Comparisons of observed and forward modeled Balmer-α intensities from WACCM-X and MSIS-00

Abstract

From 2000 to 2001, ground-based geocoronal hydrogen Balmer- (H) observations were obtained from Pine Bluff Observatory (PBO) in Wisconsin (43.07N, 270.33E) using a high spectral resolution (R 80,000) Fabry-Perot interferometer (FPI). In previous work, the PBO March 2000 observations have been compared to forward modeled MSISE-90 H intensities using the radiative transport code, lyao_rt (Bishop, Journal of Qunatitative Spectroscopy and Radiative Transfer, 1999, 61, 473–491). Results indicate that hydrogen column abundances exceed those predicted by MSISE-90 (Bishop et al., Journal of Geophysical Research, 2004, 109, 473–491). Furthermore, the PBO 2000–2001 observations have been compared to NRLMSISE-00 (MSIS-00) H intensities generated by lyao_rt. It was found that the observed dusk-to-dawn intensity variation and MSIS-00 show good agreement near the equinoxes and summer solstice, however, MSIS-00 underestimates the dusk-to-dawn asymmetry near the winter solstice (Gallant et al., Journal of Geophysical Research: Space Physics, 2019, 124, 4525–4538). More recent work has focused on forward modeling Whole Atmosphere Community Climate Model-eXtended (WACCM-X) simulations for three separate observatory locations, including PBO. Here, we investigate variations in upper thermospheric hydrogen by comparing WACCM-X and MSIS-00 forward modeled H intensities for the March equinox with the PBO March 2000 evening observations used in Bishop et al. (Journal of Geophysical Research, 2004, 109, 473–491). WACCM-X underestimates the March 2000 observations by less than a factor of 2.0 rayleighs whereas MSIS-00 underestimates the observations by 2.6 rayleighs. A further analysis suggests that WACCM-X is better at simulating the underlying hydrogen density distribution than MSIS-00. Model-model-data comparisons between the 2000–2001 PBO observations, MSIS-00, and WACCM-X simulations that are representative of the observational conditions are currently underway.

1 Introduction

Hydrogen distribution is key to understanding the chemistry and dynamics of the middle and upper atmosphere (Bishop, 2001). Atomic hydrogen in the mesosphere is a byproduct of photolysis reactions of O, , and and plays an important role in chemistry in the middle atmosphere (Brasseur and Solomon, 2005). Furthermore, as a minor species in the lower thermosphere, atomic hydrogen becomes increasingly important in the upper thermosphere, where hydrogen charge exchange reactions contribute to hydrogen kinetics and escape (Chamberlain and Hunten, 1987). In the exosphere, neutral hydrogen becomes the dominant species and resonantly scatters sunlight at Lyman- (1,216 Å; hereafter referred to as Ly-) to produce the diffuse geocorona around the Earth. Increased exospheric hydrogen contributes to the recovery of the plasmasphere after geomagnetic storms as well as ring current decay during geomagnetic storms (Krall et al., 2018). Additionally, hydrogen charge exchange reactions between hydrogen and hydrogen ions result in energetic neutral atoms (ENAs). Therefore, reliable measurements of the geocoronal density are essential for techniques such as ENA imaging to determine the ion distributions of the inner magnetosphere (Ilie et al., 2013).

