State Of Magnetosphere Research Articles

Effect of magnetospheric conditions on the morphology of Jupiter’s ultraviolet main auroral emission as observed by Juno-UVS

Auroral emissions are a reflection of magnetospheric processes, and at Jupiter, it is not entirely certain how the morphology of the UV main emission (ME) varies with magnetospheric compression or the strength of the central current sheet. This work leverages the observations from Juno-UVS to link ME variability with particular magnetospheric states. We employed novel arc-detection techniques to determine new reference ovals for the ME from perijoves 1 through 54, in both hemispheres, and analysed how the size and shape of the ME vary compared to this reference oval. The morphology and brightness of the ME vary in local time: the dawn-side ME is typically expanded, while the dusk-side ME is contracted, compared to the reference oval, and the dusk-side ME is twice as bright as the dawn-side ME. Both the northern and southern ME and the day-side and night-side ME expand and contract from their reference ovals synchronously, which indicates that the variable size of the ME is caused by a process occurring throughout the Jovian magnetosphere. The poleward latitudinal shift of the auroral footprint of Ganymede correlates with the poleward motion of the ME, whereas a similar relation is not present for the footprint of Io. Additionally, the expansion of the ME correlates well with an increase in magnetodisc current. These two results suggest that a changing current-sheet magnetic field is partially responsible for the variable size of the ME. Finally, magnetospheric compression is linked to a global ME contraction and brightening, though this brightening occurs predominantly in the day-side ME. This observation, and the observation that the dusk-side ME is typically brighter than the dawn-side ME, stands in contrast to the modelled and observed behaviour of field-aligned currents and thus weakens the theoretical link between field-aligned currents and the generation of the auroral ME.

The Ionospheric Leg of the Substorm Current Wedge: Combining Iridium and Ground Magnetometers

AbstractUtilizing magnetic field measurements made by the Iridium satellites and by ground magnetometers in North America we calculate the full ionospheric current system and investigate the substorm current wedge. The current estimates are independent of ionospheric conductance, and are based on estimates of the divergence‐free (DF) ionospheric current from ground magnetometers and curl‐free (CF) ionospheric currents from Iridium. The DF and CF currents are represented using spherical elementary current systems (SECS), derived using a new inversion scheme that ensures the current systems' spatial scales are consistent. We present 18 substorm events and find a typical substorm current wedge (SCW) in 12 events. Our investigation of these substorms shows that during substorm expansion, equivalent field‐aligned currents (EFACs) derived with ground magnetometers are a poor proxy of the actual FAC. We also find that the intensification of the westward electrojet can occur without an intensification of the FACs. We present theoretical investigations that show that the observed deviation between FACs estimated with satellite measurements and ground‐based EFACs are consistent with the presence of a strong local enhancement of the ionospheric conductance, similar to the substorm bulge. Such enhancements of the auroral conductance can also change the ionospheric closure of pre‐existing FACs such that the ground magnetic field, and in particular the westward electrojet, changes significantly. These results demonstrate that attributing intensification of the westward electrojet to SCW current closure can yield false understanding of the ionospheric and magnetospheric state.

On the Global Features of the 10–60‐Min ULF Waves in Jovian Magnetosphere: Juno Observations

AbstractIn the Jovian magnetosphere, quasi‐periodic phenomena, with quasi‐periods on the order of 10–60 min, are frequently identified using different data sets. These pulsations are a branch of ultra‐low frequency (ULF) waves, which are believed to play a crucial role in driving the energy circulation within Jupiter's magnetosphere. In this study, we utilize magnetic field data collected by Juno between 2016 and 2022 to perform a comprehensive global statistical analysis of the spatial distribution and periodic characteristics of ULF waves in the Jovian magnetosphere. Our findings reveal distinct periodic features observed at different latitudes and distances, providing valuable insights into the generation mechanisms of ULF waves. Furthermore, we establish a close relationship between the presence of these ULF wave fluctuations and the magnetospheric state, such as under conditions of solar wind compression. By combining contemporaneous ultraviolet aurora observations from the Hubble Space Telescope (HST) and magnetic field data obtained by Juno, we have discovered that the compressed magnetospheres exhibit more pronounced ULF waves and enhanced auroral activity. These results provide a global picture of the distribution, implying potential generation of ULF waves in the Jovian magnetosphere, and shedding light on the processes behind the 10–60‐min energy releases.

