# Interplanetary Hydrogen Observations of the Emirates Ultraviolet Spectrometer Onboard the Emirates Mars Mission

Raghuram Susarla, Justin Deighan, Michael S Chaffin, Eric Quémerais, Sonal Jain, Robert J Lillis, Gregory Holsclaw, Krishnaprasad Chirakkil, David Brain, Edward D Thiemann, et al.

► To cite this version:

Raghuram Susarla, Justin Deighan, Michael S Chaffin, Eric Quémerais, Sonal Jain, et al. Interplanetary Hydrogen Observations of the Emirates Ultraviolet Spectrometer Onboard the Emirates Mars Mission. Journal of Geophysical Research Space Physics, 2025, 130 (6), pp.e2025JA033903. 10.1029/2025JA033903. hal-05136521

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# JGR Space Physics

## RESEARCH ARTICLE

10.1029/2025JA033903

**Key Points:**

* The observed H Lyman-$\alpha$ and $\beta$ emission brightnesses decrease with increasing angle from the interplanetary hydrogen bulk flow's upwind direction

* Observed sky background intensities are determined by interplanetary hydrogen at aphelion, and by Martian exospheric hydrogen at perihelion

* Modeled H Lyman $\alpha$ intensities are 60% smaller than observations, while they are consistent for H Ly-$\beta$

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

R. Susarla,
raghuramsusarla@gmail.com

**Citation:**

Susarla, R., Deighan, J., Chaffin, M. S., Quemerais, E., Jain, S., Lillis, R. J., et al. (2025). Interplanetary hydrogen observations of the emirates ultraviolet spectrometer onboard the emirates mars mission. *Journal of Geophysical Research: Space Physics*, 130, e2025JA033903. https://doi.org/10.1029/2025JA033903

Received 28 FEB 2025
Accepted 31 MAY 2025

**Author Contributions:**

**Conceptualization:** R. Susarla

**Data curation:** J. Deighan, M. S. Chaffin, S. Jain, G. Holsclaw

**Formal analysis:** R. Susarla

**Funding acquisition:** J. Deighan

**Investigation:** R. Susarla, E. Quemerais

**Methodology:** R. Susarla

**Software:** R. Susarla, J. Deighan

**Supervision:** J. Deighan, M. S. Chaffin, S. Jain

**Visualization:** R. Susarla

**Writing – original draft:** R. Susarla

**Writing – review & editing:**

M. S. Chaffin

©2025. The Author(s).
This is an open access article under the
terms of the Creative Commons
Attribution License, which permits use,
distribution and reproduction in any
medium, provided the original work is
properly cited.

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## Interplanetary Hydrogen Observations of the Emirates Ultraviolet Spectrometer Onboard the Emirates Mars Mission

R. Susarla¹,² ID, J. Deighan¹ ID, M. S. Chaffin¹ ID, E. Quemerais³ ID, S. Jain¹ ID, R. J. Lillis⁴ ID, G. Holsclaw¹ ID, K. Chirakkil¹,² ID, D. Brain¹ ID, Ed. Thiemann¹ ID, F. Eparvier¹, F. Lootah⁵ ID, M. Gacesa⁶ ID, M. O. Fillingim⁴ ID, J. S. Evans⁷ ID, H. AlMazmi⁸ ID, H. AlMatroushi⁵ ID, and M. R. El-Maarry²,⁹

¹Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO, USA, ²Space & Planetary Sciences Group, Khalifa University, Abu Dhabi, UAE, ³LATMOS-OVSQ, Université Versailles Saint-Quentin, Guyancourt, France, ⁴Space Sciences Laboratory, University of California, Berkeley, CA, USA, ⁵Mohammed Bin Rashid Space Centre, Dubai, UAE, ⁶Physics Department, Khalifa University, Abu Dhabi, UAE, ⁷Computational Physics Inc. Springfield, Springfield, VA, USA, ⁸United Arab Emirates Space Agency, Abu Dhabi, UAE, ⁹Department of Earth Sciences, Khalifa University, Abu Dhabi, UAE

**Abstract** The Emirates Mars Ultraviolet Spectrometer (EMUS) aboard the Emirates Mars Mission (EMM) has been studying backscattered interplanetary hydrogen (interplanetary hydrogen (IPH)) Lyman emissions with dedicated observation strategies viz., U-OS3b and U-OS4b. These observation techniques involve looking away from Mars from the EMM's orbit. U-OS3b provides broader coverage of the Martian sky, while U-OS4b is designed to achieve high signal-to-noise measurements. Here we present analysis of the interplanetary hydrogen emission distribution across the sky, particularly at the Lyman-$\alpha$ and $\beta$ emission wavelengths. EMUS observes a well-known reduction in the brightness of interplanetary hydrogen as the observation angle increases across the sky from the hydrogen bulk flow upwind direction. Our modeled emission intensities indicate that when Mars is around aphelion, the observed emissions are dominated by IPH rather than Martian exospheric hydrogen. Our modeled intensities for H Ly-$\alpha$ emission are smaller by a factor of 1.6 compared to the observations, and they are consistent with the observations for H Ly-$\beta$ emission. The H Ly-$\alpha$ intensities measured by the Imaging Ultraviolet Spectrograph (IUVS) on board the Mars Atmosphere and Volatile EvolutionN mission were found to be 45% lower than those from EMUS measurements, likely owing to differences in the instruments' calibration factors. During the observation period, differences in the measured intensities between the Solar Wind ANisotropy (SWAN) instrument aboard the Solar and Heliospheric Observatory and EMUS were also found to vary. These discrepancies can be attributed not only to instrumental factors but also to the relative positions of Earth and Mars.

**Plain Language Summary** Hydrogen atoms in the solar system originate from sources like the solar wind, interstellar medium, and interactions that convert interstellar protons into neutral atoms. Together, these neutral hydrogen atoms within our solar system constitute Interplanetary Hydrogen (IPH) which is flowing through the solar system. The Emirates Mars Ultraviolet Spectrometer (EMUS) onboard EMM, can observe the emissions from IPH across the Martian sky. In EMUS observations, IPH emissions appear brighter when observed in the direction from which the bulk flow of IPH originates, and they appear dimmer when observed in the opposite direction, in alignment with previous findings from Solar Wind ANisotropy (SWAN) and IUVS. We also found that the Martian exosphere has a seasonal influence on our ability to observe the Martian sky background intensities. Our modeled IPH Ly-$\alpha$ emission intensities are smaller by 60% while those IPH H Ly-$\beta$ intensities are consistent with the EMUS observations made when Mars is near aphelion. The observed differences in emission intensities between EMUS, SWAN, and IUVS measurements likely stem from variations in instrument calibration, as well as the locations and viewing directions of the measurements in the inner heliosphere.

## 1. Introduction

Atomic hydrogen is a widespread element in the vast space between celestial bodies within our solar system. As the Sun moves through the interstellar medium, solar radiation, the solar wind, and solar magnetic fields
contribute to the formation of a distinct boundary known as the heliopause, separating the interstellar plasma from plasma of solar origin (solar wind), which constitutes the heliosphere. The solar wind, which flows continuously and radially outward from the Sun at supersonic speeds, encounters interstellar charged particles at the heliopause, where its pressure balances that of the interstellar medium. Before reaching the heliopause, the solar wind is decelerated in a region called the termination shock, beyond which it slows down and becomes subsonic. In the inner heliosheath, located between the termination shock and the heliopause, the decelerated solar wind protons undergo charge exchange with neutral interstellar hydrogen, producing an isotropic source of high-energy neutral hydrogen atoms that eventually move back toward the Sun. In the outer heliosheath, beyond the heliopause, the charge exchange between decelerated interstellar plasma and neutral atoms also produces hydrogen atoms that penetrate the heliosphere. Additionally, a primary population of neutral hydrogen atoms, unimpeded by solar wind and solar radiation, enters the heliosphere directly from the interstellar medium. The neutral hydrogen atoms produced through these various processes throughout the solar system are collectively known as interplanetary hydrogen (interplanetary hydrogen (IPH)). Studying the properties of IPH provides valuable insights into the interactions between the Sun and its surroundings and also useful to derive the properties of the interstellar medium (Baliukin et al., 2022; Bertaux & Blamont, 1971; Izmodenov et al., 2013; Katushkina et al., 2021; Lallement, Bertaux, & Dalaudier, 1985; Lallement, Bertaux, & Kurt, 1985; Lallement et al., 1993; Möbius et al., 2004; Quémerais & Bertaux, 2002; Quémerais et al., 2014; Quémerais, Lallement, Bertaux, et al., 2006; Thomas & Krassa, 1971; Witte, 2004).

