The VISIONS-2 sounding rockets performed in-situ measurements of the active dayside auroral region. Numerous broadband dispersed signatures up to keV energies are visible in the electron electrostatic analyser data, typical of Alfvénic precipitation. In order to characterize the region where the particles are accelerated, we estimate source altitudes based on different fits of the observed energy–time dispersions. Additionally, a method based on pitch-angle–time dispersions is developed, which relaxes the assumption that all electron energies are released simultaneously. Both approaches are found to yield similar source altitudes. For most of the analysed dispersed precipitation structures, these are found to lie between 1000 and 3000 km, and increase in height for larger electron energies. Variations across events suggest differences in the plasma and/or wave conditions in the acceleration region. Finally, a comparison with previous observational studies and theoretical predictions is performed, and our estimated source altitudes are found to be generally consistent with some inertial Alfvén wave velocity profiles, particularly those associated with relatively small O+Overall, the results presented here provide further detail about the Alfvénic auroral acceleration region on the dayside. The developed method also opens the possibility of inferring the plasma density profiles and essential wave parameters above the spacecraft.
In this work we have selected 40 corotating/stream interaction (CIR) driven geomagnetic storms that occurred between 2001 and 2016, and statistically studied their impacts on the magnetotail. The wavelet transform was applied to the interplanetary magnetic field (IMF) BzBxBz×Bx, IMF Bz×AEBx×AEBzBx≤12 h) than for the main phase (≤8 h) probably due to the presence of high-intensity long-duration continuous AE activities (HILDCAAs). From the cross-wavelet analysis (IMF Bz×Bx, IMF Bz×AEBx×periods≤4 hBzBx
Auroral forms can provide information not only on the state of near-Earth space but also on conditions in the lower-thermosphere–ionosphere. The so-called dune aurora, consisting of brighter stripes forming a wave-like pattern in the dim, diffuse green aurora, has been hypothesised as being an optical signature revealing the presence of large-scale atmospheric waves above or near the mesopause. However, only a few dune aurora events have been studied to date, leaving many open questions regarding the nature of this phenomenon. We carry out the first systematic survey of dune aurora events by collecting citizen science observations of the dunes since 2000 using the Skywarden (https://taivaanvahti.fi, last access: 25 August 2026) database of observations. From a total of 308 dune aurora observations made during 61 different events by citizen scientists from Northern Europe, North America, Australia, and New Zealand, we investigate the distribution of dune events as a function of location, month, magnetic local time (MLT), solar wind and interplanetary magnetic field (IMF) conditions, and geomagnetic activity. We compare those distributions to that of all the aurora observations reported in Skywarden since 2000. We find that the vast majority (92 %) of the dune reports were made by observers below 61° geomagnetic latitude (to be compared to 74 % for all aurora observations), suggesting that they frequently occur in the equatorward part of the auroral oval. The dune observations are mostly (89 %) made in the pre-midnight sector, with a peak between 21:00 and 22:00 MLT (between 23:00 and 00:00 MLT for all aurora observations). The months with the highest number of reports of dune observations are October and March, which could be the result of a combination of geomagnetic, atmospheric, darkness, and cloudiness conditions needed for them to be observed. Regarding the solar wind and IMF driving parameters, no statistically significant differences are found between dune and all-aurora observations, but the differences are significant for geomagnetic indices, with dune observations being made during more active conditions than for all types of aurora considered together. Finally, we investigate a possible relationship between dune aurora and equivalent current patterns derived from ground-based magnetometer measurements. We find that, for all the reported events, the dunes are observed in association with strong (in most cases eastward but occasionally westward) auroral electrojet signatures and in the vicinity of the Harang discontinuity. These results suggest that the dune aurora formation mechanism might involve an interplay between proton precipitation, atmospheric dynamics, and nightside transition region processes.
