JGR:Space physics

The day-to-day variability (“weather”) associated with the diurnal- and zonal-mean (DZM) circulation, O/N2 ratio and electron density (Ne) of the I-T system due to tidal “forcing from below” and solar flux and magnetosphere (SM) “forcing from above” during 2021 are delineated, diagnosed and quantitatively compared using a series of model simulations designed to separate these responses with respect to their origins. The external forcings are driven by actual tidal, solar wind, and solar flux observations. Both circulation systems occupy the full extent of the I-T, and the SM-forced DZM circulation is 2–3 times more vigorous in terms of vertical and meridional wind magnitudes. Tidal-driven DZM Ne reductions of up to 30%–40% with respect to those of the fully forced I-T system occur, mainly between ±30° latitude, compared to SM-driven increases of up to 15%–20%. In terms of annual variances over this latitude range, tidal-driven DZM Ne variances exceed or equal those of the SM-driven variances. The former is mainly controlled by O/N2 ratio vis-a-vis tidal-forced temperature variations above 150 km. While a similar cause-effect relation exists for the latter, this is superseded by Ne variability associated with solar production. However, DZM I-T system variability forced from below is underestimated in the simulations in two respects: the effects of gravity waves are omitted, and tidal forcing is represented by 45-day running means, as compared with the more realistic actual daily variability of SM forcing. These shortcomings should be ameliorated once multi-satellite missions planned for the future come to fruition.

The present study uncovers the fine structures of magnetosonic waves by investigating the EFW waveforms measured by Van Allen Probes. We show that each harmonic of the magnetosonic wave may consist of a series of elementary rising-tone emissions, implying a nonlinear mechanism for the wave generation. By investigating an elementary rising-tone magnetosonic wave that spans a wide frequency range, we show that the frequency sweep rate is likely proportional to the wave frequency. We studied compound rising-tone magnetosonic waves, and found that they typically consist of multiple harmonics in the source region, and may gradually become continuous in frequency as they propagate away from source. Both elementary and compound rising-tone magnetosonic waves last for ∼1 min which is close to the bounce period of the ring proton distribution, but their relation is not fully understood.

Besides the significant effects of a variety of naturally occurring magnetospheric waves on the electron dynamics in the magnetosphere, the important contribution of ground-based very-low-frequency (VLF) transmitter waves also has been gradually discovered. The VLF transmitter's wave penetrating into the topside ionosphere is its energy source injected into the magnetosphere and has been extensively investigated. In the VLF wave trans-ionospheric propagation, the main energy attenuation occurs in the lower ionosphere which is controlled by solar short-wave radiation. However, the investigation on the variation of the VLF transmitters' energy in the topside ionosphere and ionospheric reflection height with solar activity is lacking. We use 4 years electric field measurements performed by DEMETER satellite and full-wave simulations to address these concerns. The results show the electric field radiated from NWC was relatively similar from May 1 to July 31 in 2006, 2008, and 2009 in daytime and nighttime, stronger than that in 2005, because the solar activity was similar and extremely low in these years compared with that in 2005. The nighttime and daytime ionospheric reflection heights are also relatively similar in these 3 years, with about 6 km higher than that in 2005. The difference in the simulated electric field based on the electron density profile from the IRI-2016 model between 2005 and 2009 is lower compared with the observation results. However, considering 6 km added in ionospheric reflection height in 2009, the simulation results are much more consistent with the observation results both in daytime and nighttime.

The effects of plasma density on the plasmaspheric hiss amplitude and its spatial and temporal scale sizes are investigated using data from Van Allen Probes A and B. The plasmaspheric hiss amplitude and scale sizes are found to be more affected by the small-scale density variations than the overall density profile of the plasmasphere. In detail, our results suggest that both the hiss wave amplitude and its scale sizes are (a) related to the total electron density when the density is less than ∼1,000 cm−3; (b) independent of the total electron density when the density is greater than ∼1,000 cm−3; and (c) better correlated with the detrended density (removing the background plasmasphere density profile from the total electron density) regardless of the density value.

