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The present work focuses on the variability of the magnetospheric convection electric field (MCEF) during geomagnetic storms generated by coronal mass ejections (CMEs). The aim of the study was to analyze the response of the MCEF to geoeffective CMEs occurring during the maximum phase of Solar Cycle 24. A total of eight storms were selected on the basis of their intensity with respect to the Dst (disturbance storm time) and Kp (planetary geomagnetic index) indices. We examined the variations in solar wind parameters (V SW (solar wind speed), P SW (plasma pressure), and interplanetary magnetic field (IMF) Bz) and geomagnetic indices (Sym‐H (symmetric horizontal) and auroral electrojet (AE)) during these storms. We analyzed the time lag between the MCEF and AE, Sym‐H, and IMF Bz using a cross‐correlation analysis. The study shows that CMEs arriving in the magnetosphere at a speed of 800–960 km/s induce a change in MCEF of 0.95–1.29 mV/m in the initial phase, and those transiting at a speed of 520–651 km/s induce a change in MCEF of 0.78–0.91 mV/m. During storms associated with CMEs arriving at a speed of 520–651 km/s, a change in IMF Bz of 1 nT is associated with a change in MCEF of about 0.05 mV/m. During those associated with CMEs transiting at a speed of 800–960 km/s, each 1 nT change in IMF Bz corresponds to a change in MCEF of about 0.07 mV/m. Variations in the MCEF follow those in the AE index in 88% of the storms studied, with a time lag ranging from 9 to 79 min. They precede those of the ring current index (Sym‐H) in 50% of the cases analyzed, with a time lag varying from 49 to 742 min. These results provide us with some information on the complete evolution of geomagnetic storms, with a view to better prediction.

This study investigated the impact of the June 2015 geomagnetic storms on the Brazilian equatorial and low-latitude ionosphere by analyzing various data sources, including solar wind parameters from the advanced compositional explorer satellite (ACE), global positioning satellite vertical total electron content (GPS-VTEC), geomagnetic data, and validation of the SAMI2 model-VTEC with GPS-VTEC. The effect of geomagnetic disturbances on the Brazilian longitudinal sector was examined by applying multiresolution analysis (MRA) of the maximum overlap discrete wavelet transform (MODWT) to isolate the diurnal component of the disturbance dynamo (Ddyn), DP2 current fluctuations from the ionospheric electric current disturbance (Diono), and semblance cross-correlation wavelet analysis for local phase comparison between the Sq and Diono currents. Our findings revealed that the significant fluctuations in DP2 at the Brazilian equatorial stations (Belem, dip lat: −0.47° and Alta Floresta, dip lat: −3.75°) were influenced by IMF Bz oscillations; the equatorial electrojet also fluctuated in tandem with the DP2 currents, and dayside reconnection generated the field-aligned current that drove the DP2 current system. The short-lived positive ionospheric storm during the main phase on 22 June in the Southern Hemisphere in the Brazilian sector was caused by the interplay between the eastward prompt penetration of the magnetospheric convection electric field and the westward disturbance dynamo electric field. The negative ionospheric storms that occurred during the recovery phase from 23 to 29 June 2015, were attributed to the westward disturbance dynamo electric field, which caused the downward E × B drift of the plasma to a lower height with a high recombination rate. The comparison between the SAMI2 model-VTEC and GPS-VTEC indicates that the SAMI2 model underestimated the VTEC within magnetic latitudes of −9° to −24° in the Brazilian longitudinal sector from 6 to 17 June 2015. However, it demonstrated satisfactory agreement with the GPS-VTEC within magnetic latitudes of −9° to 10° from 8 to 15 June 2015. Conversely, the SAMI2 model overestimated the VTEC between ±10° magnetic latitudes from 16 to 28 June 2015. The most substantial root mean square error (RMSE) values, notably 10.30 and 5.48 TECU, were recorded on 22 and 23 June 2015, coinciding with periods of intense geomagnetic disturbance.

