A lightning event was detected by the MAJIS imaging spectrometer onboard the Jupiter Icy Moons Explorer (JUICE) spacecraft during its first Earth gravity assist maneuver. This serendipitous observation represents the first space-based spectroscopic measurement of lightning for any planetary atmosphere. The event, composed of four flashes, was registered on 2024, August, 20th in an area offshore of Sumatra island, during local nighttime, near to optically thick clouds probed by MAJIS thermal wavelengths. No coincident detection has been obtained by ground-based lightning sensor networks, yet MAJIS observations provide unambiguous evidence of neutral atomic oxygen and nitrogen emissions, identified through several diagnostic lines. A faint HαAs MAJIS is not optimized for such observations, a number of caveats related to spectral and temporal resolutions have been considered when deriving absolute quantities, such as lightning energy and temperature. Retrieved energies are overall consistent with known emission by lightning of average strength, ranging from (0.7±0.2) to (1.3±0.3) MJ in the 777 nm O I line and from (0.5±0.2) to (1.5±0.4) MJ in the 870 nm N I line. Estimates of the temperature of the lightning channel yield a broad range of values, spanning between 5000 and 20 000 K, with standard uncertainties of the order of 2000–3000 K depending on the retrieval method. This is ascribed to a higher sensitivity to biases induced by the limited measurement resolutions.Overall, this observation represents a useful benchmark for guiding detection and interpreting possible lightning events on Jupiter, a primary target of the JUICE mission. A preliminary extrapolation of the terrestrial case to the conditions of Jovian atmosphere suggests that H I emissions in the 650 and 1870 nm spectral ranges are the most promising for identifying lightning on Jupiter with the MAJIS instrument.
We investigate how the location, size, and intensity of the auroral oval is affected by the combination of the tilt of the Earth's magnetic dipole axis and a strong, stable dawn/dusk component of the interplanetary magnetic field (IMF By) during northward IMF. Sunlit auroral observations are contaminated by dayglow, and its impact on average intensity estimates remains unclear. Dayglow modelling is also accompanied by significant uncertainties that increase with increasing sunlight intensity. These difficulties motivate us to develop a new technique for isolating the auroral contribution in dayglow-subtracted UV images. This technique assumes the observed distribution of intensities consists of separate contributions from dayglow and aurora, and that the dayglow subtraction process to which the observations are subjected is imperfect and leaves a residual error. By performing a nonlinear fit to dayglow-subtracted count distributions one may extract the best-fit parameters that describe the dayglow residual error and auroral sources separately. We apply this isolation technique to dayglow-subtracted UV images derived from measurements made by the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) onboard the Defense Meteorological Satellite Program’s F16–19 satellites during 2005–2018. The isolation technique produces 30 %–40 % higher auroral intensities than a simple calculation of the distribution mean. Statistics of the auroral component show a clear and substantial (∼500 km in NH, ∼430 km in SH) dawn-dusk shift in the polar cap location depending on the sign of IMF By
Anomalous electromagnetic phenomena in the ionosphere before seismic activity have been identified as potential indicators for earthquake early warning. Data from the CSES-01 satellite were analyzed using the STL decomposition method to break down electric field time series into longitudinal, latitudinal, and residual components. To quantify disturbance intensity, we use the C-value, a spectrum-fitting index derived from the power-law relationship between electric-field power spectral density and frequency. The longitudinal and latitudinal components reveal the electric field's double periodicity, characterized by a V-shape from south to north and a bimodal shape from east to west. After isolating conventional periodic disturbances with a strength of 0.87, unconventional disturbances in the residual component were examined to identify seismic precursor anomalies. Electric field power density disturbances associated with the 11 May 2023, Tonga Islands magnitude 7.6 earthquake were extracted. Multiple significant anomalies in the ionospheric electric field were detected within 20 d prior to the earthquake: an initial anomaly with a C-value exceeding 3.5 appeared 20 d before; a persistent anomaly with a peak C-value of 3.9 occurred 13 to 11 d prior; a sharp increase to a peak C-value of 4.4, three times the standard deviation, was observed 7 d prior; disturbances decreased until a resurgence 4 d prior, with a peak C-value of 3.5 lasting 2 d; the C-value returned to baseline 1 d before the earthquake. The STL-C
This study systematically derives transport coefficients – electrical conductivity, thermoelectric, diffusion, and mobility – for a Lorentz plasma described by a standard Kappa distribution function. Within the five-moment transport framework, the standard Kappa distribution serves as the zeroth-order function. Momentum and energy collision terms are obtained via the Boltzmann collision integral for Coulomb, hard-sphere, and Maxwell molecule interactions, and incorporated into the momentum equation to formulate generalized Ohm's and extended Fick's laws, yielding the transport coefficients. This study also compares the standard Kappa, modified Kappa, and Maxwellian distributions in terms of their influence on plasma behavior. The results show that for velocity-dependent collisions, such as Coulomb collisions, significant differences arise between the standard and modified Kappa distributions. For low kappa parameter κκ
