Global Magnetohydrodynamic Simulations Research Articles

Turbulence in outer protoplanetary discs: MRI or VSI?

ABSTRACT The outer protoplanetary discs (PPDs) can be subject to the magnetorotational instability (MRI) and the vertical shear instability (VSI). While both processes can drive turbulence in the disc, existing numerical simulations have studied them separately. In this paper, we conduct global 3D non-ideal magnetohydrodynamic (MHD) simulations for outer PPDs, with ambipolar diffusion and instantaneous cooling, and hence conductive to both instabilities. Given the range of ambipolar Elsässer numbers (Am) explored, it is found that the VSI turbulence dominates over the MRI when ambipolar diffusion is strong (Am = 0.1); the VSI and MRI can co-exist for Am = 1; and the VSI is overwhelmed by the MRI when ambipolar diffusion is weak (Am = 10). Angular momentum transport process is primarily driven by MHD winds, while viscous accretion due to MRI and/or VSI turbulence makes a moderate contribution in most cases. Spontaneous magnetic flux concentration and formation of annular substructures remain robust in strong ambipolar diffusion-dominated discs (Am ≤ 1) with the presence of the VSI. Ambipolar diffusion is the major contributor to the magnetic flux concentration phenomenon rather than advection.

Formation of Alfvén Wave Ducts by Magnetotail Flow Bursts

AbstractGeomagnetic activity in Earth's outer magnetosphere stimulates intense and sometimes explosive flows of electromagnetic power propagating earthward as Alfvén waves. Observations show that among its various magnetospheric sources, Alfvénic Poynting flux from the magnetotail reaches the ionosphere with the greatest intensity. Dayside fluxes are statistically weaker though more persistent. Flankside fluxes are weakest. We show using global magnetohydrodynamic simulations that these distributions can be attributed to the formation (or not) of meridionally narrow, relatively uniform, low Alfvén conductance channels that efficiently transmit Alfvénic power to the near‐Earth space. These Alfvén ducts form naturally at the heads of flow bursts in the magnetotail but not in flankside flux tubes, where the transmission of Alfvénic power is strongly attenuated by reflections at large conductance gradients along the propagation path. The results elucidate the underlying physics that control efficient transmission of electromagnetic power from the magnetotail to low altitude to power Alfvénic aurora.

A 3D parametric Martian magnetic pileup boundary model with the effects of solar wind density, velocity, and IMF

Using global magnetohydrodynamic simulations, we construct a 3D parametric model of the Martian magnetic pileup boundary (MPB). This model employs a modified parabola function defined by four parameters. The effects of the solar wind dynamic pressure, the solar wind densities and velocities, and the intensity and orientation of the interplanetary magnetic field (IMF) are examined using 267 simulation cases. The results from our parametric model show that (1) the MPB moves closer to Mars when the upstream solar wind dynamic pressure (Pd) increases, the subsolar standoff distance decreases and the flaring degree of the Martian MPB increases with an increasing Pd according to the power-law relations. For the same Pd, a higher solar wind velocity (a lower density) leads to a farther location of the MPB from Mars, along with a larger flaring degree, which is explained by the higher solar wind convection electric fields and a stronger magnetic pileup process under these conditions. (2) Larger Y or Z components of the IMF, BY or BZ, result in a thicker pileup region and a farther MPB location from Mars, as well as a decrease in the flaring degree. The radial IMF component, BX, has little effect on the geometry of the MPB. (3) In most of the simulations used to derive the current parametric model, the strongest Martian crustal magnetic field is located on the dayside. However, for a larger value of the southward IMF, the Martian MPB is located farther away in the northern hemisphere instead of the southern hemisphere. The north-south asymmetry of the Martian MPB with the southern hemisphere being farther away is observed for other IMF directions. We suggest that the magnetic reconnection of the southward IMF with the crustal field that occurs at middle latitudes of the southern hemisphere results in different magnetic field topologies and the closer location of the MPB under these conditions. Our model results show a relatively good agreement with the previous empirical and theoretical models.

