JGR:Space physics

Rapid relativistic electron enhancements (REE) in the outer radiation belt have long been an intriguing phenomenon for space weather. In this study, we investigate rapid REE from October 2012 to December 2017 using multi-spacecraft observations. A total of 27 rapid REE events are identified from the Van Allen Probes (RBSP) measurements with a 5 times increase of MeV electrons at the center of the outer radiation belt (L = 4.5–5.5) in a half RBSP orbit (∼4.5 hr). All REE events are found to be in association with pulse-like injections of MeV electrons in the outer radiation belt. Electron fluxes in each injection at L ∼ 6.6 and the overall electron enhancements at L = 4.5–5.5 are quantified. The 500 keV and 0.8–1 MeV electron fluxes are correlated in injections and in overall enhancements. Substorm strength is more intense before/during the REE than intervals after the REE. The statistical study suggests that substorm-associated MeV electron injections are highly correlated with rapid REE in the outer radiation belt.

The in situ characterization of moon-magnetosphere interactions at Jupiter and the mapping of moon auroral footpaths require accurate global models of the magnetospheric magnetic field. In this study, we compare the ability of two widely-used current sheet models, Khurana-2005 (KK2005) and Connerney-2020 (CON2020) combined with the most recent internal magnetic field model of Jupiter (JRM33) to match representative Galileo and Juno measurements acquired at low, medium, and high latitudes. With the adjustments of the KK2005 model to JRM33, we show that in the outer and middle magnetosphere (R > 15R J), JRM33 + KK2005 is found to be the best model to reproduce the magnetic field observations of Galileo and Juno as it accounts for local time effects. JRM33 + CON2020 gives the most accurate representation of the inner magnetosphere. This finding is drawn from comparisons with Juno in situ magnetic field measurements and confirmed by contrasting the timing of the crossings of the Io, Europa, and Ganymede flux tubes identified in the Juno particles data with the two model estimates. JRM33 + CON2020 also maps more accurately the UV auroral footpath of Io, Europa, and Ganymede observed by Juno than JRM33 + KK2005. The JRM33 + KK2005 model predicts a local time asymmetry in position of the moons' footprints, which is however not detected in Juno's UV measurements. This could indicate that local time effects on the magnetic field are marginal at the orbital locations of Io, Europa, and Ganymede. Finally, the accuracy of the models and their predictions as a function of hemisphere, local time, and longitude is explored.

Critically important phenomena in Earth’s magnetosphere often occur briefly, or in small spatial regions. These processes are sampled with orbiting spacecraft or by fixed ground observatories and so rarely appear in data. Identifying such intervals can be an incredibly time consuming task. We apply a novel, powerful method by which two dimensional data can be automatically processed and embeddings created that contain key features of the data. The distance between embedding vectors serves as a measure of similarity. We apply the state-of-the-art method to two example datasets: MMS electron velocity distributions and auroral all sky images. We show that the technique creates embeddings that group together visually similar observations. When provided with novel example images the method correctly identifies similar intervals: when provided with an electron distribution sampled during an encounter with an electron diffusion region the method recovers similar distributions obtained during two other known diffusion region encounters. Similarly, when provided with an interesting auroral structure the method highlights the same structure observed from an adjacent location and at other close time intervals. The method promises to be a useful tool to expand interesting case studies to multiple events, without requiring manual data labeling. Further, the models could be fine-tuned with relatively small set of labeled example data to perform tasks such as classification. The embeddings can also be used as input to deep learning models, providing a key intermediary step—capturing the key features within the data.

Years of geomagnetic observatory data during the geomagnetic quiet days (Kp ≤ 3) at the low and middle dip latitudes from INTERMAGNET and SuperMAG were utilized. Seasonal models of Sq currents for two longitudinal chains at each year and under two solar activity levels (F 10.7 index are ∼75 and 125 sfu) were constructed by spherical harmonic analysis. It is found that there are significant seasonal variations of Sq currents in the Asian-Oceanian (A-O) and North-South American (N-SA) longitude sectors under different solar activities. First, the focus intensity J increases faster in local winter than in summer for both two hemispheres as solar activity strengthens. J of ionospheric currents in the northern hemisphere (NH) during local winter is higher than that in the southern hemisphere during its winter. This asymmetric activity in the N-SA chain increases with the solar activity becomes stronger. Second, with solar activity increases, the ionospheric current focus of N-SA chain in the NH shifts toward the lower latitudes during two solstices, while internal current focus of both two chains move toward the lower latitudes during the December solstice. Seasonal variations of Sq currents in the two chains exhibit longitudinal effects. With increasing solar activity, the two hemispheric focuses of the ionospheric currents in the A-O chain both move closer to the midday longitude sector, while in the N-SA chain, only the focus in the NH shifts toward noon. The results reveal details of the seasonal variations of Sq currents.

