Global Magnetospheric Dynamics Research Articles

Auroral, Ionospheric and Ground Magnetic Signatures of Magnetopause Surface Modes

AbstractSurface waves on Earth's magnetopause have a controlling effect upon global magnetospheric dynamics. Since spacecraft provide sparse in situ observation points, remote sensing these modes using ground‐based instruments in the polar regions is desirable. However, many open conceptual questions on the expected signatures remain. Therefore, we provide predictions of key qualitative features expected in auroral, ionospheric, and ground magnetic observations through both magnetohydrodynamic theory and a global coupled magnetosphere‐ionosphere simulation of a magnetopause surface eigenmode. These show monochromatic oscillatory field‐aligned currents (FACs), due to both the surface mode and its non‐resonant Alfvén coupling, are present throughout the magnetosphere. The currents peak in amplitude at the equatorward edge of the magnetopause boundary layer, not the open‐closed boundary as previously thought. They also exhibit slow poleward phase motion rather than being purely evanescent. We suggest the upward FAC perturbations may result in periodic auroral brightenings. In the ionosphere, convection vortices circulate the poleward moving FAC structures. Finally, surface mode signals are predicted in the ground magnetic field, with ionospheric Hall currents rotating perturbations by approximately (but not exactly) 90° compared to the magnetosphere. Thus typical dayside magnetopause surface modes should be strongest in the East‐West ground magnetic field component. Overall, all ground‐based signatures of the magnetopause surface mode are predicted to have the same frequency across L‐shells, amplitudes that maximize near the magnetopause's equatorward edge, and larger latitudinal scales than for field line resonance. Implications in terms of ionospheric Joule heating and geomagnetically induced currents are discussed.

Solar Wind Energy Input: The Primary Control Factor of Magnetotail Reconnection Site

AbstractIn this paper, we examine the solar wind energy input (expressed by −Vx × Bs, where Vx is the x component of the solar wind velocity and Bs is the southward component of the interplanetary magnetic field IMF Bz) for an onset of magnetic reconnection in the near‐Earth magnetotail. There are 41 events in which in situ observations of magnetic reconnection were made by Geotail. Magnetic reconnection in the postmidnight (premidnight) sector of the plasma sheet occurred under strong (weak) solar wind energy input conditions. Furthermore, we study temporal variations in the solar wind energy input with two different approaches using ground magnetic field observations and proton injections at geosynchronous altitude. These two analyses confirmed the preference of the postmidnight sector for the onset of magnetic reconnection under the strong solar wind energy input conditions. It is also found that the medium and weak solar wind energy input may move the onset location to earlier magnetic local times. The onset location of magnetic reconnection in the near‐Earth magnetotail is controlled by the solar wind energy input through the global magnetospheric dynamics during the loading period.

ULF Wave‐Induced Ion Pitch Angle Evolution in the Dayside Outer Magnetosphere

AbstractDrift‐bounce resonance between ultralow frequency (ULF) waves and ions is essential for ion energization in the magnetosphere. Here, we present the first comprehensive study of drift‐bounce resonance in the dayside outer magnetosphere, where off‐equatorial magnetic field minima would strongly distort ions' bounce and drift motion. A generalized theory is proposed, in which the effects of off‐equatorial minima, time‐evolving fields and ion bounce motion are taken into account. In consequence of these effects, ion pitch angle distributions undergo dramatic changes. In the presence of off‐equatorial minima, the time‐of‐flight effect of ion bounce motion forms latitude‐dependent dispersions besides “paw‐track shaped” structures, while evolving wave fields cause time‐dependent phase shifts in “paw‐tracks.” All the predicted signatures have been confirmed by 5 years of Magnetospheric Multiscale spacecraft data and numerical simulations. These results allow us to better understand the interactions between ULF waves and thermal ion species in global magnetospheric dynamics.