Response of the Jovian Magnetosphere‐Ionosphere System to the Interplanetary Magnetic Field Discontinuity: A Simulation Study

AbstractThe response of the magnetosphere‐ionosphere (MI) system at Jupiter to the interplanetary magnetic field (IMF) tangential discontinuity is studied via global Magnetohydrodynamics (MHD) simulations. Our results show that the IMF discontinuity, which has a transition front perpendicular to the Sun‐Jupiter line, has strong effects on the Jovian MI system depending on local times and radial distances to Jupiter. The magnetospheric significant response to the discontinuity is delayed by ∼2–6 hr after the encounter of the discontinuity front with the bow shock, while the delay time for the ionospheric field‐aligned currents (FACs) is ∼3–8 hr. The outer magnetosphere, particularly at dayside, tends to respond slightly quicker than the inner regions. At the ionosphere, the response of downward FACs is found to occur earlier than that of the upward ones by a time ranging from less than 1 hr to several hours, which is also more prominent at dayside sectors. Using a new MI mapping method with Jupiter's JRM09 magnetic field model, we reproduce for the first time the ionospheric FACs consistent with the morphology of the observed ultraviolet (UV) main auroral emissions. This indicates a potential application of the simulated upward FACs on the morphology of the UV main auroral ovals.

An Electric-field-driven Global Coronal Magnetohydrodynamics Simulation Model Using Helioseismic and Magnetic Imager Vector-magnetic-field Synoptic Map Data

We present the simulation methodology and results of our new data-driven global coronal magnetohydrodynamics (MHD) simulation model. In this model, the solar-surface electric field is first calculated such that the curl will satisfy both the induction equation and the given temporal variations of the solar-surface magnetic field. We use the synoptic maps of the Helioseismic and Magnetic Imager three-component vector-magnetic-field data to specify the solar-surface magnetic-field vector for a period from Carrington Rotations (CRs) 2106 to 2110. A set of whole-Sun three-component electric-field maps are obtained for each CR transition interval of about 27.3 days. Using the inverted electric field as the driving variable, our new global coronal MHD model, with the angular resolution of π/64, can trace the evolution of the three-dimensional coronal magnetic field that matches the specified time-dependent solar-surface magnetic-field maps and simultaneously satisfies the divergence-free condition. A set of additional boundary treatments are introduced to control the contribution of the horizontal components of the magnetic field at the weak-field regions. The strength of the solar-surface magnetic field is limited to 20 Gauss for the sake of computational stability in this study. With these numerical treatments, the nonpotential coronal features, such as twisted loop structures, and their eruptive outward motions are obtained. This present model, capable of introducing three-component solar-surface magnetic-field observation data to coronal MHD simulations, is our first step toward a better model framework for the solar corona and hence solar wind.

Two current systems in the preliminary phase of sudden commencements in the magnetosphere