A working Direct Analytic Method (DAM) model is envisaged to explain the normal modes of poloidal Alfven waves in the Earth's magnetosphere. The model solves the ideal, cold, magnetohydrodynamic (MHD) equations associated with transverse components of the magnetic perturbations in a dipolar magnetic field. DAM model is used to study the transverse poloidal waves in different regions of magnetosphere characterized by their L-value and different plasma variability. The plasma density distribution is assumed to be governed by the standard power law, 1/r m , where r is the geocentric distance of any point of interest on the field line and m is the density index. The eigen frequencies and spatial structures are obtained analytically under different ideal ionospheric boundary conditions and the results are compared with the numerical solutions to establish the validity of the model. DAM, being an analytic model, is used to explain the distinctive structural features of transverse poloidal waves which are obtained under different boundary conditions, for different density indices. Furthermore, the application of the analytic model in the computation of eigen frequency as well as plasma density is demonstrated under different observational scenario.

Arc width is important for understanding the generation mechanism of auroral arcs. However, the continuity or discreteness of the distribution of small and meso-large scale auroral arc widths has not been determined in previous studies. This study employs machine learning techniques to investigate the distribution of arc widths across multiple scales using multi-field-of-view (multi-FOV) auroral observations. Based on the 180°, 47°, and 19° auroral observations at the Antarctic Zhongshan Station from February to October 2012, the statistical results demonstrate that the auroral arc width spectrum is continuously distributed across small, meso, and large scales, suggesting that the mechanisms responsible for their generation are capable of producing arcs at all scales. Furthermore, the arc width distribution at each FOV can be well fitted with a log-normal function. We also find that the main widths observed at different FOVs depend on the spatial resolution of the instruments. Our work provides new observational evidence for the generation mechanism of auroral arcs.

We report the influence of the atmospheric gravity waves on medium scale traveling ionospheric disturbances (MSTIDs) that are observed during the month of September 2020, using an airglow imager over Srinagar, Kashmir. Several cases of nighttime MSTIDs at ∼250 km altitude are presented which propagate either in northwestward, northward or northeastward direction. Either the phase fronts of the observed MSTIDs are not aligned in the NW-SE direction, or the MSTIDs are not propagating in the southwest direction, these are believed to be non-electrified MSTIDs which are generally associated with gravity waves (GWs). The average horizontal wavelengths of these MSTIDs range from 185 to 469 km, horizontal phase speeds of about 162–521 m/s while the time periods range from 13 to 24 min considered as very short-period ionospheric disturbances. The detection of GWs at ∼97 and ∼85 km heights during the nights of MSTID detection leads to the inference that there is a strong correlation between the occurrences of these MSTIDs with mesospheric GWs. By using satellite data, including INSAT-3DR and the Atmospheric Infrared Sounder, the detection of convective clouds near the locations of the imager is observed, and by utilizing the kinetic temperature data from the Sounding of the Atmosphere using Broadband Emission Radiometry satellite, the presence of GWs near the convective systems is also seen. Such GWs are also observed in the vicinity of the imager location and it is concluded that the lower atmospheric convectively-generated GWs could be a leading factor for the generation of poleward propagating MSTIDs.

We have modeled the individual harmonic frequencies of the Ionospheric Alfvén Resonator (IAR) at Eskdalemuir by solving a one-dimensional wave equation and using non-uniform modeled Alfvén velocity profiles. By comparing the results of the modeling alongside harmonics obtained from the Eskdalemuir, UK, data set from 2013 to 2021, the effects of the non-uniformity of the Alfvén velocity profile on the IAR are considered. We calculated the offset between the fundamental frequency and the harmonic frequency separation and found that this is not constant. From this parameter, we infer that the lower boundary condition of the electric field of the IAR is closest to a node, which agrees with previous studies. We compare the results of the non-uniform model with previous uniform models and evaluate their interpretations and the implications for the lower boundary condition.