We present the first global geospace simulation to reproduce auroral giant undulations (GUs). To identify their magnetospheric drivers, we employ the MAGE (Multiscale Atmosphere‐Geospace Environment) model in a case study of a geomagnetic storm for which there were spacecraft‐ and ground‐based observations of GUs. The model reproduces the spatial and temporal scales of the GUs as well as the presence of duskside subauroral polarization streams (SAPS) and plasmapause undulations. Based on our modeling, we are able to identify the magnetospheric drivers of GUs as mesoscale ring current injections which, after drifting westward, create inverted regions of flux‐tube entropy (FTE) and subsequent interchange instability. Outward‐protruding interchange fingers disrupt shielding of the inner magnetosphere, creating longitudinally localized ripples in magnetospheric convection equatorward of the magnetospheric instability, which structure the plasmapause and duskside diffuse precipitation. While not causal, SAPS and plasmapause undulations are a consequence of the unstable magnetospheric configuration. Plain Language Summary The visually dazzling display of the aurora during active periods is caused primarily by the precipitation of energetic electrons from the magnetosphere into the ionosphere. The auroral oval plays host to a variety of morphological features, or auroral forms, that are a reflection of magnetospheric processes and therefore a powerful tool for understanding the cross‐scale processes that bind together different geospace domains. Unlocking that power, however, requires an understanding of how magnetospheric processes are reflected in the aurora. Despite decades of study, that understanding has remained elusive, primarily due to limited in situ measurements and uncertainty in the magnetic mapping connecting them to the ionosphere. Only recently have new global geospace models emerged that can provide this understanding. In this letter we identify the magnetospheric driver of auroral giant undulations (GUs), wave‐like trains of undulations that form on the equatorward edge of the diffuse aurora with typical spatial scales of 100 km. We show that GUs are the consequence of a “buoyancy imbalance” formed during the buildup of the ring current and the subsequent disruption of the ionospheric current systems that typically shield the inner magnetosphere. Key Points We present the first global geospace simulation to reproduce auroral giant undulations (GUs) Model shows GUs result from localized under‐shielding as a consequence of interchange instability during the buildup of the ring current Interchange‐unstable regions drive ripples in magnetospheric convection, structuring the plasmapause and duskside diffuse precipitation

Injection flux tubes, characterized by localized equatorial magnetic field enhancements and concomitant hot plasma populations, contribute to Saturn's magnetospheric convection cycle by transporting magnetic flux radially inward. The sharp magnetic gradients at the flux‐tube edges have been demonstrated to enable the trapping of equatorially mirroring particles, leading to their energy‐dispersionless signatures in spacecraft observations. Here, we present a statistical distinction between flux tubes with and without particle‐trapping features in the electron cyclotron harmonic (ECH) wave properties. The particle‐trapping flux tubes carry stronger ECH waves in the high‐harmonic bands, whereas the other category is usually accompanied only by fundamental‐mode waves. This distinction is largely attributed to the higher content of energetic electrons within the particle‐trapping flux tubes. These results improve our understanding of the association between injection flux tubes and the high‐band ECH waves therein, suggesting a unique role of particle‐trapping flux tubes in Saturnian magnetospheric dynamics. Plain Language Summary In Saturn's magnetosphere, the plasma convection involves the inward motion of localized injection flux tubes characterized by enhanced magnetic field strength and energetic particle fluxes. The flux enhancements of particles at different energies are usually asynchronous because of their energy‐dependent drift speeds around Saturn. In certain events, however, this picture could be modified by sharp magnetic gradients at flux‐tube edges, which lead to the trapping of near‐equatorial particles manifested by their simultaneous flux enhancements over a wide energy range. Here, we analyze the Cassini observations of injection flux tubes, categorize them into those with and without particle‐trapping features, and examine their associated electrostatic wave properties. The particle‐trapping flux tubes are statistically associated with waves at discrete frequencies between harmonics of the electron gyrofrequency, known as electron cyclotron harmonic waves. These emissions occur in both the fundamental‐mode and high‐harmonic bands. In contrast, non‐trapping flux tubes are predominantly accompanied by fundamental‐mode waves only. The different wave properties are attributed to the higher content of energetic electrons in particle‐trapping flux tubes. These findings highlight the role of injection flux tubes in Saturn's magnetospheric dynamics. Key Points Particle‐trapping flux tubes with energy‐dispersionless features of equatorially mirroring particles carry intense electron cyclotron harmonic (ECH) waves in high bands Particle‐trapping flux tubes correspond to stronger magnetic field enhancements and higher content of energetic electrons than other events Parametric analysis demonstrates the important role of energetic electrons above 1 keV in the excitation of ECH waves in high harmonic bands

The electron rolling pin distribution, showing electron pitch angles primarily at 0°, 90°, and 180°, has been recently observed behind dipolarization fronts (DFs) in the magnetosphere of Earth. However, the relation between such distribution and the leading edge of tailward magnetic reconnection jets, also known as anti‐dipolarization fronts (ADFs), is still unclear. Here, by utilizing high‐resolution data of the Magnetospheric Multiscale (MMS) mission, we provide the first observation of electron rolling pin distribution behind ADF. Such distribution of Maxwellian electrons appears in 1.3–5 keV and is modulated by firehose fluctuations: electron fluxes are high at wave troughs (|B|‐minima) and are low at wave crests. The results of Liouville mapping and the electron loss cone angles are consistent with the spacecraft observations respectively, indicating such distribution is formed by the combination of global‐scale Fermi acceleration and local‐scale electron trapping. These findings highlight the importance of ADFs in magnetospheric convection.