The post-launch spectral and radiometric performances of MAJIS, the VISNIR imaging spectrometer of the ESA Jupiter Icy Moon Explorer (JUICE), have been evaluated using observations performed during the Lunar-Earth Gravitational Assist (LEGA) of 19–20 August 2024 and observations of the Internal Calibration Unit (ICU). Observations of the Earth provided a comprehensive check of the spectral performances taking advantage of narrow atmospheric absorption bands over the full wavelength range of MAJIS (0.5–5.56 µm). This was of particular interest for wavelengths larger than 3.5 µm due to limitations of the ground calibration setup in this wavelength range. The radiometric performance of MAJIS has been reassessed considering the updated spectral calibration and the comparison of ICU observations before and after launch. On this basis, the observations of the Earth and Moon by MAJIS were compared to that of other instruments. The very good agreement with atmospheric spectral features observed by Earth observation instruments validate the updated spectral calibration of MAJIS. Comparing radiances for the Earth is not straightforward due to the very specific photometric angles for MAJIS observations (phase ∼ 90°, “glint spot”) and the high time variability of cloud patterns. A good agreement has been obtained within these limitations and the MAJIS radiance evaluations for the Moon are also consistent with that obtained by instruments on lunar orbiters, which indicates that the post-launch absolute radiometric calibration of MAJIS is close to the mark. These comparisons benefited from the high quality of the MAJIS data obtained during the LEGA with a very high operability and a signal to noise ratio (SNR) up to 400 (more with stacking). Extrapolating the operating conditions at 1 AU to those at 5 AU confirm that MAJIS will obtain high quality data during the science operations phase around Jupiter.
We identify a previously unrecognized population of rapid geomagnetic variations associated with cusp dynamics. These variations produce distinct summertime signatures in the high–latitude ground magnetometer observations near the cusp. In this study, we apply four years of data from the high time resolution West Greenland magnetometer chain and perform a statistical analysis of rapid geomagnetic variations in the ultra low frequencyWe find that rapid geomagnetic variability at the highest latitudes, in the cusp and beyond, is sensitive to seasonal change, indicating that the ionospheric currents generating the observed ground magnetic signatures depend on solar illumination to obtain sufficient conductivities. This effect, in concert with dipole tilt, was investigated, and a clear cusp-related population during summer was found. In winter, this population weakens owing to a lack of sunlight and merges with other signals such as ULF waves associated with Alfvénic interhemispheric bouncing further south, and, thus, becomes indiscernible. Furthermore, we discuss other aspects of our statistical analysis and briefly address relationships between our detected signals and other known cusp phenomena.
On 19 and 20 August 2024 the Jupiter Icy Moons Explorer (Juice) executed during its cruise phase towards Jupiter a combined Lunar Earth Gravity Assist (LEGA) maneuver. These close flybys of the Moon and the Earth provided so far the best opportunity to test the behavior, performance, and calibration of the Submillimetre Wave Instrument (SWI) onboard Juice. This paper shows typical data taken during the Earth Gravity Assist and the following few days. Data quality and problems resulting from unexpected behavior of the hardware are discussed.
The energy flow and energy balance in the Lower Thermosphere–Ionosphere (LTI) is governed by a number of processes that are driven by interactions between ions, neutrals and electrons. Even though these processes are well understood theoretically, and even though the framework to implement these processes exists in current global circulation models, the energy estimates for the different processes show large discrepancies between models, in large part because of limitations in available data sets. In this study, we explore numerically the energy inputs and energy transfer between ions, neutrals and electrons during the 2015 St. Patrick's day geomagnetic super-storm. We use NCAR's Thermosphere Ionosphere Electrodynamics General Circulation Model, version 2.0 (TIE-GCM 2.0) for estimating energy sources and sinks, energy transfer rates and the energy partitioning between species. Two independent TIE-GCM runs were executed: the first one used the Weimer 2005 empirical model, and the second used the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) data assimilative technique. The resulting energy budget and the corresponding partitioning of energy between species are inter-compared between the two runs, before and at the peak of the storm. The energy budget is investigated individually for neutrals, ions and electrons, as different heating and cooling mechanisms are associated with each species. Discrepancies between the model runs are discussed and the way forward to close the gaps in present knowledge is highlighted.
Solar eclipses are transient atmospheric events that cause rapid localized reduction in solar radiation, which produces complex changes associated with the vertical coupling of the layers. This study investigates how the neutral atmosphere responded vertically to the 14 October 2023 annular solar eclipse over Natal, Brazil (5.79° S, 35.2° W). This event presented a unique setup i.e., the maximum obscuration of ∼ 88.5 % occurred near sunset in a coastal transition between land and ocean. A stratospheric balloon sounding launched immediately before the umbra reached Natal was used. The balloon collected atmospheric profiles (temperature, pressure, relative humidity, and ozone concentration) over 1.45 h, with the umbra passing over the balloon at ∼ 22.35 km altitude for 3 min and 36 s. Comparison with average October profiles, model and reanalysis data revealed clear vertical patterns: (i) A notable cooling of 4–5 K in the tropopause region (14 km altitude); (ii) increased vertical temperature fluctuations, especially above 14 km; (iii) a decrease in atmospheric pressure of 0.2–0.7 hPa above 12 km; (iv) strong vertical oscillation in ozone concentration, with an increase of up to 1.7 ppm above 20 km altitude; (v) higher relative humidity between 5 and 18 km altitude compared to control profiles. These observations of complex, small-scale fluctuations and clear responses in the vertical atmospheric field align with previous reports and theoretical expectations of solar eclipse effects, confirming the importance of an annular eclipse as a significant driver of localized atmospheric dynamics.
