Local Time Asymmetry Research Articles

Local Time and Hemispheric Asymmetries of Field‐Aligned Currents and Polar Electrojet During May 2024 Superstorm Periods

AbstractThis study examines field‐aligned currents (FACs) and polar electrojet (PEJ) characteristics during the extreme May 2024 geomagnetic storms across dawn, dusk, daytime, and nighttime in both hemispheres. FACs and PEJ intensities were up to 9 times greater than usual, with equatorward FACs reaching −44° Magnetic Latitude. Maximum FACs and PEJ are larger at dawn than dusk in the Northern Hemisphere but larger at dusk than at dawn in the Southern Hemisphere. Dawn and duskside FACs correlate best with Dst or solar wind dynamic pressure (Pd) in both hemispheres. On the dayside (nightside), most FACs in both hemispheres are primarily correlated with Pd (merging electric field, Em or Pd). The PEJs correlate largely with Dst and partly with Em and Pd. Duskside (nighttime) currents are located at lower latitudes than dawnside (daytime), and northern currents are positioned more poleward than southern currents. The latitudes of peak FACs are most strongly correlated with Dst or Pd in both hemispheres. However, in the northern daytime sector, they are primarily influenced by Em. The latitudes of peak PEJ show the strongest correlation with Dst or Pd in both hemispheres, except on the northern dawnside, where they are primarily influenced by Em. The qualitative relationships between peak current density, corresponding latitude, solar wind parameters, and the Dst index are derived.

Study on the local time asymmetry of the Earth’s ring currents based on multiple low-Earth-orbit satellite magnetic observations

Study on the local time asymmetry of the Earth’s ring currents based on multiple low-Earth-orbit satellite magnetic observations

The Global Characteristics of Electrons Near the Inner Edge of the Inner Radiation Belt

AbstractEnergetic electrons around the inner edge of the inner radiation belt (L1.3) have not been studied to the extent of the rest of the inner belt and certainly not to that of the outer radiation belt. Yet, the behavior of electrons at the inner edge of the inner belt could improve our understanding of important dynamics deep within Earth's magnetosphere. Previous work has analyzed these electrons during either isolated events or via significantly limited statistical studies, and it remains unclear what the particular drivers of electron flux enhancements in this region may be. Here, we present a broad statistical study of electrons around the inner edge of the inner radiation belt, utilizing the 7‐year data set of the MagEIS particle detectors aboard both Van Allen Probes spacecraft. Moreover, we separate electron flux measurements into stably trapped and quasi‐trapped electrons and directly compare their behavior. We find both populations to be variable in L and energy, as well as showing a clear magnetic local time (MLT) asymmetry. Potential drivers are also explored, where we demonstrate an association between increased electron fluxes in this region and heightened solar wind speed, Sym‐H, dynamic pressure, southward IMF and most prominently with the AU and AL indices.

Modeling the Dynamic Global Distribution of the Ring Current Oxygen Ions Using Artificial Neural Network Technique

AbstractThe ring current is an important component of the Earth's near‐space environment, as its variations are the direct driver of geomagnetic storms that can disrupt power grids, satellite communications, and navigation systems, thereby impacting a wide range of technological and human activities. Oxygen ions (O+) are one of the major components of the ring current and play a significant role in both the enhancement and depletion of the ring current during geomagnetic storms. Although a standard statistical study can provide average global distributions of ring current ions, it can't offer insight into the short‐term dynamic variations of the global distribution. Therefore, we employed the Artificial Neural Network technique to construct a global ring current O+ ion model based on the Van Allen Probes observations. Through optimization of the combination of input geomagnetic indices and their respective time history lengths, the model can well reproduce the spatiotemporal variation of the oxygen ion flux distributions and demonstrates remarkable accuracy and minimal errors. Additionally, the model effectively reconstructs the temporal variation of ring current O+ ions for non‐training set data. Furthermore, the model provides a comprehensive and dynamic representation of global ring current O+ ion distribution. It accurately captures the dynamics of O+ ions during a geomagnetic storm with the oxygen ion fluxes enhancement and decay, and reveals distinct characteristics for different energy levels, such as injection from the plasma sheet, outflow from the ionosphere, and magnetic local time asymmetry.