Superthermal Electron Observations at Mars During the December 2022 Disappearing Solar Wind Event

AbstractOn 26–27 December 2022, Mars experienced an extremely low‐density solar wind stream, which was encountered first by Earth because of the radial alignment of the two planets (i.e., Mars opposition). During this event, two important properties of the ionospheric and magnetospheric states changed significantly in response to the low solar wind ram pressure, as inferred from the superthermal electron observations from the Mars Atmospheric and Volatile EvolutioN (MAVEN) mission. The interface between the ionosphere and magnetosphere expanded to thousands of kilometers, outside of the nominal bow shock locations, coinciding with the expansion of the cold planetary ions. Meanwhile, the ambipolar electrostatic potential arising from the ionospheric electron pressure gradient increased from the nominal ∼ −0.7 to ∼ −2 V (relative to the lower ionosphere). This enhanced ambipolar potential likely facilitated the observed ionosphere expansion.

Identifying the Variety of Jovian X‐Ray Auroral Structures: Tying the Morphology of X‐Ray Emissions to Associated Magnetospheric Dynamics

AbstractWe define the spatial clustering of X‐rays within Jupiter's northern auroral regions by classifying their distributions into “X‐ray auroral structures.” Using data from Chandra during Juno's main mission observations (24 May 2016 to 8 September 2019), we define five X‐ray structures based on their ionospheric location and calculate the distribution of auroral photons. The morphology and ionospheric location of these structures allow us to explore the possibility of numerous X‐ray auroral magnetospheric drivers. We compare these distributions to Hubble Space Telescope (HST) and Juno (Waves and MAG) data, and a 1D solar wind propagation model to infer the state of Jupiter's magnetosphere. Our results suggest that the five sub‐classes of “X‐ray structures” fall under two broad morphologies: fully polar and low latitude emissions. Visibility modeling of each structure suggests the non‐uniformity of the photon distributions across the Chandra intervals are likely associated with the switching on/off of magnetospheric drivers as opposed to geometrical effects. The combination of ultraviolet (UV) and X‐ray morphological structures is a powerful tool to elucidate the behavior of both electrons and ions and their link to solar wind/magnetospheric conditions in the absence of an upstream solar monitor. Although much work is still needed to progress the use of X‐ray morphology as a diagnostic tool, we set the foundations for future studies to continue this vital research.

Universal Time Effects on Substorm Growth Phases and Onsets

AbstractUniversal Time (UT) variations in many magnetospheric state indicators and indices have recently been reviewed by Lockwood and Milan (2023, https://doi.org/10.3389/fspas.2023.1139295). Key effects are introduced into magnetospheric dynamics by the eccentric nature of Earth's magnetic field, features that cannot be reproduced by a geocentric field model. This paper studies the UT variation in the occurrence of substorm onsets and uses a simple Monte‐Carlo model to show how it can arise for an eccentric field model from the effect of the diurnal motions of Earth's poles on the part of the geomagnetic tail where substorms are initiated. These motions are in any reference frame that has an X axis that points from the center of the Earth to the center of the Sun and are caused by Earth's rotation. The premise behind the model is shown to be valid using a super‐posed epoch study of the conditions leading up to onset. These studies also show the surprising degree of preconditioning ahead of the growth phase that is required, on average, for onset to occur. A key factor is the extent to which pole motions caused by Earth's rotation influence the near‐Earth tail at the relevant X coordinate. Numerical simulations by a global MHD model of the magnetosphere reveal the effect required to generate the observed UT variations and with right order of amplitude, albeit too small by a factor of about one third. Reasons why this discrepancy may have arisen for the simulations used are discussed.

