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We show that atmospheric gravity waves can generate plasma ducts and irregularities in the plasmasphere using the coupled SAMI3/WACCM‐X model. We find the equatorial electron density is irregular as a function of longitude which is consistent with CRRES measurements (Clilverd et al., 2007, https://doi.org/10.1029/2007ja012416). We also find that plasma ducts can be generated for L‐shells in the range 1.5–3.0 with lifetimes of ∼ 0.5 hr; this is in line with observations of ducted VLF wave propagation with lifetimes of 0.5–2.0 hr (Clilverd et al., 2008, https://doi.org/10.1029/2007ja012602; Singh et al., 1998, https://doi.org/10.1016/s1364-6826(98)00001-7). Plain Language Summary Electromagnetic plasma waves, known as whistler waves, are observed to propagate in the ionosphere/plasmasphere system where the ionosphere is nominally defined as the partially ionized gas surrounding the earth in the altitude range 90–1,000 km and the plasmasphere is essentially the extension of the ionosphere 1,000s of km into space along closed geomagnetic field lines. Whistler wave propagation has been characterized as ducted and non‐ducted. Ducted propagation is guided along the magnetic field by density tubes in which the plasma density is lower or higher than the background plasma. However, the physical processes that generate these ducts has remained unclear. We show that these plasma ducts can be generated by atmospheric gravity waves that perturb the ionosphere and plasmasphere electron density using the coupled SAMI3/WACCM‐X model. Key Points Atmospheric gravity waves can generate plasma ducts and irregularities in the plasmasphere using the coupled SAMI3/WACCM‐X model The electron density in the equatorial plasmasphere is irregular as a function of longitude Plasma ducts can be generated for L‐shells in the range 1.5–3.0 with lifetimes of 0.5–2.0 hr

Plasma content and distribution are key parameters in the dynamics of the inner magnetosphere. The plasmasphere contributes, for the most part, to the plasma mass density, and its properties are very dependent on the history of the magnetosphere and geomagnetic activity. In this work, we investigated plasmasphere dynamics and plasmasphere–ionosphere coupling, focusing on the refilling process that followed the geomagnetic storm that occurred on 1 June 2013. The equatorial plasma mass density used to evaluate the refilling rates was remotely sensed by observation of the field line resonance (FLR) frequencies of the geomagnetic field, driven by ultra-low-frequency magnetic waves. The FLR frequencies were retrieved by performing an analysis of signals detected by several station pairs of the European quasi-Meridional Magnetometer Array. We estimated the rate at which the refilling process occurred, concentrating on both the diurnal and the day-to-day refilling rates. The estimated contraction rate during the main phase of the storm was higher than ∼3.5 REd−1, while the average expansion rate was ∼0.4 REd−1. We investigated the radial dependence of the refilling rates, using a novel approach based on fit plasma mass density profiles, and we related their variation to the plasmasphere boundary layer and the zero-energy Alfvén boundary. We found evidence supporting the idea that flux tubes mapping in the region between these two boundaries experience an enhanced refilling process.

On 3 November 2021, an interplanetary coronal mass ejection impacted the Earth’s magnetosphere leading to a relevant geomagnetic storm (Kp = 8-), the most intense event that occurred so far during the rising phase of solar cycle 25. This work presents the state of the solar wind before and during the geomagnetic storm, as well as the response of the plasmasphere–ionosphere–thermosphere system in the European sector. To investigate the longitudinal differences, the ionosphere–thermosphere response of the American sector was also analyzed. The plasmasphere dynamics was investigated through field line resonances detected at the European quasi-Meridional Magnetometer Array, while the ionosphere was investigated through the combined use of ionospheric parameters (mainly the critical frequency of the F2 layer, foF2) from ionosondes and Total Electron Content (TEC) obtained from Global Navigation Satellite System receivers at four locations in the European sector, and at three locations in the American one. An original method was used to retrieve aeronomic parameters from observed electron concentration in the ionospheric F region. During the analyzed interval, the plasmasphere, originally in a state of saturation, was eroded up to two Earth’s radii, and only partially recovered after the main phase of the storm. The possible formation of a drainage plume is also observed. We observed variations in the ionospheric parameters with negative and positive phase and reported longitudinal and latitudinal dependence of storm features in the European sector. The relative behavior between foF2 and TEC data is also discussed in order to speculate about the possible role of the topside ionosphere and plasmasphere response at the investigated European site. The American sector analysis revealed negative storm signatures in electron concentration at the F2 region. Neutral composition and temperature changes are shown to be the main reason for the observed decrease of electron concentration in the American sector.

