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The ion density and velocity measured by the advanced ionospheric probe (AIP) onboard FORMOSAT‐5 (F5) and ion velocity meter (IVM) onboard FORMOSAT‐7/COSMIC‐2 (F7C2), and the electron density assimilated by the global ionospheric specification (GIS) are employed to study the ionospheric plasma structure and dynamics during the 10 May 2024 Mother's Day storm (Dst −412 nT). The F5/AIP and F7C2/IVM display a large‐scale hole over the magnetic equator in the Atlantic Ocean area (−10° to 25°N, −60° to 20°E) during the local midnight period, with the minimum ion density of 1.7 × 104 #/cm3 and 1.6 × 103 #/cm3, respectively. In the hole area, F5/AIP and F7C2/IVM reveal upward ion velocities at 720 km altitude and downward ones at 550 km altitude, respectively, while GIS profiles show that the electron density yields the lower peak at ∼440 km and upper peak at ∼760 km altitude. This suggests that the downward and upward ion velocities result in the double‐peak feature.

Astrophysical collisionless shocks are among the most powerful particle accelerators in the Universe. Generated by violent interactions of supersonic plasma flows with the interstellar medium, supernova remnant shocks are observed to amplify magnetic fields 1 and accelerate electrons and protons to highly relativistic speeds 2 – 4 . In the well-established model of diffusive shock acceleration 5 , relativistic particles are accelerated by repeated shock crossings. However, this requires a separate mechanism that pre-accelerates particles to enable shock crossing. This is known as the ‘injection problem’, which is particularly relevant for electrons, and remains one of the most important puzzles in shock acceleration 6 . In most astrophysical shocks, the details of the shock structure cannot be directly resolved, making it challenging to identify the injection mechanism. Here we report results from laser-driven plasma flow experiments, and related simulations, that probe the formation of turbulent collisionless shocks in conditions relevant to young supernova remnants. We show that electrons can be effectively accelerated in a first-order Fermi process by small-scale turbulence produced within the shock transition to relativistic non-thermal energies, helping overcome the injection problem. Our observations provide new insight into electron injection at shocks and open the way for controlled laboratory studies of the physics underlying cosmic accelerators. In laser–plasma experiments complemented by simulations, electron acceleration is observed in turbulent collisionless shocks. This work clarifies the pre-acceleration to relativistic energies required for the onset of diffusive shock acceleration.

This study investigates the F‐region plasma density and irregularities response during the May‐2024 storm using Swarm and Gravity Recovery and Climate Experiment Follow‐On (GRACE‐FO) data. Swarm‐B provided measurements at 11/23 LTs. Data from GRACE‐FO and Swarm‐A were collected at 05/17 and 07/19 LTs. Following the storm's commencement on May 10, the equatorial ionization anomaly (EIA) was particularly strong (depleted) on the dayside (nightside). An intensification of the EIA was observed during the early morning 05/07 LTs. Variability during the storm's initial and recovery phases indicate the strong influence of electric fields and thermospheric winds. The magnitude of the EIA was greater during the Halloween storm, but the EIA's relative variabilities were of similar order. Equatorial plasma depletion (EPD) was observed, with apex altitudes reaching ∼${\\sim} $ 5,000 km before midnight on May 11 and ∼${\\sim} $ 3,400 km in the early morning. We suggest that the morning EPDs are generated after mid‐night rather than being remnants from the previous evening.

An annular solar eclipse was visible on 14 October 2023 from 15:00–21:00 UT as its path traveled across North, Central, and South America. In this letter, we present the first multi‐frequency Super Dual Auroral Radar Network (SuperDARN) observations of the bottomside ionospheric response to a solar eclipse using a novel experimental mode designed for the October 2023 annular eclipse. We compare our results from the mid‐latitude Christmas Valley East radar with measurements of the vertical electron density profile from the nearby Boulder Digisonde, finding the changes in 1‐ and 2‐hop ground scatter skip distance are well correlated with the F2${F}_{2}$ ‐layer density response, which lags the peak obscuration by ∼${\\sim} $ 30 min. Changes in the line‐of‐sight Doppler shifts are better aligned with the time derivative of eclipse obscuration. Plain Language Summary During a solar eclipse when the moon passes between our planet and the sun, the amount of incoming solar extreme ultraviolet radiation is reduced over the shadowed region of Earth. As a result, temperatures can become cooler and the rate of plasma production in the ionized layers of the Earth's upper atmosphere, known as the ionosphere, can also decrease. Users of high‐frequency radio systems, such as over‐the‐horizon radar and amateur radio, depend on knowledge of the ionospheric plasma density, and are therefore susceptible to abrupt changes due to space weather phenomena such as a solar eclipse. While not attracting as much media attention as the total solar eclipses over North America in August 2017 and April 2024, the annular solar eclipse of October 2023 nevertheless presented a unique opportunity to study the response of Earth's upper atmosphere using a diverse array of ground‐based instruments in a similar region but at a different time of day. For this event, we designed a new experimental mode for a pair of space weather radars located in central Oregon to transmit signals at a range of frequencies in the HF band to monitor their evolution as the eclipse shadow passed nearby. Key Points A novel multi‐frequency experiment was conducted with the Christmas Valley East SuperDARN radar during the 14 October 2023 annular eclipse Our comparison with the nearby Boulder Digisonde shows a strong correlation in 1‐hop skip distance with F2‐layer critical frequency The transition from negative to positive Doppler shifts is clearly aligned with the time derivative of eclipse shadow