This study investigates the ionospheric total electron content (TEC) responses in the 2-D spatial domain and electron density variations in the 3-D spatial domain during the annular solar eclipse on 14 October 2023, using ground-based Global Navigation Satellite System (GNSS) observations, a novel TEC-based ionospheric data assimilation system (TIDAS), ionosonde measurements, and satellite in situ data. The main results are summarized as follows: (a) The 2-D TEC responses exhibited distinct latitudinal differences. The mid-latitude ionosphere exhibited a more substantial TEC decrease of 25%–40% along with an extended recovery time of 3–4 hr. In contrast, the equatorial and low-latitude ionosphere experienced a smaller TEC reduction of 10%–25% and a faster recovery time of 20–50 min. The minimal eclipse effect was observed near the northern equatorial ionization anomaly crest region. (b) The ionospheric electron density variations during the eclipse were effectively reconstructed by TIDAS data assimilation in the 3-D domain, providing important altitude information with validity. (c) The ionospheric electron density variations showed a notable altitude-dependent feature. The eclipse led to a substantial electron density reduction of 30%–50%, with the maximum depletion occurring around the ionospheric F2-layer peak height (hmF2) of 250–350 km. The post-eclipse recovery of electron density exhibited a relatively slower pace near the F2-layer peak height than that at lower and higher altitudes.

Based on Global Coupled Ionosphere-Thermosphere-Electrodynamics Model, the solution of the 3-dimensional current in the ionospheric region, the equivalent sheet current and filed-aligned current are examined. The simulation study enables a comprehensive analysis of the effect of the geomagnetic field configuration, especially the non-dipole component and tilt angle, on the ionospheric electrodynamics phenomena. Different geomagnetic field configurations are specified in the present work, including realistic geomagnetic field (RGF), tilted dipole geomagnetic field (TDGF) and zero-declination dipole geomagnetic field (ZDGF). Our simulation focuses on the seasonal variation of Sq current, primarily governed by annual and semi-annual variation. The modulation of the tilt angle of the geomagnetic field is globally distributed, whereas the modulation of the geomagnetic anomaly is localized. At mid-latitudes, the annual mean and semi-annual amplitudes of the Sq current are negatively correlated with the magnetic field strength, especially shown in geomagnetic anomaly area, while there is the opposite effect in the geomagnetic conjugate regions of the opposite hemispheres. The annual variation of Sq current system is more affected by the offset of the geomagnetic latitudes and geographical latitudes. The seasonal variation of the total Sq current is also modulated by the geomagnetic field. The annual mean, the annual and semi-annual components of the total Sq current are negatively correlated with the magnetic field strength, while the annual variation is also controlled by the tilt angle of the geomagnetic field. The solar radiation affects the semi-annual variation of the current more strongly than the annual variation.

Narrowband (Δf ≲ 0.1f cp ), high-frequency (0.9f cp  ≲ f < f cp ) electromagnetic ion cyclotron (EMIC) waves, or HFEMIC waves for short, are a relatively new type of EMIC waves, where f cp is the proton cyclotron frequency. Here, we investigate the instability threshold conditions and the nonlinear evolution of HFEMIC waves at parallel propagation. First, linear theory analysis is extended to a regime relevant to HFEMIC waves (parallel proton beta β ‖p  < 0.01). The instability threshold follows a similar anisotropy-β ‖p relation and requires a large value of anisotropy (T ⊥p /T ‖p  ≳ 10) for wave growth. As a result of decreasing group velocity, the convective growth rate at a fixed threshold exhibits a similar inverse relation with β ‖p . Heavy ions affect the instability only weakly, primarily through the introduction of stop bands. Second, we carry out one-dimensional hybrid simulations in a parabolic background magnetic field, with initial parameters constrained by observation. Despite the narrow source region (within about ±3° latitude), HFEMIC waves can grow well above the thermal noise level due in large part to a small group velocity of HFEMIC waves. The saturation level is well within the range of observational amplitudes, and the quasilinear process primarily determines the wave evolution. Finally, we demonstrate that the present results compare favorably to the recent statistical results, thereby supporting anisotropic low-energy protons as free energy source for HFEMIC waves.