Mesoscale density structures in the solar wind are often periodic, with f ∼ 0.1–5 mHz. They are trains of advected density structures with radial length scales of ∼100–10,000 Mm. While studies have shown that these periodic density structures (PDSs) are often formed at the Sun and released into the solar wind, it is unknown what percent of the solar wind at 1 AU is comprised of PDSs from the Sun, as opposed to periodicities formed through dynamics en route. We expand on Kepko et al. (2020, https://doi.org/10.1029/2020ja028037) which analyzed 25 years of in situ solar wind proton data, and include here alphas to examine the compositional characteristics of PDSs. Compositional changes, such as the alpha-to-proton ratio (α/p), are frozen into the solar wind plasma low in the corona, and so do not evolve as the solar wind advects and fills the Heliosphere. We find a broad occurrence enhancement in both the proton and α/p distributions between 1 and 3 mHz, centered near ∼2.1 mHz, and demonstrate that this distribution can be modeled assuming ∼30% of the solar wind segments contain a PDS. We find a distinct distribution below 1 mHz, with markedly different α/p characteristics. The α/p indicates that both populations are from the Sun, with likely different generation mechanisms. We conclude by summarizing mechanisms at the Sun that could produce periodic mass release, namely, periodic magnetic reconnection. The lower frequency PDSs likely involve reconnection at S-web arcs including the heliospheric current sheet, while the higher frequency PDSs may be driven by p-mode-related transverse coronal oscillations.

Recent simulations and in-situ observations have shown that magnetic reconnection is an active dissipation mechanism in the transition region of collisionless shocks. The generation mechanisms and upstream conditions enabling reconnection have been studied numerically. However, these numerical studies have been limited to the case of a steady, uniform upstream. The effect upstream discontinuities have on shock reconnection remains poorly understood. Here, we use local hybrid (fluid electron, particle ion) simulations with time-varying upstream conditions to study the influence upstream rotational discontinuities (RDs) have on the formation of reconnected magnetic structures in the shock transition region. Our results show that bursts of reconnection can occur when RDs interact with the shock. This effect is much more significant at initially quasi-parallel shocks than quasi-perpendicular shocks, as the interaction between the RDs and the foreshock (only present in the quasi-parallel case) can lead to the generation of foreshock bubbles, in which we observe an enhanced reconnection occurrence. The enhanced fluxes of accelerated ions within the foreshock bubble are likely a contributing factor to the increased reconnection occurrence. In addition, we find that the RDs with large magnetic shear are prone to reconnect upon reaching the shock, resulting in the generation of large magnetic islands. Our findings illustrate that upstream discontinuities can significantly increase the amount of reconnected magnetic structures at the bow shock, suggesting that reconnection might be a particularly important dissipation mechanism during periods of dynamic upstream conditions.

Development of new plasma instruments is needed to enable constellation- and small satellite-based missions. Key steps in the development pathway of ultra-compact plasma instruments employing lithographically patterned wafers are the implementation of layer-to-layer electrical interconnects and demonstration of massively parallel measurements, that is, simultaneous measurements through multiple identical plasma analyzer structures. Here we present energy resolved measurements of electron beams using a 5-layer stack of wafer-based, energy-per-charge, electrostatic analyzers. Each layer has eight distinct analyzer groups that are comprised of multiple micron scale energy-per-charge analyzers. The process of fabricating the electrical interconnects between the layers is described and the measured energy resolution and the angular resolution compared to theoretical predictions. The measurements demonstrate successful operation of 400 micron scale analyzers operating in parallel.

An LMN coordinate system for magnetic reconnection events is sometimes determined by defining N as the direction of the gradient across the current sheet and L as the direction of maximum variance of the magnetic field. The third direction, M, is often assumed to be the direction of zero gradient, and thus the orientation of the X line. But when there is a guide field, the X line direction may have a significant component in the L direction defined in this way. For a 2D description, a coordinate system describing such an event would preferably be defined using a different coordinate direction M′ oriented along the X line. Here we use a 3D particle-in-cell simulation to show that the X line is oriented approximately along the direction bisecting the asymptotic magnetic field directions on the two sides of the current sheet. We describe two possible ways to determine the orientation of the X line from spacecraft data, one using the minimum gradient direction from Minimum Directional Derivative analysis at distances of the order of the current sheet thickness from the X line, and another using the bisection direction based on the asymptotic magnetic fields outside the current sheet. We discuss conditions for validity of these estimates, and we illustrate these conditions using several Magnetospheric Multiscale (MMS) events. We also show that intersection of a flux rope due to secondary reconnection with the primary X line can destroy invariance along the X line and negate the validity of a two-dimensional description.