Dayside Pc2 Waves Associated With Flux Transfer Events in a 3D Hybrid‐Vlasov Simulation

AbstractFlux transfer events (FTEs) are transient magnetic flux ropes at Earth's dayside magnetopause formed due to magnetic reconnection. As they move across the magnetopause surface, they can generate disturbances in the ultralow frequency (ULF) range, which then propagate into the magnetosphere. This study provides evidence of ULF waves in the Pc2 wave frequency range (>0.1 Hz) caused by FTEs during dayside reconnection using a global 3D hybrid‐Vlasov simulation (Vlasiator). These waves resulted from FTE formation and propagation at the magnetopause are particularly associated with large, rapidly moving FTEs. The wave power is stronger in the morning than afternoon, showing local time asymmetry. In the pre and postnoon equatorial regions, significant poloidal and toroidal components are present alongside the compressional component. The noon sector, with fewer FTEs, has lower wave power and limited magnetospheric propagation.

Water Vapor Variability in the Thermosphere of Mars During Mars Years 32–36

AbstractUsing Mars Atmosphere and Volatile Evolution observations, we characterize the variability of water vapor in the Martian thermosphere during Mars Years 32–36. Near a fixed atmospheric pressure level of ∼5 × 10−7 Pa, the typical water density is 1.7 (±1.0) × 103 cm−3 and the typical water mixing ratio is 14.6 (±9.0) ppm. Thermospheric water levels are higher during the southern spring and summer seasons when Mars is near perihelion and there is significant dust loading in the lower atmosphere. However, the seasonal variation is not the same from year‐to‐year, likely due to annual differences in dust loading. Water vapor abundance is highly correlated with the amount of dust in the lower atmosphere, and increases during both regional and global dust storms. Additionally, there is a local time asymmetry with higher mixing ratios observed during morning local times, potentially caused by atmospheric circulation. Our results support previous work that found increased dust levels allow more water to be supplied directly to the thermosphere.

Influence of the Jovian Current Sheet Models on the Mapping of the UV Auroral Footprints of Io, Europa, and Ganymede

AbstractThe 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 > 15RJ), 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.

A Model of Hourly Variations of the Near‐Earth Magnetic Field Generated in the Inner Magnetosphere and Its Induced Counterpart

AbstractWe present a new model of the near‐Earth magnetospheric field produced by electric currents in the inner magnetosphere and the associated induced magnetic field. The model is designed to track hourly variations of these fields and accounts for their local time asymmetries. It is built by applying spherical harmonic analysis to vector measurements from the ground observatory network at low and mid‐latitudes. The primary and induced fields are separated with an approach in the time domain that uses a a priori radially‐symmetric electric conductivity model of the Earth. The model coefficients are computed at one‐hour time steps between 1997 and 2022. This model is shown to be consistent to within a few nT with previously developed indices which track the magnetospheric ring current. It is also validated against data from the Swarm, CHAMP and Øersted satellites. The fit to satellite data is comparable to that of the CHAOS‐7.15 model for geomagnetically quiet times, and improved by up to 20% on some components for geomagnetically moderate and active times. We attribute these differences mostly to a better representation of local time asymmetries, both on average and during individual geomagnetic storms. This model can be used in various applications, such as investigating the properties of the magnetospheric field and its sources and separating the magnetospheric field from the fields of other sources in geomagnetic field modeling.