A system science methodology develops a new composite highly predictable index of magnetospheric activity for the community: the whole-Earth index E(1)

For community use, a new composite whole-Earth index E(1) and its matching composite solar wind driving function S(1) are derived. A system science methodology is used based on a time-dependent magnetospheric state vector and a solar wind state vector, with canonical correlation analysis (CCA) used to reduce the two state vectors to the two time-dependent scalars E(1)(t) and S(1)(t). The whole-Earth index E(1) is based on a diversity of measures via six diverse geomagnetic indices that will be readily available in the future: SML, SMU, Ap60, SYMH, ASYM, and PCC. The CCA-derived composite index has several advantages: 1) the new “canonical” geomagnetic index E(1) will provide a more powerful description of magnetospheric activity, a description of the collective behavior of the magnetosphere–ionosphere system. 2) The new index E(1) is much more accurately predictable from upstream solar wind measurements on Earth. 3) Indications are that the new canonical geomagnetic index E(1) will be accurately predictable even when as-yet-unseen extreme solar wind conditions occur. The composite solar wind driver S(1) can also be used as a universal driver function for individual geomagnetic indices or for magnetospheric particle populations. To familiarize the use of the new index E(1), its behavior is examined in different phases of the solar cycle, in different types of solar wind plasma, during high-speed stream-driven storms, during CME sheath-driven storms, and during superstorms. It is suggested that the definition of storms are the times when E(1) >1.

Application of Recurrent Neural Network to Modeling Earth's Global Electron Density

AbstractThe total electron density is a fundamental quantity in the Earth's magnetosphere and plays an important role in a number of physical processes, but its dynamic global evolution is not fully quantified yet. We present an implementation of a specific type of recurrent neural network (encoder‐decoder), which is distinct from previous models, to construct global electron density based on the multiyear data from Van Allen Probes. The history of geomagnetic indices is first encoded into a hidden state H, then together with auxiliary information (satellite location), they are decoded into the quantity of interest (total electron density in this study). In this process the input of historical geomagnetic indices is detangled from the satellite location and is processed chronologically by the encoder. As a result, time evolution of geomagnetic indices is explicitly embedded in the structure and the encoded hidden state H can be viewed as the representation of the inner magnetospheric state. The magnetospheric state is then decoded to predict global electron density evolution. Our results show that the model can capture the dynamical evolution of total electron density with the formation and evolution of stable and evident plume configurations that roughly agree with global observations. Our findings demonstrate the importance of applying recurrent neural networks to specify the inner magnetospheric state in a novel way, which will potentially improve our fundamental understanding of wave and particle dynamics in the Earth's magnetosphere.

Modeling Radiation Belt Electrons With Information Theory Informed Neural Networks

AbstractAn empirical model of radiation belt relativistic electrons (μ = 560–875 MeV G−1 and I = 0.088–0.14 RE G0.5) with average energy ∼1.3 MeV is developed. The model inputs solar wind parameters (velocity, density, interplanetary magnetic field (IMF) |B|, Bz, and By), magnetospheric state parameters (SYM‐H and AL), and L*. The model outputs the radiation belt electron phase space density (PSD). The model is operational from L* = 3 to 6.5. The model is constructed with neural networks assisted by information theory. Information theory is used to select the most effective and relevant solar wind and magnetospheric input parameters plus their lag times based on their information transfer to the PSD. Based on the test set, the model prediction efficiency (PE) increases with increasing L*, ranging from −0.043 at L* = 3 to 0.76 at L* = 6.5. The model PE is near 0 at L* = 3–4 because at this L* range, the solar wind and magnetospheric parameters transfer little information to the PSD. Using solar wind observations at L1 and magnetospheric index (AL and SYM‐H) models solely driven by solar wind, the radiation belt model can be used to forecast PSD 30–60 min ahead. This baseline model can potentially complement a class of empirical models that input data from low earth orbit (LEO).

Reconstructing Substorms via Historical Data Mining: Is It Really Feasible?