Electron density (N e) and electron temperature (T e) observations collected by Langmuir Probes on board the European Space Agency (ESA) Swarm B satellite are used to characterize their correlation in the topside ionosphere at an altitude of about 500 km. Spearman correlation coefficient values (R Spearman) are calculated on joint probability distributions between N e and T e for selected conditions. The large data set of Swarm B observations at 2-Hz rate, covering the years 2014–2022, allowed investigating the correlation properties of the topside ionospheric plasma on a global scale, for different diurnal and seasonal conditions, with both a coverage and a detail never reached before. Results are given as maps of R Spearman as a function of the Quasi-Dipole (QD) magnetic latitude and magnetic local time (MLT) coordinates. The characterization of the correlation at high latitudes, along with the description of the diurnal trend at all latitudes, are the new findings of this study. The main correlation features point out a negative correlation at the morning overshoot, during daytime at mid latitudes, and during nighttime at the ionospheric trough and subauroral latitudes. Conversely, a positive correlation dominates the nighttime hours at mid and low latitudes and, to a minor extent, the low latitudes from 09 MLT onwards. A seasonal dependence of the correlation is noticeable only at very high latitudes where the general pattern of the negative correlation does not hold around ±75° QD latitude in the summer season. Results from Swarm B have been statistically compared and discussed with observations from the Arecibo, Jicamarca, and Millstone Hill incoherent scatter radars.

The full spatiotemporal distribution of chorus wave-induced relativistic electron microburst is modeled for chorus waves originated from different L shells and MLTs, based on the newly developed numerical precipitation model (Kang et al., 2022, https://doi.org/10.1029/2022gl100841). The wave-particle interaction process that induces each microburst is analyzed in detail, and its relation to the chorus wave propagation effects is explained. The global distribution of maximum precipitation fluxes and scale sizes of relativistic microbursts is then obtained by modeling chorus waves at different L-shells and local times. The characteristics of dawn and midnight sector microbursts have little difference, but the noon sector has much larger maximum flux and much smaller full width at half maximum, which may be due to dayside's low electron flux in the Landau resonance range. This suggests the controlling effect of keV electrons on the MeV electron precipitation intensity and properties and the overall relativistic electron loss in the outer radiation belt.

Magnetic reconnection is a critically important process in defining the dynamics and energy transport within plasma environments. In near-Earth space we may track where and when reconnection occurs by identifying associated coherent magnetic structures. On a global scale these structures facilitate the flow of mass and magnetic flux into, within, and out of the magnetospheric system, whilst contributing to local plasma heating. In the Earth's magnetotail there are two similar structures we identify in this work: magnetic flux ropes and loops. We present a robust, automated and model independent method by which encounters with such structures may be identified using the Magnetospheric Multiscale (MMS) mission. The magnetic structures are first identified through their magnetic field signatures at a single spacecraft (MMS1), including checks on the local minimum variance coordinate system. Next, the local curvature of the magnetic field is evaluated with all four MMS spacecraft. Finally, the plasma conditions are checked to ensure that the interpretation is fully self-consistent. We evaluate the data obtained by MMS between 2017 and 2022. In total we find 181 self-consistent magnetic flux ropes and 263 magnetic loops, which fit an exponentially decaying size distribution with a scale size comparable to the ion gyroradius (∼0.23 R E /1,400 km). If we remove the requirements on the plasma properties of the structure, we locate 648 potential magnetic flux ropes and 1,073 magnetic loops. The magnetic structures are preferentially observed in the pre-midnight region of the magnetotail, with most identifications occurring beyond 20 R E . All catalogs are provided to the community.