We present a newly developed empirical model of the plasma density in the plasmasphere. It is based on more than 700 density profiles along field lines derived from active sounding measurements made by the radio plasma imager on IMAGE between June 2000 and July 2005. The measurements cover all magnetic local times and vary from L = 1.6 to L = 4 spatially, with every case manually confirmed to be within the plasmasphere by studying the corresponding dynamic spectrogram. The resulting model depends not only on L‐shell but also on magnetic latitude and can be applied to specify the electron densities in the plasmasphere between 2000 km altitude and the plasmapause (the plasmapause location itself is not included in this model). It consists of two parts: the equatorial density, which falls off exponentially as a function of L‐shell; and the field‐aligned dependence on magnetic latitude and L‐shell (in the form of invariant magnetic latitude). The fluctuations of density appear to be greater than what could be explained by a possible dependence on magnetic local time or season, and the dependence on geomagnetic activity is weak and cannot be discerned. The solar cycle effect is not included because the database covers only a fraction of a solar cycle. The performance of the model is evaluated by comparison to four previously developed plasmaspheric models and is further tested against the in situ passive IMAGE RPI measurements of the upper hybrid resonance frequency. While the equatorial densities of different models are mostly within the statistical uncertainties (especially at distances greater than L = 3), the clear latitudinal dependence of the RPI model presents an improvement over previous models. The model shows that the field‐aligned density distribution can be treated neither as constant nor as a simple diffusive equilibrium distribution profile. This electron density model combined with an assumed model of the ion composition can be used to estimate the time for an Alfven wave to propagate from one hemisphere to the other, to determine the plasma frequencies along a field line, and to calculate the raypaths for high frequency waves propagating in the plasmasphere. Key Points Improved empirical model of the Earth's plasmasphere Realistic description of the field‐aligned electron density distribution Possible applications in studies of the inner magnetosphere and wave propagation

Hiss waves frequently occur in the plasmasphere or plumes, playing a key role in energetic electron loss in the Earth's inner magnetosphere. While previous studies have linked hiss wave enhancements in the outer plasmasphere (just inside the plasmapause) to electron injections during substorms, their evolution across various substorm phases remains unclear. Using Van Allen Probes observations over 2013–2019, we evaluate hiss wave evolution during various phases of substorm activity. At L > 4, both hiss wave intensity and energetic electron flux increase shortly after substorm onset, first on the morning side, then progress to later magnetic local times (MLTs) at a rate of ∼1–3 hr MLT per hr in universal time (UT), eventually stabilizing near 13 MLT. Stronger substorms result in larger and faster intensification in hiss wave intensity and have more significant impact at lower L‐shells. Our results highlight the global variation of hiss waves during substorms.

Since their discovery more than 50 years ago, Earth's Van Allen radiation belts have been considered to consist of two distinct zones of trapped, highly energetic charged particles. The outer zone is composed predominantly of megaelectron volt (MeV) electrons that wax and wane in intensity on time scales ranging from hours to days, depending primarily on external forcing by the solar wind. The spatially separated inner zone is composed of commingled high-energy electrons and very energetic positive ions (mostly protons), the latter being stable in intensity levels over years to decades. In situ energy-specific and temporally resolved spacecraft observations reveal an isolated third ring, or torus, of high-energy (>2 MeV) electrons that formed on 2 September 2012 and persisted largely unchanged in the geocentric radial range of 3.0 to ~3.5 Earth radii for more than 4 weeks before being disrupted (and virtually annihilated) by a powerful interplanetary shock wave passage.

As a unique structure of plasmaspheric hiss emissions, banded hiss waves generally consist of an upper band above ∼200 Hz, a lower band below ∼100 Hz and a power gap in between. However, the generation mechanism of banded hiss remains unclear. Here we present a representative event of banded hiss co‐existing in both the plasmaspheric plume structure and the plasmasphere. The wave Poynting flux measurements and linear instability analysis suggest that banded hiss can be generated in the equatorial plume region. The ray tracing simulations further show that the plume structure enables banded hiss to propagate along the plasma density crest to higher latitudes, followed by the penetration into the plasmasphere. Our data analysis and modeling efforts provide a plausible mechanism for the generation of banded hiss, and also highlight the significant role of plume structure in the generation and propagation of magnetospheric whistler‐mode waves. Plain Language Summary Plasmaspheric hiss is a whistler‐mode emission usually observed within the plasmasphere and the plume structure, with frequency ranging from ∼20 to ∼2,000 Hz. Hiss can scatter energetic electrons from ∼10s keV to several MeV, which is known as the main mechanism of the formation of radiation belt slot region. Hiss can be excited from various source regions, which leads to differences of plasmaspheric hiss waves on frequency, morphology, and scattering effects. In this study, we use the wave and particle measurements from Van Allen Probes to investigate the generation mechanism of banded hiss, which usually shows an upper band above ∼200 Hz and a lower band below ∼100 Hz. Based on the analysis of banded hiss event and ray tracing method, the banded hiss is found to be generated in the equatorial plume region, propagate to higher latitudes along the plume structure, and eventually penetrate into the plasmasphere. The plume structure plays an important role in the generation and propagation of banded hiss, which can subsequently impact the wave‐particle interaction processes and their consequences in Earth's radiation belts. Key Points A physical mechanism is proposed to explain the generation of banded plasmaspheric hiss waves Wave growth rate calculations based on electron distributions suggest that banded hiss is likely generated in the equatorial plume region Ray tracing results indicate that the plume structure plays a crucial role in guiding the propagation of banded hiss into the plasmasphere