The distribution of interplanetary hydrogen in the heliosphere is influenced by several factors, including solar radiation pressure, gravitational force, and the ionization power of the solar wind. Near the vicinity of the Sun, both solar radiation and solar wind ionize the hydrogen atoms, resulting in the formation of an ionization cavity. The cavity size is defined by where the hydrogen concentration drops below 1/e of the unperturbed density in the local interstellar medium. The cavity is smaller in the upwind direction and larger in the downwind direction, with its size also influenced by solar activity (Izmodenov et al., 2013; Quémerais, 2000; Quémerais et al., 2008; Ruciński & Bzowski, 1995; Thomas & Krassa, 1971). The cavity size changes with solar cycle; it extends to ~4–5 AU in the upwind direction, while it can reach up to 25 AU along the downwind direction (Ruciński & Bzowski, 1995; Sokół et al., 2019).

Mapping the distribution of hydrogen involves measuring the intensity of H Ly-α photons at various look directions across the sky. The combined effects of intense solar radiation and the large oscillator strength of atomic hydrogen at a wavelength of 121.6 nm (0.416, Wiese & Fuhr, 2009) cause solar photons to scatter resonantly in all directions off hydrogen throughout the solar system. The backscatter IPH radiation at H Ly-α wavelength is detected by numerous instruments across diverse locations within the solar system and under varying solar activity conditions (Baliukin et al., 2022; Bertaux & Blamont, 1971; Bertaux et al., 1976, 1995; Broadfoot et al., 1977; Bzowski et al., 2012; Clarke et al., 1995, 1998; Mayyasi, Quémerais, et al., 2023; Vincent et al., 2014). It should be kept in mind that the measured IPH intensities of Lyman emissions at a given time depend both on the location of the observer in the solar system and the direction of the line-of-sight, as can be inferred from the modeled distribution of H emissivity.

The Imaging Ultraviolet Spectrograph (IUVS) aboard the Mars Atmosphere and Volatile EvolutionN (MAVEN) mission (McClintock et al., 2015) has measured interplanetary hydrogen (IPH) at the H Ly-α wavelength at Mars during different time periods spanning around four Martian years (MY) (Mayyasi, Clarke, et al., 2023; Mayyasi et al., 2017; Mayyasi, Quémerais, et al., 2023). The high spectral resolution of IUVS/MAVEN, when operating in echelle mode, allows it to separate the IPH Ly-α emission from the Martian exospheric component, enabling the study of IPH properties in the observed region along the instrument's line of sight. These observations show that IPH brightness increases linearly with solar irradiance, and also found that bulk flow brightness is fainter in the downwind direction compared to upwind, and also varies during Solar Cycle 24 (Mayyasi, Quémerais, et al., 2023).

The Solar Wind ANisotropy (SWAN) instrument, aboard the Solar and Heliospheric Observatory (SOHO) spacecraft in a halo orbit around the Sun-Earth L1 Lagrange point, has been extensively mapping the entire sky at the H Ly-α wavelength over last 25 years under different solar activity conditions (Bertaux et al., 1995). Additionally, several other space-based instruments have contributed to the data set of IPH H Lyman observations (Baliukin et al., 2022). If the solar radiation flux from the side of the Sun facing the observer increases, which is a typical response of the Sun during a flare, the IPH shows an increase in the backscattered radiation encountering
that zone (Bertaux et al., 2000; Quémerais & Bertaux, 2002). Previous SWAN/SOHO observations have shown higher intensity at the heliospheric equator during solar maximum, whereas during solar minimum, the intensity at the heliospheric poles exceeds that at the equator (Katushkina et al., 2019). Additionally, these intensities vary according to solar activity conditions. Hence, these observed H Ly-α emission intensity maps, combined with theoretical models, serve as indirect diagnostic tools for studying solar wind parameters such as number density and velocity as a function of heliographic latitude. Moreover, the analysis of IPH emission intensity maps can also help forecast varying solar activity conditions (Bertaux et al., 1996; Katushkina et al., 2019; Quémerais, Lallement, Ferron, et al., 2006). Baliukin et al. (2022) have provided a comprehensive review of SWAN/SOHO and other instrument observations, along with key findings and the pertinent model results.

The “Hope” probe, launched by the Emirates Mars Mission (EMM) (Almatroushi et al., 2021; Amiri et al., 2022) on 20 July 2020, is equipped with three remote sensing instruments: the Emirates eXploration Imager, the Emirates Mars InfraRed Spectrometer, and the Emirates Mars Ultraviolet Spectrometer (EMUS, Holsclaw et al., 2021). The spacecraft's orbit, with a periapsis of ~20,000 km and an apoapsis of ~43,000 km, along with its low orbital inclination of 25°, enables global views of both the Martian lower and upper atmosphere at all local times. EMUS, a far-ultraviolet spectrograph, is designed to remotely sense the emissions from different atmospheric species (CO, Ar, N, C, H, and O) within the wavelength range of ~85–180 nm (Holsclaw et al., 2021). The placement of EMM in a unique orbit with a period of approximately 55 hr enables EMUS to scan emissions from these atmospheric species across the Martian disk and along tangential altitudes ranging from 130 km to over 6 Martian radii. EMUS was specifically designed to analyze the three-dimensional spatial structure and variability of Martian atmospheric species, which fluctuate with the changing seasons. EMUS’ detector is sensitive to extreme ultraviolet wavelengths (<121 nm) and it is the first Mars-orbiting spectrometer working at a high-altitude orbit. EMUS employs various observational modes, each tailored to specific spectral and spatial resolution requirements, depending on the target emission features. These observations extend from the Martian disk to higher altitudes, encompassing both the middle atmosphere and the outer exosphere. The observable emissions for a given target region, along with the necessary instrument requirements, are provided in Table 2, while the instrument design parameters are summarized in Table 5 of Holsclaw et al. (2021). The extremely low background level of the cross-delay line detector used by EMUS has been demonstrated through observations made by a different spectrometer, which employs the same detector design, in measuring faint H₂ EUV/FUV emissions from the lunar atmosphere (Stern et al., 2013). EMUS was designed to measure emission intensities as low as 0.1 Rayleigh and, with sufficient integration time, can observe even fainter signals owing to its low-background photon-counting detector making it particularly suited for studying variations in exospheric emission intensity with a sufficient signal-to-noise ratio (Holsclaw et al., 2021).

Through the examination of emissions from key species in the exosphere, EMUS has been exploring the daylight atmosphere using the cross-exospheric observation strategy known as U-OS4a mode. During U-OS4a cross-exospheric observations, the instrument scans across the daylight Martian exosphere, with the instrument boresight vector in the plane of the spacecraft orbit, perpendicular to both the Mars-Sun line and orbit normal. Similarly, to study three-dimensional structure and temporal variability of hydrogen distribution in the middle and outer exosphere, EMUS scans the Martian atmosphere in asterisk pattern known as U-OS3a observation mode (Holsclaw et al., 2021). To support the U-OS4a and U-OS3a foreground observations, EMUS also incorporates two dedicated observation strategies, viz., U-OS4b and U-OS3b, which enables the observation of the emissions from sky background from various points of Mars orbit, and during different seasons of Mars. The primary objective of designing these observation modes is to differentiate between the hydrogen emission intensities originating from the Martian exosphere and those from the hydrogen that pervades the solar system. Both these modes of EMUS are intended to observe the Martian sky background from the EMM orbit by looking away from the planet. A cartoon depiction of the distribution of atomic hydrogen in the inner solar system, as well as the EMM/EMUS Martian sky-background observation scenarios are presented in Figure 1.

In this study, we presented the EMUS-observed sky background emission intensities of atomic hydrogen as seen from the orbit of Mars and their variations during different Martian seasons. The description of the EMUS observations is outlined in Section 2, and the observed sky emission intensity maps are detailed in Section 3. The variability of these emission intensities across different Martian seasons is discussed in Section 4, and the modeling of U-OS4b observed emission intensities is presented in Section 5. We compared our observed H Ly-α emission intensities with SWAN/SOHO and IUVS/MAVEN measurements that occurred during the EMUS
<figure><img src="image_4.png" /><figcaption>**Figure 1.** An illustrative depiction of interplanetary hydrogen flow and EMUS observations. The central yellow circle represents the Sun, while the red circles along the elliptical orbit indicate Mars' positions at various solar longitudes (Ls). Black circles around Mars represent EMM's orbit. Small gray circles with arrows depict hydrogen flow within the solar system while the green elliptical region highlights the ionization cavity. Sky scanning swaths of EMUS in U-OS3b (blue asterisk pattern) and U-OS4b (blue rectangle) observation modes are also shown here. This portrayal is not drawn to scale.</figcaption></figure>

observation period in Section 6. These findings are summarized, along with future aspects of this work, in Section 7.