AbstractThe evolution of the low‐latitude magnetosphere over the substorm cycle is reconstructed based on a new high‐resolution 3D representation of the magnetic field and nearest‐neighbor data mining. The study covers radial distances 2.5–25 and employs a record‐large pool of spacecraft data taken during 1995–2019. The magnetospheric state is quantified by four indices, representing the ground geomagnetic activity and its temporal trends in the entire range of geomagnetic latitude: the SuperMAG SMR, the midlatitude positive bay MPB, the auroral SML, and the polar cap PC index. The developed technique has been tested on specific substorm events, with the results presented in the form of 5‐min cadence diagrams and animations of the magnetic field line configurations and electric current distributions. In all the analyzed events, the initial intensification and radial expansion of the inner tail current is accompanied by a gradual stretching of the magnetic field, followed by its sudden collapse, dramatic depletion of the current beyond , and a large‐scale dipolarization of the field around the time of MPB peak, after which the system recovers and tends to its pre‐substorm state.

Multi‐Fluid MHD Simulations of Europa's Plasma Interaction Under Different Magnetospheric Conditions

AbstractEuropa hosts a periodically changing plasma interaction driven by the variations of Jupiter's magnetic field and magnetospheric plasma. We have developed a multi‐fluid magnetohydrodynamic (MHD) model for Europa to characterize the global configuration of the plasma interaction with the moon and its tenuous atmosphere. The model solves the multi‐fluid MHD equations for electrons and three ion fluids (Jupiter's magnetospheric O+, as well as O+ and O2+ originating from Europa's atmosphere) while incorporating sources and losses in the MHD equations due to electron impact and photo‐ionization, charge exchange, recombination and other relevant collisional effects. Using input parameters constrained by the Galileo magnetic field and plasma observations, we first demonstrate the accuracy of our model by simulating the Galileo E4 and E14 flybys, which took place under different upstream conditions and sampled different regions of Europa's interaction. Our model produces 3D magnetic field and plasma bulk parameters that agree with and provide context for the flyby observations. We next present the results of a parameter study of Europa's plasma interaction at three different excursions from Jupiter's central plasma sheet, for three different global magnetospheric states, comprising nine steady‐state simulations. By separately tracking multiple ion fluids, our MHD model allows us to quantify the access of the Jovian magnetospheric plasma to Europa's surface and determine how that access is affected by changing magnetospheric conditions. We find that the thermal magnetospheric O+ precipitation rate ranges from (1.8–26) × 1024 ions/s, and that the precipitation rate increases with the density of the ambient magnetospheric plasma.

A survey of geomagnetic and plasma time lags in the solar-wind-driven magnetosphere of earth

Sixteen variables describing the state of the magnetosphere are examined in the years 1991–2007; the sixteen variables include nine geomagnetic indices plus seven measures of electron and ion precipitation into the atmosphere, the rate of substorm occurrence, the pressure and number density of the ion plasma sheet, the flux of substorm-injected electrons, and the radiation-belt electron flux. Eight other variables are used to represent the properties of the solar wind at Earth. To estimate the time lags between magnetospheric and solar-wind variables bivariate (two-variable) linear correlations, multivariate linear correlations, and vector-vector correlations are utilized. Using bivariate correlation the lag time of each magnetospheric variable with respect to each solar-wind variable is varied to obtain the maximum correlation coefficient. Using multivariate linear correlations, the time series of each magnetospheric variable is correlated with a linear combination of the eight time series of the eight solar-wind variables, with eight independent lead times on the eight solar-wind variables optimized to produce the largest multivariate correlation coefficient. The resulting time lag between each magnetospheric variable and each solar-wind variable is then catalogued. Additionally, the solar-wind variables that most-strongly affect each of the 16 magnetospheric variables are noted. A similar process of optimizing lag times on magnetospheric variables is performed during vector-vector correlations between a 9-dimensional magnetospheric state vector and an 8-dimensional solar-wind state vector.

