Alexandre Winant

When high-energy particles of cosmic origin penetrate the atmosphere, they collide with atmospheric atoms and molecules, triggering cascades of secondary particles that can propagate down to the surface. These energetic particles constitute a radiation background in the atmosphere with direct implications for aviation exposure safety and influence atmospheric chemistry through the ionization of ambient air. For these reasons, the development of secondary particle showers has been extensively studied, and considerable effort has been devoted to Monte Carlo-based models capable of computing secondary particle fluxes in the atmosphere. However, the atmospheric models in which particle transport is simulated are generally standard atmospheric models, which can differ substantially from actual atmospheric conditions. In this work, the Atmospheric Radiation Interaction Simulator (AtRIS) is used in conjunction with the NRLMSIS-2.1 atmospheric model to investigate the effects of varying density, pressure, and temperature profiles on secondary particle fluxes at different atmospheric levels. For a given incident radiation spectrum, the AtRIS simulations reveal substantial differences between atmospheric profiles when a uniform vertical cutoff rigidity is prescribed across all latitudes, effectively isolating atmospheric effects from geomagnetic shielding. Below ∼20 km, secondary particle fluxes are consistently higher in the polar atmosphere than at the equator. At ∼7 km, the difference between the equatorial and southern polar winter profiles reaches ∼40% for protons, photons, electrons, and positrons, and ∼30% for neutrons and muons.

The exceptionally strong geomagnetic storm of 10–11 May 2024 injected new energetic protons and electrons into the terrestrial radiation belts, creating extraordinary conditions to study the loss mechanisms scattering these particles into the atmosphere after the storm. For the first time, four electron belts were observed during several weeks. We show that this structure was due to electron loss, highly dependent on specific positions. Using the proton and electron fluxes measured by the Energetic Particle Telescope, EPT, on board PROBA-V, we determine the lifetimes of these populations depending on their energy ranges and positions. We show that the lifetimes are much longer for protons than for electrons, which enables us to determine their time variations independently. For electrons, the wave–particle loss mechanisms depend on the background ionosphere–plasmasphere density. The lifetimes determined after the May 2024 and 10 October 2024 big events are compared with average ones to understand their unusual specificity for the formation of four and three belts, respectively. For the injected protons of 9.5 to 13 MeV, the lifetime is minimum at L~1.9, where the fluxes are maximum, showing a lifetime depending on the flux intensity. Loss is due to pitch angle diffusion and collisions with electrons and nuclei in the ambient plasma and neutral atmosphere. At the outer edge of the proton belt, the flux is depleted at all energies after the geomagnetic perturbation, and we determine that the progressive time of refilling after the storm generally reaches more than 40 days. There is an excellent discrimination between the different populations of energetic electrons (0.5–8 MeV) and the injected protons (9.5–13 MeV) that are still observed several months after the event. Such results contribute to advancing understanding of the interactions between the terrestrial atmosphere and space radiation.

The exceptionally strong geomagnetic storm of 10-11 May 2024 injected new energetic protons and electrons in the terrestrial radiation belts, creating extraordinary conditions to study the loss mechanisms scattering these particles into the atmosphere after the storm. For the first time, four electron belts were observed during several weeks. We show that this structure was due to electron loss highly depending on specific positions. Using the proton and electron fluxes measured by the Energetic Particle Telescope EPT on board PROBA-V, we determine the lifetimes of these populations depending on their energy ranges and positions. We show that the lifetimes are much longer for protons than for electrons, which allows us to determine their time variations independently. For electrons, the wave-particle loss mechanisms depend on the background ionosphere-plasmasphere density. The lifetimes determined after the May 2024 and 10 October 2024 big events are compared with average ones to understand their unusual specificity for the formation of four and three belts, respectively. For the injected protons of 9.5 to 13 MeV, the lifetime is minimum at L~1.9 where the fluxes are maximum, showing a lifetime depending on the flux intensity. Loss is due to pitch angle diffusion and collisions with electrons and nuclei in the ambient plasma and neutral atmosphere. At the outer edge of the proton belt, the flux is depleted at all energies after the geomagnetic perturbation, and we determine that the progressive time of refilling after the storm reaches generally more than 40 days. There is an excellent discrimination between the different populations of energetic electrons (0.5-8 MeV) and the injected protons (9.5-13 MeV) that are still observed several months after the event. Such results contribute to advancing understanding of the interactions between the terrestrial atmosphere and space radiation.