The preliminary impulse of the sudden commencement is simply explained by the generation of the compressional wave due to sudden compression of the dayside magnetopause and mode conversion from the compressional wave to the Alfvén wave in the magnetosphere. However, this simple model cannot explain a time delay of the peak displacement and longer duration time in the higher latitudes in the pre-noon and post-noon sectors of the polar region. Based on the global magnetohydrodynamic simulation of the magnetosphere–ionosphere system reveals that this peculiar behavior of the geomagnetic variation of the preliminary impulse is associated with temporal deformation of the ionospheric field-aligned current distribution of the preliminary impulse into a crescent shape; its lower-latitude edge extends toward the anti-sunward direction, and its higher-latitude edge almost stays on the same longitude near noon. Numerical simulations revealed that the deformation of the field-aligned current distribution is derived from different behaviors of the two current systems of the preliminary impulse. The first current system consists of the field-aligned current connected to the field-aligned current of the preliminary impulse in the lower latitude side of the ionosphere, the cross-magnetopause current, and the magnetosheath current (type L current system). The cross-magnetopause current is the inertia current generated in the acceleration front of the solar wind due to the sudden compression of the magnetosheath. Thus, the longitudinal speed of the type L current system in the ionosphere is the solar wind speed in the magnetosheath projected into the ionosphere. In contrast, the current system of the preliminary impulse connected to the field-aligned current of the preliminary impulse at higher latitude (type H current system) consists of the upward/downward field-aligned current in the pre-noon/post-noon sector, respectively, and dawn-to-dusk field-perpendicular current along the dayside magnetopause. The dawn-to-dusk field-perpendicular current moves to the higher latitudes in the outer magnetosphere over time. The field-aligned current of the type H current system is converted from the field-perpendicular current due to convergence of the return field-perpendicular current heading toward the sunward direction in the outer magnetosphere; the return field-perpendicular current is the inertia current driven by the magnetospheric plasma flow associated with compression of the magnetopause behind the front region of the accelerated solar wind. The acceleration front spreads concentrically from the subsolar point. Consequently, as the return field-perpendicular current is converted to the field-aligned current of the type H current system, it does not move much in the longitudinal direction over time because the dawn-to-dusk field-perpendicular current of the type H current system moves to the higher latitudes. Therefore, the high-latitude edge of the current distribution of the preliminary impulse in the ionosphere moves only slightly. Finally, we clarified that the conversion between field-perpendicular current and field-aligned current of the type L current system mainly occurs in the region where the Alfvén speed starts to increase toward the Earth. A region with a steep gradient of the Alfvén speed like the plasmapause is not always necessary for conversion from the field-perpendicular current to the field-aligned current. We also suggest the possible field-aligned structure of the standing Alfvén wave that may occur in the preliminary impulse phase.Graphical

Assessing the Influence of Input Magnetic Maps on Global Modeling of the Solar Wind and CME‐Driven Shock in the 2013 April 11 Event

AbstractIn the past decade, significant efforts have been made in developing physics‐based solar wind and coronal mass ejection (CME) models, which have been or are being transferred to national centers (e.g., SWPC, Community Coordinated Modeling Center) to enable space weather predictive capability. However, the input data coverage for space weather forecasting is extremely limited. One major limitation is the solar magnetic field measurements, which are used to specify the inner boundary conditions of the global magnetohydrodynamic (MHD) models. In this study, using the Alfvén wave solar model, we quantitatively assess the influence of the magnetic field map input (synoptic/diachronic vs. synchronic magnetic maps) on the global modeling of the solar wind and the CME‐driven shock in the 11 April 2013 solar energetic particle event. Our study shows that due to the inhomogeneous background solar wind and dynamical evolution of the CME, the CME‐driven shock parameters change significantly both spatially and temporally as the CME propagates through the heliosphere. The input magnetic map has a great impact on the shock connectivity and shock properties in the global MHD simulation. Therefore this study illustrates the importance of taking into account the model uncertainty due to the imperfect magnetic field measurements when using the model to provide space weather predictions.

Where Is Region 1 Field‐Aligned Current Generated?

AbstractWe investigate generation regions of the Region 1 field‐aligned currents (FACs) by using global magnetohydrodynamics (MHD) simulation for steady and purely southward interplanetary magnetic field (IMF). Unlike conventional means, we consider Alfvén traveling time and background motion of plasma. We trace “packets,” which are supposed to carry the perturbations associated with FACs, backward in time from the ionosphere. The following criteria are used to identify generation regions. (a) Conversion between field‐aligned and field‐perpendicular currents takes place (as required from current continuity). (b) Plasma performs negative work against the magnetic tension force (as required from energy conservation). (c) Rate of change in FACs is nonzero (as required from Ampère's and Faraday's laws). Flank (low‐latitude) magnetopause is identified to be a major generator region, in which solar wind‐originated plasma pulls the newly reconnected magnetic field lines. Magnetosphere‐originated plasma, which is indirectly pulled by the solar wind‐originated plasma, also participates in the generation. The involvement of the reconnection can explain the general dependence on the southward IMF. The generation of Region 1 FACs is associated with field‐aligned gradient of space charge ∇||(∇⋅E), where E is the electric field, but the contribution from the term (∇2E)|| is also significant because of curved magnetic field lines. The flank generator can reasonably explain the latitudinal and the longitudinal extensions of the Region 1 FACs because of the finite travel time of the Alfvén waves under convective motion. The generation region is located apart from the instantaneous magnetic field lines extending from the ionosphere.