Multiple studies have analyzed geocoronal hydrogen ground- and space-based measurements and how they compare to the NRLMSISE-00 [(Picone et al., 2002); hereafter referred to as MSIS-00] empirical model. Gallant et al. (2019) compared ground-based geocoronal hydrogen Balmer- (6,563 Å; hereafter referred to as H) observations from a Fabry-Perot interferometer (FPI) located at Pine Bluff Observatory (PBO) in 2000–2001 (Mierkiewicz et al., 2012) to MSIS-00 forward modeled H intensities. The results indicate a seasonal trend in the observed dusk-to-dawn intensity variation consistent with a diurnal variation in the underlying thermospheric hydrogen density distribution. Furthermore, they found that the observed H emission intensities are higher than the MSIS-00 forward modeled intensities by 1.5–2.5 rayleighs indicating a discrepancy in the underlying MSIS-00 hydrogen distribution (Gallant et al., 2019). Nossal et al. (2012) compared ground-based H observations from the Wisconsin H-Alpha Mapper (WHAM) FPI to MSIS-00 forward modeled H intensities over an entire solar cycle. The MSIS-00 forward modeled intensities show a greater solar cycle variation than the observed emission intensities and, therefore, indicate possible discrepancies in the underlying hydrogen distribution for solar minimum and maximum during nighttime conditions (Nossal et al., 2012). Moreover, Waldrop and Paxton (2013) analyzed Ly- measurements from the Global Ultraviolet Imager (GUVI) instrument aboard NASA’s Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) spacecraft and found that MSIS-00 overestimates GUVI upper thermospheric hydrogen density by 36%–67% at solar minimum and 42%–74% at solar maximum during dayside conditions. Wan et al. (2022) demonstrated that exobase hydrogen densities derived from GUVI Ly- observations exhibit solar cycle, seasonal, and local time variability not captured by MSIS-00 or NRLMSIS 2.0 (Emmert et al., 2021), including a weakening dawn-dusk asymmetry towards solar minimum. In contrast, Joshi et al. (2019) used MSIS-00 to prescribe the background thermosphere within a radiative transfer model inversion of Ly- emission measured by GUVI. They found that the vertical flux of thermospheric hydrogen is nearly constant over a large range of solar activity and typically exceeds the calculated thermal evaporative flux. These studies emphasize the need for improved representations of upper thermospheric hydrogen transport and coupling in empirical models across various temporal scales.

WACCM-X (Whole Atmosphere Community Climate Model-eXtended) is a comprehensive numerical model that simulates the entire atmosphere and ionosphere from the Earth’s surface to 700 km (Liu et al., 2018). Previous work by Qian et al. (2018) compared WACCM-X simulations of atomic hydrogen for different temporal and spatial scales to observations from TIMED’s Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument as well as abundances specified by MSIS-00. They found that in the upper thermosphere, hydrogen density is predicted to be higher in the winter than summer, higher at solar minimum than solar maximum, and higher during the night than day.

We forward model WACCM-X hydrogen density and temperature profiles using the radiative transport code, lyao_rt (Bishop, 1999) to obtain H column emission intensities. Intensities are reported in rayleighs (R), where 1 R is defined as an apparent column emission rate of ph (Chamberlain, 1961), which has been shown (Baker and Romick, 1976) to be numerically equivalent to a specific intensity (or surface brightness) of /4 ph . WHAM observations are calibrated against the North American Nebula (NGC 7000) using the 850 R absolute intensity standard (Scherb, 1981). Additionally, the WACCM-X simulations used in this study are from Solomon et al. (2019) for the years 2001–2005 run for perpetual solar maximum conditions and were selected to be compared qualitatively to the Mierkiewicz et al. (2012) ground-based H observations from 2000 to 2001. Here, we compare these WACCM-X model runs for the March equinox with observations from March 2000. Although these simulations do not reproduce the observational conditions for March 2000, they are appropriate for a qualitative seasonal comparison. WACCM-X is chosen over other models like the Thermosphere Ionosphere Electrodynamics-General Circulation Model (TIE-GCM), Whole Atmosphere Model (WAM), and Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA) for simulating upper atmosphere temperature, winds, and neutral species densities, including hydrogen density, because of its vertical coverage from the surface to the exobase (700 km depending on solar activity), advanced chemistry modules, including multi-species hydrogen chemistry, and self-consistent neutral and ionized species coupling. It captures key processes such as hydrogen transport, ion-neutral interactions, and variability driven by tides and planetary waves more holistically than models with limited altitude ranges (TIE-GCM) or simpler chemistry and ion-neutral coupling (WAM and GAIA). Additionally, this work aims to build off the previous work of Gallant et al. (2019) that compared the 2000–2001 PBO observations to forward modeled MSIS-00 results. The broader significance of this work lies in establishing a framework for examining geocoronal hydrogen variability over a wide range of temporal scales, including during geomagnetic storm conditions.

The 2000–2001 ground-based H observations from Mierkiewicz et al. (2012) are described in Section 2.1, the WACCM-X simulations used in this study are described in Section 2.2, and the forward modeling technique to produce WACCM-X and MSIS-00 H emission intensity is described in Section 2.3. Results are presented in Section 3 and discussed in Section 4.

