α emissions.

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.