The effect of cosmic variance on the characteristics of dust polarization power spectra

In the context of cosmic microwave background polarization studies and the characterization of the Galactic foregrounds, the power spectrum analysis of the thermal dust polarization sky has led to intriguing evidence of an E∕B asymmetry and a positive TE correlation. In this work, we produce synthesized dust polarization maps from a set of global magneto-hydrodynamic (MHD) simulations of Milky-Way-sized galaxies, and analyze their power spectra at intermediate angular scales (intermediate angular multipoles ℓ∈[60, 140]). We study the role of the initial configuration of the large-scale magnetic field, its strength, and the feedback on the power spectrum characteristics. Using full-galaxy MHD simulations, we were able to estimate the variance induced by the peculiar location of the observer in the galaxy. We find that the polarization power spectra sensitively depend on the observer’s location, impeding a distinction between different simulation setups. In particular, there is a clear statistical difference between the power spectra measured from within the spiral arms and those measured from the inter-arm regions. Also, power spectra from within supernova-driven bubbles share common characteristics, regardless of the underlying model. However, no correlation was found between the statistical properties of the polarization power spectra and the local (with respect to the observer) mean values of physical quantities such as the density and the strength of the magnetic field. Finally, we find some indications that the global strength of the magnetic field may play a role in shaping the power spectrum characteristics; as the global magnetic field strength increases, the E∕B asymmetry and the TE correlation increase, whereas the viewpoint-induced variance decreases. However, we find no direct correlation with the strength of the local magnetic field that permeates the mapped region of the interstellar medium.

Steady-state accretion in magnetized protoplanetary disks

Context. The transition between magnetorotational instability (MRI)-active and magnetically dead regions corresponds to a sharp change in the disk turbulence level, where pressure maxima may form, hence potentially trapping dust particles and explaining some of the observed disk substructures. Aims. We aim to provide the first building blocks toward a self-consistent approach to assess the dead zone outer edge as a viable location for dust trapping, under the framework of viscously driven accretion. Methods. We present a 1+1D global magnetically driven disk accretion model that captures the essence of the MRI-driven accretion, without resorting to 3D global nonideal magnetohydrodynamic (MHD) simulations. The gas dynamics is assumed to be solely controlled by the MRI and hydrodynamic instabilities. For given stellar and disk parameters, the Shakura–Sunyaev viscosity parameter, α, is determined self-consistently under the adopted framework from detailed considerations of the MRI with nonideal MHD effects (Ohmic resistivity and ambipolar diffusion), accounting for disk heating by stellar irradiation, nonthermal sources of ionization, and dust effects on the ionization chemistry. Additionally, the magnetic field strength is numerically constrained to maximize the MRI activity. Results. We demonstrate the use of our framework by investigating steady-state MRI-driven accretion in a fiducial protoplanetary disk model around a solar-type star. We find that the equilibrium solution displays no pressure maximum at the dead zone outer edge, except if a sufficient amount of dust particles has accumulated there before the disk reaches a steady-state accretion regime. Furthermore, the steady-state accretion solution describes a disk that displays a spatially extended long-lived inner disk gas reservoir (the dead zone) that accretes a few times 10−9 M⊙ yr−1. By conducting a detailed parameter study, we find that the extent to which the MRI can drive efficient accretion is primarily determined by the total disk gas mass, the representative grain size, the vertically integrated dust-to-gas mass ratio, and the stellar X-ray luminosity. Conclusions. A self-consistent time-dependent coupling between gas, dust, stellar evolution models, and our general framework on million-year timescales is required to fully understand the formation of dead zones and their potential to trap dust particles.

How a Realistic Magnetosphere Alters the Polarizations of Surface, Fast Magnetosonic, and Alfvén Waves.