2 Methods

2.1 2000-2001 ground-based H observations

From 2000 to 2001, a comprehensive data set of 20 dark Moon periods consisting of multiple nighttime observations was obtained at PBO in Wisconsin (43.07N, 270.33E). These nighttime geocoronal H line profile observations were collected using a 15 cm aperture, dual etalon FPI. The PBO FPI is coupled to a siderostat with a circular 1.5 field of view on the sky and has a resolving power of R = 80,000 and a spectral resolution of 0.08 Å at H (Mierkiewicz et al., 2012). Furthermore, observations were made during optimal dark sky conditions where the Sun was at least 10 below the horizon (solar depression angle 10) and on clear nights within 2 weeks centered around the new Moon (Mierkiewicz et al., 2012). Additionally, the siderostat allowed observations of a wide range of viewing geometries while avoiding regions of significant Galactic H emission (Haffner et al., 2003). Therefore, all observations were obtained at least 10 away from the Galactic plane but as close to the zenith as possible. See Mierkiewicz et al. (2012) for more information regarding the 2000–2001 PBO observations and Mierkiewicz et al. (2006) for more information regarding the technique of Fabry-Perot spectroscopy applied to geocoronal hydrogen.

Here, we focus on PBO observations from March 2000. This observation period was the focus of the model-data comparisons in Bishop et al. (2004) and lasted from 28 February to 6 March, with the new Moon on 6 March. Solar conditions include the index varying between 197 and 219 with an 81-day average of 182. Geophysical conditions were generally quiet with 28 February being the only day with an Ap index 12 (Mierkiewicz et al., 2012).

2.2 WACCM-X simulations

Solomon et al. (2018), Solomon et al. (2019) conducted the first whole atmosphere simulations of global variability using WACCM-X over a 30-year period, where solar and geomagnetic conditions were controlled in order to isolate atmospheric responses to changes in radiatively active gases. The WACCM-X simulations were run for perpetual solar and geomagnetic conditions representative of solar minimum ( = 70 and Kp = 0.3) (Solomon et al., 2018) and solar maximum ( = 200 and Kp = 3.0) (Solomon et al., 2019). These studies found that as the lower atmosphere gradually warms, the upper atmosphere shows a cooling trend (Solomon et al., 2018; 2019).

In this study, the Solomon et al. (2019) perpetual solar maximum simulations for the years 2001–2005 are used, but specifically for the PBO geographic location. Furthermore, these WACCM-X simulations are characterized for all seasons at UT = 0 which corresponds to local evening twilight (6 p.m. or 7 p.m. depending on the season) at PBO; March equinox comparisons are presented here. WACCM-X model inputs are summarized in Table 1. See Solomon et al. (2019) for more information regarding these WACCM-X simulations.

2.3 Forward modeling WACCM-X & MSIS-00 H emission intensity

Geocoronal H column emission intensity is modeled using the atomic resonance radiative transport code, known as lyao_rt, developed by James Bishop (Bishop, 1999). These H column emission intensities are calculated assuming a non-isothermal, spherically symmetric exosphere and depend on the hydrogen density [H] profile, solar Lyman- line center flux, and observational viewing geometry. Geocoronal H emission is excited by solar Lyman- (1,026 Å; hereafter referred to as Ly-), however, Ly- is fully absorbed by at approximately 102 km. Therefore, the Earth’s shadow is a cylinder with a radius equal to the radius of the Earth (6,378 km) plus 102 km. Because singly scattered H emission originates in the sunlit atmosphere, the Earth’s shadow is used as a means to probe the altitude structure of the exosphere. As a result, H emission intensity is expressed as a function of shadow altitude, where shadow altitude is the altitude of the crossing point between the Earth’s shadow and the ground-based FPIs line-of-sight. Bishop et al. (2004) found that shadow altitude is the more advantageous independent parameter compared to solar zenith angle (see Figure 1 in Bishop et al. (2004)). Furthermore, Mierkiewicz et al. (2012) have also shown that H emission intensity varies significantly with shadow altitude with 2 R at midnight and 15 R at dawn and dusk.

In this study, lyao_rt was set to use WACCM-X thermospheric temperature and density profiles, including H, O, , and , extending from 102–494 km [nominal exobase in the Bishop et al. (2004) study], as inputs for a model thermosphere. Lyao_rt first extends the WACCM-X hydrogen model thermosphere to exospheric altitudes using either the Chamberlain model exosphere (Chamberlain, 1963) or the Bishop analytic exosphere model (Bishop, 1991). Lyao_rt then uses the extended [H] profile and observational viewing geometry to calculate H column emission intensities. Lyao_rt also computes line-of-sight apparent column emission rates of hydrogen for specified look directions. Synthetic line-of-sight geometries for the zenith look direction are used in this work. To extend the WACCM-X thermospheric hydrogen density profile to exospheric altitudes, the “evaporative” case of the Bishop analytic exosphere model (Bishop, 1999) is used, where the population of hydrogen atoms traveling on satellite orbits is determined using the kinetic distribution of the hydrogen atoms traveling on ballistic orbits. This population is calculated using two parameters specifying the extent of the satellite atom component, the satellite component temperature and density (Bishop, 1999). However, in the evaporative case, the satellite atom component is calculated using the nominal exobase temperature and number density provided by WACCM-X.