The production, attenuation, and absorption of secondary cosmic rays (SCR) are influenced by atmospheric parameters such as air pressure and temperature. To reliably correlate SCR flux measurements with atmospheric ionization driven by energetic particle precipitation, these dependencies must be quantified. Monte Carlo simulations enable detailed modeling of stochastic interactions between cosmic radiation and atmospheric components, providing a robust framework for analyzing underlying physical processes and predicting SCR flux under varying atmospheric conditions. This study introduces a simulation model based on the Monte Carlo N‐Particle (MCNP) code, integrating atmospheric profiles from radiosonde data to model the production, absorption, and attenuation of SCR. The model's accuracy was validated through comparisons with the PHITS (Particle and Heavy Ion Transport code System)‐based Analytical Radiation Model in the Atmosphere (PARMA) and experimental ground‐based muon count measurements. It was subsequently used to investigate the dependence of muon flux on atmospheric pressure and temperature up to 20 km altitude. Results reveal a complex relationship between muon flux and atmospheric variables, particularly in the troposphere and lower stratosphere, where pressure correlations and barometric coefficients exhibit both positive and negative values depending on altitude. The model provides a valuable tool for investigating interactions between SCR and climate variables such as humidity and cloud coverage. Furthermore, the model can be coupled with dosimetry models to assess the biological effects of SCR, including deoxyribonucleic acid (DNA) damage, genomic instability, cellular dysfunction, and long‐term health risks such as cancer.

On 10 May 2024 at 17 h:07 UTC, the simultaneous arrival of several solar coronal mass ejections (CMEs) generated the strongest geomagnetic storm of the last twenty years, with a minimum Dst = −412 nT, usually referred to as the Mother’s Day event. On 10 October 2024, the second strongest event of solar cycle 25 appeared with a Dst = −335 nT, preceded on 8 October by an event with a Dst = −153 nT. In the present work, with measurements of the vertical total electron content and with ionosonde observations from Europe, USA, and South Korea, we show that the ionization of the upper atmosphere shortly increased at the arrival of the CME for these different events, followed by a fast decrease at all latitudes. The ionization remained very low for more than a full day. While the recovery started at the beginning of the second day after the onset for both events in October, the sudden recovery in the middle of the second day on 12 May is much more unusual. The analysis of the observations at different latitudes and longitudes shows that the causes of the ionization variations during the superstorms were mainly due to strong perturbations in the ionospheric F layer, amplified by the plasmasphere’s influence on the vertical total electron content (VTEC). The erosion of the plasmasphere during these two strong events led to a plasmapause located at exceptionally low radial distances smaller than 2 Re (Earth’s radii) in the post-midnight sector and a rotating plume in the afternoon–dusk sector clearly visible in the BSPM plasmasphere model. It took several days after the storms to recover normal ionization rates.

On 10 May 2024 at 17h:07 UTC, the arrival of solar a coronal mass ejection (CME) generated the strongest geomagnetic storm of the last twenty years with a minimum Dst = -412 nT, usually referred to as the Mother’s Day event. On 10 October 2024, the second strongest event of the solar cycle 25 appeared with Dst=-335 nT, preceded on 8 October by an event with Dst=-153nT. In the present work, we show with measurements of vertical total electron contents and with ionosonde observations from Europe, USA and South Korea that the ionization of the upper atmosphere shortly increased at the arrival of the CME for these different events, followed by a fast decrease at all latitudes. The ionization remained very low for more than a full day. While the recovery appears at the beginning of the second day after the onset for both events in October, the sudden recovery in the middle of the second day on 12 May is much more unusual. The analysis of the observations at different latitudes and longitudes shows that the causes of the ionization variations during the superstorms were mainly due strong perturbations in the ionospheric F layer, and amplified by the plasmasphere’s influence on the vertical total electron content (VTEC). The erosion of the plasmasphere during these two strong events led to a plasmapause located to exceptionally low radial distances smaller than 2 Re (Earth’s radii) in the post-midnight sector, and a rotating plume in the afternoon-dusk sector clearly visible in the BSPM plasmasphere model. Several days after the storms were necessary to recover normal ionization rates.