Magnetopause ripples going against the flow form azimuthally stationary surface waves

Surface waves process the turbulent disturbances which drive dynamics in many space, astrophysical and laboratory plasma systems, with the outer boundary of Earth’s magnetosphere, the magnetopause, providing an accessible environment to study them. Like waves on water, magnetopause surface waves are thought to travel in the direction of the driving solar wind, hence a paradigm in global magnetospheric dynamics of tailward propagation has been well-established. Here we show through multi-spacecraft observations, global simulations, and analytic theory that the lowest-frequency impulsively-excited magnetopause surface waves, with standing structure along the terrestrial magnetic field, propagate against the flow outside the boundary. Across a wide local time range (09–15h) the waves’ Poynting flux exactly balances the flow’s advective effect, leading to no net energy flux and thus stationary structure across the field also. Further down the equatorial flanks, however, advection dominates hence the waves travel downtail, seeding fluctuations at the resonant frequency which subsequently grow in amplitude via the Kelvin-Helmholtz instability and couple to magnetospheric body waves. This global response, contrary to the accepted paradigm, has implications on radiation belt, ionospheric, and auroral dynamics and potential applications to other dynamical systems.

Vlasov simulation of electrons in the context of hybrid global models: an eVlasiator approach

Abstract. Modern investigations of dynamical space plasma systems such as magnetically complicated topologies within the Earth's magnetosphere make great use of supercomputer models as well as spacecraft observations. Space plasma simulations can be used to investigate energy transfer, acceleration, and plasma flows on both global and local scales. Simulation of global magnetospheric dynamics requires spatial and temporal scales currently achievable through magnetohydrodynamics or hybrid-kinetic simulations, which approximate electron dynamics as a charge-neutralizing fluid. We introduce a novel method for Vlasov-simulating electrons in the context of a hybrid-kinetic framework in order to examine the energization processes of magnetospheric electrons. Our extension of the Vlasiator hybrid-Vlasov code utilizes the global simulation dynamics of the hybrid method whilst modelling snapshots of electron dynamics on global spatial scales and temporal scales suitable for electron physics. Our eVlasiator model is shown to be stable both for single-cell and small-scale domains, and the solver successfully models Langmuir waves and Bernstein modes. We simulate a small test-case section of the near-Earth magnetotail plasma sheet region, reproducing a number of electron distribution function features found in spacecraft measurements.

Global Dynamics of the Earth's Magnetosphere During Northward IMF Conditions in the Era of the Heliophysics System Observatory

Global Dynamics of the Earth's Magnetosphere During Northward IMF Conditions in the Era of the Heliophysics System Observatory

Comparative Analysis of the Vlasiator Simulations and MMS Observations of Multiple X-Line Reconnection and Flux Transfer Events.

The Vlasiator hybrid‐Vlasov code was developed to investigate global magnetospheric dynamics at ion‐kinetic scales. Here we focus on the role of magnetic reconnection in the formation and evolution of magnetic islands at the low‐latitude magnetopause, under southward interplanetary magnetic field conditions. The simulation results indicate that (1) the magnetic reconnection ion kinetics, including the Earthward pointing Larmor electric field on the magnetospheric side of an X‐point and anisotropic ion distributions, are well‐captured by Vlasiator, thus enabling the study of reconnection‐driven magnetic island evolution processes, (2) magnetic islands evolve due to continuous reconnection at adjacent X‐points, “coalescence” which refers to the merging of neighboring islands to create a larger island, “erosion” during which an island loses magnetic flux due to reconnection, and “division” which involves the splitting of an island into smaller islands, and (3) continuous reconnection at adjacent X‐points is the dominant source of magnetic flux and plasma to the outer layers of magnetic islands resulting in cross‐sectional growth rates up to + 0.3 RE 2/min. The simulation results are compared to the Magnetospheric Multiscale (MMS) measurements of a chain of ion‐scale flux transfer events (FTEs) sandwiched between two dominant X‐lines. The MMS measurements similarly reveal (1) anisotropic ion populations and (2) normalized reconnection rate ~0.18, in agreement with theory and the Vlasiator predictions. Based on the simulation results and the MMS measurements, it is estimated that the observed ion‐scale FTEs may grow Earth‐sized within ~10 min, which is comparable to the average transport time for FTEs formed in the subsolar region to the high‐latitude magnetopause. Future simulations shall revisit reconnection‐driven island evolution processes with improved spatial resolutions.