Analysis of Muon Flux Variations Caused by High-Speed Solar Wind During Periods of Low Solar Activity

The influence of a high-speed solar wind on the state of magnetosphere, as well as on the cosmic ray flux registered on the Earth’s surface by the muon hodoscope URAGAN (MEPhI) is discussed. The results of the analysis of the parameters of muon flux local anisotropy in the periods of low solar activity (2008–2009 and 2017–2018) during geomagnetic disturbances caused by the high-speed solar wind are presented.

Does the Solar Wind Electric Field Control the Reconnection Rate at Earth's Subsolar Magnetopause?

AbstractWe demonstrate that the reconnection rate at the subsolar magnetopause is strongly controlled by the solar wind electric field and depends weakly on the local properties of the dissipation region. Our approach is to match the solar wind and magnetospheric states in an internal boundary layer described by the Cassak and Shay (2007, https://doi.org/10.1063/1.2795630) expression for two‐dimensional asymmetric reconnection. Faraday's law along the Sun‐Earth line determines the variation of the solar wind electric field from the bow shock to the magnetopause. While the magnetospheric plasma exerts some control over the reconnection rate, magnetic flux pileup in the sheath partially compensates for any local reduction in the reconnection rate. For a fixed magnetospheric state, the reconnection rate is shown to be directly proportional to the solar wind electric field, thus explaining why the solar wind electric field correlates well with geomagnetic indices.

Energy Release from Magnetospheres Deformed by Gravitational Waves

Abstract In this work, we consider the possibility of energy release in pulsar magnetospheres deformed by gravitational waves (GWs) from nearby sources. The strong electromagnetic fields in the magnetospheres may release non-negligible energy despite the weakness of the GW. When the background spacetime is perturbed due to the passage of a GW, the original force-free state of the inner magnetosphere will be slightly violated. The plasma-filled magnetosphere tends to evolve into new force-free states as the spacetime varies with time. During this process, a small portion of the electromagnetic energy stored in the magnetosphere will be released to the acceleration of charged particles along the magnetic field lines. When the pulsar is close enough to the GW source (e.g., pc to the GW sources observed recently), the resulting energy loss rate is comparable with the radio luminosity of the pulsar. It is also noticed that, under very stringent conditions (for magnetars with much shorter distance to the sources), the released energy can reach the typical energy observed from fast radio bursts.

Exploration of a Composite Index to Describe Magnetospheric Activity: Reduction of the Magnetospheric State Vector to a Single Scalar

AbstractGeomagnetic activity is usually gauged by a single time‐dependent geomagnetic index. One drawback is that an individual geomagnetic index measures only one aspect of the activity in the Earth's magnetosphere. Here we construct a time‐dependent 11‐element magnetospheric state vector E for Earth that consists of measures of high‐latitude currents, polar cap current, magnetospheric convection, plasma pressure, ion and electron precipitation rates, the intensity of substorm‐injected electrons, and the elapsed time since the last substorm onset. At the same time we construct an eight‐element time‐dependent solar‐wind state vector S from the measured properties of the solar wind at Earth. In this S → E picture a time‐dependent “magnetospheric‐activity index” E(1) is created from a weighted sum of the 11 variables in the magnetospheric state vector, with the weights chosen by the vector correlation properties between the magnetospheric state vector E and the solar wind state vector S. The properties and advantages of this E(1) index are examined for the years 1991–2007. Comparing fits made on different subsets of the data demonstrates the robustness of the definition of E(1); tests using out‐of‐sample data demonstrate the accuracy of solar wind predictions of E(1). The advantages of the composite index E(1) include a more direct association with the solar wind, linearity, high predictability, and versatility with respect to (a) storm‐versus‐quiet intervals, (b) solar maximum versus solar minimum, and (c) the various types of solar wind plasma. Defining magnetospheric storms with the composite index E(1) is explored, and categorizing magnetospheric storms using the magnetospheric state vector E is considered.