On 26 December 2022 the solar wind density dropped by over an order of magnitude and remained low for about a day. We have utilized in-situ plasma measurements made by the Mars Atmosphere and Volatile EvolutioN mission to determine how this change affected the Mars-solar wind interaction. During this time period, on inbound orbit segments, MAVEN sampled the terminator ionosphere, which switched from a magnetized to unmagnetized state immediately following the minimum in solar wind density. The magnetic field amplitude was typically 5–10 nT within the upper ionosphere prior to the event and consistently <1 nT after. During the event the magnetic pressure dominated immediately above the ionosphere while within the ionosphere the ionospheric plasma pressure dominated. The high altitude terminator ionosphere remained in this unmagnetized state throughout the event, suggesting that it was the new equilibrium state of the system. The terminator upper ionosphere returned to its original magnetized state once the solar wind density had recovered. The outbound orbit segments sampled the dayside subsolar region which remained magnetized throughout the event: the magnetization state of the ionosphere varied locally, dependent upon the solar zenith angle and corresponding incident solar wind dynamic pressure. Such conditions are different to the commonly reported unmagnetized ionospheric state at Venus during solar maximum conditions, where the interplanetary magnetic field is repelled from the entire dayside ionosphere. Drastic changes in the upstream solar wind are able to change the Mars-solar wind interaction state on timescales less than one MAVEN orbit (∼3.5 hr).

Using observations from multi-satellites at different altitudes, including the CHAllenging Minisatellite Payload (CHAMP), the Gravity Recovery and Climate Experiment, Swarm B, and Defense Meteorological Satellites Program F17, the ionospheric post-midnight enhancement at mid-latitudes and the associated inter-hemispheric asymmetry during equinox are investigated in this study. During equinox months, the ionospheric electron density enhancement during post-nighttime at mid-latitudes is visible in both hemispheres, however, it is asymmetric between the Northern and Southern Hemispheres. At most longitudes, inter-hemispheric asymmetry of Mid-latitude Ionospheric Post-midnight Enhancement (MIPE) reverses with altitudes, from a stronger electron density in the Northern Hemisphere at CHAMP altitude to a stronger electron density in the Southern Hemisphere at top ionosphere during equinox. The reversal altitude and reversal time have significant longitudinal differences. The effective ionospheric uplifting induced by the combination of neutral winds and geomagnetic field configuration is the main contribution to the asymmetry reversal of MIPE at lower altitudes, as shown in the simulations from the SAMI2 and HWM14 models. In comparison with that in the Southern Hemisphere, the stronger neutral winds in the Northern Hemisphere move the plasma along the geomagnetic field lines to a higher altitude with lower chemical recombination, resulting in the enhancement of electron density.

During the first flyby of the BepiColombo composite spacecraft at Mercury in October 2021 ion spectrometers observed two intense spectral lines with energies between 10 and 70 eV. The spectral lines persisted also at larger distances from Mercury and were observed again at lower intensity during cruise phase in March 2022 and at the second and third Mercury flyby as a single band. The ion composition indicates that water is the dominant gas source. The outgassing causes the composite spacecraft to charge up to a negative potential of up to −50 V. The distribution and intensity of the lower energy signal depends on the intensity of low energy electron fluxes around the spacecraft which again depend on the magnetic field orientation. We interpret the observation as being caused by water outgassing from different source locations on the spacecraft being ionized in two different regions of the surrounding potential. The interpretation is confirmed by two dimensional particle-in-cell simulations.

To study the emission of energetic neutral atoms (ENAs) at Titan, we have developed a novel model that takes into account a spacecraft detector's limited field of view and traces energetic magnetospheric particles backward in time. ENAs are generated by charge exchange between Titan's atmospheric neutrals and energetic magnetospheric ions. By tracing these ions through the draped electromagnetic fields in Titan's environment, we generate synthetic ENA images and compare them to Cassini observations from the TA flyby. Our model can reproduce the intensity and morphology of the observed images only when field line draping is included. Using a realistic detector geometry is necessary to determine the influence of this draping on the ENA images: the non-uniform fields eliminate a localized feature of increased ENA flux, which is a different effect than in models utilizing an infinitely extended detector. We demonstrate that ENA observations from TA contain signatures of the time-varying Saturnian magnetospheric environment at Titan: the modeled ENA emission morphology and the effect of field line draping are different for the background field vectors measured during the inbound and outbound legs of TA. The visibility and qualitative effect of the draping on observed ENA images vary strongly between different detector locations and pointings. Depending on the viewing geometry, field line draping may add segments of elevated flux to the synthetic ENA images, remove such segments, or have no qualitative effect at all. Our study emphasizes the challenges and the potential for remote sensing of Titan's interaction region using ENA imaging.

