Thermospheric response to solar EUV variability probed with orbital-energy decay from NinjaSat CubeSat GNSS telemetry
8 affiliations
- 1Department of Physics, School of Science, Institute of Science Tokyo
- 2Advanced Research Laboratories, Tokyo City University
- 3Ibaraki University
- 4RIKEN Pioneering Research Institute
- 5Department of Physics, Kyoto University
- 6International Center for Hadron Astrophysics, Chiba University
- 7Graduate School of Advanced Science and Engineering, Hiroshima University
- 8RIKEN Nishina Center for Accelerator-Based Science
Abstract
Solar extreme ultraviolet (EUV) radiation heats and expands the thermosphere, modulating atmospheric density at altitudes in low Earth orbit and thereby increasing satellite drag and accelerating orbital decay. Previous studies have relied on empirical solar proxies, such as the F10.7 cm radio flux and Two-Line Element (TLE)-derived orbit solutions, which lack sufficient spectral resolution and temporal cadence to resolve wavelength-dependent thermospheric responses, and on orbital-decay metrics—most commonly the satellite altitude itself—that are not conserved quantities along the orbit and have rarely been validated against independent-density measurements. Here, we combine spectrally resolved EUV irradiance from the GOES-18 Extreme Ultraviolet and X-ray Irradiance Sensors at seven wavelengths (25.6–140.5 nm) with high-cadence GNSS position and velocity data from NinjaSat, an X-ray-astronomy 6U CubeSat orbiting at approximately 500–530 km altitude, over an 8-month period (April–November 2024). We derive the orbital-decay signal from the J2 J 2 -corrected specific mechanical energy computed directly from the GNSS position and speed telemetry—a conserved quantity whose within-orbit scatter is about 50 times smaller than that of an altitude-based (point-mass energy) decay metric and which, unlike TLE/SGP4-based approaches, requires no propagated reference orbit. This energy-based decay rate correlates with independent thermospheric-density retrievals from the Swarm and GRACE-FO missions with Spearman coefficients of 0.85–0.92 (daily means), and its systematic-error budget (radiation pressure, unmodeled gravitational perturbations) remains below the statistical uncertainty of the daily decay rate. Cross-correlation analysis shows that the decay rate is strongly related to EUV and far-ultraviolet (FUV) irradiance (Spearman coefficients up to ∼ ∼ 0.8), with a positive lag of order 1–3 days. Using a block-bootstrap procedure that accounts for the strong autocorrelation of both series, we find that the 95% confidence intervals of the estimated lag overlap across all seven wavelengths, extending from the 6-h search floor to approximately 100 h, while the central lag estimates lie at roughly 30–70 h depending on wavelength and analysis choice; a wavelength-dependent separation of the response times is not statistically resolved with the present dataset and remains a plausible but unconfirmed hypothesis pending independent constraints from physics-based thermosphere models. These results demonstrate that standard GNSS telemetry from a CubeSat, combined with a conserved orbital-energy metric and independent-mission validation, can quantitatively characterize the thermospheric drag response to solar EUV forcing, offering a scalable observational approach for space-weather monitoring and satellite drag modeling. Graphic Abstract
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BibTeX
@article{tanaka2026thermospheric,
title = {{Thermospheric response to solar EUV variability probed with orbital-energy decay from NinjaSat CubeSat GNSS telemetry}},
author = {Hayato Tanaka and Takao Nakagawa and Satoshi Nozawa and Toru Tamagawa and Teruaki Enoto and Takao Kitaguchi and Wataru Iwakiri and Tatehiro Mihara and Tomoshi Takeda and Naoyuki Ota and Amira Aoyama and Satoko Iwata and Takuya Takahashi and Kaede Yamasaki},
journal = {Earth, Planets and Space},
year = {2026},
month = oct,
doi = {10.1186/s40623-026-02557-x},
url = {https://doi.org/10.1186/s40623-026-02557-x},
note = {CC BY},
}
RIS
TY - JOUR
TI - Thermospheric response to solar EUV variability probed with orbital-energy decay from NinjaSat CubeSat GNSS telemetry
AU - Hayato Tanaka
AU - Takao Nakagawa
AU - Satoshi Nozawa
AU - Toru Tamagawa
AU - Teruaki Enoto
AU - Takao Kitaguchi
AU - Wataru Iwakiri
AU - Tatehiro Mihara
AU - Tomoshi Takeda
AU - Naoyuki Ota
AU - Amira Aoyama
AU - Satoko Iwata
AU - Takuya Takahashi
AU - Kaede Yamasaki
PY - 2026
DA - 2026/10/25
JO - Earth, Planets and Space
DO - 10.1186/s40623-026-02557-x
UR - https://doi.org/10.1186/s40623-026-02557-x
AB - Solar extreme ultraviolet (EUV) radiation heats and expands the thermosphere, modulating atmospheric density at altitudes in low Earth orbit and thereby increasing satellite drag and accelerating orbital decay. Previous studies have relied on empirical solar proxies, such as the F10.7 cm radio flux and Two-Line Element (TLE)-derived orbit solutions, which lack sufficient spectral resolution and temporal cadence to resolve wavelength-dependent thermospheric responses, and on orbital-decay metrics—most commonly the satellite altitude itself—that are not conserved quantities along the orbit and have rarely been validated against independent-density measurements. Here, we combine spectrally resolved EUV irradiance from the GOES-18 Extreme Ultraviolet and X-ray Irradiance Sensors at seven wavelengths (25.6–140.5 nm) with high-cadence GNSS position and velocity data from NinjaSat, an X-ray-astronomy 6U CubeSat orbiting at approximately 500–530 km altitude, over an 8-month period (April–November 2024). We derive the orbital-decay signal from the J_2 J 2 -corrected specific mechanical energy computed directly from the GNSS position and speed telemetry—a conserved quantity whose within-orbit scatter is about 50 times smaller than that of an altitude-based (point-mass energy) decay metric and which, unlike TLE/SGP4-based approaches, requires no propagated reference orbit. This energy-based decay rate correlates with independent thermospheric-density retrievals from the Swarm and GRACE-FO missions with Spearman coefficients of 0.85–0.92 (daily means), and its systematic-error budget (radiation pressure, unmodeled gravitational perturbations) remains below the statistical uncertainty of the daily decay rate. Cross-correlation analysis shows that the decay rate is strongly related to EUV and far-ultraviolet (FUV) irradiance (Spearman coefficients up to ∼ ∼ 0.8), with a positive lag of order 1–3 days. Using a block-bootstrap procedure that accounts for the strong autocorrelation of both series, we find that the 95% confidence intervals of the estimated lag overlap across all seven wavelengths, extending from the 6-h search floor to approximately 100 h, while the central lag estimates lie at roughly 30–70 h depending on wavelength and analysis choice; a wavelength-dependent separation of the response times is not statistically resolved with the present dataset and remains a plausible but unconfirmed hypothesis pending independent constraints from physics-based thermosphere models. These results demonstrate that standard GNSS telemetry from a CubeSat, combined with a conserved orbital-energy metric and independent-mission validation, can quantitatively characterize the thermospheric drag response to solar EUV forcing, offering a scalable observational approach for space-weather monitoring and satellite drag modeling. Graphic Abstract
SN - 1880-5981
SN - 1343-8832
ER -