Christopher J. Mertens

Exposure to ionizing radiation from galactic cosmic rays and solar energetic particles at aviation flight altitudes can have an adverse effect on human health. Although airline crews are classified as radiation workers by the International Commission on Radiological Protection (ICRP) , in most countries, their level of exposure is unquantified and undocumented throughout the duration of their career. As such, there is a need to assess aviation crew ionizing radiation exposure. The Nowcast of Aerospace Ionizing RAdiation System (NAIRAS) is a real‐time, global, physics‐based model currently used to assess such exposure. To evaluate recent model updates, radiation measurements from the Automated Radiation Measurements for Aerospace Safety (ARMAS) flight inventory are utilized. The inventory contains 1,324 flights at typical commercial aviation cruising altitudes during the most recent solar cycle, and covers latitudes 85°N to 85°S. Overall, while NAIRAS dose rates are uniformly biased high relative to the ARMAS dosimeter measurements, there is no bias relative to solar activity. Inspection of the model differences suggests that the TS05 magnetospheric magnetic field model may produce cutoff rigidities that are too low during quiet geomagnetic periods, particularly at mid‐to‐low latitudes, resulting in higher dose rates in NAIRAS. The difference in the altitude unit is also a source of error; the ARMAS flight database reports aircraft altitudes in GPS coordinates, while barometric pressure altitudes are required for NAIRAS. Future aircraft flights are being designed to constrain the uncertainties in the dosimetric measurements and model calculations.

Nowcasting and forecasting of the radiation environment in the Earth's lower atmosphere are critical for the safety of aircraft and spacecraft crews and passengers. Currently, this problem is addressed by employing statistical and physics‐based models that take into account particle transport and precipitation. However, given the increased number of radiation measurements available to the community, it is possible to start developing data‐driven approaches. We prepared Machine Learning‐ready (ML‐ready) data sets to nowcast the effective dose rates at aviation altitudes. The presented data sets contain 92,476 individual measurements from 589 flights obtained by the Automated Radiation Measurements for Aerospace Safety (ARMAS) experiment from 2013 to 2023. The ARMAS measurements are augmented with the properties of the Geospace environment, such as solar soft X‐ray and proton fluxes, solar wind properties, secondary cosmic ray neutrons, space weather indexes, and global solar activity indicators (such as daily sunspot number). ARMAS data are separated into three partitions, ensuring that (a) the data points from a single flight remain within the same partition, and (b) each partition samples the flight locations and Geospace environment conditions equally. Several versions of the data sets allow predictions based on point‐in‐time measurements and use up to 24 hr of Geospace parameter history. The test of the use case demonstrates a possibility of nowcasting ARMAS measurements with accuracies slightly better than the considered physics‐based models. The publicly available ML‐ready data sets could serve as the first step in data preparation for ML‐driven nowcasting and forecasting of the radiation environment.

The Nowcast of Aerospace Ionizing RAdiation System (NAIRAS) model predicts the radiation environment from the Earth's surface to free‐space. The model output provides dosimetric and particle flux quantities required to assess the hazardous radiation impacts to human health and adverse effects on vehicle electronic systems. The four sources of ionizing radiation included in NAIRAS are galactic cosmic rays (GCR), solar energetic particles (SEP), trapped protons, and trapped electrons. The focus of this paper is to present an advance in the state‐of‐the‐art in SEP radiation modeling by extending the NAIRAS nowcast capability to a forecast capability. A new SEP peak effective dose rate forecast model (for brevity, denoted SEP peak dose forecast) is developed by coupling peak integral proton flux forecast products provided by the University of Málaga Solar particle Event Predictor (UMASEP) model to the input data stream of the NAIRAS model. The UMASEP‐NAIRAS SEP peak dose forecast approach is assessed by analyzing the September 2017 and the May 2024 SEP events. The model forecast uncertainties are characterized at commercial and military aviation altitudes and at low‐Earth orbit (LEO) trajectories similar to the International Space Station (ISS) orbit. The analysis of the two SEP events demonstrates accurate SEP peak dose forecasts bolstering the viability of this forecast approach. The future plan is to make the UMASEP‐NAIRAS SEP peak dose forecast products available in real‐time at the Community Coordinated Modeling Center (CCMC) Integrated Space Weather Analysis Web‐based dissemination system.

Time series of infrared radiated power by thermospheric nitric oxide (NO) and carbon dioxide (CO₂), and their total power (NO + CO₂) (measured by the SABER instrument on the NASA TIMED satellite from 2002 through 2023) have been analyzed for long‐term trends. Solar and geomagnetic indices (F_10.7, Ap, Dst) are used with multiple linear regression to provide trend approximations for the radiated infrared power. There are no statistically significant trends observed for NO and total radiated power, and the trend in CO₂ radiated power is not statistically significant at 2‐sigma (95%) confidence level. The thermosphere continues to radiate a constant amount of infrared energy over time after accounting for natural variability, despite more than two decades of cooling due to increasing CO₂. These results demonstrate energy conservation on decadal timescales and validate the mechanism of thermospheric greenhouse cooling. The long‐term radiometric stability of the SABER instrument enables these results.