This work involves an investigation of high-latitude medium scale traveling ionospheric disturbances (MSTIDs) over the newly established EISCAT-3D radar site. We have used the ground backscatter data from an HF radar located at Hankasalmi, Finland (∼62.3° N, ∼26.61° E geographic coordinates), which is a part of the SuperDARN (Super Dual Auroral Radar Network). Data from solar maximum (2001 & 2014) and minimum (1996 & 2009) years from solar cycles 23 and 24 have been used to investigate the characteristics, seasonal variation, and possible generating sources of high-latitude daytime MSTIDs. Irrespective of the seasons and solar activity conditions, a dominant fraction of MSTIDs propagates equatorward with velocity in the range of 50–150 m s−1∼72 %) than during solar minimum years (below 50 %). Furthermore, the MSTIDs' occurrence shows a dependence on IMF Bz, being generally higher during intervals of prolonged northward or southward IMF Bz, and lower during small or fluctuating IMF Bz
A transatlantic scientific balloon flight (TRANSAT) was conducted between 22 and 26 June 2024. The TRANSAT balloon, operated by the French Space Agency (CNES), floated in the stratosphere at approximately 40 km altitude between Esrange (Sweden) and Baffin Island (Canada) for about 3.8 d. The scientific payload comprised nine instruments, including an optical imager for noctilucent cloud (NLC) studies from the Swedish Institute of Space Physics. The NLC imager consisted of three identical visible-range optical cameras, one of which operated successfully throughout the entire flight, capturing thousands of NLC images. The TRANSAT balloon campaign was supported by ground-based lidar measurements and spaceborne observations from the Swedish MATS satellite. Here, we describe the technical characteristics of the balloon experiment and present early results. Nearly continuous observations of NLC were obtained during the entire flight. A localized warm region in the mesopause was identified as the cause of temporary NLC disappearance, while complex NLC structures exhibiting different motions were found to probably result from horizontal wind rotation with altitude within the mesopause region.
The Next-generation Ionospheric Model for Operations (NIMO) is an assimilative geospace model developed to address the space weather operational needs in the ionosphere. NIMO harnesses contributions from both near real-time data and state-of-the-art implementation of ionospheric theory to provide hindcasts, nowcasts, and forecasts for operational or research purposes. NIMO is currently configured to assimilate various types of electron density measurements through the Ionospheric Data Assimilation Four-Dimensional (IDA-4D) data assimilation schema. Information from the neutral atmosphere is provided by empirical models. The ionospheric chemistry and transport calculations are handled within NIMO using a version of SAMI3 is also a Model of the Ionosphere (SAMI3) designed to have a realistic geomagnetic field and work effectively on a parallel processing system. NIMO was designed to be more adaptive than previous systems that couple first-principle and assimilative models. This article discusses how NIMO is configured, demonstrates potential use cases for the research community, and validates hindcast runs using a new suite of metrics designed to allow repeatable, quantitative, model-independent evaluations against publicly available observations that may be adopted by any ionospheric global circulation or regional space weather model. Future versions of NIMO and other empirical, first-principle, or assimilative models may compare their performance against these results.
Dynamic loads in planetary mantles have the potential to deform the core-mantle boundary (CMB). On Earth, subducting slabs primarily induce a degree 2–order 2 deformation of the CMB in the spherical harmonic (SH) reference system. On Mars, the presence of the dichotomy and of the Tharsis region could produce loading across multiple degrees and orders, including degree-1, degree 2–order 2, degree 2–order 0, and degree 3–order 3 components. Thanks to the InSight (Interior exploration using Seismic Investigations, Geodesy, and Heat Transport) mission's radio science experiment, observations of Mars' nutations are now available. Periodic length-of-day (LOD) variations of Mars have been detected first by radio tracking the Viking landers, and InSight data have indicated the presence of a secular trend in LOD. In the case of nutations, the Martian core's non-hydrostatic flattening plays a first-order role in determining nutation amplitudes. In this study, we explore second-order effects arising from dynamic topography at the CMB. We compute the pressure exerted on the CMB topography inside Mars' liquid core and evaluate the resulting topographic pressure torque acting on the boundary, which can influence both nutations and LOD variations. Our results show that, albeit at microarcsecond (µarcsec) level – well below current observational thresholds, the most significant contribution to nutations arises from degree 2–order 2 component. As for LOD variations, while Earth exhibits notable contributions from inertial wave resonances, the situation on Mars is different. The planet's tidal LOD variations have periods that are either too long or too far apart from those of inertial waves. Consequently, the associated contributions fall below the level of detectability.