We present a statistical analysis of electrostatic solitary waves observed aboard Magnetospheric Multiscale spacecraft in the Earth's magnetosheath. Applying single-spacecraft interferometry to several hundred solitary waves collected in about 2-minute interval, we show that almost all of them have the electrostatic potential of positive polarity and propagate quasi-parallel to the local magnetic field with plasma frame velocities of the order of 100 km/s. The solitary waves have typical parallel half-widths from 10 to 100 m that is between 1 and 10 Debye lengths and typical amplitudes of the electrostatic potential from 10 to 200 mV that is between 0.01% and 1% of local electron temperature. The solitary waves are associated with quasi-Maxwellian ion velocity distribution functions, and their plasma frame velocities are comparable with ion thermal speed and well below electron thermal speed. We argue that the solitary waves of positive polarity are slow electron holes and estimate the time scale of their acceleration, which occurs due to interaction with ions, to be of the order of one second. The observation of slow electron holes indicates that their lifetime was shorter than the acceleration time scale. We argue that multi-spacecraft interferometry applied previously to these solitary waves is not applicable because of their too-short spatial scales. The source of the slow electron holes and the role in electron-ion energy exchange remain to be established.

Mercury's planetary magnetic field models (PMFMs) agree on a majorly dipolar field structure with a northward shift of the magnetic equator. However, due to the northerly biased orbit coverage of past spacecraft missions and different data analyzing methods, the available PMFMs differ in the determined multipole magnitudes for the dipole, quadrupole and octupole moments. While the PMFMs agree well with northern observations, we find that the predicted magnetic field values differ in the unexplored equatorial and southern regions. In the forward modeling approach of this study, we apply three different PMFM representatives, differing in their values of the dipole, quadrupole and octupole moments, and model the resulting solar wind interaction via a global hybrid model with sets of the 4 most common interplanetary magnetic field (IMF) directions under otherwise average solar wind conditions. Extracting our modeled fields along the flybys of MESSENGER and BepiColombo, as well as along four representative orbits of the Mercury Planetary Orbiter (MPO), allows us to estimate local field variations due to IMF and PMFM influence. Our modeled magnetic field components separate significantly by up to 60 nT between the PMFMs in low-altitude southern regions, leading to displacements of magnetopause, cusps and current sheet locations. We find that MESSENGER flyby observations agree best with modeled field ranges resulting from a quadrupolar PMFM and that certain nightside regions are useful to estimate upstream IMF polarity. We also demonstrate how comparing BepiColombo swingby and MPO orbit phase observations with modeled results can help determine the correct PMFM for Mercury.

Multiple Large-Scale Traveling Ionospheric Disturbances (LSTIDs) are observed in the European sector in both day-time and night-time during the magnetic storm on March 23–24, 2023. The Total Electron Content (TEC) observation from a network of GNSS receivers shows the propagation of LSTIDs with amplitudes between around 0.5 and 1 TECU originating from auroral and polar cusp regions down to southern Europe (35°N) with velocities between around 500 and 1,600 [m/s]. We study the energy deposition to the LSTIDs in the source regions and the resulting horizontal propagation over storm-time background density by using continuous measurements of EISCAT incoherent scatter radars in northern Norway and Svalbard that allow for estimating the source energy to the thermosphere-ionosphere system via Joule heating and particle precipitation. Both EISCAT and GNSS TEC data show that the electron density decreased to 50% in the auroral zone after the storm onset. The ionospheric heating caused a nearly 250% increase in the electron temperature above 200 km altitude and the ion temperature above 100 km altitude. We find that Joule Heating acts as a primary energy source for the night-time LSTIDs triggered in the auroral region, while the day-time LSTIDs can be also driven by precipitating particles in the polar cusp. We also find that a significant background density decrease over the whole European sector is caused by this storm for the following day, during which almost no clear LSTIDs are observed.

Short large-amplitude magnetic structures (SLAMS) frequently appear near the bow shock. Inside steepening SLAMS, whistler-mode waves are coherently generated at their leading edges. These waves are crucial for electron dynamics, energy conversion and magnetic reconnection near the shock. Nevertheless, the characteristics and generation of the whistler-mode waves inside SLAMS are still unclear. In this study, we conducted a statistical analysis of whistler-mode waves within SLAMS near the Earth's bow shock to investigate their properties and generation mechanisms. We found that these waves are mainly excited by electron temperature anisotropy with 99% of them falling below the nonlinear resonant threshold. The combination of a low background magnetic field and intense steepening at SLAMS' leading edge makes it the main source region of whistler-mode waves and may induce the nonlinear resonance of whistler-mode waves.