## 2. EMUS Sky Background Observations and Data Analysis

EMUS observes the emission intensities of various spectral features in the Martian sky in two observation modes: U-OS3b and U-OS4b. EMUS systematically scanned the Martian sky background in these modes from the last week of April 2021 to the end of December 2023, covering approximately 1.4 MY. The frequency of U-OS3b observations averages about 4 per month, while U-OS4b mode averages about 24 observations per month. The major differences between these observation modes are the areas of the Martian sky they scan and the exposure times for each observation. U-OS3b scans a larger area compared to U-OS4b. U-OS3b observations scan the sky in four swaths, creating an asterisk-shaped coverage pattern, while U-OS4b mode observations covers a smaller region when the instrument pointed outward in the plane of the spacecraft's orbit, as presented in Figure 1. The different scanned regions of the sky and number of spectra obtained during these observation modes for those look directions is described in Figure S1 in Supporting Information S1. Furthermore, U-OS3b observations are much shorter at 0.7 s per integration, whereas U-OS4b mode employs a long-exposure integration time of about 55 s. The spectral resolution of EMUS for these background U-OS3b and U-OS4b observations is 1.8 nm. Sample spectra obtained in these modes are presented in Figure S2 in Supporting Information S1.

In this analysis we have utilized the Level 2a version 4.0 data sets of U-OS3b and U-OS4b observations. Throughout this observational period, the positions of the EMM spacecraft in the X-Y plane of the Mars-centered Solar Orbital (MSO) coordinate system, alongside EMUS' scanned sky coverage in Ecliptic J2000 coordinates are presented in Figure 2. The MSO frame consists of +X axis points from Mars to the Sun, +Z axis perpendicular to Mars' orbital plane (not far from the ecliptic plane), and +Y completing the right-handed coordinate system and oriented opposite the direction of motion of the planet. The origin of this frame of reference is located at center of mass of Mars. The top-left panel of Figure 2 reveals that U-OS3b observations were conducted with the instrument's line-of-sight (LOS) vector $\vec{l}_{emus}$ directed

away from the planet, and scanning the sky background with $\vec{l}_{emus} \cdot \hat{x}_{mso} < 0$ (roughly oriented away from the Sun) and with $\vec{l}_{emus} \cdot \hat{y}_{mso} > 0$ (roughly oriented in the direction of the planet's motion along its orbit). Conversely, for U-OS4b, the instrument observes the sky background with $\vec{l}_{emus} \cdot \hat{y}_{mso} < 0$ (roughly oriented in the opposite direction of the planet's motion along its orbit) and with $\vec{l}_{emus} \cdot \hat{y}_{mso} > 0$, while also pointing away from the planet, as shown in the top right panel in this figure. Here $\hat{x}_{mso}$ and $\hat{y}_{mso}$ are the unit vectors in MSO coordinate system. In each orbit, LOS look directions of each EMUS observation mode enable to scan various regions of the sky background. It is important to note that the instrument's boresight vector follows a designated pointing strategy, covering different sky areas rather than being restricted to a single fixed axis. When EMUS' LOS look direction is oriented with $\pm \vec{l}_{emus} \cdot \hat{y}_{mso}$ ($\vec{l}_{emus} \cdot \hat{x}_{mso}$), the boresight covers a small portion of the sky background within the MSO-XZ (MSO-YZ) plane during each orbit. As Mars moves through different solar longitudes, the EMUS sky background coverage shifts, allowing observations of a broader range of celestial regions over time for corresponding look directions as presented in middle and bottom panels of Figure 2.

The EMUS coverage of background sky for corresponding instrument look directions is depicted in the middle and bottom panels of Figure 2, highlighting that U-OS3b mode coverage is notably larger compared to that of U-OS4b. For a given observation, EMUS U-OS3b (U-OS4b) mode covers about 100° (<1°) in ecliptic longitudes and about 90° (~ 10°) in ecliptic latitudes. Moreover, it should be noted that EMM orbit is inclined about 25° with Martian equator which allows the EMUS instrument to cover off-ecliptic parts of the sky background depending
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Emirates Mars mission (EMM) locations and EMUS coverage of sky background for the U-OS3b and U-OS4b modes over 1.4 Martian years observations. Top panels: Locations of EMM/EMUS around Mars in the Mars-centered solar orbital (MSO) X-Y plane during U-OS3b and U-OS4b observation scenarios. Arrows in these panels indicate the orientation of the instrument's line of sight ($\vec{l}$) directions. Middle and Bottom panels: Sky coverage maps of EMUS background observation modes in Ecliptic J2000 coordinates. Red, black, and Blue colored areas represent the instruments' sky coverage for corresponding look directions for each observation mode. White curves in the U-OS3b sky coverage represent the time progression of the center of the swaths, and dashed magenta curves represent repeated coverage region. Black dots over the scanned areas represent the starting swath center at the initiation of the mission. Curved arrows in middle and bottom panels indicate the direction of movement of the swath center with time, and yellowish-green shaded regions indicate repeated observed regions in the sky coverage since the beginning of observations. Red and blue stars represent the interplanetary hydrogen upwind and downwind locations, respectively. $\vec{l}_{emus}$ is the line of sight vector and $\hat{x}_{mso}$ and $\hat{y}_{mso}$ are unit vectors of MSO coordinate system.</figcaption></figure>

on the location of Mars around the Sun. Due to this EMM orbital inclination, the center of swath shifts between +40° and -40° ecliptic latitudes over extended periods, as illustrated in the middle bottom panels of Figure 2.

Over the course of 1.4 Mars years, the instrument revisits ~40% of the same portions of the sky as observed during the early mission period, denoted by areas shaded in yellow in Figure 2. In this figure we also illustrate that the starting points of EMUS' sky coverage differ for the two different LOS look directions of a given observation mode. It can be noticed here that none of these modes covers the complete sky during the observation period. U-OS4b observations are limited to within ±45° in ecliptic latitude, while U-OS3b observations leave some areas uncovered near the southern ecliptic pole and parts of the Northern Hemisphere. This is mainly because these observation strategies are primarily tailored to support the foreground U-OS4a and U-OS3a observation modes, which are designed to study the variability of exospheric hydrogen, and are not intended to cover the entire sky.
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Temporal progression of spacecraft altitudes and upwind phase angles for U-OS3b (left column panels) and U-OS4b (right column panels) observation modes for the instrument's different LOS look directions throughout the observation period. Vertical bars in the bottom panels represent the upwind phase angle coverage for a given observation. Red, black, and blue colors represent the parameters for corresponding instrument LOS look directions are oriented with $\vec{l}_{emus} \cdot \hat{y}_{mso} > 0$, $\vec{l}_{emus} \cdot \hat{x}_{mso} < 0$, and with $\vec{l}_{emus} \cdot \hat{y}_{mso} < 0$, respectively.</figcaption></figure>

The IPH upwind direction is located at ecliptic longitude and latitude of 252.5° and 8.9°, respectively, whereas the downwind direction is at ecliptic longitude and latitude of 72.5° and -8.9°, respectively in ecliptic J2000 co-ordinates (Lallement et al., 2010). We calculated the angle between the instrument's line of sight and the IPH upwind direction for all U-OS3b and U-OS4b observations, which we refer to as the upwind phase angle. An upwind phase angle of zero indicates that EMUS is observing toward the IPH upwind direction, while an angle of 180° corresponds to looking along IPH downwind flow, and an angle of 90° represents a crosswind direction. The upwind phase angle distribution in the bottom panels of Figure 3 indicate that U-OS4b observations do not directly observe the IPH upwind and downwind directions but cover the cross wind at some periods of obser-vations, whereas U-OS3b mode covers the upwind, downwind and cross wind regions at different Martian seasons. During Mars Year 36, as Mars crossed solar longitudes (Ls) 170° and 350° (Mars ecliptic longitudes 255° and 75°, respectively), EMUS U-OS3b mode mostly focused on the upwind and downwind regions, while the U-OS4b mode observed the cross-wind regions (see bottom panels of Figure 3). When the instrument's LOS look direction is oriented with $\pm \vec{l}_{emus} \cdot \hat{y}_{mso}$, sky background coverage in U-OS4b leads to observation of opposite regions of the sky from the EMM location. This leads to a phase shift of around 180° in the observed upwind phase angles for these look directions, as shown in the lower right panel of Figure 3.