Variability of the Auroral Footprint of Io Detected by Juno‐JIRAM and Modeling of the Io Plasma Torus

AbstractOne of the auroral features of Jupiter is the emission associated with the orbital motion of its moon Io. The relative velocity between Io and the surrounding plasma trigger perturbations that travels as Alfvén waves along the magnetic field lines toward the Jovian ionosphere. These waves can accelerate electrons into the atmosphere and ultimately produce an auroral emission, called the Io footprint. The speed of the Alfvén waves—and hence the position of the footprint—depends on the magnetic field and on the plasma distribution along the field line passing through Io, whose SO2‐rich atmosphere is the source of a dense plasma torus around Jupiter. Since 2016, the Jovian InfraRed Auroral Mapper (JIRAM) onboard Juno has been observing the Io footprint with a spatial resolution of ∼few tens of km/pixel. JIRAM detected evidences of variability in the Io footprint position that are not dependent on the System III longitude of Io. The position of the Io footprint in the JIRAM images is compared with the position predicted by a model of the Io Plasma Torus and of the magnetic field. This is the first attempt to retrieve quantitative information on the variability of the torus by looking at the Io footprint. The results are consistent with previous observations of the density and temperature of the Io Plasma Torus. However, we found that the plasma density and temperature exhibit considerable non‐System III variability that can be due either to local time asymmetry of the torus or to its temporal variability.

Modeling Ring Current Proton Fluxes Using Artificial Neural Network and Van Allen Probe Measurements

AbstractTerrestrial ring current dynamics are a critical part of the near‐space environment, in that they directly drive geomagnetic field variations that control particle drifts, and define geomagnetic storms. The present study aims to specify a global and time‐varying distribution of ring current proton using geomagnetic indices and solar wind parameters with their history as input. We train an artificial neural network (ANN) model to reproduce proton fluxes measured by the Radiation Belt Storm Probes Ion Composition Experiment instrument onboard Van Allen Probes. By choosing optimal feature parameters and their history length, the model results show a high correlation and a small error between model specifications and satellite measurements. The modeled results well capture energy‐dependent proton dynamics in association with geomagnetic storms, including inward radial diffusion, acceleration and decay. Our ANN model produces proton fluxes with their corresponding 3D spatiotemporal variations, capturing the latitudinal distribution and local time asymmetry that are consistent with observations and that can further inform theory.

Variability of Jupiter's Main Auroral Emission and Satellite Footprints Observed With HST During the Galileo Era

AbstractHubble Space Telescope images of Jupiter's UV aurora show that the main emission occasionally contracts or expands, shifting toward or away from the magnetic pole by several degrees in response to changes in the solar wind dynamic pressure and Io's volcanic activity. When the auroral footprints of the Galilean satellites move with the main emission this indicates a change in the stretched field line configuration that shifts the ionospheric mapping of a given radial distance at the equator. However, in some cases, the main emission shifts independently from the satellite footprints, indicating that the variability stems from some other part of the corotation enforcement current system that produces Jupiter's main auroral emissions. Here, we analyze HST images from the Galileo era (1996–2003) and compare latitudinal shifts of the Ganymede footprint and the main auroral emission. We focus on images with overlapping Galileo measurements because concurrent measurements are available of the current sheet strength, which indicates the amount of field line stretching and can influence both the main emission and satellite footprints. We show that the Ganymede footprint and main auroral emission typically, but do not always, move together. Additionally, we find that the auroral shifts are only weakly linked to changes in the current sheet strength measured by Galileo. We discuss implications of the observed auroral shifts in terms of the magnetospheric mapping. Finally, we establish how the statistical reference main emission contours vary with central meridian longitude and show that the dependence results from magnetospheric local time asymmetries.