On May 11^th 2024, a succession of coronal mass ejections that merged together struck the Earth and induced large scale perturbations in the magnetosphere. During this event, satellite observations showed a large solar energetic proton (SEP) event associated to an extreme geomagnetic storm. At the same time, satellite observations of atmospheric ozone have been performed by AURA/MLS. In this work, we present the first observations of the effect of the storm of May and the following SEP of June 8^th on ozone concentration throughout the atmosphere. Observations of the MLS show that the event of May lead to stronger depletion of O₃ in the upper part of the atmosphere than in June. This difference is explained by the type of particle precipitation that occurred during the two events, with both protons and electrons in May and only protons in June. Neither event caused ozone depletion in the stratosphere while strong decreases are observed in the mesosphere. In May, mesospheric ozone depletion is observed during 18 days and reaches a maximum of 60 %. In addition, the storm of May also caused a noticeable decrease in ozone concentration (up to 20 %) at altitudes above 90 km.

The month of May 2024 was characterized by solar energetic particles events directed towards the Earth, especially the big event causing a strong terrestrial geomagnetic storm during the night from 10 to 11 May 2024, with auroras observed everywhere in Europe. This was the strongest storm for the last 20 years with a Dst < -400 nT. In the present work, we show with observations of GOES, PROBA-V/EPT and MetOP/MEPED that this exceptional event was associated with the injection of energetic protons in the proton radiation belt, with important consequences for the South part of the South Atlantic Anomaly (SAA). In addition, the geomagnetic storm caused by the solar eruption has had tremendous impacts on the electron radiation belts. Indeed, we show that for 0.3 to 1 MeV electrons, the storm led to a long lasting four belts configuration which was not observed before with EPT launched in 2013, until a smaller geomagnetic storm on June 15 took place. Moreover, for the first time since its launch, observations of the EPT show that ultra-relativistic electrons with E>2 MeV have been injected into the inner belt until L=2.4, violating the impenetrable barrier previously estimated to be located at L=2.8.

When high-energy particles originating from space penetrate the atmosphere, they may interact with atoms and molecules, initiating air showers composed of secondary and tertiary particles propagating towards the ground. They can cause ionization of the atmosphere and contribute to the radiation dose at low altitudes. This work uses the GEANT-4-based Atmospheric Radiation Interaction Simulator (AtRIS) toolkit to compute these quantities in the Earth’s atmosphere. We take advantage of the unique Planet Specification File (PSF) of the Atmospheric Radiation Interaction Simulator (AtRIS) to investigate the effect of the state of the atmosphere on the resulting induced ionization and absorbed dose rates from the top of the atmosphere (at 100 km) down to the surface. The atmospheric profiles (density, pressure, temperature, and composition) are computed with the NRLMSISE-00 model at various latitudes and for every month of 2014, corresponding to the last maximum of solar activity. The resulting ionization and dose rates present different profiles that vary with latitude in the atmosphere, with the relative difference between equatorial and high latitude ionization rates reaching 68% in the Pfotzer maximum. We obtain differences of up to 59% between the equator and high latitudes observed at commercial flight altitudes for the radiation dose. Both ionization and absorbed dose rates also feature anti-phased seasonal variations in the two hemispheres throughout 2014. Based on these results, we computed global maps of the ionization and dose rates at fixed altitudes in the atmosphere by using precomputed maps of the effective vertical cutoff rigidities and the results of three AtRIS simulations to consider the effect of latitude. While sharing the same general structure as maps created with a single profile, these new maps also show a clear asymmetry in the ionization and absorbed dose rates in the polar regions.