The interaction of a solar wind discontinuity with the backstreaming particles of the Earth’s ion foreshock can generate hot, tenuous plasma transients such as foreshock bubbles (FB) and hot flow anomalies (HFA). These transients are known to have strong effects on the magnetosphere, distorting the magnetopause (MP), either locally during HFAs or globally during FBs. However, previous studies on the global impact of FBs have not been able to determine whether the response stems directly from the transverse scale size of the phenomenon or its fast motion over the magnetosphere. Here we present the observation of an FB and its impact on the magnetosphere from different spacecraft scattered over the dayside magnetosphere. We are able to constrain the size of the transverse scale of an FB from direct observations to be about 10 R E. We go on to discuss how the magnetosphere responds to this transient, which seems to have a similar scale across the dayside.

The Kelvin-Helmholtz (KH) instability can transport mass, momentum, magnetic flux, and energy between the magnetosheath and magnetosphere, which plays an important role in the solar-wind-magnetosphere coupling process for different planets. Meanwhile, strong density and magnetic field asymmetry are often present between the magnetosheath (MSH) and magnetosphere (MSP), which could affect the transport processes driven by the KH instability. Our magnetohydrodynamics simulation shows that the KH growth rate is insensitive to the density ratio between the MSP and the MSH in the compressible regime, which is different than the prediction from linear incompressible theory. When the interplanetary magnetic field (IMF) is parallel to the planet's magnetic field, the nonlinear KH instability can drive a double mid-latitude reconnection (DMLR) process. The total double reconnected flux depends on the KH wavelength and the strength of the lower magnetic field. When the IMF is anti-parallel to the planet's magnetic field, the nonlinear interaction between magnetic reconnection and the KH instability leads to fast reconnection (i.e., close to Petschek reconnection even without including kinetic physics). However, the peak value of the reconnection rate still follows the asymmetric reconnection scaling laws. We also demonstrate that the DMLR process driven by the KH instability mixes the plasma from different regions and consequently generates different types of velocity distribution functions. We show that the counter-streaming beams can be simply generated via the change of the flux tube connection and do not require parallel electric fields.

We present the first large-scale statistical survey of the Jovian main emission (ME) to map auroral properties from their ionospheric locations out into the equatorial plane of the magnetosphere, where they are compared directly to in-situ spacecraft measurements. We use magnetosphere-ionosphere (MI) coupling theory to calculate currents from the auroral brightness as measured with the Hubble Space Telescope and from plasma flow speeds measured in-situ with the Galileo spacecraft. The effective Pedersen conductance of the ionosphere ΣP∗ $\left({{\Sigma }}_{P}^{\ast }\right)$ remains a free parameter in this comparison. We calculate the Pedersen conductance from the combined data sets, and find it ranges from 0.03<ΣP∗<2.40 $0.03< {{\Sigma }}_{P}^{\ast }< 2.40$ mho overall with averages of 0.13−0.07+0.26 $0.1{3}_{-0.07}^{+0.26}$ mho in the north and 0.16−0.10+0.34 $0.1{6}_{-0.10}^{+0.34}$ mho in the south. Considering the HST-derived field-aligned currents per radian of azimuth only, we find values of I‖=9.34−3.54+5.72 ${I}_{\Vert }=9.3{4}_{-3.54}^{+5.72}$ MA rad−1 and I‖=8.61−3.05+6.77 ${I}_{\Vert }=8.6{1}_{-3.05}^{+6.77}$ MA rad−1 in the north and south, respectively, in general agreement with previous results. Taking the currents and effective Pedersen conductance together, we find that the average ME intensity and plasma flow speed in the middle magnetosphere (10–30 R J ) are broadly consistent with one another under MI coupling theory. We find evidence for peaks in the distribution of ΣP∗ ${{\Sigma }}_{P}^{\ast }$ near dawn, then again near 12 and 14 hr magnetic local time (MLT). This variation in Pedersen conductance with MLT may indicate the importance of conductance in modulating MLT- and local-time-asymmetries in the ME, including the apparent subcorotation of some auroral features within the ME.