An empirical model of the current sheet in Jupiter's magnetosphere

Based on the Magnetometer data from the Galileo and Juno mission, we construct an analytical ring current model that provides the azimuthal current (Jφ) distribution in radial (ρ), height (z), and local time (LT) dimensions. After binning the in-situ magnetic field data into 2 RJ ​× ​2 RJ ​× ​3 ​h (RJ is the radius of Jupiter) bins, the three-dimensional Jφ is calculated by differencing of grid values from the Maxwell equation. The modeled Jφ is obtained by fitting it to a lognormal distribution in ρ and a Gaussian distribution in z. The local time dependence is included by fitting the coefficients of the lognormal using a harmonic expansion. The results show a local time asymmetry of the ring current. Jφ is weaker and the current sheet is compressed at the noon and dusk sectors, comparing with the situation at the midnight and dawn sectors. The ring current peaks at ∼1.5MA/RJ2(2.94×10−1nA/m2) near midnight. Furthermore, the current sheet is thickest near ρ ​= ​10 RJ. The thickness decrease with ρ when ρ ​< ​35 RJ and increase slightly again when ρ ​> ​35 RJ. This empirical model of Jupiter's current sheet is more realistic than previous models.

Thermospheric Neutral Winds as the Cause of Drift Shell Distortion in Earth’s Inner Radiation Belt

Recent analysis of energetic electron measurements from the Magnetic Electron Ion Spectrometer instruments onboard the Van Allen Probes showed a local time variation of the equatorial electron intensity in the Earth’s inner radiation belt. The local time asymmetry was interpreted as evidence of drift shell distortion by a large-scale electric field. It was also demonstrated that the inclusion of a simple dawn-to-dusk electric field model improved the agreement between observations and theoretical expectations. Yet, exactly what drives this electric field was left unexplained. We combine in-situ field and particle observations, together with a physics-based coupled model, the Rice Convection Model (RCM) Coupled Thermosphere-Ionosphere-Plasmasphere-electrodynamics (CTIPe), to revisit the local time asymmetry of the equatorial electron intensity observed in the innermost radiation belt. The study is based on the dawn-dusk difference in equatorial electron intensity measured at L = 1.30 during the first 60 days of the year 2014. Analysis of measured equatorial electron intensity in the 150–400 keV energy range, in-situ DC electric field measurements and wind dynamo modeling outputs provide consistent estimates of the order of 6–8 kV for the average dawn-to-dusk electric potential variation. This suggests that the dynamo electric fields produced by tidal motion of upper atmospheric winds flowing across Earth’s magnetic field lines - the quiet time ionospheric wind dynamo - are the main drivers of the drift shell distortion in the Earth’s inner radiation belt.

Kronian Magnetospheric Reconnection Statistics Across Cassini’s Lifetime

AbstractMagnetic reconnection is a fundamental physical process in planetary magnetospheres, in which plasma can be exchanged between the solar wind and a planetary magnetosphere, and material can be disconnected and ultimately lost from a magnetosphere. Magnetic reconnection in a planetary magnetotail can result in the release of plasmoids downtail and dipolarizations planetward of an x‐line. The signatures of these products include characteristic deflections in the north‐south component of the magnetic field which can be detected by in‐situ spacecraft. These signatures have been identified by eye, semi‐automated algorithms, and recently machine learning (ML) methods. Here, we apply statistical analysis to the most thorough catalog of Kronian magnetospheric reconnection signatures created through ML methods to improve understanding of magnetospheric evolution. This research concludes that no quasi‐steady position of the magnetotail x‐line exists within 70 . This research introduces prediction equations to estimate the distribution of duration of plasmoid passage over the spacecraft (, bin width = 1 min) and north‐south field deflection (, bin width = 0.25 nT) expected to be identified by an orbiting spacecraft across a year of observations. Furthermore, this research finds a local time asymmetry for reconnection identifications, with a preference for dusk‐side over dawn‐side. This may indicate a preference for Vasyliunas style reconnection over Dungey style for Saturn. Finally, through these distributions, the reconnection rate of Saturn’s magnetotail can be estimated as 3.22 reconnection events per day, with a resulting maximum mass loss from plasmoids of 44.87 kg on average, which is comparable with the magnetospheric mass loading from Enceladus (8–250 kg ).