Figure 3 shows that during the Martian perihelion season, several observations were conducted when the spacecraft was orbiting at lower altitudes. During perihelion passage, the Martian exosphere density increases due to heightened insolation and dust activity. This increase in hydrogen density near perihelion seasons was already also noted by various studies (Bhattacharyya et al., 2015; Chaffin et al., 2014; Clarke et al., 2014, 2017, 2024; Halekas, 2017). This may result in the observation of higher emission intensities in these observation modes at these times, because of some exospheric contribution. The impact of spacecraft altitudes on the observations and the effect of seasonal variation on the measured intensities are discussed in Section 4.

The Martian sky background observations occasionally encounter bright stars, particularly near the galactic plane, resulting in intense signal features at different wavelengths. Additionally, H Ly-α photons make a significant contribution as underlying signals to other emission features, which is due to the brightness of Ly-α and the broad wings of the instrument's line spread function (LSF) (see Figure S2 in Supporting Information S1). During the calculation of emission intensities at H Ly-β and Ly-γ wavelengths, we employed the star-removal technique and H Ly-α wing-subtraction, as described in Chirakkil et al. (2024) and Susarla et al. (2024). Additional information
regarding subtraction of H Ly-α stray background and star-removal method is provided in Sections 2 and 3 in SI, respectively.

Unfortunately, the sensitivity of the micro-channel plate detector at H Ly-α significantly diminished after January 2022, leading to reduced reported emission intensities for observations conducted after this date. The impact of this decline in sensitivity is negligible for earlier observations. Therefore, we chose to consider only the H Ly-α emission intensities measured before Jan 2022 when calculating the sky map intensities and for other calculations. Our analysis of H Ly-α focuses on the period when Mars was traveling between Ls 36° and Ls 150° in MY 36, which includes nine months of data from the starting observation to before the onset of the detector burn-in period at H Ly-α wavelength.

The Lyman-α emission is produced by the resonant scattering of solar photons by both atomic hydrogen and its isotope deuterium (D), with a small wavelength difference of 0.033 nm corresponding to the small isotopic difference in energy levels between them (Wiese & Fuhr, 2009). The D/H ratio in the Martian atmosphere has been studied through various observations (Clarke et al., 2017, 2024; Krasnopolsky et al., 1998; Vandaele et al., 2019), yielding an average value of approximately $10^{-3}$ (Clarke et al., 2024). These studies indicate that only high-resolution measurements ($<0.001$ nm, Clarke et al., 2017) can distinguish deuterium's contribution from hydrogen. Typically, the radiance of Lyman-α photons from deuterium is at least an order of magnitude smaller than that from hydrogen when looking at the sunlit thermosphere (Clarke et al., 2017). Moreover, the D/H brightness ratio is expected to be further reduced at EMM spacecraft altitudes due to the mass difference between these species, as D atoms are more strongly gravitationally bound to Mars compared to H atoms. In the optically thin corona region, the observed brightness ratio of deuterium to hydrogen Lyman series emissions cannot exceed the actual deuterium-to-hydrogen abundance, which is very low. Therefore, D exospheric emission is certainly weak and can be neglected.

Similarly, the detection of deuterium in the interplanetary medium suggests that the D/H ratio at 1 au is on the order of $10^{-5}$ (Kubiak et al., 2024; Rodríguez Moreno et al., 2013). Therefore, resolving deuterium's contribution in Lyman-α emission is only possible in high-resolution observations. EMUS was not designed for this purpose. However, since the IPH emission is not far from the optically thin regime and the D/H ratio of emissions is near the D/H density ratio of $10^{-5}$, putting the Ly-α at less than 0.01 R. As a result, the contribution of D in these observations was considered negligible and omitted from this analysis.

### 3. H Lyman α and β Emission Maps

The EMUS observed background emission intensities, in both U-OS3b and U-OS4b modes, at the H Lyman wavelengths for a given look direction could be influenced by several factors: the emission intensity of inter-planetary hydrogen, which varies based on the instrument's alignment with the IPH flow; stellar contributions from the observed region of the sky, and the position of the observer (at Mars) in the solar system (see Figure 10); and the Martian exosphere, which exhibits seasonal variability, even at the spacecraft's altitude. As previously discussed, H Ly-α photons can also contribute as stray light signals to the observed emission intensities at the H Ly-β wavelength. Without applying the star-removal technique and H Ly-α wing-subtraction to the observed data set, the calculated intensity maps of H Ly-α and β emissions averaged over the EMUS observation period are shown in Figure 4. While calculating the H Ly-α intensity map, we use the EMUS data set from the beginning of the mission to before the onset of detector decline at this wavelength. This limitation results in fewer regions of the sky map being available for this emission line compared to the H Ly-β emission map (see Figure 4). In this figure, we can observe intense H Ly-β features associated with hot stars in the galactic plane. In the case of H Ly-α map, a few bright regions are discernible within the galactic plane. The bright features of H Ly-β observed in U-OS4b around ecliptic longitude 80° and ecliptic latitude 20°S sky maps are also associated with the emission features of hot stars present in the Milky Way. Due to insufficient spatial coverage and deletion of planned U-OS4b observations that would observe the bright stars, we could not see any stars in U-OS4b H Ly-α sky map. Deleting bright star observations is essential to safeguard the instrument's sensitive detector from damage. The presence of these bright emission regions associated with the galactic plane is consistent with previous observations made by SWAN/SOHO and other instruments (Baliukin et al., 2022; Quémerais, Lallement, Ferron, et al., 2006).
<figure><img src="image_7.png" /><figcaption>**Figure 4.** Mean emission intensity sky maps for H Lyman α and β emissions without excluding stars and without wing-subtraction in U-OS3b (left panels) and U-OS4b (right panel) observation modes. The mean H Ly-α emission intensity maps are computed using data from the beginning of the mission up until January 2022 in both the observation modes. In contrast, the mean emission intensity for H Ly-β is calculated over the entire EMUS observation period. Red and green points indicate the interplanetary hydrogen bulk flow upwind and downwind locations, respectively. Refer the main text for the explanation of observation period and data gaps in the H Ly-α emission sky map.</figcaption></figure>

By implementing the star-removal technique and H Ly-α stray light subtraction methods as described in Sections 2 and 3 of the SI, the emission intensity maps of H Lyman emissions derived from U-OS3b and U-OS4b observations averaged over the EMUS observation period are plotted in Figure 5. In this Figure, we considered only the data that is observed before the beginning of detector burn-in period while calculating the H Ly-α intensity maps. While our star-removal technique effectively eliminates bright star regions, it may encounter difficulty with faint stars whose intensities are closer to the background IPH intensity. However, we have noticed that their impact on calculating IPH intensities remains minimal and contributes less than 1% to the total signal at H Lyman wavelengths. These observations underscore that the highest Lyman emission intensities are concentrated in the region of IPH upwind direction, while weaker intensities are observed in the opposite, downwind direction. The diminished intensities observed in the downwind direction are attributed to the reduced hydrogen density within the ionization cavity (see Figure 10). As hydrogen atoms traverse the heliosphere, most become ionized in the vicinity of the Sun, leading to a decline in hydrogen density downstream of the IPH (Quémerais, 2000). These findings are consistent with previous measurements obtained from SWAN/SOHO, IUVS/MAVEN, and other interplanetary IPH observations (Baliukin et al., 2022; Mayyasi, Quémerais, et al., 2023). As previously mentioned, these outward looking emission intensities may be influenced by variations in hydrogen within the Martian exosphere over different time scales, driven by seasonal changes, fluctuations in solar radiation, and dust storms. Hence, we evaluated the H Lyman emission intensities at different seasons of Mars and also modeled these emission intensities, as explained in the following sections.