EXTRASOLAR GIANT MAGNETOSPHERIC RESPONSE TO STEADY-STATE STELLAR WIND PRESSURE AT 10, 5, 1, AND 0.2 au

ABSTRACT A three-dimensional, multifluid simulation of a giant planet’s magnetospheric interaction with steady-state stellar wind from a Sun-like star was performed for four different orbital semimajor axes—10, 5, 1, and 0.2 au. We simulate the effect of the increasing, steady-state stellar wind pressure related to the planetary orbital semimajor axis on the global magnetospheric dynamics for a Saturn-like planet, including an Enceladus-like plasma torus. Mass-loss processes are shown to vary with orbital distance, with the centrifugal interchange instability displayed only in the 10 and 5 au cases, which reach a state of mass-loss equilibrium more slowly than the 1 or 0.2 au cases. The compression of the magnetosphere in the 1 and 0.2 au cases contributes to the quenching of the interchange process by increasing the ratio of total plasma thermal energy to corotational energy. The strength of field-aligned currents, associated with auroral radio emissions, is shown to increase in magnitude and latitudinal coverage with a corresponding shift equatorward from increased dynamic ram pressure experienced in the hotter orbits. Similar to observed hot Jovian planets, the warm exo-Saturn simulated in the current work shows enhanced ion density in the magnetosheath and magnetopause regions, as well as the plasma torus, which could contribute to altered transit signals, suggesting that for planets in warmer (>0.1 au) orbits, planetary magnetic field strengths and possibly exomoons—via the plasma torus—could be observable with future missions.

Magnetic reconnection in Saturn's magnetotail: A comprehensive magnetic field survey.

Reconnection within planetary magnetotails is responsible for locally energizing particles and changing the magnetic topology. Its role in terms of global magnetospheric dynamics can involve changing the mass and flux content of the magnetosphere. We have identified reconnection related events in spacecraft magnetometer data recorded during Cassini's exploration of Saturn's magnetotail. The events are identified from deflections in the north‐south component of the magnetic field, significant above a background level. Data were selected to provide full tail coverage, encompassing the dawn and dusk flanks as well as the deepest midnight orbits. Overall 2094 reconnection related events were identified, with an average rate of 5.0 events per day. The majority of events occur in clusters (within 3 h of other events). We examine changes in this rate in terms of local time and latitude coverage, taking seasonal effects into account. The observed reconnection rate peaks postmidnight with more infrequent but steady loss seen on the dusk flank. We estimate the mass loss from the event catalog and find it to be insufficient to balance the input from the moon Enceladus. Several reasons for this discrepancy are discussed. The reconnection X line location appears to be highly variable, though a statistical separation between events tailward and planetward of the X line is observed at a radial distance of between 20 and 30R S downtail. The small sample size at dawn prevents comprehensive statistical comparison with the dusk flank observations in terms of flux closure.

Assessing the role of oxygen on ring current formation and evolution through numerical experiments

We address the effect of ionospheric outflow and magnetospheric ion composition on the physical processes that control the development of the 5 August 2011 magnetic storm. Simulations with the Space Weather Modeling Framework are used to investigate the global dynamics and energization of ions throughout the magnetosphere during storm time, with a focus on the formation and evolution of the ring current. Simulations involving multifluid (with variable H+/O+ ratio in the inner magnetosphere) and single‐fluid (with constant H+/O+ ratio in the inner magnetosphere) MHD for the global magnetosphere with inner boundary conditions set either by specifying a constant ion density or by physics‐based calculations of the ion fluxes reveal that dynamical changes of the ion composition in the inner magnetosphere alter the total energy density of the magnetosphere, leading to variations in the magnetic field as well as particle drifts throughout the simulated domain. A low oxygen to hydrogen ratio and outflow resulting from a constant ion density boundary produced the most disturbed magnetosphere, leading to a stronger ring current but misses the timing of the storm development. Conversely, including a physics‐based solution for the ionospheric outflow to the magnetosphere system leads to a reduction in the cross‐polar cap potential (CPCP). The increased presence of oxygen in the inner magnetosphere affects the global magnetospheric structure and dynamics and brings the nightside reconnection point closer to the Earth. The combination of reduced CPCP together with the formation of the reconnection line closer to the Earth yields less adiabatic heating in the magnetotail and reduces the amount of energetic plasma that has access to the inner magnetosphere.