Energetic electron precipitation (EEP) associated with pulsating aurora can transfer greater than 30 keV electrons from the outer radiation belt region into the upper atmosphere and can deplete atmospheric ozone via collisions that produce NOx and HOx molecules. Our knowledge of exactly how EEP occurs is incomplete. Previous studies have shown that pitch angle scattering between electrons and lower-band chorus waves can cause pulsating aurora associated with EEP and that substorms play an important role. In this work, we quantify the timescale of chorus wave decay following substorms and compare that to previously determined timescales. We find that the chorus decay e-folding time varies based on magnetic local time (MLT), magnetic latitude, and wave frequency. The shortest timescales occur for lower-band chorus in the 21 to 9 MLT region and compares, within uncertainty, to the energetic pulsating aurora timescale of Troyer et al. (2022, https://doi.org/10.3389/fspas.2022.1032552) for energetic pulsating aurora. We are able to further support this connection by modeling our findings in a quasi-linear diffusion simulation. These results provide observations of how chorus waves behave after substorms and add additional statistical evidence linking energetic pulsating aurora to substorm driven lower-band chorus waves.

We present a unified approach to subauroral arcs within intense subauroral ion drifts (SAID), which explains the observed transition of a precursor Stable Auroral Red (SAR) arc into Strong Thermal Emission Velocity Enhancement (STEVE). This approach is based on the short-circuiting concept of fasttime SAID as an integral part of a magnetospheric voltage generator between the innermost boundaries of the freshly injected plasma sheet electrons and ring current ions. Here, enhanced plasma turbulence rapidly heats the bulk plasma and accelerates suprathermal non-Maxwellian “tails.” Heat and suprathermal electron transport rapidly elevate the ionospheric electron temperature—the source of a bright SAR arc. Through a substorm, the density altitude profile within the evolving ionospheric SAID channel transforms into a “fresh” F-region trough with the E-region valley. The ionospheric feedback instability within the depleted-density SAID channel generates small-scale, field-aligned currents with parallel electric fields sufficient to produce the suprathermal electron population, exciting the STEVE and Picket Fence emissions. This approach also explains the inner electromagnetic structure of intense SAID, which is consistent with fine optical structures in STEVE and Picket Fence.

Resonant interactions between relativistic electrons and electromagnetic ion cyclotron (EMIC) waves provide an effective loss mechanism for this important electron population in the outer radiation belt. The diffusive regime of electron scattering and loss has been well incorporated into radiation belt models within the framework of the quasi-linear diffusion theory, whereas the nonlinear regime has been mostly studied with test particle simulations. There is also a less investigated, nonresonant regime of electron scattering by EMIC waves. All three regimes should be present, depending on the EMIC waves and ambient plasma properties, but the occurrence rates of these regimes have not been previously quantified. This study provides a statistical investigation of the most important EMIC wave-packet characteristics for the diffusive, nonlinear, and nonresonant regimes of electron scattering. We utilize 3 years of observations to derive distributions of wave amplitudes, wave-packet sizes, and rates of frequency variations within individual wave-packets. We demonstrate that EMIC waves typically propagate as wave-packets with ∼10 wave periods each, and that ∼3–10% of such wave-packets can reach the regime of nonlinear resonant interaction with 2–6 MeV electrons. We show that EMIC frequency variations within wave-packets reach 50–100% of the center frequency, corresponding to a significant high-frequency tail in their wave power spectrum. We explore the consequences of these wave-packet characteristics for high and low energy electron precipitation by H-band EMIC waves and for the relative importance of quasi-linear and nonlinear regimes of wave-particle interactions.

We examine a Dipolarization Front (DF) event with an embedded electron diffusion region (EDR), observed by the Magnetospheric Multiscale (MMS) spacecraft on 08 September 2018 at 14:51:30 UT in the Earth's magnetotail by applying multi-scale multipoint analysis methods. In order to study the large-scale context of this DF, we use conjunction observations of the Cluster spacecraft together with MMS. A polynomial magnetic field reconstruction technique is applied to MMS data to characterize the embedded electron current sheet including its velocity and the X-line exhaust opening angle. Our results show that the MMS and Cluster spacecraft were located in two counter-rotating vortex flows, and such flows may distort a flux tube in a way that the local magnetic shear angle is increased and localized magnetic reconnection may be triggered. Using multi-point data from MMS we further show that the local normalized reconnection rate is in the range of R ∼ 0.16 to 0.18. We find a highly asymmetric electron in- and outflow structure, consistent with previous simulations on strong guide-field reconnection events. This study shows that magnetic reconnection may not only take place at large-scale stable magnetopause or magnetotail current sheets but also in transient localized current sheets, produced as a consequence of the interaction between the fast Earthward flows and the Earth's dipole field.