## 4. Martian Seasonal Influence on the Observed IPH Intensities

The EMUS observation period for U-OS3b and U-OS4b modes spans 1.4 MY, starting from Ls 36° in MY 36 to Ls 170° in MY 37. As noted from previous observations, the Martian atmosphere experiences seasonal fluctuation due to varying solar radiation (Bhattacharyya et al., 2015, 2017; Chaffin et al., 2014; Clarke et al., 2014, 2017; Halekas, 2017). Throughout this period, EMM altitudes ranged from 20,000 to 43,000 km above Mars' surface, as shown in the top panels of Figure 3. Even at these altitudes, hydrogen density can vary due to seasonal changes in
<figure><img src="image_8.png" /><figcaption>**Figure 5.** H Lyman emission sky maps similar to the previous figure, but in these plots the mean H Ly-α intensity maps are calculated after applying the star-removal technique. For the H Ly-β mean intensity maps, both H Ly-α stray light subtraction and the star-removal technique were applied. The mean emission intensity maps for H Ly-α is calculated considering the data from the beginning of the mission up until January 2022, whereas for H Ly-β it is calculated over the complete EMUS observation period. Refer the main text for the explanation of observation period and data gaps in the H Ly-α emission sky map.</figcaption></figure>

the Martian atmosphere (Bhattacharyya et al., 2015, 2017). The H number density and the temperature ($T_{ex}$) at the exobase dictate the whole distribution of H in the exosphere, decreasing with altitude, faster for cooler temperatures $T_{ex}$. Consequently, at lower altitudes, the EMUS encounters higher hydrogen column densities along the observation line of sight compared to at higher altitudes. The variability in Martian exospheric hydrogen density, influenced by both seasonal changes and spacecraft altitude, may affect the observed background emission intensities both in U-OS3b and U-OS4b modes. Using the simultaneously retrieved exobase number density and temperature from disk observations, the calculated column densities for the EMUS U-OS4b observation geometry, as shown in Figure S5 in Supporting Information S1, exhibit a strong seasonal variation during the EMUS observation period. This figure also highlights the influence of spacecraft altitude on column densities, indicating that EMUS encounters higher values while orbiting at lower altitudes and lower column densities at higher altitudes. Therefore, the EMUS observed emission intensities result from the addition of IPH and Martian exospheric hydrogen emissions, which is not negligible in some cases. To evaluate the potential impact of Martian-originated hydrogen emissions on the observed IPH Lyman intensities, we analyzed these emissions across different Martian seasons for both U-OS4b and U-OS3b observation modes.

In Figure 6, we illustrate the observed IPH emission intensities at H Lyman wavelengths for instrument's LOS look directions are oriented with $\vec{l}_{emus} \cdot \hat{y}_{mso} > 0$ and $\vec{l}_{emus} \cdot \hat{y}_{mso} < 0$ in U-OS4b mode. The periodic variations observed in both H Ly-α and β during the earlier observation period primarily stem from the solar rotation effect. To understand these variations better, we used the broadband averaged solar irradiance obtained from the L3 data product of the EUVM/MAVEN instrument, based on the FISM-M model (Thiemann et al., 2017). These values were then normalized by dividing them by the minimum solar irradiance value corresponding to the wavelength for the EMUS observation period and plotted on the right y-axes of Figure 6. It can be noticed in this figure that the solar fluxes exhibit periodic variations associated with solar rotation, and these fluctuations are also reflected in the EMUS observed IPH Ly-α and β emission intensities.

In U-OS4b observations, during the Mars perihelion season specifically Ls 210–340° in MY 36, we observed an increase in emission intensities at H Ly-β and γ wavelengths. This increase can be attributed to three factors: an increase in solar insolation (see Figure 6), heightened hydrogen density due to Mars' seasonal effects, and the
<figure><img src="image_9.png" /></figure>

**Figure 6.** EMUS observed Martian sky background H Lyman emission intensities (top three panels) and upwind phase angle coverage (bottom panel) for instrument's LOS look directions oriented with $\vec{\ell}_{emus} \cdot \hat{y}_{mso} > 0$ (Red points) and with $\vec{\ell}_{emus} \cdot \hat{y}_{mso} < 0$ (Blue points) in the U-OS4b observation mode. Magenta points in the final panel represent the IUVS/MAVEN coverage during MY 36. The observed H Ly-$\alpha$ emissions are shown only prior to the onset of the detector burn-in period, which is marked by the vertical dashed line in the third panel. Error bars of the observed radiance depict a $1-\sigma$ standard error of the mean in the sample, which are too small to be noticeable in H Ly-$\beta$ and $\alpha$ emissions. The EUVM/MAVEN measured solar irradiance fluxes (Thiemann et al., 2017) are normalized and plotted on the right-y axes in gray color and the normalization processes is described in the main text.

spacecraft orbiting within the expanded exosphere at lower altitudes (see Figure 3). Additionally, during this period, the instrument primarily observed along the IPH upwind and downwind directions. As presented in the final panel of Figure 6, it can be noticed that the EMUS was oriented toward closer to the upwind and downwind directions of the IPH during August-September in 2021, 2022, and 2023. But the observed emission intensities are largest only during August-September 2022 and not at other periods, which is mainly due to exospheric seasonal effects on the observed emission intensities. It can be seen in this figure that some data points are missing in the H Ly-$\gamma$ emission intensities during July-August of 2021, 2022, and 2023, during which the instrument was oriented toward the IPH downwind direction. For IPH downwind looking directions, the radiance of H Ly-$\gamma$ is lower compared to the background contribution. This is mainly due to strong continuum signals due to hot stars present in those look directions. This results in greater wing subtraction, which eventually leads to negative emission intensities (see Section 3 and Figure S3 in Supporting Information S1).

As shown in Figure 6, the H Ly-$\gamma$ emission intensities, though relatively weak ($<0.1$ R), exhibit a seasonal variation similar to H Ly-$\beta$. It is important to emphasize that the U-OS4b observations were conducted with a long-exposure integration time ($\sim55$ s), enabling the detection of clear emission features at the H Ly-$\gamma$
<figure><img src="image_10.png" /></figure>

**Figure 7.** An intra-annual comparison of the observed radiances of H Ly-γ and H Ly-β emissions in the U-OS4b sky maps, for the observations made when the instrument’s LOS look directions are oriented with $\vec{l}_{emus} \cdot \hat{y}_{mso} > 0$ and with $\vec{l}_{emus} \cdot \hat{y}_{mso} < 0$ look directions. Bottom panels denote the sky coverage in U-OS4b observation mode in ecliptic J2000 coordinates. Red and blue colors represent observations that began when Mars was at Ls 36° in Martian year 36, while black colors indicate observations from overlapping sky coverage measured after Ls 36° in Martian year 37 for corresponding look directions. The gray error bars in top panels represent the standard error of the mean which are too small to be noticeable for H Ly-β emission. Black circles with arrows in the bottom panels denote the instrument scan direction of the Martian sky indicating starting point of the observation. Red and blue stars represent the interplanetary hydrogen upwind and downwind locations, respectively.

wavelength. The observed spectra at this wavelength also exhibit the same LSF as the well-established H Ly-α and H Ly-β emissions (see Figure S2 in Supporting Information S1). Even after subtracting the H Ly-α stray signal beneath this emission feature, a residual emission remains, maintaining the expected LSF shape. Furthermore, after accounting for contributions from scattered components, the H Ly-γ variation continues to reflect both Martian seasonal changes and IPH upwind and downwind asymmetries, aligning with the variations observed in H Ly-α and H Ly-β emissions. This strong correlation reinforces that the detected H Ly-γ feature is astrophysical in nature rather than an instrumental artifact. We also estimated the detector counts based on the instrument's sensitivity and found them to be comparable to our observed value, as discussed in Section 7 of the SI.