MSIS-00 thermospheric temperature and density profiles were also used with lyao_rt. Similarly to WACCM-X, the evaporative case of the Bishop analytic exosphere model is applied to extend the MSIS-00 model hydrogen distribution to exospheric altitudes. Lyao_rt can also allow for a user-specified [H] profile that can be substituted for the MSIS-00 [H] profile. This “modified” MSIS-00 [H] profile is built from 3 parameters described in Bishop (2001), known as the [H](z) thermospheric parameters: exobase density , photochemical upward flux , and mesospheric peak density . This is extensively studied by Bishop et al. (2004) who substituted user-specified [H] profiles for MSISE-90 (Hedin, 1991) [H] profiles to fit the model to the PBO 4 March 2000 observations. Here, the March equinox MSIS-00 [H] profile is adjusted using the same satellite atom and [H](z) thermospheric parameter values for PM conditions from Figure 3a in Bishop et al. (2004). This modified MSIS-00 profile is used as a proxy to the March 2000 observations which allows us to evaluate the relative agreement between WACCM-X, MSIS-00, and the observations. See Table 2 for the lyao_rt input parameters used for each forward modeled run and the satellite atom and [H](z) thermospheric parameter values used to calculate modified MSIS-00 H emission intensities. See Bishop et al. (2004) for more information regarding the search procedure used to determine these [H](z) thermospheric parameters as well as a sensitivity analysis of the search procedure.

3 Results

In order to investigate variations of upper thermospheric hydrogen, WACCM-X thermospheric temperature and density profiles for the conditions outlined in Table 1 are used as inputs to lyao_rt. MSIS-00 forward modeled simulations are also conducted for the conditions in Table 2. For the MSIS-00 case, lyao_rt uses the Ap index as opposed to the Kp index used by WACCM-X. Therefore, to be consistent with WACCM-X, Kp = 3 is converted to Ap = 15 using NOAA’s Geomagnetic Kp and Ap Indices. Furthermore, the solar Ly- line center flux for high solar activity is estimated to be ph . This value was calculated by deriving the scaling needed to match the forward modeled H column emission intensity with the full set of March 2000 PBO observations (Myers et al., 2024) and is within range of the values reported in Bishop et al. (2004). Lastly, observational uncertainty in the geocoronal signal arises from tropospheric scattering by molecules and aerosols in the lower atmosphere (Bishop et al., 2004) as well as cascade enhancements of the Balmer- line profile (Roesler et al., 2014; Mierkiewicz et al., 2012). The March 2000 observations are not adjusted to account for estimates of tropospheric scattering and the model runs do not account for cascade enhancements. These effects will be carefully considered in future work.

3.1 Case study: march equinox

Figure 1 illustrates the 2001–2005 WACCM-X vertical [H] profiles and forward modeled H intensities for the March equinox at PBO. Figure 1a compares the WACCM-X forward modeled H intensities to an MSIS-00 forward modeled simulation and the March 2000 PBO observations. At 400 km shadow altitude, the forward modeled WACCM-X and MSIS-00 March equinox H intensities are lower than the observed H emission intensities by approximately 1.9 R and 2.6 R, respectively. These results show that WACCM-X is in better agreement in terms of magnitude of intensity to the observations than MSIS-00. In Figure 1b, a modified MSIS-00 simulation characterized by the [H](z) thermospheric parameters listed in Table 2 is added. Here, it is evident that the modified MSIS-00 intensity profile shows better agreement with the observations than both WACCM-X and MSIS-00. Figure 1c shows extended vertical [H] profiles for all March equinox simulations. The [H] profiles are “zoomed in” to an altitude range of 300–500 km in Figure 1d to better illustrate how each profile behaves near the exobase (500 km). According to Figure 1c, the modified MSIS-00 thermospheric [H] profile agrees more closely with WACCM-X. However, Figure 1d shows that, near the exobase, MSIS-00 is in better agreement with WACCM-X than the modified MSIS-00 case. Therefore, the March equinox WACCM-X and MSIS-00 [H] profiles appear to disagree in the thermosphere but converge near the exobase.