Mercury is surrounded by a tenuous neutral exosphere composed primarily of sodium atoms, which can be continuously ionized. The production of sodium ions is concentrated on the dayside, and these ions can subsequently be transported to the magnetotail and flanks. MESSENGER spacecraft observations revealed dawn‐dusk asymmetric distributions of sodium ions Na+$N{a}^{+}$ . In this study, we investigate the Na+$N{a}^{+}$circulation, distribution, and its influence on global magnetospheric convection with a two‐fluid MHD model, which is coupled with an empirical sodium exosphere profile as the source of Na+$N{a}^{+}$ . In particular, we aim to investigate if the dawn‐dusk asymmetries in Na+$N{a}^{+}$distributions near the equator can be driven by internal mechanisms within the magnetosphere. Our findings indicate that (a) the observed dawn‐dusk asymmetric Na+$N{a}^{+}$distributions can be driven by the separation of H+${H}^{+}$and Na+$N{a}^{+}$flows, and (b) the Hall‐driven global convection preferentially transporting Na+$N{a}^{+}$ions to the morning sector. Plain Language Summary Mercury's weak magnetic field and proximity to the Sun make its magnetosphere much smaller and more dynamic than Earth's. With no substantial ionosphere, the magnetosphere is dominated by solar wind protons rather than planetary ions. However, Mercury has a tenuous sodium exosphere, and these neutral sodium atoms can become ionized on the dayside. This study uses a two‐fluid MHD model that couples magnetospheric plasma flows with Mercury's neutral sodium exosphere to study the transport and distribution of the sodium ions. It shows the Hall effect and the velocity separation between the proton fluid and sodium fluid can produce dawn‐dusk asymmetric distributions of sodium ions. Key Points Developed a multi‐fluid MHD model to study the dawn‐dusk asymmetric distributions of sodium ions in Mercury's magnetosphere Velocity separation between proton and sodium ion fluids can drive sodium ion dawn‐dusk asymmetric distributions Hall effects can transport more sodium ions to the morning sector

Among nearly 300 near‐Mercury tail current sheet crossings performed by the MESSENGER spacecraft, we identified 37 traversals of an asymmetric current sheet, wherein the lobe densities on opposite sides differ by a factor of three or more. These asymmetric current sheet crossings primarily occur on the dawnside. A global magnetohydrodynamic (MHD) simulation was found to be in excellent agreement with the observations. The results suggest that the north–south density asymmetry is caused by solar wind entering via an upstream‐connected window in one hemisphere. Furthermore, the Parker spiral interplanetary magnetic field (IMF) controls the near‐tail density asymmetries, whereas Mercury's offset dipole magnetic field controls those in mid‐ or distant‐tail regions. We propose that hemispheric asymmetries in Mercury's magnetospheric convection occur under strong IMF conditions. Plain Language Summary Mercury possesses a small magnetosphere owing to its weak planetary magnetic field and strong interactions with the solar wind in the inner heliosphere. The transport process of the solar wind mass and energy into its magnetosphere remains unclear. Previous MESSENGER observations suggest that although the Earth‐like plasma mantle is detected inside the near‐tail magnetopause in normal IMF magnitudes, it is not a permanent feature of Mercury's magnetosphere. Here we report, for the first time, that solar wind ions can enter deep into the near‐tail region via an upstream‐connected window in one hemisphere and form a density‐asymmetric current sheet under strong IMF conditions. Through MHD simulations, we revealed tail dawn–dusk asymmetries during the transport of solar wind plasma. Advanced data expected from BepiColombo will further improve our understanding of the solar wind–magnetosphere coupling. Key Points North–south asymmetries exist in Mercury's magnetotail Both the interplanetary magnetic field (IMF) BX polarity and intrinsic offset dipole magnetic field control the north–south density asymmetry in Mercury's tail The IMF Parker spiral results in a dawnside preference for the plasma asymmetric current sheet