Field dipolarization in Saturn's magnetotail with planetward ion flows and energetic particle flow bursts: Evidence of quasi-steady reconnection.

We present a case study of an event from 20 August (day 232) of 2006, when the Cassini spacecraft was sampling the region near 32 RS and 22 h LT in Saturn's magnetotail. Cassini observed a strong northward‐to‐southward turning of the magnetic field, which is interpreted as the signature of dipolarization of the field as seen by the spacecraft planetward of the reconnection X line. This event was accompanied by very rapid (up to ~1500 km s−1) thermal plasma flow toward the planet. At energies above 28 keV, energetic hydrogen and oxygen ion flow bursts were observed to stream planetward from a reconnection site downtail of the spacecraft. Meanwhile, a strong field‐aligned beam of energetic hydrogen was also observed to stream tailward, likely from an ionospheric source. Saturn kilometric radiation emissions were stimulated shortly after the observation of the dipolarization. We discuss the field, plasma, energetic particle, and radio observations in the context of the impact this reconnection event had on global magnetospheric dynamics.

Circulation of Heavy Ions and Their Dynamical Effects in the Magnetosphere: Recent Observations and Models

Knowledge of the ion composition in the near-Earth’s magnetosphere and plasma sheet is essential for the understanding of magnetospheric processes and instabilities. The presence of heavy ions of ionospheric origin in the magnetosphere, in particular oxygen (O+), influences the plasma sheet bulk properties, current sheet (CS) thickness and its structure. It affects reconnection rates and the formation of Kelvin-Helmholtz instabilities. This has profound consequences for the global magnetospheric dynamics, including geomagnetic storms and substorm-like events. The formation and demise of the ring current and the radiation belts are also dependent on the presence of heavy ions. In this review we cover recent advances in observations and models of the circulation of heavy ions in the magnetosphere, considering sources, transport, acceleration, bulk properties, and the influence on the magnetospheric dynamics. We identify important open questions and promising avenues for future research.

Solar Wind and Internally Driven Dynamics: Influences on Magnetodiscs and Auroral Responses

The dynamics of the giant planet magnetodiscs are strongly influenced by planetary rotation. Yet the solar wind must ultimately remove plasma from these rapidly rotating magnetodiscs at the same rate that plasma is transported radially outward from the source regions: the Io and Enceladus plasma tori. It is not clear how the solar wind influences magnetospheric dynamics when the dynamics are dominated by rotation. However, auroral observations provide important clues. We review magnetodisc sources and radial transport and the solar wind interaction with the giant magnetospheres of Jupiter and Saturn in an attempt to connect auroral features with specific drivers. We provide a discussion of auroral signatures that are related to the solar wind interaction and summarize with a discussion of global magnetospheric dynamics as illustrated by global MHD simulations. Many questions remain and it is the intent of this review to highlight several of the most compelling questions for future research.

Saturn's dynamic magnetotail: A comprehensive magnetic field and plasma survey of plasmoids and traveling compression regions and their role in global magnetospheric dynamics