Energetic electron precipitation from the equatorial magnetosphere into the atmosphere plays an important role in magnetosphere‐ionosphere coupling: precipitating electrons alter ionospheric properties, whereas ionospheric outflows modify equatorial plasma conditions affecting electromagnetic wave generation and energetic electron scattering. However, ionospheric measurements cannot be directly related to wave and energetic electron properties measured by high‐altitude, near‐equatorial spacecraft, due to large mapping uncertainties. We aim to resolve this by projecting low‐altitude measurements of energetic electron precipitation by ELFIN CubeSats onto total electron content (TEC) maps serving as a proxy for ionospheric density structures. We examine three types of precipitation on the nightside: precipitation of <200 keV electrons in the plasma sheet, bursty precipitation of <500 keV electrons by whistler‐mode waves, and relativistic (>500 keV) electron precipitation by EMIC waves. All three types of precipitation show distinct features in TEC horizontal gradients, and we discuss possible implications of these features. Plain Language Summary Bursty precipitation of energetic electrons, via pitch‐angle scattering by whistler‐mode waves from the magnetosphere to the ionosphere, is an important factor in the global magnetosphere‐ionosphere coupling. It induces local modifications of ionospheric density and chemical composition. A recurrent problem in the investigation of this process is the presence of large uncertainties in the field‐line mapping between ionospheric density structures and high altitude satellites measuring electron fluxes in the magnetosphere. In the present study, such uncertainties are significantly reduced by making use of precipitating electron fluxes recorded by ELFIN CubeSats at low altitudes (450 km) just above the ionosphere and comparing them with maps of the corresponding ionospheric density structures. We identify three different types of electron precipitation on the nightside, corresponding to low, moderate, and high energy precipitating electrons. We show that each type of the precipitation is characterized by particular plasma density gradients in the ionosphere, suggesting a key role of wave ducting by plasma density gradients in fostering the precipitation of 300–500 keV electrons by whistler‐mode waves, and the potential importance of midnight plasma injections in generating EMIC waves that can further precipitate 0.5–2 MeV electrons far away from the plasmasphere. Key Points Total electron content (TEC) maps are examined during energetic electron precipitation at the conjugate low altitude Night‐side whistler‐mode wave driven precipitation is often associated with local horizontal TEC gradients Night‐side EMIC wave driven precipitation is often poleward of the TEC minima associated with the plasmapause projection

FORMOSAT‐7/COSMIC‐2 is the largest equatorial multi‐satellite constellation of six full‐size satellites to study the equatorial ionosphere. Each satellite is equipped by an ion velocity meter (IVM) instrument to provide high rate in situ plasma density observations along the low‐inclined satellite orbits at ∼530–550 km altitude. Six satellites provide an unprecedented dense coverage of the entire equatorial region around the globe and allow reliable detection of equatorial plasma bubbles (EPBs) and plasma density irregularities at different local times/longitudinal sectors simultaneously. We present a method for detection of EPBs in FORMOSAT‐7/COSMIC‐2 in situ plasma density data and construction of the global maps of EPB geolocations. The results in the form of time series and IVM‐based global Bubble Maps have a great potential for both near real‐time monitoring of space weather conditions and long‐term statistical analysis of EPB occurrence in regional or global scales. We present first FORMOSAT‐7/COSMIC‐2 derived climatological characteristics of the post‐sunset and post‐midnight EPBs occurrence probability and their apex altitudes during a period of low solar activity. Also, we demonstrate the good performance of the FORMOSAT‐7/COSMIC‐2 IVM‐based Bubble Maps when compared to optical images and ground‐based ionosonde observations.