The cold ions, which are generally “invisible” to most instruments, have strong impacts on plasma wave and magnetic reconnection. Under particular situations, these cold ions could be accelerated and thus become detectable. In this study, we statistically investigated the properties of background cold ions based on Van Allen Probe observations. The cold ions could often be detected near the dusk sector, and a clear dawn‐dusk asymmetry is observed for all ion species with higher density at the dusk side, showing plasmaspheric plume‐like structures. Similar to the cold electrons, cold proton ions show a clear boundary of plasmapause with its location moving toward the Earth as geomagnetic activity increases. Furthermore, the percentage of oxygen increases, and the percentage of protons decreases as geomagnetic activity increases whereas the helium composition is generally small. Our results provide important information on ion compositions for the understanding of cold‐plasma dynamics in the inner magnetosphere. Plain Language Summary The cold ions play an important role in magnetospheric dynamics since they are the source of thermal plasma and they could affect the magnetic reconnection and wave generation. However, the main population of cold ions is difficult to measure due to their low energy and spacecraft charging. Magnetospheric convection and/or induced electric field could increase the energy of cold ions sufficiently above the spacecraft potential so that these ions can be detected by particle instrument. In this study, we investigate the properties of background cold ions when the total ion density is comparable to the background electron density. We found the cold ion could often be measured near the dusk sector and a clear dawn‐dusk asymmetry is observed for all ion species. Similar to the cold electrons, cold protons also show a clear boundary of plasmapause with its location moving toward the Earth as geomagnetic activity increases. Furthermore, the percentage of oxygen ions increases, and the percentage of protons decreases as geomagnetic activity increases whereas the percentage of helium ions is generally small. Our results provide important information on cold ion density for the study of wave‐particle interactions and magnetic reconnection in the Earth's magnetosphere. Key Points We statistically analyzed the cold ion densities and compositions based on Van Allen Probe observations The density above L = 3 decreases as geomagnetic activity increases for all three ion species, suggesting the shrinking of plasmasphere The percentage of cold oxygen ions increases as geomagnetic activity increases

We compare Hubble Space Telescope observations of Jupiter's FUV auroras with contemporaneous conjugate Juno in situ observations in the equatorial middle magnetosphere of Jupiter. We show that bright patches on and equatorward of the main emission are associated with hot plasma injections driven by ongoing active magnetospheric convection. During the interval that Juno crossed the magnetic field lines threading the complex of auroral patches, a series of energetic particle injection signatures were observed, and immediately prior, the plasma data exhibited flux tube interchange events indicating ongoing convection. This presents the first direct evidence that auroral morphology previously termed “strong injections” is indeed a manifestation of magnetospheric injections, and that this morphology indicates that Jupiter's magnetosphere is undergoing an interval of active iogenic plasma outflow. Plain Language Summary Auroras, known as the “Northern (or Southern) Lights” on Earth, are spectacular manifestations of energetic processes occurring in the space environment of a planet. The behavior of Jupiter's magnetosphere is dominated by the planet's rapid rotation, along with the centrifugally‐driven outflow of plasma (ionized gas) originating from active volcanoes on the moon Io. A prominent auroral feature on Jupiter has for many years been interpreted as a sign that Jupiter's magnetosphere is undergoing active convection, in which plasma from Io “falls” away from the planet, to be replaced by hot, relatively empty “bubbles” known as injections, moving inward. This feature comprises prominent patches of bright emission that are often observed in Jupiter's auroras, though the evidence associating them with injections has been largely circumstantial. Here we show that the NASA Juno spacecraft flew through such injections in the equatorial magnetosphere on magnetic field lines mapping to a cluster of auroral patches as observed by HST. The Juno data also indicated the interval was characterized by signatures of convection and outflow of plasma originating from Io. This demonstrates that auroral patches are signatures of injections, and that auroral emissions are an important tool for diagnosing the behavior of planetary magnetospheres. Key Points Bright FUV auroral patches on Jupiter are associated with magnetospheric injections and magnetospheric convection Hubble Space Telescope and Juno equatorial data show a cluster of patches is magnetically conjugate with energetic particle injections The interval also exhibits flux tube interchange and lagging magnetic field associated with plasma mass outflow

Magnetohydrodynamic simulations of the Alfvénic magnetosphere‐ionosphere (M‐I) coupling reveal a comprehensive picture of nonlinear development of the feedback instability, demonstrating spontaneous growth of auroral structures of density and current enhancements, vortex formation, turbulence transition, and energy cascade. Turbulent energy spectra consistent with the −5/3 ${-}5/3$‐law and the critical balance are self‐organized in the feedback M‐I coupling with ionospheric density change. Simultaneously, field‐aligned current and vorticity fluctuations in the auroral ionosphere show different power laws in consistent with observations. The growth of feedback instability and the turbulent energy cascade transfer the kinetic and magnetic energy from a large magnetospheric convection scale to a small dissipation scale.