ULF Wave Driven Radial Diffusion During Geomagnetic Storms: A Statistical Analysis of Van Allen Probes Observations

AbstractThe impact of radial diffusion in storm time radiation belt dynamics is well‐debated. In this study we quantify the changes and variability in radial diffusion coefficients during geomagnetic storms. A statistical analysis of Van Allen Probes data (2012–2019) is conducted to obtain measurements of the magnetic and electric power spectral densities for Ultra Low Frequency (ULF) waves, and corresponding radial diffusion coefficients. The results show global wave power enhancements occur during the storm main phase, and continue into the recovery phase. Local time asymmetries show sources of wave power are both external solar wind driving and internal sources from coupling with ring current ions and substorms. Wave power enhancements are also observed at low L values (L &lt; 4). The accessibility of wave power to low L is attributed to a depression of the Alfvén continuum. The increased wave power drives enhancements in both the magnetic and electric field diffusion coefficients by more than an order of magnitude. Significant variability in diffusion coefficients is observed, with values ranging over several orders of magnitude. A comparison to the Kp parameterized empirical model of Ozeke et al. (2014) is conducted and indicates important differences during storm times. Although the electric field diffusion coefficient is relatively well described by the empirical model, the magnetic field diffusion coefficient is approximately ∼10 times larger than predicted. We discuss how differences could be attributed to data set limitations and assumptions. Alternative storm‐time radial diffusion coefficients are provided as a function of L* and storm phase.

Challenging the Use of Ring Current Indices During Geomagnetic Storms

AbstractThe ring current experiences dramatic enhancements during geomagnetic storms; however, understanding the global distribution of ring current energy content is restricted by spacecraft coverage. Many studies use ring current indices as a proxy for energy content, but these indices average over spatial variations and include additional contributions. We have conducted an analysis of Van Allen Probes' data, identifying the spatial distribution and storm‐time variations of energy content. Ion observations from the HOPE and RBSPICE instruments were used to estimate energy content in L‐MLT bins. The results show large enhancements particularly in the premidnight sector during the main phase, alongside reductions in local time asymmetry and intensity during the recovery phase. A comparison with estimated energy content using the Sym‐H index was conducted. In agreement with previous results, the Sym‐H index significantly overestimates (by up to ∼4 times) the energy content, and we attribute the difference to contributions from additional current systems. A new finding is an observed temporal discrepancy, where energy content estimates from the Sym‐H index maximize 3–9 h earlier than in‐situ observations. Case studies reveal a complex relationship, where variable degrees of agreement between the Sym‐H index and in‐situ measurements are observed. The results highlight the drawbacks of ring current indices and emphasize the variability of the storm time ring current.

Statistical Study on Locally Generated High‐Frequency Plasmaspheric Hiss and Its Effect on Suprathermal Electrons: Van Allen Probes Observation and Quasi‐linear Simulation

AbstractThe local generation of high‐frequency plasmaspheric hiss has recently been reported by a case study (He et al., 2019, https://doi.org/10.1029/2018GL081578). In this research, we perform statistics of global distributions of the locally generated high‐frequency plasmaspheric hiss (LHFPH) for different levels of substorm activity, using 6‐year observational data from Van Allen Probes. The statistics find that the LHFPH amplitude presents a strong magnetic local time (MLT) asymmetry and highly depends on substorm activity, and intense LHFPHs occur from predawn to dusk side and can penetrate into inner plasmasphere of L ∼ 3 during AE &gt; 300 nT. The statistical LHFPH spectrum shows that its frequency increases with the ambient magnetic field, with peaked wave powers between 0.1 and 0.5 fce. Based on the statistical properties of LHFPH, we evaluate the electron diffusion coefficients using quasi‐linear theory. Those results suggest that electron pitch angle scattering driven by LHFPH could be a potential mechanism for the precipitation loss of suprathermal electrons of 0.1 keV to tens of keV, which can impact the ionization and chemical changes in the upper atmosphere.