AbstractWe present a comprehensive study of the magnetic field and plasma signatures of reconnection events observed with the Cassini spacecraft during the tail orbits of 2006. We examine their “local” properties in terms of magnetic field reconfiguration and changing plasma flows. We also describe the “global” impact of reconnection in terms of the contribution to mass loss, flux closure, and large‐scale tail structure. The signatures of 69 plasmoids, 17 traveling compression regions (TCRs), and 13 planetward moving structures have been found. The direction of motion is inferred from the sign of the change in the Bθ component of the magnetic field in the first instance and confirmed through plasma flow data where available. The plasmoids are interpreted as detached structures, observed by the spacecraft tailward of the reconnection site, and the TCRs are interpreted as the effects of the draping and compression of lobe magnetic field lines around passing plasmoids. We focus on the analysis and interpretation of the tailward moving (south‐to‐north field change) plasmoids and TCRs in this work, considering the planetward moving signatures only from the point of view of understanding the reconnection x‐line position and recurrence rates. We discuss the location spread of the observations, showing that where spacecraft coverage is symmetric about midnight, reconnection signatures are observed more frequently on the dawn flank than on the dusk flank. We show an example of a chain of two plasmoids and two TCRs over 3 hours and suggest that such a scenario is associated with a single‐reconnection event, ejecting multiple successive plasmoids. Plasma data reveal that one of these plasmoids contains H+ at lower energy and W+ at higher energy, consistent with an inner magnetospheric source, and the total flow speed inside the plasmoid is estimated with an upper limit of 170 km/s. We probe the interior structure of plasmoids and find that the vast majority of examples at Saturn show a localized decrease in field magnitude as the spacecraft passes through the structure. We take the trajectory of Cassini into account, as, during 2006, the spacecraft's largely equatorial position beneath the hinged current sheet meant that it rarely traversed the center of plasmoids. We present an innovative method of optimizing the window size for minimum variance analysis (MVA) and apply this MVA across several plasmoids to explore their interior morphology in more detail, finding that Saturn's tail contains both loop‐like and flux rope‐like plasmoids. We estimate the mass lost downtail through reconnection and suggest that the apparent imbalance between mass input and observed plasmoid ejection may mean that alternative mass loss methods contribute to balancing Saturn's mass budget. We also estimate the rate of magnetic flux closure in the tail and find that when open field line closure is active, it plays a very significant role in flux cycling at Saturn.

Solar cycle variations in polar cap area measured by the superDARN radars

AbstractWe present a long‐term study, from January 1996 to August 2012, of the latitude of the Heppner‐Maynard Boundary (HMB) measured at midnight using the northern hemisphere Super Dual Auroral Radar Network (SuperDARN). The HMB represents the equatorward extent of ionospheric convection and is used in this study as a measure of the global magnetospheric dynamics. We find that the yearly distribution of HMB latitudes is single peaked at 64° magnetic latitude for the majority of the 17 year interval. During 2003, the envelope of the distribution shifts to lower latitudes and a second peak in the distribution is observed at 61°. The solar wind‐magnetosphere coupling function derived by Milan et al. (2012) suggests that the solar wind driving during this year was significantly higher than during the rest of the 17 year interval. In contrast, during the period 2008–2011, HMB distribution shifts to higher latitudes, and a second peak in the distribution is again observed, this time at 68° magnetic latitude. This time interval corresponds to a period of extremely low solar wind driving during the recent extreme solar minimum. This is the first long‐term study of the polar cap area and the results demonstrate that there is a close relationship between the solar activity cycle and the area of the polar cap on a large‐scale, statistical basis.

Exploring the influence of ionospheric O+ outflow on magnetospheric dynamics: dependence on the source location

Heavy ions of ionospheric origin (O+) play an important role in altering global magnetospheric dynamics. While the heavy ions mainly originate from the dayside cusp and the nightside auroral region, the impact of these heavy ions on magnetospheric dynamics has not been differentiated. Controversy also remains on the role of heavy ions on tail stability and their energization mechanism in the magnetosphere. Two global MHD simulations are carried out to investigate the influence of heavy ion outflow from different source regions on reconnection rates, tail stability, and ring current energization. The local reconnection rate at the subsolar point and the total dayside reconnection rate are reduced after the outflow begins, but the decrease is more significant when the outflow comes out of the cusp region. Furthermore, the magnetotail is more disturbed when heavy ions flow out of the dayside cusp region as opposed to the nightside auroral zone. This implies that the role of O+ on tail stability is not definitively positive or negative; instead, the location of the source of heavy ions may be important in determining tail dynamics. Finally, the simulation reveals that the heavy ions originating from the dayside cusp region experience first adiabatic heating while traveling from the tail reconnection site toward the Earth and then further energization caused by flow braking near the outer boundary of the ring current.