In this work, we present a statistical study of substorms covering a five-year period 2016–2020. Substorm phases were identified from time series of the SuperMAG AL (SML) index using a list of 5,077 previously identified substorm onsets, the SML peak value marking transition from expansion to recovery phase, and the recovery identified as return to activity less than −100 nT in the SML index. Magnetic field observations from THEMIS, RBSP, and MMS missions were used to study the magnetotail characteristics during the substorm evolution. A superposed epoch analysis indicates that the substorm onset occurs almost simultaneously with a few minutes of uncertainty throughout the magnetotail, ranging from geostationary orbit to 20 RE. The onset in the transition region precedes the ground onset by a few minutes. The peak SML time coincides with the peak of the outer transition region ΔB Z , which suggests that the field-aligned currents driving the SML activity arise from the outer transition region. Analysis of 2D maps of the tail magnetic field shows that the magnetotail current changes are limited to the center of the tail within |Y| < 10R E . The substorm recovery is fastest in the inner transition region and lasts longer when moving further out. We did not find major asymmetries in the substorm signatures associated with IMF B Y or B Z .

Electromagnetic ion cyclotron (EMIC) waves lead to rapid scattering of relativistic electrons in Earth's radiation belts, due to their large amplitudes relative to other waves that interact with electrons of this energy range. A central feature of electron precipitation driven by EMIC waves is deeply elusive. That is, moderate precipitating fluxes at energies below the minimum resonance energy of EMIC waves occur concurrently with strong precipitating fluxes at resonance energies in low-altitude spacecraft observations. This paper expands on a previously reported solution to this problem: nonresonant scattering due to wave packets. The quasi-linear diffusion model is generalized to incorporate nonresonant scattering by a generic wave shape. The diffusion rate decays exponentially away from the resonance, where shorter packets lower decay rates and thus widen the energy range of significant scattering. Using realistic EMIC wave packets from δf particle-in-cell simulations, test particle simulations are performed to demonstrate that intense, short packets extend the energy of significant scattering well below the minimum resonance energy, consistent with our theoretical prediction. Finally, the calculated precipitating-to-trapped flux ratio of relativistic electrons is compared to ELFIN observations, and the wave power spectra is inferred based on the measured flux ratio. We demonstrate that even with a narrow wave spectrum, short EMIC wave packets can provide moderately intense precipitating fluxes well below the minimum resonance energy.

Energetic electron losses by pitch-angle scattering and precipitation to the atmosphere from the radiation belts are controlled, to a great extent, by resonant wave particle interactions with whistler-mode waves. The efficacy of such precipitation is primarily modulated by wave intensity, although its relative importance, compared to other wave and plasma parameters, remains unclear. Precipitation spectra from the low-altitude, polar-orbiting ELFIN mission have previously been demonstrated to be consistent with energetic precipitation modeling derived from empirical models of field-aligned wave power across a wide swath of local-time sectors. However, such modeling could not explain the intense, relativistic electron precipitation observed on the nightside. Therefore, this study aims to additionally consider the contributions of three modifications—wave obliquity, frequency spectrum, and local plasma density—to explain this discrepancy on the nightside. By incorporating these effects into both test particle simulations and quasi-linear diffusion modeling, we find that realistic implementations of each individual modification result in only slight changes to the electron precipitation spectrum. However, these modifications, when combined, enable more accurate modeling of ELFIN-observed spectra. In particular, a significant reduction in plasma density enables lower frequency waves, oblique, or even quasi field-aligned waves to resonate with near ∼1 MeV electrons closer to the equator. We demonstrate that the levels of modification required to accurately reproduce the nightside spectra of whistler-mode wave-driven relativistic electron precipitation match empirical expectations and should therefore be included in future radiation belt modeling.