Jupiter's Aurora Observed With HST During Juno Orbits 3 to 7

AbstractA large set of observations of Jupiter's ultraviolet aurora was collected with the Hubble Space Telescope concurrently with the NASA‐Juno mission, during an eight‐month period, from 30 November 2016 to 18 July 2017. These Hubble observations cover Juno orbits 3 to 7 during which Juno in situ and remote sensing instruments, as well as other observatories, obtained a wealth of unprecedented information on Jupiter's magnetosphere and the connection with its auroral ionosphere. Jupiter's ultraviolet aurora is known to vary rapidly, with timescales ranging from seconds to one Jovian rotation. The main objective of the present study is to provide a simplified description of the global ultraviolet auroral morphology that can be used for comparison with other quantities, such as those obtained with Juno. This represents an entirely new approach from which logical connections between different morphologies may be inferred. For that purpose, we define three auroral subregions in which we evaluate the auroral emitted power as a function of time. In parallel, we define six auroral morphology families that allow us to quantify the variations of the spatial distribution of the auroral emission. These variations are associated with changes in the state of the Jovian magnetosphere, possibly influenced by Io and the Io plasma torus and by the conditions prevailing in the upstream interplanetary medium. This study shows that the auroral morphology evolved differently during the five ~2 week periods bracketing the times of Juno perijove (PJ03 to PJ07), suggesting that during these periods, the Jovian magnetosphere adopted various states.

Radiation belt electron scattering by whistler-mode chorus in the Jovian magnetosphere: Importance of ambient and wave parameters

Whistler-mode chorus waves are regarded as an important acceleration mechanism contributing to the formation of relativistic and ultra-relativistic electrons in the Jovian radiation belts. Quantitative determination of the chorus wave driven electron scattering effect in the Jovian magnetosphere requires detailed information of both ambient magnetic field and plasma density and wave spectral property, which however cannot be always readily acquired from observations of existed missions to Jupiter. We therefore perform a comprehensive analysis of the sensitivity of chorus induced electron scattering rates to ambient magnetospheric and wave parameters in the Jovian radiation belts to elaborate to which extent the diffusion coefficients depend on a number of key input parameters. It is found that quasi-linear electron scattering rates by chorus can be strongly affected by the ambient magnetic field intensity, the wave latitudinal coverage, and the peak frequency and bandwidth of the wave spectral distribution in the Jovian magnetosphere, while they only rely slightly on the background plasma density profile and the peak wave normal angle, especially when the wave emissions are confined at lower latitudes. Given the chorus wave amplitude, chorus induced electron scattering rates strongly depend on Jovian L-shell to exhibit a tendency approximately proportional to LJ3. Our comprehensive analysis explicitly demonstrates the importance of reliable information of both the ambient magnetospheric state and wave distribution property to understanding the dynamic electron evolution in the Jovian radiation belts and therefore has implications for future mission planning to explore the extreme particle radiation environment of Jupiter and its satellites.

Abrupt changes in pulsar pulse profile through multiple magnetospheric state switching

A purely magnetospheric model is introduced for observed abrupt changes in pulsar radio profile. Motion of magnetospheric plasma is described by a drift frequency, ωdr, that depends on a parameter 0 ≤ y ≤ 1, and a change in the magnetospheric state corresponds to a change in y. Emission is assumed to arise from m spots distributed uniformly around the magnetic axis, so that spots drift by at the rate mωdr. Observable features, such as subpulses, appear to rotate as ωR = ωdr − mωV. The motion of the visible point, ωV, is ignored in a “standard” version of the viewing geometry that assumes a fixed line of sight (rather than a fixed line-of-sight direction), implying ωV = 0. With ωV ≠ 0, the apparent motion of subpulses is not constant. An abrupt (or more gradual) change in y implies a change in ωR, which affects the observed pulse structure and the average profile. We apply the model for profile shifts observed with PSR B0919+06.

Magnetospheric Switching in PSR B1828–11

AbstractPSR B1828–11 is a young pulsar once thought to be undergoing free precession and recently found instead to be switching magnetospheric states in tandem with spin-down changes. Here we show the two extreme states of the mode-changing found for this pulsar and comment briefly on its interpretation.