System‐scale magnetohydrodynamic (MHD) waves within Earth's magnetosphere are often understood theoretically using box models. While these have been highly instructive in understanding many fundamental features of the various wave modes present, they neglect the complexities of geospace such as the inhomogeneities and curvilinear geometries present. Here, we show global MHD simulations of resonant waves impulsively excited by a solar wind pressure pulse. Although many aspects of the surface, fast magnetosonic (cavity/waveguide), and Alfvén modes present agree with the box and axially symmetric dipole models, we find some predictions for large‐scale waves are significantly altered in a realistic magnetosphere. The radial ordering of fast mode turning points and Alfvén resonant locations may be reversed even with monotonic wave speeds. Additional nodes along field lines that are not present in the displacement/velocity occur in both the perpendicular and compressional components of the magnetic field. Close to the magnetopause, the perpendicular oscillations of the magnetic field have the opposite handedness to the velocity. Finally, widely used detection techniques for standing waves, both across and along the field, can fail to identify their presence. We explain how all these features arise from the MHD equations when accounting for a non‐uniform background field and propose modified methods that might be applied to spacecraft observations.

Mesospheric ionization during substorm growth phase

Many studies have been conducted about the impact of energetic charged particles on the atmosphere during geomagnetically active times, while quiet time effects are poorly understood. We identified two energetic electron precipitation (EEP) events during the growth phase of moderate substorms and estimated the mesospheric ionization rate for an EEP event for which the most comprehensive dataset from ground-based and space-born instruments was available. The mesospheric ionization signature reached below 70 km altitude and continued for ~15 min until the substorm onset, as observed by the PANSY radar and imaging riometer at Syowa Station in the Antarctic region. We also used energetic electron flux observed by the Arase and POES 15 satellites as the input for the air-shower simulation code PHITS to quantitatively estimate the mesospheric ionization rate. The calculated ionization level due to the precipitating electrons is consistent with the observed value of cosmic noise absorption. The possible spatial extent of EEP is estimated to be ~8 h MLT in longitude and ~1.5° in latitude from a global magnetohydrodynamic simulation REPPU and the precipitating electron observations by the POES satellite, respectively. Such a significant duration and spatial extent of EEP events suggest a non-negligible contribution of the growth phase EEP to the mesospheric ionization. Combining the cutting-edge observations and simulations, we shed new light on the space weather impact of the EEP events during geomagnetically quiet times, which is important to understand the possible link between the space environment and climate.

Kelvin-Helmholtz Instability Associated With Reconnection and Ultra Low Frequency Waves at the Ground: A Case Study

The Kelvin-Helmholtz instability (KHI) and its effects relating to the transfer of energy and mass from the solar wind into the magnetosphere remain an important focus of magnetospheric physics. One such effect is the generation of Pc4-Pc5 ultra low frequency (ULF) waves (periods of 45–600 s). On July 3, 2007 at ∼ 0500 magnetic local time the Cluster space mission encountered Pc4 frequency Kelvin-Helmholtz waves (KHWs) at the high latitude magnetopause with signatures of persistent vortices. Such signatures included bipolar fluctuations of the magnetic field normal component associated with a total pressure increase and rapid change in density at vortex edges; oscillations of magnetosheath and magnetospheric plasma populations; existence of fast-moving, low-density, mixed plasma; quasi-periodic oscillations of the boundary normal and an anti-phase relation between the normal and parallel components of the boundary velocity. The event occurred during a period of southward polarity of the interplanetary magnetic field according to the OMNI data and THEMIS observations at the subsolar point. Several of the KHI vortices were associated with reconnection indicated by the Walén relation, the presence of deHoffman-Teller frames, field-aligned ion beams observed together with bipolar fluctuations in the normal magnetic field component, and crescent ion distributions. Global magnetohydrodynamic simulation of the event also resulted in KHWs at the magnetopause. The observed KHWs associated with reconnection coincided with recorded ULF waves at the ground whose properties suggest that they were driven by those waves. Such properties were the location of Cluster’s magnetic foot point, the Pc4 frequency, and the solar wind conditions.