The observation of U-OS4b emission intensities commenced in the last week of April 2021, corresponding to Ls 36° in MY 36. The sky background coverage in this mode for different LOS look directions is presented in the bottom panels of Figure 7. Here, we have also compared the observed emission intensities of the same region of the sky at consecutive MYs, that is, 36 and 37, starting from Ls 36°. In this Figure, red and blue colors represent observations carried out in MY 36, while black colors denote the observations from MY 37 for the corresponding portions of the same observed sky. When the instrument revisited the same part of the sky during the same Martian seasons in consecutive years, the observed emission intensities are consistent with measurements from
<figure><img src="image_11.png" /><figcaption>**Figure 8.** Seasonal variation of U-OS3b Martian sky background H Lyman α and β emission intensities as a function of upwind phase angle. The observed intensities averaged across 20° upwind phase angle bins and over different seasons (in Ls 60° bins) of Mars across the observation period during MY 36 and MY 37. Vertical gray bars represent 1-σ standard error of the mean in the sample, which are negligible in H Ly-α profiles. While averaging we have used H Ly-α emission intensities before the beginning of burn-in period (i.e., before Ls 150, MY 36) at H Ly-α wavelength.</figcaption></figure>

previous MY. This can be noticed from the figure by comparing emission intensities for the corresponding look directions observed during Mars years MY36 and MY 37. The consistency in observed emission intensities at two years interval suggests a stable and predictable behavior of the IPH emissions over two years. This consistency also indicates that the underlying processes driving these emissions are relatively stable and follow predictable patterns during this observation period. However, it should be noted that solar flux at H Lyman wavelengths in MY 37 is relatively higher compared to earlier years by about 50% (see Figure 6). We did not find any significant increment in the observed emission intensities due to increased solar flux in MY 37, except a small increment in the observed H Ly-β intensities when the instrument's LOS look direction is oriented with $\tilde{\nu}_{emus} \cdot \hat{y}_{mso} < 0$, while EMUS observations are orientated along the IPH upwind direction in MY 37 (see right middle panel in Figure 7 and bottom panel of Figure 6). Considering the increased solar irradiance in MY 37, this variation in the observed intensities cannot be explained by assuming a symmetric hydrogen density distribution along IPH flow direction axis in the heliosphere. We hypothesize that hydrogen column densities decreased by 50% during MY 37 in the interplanetary hydrogen flow, as this reduction is necessary to explain the observed variation.

Figure 8 shows the calculated mean observed brightness across 20° upwind angle bins over different seasons of Mars (Ls 60° bins) using U-OS3b observed emission intensities. It is evident from the figure that the calculated mean brightness of H Ly-α emission increases toward the interplanetary hydrogen (IPH) upwind direction. Quémerais et al. (2014) identified the Maximum Emissivity Region (MER) occur at an ecliptic longitude of 253.2 ± 2.0°, which is also confirmed by long-term observations from SWAN/SOHO (Koutroumpa et al., 2017). A model of emissivity distribution is shown on Figure 10, represented by iso-emissivity contours. The largest emissivity contour encompasses a white area along the upwind, which may be considered as the MER. Our calculated emission intensities, using ecliptic longitude 252.5° as the IPH upwind location are consistent with this model.

A similar variation of upwind to downwind in the calculated H Ly-β emission intensity profiles also can be noticed. But the calculated emission intensity profile of H Ly-β is notably higher during Mars' perihelion period (Ls 240–300°, MY 36) compared to those calculated at other seasons of Mars and does not show the expected upwind to downwind variation. During this period, the EMUS observed emission intensities are influenced by the Mars' seasonal hydrogen variation. It should be noted that the H Ly-β emission profiles in Figure 8 are calculated after subtracting the H Ly-α stray light contribution. Both H Ly-β and H Ly-α sky intensities vary with upwind phase angles and Martian seasons. Consequently, the calculated H Ly-β emission intensity depends on the variation of both the H Ly-β and H Ly-α background intensities for a given observation. At upwind phase angles close to zero, where the H Ly-α emission intensity is at its maximum, the background subtraction is most significant compared to other upwind phase angles. The averaged H Ly-β emission profiles during seasons $L_s = 60°–240°$ show a slight decrease for upwind phase angles smaller than 50°, primarily due to the relatively higher subtraction of H Ly-α stray light contribution. However, these profiles still show the expected upwind-to-downwind variation. It is important to note that the variations in the observed radiance also depend on factors such as spatial coverage, solar flux, the influence of Martian hydrogen column density along the corresponding look direction, and spacecraft altitudes. A detailed modeling of these profiles considering these factors will be the subject of a future work.

## 5. Modeling U-OS4b Sky Background H Lyman Emissions

We have noticed in the observations that the sky background emission intensities are significantly higher when Mars is near perihelion, which could be potentially influenced by seasonal variation of Martian exosphere. To assess the contribution of exospheric emission intensities during the EMUS observation period, we have carried out a simple forward model calculation. This model computes H Lyman emission intensities of the Martian exosphere corresponding to each U-OS4b observation considering the EMUS' viewing geometry and EMM spacecraft altitude. To achieve this, we have used the simultaneously retrieved exobase parameters at 200 km
<figure><img src="image_12.png" /><figcaption>**Figure 9.** Comparison between observed and modeled radiances for sky H Ly-α and β emissions for U-OS4b observation mode. The left and right column panels represent EMUS observations for the corresponding LOS look directions are oriented with $\vec{l}_{emus} \cdot \hat{y}_{mso} > 0$ and with $\vec{l}_{emus} \cdot \hat{y}_{mso} < 0$, respectively. Red and blue circles represent the observed EMUS U-OS4b sky emission intensities. The modeled exospheric (Exo) and interplanetary hydrogen (IPH) emission intensities are presented using lower gray triangles and crosses, respectively. The modeled total sky intensities, which are the sum of exospheric and IPH intensities, are presented as open black circles. The calculated ratios of the modeled total sky intensities to the observed values are presented in the second and fourth row panels for H Ly-β and H Ly-α emissions, respectively. Modeled IPH Ly-α emission intensities in the third row panels (orange plus symbols) are plotted after being multiplied by a factor of 1.6. EMUS observed intensities and the calculated modeled-to-observed emission ratios for H Ly-α are shown only before the onset of the detector burn-in period, which is indicated by vertical dashed lines in the third and fourth row panels.</figcaption></figure>

altitude, viz., atomic hydrogen density and temperature based on the observation of EMUS U-OS2 disk observations. The retrieval mechanism is based on the earlier model calculations which are employed while studying the H corona of the Earth and Mars (Bishop, 1999; Chaffin et al., 2014, 2018, 2021; Chaufray et al., 2008). These derived exobase number density and exobase temperature at 200 km altitude facilitated the computation of atomic hydrogen density profiles of Martian exosphere using the analytical expression from Chamberlain (1963). In calculating the hydrogen density profiles, the model assumes a uniform exobase density and temperature, with no consideration of a hot hydrogen component. The model also assumes a spherically symmetric hydrogen distribution around the planet. These hydrogen density profiles are used to calculate the column densities (N) for the instrument position and direction of view, which are presented in Figure S5 in Supporting Information S1. For each U-OS4b observation, we calculate the fluorescence emission rate (g-factor) utilizing solar flux data from EUVM/MAVEN observations (Thiemann et al., 2017). The volume emission rates are then determined by multiplying these g-factors with the number density profiles. We calculated the emission intensities by integrating volume emission rates along the EMUS observed line of sight for each U-OS4b observation under optically thin approximation. In such a case the intensity I in Rayleigh is related to the column density N by the simple formula:
$$I(R) = 10^{-6} \text{ g N.}$$
<figure><img src="image_13.png" /><figcaption>**Figure 10.** Earth (inner circle points) and Mars (outer circle points) positions around the Sun (central red dot) during the EMUS observation period before the onset of the decay of detector sensitivity at H Ly-α wavelength. The locations of Mars during Imaging Ultraviolet Spectrograph/Mars Atmosphere and Volatile Evolution observations are represented with three magenta points on Mars orbit. The black-and-white contour map represents modeled typical emission emissivity distribution of interplanetary hydrogen (IPH) Ly-α in the ecliptic plane, where white indicates the region of maximum emissivity. Gray arrow represents the IPH flow direction. Color scale represents the date of the EMUS observation and its corresponding solar longitude (Ls) of Mars. White and yellow plus symbols on Earth and Mars orbits represents perihelion and aphelion positions of the planets, respectively. Outermost circle indicates ecliptic longitudes. Sun, Earth and Mars sizes are not plotted to the scale.</figcaption></figure>

Also, we have utilized an IPH model to quantify the H Ly-β and α emission intensities for the corresponding U-OS4b observed sky locations during the EMUS observation period. The IPH density distribution model used in this work is based on Lallement, Bertaux, & Dalaudier (1985). It takes into account the effects of solar gravitation, radiation pressure, and solar ionization on hydrogen atoms as they propagate in the inner heliosphere. For a given set of solar parameters, this model computes the hydrogen number density and velocity distribution at any point in the heliosphere. The resulting distribution is combined with a Monte Carlo radiative transfer code described by Quémerais (2000), which allows computation of the H Ly-α interplanetary brightness corresponding to EMUS U-OS4b observation conditions. The same code was used for calculating H Ly-β emission brightness, but the oscillator strength, and therefore the scattering cross-section, was adjusted according to the value given by Wiese and Fuhr (2009). The absorption oscillator strengths used in our code for H Ly-α, β and γ transitions are 0.416, 0.079, and 0.029, respectively (Wiese & Fuhr, 2009). The computation has been done for the hot model with input parameters of atomic hydrogen density at infinity of 0.1 cm⁻³, a mean bulk velocity of IPH of 22 km/s, and a temperature of 13,000 K for calculating the H Lyman emission intensities. We considered the ratio of solar radiation pressure to gravitation force (μ) as one and ionization rate (β) as 8.33 × 10⁻⁷ s⁻¹, at 1 AU respectively. More details of this model calculations are presented in Quémerais (2000). In our model calculations within the heliosphere, we found that the mean free paths of H Ly-α and β photons are approximately 10 and 50 AU, respectively. Additionally, we found that multiple scattering has marginal effect on H Ly-β photons and can be safely disregarded in the case for H Ly-γ photons. The H Ly-α emissivity distribution for such a model is presented in Figure 10, together with the orbit of Mars and Earth.