Using a coupled EUHFORIA–Gorgon-Space modelling chain, we simulate the magnetosphere–ionosphere coupled response to two coronal mass ejections (CMEs) that impacted Earth on 23 and 28 June 2015. The first CME (CME1), launched on 21 June, triggered a major geomagnetic storm (Kp = 8), driving strong magnetosphere–ionosphere coupling characterised by a cross-polar cap potential of 160 kV and intense field-aligned currents exceeding 23 MA, key indicators of energy transfer from the solar wind into the high-latitude ionosphere. In contrast, the faster CME2 (launched on 25 June) produced only a weak disturbance (Kp ≈ 2) due to the absence of a prolonged southward interplanetary magnetic field (IMF), resulting in minimal magnetospheric driving and ionospheric feedback. These results validate an end-to-end space-weather forecasting framework and demonstrate that CME geoeffectiveness is governed primarily by the IMF orientation and reconnection-driven magnetosphere–ionosphere energy coupling, rather than by kinematic properties such as speed alone.
The JUpiter ICy moons Explorer (JUICE) mission, launched in April 2023 by the European Space Agency (ESA), is designed to investigate Jupiter and its largest icy moons, Ganymede, Callisto, and Europa, with a focus on assessing their potential habitability and investigating subsurface oceans. During its eight-year interplanetary cruise to the Jovian system, JUICE is scheduled to perform several flybys. The first of these, the combined Lunar-Earth Gravity Assist (LEGA), took place in August 2024. The spacecraft is equipped with a High Accuracy Accelerometer (HAA) that is part of the Gravity and Geophysics of Jupiter and the Galilean Moons (3GM) radio science instrument. During LEGA operations, HAA collected two hours of scientific data centered across the Moon's closest approach. We present here a detailed analysis of the HAA calibrated measurements that show a strong agreement with predicted non-gravitational accelerations, including those related to spacecraft deformation caused by the Moon's gravity gradient and thermoelastic displacements of the solar arrays during penumbra transitions. Additionally, unexpected dynamic responses were observed, including structural vibrations excited by the movement of the steerable telescope of the Submilimetre Wave Instrument (SWI) and a distinct outgassing event detected shortly after crossing the lunar terminator. The outgassing, likely involving sublimated water ice on the spacecraft, resulted in a measurable velocity change of 0.7 ± 0.1 mm s−1Z
In many studies of the electrodynamics of the coupled ionosphere-thermosphere (IT) system at high latitudes, the ionosphere is represented as a two-dimensional spherical shell and the height-integrated ionospheric Ohm's law is used to understand IT electrodynamic coupling. Thermospheric winds play a central role in IT electrodynamics, but they are generally ignored in existing empirical models and assimilative methods. While the primary issue is a lack of comprehensive wind measurements, there is also a gap in the literature on how to represent the thermospheric winds – which often exhibit strong variations with altitude – in a height-integrated description of high-latitude IT electrodynamics, and what the associated sources of error might be. Here we highlight that there is in general no single suitable definition of the neutral wind term in high-latitude, height-integrated IT electrodynamics. Instead, two neutral wind terms weighted by Hall and Pedersen conductivities appear in the height-integrated Ohm's law. Using altitude profiles of neutral winds and ionospheric conductivities respectively derived from sounding rocket chemical release experiments near Poker Flat, Alaska, and Poker Flat Incoherent Scatter Radar (PFISR) measurements, we find magnitude differences of order 10–100 m s−1∼100–120 kmnm
We developed a numerical model for solving the linearized gravity-wave equations using a multilayer approach that explicitly accounts for viscosity, thermal conduction, and ion drag. The solution strategy is based on a matrix-exponential formalism and comprises two classes of methods: global matrix methods and scattering matrix methods. The model supports both single-frequency waves and time-dependent wave packets. Particular emphasis is placed on the global matrix method, which exploits the structured form of the multilayer system to achieve high computational efficiency while maintaining numerical accuracy. Numerical experiments demonstrate that all methods yield identical accuracy, although the global matrix method is significantly more efficient than the scattering matrix method, especially for time-dependent wave packets. The impact of ion drag on wave characteristics is quantified within this framework. The implementation is freely available as open-source code at https://github.com/AlexandruDoicu/Gravity-Waveshttps://doi.org/10.5281/zenodo.21410453(Doicu and Efremenko, 2026).
