- The Catholic University of America located at Solar Physics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, United States
Solar variability and solar spectral irradiance (SSI) are important for studying planetary atmospheres, particularly the ionosphere–thermosphere–mesosphere (ITM) system, and planetary exospheres. This paper introduces new SSI time series from the CODET model, obtained at different geocentric distances, namely,
1 Introduction
Solar spectral irradiance (SSI) plays an important role in the ionosphere–thermosphere–mesosphere (ITM) system and in the Earth’s exosphere. Characteristics such as the exospheric density provides clues about the past, present, and future of Earth’s atmosphere and also offer insights into the atmospheres of other planets. In general, the exosphere connects Earth’s atmosphere to the interplanetary space. The exosphere can provide key insights into Earth’s atmosphere loss mechanisms resulting from Sun–Earth interactions. Some exospheric neutrals are lost to the interplanetary space due to the influence of solar extreme ultraviolet (EUV) photoionization (Connor et al., 2023). Additionally, the exosphere is dynamic and directly affected by the solar activity and during geomagnetic storms (Cucho-Padin and Waldrop, 2019; Qin et al., 2017).
SSI is important to determine and follow changes in the exospheric density (Connor et al. (2023) and references therein) and for understanding how periods with high solar activity, such as the solar maximum, can affect the thermosphere and the heating process present there. Although the importance of SSI in the planetary atmospheres is well-known, there is no general agreement on how it can impact the Earth’s exosphere during the maximum and minimum solar activity or in the upper atmospheres of other planets such as Mars and Venus. Solar variability is associated with solar magnetic activity and occurs across different timescales. Long time-scales are related to the solar cycle modulation (
Thus, understanding solar variability and its impacts on planetary atmospheres (including the ITM system and their exospheres) is important, especially for future human exploration of the Moon and Mars. In addition to the influence of EUV and X-ray fluxes in the era of exoplanetary exploration, these fluxes play an important role. They can help characterize exoplanet atmospheres and understand the variability of the host star (Krishnamurthy and Cowan, 2024; Linsky, 2014).
This paper aims to show the importance of SSI in EUV wavelengths using the COronal DEnsity and Temperature (CODET) model, particularly when observational data are unavailable. Specifically, this study uses the CODET model versions 1.0 (Rodríguez-Gómez, 2017; Rodríguez Gómez et al., 2018; 2019) and 1.1 (Rodríguez Gómez, 2025) (details in Section 2). New SSI time series from the CODET
2 SSI from the CODET model
The CODET model is a physics-based model (Rodríguez Gómez, 2025; Rodríguez Gómez et al., 2018; Rodríguez-Gómez, 2017). It uses the relationship between the solar magnetic field, density, temperature, and emission. The CODET model uses the solar photospheric magnetic field from SOHO/MDI (Scherrer et al., 1995) and SDO/HMI (Scherrer et al., 2012), a flux transport model (Schrijver, 2001), and a coronal magnetic field extrapolation (PFSS) model (Schrijver, 2001; Schrijver and De Rosa, 2003) to obtain the solar atmosphere’s magnetic structure. The plasma temperature and density are derived from scaling laws and used as inputs to the emission model, which retrieves daily SSI in EUV wavelengths (i.e., the mean full-disc intensity) (Rodríguez-Gómez, 2017; Rodríguez Gómez et al., 2018). This model accurately describes the solar coronal emission on scales from days to solar cycles. The original version of the CODET model, version 1.0 (Rodríguez-Gómez, 2017; Rodríguez Gómez et al., 2018), used TIMED/SEE data (Hock and Eparvier, 2008; Woods et al., 2005) to compare modeled SSI. This version model provides SSI in
3 SSI modeled at different geocentric distances and its potential applications for studying the Earth’s exosphere
SSI plays a key part in determining the density of the exosphere. Exospheric H atoms resonantly scatter the near-line-center solar Lyman-
Figure 1 shows the SSI time series at