The Nowcast of Aerospace Ionizing RAdiation System (NAIRAS) model predicts dosimetric and radiative flux quantities for assessing human radiation exposure levels and radiation effects on flight electronic systems from the surface of the Earth to deep space. NAIRAS version 3 includes many updates and improvements to the model. In this paper, NAIRAS version 3 dosimetric quantities are compared to observations of the atmospheric radiation environment taken during the Radiation Dosimetry Experiment (RaD‐X) flight campaign. The RaD‐X campaign consisted of a high‐altitude balloon flight, with altitudes up to 40 km, and four coordinated aircraft flights covering an altitude range from 6 to 20 km. The RaD‐X measurement data from the different flight platforms were acquired in September 2015 and at nearly the same vertical cutoff rigidity (3–4 GV). High quality, flight‐averaged measurement data were provided at seven altitudes, which correspond to regions of fundamentally different radiation transport and collisional interaction physics. The NAIRAS model agrees with the RaD‐X measurements to within 30 over the entire altitude domain of the flight campaign (0–40 km), which is within the measurement uncertainty.

Earth experienced the strongest geomagnetic storm in 20 years over 10–13 May 2024. The Ap and Dst geomagnetic indices were 273 and −291.94 nT on 11 May. The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics satellite observed significant enhancement in thermospheric infrared emission at 15, 5.3, 4.3, and 1.27 μm. On 11 May the daily global power radiated by nitric oxide (NO) at 5.3 μm was 1.41 TW and by carbon dioxide (CO ₂ ) at 15 μm was 1.35 TW. These are the largest single day power values observed by SABER in 22 years and the first time the daily power radiated by NO exceeded that of CO ₂ . The total infrared power (above background) radiated during the storm was 2.64 TW (2.28 × 10 ¹⁷ J). Significant enhancement in limb radiance observed at 4.3 μm (to 250 km tangent height) is likely indicative of NO + formation during the storm.

Time series of radiative cooling of the upper mesosphere and lower thermosphere (UMLT) by carbon dioxide (CO ₂ ) are examined for evidence of trends over 20 years. Radiative cooling rates in K day ⁻¹ provided by the SABER instrument are converted to time series of infrared power radiated from three distinct layers between 0.1 hPa and 0.0001 hPa (65–105 km). Linear regression against time and a predictor for solar variability provides estimates of the trend in exiting longwave radiation (ELR) from these layers. Trends in ELR are not significantly different from zero at 95% or 99% confidence in each layer. These results demonstrate energy conservation in the UMLT on decadal time scales and show that the UMLT continues to radiate the same amount of energy it receives despite cooling and contracting over two decades. These results are enabled by the long‐term stability of the SABER instrument calibration.

Twenty‐two years (2002–2023) of infrared radiative cooling rate data derived from the SABER instrument on the NASA TIMED satellite are presented. Global daily and global annual infrared power (Watts, W) emitted by nitric oxide (NO) and carbon dioxide (CO ₂ ) illustrate the variability of the geospace environment on timescales from days to decades. The 11‐year solar cycle (SC) is evident in the global power data and in vertical profiles of infrared cooling rates (nW/m ³ ). The global annual power radiated by NO and CO ₂ are larger in 2023 than at any time since 2003 and 2002, respectively. The to‐date peak in NO infrared power in SC 25 is larger than in SC 24, is comparable to SC 20, but is less than in SCs 18–19 and 21–23. Two geomagnetic storms in 2023 radiated more than 1 TW and are in the top 10 strongest storms observed by SABER.

Exposure to ionizing radiation from galactic cosmic rays (GCR) and solar energetic particles (SEP) at aircraft flight altitudes can have an adverse effect on human health. Although airline crews are classified as radiation workers by the International Commission on Radiological Protection (ICRP), in most countries, their level of exposure is unquantified and undocumented throughout the duration of their career. As such, there is a need to assess pilot ionizing radiation exposure. The Nowcast of Aerospace Ionizing RAdiation System (NAIRAS), a real‐time, global, physics‐based model is used to assess such exposure. The Automated Radiation Measurements for Aerospace Safety (ARMAS) measurement data set consists of high latitude, high altitude, and long‐duration aircraft flights between 2013 and 2023. Here, we characterize radiation exposure at aviation flight altitudes using the NAIRAS model and compare with 45 flight trajectories from the recent ARMAS flight measurement inventory.