To examine the temporal and spatial evolution of cold ions within the inner magnetosphere during geomagnetic storms, we conducted superposed epoch analyses on the electron density (Ne) and the partial ion densities of H+, He+, and O+ within the instrument frame energy range of 1–10 eV, using data obtained from the Van Allen Probes. The analyses were performed on a set of 14 large storms (SYM‐H < −100 nT) over a 5‐year period from 2013 to 2017. We found that the electron density and cold H+ and He+ densities outside the plasmasphere decrease as large storms develop. However, the cold O+ density outside the plasmasphere remains largely unchanged during the storm's main and early recovery phases. Based on these observations, we suggest that the less‐dense cold population outside the plasmasphere during storm activities contains more cold O+ ions.

The spatial distribution of electron density in the ionosphere exhibits notable variability and undergoes considerable changes during storms and substorms driven by solar wind disturbances. Electron density variations and irregularities can cause total signal blackouts of broadcast waves during strong scintillation periods and enhance satellite positioning errors. We analyzed Global Navigation Satellite System (GNSS)—total electron content (TEC) and Arase satellite observation data to elucidate the characteristics of the electron density variation in the plasmasphere and ionosphere during the May 2024 super storm. To identify the electron density variation in the ionosphere, we calculated the ratio of the TEC difference (rTEC), which is defined as the difference from the 10-quiet-day average TEC divided by the average value. Additionally, we estimated the electron density in the plasmasphere and inner magnetosphere from the upper frequency limit of the upper hybrid resonance (UHR) waves observed by the Arase satellite. Consequently, an L–t plot of the electron density showed that the plasmasphere contracted from L  = 7.0 to L  = 1.5 within 9 h after a sudden commencement. During the storm recovery phase, the plasmapause gradually shifted to a higher L-shell. The electron density in the plasmasphere recovered to the geomagnetically quiet-time level on a 4-day scale. The timescale of the plasmaspheric refilling was much longer than that of other coronal mass ejection (CME)-driven storms during the Arase era. The rTEC in the Northern Hemisphere showed that an enhancement in the rTEC value occurred at high latitudes [60°–70° in magnetic latitude (MLAT)] in the daytime [10–14 in magnetic local time (MLT)], approximately 1 h after the storm onset. Subsequently, a tongue of ionization (TOI) formed in the polar cap owing to the effect of the solar wind and magnetosphere in driving horizontal flows in the ionosphere. The rTEC was globally depleted during the storm recovery phase. The depletion indicates the occurrence of a negative storm owing to a neutral composition (O/N 2 ) change driven by the energy input from the magnetosphere in the high-latitude thermosphere. The coincidence of the long refilling timescale of the plasmasphere and the depletion of the rTEC suggests that a strong negative storm impedes plasmaspheric refilling. Graphical Abstract

We present the analysis of the first lunar‐based observational characterization of the Earth's plasmasphere and ionosphere using Global Navigation Satellite Systems signals tracked from the lunar surface by the Lunar GNSS Receiver Experiment (LuGRE). The Earth‐Moon geometry enables limb sounding of the plasmasphere at altitudes exceeding 3,000 km, bridging a critical observational gap. We compared Total Electron Content measurements from GPS and Galileo satellites' signals with predictions from the Global Core Plasma Model. While the model captures the general morphology of the plasmasphere, significant discrepancies emerge in the ionosphere/plasmasphere transition region. Specifically, LuGRE data reveal a overestimation of electron density by the model during the dayside phase indicating a lower plasma refilling efficiency than currently parameterized and an underestimation during the nightside phase. These results demonstrate the capability of lunar‐based GNSS measurements to continuously monitor the global plasma environment, paving the way for future permanent observatories on the Moon.