Enhanced Densities in the Martian Thermosphere Associated With the 2018 Planet‐Encircling Dust Event: Results From MENCA/MOM and NGIMS/MAVEN

AbstractResponse of the Martian upper thermosphere to the lower atmospheric dust activity is studied using unique observations made together by the Mars Orbiter Mission (MOM) and the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. The Mars Exospheric Neutral Composition Analyzer (MENCA)/MOM and the Neutral Gas and Ion Mass Spectrometer (NGIMS)/MAVEN have simultaneously (on the same day) measured the neutral densities in the Martian thermosphere on 5, 8, 10, 13, 16, and 29 June 2018. The measurement period falls in the onset and progression time of the planet‐encircling dust event (PEDE) in the Mars lower atmosphere. During this time, the inbound trajectories of MAVEN and MOM spacecraft occurred on the dawnside and duskside, respectively. Using these observations, we found that thermospheric densities both on the dawnside and duskside are enhanced associated with the onset and growth of PEDE‐2018. The enhancement, however, is more on the duskside than on the dawn, showing the dawn‐dusk asymmetry. The densities on the duskside reach their maximum on 29 June 2018, close to the time of peak dust activity. These results are explained by considering the local time asymmetries in radiative heating of the lower atmosphere and subsequent expansion of the thermosphere due to PEDE‐2018. Furthermore, O/CO2 ratios below 220‐km altitude become one on the dawnside, whereas they are always &lt;0.2 at the dusk. This indicates that radiative cooling is more effective on the dawnside than the duskside.

Local Time Asymmetries in Jupiter's Magnetodisc Currents

AbstractWe present an investigation into the currents within the Jovian magnetodisc using all available spacecraft magnetometer data up until 28 July 2018. Using automated data analysis processes as well as the most recent intrinsic field and current disk geometry models, a full local time coverage of the magnetodisc currents using 7,382 lobe traversals over 39 years is constructed. Our study demonstrates clear local time asymmetries in both the radial and azimuthal height‐integrated current densities throughout the current disk. Asymmetries persist within 30 R where most models assume axisymmetry. Inward radial currents are found in the previously unmapped dusk and noon sectors. Azimuthal currents are found to be weaker in the dayside magnetosphere than the nightside, in agreement with global magnetohydrodynamic simulations. The divergence of the azimuthal and radial currents indicates that downward field‐aligned currents exist within the outer dayside magnetosphere. The presence of azimuthal currents is shown to highly influence the location of the field‐aligned currents, which emphasizes the importance of the azimuthal currents in future magnetosphere‐ionosphere coupling models. Integrating the divergence of the height‐integrated current densities, we find that 1.87 MA R of return current density required for system closure is absent.

Dawn‐Dusk Asymmetries in the Martian Upper Atmosphere

AbstractIn this paper, we report the first comprehensive observations of local time asymmetries in densities and scale heights (temperatures) of the Martian upper atmosphere (between 150 and 300 km) measured by the Mars Atmosphere and Volatile Evolution mission/Neutral Gas and Ion Mass Spectrometer. For this purpose, we use the densities and temperatures of Ar. In general, the daytime densities and temperatures are greater than the nighttime values. The maximum and minimum values, however, are observed at the dusk and dawn terminators, respectively. An enhancement at the dusk terminator is persistently observed at all altitudes; however, the time of the peak enhancement shifts toward sunlit hours with increasing altitude. At the dawn terminator, a minimum in density is observed at altitudes of 150–170 km. At higher altitudes, the minimum is observed close to midnight. Accordingly, the dawn‐dusk asymmetry is more prominent at 150–170 km and decreases with increasing altitude. A maximum ratio of six is observed between the dusk and dawn densities at 160 km. In addition, the local time for the maximum ratio at each altitude moves toward sunlit hours with increasing altitude. The observed asymmetry is explained in terms of dynamical heating and cooling due to convergent and divergent winds at the dusk and dawn terminators, respectively. In addition, upward propagating gravity waves generated by the solar terminator wave and O/CO2 radiative cooling are also proposed as important mechanisms contributing to the observed asymmetry.