In Figure 9, we present a comparison between the modeled Martian exospheric (Exo) and IPH Ly-α and β emission intensities alongside the U-OS4b observed values. For the input parameters described above, the modeled IPH Lyman-β emission intensities are consistent with the observations for Ls < 150° in MY 36, while H Ly-α intensities are smaller by a factor of 1.6 when compared to the corresponding observations. To match the observations, we multiplied our modeled H Ly-α emission intensities by a factor of 1.6 and added the modeled exospheric emission intensities to determine the total sky H Lyman emission intensities as should be seen from EMUS/EMM at Mars. The solar flux at H Lyman wavelengths is a key factor in determining emission intensities, as other parameters such as hydrogen density distribution, temperature, and flow speed in the IPH model remain constant. Thus, the discrepancy between the modeled H Ly-α and the observations could be attributed to the input solar flux used in the IPH model at H Ly-α wavelength and any changes in the other input parameters of the IPH model would affect the intensities of both Ly-α and Ly-β emission intensities.

During the early observation period (i.e., Ls < 150°, MY 36) it is evident that the contribution from exospheric emission intensities is expected to be minor to the observed emissions and the primary contribution is originating from IPH emissions. However, as Mars moves closer to the Sun, the exospheric hydrogen emission intensity becomes a dominant contributor to the total observed emission intensity. By the time Mars progresses toward aphelion, the contribution of the Martian exosphere to the total observed emission intensity diminishes for the observations exceeding Ls more than 30° in MY 37.

Our calculated intensity ratios of modeled intensities to the observed values for H Ly-β emission are about a factor of 2 during the perihelion period (see second row panels in Figure 9). This discrepancy could be due to the uncertainty in input model parameters such as exobase density and temperature. By reducing the input exobase density by a factor of about two in the model, we find that the calculated emission intensities are consistent with the observations. As previously mentioned, the substantial decay of the EMUS detector at the H Ly-α region poses a challenge for comparing observed and modeled emission intensities after January 2022, so we have chosen not to include such comparisons for that period.
We also estimated the Ly-γ emission intensities to interpret the EMUS-observed emission across different look directions. This was achieved using g-factor ratios of Ly-α to Ly-γ. Our results indicate that the modeled intensities are higher by a factor of 3 compared to the observed values. This discrepancy is likely attributed to our assumption of an optically thin approximation in the calculation of H Ly-α intensities. Moreover, Pritchard and Furlanetto (2006) discussed that the excitation of hydrogen atoms by Ly-γ photons results in cascades to lower excited states, producing Ly-β and Ly-α emissions before transitioning directly to the ground state. Their calculations suggest that approximately 25% of Ly-γ photons contribute to Ly-α emission through this cascading process. The cascading of Ly-β into the production of Ly-α photons after one H-α transition is a forbidden transition (Pritchard & Furlanetto, 2006) and, therefore, does not influence our modeled H Ly-β emission intensities. However, a comprehensive analysis of these emission lines, incorporating radiative transfer effects and cascading processes for the specific EMUS observation geometry, is beyond the scope of this study. We plan to address these complexities in future work.

It is important to acknowledge that our model calculations have limitations, which could contribute to the discrepancies between the observed and calculated H Lyman intensities. Our calculations for exospheric intensity assume optically thin conditions for both H Lyman β and α emissions. This assumption may lead to underestimation of intensities, especially during the southern summer period when hydrogen exosphere densities peak. Additionally, the retrieved exobase parameters, viz., temperature and number density, are based on assumptions such as a constant temperature profile of the diffusion-dominated thermosphere, spherical symmetry in the hydrogen distribution around the planet, and a negligible population of satellite H atoms in the exosphere. Consequently, these uncertainties may result in under- or overestimating emission intensities, particularly significant during Martian perihelion. In light of these model limitations the determined radiance around the southern summer (near perihelion) should be interpreted carefully.

## 6. Comparison of EMUS Observations With SWAN/SOHO and IUVS/MAVEN

Before the onset of the detector decay at H Ly-α wavelength, the EMUS-measured H Ly-α emission intensities can be compared with observations from SWAN/SOHO and IUVS/MAVEN. Both EMUS and IUVS/MAVEN provide Mars-based observations, whereas SWAN/SOHO measures emission intensities from its position at the Sun-Earth L1 point. Figure 10 presents the locations of Earth and Mars during this EMUS observation period, along with the modeled typical IPH emissivity (photons cm⁻³ s⁻¹) distribution in the ecliptic plane of the inner heliosphere. The 2D emissivity distribution depicted in this figure illustrates how the instruments observe regions of different emissivity for a given sky location while observing from different view directions. It can be noticed in this figure that during the initial observation period, SWAN/SOHO was positioned closer to the region of MER compared to Mars. Over time, Mars moved nearer to this brighter region, while Earth shifted toward the IPH downwind, lower emissivity region. By considering the instrument locations and their observation directions at specific times, we tried to clarify the differences in emission intensities measured by these instruments.

The comparison between the measured H Ly-α emission intensities by EMUS/EMM, SWAN/SOHO and IUVS/MAVEN is presented in Figure 11. In this figure, the observed emission intensities are presented averaged over different upwind phase angle bins. For this analysis, we used daily measured H Ly-α intensity maps from SWAN/SOHO and considered only areas of the SWAN/SOHO sky maps that overlap with EMUS observations (U-OS4b and U-OS3b). We also applied a calibration factor accounting for SWAN/SOHO sensitivity degradation which is based on the previous observations between 2001 and 2024, obtained from Eric Quémerais. The method employed in calculating the calibration factor for SWAN/SOHO degradation will be discussed as a separate work. Additionally, during this period, IUVS/MAVEN also measured H Ly-α intensities on three dates in Martian year 36 (June 21-22, September 3, and 30 September 2021) for upwind phase angles between 20° and 60° (Mayyasi, Quémerais, et al., 2023), which were used to compare with EMUS and SWAN/SOHO observations.

It can be noticed in Figure 11 that the trend in the SWAN/SOHO daily measured intensity shows small variations for upwind phase angles below 60° and above 160° during the EMUS observation period (see gray dashed curves in the corresponding panels of Figure 11). For other angles, SWAN/SOHO measured intensities decrease over time. This trend can be explained based on the Earth's changing location and view directions in the inner heliosphere at the time of observation (see Figure 10). SWAN/SOHO's observed H Ly-α emission intensity is a cumulative response from different emissivity zones for a given look direction as illustrated in Figure 10. The region of maximum emissivity is located beyond Earth's orbit, whereas the region of minimum emissivity lies in
<figure><img src="image_14.png" /><figcaption>**Figure 11.** A comparison of H Ly-α emission intensities measured by EMUS/Emirates Mars Mission (closed green and closed purple circles), Solar Wind ANisotropy (SWAN)/Solar and Heliospheric Observatory (SOHO) (open green and open purple circles) averaged over 20° upwind phase angle bins (Up Ang) at different season of Mars before the beginning of detector burn-in period. Green and purple circles represent emission intensities for U-OS3b and U-OS4b modes, respectively. The observed emission intensities from Imaging Ultraviolet Spectrograph/Mars Atmosphere and Volatile EvolutionN during this period (closed magenta circles in top middle and top right panels) are taken from Mayyasi, Quémerais, et al. (2023). Dashed gray curve represents the daily measured SWAN/SOHO H Ly-α emission intensities for corresponding upwind angle bins. SWAN-OS3b and SWAN-OS4b represent the calculated intensities from SWAN sky maps for the sky areas as observed by EMUS in U-OS3b and U-OS4b modes, respectively.</figcaption></figure>

the opposite radial direction along the line connecting the Sun and the MER. Consequently, the emission intensities observed by SWAN/SOHO are largely independent of Earth's location when looking directly along or closer to the maximum or minimum emissivity regions, leading to smaller changes in the observed intensities for upwind phase angles below 60 and above 160°. In contrast, the measured emission intensities for other upwind phase angles are influenced by Earth's position at the time of observation. During the initial period, Earth was closer to the MER, resulting in higher SWAN/SOHO intensities when observing along the other upwind phase angles. Later in the observation period, as Earth moved into lower-emissivity regions, the cumulative intensities for the look directions along the crosswind phase angles decreased, leading to lower SWAN/SOHO emission intensities.