Particle measurements from the Polar Operational Environmental Satellites (POES) and the Meteorological Operational (Metop) satellite program, are widely used for various scientific applications. While most studies focus on the Medium Energy Proton and Electron Detector (MEPED), the low-energy (eV and keV) counterpart, the Total Energy Detector (TED), has received comparatively less attention. However, the recent rise in the altitudes considered in ionization and climate models has increased interest in low-energy particle measurements as inputs for atmospheric ionization models.This study analyzes TED particle data (together with selected MEPED channels) from 2001 to 2025 and shows that the TED 0° proton channels, in particular, are contaminated by energetic electrons at L<6, with weaker contamination observed in other TED channels. In some cases, the contaminated fluxes exceed typical auroral flux levels. The affected regions were cross-validated using auroral UV emissions and occurrences of GNSS derived S4 index to rule out the possibility that the observed fluxes correspond to real particle precipitation.As correction approach, we provide a simple Kp- and channel-dependent latitude boundary that may serve as preliminary cut-off criterion for the contaminated regions. In a more advanced step, we identified the contamination characteristics of each particle channel on each satellite. The outcome is a list of problematic channels that should be neglected and a correction method based on background counts for the other channels. The corrected fluxes are in good agreement with UV emissions and the method is available in the additional material.
Sweden’s communication and power systems have been impacted by major space weather events in the past. For instance, the May 1921 storm caused a fire at the telegraph and telephone station in Karlstad, and the 2003 Halloween storm led to a blackout in Malmö. In this study, we present the first comprehensive assessment of the potential impacts of a 1-in-100-year extreme geoelectric field on the entire Swedish power grid. Using magnetic field observations from the 30 October 2003 event as a baseline, we constructed two extreme scenarios. In Case 1, we used the observed magnetic field across Fennoscandia. In Case 2, we assume an idealized ionospheric current system in which all stations share the same temporal magnetic field pattern. Then the estimated 3D geoelectric field was scaled using region-specific scaling factors derived from recent statistical analyses of geoelectric field extremes in Sweden. The scaled geoelectric field and power line voltages are computed using the recently developed RAISE model, which includes realistic ground conductivity and power line topology. Our results show that the largest-magnitude horizontal geoelectric fields, around 12 V km−1, occur within the 5558°60°
The dynamics of the inner magnetosphere and magnetotail are determined by a number of factors such as magnetic reconnection, plasma instabilities, and large-scale plasma motion. We use the global hybrid-Vlasov simulation Vlasiator to study these dynamics as well as their signatures in the ionosphere. We observe magnetic reconnection, fast flows, and vorticity in the transition region between the Earth's dipolar field and the magnetotail. In our simulation, reconnection is first triggered at the dawn and dusk sides of the magnetotail current sheet. It then spreads across the current sheet. Concurrently, an azimuthally periodic, wave-like density structure develops in the transition region along with fast Earthward flows and enhanced vorticity patterns. The Earthward flows and vorticity induce field-aligned currents, which map onto the ionospheric simulation domain, creating a patchy current distribution. We find that the event is driven by the combination of reconnection-induced fast flows and the ballooning/interchange instability.
We investigate the response of space weather events on Earth's upper atmosphere over the polar regions by studying their effect on the drag of the CHAMP and GRACE satellites. Increasing solar activity that results in heating and the expansion of the upper atmosphere threatens low Earth orbit (LEO) satellites. Auroral events are closely related to the stellar energy deposition of solar EUV radiation and precipitating energetic electrons, which influence photochemical processes such as the production of nitric oxide (NO) in the upper atmosphere. To study the production of NO molecules and their influence on the thermospheric structure and satellite drag, we first model Earth's background thermosphere structure with the 1D upper atmosphere model Kompot by considering the incident X-ray, EUV, and IR radiation during selected space weather events. For investigating the effect of electron precipitation in the production of NO molecules in the polar thermosphere, we apply a Monte Carlo model that takes into account the stochastic nature of collisional scattering of auroral electrons in collisions with the surrounding N2-O2
Tropopause height and temperature play a crucial role in atmospheric chemistry and radiative forcing and serve as key indicators of anthropogenic climate change. However, accurately determining this parameter requires advanced remote sensing techniques. This study compares tropopause height and temperature estimated from in-situ and remote sensing instruments (SHADOZ and COSMIC-1) with reanalysis data from MERRA-2 over Réunion from 2006 to 2020. The results reveal strong agreement between vertical temperature profiles obtained from SHADOZ and COSMIC-1, demonstrating that both can reliably estimate tropopause height using the Cold Point Temperature (CPT) and/or Lapse Rate Temperature (LRT) methods. Conversely, while MERRA-2 assimilates data from these sources, its fixed vertical resolution limits its ability to capture tropopause height variations accurately. Given the consistency between SHADOZ and COSMIC-1, their data were combined to construct a more refined dataset, which was then used to assess temperature trends. The analysis indicates a high influence of annual and semi-annual oscillations in Tropopause height dynamics, as well as, a decreasing trend in CPT and a slight increase in the Lapse Rate Tropopause (LRT) height.
