Abstract. 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.

Abstract. 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 atmosphere, including the production of suprathermal N atoms. The observed effect of the atmospheric drag on the CHAMP and GRACE spacecraft during the two studied events indicates that a sporadic enhancement of NO molecule production in the polar thermosphere and its IR-cooling capability, which counteracts thermospheric expansion and can lead to an “overcooling” with decreased density after the space weather event, can have a protective effect on LEO satellites. Their production efficiency, however, is highly dependent on the energy flux of the precipitating electrons. Our results have direct implications for empirical satellite orbit prediction models, as our simulations highlight the need to consider precipitation-induced NO production to improve the predictive power of these models.

Abstract. 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.

Abstract. 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.

Abstract. 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.

Abstract. 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.

Abstract. 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.

Abstract. Data from the FAST spacecraft are used to study the temporal progression of the energy inputs to the dayside cusp and the nightside aurora, including Poynting flux, electron number flux and amplitude of extremely low frequency (ELF) waves, during a storm driven by CME (Coronal Mass Ejections), and the resulting H+ and O+ outflows. The results show that (1) On the dayside, Poynting flux, ELF waves activity and soft electron precipitation are all enhanced during the initial and main phases of the storm, and decrease during the recovery phases. On the nightside, the Poynting flux increases during the initial and main phase, but the enhancements are smaller than on the dayside. The variations in the ELF wave activity and electron precipitation are similar before and during the storm. (2) The energy inputs are strongly correlated with the solar wind – magnetosphere coupling functions, dΦMP/dt and p1/2dΦMP/dt, especially in the dayside cusp region where the energy inputs and the ion outflows are localized. (3) The O+ and H+ ion outflow flux, fO+ and fH+, and the flux ratio fO+/fH+ all increase during the storm. Both the fluxes and the flux ratio reach their peaks on the initial phase and are enhanced during the main phase. Nightside auroral H+ and O+ outflows have lower outflow number fluxes than that in the dayside cusp region. These observations show how the solar wind changes characteristics of CME storms and results in strong sustained ion outflow during the initial and main phases.

Abstract. Magnetospheric sawtooth events are characterized by periodic particle injections and magnetic dipolarizations spread quasi-simultaneously across a wide range of magnetic local times. We present a comprehensive statistical study of magnetospheric sawtooth events (STEs) during solar cycle 24 (2008–2016), extending previous catalogs and enabling solar cycle comparisons. Our results confirm that STEs predominantly occur during the rising and declining phases of the solar cycle, and are strongly associated with geomagnetic storms. Superposed epoch analysis reveals near-simultaneous particle injections across all magnetic local time sectors, but magnetic field dipolarization confined to the midnight region. These results support a scenario in which nightside tail reconnection and enhanced convection are the primary drivers of sawtooth oscillations. The localization of magnetic dipolarizations during STEs challenges global instability interpretations and suggests that STEs represent a stormtime substorm mode triggered under specific solar wind and magnetotail conditions. Superposed epoch analyses also show enhanced oxygen content in the magnetosphere during sawtooth events, but do not show a significant difference from geomagnetic storms that do not exhibit periodic behavior.

Abstract. Joule heating is a primary energy dissipation mechanism of the solar wind in the Earth's upper atmosphere. However there are large discrepancies in the computation of Joule heating between models. In this study, we perform a comparison of the Joule heating rates between two of the most commonly used physics-based Global Circulation Models (GCM) of the Earth's upper atmosphere: the Global Ionosphere/Thermosphere Model (GITM) and the Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM). Both GCMs are externally driven by models that provide the specification of high-latitude electric fields as well as auroral precipitation. In this study, each model is driven by two different specifications of high-latitude electric fields, namely the Weimer 2005 and the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) models. Several empirical formulations are also commonly used to estimate Joule heating rates as a function of various indices of solar and geomagnetic activity; a further comparison is performed between these empirical formulations and the GCMs. We find that the empirical formulations generally give lower estimates of Joule heating rates compared to both GCMs, GITM and TIE-GCM. We also find that TIE-GCM provides lower estimates of the heating rates compared to GITM when the Weimer 2005 model is used as driver, whereas TIE-GCM and GITM give rather similar estimates when the AMIE model is used, with TIE-GCM occasionally giving higher estimates. Estimates of Joule heating rates separately for the two hemispheres indicate that higher Joule heating rates are observed in the Southern Hemisphere when the Weimer model is used, both in GITM and TIE-GCM. However, when the AMIE method is used, higher Joule heating rates are calculated for the Northern Hemisphere. The comparisons between the two Global Circulation models and the empirical models are discussed.