Similarly, the trend in observed EMUS/EMM intensity can also be explained by Mars relative positions in the inner heliosphere while it is moving closer to the MER and scanning different parts of the sky. As shown in Figure 11, EMUS-measured emission intensities are gradually increasing with time for upwind phase angles greater than 40°. With the progression of time Mars is getting closer to the MER, which results in an increase in the observed emission intensities for higher upwind phase angles.

As shown in Figure 11, SWAN/SOHO measured emission intensities were 25%–80% higher than those measured by EMUS for observations carried out before Ls 120° in MY 36, with the differences varying depending on the instrument's viewing direction. For observations after Ls 120°, SWAN/SOHO intensities became consistent with
EMUS for upwind phase angles greater than 60°. These differences between SWAN/SOHO and EMUS/EMM can be explained by the relative positions of Earth and Mars. Before Ls 120°, SWAN/SOHO was gradually moving away from higher emissivity regions toward lower emissivity regions, while EMUS was approaching the MER. As a result, SWAN/SOHO observed higher intensities during this period for all look directions, as it scanned the sky being in the brighter emissivity zone. After Ls 120°, SWAN/SOHO moved into lower-intensity regions, while EMUS approached the MER (see Figure 10). As mentioned earlier, although these instruments observe the same sky regions (in the same direction) at a given time, the differences in their location, along with the strong variation in the IPH emissivity distribution in the inner heliosphere, must be considered when comparing the measured emission intensities.

As shown in the bottom panel of Figure 6, the IUVS/MAVEN observation on 3 September 2021, aligns most closely with the EMUS U-OS4b coverage in terms of upwind phase angle (20–40°). On this date, the IUVS/MAVEN measured intensity was about 45% smaller than the nearby EMUS measured intensity (see top middle panel in Figure 11). A similar discrepancy was also observed at upwind phase angles between 40° and 60° when compared with nearby EMUS U-OS3b observations (see top right panel in Figure 11). Since both IUVS/MAVEN and EMUS/EMM are Mars-based observations, these differences in the reported brightness are likely best attributed to improper estimates of the respective photometric calibration factors. Usually, the calibration factor at H Ly-α may be estimated by observations of hot stars. However, passing from the sensitivity to a point source to an extended source presents some difficulties, depending on spatial and spectral resolutions which are different for the various instruments.

Instrumental factors may also contribute to these measurement discrepancies, including spatial and spectral resolutions and calibration factors. SWAN/SOHO is a broadband photometer with a large bandwidth (110–160 nm) and a field of view of 1° by 1° (Bertaux et al., 1995), while IUVS/MAVEN operates at high spectral resolution with a resolving power of 15,000–20,000 (Mayyasi, Quémerais, et al., 2023; McClintock et al., 2015). Notably, IUVS/MAVEN was calibrated using SOHO observations (Mayyasi et al., 2017), so differences between SWAN/SOHO and IUVS/MAVEN may be linked to Earth and Mars positions and the viewing geometry at the time of observation. Additionally, increasing solar activity during this period may have expanded the ionization cavity away from the Sun during this observation period. A more detailed comparison of these data sets, accounting for all these factors, lies beyond the scope of this study and will be addressed in future work.

## 7. Discussion and Summary

In this study, we examined EMUS observations of interplanetary hydrogen at Ly-α and β wavelengths in the Martian sky, utilizing dedicated observation strategies known as U-OS3b and U-OS4b, which involve looking away from the planet in order to minimize the Mars exospheric contribution. These observations spanned for more than a Martian year, starting from Ls 37° MY 36 to Ls 172° MY 37, covering different Martian seasons. The U-OS4b observations were tailored to achieve a high signal-to-noise ratio but covered a smaller region, while U-OS3b offered broader coverage of the Martian sky. Notably, we observed a distinct decrease in IPH brightness as the angle increased with respect to the upwind location in both H Ly-α and β emission intensity sky maps. These evaluated H Lyman brightness maps align with previous measurements conducted by various space-based instruments (Baliukin et al., 2022; Bertaux et al., 1995; Quémerais et al., 2014). During the perihelion season, we find an increment in the observed outward emission intensities. This increase can be attributed to the additional contribution of the expanding Martian hydrogen exosphere component to the EMUS observations. During this period, EMM is also orbiting at lower altitudes, and insolation is increased, potentially enhancing the observed emission intensities. We also found that the observed emission intensities remained consistent with repeated sky coverage in spite of an increased solar activity. A concomitant reduction in the hydrogen column density along the observed line of sight is a plausible explanation for these observations.

Our modeled emission intensities corresponding to U-OS4b observation conditions show that the contribution from Martian exospheric hydrogen is not significant for the observed emission intensities and is mostly controlled by IPH emissions for the periods with Ls < 150° in MY 36 and after Ls > 30° in MY 37, that is, when Mars is crossing the aphelion period. The modeled intensities of H Ly-β emission align well with the observed intensities based on the instrument's viewing geometry (see Figure 9). However, a discrepancy exists between the modeled Ly-α emission intensities and the EMUS observations, which can be attributed to the input solar flux used in the
calculations. The comparison between EMUS and IUVS measured emission intensities shows that the former values are higher by 45% compared to the latter, which can be attributed to differences in instrumental factors.

In the future, we aim to investigate how seasonal variations in the Martian exosphere influence the observed H Ly-α and β emission maps from the U-OSb observations. This study will integrate the theoretical predictions of the IPH model calculations with EMUS observed emission intensities. Our objective is to enhance our understanding of the observed IPH Lyman emission intensities from EMUS, IUVS/MAVEN, and SWAN/SOHO by integrating more accurate calculations of Martian exospheric intensities. This combined approach of model calculations and comparison with observations will offer valuable insights into the distribution and dynamics of IPH within the inner heliosphere under different solar conditions.

## Data Availability Statement

The EMM/EMUS 12a data we analyze here are available at the EMM Science Data Center (SDC, http://sdc.emiratesmarsmission.ae). This location is designated as the primary repository for all data products produced by the EMM team and is designated as long-term repository as required by the UAE Space Agency. The data available http://sdc.emiratesmarsmission.ae include ancillary spacecraft data, instrument telemetry, Level 1 (raw instrument data) to Level 3 (derived science products), quicklook products, and data users guides at http://sdc.emiratesmarsmission.ae to assist in the analysis of the data. Following the creation of a free login, all EMM data are searchable via parameters such as product file name, solar longitude, acquisition time, sub-spacecraft latitude and longitude, instrument, data product level, and etc. Data products can be browsed within the SDC via a standardized file system structure that follows the convention: /emm/data/<Instrument>/<DataLevel>/<Mode>/<Year>/<Month> Data product filenames follow a standard convention: emm_<Instrument>_<DataLevel>_<StartTimeUTC>_<OrbitNumber>_<Mode>_<Descript-tion>_<KernelLevel>_<Version>.<FileType> Emirates Mars Ultraviolet Spectrograph (EMUS) data and users guides are available at https://sdc.emiratesmarsmission.ae/data/emus. The MAVEN EUVM L3 data are publicly available at the NASA Planetary Data System through daily measurements of Solar irradiance at https://pds-ppi.igpp.ucla.edu/data/maven-euv-modelled/. SWAN/SOHO daily measured H Ly-α intensity maps data set can be obtained from http://swan.projet.latmos.ipsl.fr/.

## Acknowledgments

Funding for development of the EMM mission is provided by the UAE government, and to co-authors outside of the UAE by MBRSC. RS and KC are supported by the grant (8474000332-KU-CU-LASP Space Sci).

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