The Lunar-Earth Gravitational Assist (LEGA) of 19–20 August 2024 marked the first in-flight opportunity beyond functional checks to perform MAJIS (Moons and Jupiter Imaging Spectrometer) observations on-board the ESA's Jupiter Icy Moons Explorer (JUICE) spacecraft. This unique double flyby involved sequential close approaches to the Moon and Earth, offering an unprecedented configuration to evaluate MAJIS under high radiance, rapidly changing geometric, and operationally constrained conditions. A total of 24 hyperspectral image cubes were acquired (5 targeting the Moon and 19 the Earth) providing a dataset of approximately 7.5 Gbit. This work presents the primary goal of this observation campaign, which was to verify key aspects of MAJIS performance, including radiometric and spectral calibration, straylight behavior, geometric alignment, the use of onboard browse products, and interference tests with other JUICE instruments. This event also enabled assessment of thermal behavior and susceptibility to electromagnetic interference, and provided a first operational benchmark for MAJIS and a basis for refining future observation strategies and data analyses during JUICE's cruise and science phases. In addition, despite limited spatial and temporal coverage of the observations, the analyses presented here and in a series of companion papers of the special issue “The first-ever lunar-Earth flyby: a unique test environment for JUICE” demonstrated the instrument's ability to characterize mineralogical features on the Moon and atmospheric constituents on Earth. Observations include detection of mafic minerals (some associated to fresh excavated materials), thermal emission, and emissivity variations on the Moon at spatial scale of 100–200 m. Characterization of atmospheric absorption features, thermal brightness, icy cloud properties are captured for the Earth at km-scale and briefly discussed in the framework of the atmospheric biosignatures relevant to exoplanet habitability studies. Near-coincident acquisitions with other JUICE instruments and Earth-orbiting spectrometers provided valuable inter-calibration and cross-validation opportunities.
From the sum total dissipation of unstable wave energy in geospace, a frequent and efficient channel of dissipation is opened up by particle precipitation. The phenomenon, which is part of a complicated cascade of unstable magnetohydrodynamic wave modes, consists of charged particles that intermittently rain down into Earth's dense atmosphere. The atmospheric penetration depth of the precipitating particles in aurorae dictates the altitude profile of plasma ionization. Absent of sunlight, this profile governs the crucial ratio of bottomside- to topside (E- to F-region) electrical conductance, which can act as a primary regulator of plasma turbulence growth rates by modulating the efficiency of electric field short-circuiting as well as ambipolar diffusion. Analyzing a large database of Defense Meteorological Satellite Program (DMSP) particle spectra from the dark, high-latitude ionosphere, we systematically map the response of this conductance ratio to varying geomagnetic activity. We reveal a characteristic spatial organization: during active conditions, the dayside cusp region is systematically drained of high-energy particles, creating a low-conductivity environment that favors the persistence of F-region turbulence, which starkly contrasts with the nightside auroral oval where elevated Pedersen conductivity in the E-region may actively dampen the growth of turbulence in the F-region. These findings indicate that the specific character of the magnetospheric energy input shapes the electrodynamics of specific regions, with implications for whether the ionosphere acts as a source or a sink for small-scale structuring.
Observations of the plasma flow direction in the Earth's magnetosheath are compared with the help of three analytical magnetic-field models, namely Kobel and Flückiger (1994), Romashets and Vandas (2019), and Vandas and Romashets (2019), which all assume current-free fields in the magnetosheath. 47 magnetosheath passages by spacecraft are analyzed in detail and performance of the models are evaluated. It is concluded that the performances measured by mean angles between model and observed flow directions are comparable among the models (the difference of the mean angles is below about 1°), and that they are satisfactory on average (overall mean angles are below 5°). Therefore, a usage of the model by Kobel and Flückiger (1994) is recommended, because it is the simplest one and yields results much faster.