FIGURE 1

Bishop et al. (2004). (b) Same as (a) but now compared to a forward modeled modified MSIS-00 simulation (gray circles) where the [H] profile used is the best fit to the March 2000 observations. Parameter values for the modified MSIS-00 simulation are outlined in Table 2. (c) Extended vertical [H] profiles for all March equinox simulations. The solid lines indicate the input MSIS-00 [H] profile and one of the input WACCM-X [H] profiles (2004). (d) Vertical [H] profiles for all March equinox simulations for the altitude range 300–500 km.

Using the same viewing geometry as WACCM-X and MSIS-00, the modified MSIS-00 forward model run is used to represent the observed H intensity from PBO. Figure 2 shows ratios of the modified MSIS-00 intensity profile to the WACCM-X and MSIS-00 intensity profiles. Figure 2a illustrates three intensity profiles, including the mean WACCM-X, MSIS-00, and modified MSIS-00 intensity profiles, used to calculate the ratios in Figure 2b. Similarly, Figure 2c shows all the intensity profiles used to calculate the ratios in Figure 2d. As shown in Figure 2c, individual WACCM-X intensities for each year (2001–2005) are used to calculate the ratios. The ratios of the reference profile to both the mean and individual WACCM-X intensities show that WACCM-X is closer in intensity value to the observations than MSIS-00, however, the slope of the reference profile to the individual WACCM-X runs has a range of - to -. The slope of reference profile/MSIS-00 falls just outside the less negative end of this range with a value of -. This suggests that the MSIS-00 intensity profile offers slightly better agreement to the reference profile intensity profile compared to WACCM-X in terms of the slope. The difference between WACCM-X and MSIS-00, however, is not extreme and demonstrates that more comparisons between observations and modeled simulations are needed.

FIGURE 2

4 Discussion

We present an early application of forward modeling WACCM-X simulations to compute H emission intensities. Here, we also include a comparative analysis with MSIS-00 and its “modified version.” The PBO WACCM-X simulations used in this work were run for March equinox (day of year 79 or 80) during the years 2001–2005, perpetual solar maximum conditions, and UT = 0. In the present work, the lyao_rt simulations assume a spherically symmetric hydrogen density, which represents a first-order approximation intended to isolate the primary dependencies of the H signal on the global hydrogen density structure. While this assumption enables efficient forward modeling and comparison with observations, it may introduce systematic biases in the modeled H intensities if strong asymmetries are present (e.g., along the Sun-Earth line). As an intermediate approximation, some effects of time-dependent asymmetry could be explored through a sequence of lyao_rt simulations that each assume spherical symmetry but allow the hydrogen density to evolve in time, with the resulting H intensities interpreted in an averaged sense. Such an approach may capture aspects of the net variability in global hydrogen abundance while avoiding the computational complexity of fully asymmetric radiative transfer. However, this approximation would still be unable to reproduce directional scattering effects explicitly tied to the Sun-Earth geometry. Addressing these effects more rigorously would require extending lyao_rt to incorporate asymmetric hydrogen distributions, for example, through parameterized day-night density contrasts or coupling to two- or three-dimensional exospheric models along the Sun-Earth direction. Such developments are beyond the scope of the current study but represent an important avenue for future work aimed at improving the physical realism of H modeling and reducing potential biases associated with symmetry assumptions. In future work, we plan to increase the accuracy of these model-model-data comparisons by comparing the 2000–2001 PBO observations to WACCM-X simulations that are representative of observational conditions, including correct dates, solar conditions, and nudged with meteorology analysis fields in the troposphere-stratosphere so the model atmosphere remains close to observed meteorological conditions. Model uncertainties and observational errors will also be explored more in depth. We will also run lyao_rt with line-of-sight viewing geometries consistent with the observations as opposed to the synthetic viewing geometry used in this work. Other future work includes producing line widths for WACCM-X simulations as well as coupling NRLMSIS 2.0 (Emmert et al., 2021) with lyao_rt to conduct further comparisons with this new MSIS model version.

Radiative transfer processes, such as the resonant scattering of solar Ly- by atomic hydrogen and multiple scattering, rely on an accurate representation of hydrogen thermospheric density and temperature where these profiles directly control the populations of energetic states and the resulting exospheric hydrogen intensity. As a result, comparisons between forward modeled WACCM-X H intensities and ground-based observations contribute to validating thermospheric hydrogen variability within the WACCM-X numerical model.

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