The ACES‐II Low sounding rocket, which launched from Andøya Space Center in Andenes, Norway, on 20 November 2022, made low altitude observations consistent with characteristics of the Ionosphere Feedback Instability (IFI) as it traversed a quiet, discrete auroral arc. Small scale Alfvénic signatures are observed in regions of depleted ionospheric plasma density, large perpendicular ionospheric electric fields, and matched Pedersen and Alfvén conductivities—all observational preconditions required for the formation of the IFI. These signatures are consistent with those of standing Alfvén wave modes in the ionospheric resonant cavity driven by the IFI. The observed Alfvénic structures are correlated with small scale perturbations in the background plasma density. The observed features are similar to the predictions of recent numerical simulations of resonant Alfvén waves generated by the IFI. These observations suggest that the IFI mechanism plays a role in the formation and structuring of this discrete auroral arc. Plain Language Summary The ACES‐II‐Low rocket experiment was launched into the northern lights on 20 November 2022, from Andenes, Norway. It measured small scale changes in the upper atmosphere's density and electric and magnetic fields on the edges of the northern lights. These features are consistent with a model of northern lights by which the changes in upper atmosphere play a role in creating these small‐scale features. We hope that these measurements will help to understand how the upper atmosphere helps to create these features. Key Points Small scale Alfvén wave structures are observed on the equatorward and poleward edges of a quiet, discrete auroral arc These structures coincide with perturbations in plasma density in the lower ionosphere These observations are consistent with signatures of the ionosphere Alfvén resonator driven by the ionosphere feedback instability

Equatorial plasma bubble (EPB) is a phenomenon characterized by depletions in ionospheric plasma density being formed during post-sunset hours. The ionospheric irregularities can lead to disruptions in trans-ionospheric radio systems, navigation systems and satellite communications. Real-time detection and classification of EPBs are crucial for the space weather community. Since 2020, the Prachomklao radar station, a very high frequency (VHF) radar station, has been installed at Chumphon station (Geographic: 10.72° N, 99.73° E and Geomagnetic: 1.33° N) and started to produce radar images ever since. In this work, we propose two real-time plasma bubble detection systems based on support vector machine techniques. Two designs are made with the convolutional neural network (CNN) and singular value decomposition (SVD) used for feature extraction, the connected to the support vector machine (SVM) for EPB classification. The proposed models are trained using quick look (QL) plot images from the VHF radar system at the Chumphon station, Thailand, in 2017. The experimental results show that the combined CNN-SVM model, using the RBF kernel, achieves the highest accuracy of 93.08% while the model using the polynomial kernel achieved an accuracy of 92.14%. On the other hand, the combined SVD-SVM models yield the accuracies of 88.37% and 85.00% for RBF and polynomial kernels of SVM, respectively.

On July 14th, 2017, the first Norwegian scientific satellite NorSat-1 was launched into a high-inclination (98 ∘ ), low-Earth orbit (600 km altitude) from Baikonur, Kazakhstan. As part of the payload package, NorSat-1 carries the multi-needle Langmuir probe (m-NLP) instrument which is capable of sampling the electron density at a rate up to 1 kHz, thus offering an unprecedented opportunity to continuously resolve ionospheric plasma density structures down to a few meters. Over the coming years, NorSat-1 will cross the equatorial and polar regions twice every 90 minutes, providing a wealth of data that will help to better understand the mechanisms that dissipate energy input from larger spatial scales by creating small-scale plasma density structures within the ionosphere. In this paper we describe the m-NLP system on board NorSat-1 and present some first results from the instrument commissioning phase. We show that the m-NLP instrument performs as expected and highlight its unique capabilities at resolving small-scale ionospheric plasma density structures.

During the main phase of geomagnetic storms, large positive ionospheric plasma density anomalies arise at middle and polar latitudes. A prominent example is the tongue of ionisation (TOI), which extends poleward from the dayside storm-enhanced density (SED) anomaly, often crossing the polar cap and streaming with the plasma convection flow into the nightside ionosphere. A fragmentation of the TOI anomaly contributes to the formation of polar plasma patches partially responsible for the scintillations of satellite positioning signals at high latitudes. To investigate this intense plasma anomaly, numerical simulations of plasma and neutral dynamics during the geomagnetic superstorm of 20 November 2003 are performed using the Thermosphere Ionosphere Electrodynamics Global Circulation Model (TIE-GCM) coupled with the statistical parameterisation of high-latitude plasma convection. The simulation results reproduce the TOI features consistently with observations of total electron content and with the results of ionospheric tomography, published previously by the authors. It is demonstrated that the fast plasma uplift, due to the electric plasma convection expanded to subauroral mid-latitudes, serves as a primary feeding mechanism for the TOI anomaly, while a complex interplay between electrodynamic and neutral wind transports is shown to contribute to the formation of a mid-latitude SED anomaly. This contrasts with published simulations of relatively smaller geomagnetic storms, where the impact of neutral dynamics on the TOI formation appears more pronounced. It is suggested that better representation of the high-latitude plasma convection during superstorms is needed. The results are discussed in the context of space weather modelling.