Radio Remote Sensing of Magnetospheric Plasmas

With recent advances in radio transmitter and receiver design, and modern digital processing techniques it should be possible to perform remote radio sounding of the magnetosphere utilizing methods perfected for ionospheric sounding over the last two decades. Like ionospheric sounding, free-space electromagnetic waves, launched within a low density region will reflect at remote plasma cutoffs. The location and characteristics of the plasma at the remote reflection point can then be derived from measurements of the delay time, frequency, and direction of an echo. A magnetospheric radio sounder, operating at frequencies between 3 kHz to 3 MHz could provide quantitative electron density profiles simultaneously in several different directions on a time scale of minutes or less. The test of this technique will not have to wait long, since the first magnetospheric radio sounder will fly on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) mission to be launched in the year 2000. A simulation of radio remote sensing of the magnetosphere from the IMAGE orbit, reported here, was accomplished by using ray tracing calculations combined with specific radio sounder instrument characteristics. The radio sounder technique should provide a truly exciting opportunity to study global magnetospheric dynamics in a way which was never before possible.

Association of cusp energetic ions with geomagnetic storms and substorms

Abstract. Energetic ions observed in the cusp have been explained as a result of processes within the magnetosphere, but also proposed as a driver of some of those same processes. This study assesses potential connections between energetic ions observed in the cusp and geomagnetic storm and substorm activity. These connections may suggest sources of cusp energetic particles (CEPs), or imply effects of these particles on magnetospheric dynamics. We identify CEPs from six years of cusp crossings by the Polar satellite, relating them to storm and substorm onsets. CEPs showed no significant dependence on storms but did show a weak, statistically significant, increase after substorm onsets. CEPs had no significant association with subsequent storm or substorm onsets. We conclude that substorm acceleration may contribute to CEPs but CEPs are unlikely to contribute to global magnetospheric dynamics.

Multiscale modeling of magnetospheric reconnection

In our efforts to bridge the gap between small‐scale kinetic modeling and global simulations, we introduced an approach that allows to quantify the interaction between large‐scale global magnetospheric dynamics and microphysical processes in diffusion regions near reconnection sites. We use the global MHD code BATS‐R‐US and replace an ad hoc anomalous resistivity often employed by global MHD models with a physically motivated dissipation model. The primary kinetic mechanism controlling the dissipation in the diffusion region in the vicinity of the reconnection site is incorporated into the MHD description in terms of nongyrotropic corrections to the induction equation. We developed an algorithm to search for reconnection sites in north‐south symmetric magnetotail. Spatial scales of the diffusion region and magnitude of the reconnection electric field are calculated consistently using local MHD plasma and field parameters. The locations of the reconnection sites are constantly updated during the simulations. To clarify the role of nongyrotropic effects in the diffusion region on the global magnetospheric dynamics, we perform simulations with steady southward interplanetary magnetic field driving of the magnetosphere. Ideal MHD simulations with magnetic reconnection supported by numerical resistivity often produce quasi‐steady configuration with almost stationary near‐Earth neutral line (NENL). Simulations with nongyrotropic corrections demonstrate dynamic quasi‐periodic response to the steady driving conditions. Fast magnetotail reconnection supported by nongyrotropic effects results in tailward retreat of the reconnection site with average speed of the order of 100 km/s followed by a formation of a new NENL in the near‐Earth thin current sheet. This approach allowed to model for the first time loading/unloading cycle frequently observed during extended periods of steady low‐mach‐number solar wind with southward interplanetary magnetic field.

Harmonic structure of field line eigenmodes generated by ionospheric feedback instability

Ionospheric feedback instability generates field line eigenmodes under favorable conditions of low ionospheric conductivity and high electric convection. A numerical study has been performed using a two‐dimensional model in dipole magnetic field geometry that includes active ionospheric feedback and shear Alfvén wave dynamics of the magnetospheric response. Strong parallel inhomogeneities of the magnetospheric parameters included in the numerical model permit the simultaneous development of local ionospheric resonator modes (fast feedback) trapped between the ionosphere and Alfvén speed maximum above it and field line eigenmodes (slow feedback) that stand along the entire magnetic field line between two conjugate ionospheres. Fourier analysis of the numerical solution has been used to investigate the relative contribution of fast and slow feedback in the global dynamics of magnetosphere–ionosphere coupling. While many harmonics of the global magnetospheric resonator can be simultaneously generated by the ionospheric feedback instability, all modes above the fundamental eigenmode are strongly suppressed by Alfvén wave dispersion. The numerical results suggest that ionospheric feedback is a generating mechanism for the formation of narrow field line resonances.