Figure 1. SSI time series at
3.1 Impact of SSI from the CODET model on Earth’s upper atmosphere
The Sun–Earth interaction contributes to the atmosphere’s loss, and the Earth’s exosphere can provide important information regarding the loss mechanism. The composition of the Earth’s exosphere is dominated mainly by hydrogen, helium, and oxygen. These neutrals are lost to the interplanetary space by solar EUV photoionization and charge exchange with plasmas from the magnetosphere and the interplanetary medium (Connor et al., 2023). Solar variability, especially some events such as coronal mass ejections (CMEs), flares, or solar winds, can affect the Earth’s atmosphere; for example, geomagnetic storms can directly affect the dynamics of the exosphere (Cucho-Padin and Waldrop, 2019; Qin et al., 2017). Additionally, changes in solar activity are related to some processes such as heating efficiency, radiative cooling, thermal conduction, and dynamics in planetary exospheres (Forbes et al., 2008). The interaction of atomic hydrogen Lyman-
Figure 2 shows SSI from the CODET model version 1.0 and SSI from the solar chromosphere using the Lyman-

Figure 2. SSI time series at
To analyze the impact of geomagnetic storms through Dst and Ap indices, two different geomagnetic regimes were defined for strong Dst
Additionally, the chromospheric emission in Lyman-

Figure 3. Scatter plots of SSI from the CODET model at 21.1 nm (left) and 19.3 nm (right) versus SSI in Lyman-α from 1 July 1996 to 29 March 2025.
It is well-known that solar irradiance affects the exosphere. However, the duration and extent of the changes in exospheric density due to variations in solar irradiance and during geomagnetic storms throughout the solar cycles remain unclear. Recently, Zoennchen and Cucho-Padin (2025) showed that density distributions during weak geomagnetic disturbances at
4 SSI modeled at Mars
The SSI modeled at Mars can be obtained using an adapted version of the CODET model version 1.0 and 1.1 at

Figure 4. SSI time series at
The best model performance was obtained at both wavelengths from 6 July 2016 to 15 October 2022 (during the minimum between the solar cycles 24 and 25). In general, SSI from

Figure 5. Solar spectral irradiance observed by MAVEN/EUVM and modeled using the
5 Summary and discussion
Due to the CODET model’s versatility, it is possible to obtain SSI time series at different geocentric and heliocentric distances, providing a remarkable opportunity to study planetary atmospheres even when no observational data are available. The results of this study can be summarized as follows.
Data availability statement
The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found at: https://github.com/Rodriguez-Gomez/CODET_geocentric.git https://github.com/Rodriguez-Gomez/CODET_Mars.git.
Author contributions
JR-G: Funding acquisition, Validation, Software, Methodology, Formal Analysis, Writing – original draft, Conceptualization, Investigation.
Funding
The author(s) declare that financial support was received for the research and/or publication of this article. This work was supported by NASA Living With a Star (LWS) Program, Focused Science Topic: “Beyond F10.7: Quantifying Solar EUV Flux and its Impact on the Ionosphere–Thermosphere–Mesosphere System,” No. 80NSSC23K0900.
Conflict of interest
The author declares that this research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Generative AI statement
The author(s) declare that no Generative AI was used in the creation of this manuscript.
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Footnotes
1https://wdc.kugi.kyoto-u.ac.jp/dstae/index.html
2https://www.gfz.de/en/section/geomagnetism/data-products-services/geomagnetic-kp-index
3https://lasp.colorado.edu/lisird/data/lyman_alpha_model_ssi
4https://lasp.colorado.edu/lisird/data/mvn_euv_l3_daily
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Keywords: solar spectral irradiance, exosphere, Earth, Mars, geomagnetic storms
Citation: Rodríguez-Gómez JM (2025) Solar spectral irradiance from the CODET model for studying planetary exospheres: Earth and Mars. Front. Astron. Space Sci. 12:1638510. doi: 10.3389/fspas.2025.1638510
Received: 30 May 2025; Accepted: 06 August 2025;
Published: 28 August 2025.
Edited by:
Shingo Kameda, Rikkyo University, JapanReviewed by:
Debi Prasad Choudhary, California State University, Northridge, United StatesThomas Woods, University of Colorado Boulder, United States
Copyright © 2025 Rodríguez-Gómez. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
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