Global Non-ideal Magnetohydrodynamic Simulations of Protoplanetary Disks with Outer Truncation

Abstract It has recently been established that the evolution of protoplanetary disks is primarily driven by magnetized disk winds, requiring a large-scale magnetic flux threading the disks. The size of such disks is expected to shrink with time, as opposed to the conventional scenario of viscous expansion. We present the first global 2D non-ideal magnetohydrodynamic simulations of protoplanetary disks that are truncated in the outer radius, aiming to understand the interaction of the disk with the interstellar environment, as well as the global evolution of the disk and magnetic flux. We find that as the system relaxes, the poloidal magnetic field threading the disk beyond the truncation radius collapses toward the midplane, leading to a rapid reconnection. This process removes a substantial amount of magnetic flux from the system and forms closed poloidal magnetic flux loops encircling the outer disk in quasi-steady state. These magnetic flux loops can drive expansion beyond the truncation radius, corresponding to substantial mass loss through a magnetized disk outflow beyond the truncation radius analogous to a combination of viscous spreading and external photoevaporation. The magnetic flux loops gradually shrink over time, the rates of which depend on the level of disk magnetization and the external environment, which eventually governs the long-term disk evolution.

The Role of Mesoscale Plasma Sheet Dynamics in Ring Current Formation

During geomagnetically active periods ions are transported from the magnetotail into the inner magnetosphere and accelerated to energies of tens to hundreds of keV. These energetic ions, of mixed composition with the most important species being H+ and O+, become the dominant source of plasma pressure in the inner magnetosphere. Ion transport and acceleration can occur at different spatial and temporal scales ranging from global quasi-steady convection to localized impulsive injection events and may depend on the ion gyroradius. In this study we ascertain the relative importance of mesoscale flow structures and the effects of ion non-adiabaticity on the produced ring current. For this we use: global magnetohydrodynamic (MHD) simulations to generate self-consistent electromagnetic fields under typical driving conditions which exhibit bursty bulk flows (BBFs); and injected test particles, initialized to match the plasma moments of the MHD simulation, and subsequently evolved according to the kinetic equations of motion. We show that the BBFs produced by our simulation reproduce thermodynamic and magnetic statistics from in situ measurements and are numerically robust. Mining the simulation data we create a data set, over a billion points, connecting particle transport to characteristics of the MHD flow. From this we show that mesoscale bubbles, localized depleted entropy regions, and particle gradient drifts are critical for ion transport. Finally we show, using identical particle ensembles with varying mass, that O+ non-adiabaticity creates qualitative differences in energization and spatial distribution while H+ non-adiabaticity has non-negligible implications for loss timescales.

Shock Properties and Associated Characteristics of Solar Energetic Particles in the 2017 September 10 Ground-level Enhancement Event

Abstract The solar eruption on 2017 September 10 was accompanied by a fast coronal mass ejection (∼3000 km s−1) and produced a ground-level enhancement (GLE) event at Earth. Multiple-viewpoint remote sensing observations are used to find the three-dimensional (3D) structure of the shock. We determine the shock parameters by combining the 3D shock kinematics and the solar wind properties obtained from a global magnetohydrodynamic (MHD) simulation, in order to compare them with the characteristics of the solar energetic particles (SEPs). We extract the magnetic connectivities of the observers from the MHD simulation and find that L1 was magnetically connected to the shock flank (rather than the nose). Further analysis shows that this shock flank propagates through the heliospheric current sheet (HCS). The weak magnetic field and relatively dense plasma around the HCS result in a large Mach number of the shock, which leads to efficient particle acceleration even at the shock flank. We conclude that the interaction between the shock and HCS provides a potential mechanism for production of the GLE event. The comparison between the shock properties and the characteristics of SEPs suggests an efficient particle acceleration in a wide spatial range by the shock propagating through the highly inhomogeneous coronal medium.

Modeling Kelvin-Helmholtz Instability at the High-Latitude Boundary Layer in a Global Magnetosphere Simulation.