Fourier spectral analysis is applied to the planetary geomagnetic index Ap for the February 2001, 2003, and 2017 time intervals. We investigate how Fourier coefficients change in time with respect to geomagnetic activity. A detailed analysis of the 13–14 February 2001 substorm revealed that higher harmonics were suppressed during the event, contrary to what was expected.
We present a comprehensive study of the first observations of ionospheric ion upflow generated by ultra-low frequency (ULF) wave driven auroral arcs (UAAs). Ground- and space-based instrumentation, together with inversion models, allow us to study the event at different length scales. This shows the complex dynamics of UAAs and their role in the ionosphere-magnetosphere coupling via ion upflow, field-aligned currents (FACs), and energy dissipation. The UAA event was observed as a series of six poleward moving arcs, primarily in the 630.0 nm emission line. At the northern extent of the arcs incoherent scatter radar (ISR) data indicated that the UAAs have driven type 2 ion upflow with low to medium fluxes of around 3.3×1013−2 s−1. Data from the ISR, spacecraft, and models, result in FAC magnitudes up to 6 µA m−2, total energy fluxes up to 12 mW m−2, and Joule heating rates up to 11 mW m−2
The Jupiter Icy moons Explorer (Juice) was the first spacecraft ever that performed a combined gravity assist using both the Moon and Earth in succession. The double flyby required highly precise navigation to succeed. The LEGA allowed Juice to make a shortcut through the inner solar system on its way to Jupiter, using less fuel than would have been otherwise required. On 19 August 2024, Juice had its closest approach to the Moon. This first part of the manoeuvre accelerated the spacecraft by approximately 0.9 km s−1km s−1µm
Solar eclipses offer a unique natural experiment to probe ionospheric responses to sudden reductions in solar radiation. This study reports the recovery of historical ionogram records to analyze the ionospheric response to solar eclipses spanning several decades over Concepción (36.79° S, 73.03° W)/Chillán (36.64° S, 71.99° W). Out of 21 identified events between 1958 and 2024, data from 16 (76 %) cases were rescued, many originally on fragile or hazardous 35 mm film, emphasizing the scientific value of long-term datasets. Critical frequencies (foE, foF1, foF2) and virtual heights (h'E, h'F1, h'F/F2) were extracted from digitized and scaled ionograms to quantify eclipse-induced perturbations. Diurnal variations show typical dips in the E- and F1-layer critical frequencies, while F2-layer responses are more complex and variable. Regression analysis was performed exclusively on critical frequencies, revealing a nearly linear decrease of foE and foF1 while the maximum obscuration percentage of the eclipse is higher, whereas inconsistent behavior was observed on foF2. High-cadence observations, available for select events, provided a significantly clearer depiction of the response to the eclipses than 1 h resolution historical data. Only the 2 July 2019 and 14 December 2020 eclipse responses had been previously published. Predictions for the 6 February 2027 eclipse indicate an expected %ΔfoE decrease of ∼ 28 % and a %ΔfoF1 decrease of ∼ 24 % at Chillán, offering a timely opportunity to validate the regression models and assess predictive skill.