The ion composition in the E-region is modified by auroral precipitation. This affects the inversion of electron density profiles from field-aligned incoherent scatter radar measurements to differential energy spectra of precipitating electrons. Here a fully dynamic ionospheric chemistry model (IonChem) is developed that integrates the coupled continuity equations for 6 ion and 9 neutral species, modeling the rapid ionospheric variability during active aurora. IonChem is used to produce accurate, time-dependent recombination rates for ELSPEC to improve the inversion of electron density profiles to primary electron energy spectra. The improvement of the dynamic recombination rates on the inversion is compared with static recombination rates from the International Reference Ionosphere (IRI) and the steady-state recombination rates from an ionospheric chemistry model, FlipChem. A systematic overestimation at high electron energies can be removed using a dynamic model. The comparison with FlipChem shows that short-timescale density variations are missed in a steady-state chemistry model.
In the ionosphere, a sustained population of suprathermal electrons arises due to photoionization or electron precipitation. The presence of such a population enhances the scattered power in the plasma line spectrum, thus making it possible to detect them. Plasma line measurements improve the accuracy of electron density and temperature estimates. We investigate plasma line enhancements in EISCAT Tromsø UHF radar observations, using two image processing methodologies for detection: a supervised image morphological processing technique and an unsupervised connected component analysis. The supervised methodology detects more plasma lines, demonstrating higher sensitivity. We determine the times and altitudes with enhancements and model the spectrum with a Gaussian function. The radar beam points in the field-aligned direction for 25 % of the total observational time, is directed east for another 25 % and is oriented in the vertical direction for the remaining 50 %. Plasma lines are detected 26 % of the time when the radar is pointed in the field-aligned direction, 5 % of the time in the east direction and 5 % of the time in the vertical direction. Most plasma lines are detected around the F-region altitude where the electron density is maximum, typically between 230–260 km, with a simultaneous increase in the electron density estimates from the ion line. Plasma line intensity is maximum around noon. It decreases as the aspect angle increases. Both detection methodologies' advantages and disadvantages are discussed, and plasma line intensity variations are analyzed as a function of altitude, aspect angle and phase energy.
Accurate estimates of the ionospheric electron density are essential for various space-weather applications but are challenging at high latitudes due to strong spatial and temporal variability driven by auroral precipitation and complex ionospheric convection. This study presents an assimilative empirical model designed to improve regional electron-density estimates in Northern Scandinavia. The model uses ionogram images, the local magnetic field, the auroral electrojet, the ring current and solar-activity indices as inputs. These inputs are fused by a multimodal neural network and trained with incoherent-scatter-radar (ISR) observations of electron density profiles as the target. The model remains functional with only a subset of input, with modest accuracy degradation. Comparative analysis demonstrates that our neural-network-based assimilative model outperforms the ARTIST 4.5 ionogram scaler and the state-of-the-art E-CHAIM model, especially during auroral activity. Overall, our model achieves an R2 score of 0.74 on an independent test dataset, whereas ARTIST 4.5 and E-CHAIM obtain R2 values of −0.08 and 0.34, respectively. These results indicate that the model can provide reliable, continuous electron-density estimates at high latitudes, even under auroral conditions. This methodology can be extended to develop empirical ionospheric models for other regions with historical ISR data and to invert ionograms to electron-density profiles when ISR observations are unavailable.
We analyzed fragmented auroral-like emissions (FAEs) and picket fence structures observed in northern Scandinavia during a magnetic storm on 1 January 2025. The analysis is based on ground-based high-sensitivity optical observations and in-situ measurements from the Swarm satellites. While FAEs and picket fences have previously been reported in the polar cap and subauroral region, respectively, this study reports simultaneous occurrences of both phenomena in auroral latitudes, near the poleward edge of the oval. Ground-based camera observations revealed that some FAEs exhibited orientations closely aligned with the modeled local magnetic field in the image plane and appeared simultaneously at multiple longitudinally separated locations. Furthermore, the FAEs appeared to follow the motion of red auroras, suggesting that the background electric field structure and spatial gradients in the electron density may influence their formation. Consistent with previous studies, the generation of FAEs is considered to be due to local acceleration of electrons in the ionosphere rather than electron precipitation from the magnetosphere. While we could not clearly identify the generation mechanisms, the morphological diversity observed in this event suggests that multiple plasma instabilities may be involved in the generation of both FAEs and picket fence structures.