The Kelvin‐Helmholtz instability at the magnetospheric boundary plays a crucial role in solar wind‐magnetosphere‐ionosphere coupling, particle entry, and energization. The full extent of its impact has remained an open question due, in part, to global models without sufficient resolution to capture waves at higher latitudes. Using global magnetohydrodynamic simulations, we investigate an event when the Magnetospheric Multiscale (MMS) mission observed periodic low‐frequency waves at the dawn‐flank, high‐latitude boundary layer. We show the layer to be unstable, even though the slow solar wind with the draped interplanetary magnetic field is seemingly unfavorable for wave generation. The simulated velocity shear at the boundary is thin (∼0.65RE) and requires commensurately high spatial resolution. These results, together with MMS observations, confirm for the first time in fully three‐dimensional global geometry that KH waves can grow in this region and thus can be an important process for energetic particle acceleration, dynamics, and transport.

Drift Orbit Bifurcations and Cross‐Field Transport in the Outer Radiation Belt: Global MHD and Integrated Test‐Particle Simulations

AbstractEnergetic particle fluxes in the outer magnetosphere present a significant challenge to modeling efforts as they can vary by orders of magnitude in response to solar wind driving conditions. In this study, we demonstrate the ability to propagate test particles through global magnetohydrodynamic (MHD) simulations to a high level of precision and use this to map the cross‐field radial transport associated with relativistic electrons undergoing drift orbit bifurcations (DOBs). The simulations predict DOBs primarily occur within an Earth radius of the magnetopause loss cone and appear significantly different for southward and northward interplanetary magnetic field orientations. The changes to the second invariant are shown to manifest as a dropout in particle fluxes with pitch angles close to and indicate DOBs are a cause of butterfly pitch angle distributions within the night‐time sector. The convective electric field, not included in previous DOB studies, is found to have a significant effect on the resultant long‐term transport, and losses to the magnetopause and atmosphere are identified as a potential method for incorporating DOBs within Fokker‐Planck transport models.

Association of Auroral Streamers and Bursty Bulk Flows During Different States of the Magnetotail: A Case Study

AbstractIn‐situ measurements in the magnetotail are sparse and limited to single points. On the other hand, there is a broad range of observations, including magnetometers, aurora imagers, and radars, in the ionosphere. Since the nightside ionosphere resembles the magnetotail plasma sheet dynamics projection, it can be used to monitor the tail's dynamics. A proper interpretation of the ionosphere and ground observations necessitates understanding the coupling processes between the ionosphere and the magnetosphere. Here, we use the global magnetohydrodynamic simulation model, OpenGGCM‐CTIM‐RCM, to investigate these coupling processes during periods when the magnetic flux is transported through the tail via narrow fast flow channels, typically called bursty bulk flows (BBFs). We consider three relevant states of the magnetotail: immediately before the substorm's onset, during the expansion of the substorm, and during a steady magnetic convection (SMC) event. The number of flow channels increases, and they penetrate closer to the Earth during SMC. The IMF direction influences fast flows' location but not their orientation in the plasma sheet during this event. However, the dimension of fast flows and streamers do not depend on the IMF conditions and state of the magnetosphere.

Interplanetary Shock‐Induced Magnetopause Motion: Comparison Between Theory and Global Magnetohydrodynamic Simulations

AbstractThe magnetopause marks the outer edge of the Earth's magnetosphere and a distinct boundary between solar wind and magnetospheric plasma populations. In this study, we use global magnetohydrodynamic simulations to examine the response of the terrestrial magnetopause to fast‐forward interplanetary shocks of various strengths and compare to theoretical predictions. The theory and simulations indicate the magnetopause response can be characterized by three distinct phases; an initial acceleration as inertial forces are overcome, a rapid compressive phase comprising the majority of the distance traveled, and large‐scale damped oscillations with amplitudes of the order of an Earth radius. The two approaches agree in predicting subsolar magnetopause oscillations with frequencies 2–13 mHz but the simulations notably predict larger amplitudes and weaker damping rates. This phenomenon is of high relevance to space weather forecasting and provides a possible explanation for magnetopause oscillations observed following the large interplanetary shocks of August 1972 and March 1991.