Disturbances to the magnetopause location driven by upstream pressure variations or flow shear instabilities may be described as surface waves, which act as localised sources of field-aligned currents coupling the magnetosphere to the ionosphere. However, their impacts on the ionosphere and ground across representative ranges of wave and system properties are poorly understood. We, therefore, develop a simple numerical model for dispersionless mesoscale magnetopause surface waves within the coupled magnetosphere–ionosphere–ground system to gain insight into how their amplitudes and spatial scales throughout the system might vary with conditions. In general, the impacts of finite wave packets can be decomposed into periodic fluctuations (with matching wavelength to that directly above in the magnetosphere) along with slowly-varying trends that result from finite wave effects. Finite wave packets act in the far-field like a string of alternating field-aligned currents well described both in the ionosphere and on the ground as a two-dimensional current dipole. In the ionosphere, near-field periodic fluctuations exponentially decay over the reduced wavelength latitudinally away from the projected magnetopause boundary layer flux tubes, which may limit how well they can be resolved by radar. The relationship between the magnetic field above and below the ionosphere becomes more complicated for surface waves than infinite plane Alfvén waves due to the additional spatial structure, which introduces interference across the spectrum of wavenumbers present. This modifies how the ionosphere screens, rotates, and spatially smears magnetic field perturbations across all three components in different ways. For mesoscale wavelengths this importantly results in latitudinal scales of amplitude and polarisation variation smaller than typical ground magnetometer spacings, motivating the need for denser networks. A range of effective skin depths in the ground are applicable to surface waves, meaning ground induction can vary between a near-perfect insulator to a good conductor, affecting both observable ground magnetic fields and resulting geoelectric fields. The predicted peak amplitudes of surface waves' impacts suggest they may act as significant sources of ionospheric/thermospheric Joule heating and geoelectric fields in the ground, thereby contributing to space weather impacts. These are, however, highly localised latitudinally when considering typical mesoscale waves. Our results provide key insight into interpreting ground-based observations, of particular timeliness with the rollout of new digital ionospheric radars and the SMILE mission's planned conjugate ground–space campaigns.
Observation-based characteristics of the dayside ionosphere are important for the knowledge of the coupling between the solar wind, magnetosphere and ionosphere. Therefore, this paper presents descriptions and quantitative analyses of characteristics of the polar dayside ionosphere during the winter. We use EISCAT Svalbard radar (ESR) fast elevation scans to obtain both altitudinal and latitudinal information of the ionospheric parameters electron density Ne, electron temperature Te, and ion temperature Ti. We determine the location of the open-closed field line boundary (OCB) and divide the ionosphere into three regions based on their position relative to the OCB: on closed field lines, along the OCB, and in the polar cap. We first show two case examples, illustrative of the method and the dynamic response of the ionosphere to variable solar wind. We then statistically investigate how the parameters vary from closed to open field lines across the OCB and with altitude in the three regions. Finally, we compare the obtained OCB latitudes with the ones obtained in previous studies. Overall, significant differences in the ionospheric parameters can be seen between the three latitude regions. In general, observed enhancements in Te°TeMLTTeNekmNeNeTi
We present an analysis of four mid-infrared observations of the lunar surface acquired by the MAJIS instrument during the Jupiter Icy Moons Explorer (JUICE) gravity assist in August 2024. The data span 0.49–5.56 µmThe study explores three independent approaches to model and retrieve surface temperature and emissivity: (i) a Bayesian inversion framework, (ii) an empirical thermal correction method, and (iii) a roughness-informed thermophysical model. Rather than constituting a formal instrument validation, this paper provides a methodological consistency assessment of thermal retrieval strategies when applied to MAJIS mid-infrared data.Retrieved temperature distributions are compared with expectations from established lunar thermal behaviour, and emissivity spectra are analysed in relation to known compositional contrasts between mare and highland terrains. The analysis highlights the sensitivity of the 3–5 µm crossover regime to modelling assumptions, temperature–emissivity coupling, and surface roughness parameterization.Overall, the results demonstrate that MAJIS mid-infrared observations can be interpreted within physically consistent thermal modelling frameworks, while also revealing limitations and degeneracies inherent to this wavelength range. This workflow is directly transferable to future MAJIS observations of Jovian moons in the reflected–thermal crossover regime.
This study examines the low-latitude ionospheric response to four intense geomagnetic storms during Solar Cycle 25 (March, April, November 2023, and May 2024), focusing on Equatorial Ionization Anomaly (EIA) variations and post-sunset plasma irregularities. We used the Weimer2005)JH), Madrigal total electron content (TEC) maps, and GNSS-derived ROTI to analyze storm-time changes in EIA structure and equatorial plasma bubbles (EPBs). The May 2024 storm exhibited the strongest post-sunset JH, particularly near the June solstice, while March and April storms showed moderate JHJHJHmW m−2, November), moderate (30–50 mW m−2, March/April) and strong (>50 mW m−2, May). JH, together with storm-time electric fields and equatorial meridional winds, influence the location, strength, hemispheric asymmetry, and the generation or suppression of plasma irregularities of the EIA crest. The generation of ionospheric plasma irregularities and their geographical distribution strongly depend on EIA's density gradients and general structure. Well-developed double-crest EIAs with steep density gradients favor post-sunset irregularities, while single-crest or merged EIAs are less favorable. Fluctuations in the IMF Bz