Efficient simulation of plasmas in various contexts often involves the use of meshes that conform to the intrinsic geometry of the system under consideration. We present here a description of a new ...magnetohydrodynamic code, GAMERA (Grid Agnostic MHD for Extended Research Applications), designed to combine geometric flexibility with high-order spatial reconstruction and constrained transport to maintain the divergence-free magnetic field. GAMERA carries on the legacy of its predecessor, the LFM (Lyon-Fedder-Mobarry), a research code whose use in space physics has spanned three decades. At the time of its initial development, the LFM code had a number of novel features: eighth-order centered spatial differencing, the Partial Donor Cell Method limiter for shock capturing, a non-orthogonal staggered mesh with constrained transport, and conservative averaging-reconstruction for axis singularities. The capability to handle multiple ion species was also added later. GAMERA preserves the core numerical philosophy of LFM while also incorporating numerous algorithmic and computational improvements. The upgrades in the numerical schemes include accurate grid metric calculations using high-order Gaussian quadrature techniques, high-order upwind reconstruction, non-clipping options for interface values, and improved treatment of axis singularities. The improvements in the code implementation include the use of data structures and memory access patterns conducive to aligned vector operations and the implementation of hybrid parallelism, using MPI and OMP. GAMERA is designed to be a portable and easy-to-use code that implements multidimensional MHD simulations in arbitrary non-orthogonal curvilinear geometries on modern supercomputer architectures.
Grid-based magnetohydrodynamic (MHD) simulations have proven invaluable for the study of astrophysical accretion disks. However, the fact that angular momentum transport in disks is mediated by MHD ...turbulence (with structure down to very small scales) raises the concern that the properties of the modeled accretion disks are affected by the finite numerical resolution of the simulation. By implementing an orbital advection algorithm into the Athena code in cylindrical geometry, we have performed a set of global (but unstratified) Newtonian disk simulations extending up to resolutions previously unattained. We study the convergence of these models as a function of spatial resolution and initial magnetic field geometry. The usual viscosity parameter ( alpha ) or the ratio of thermal-to-magnetic pressure ( beta ) is found to be a poor diagnostic of convergence, whereas the average tilt angle of the magnetic field in the (r, phi)-plane is a very good diagnostic of convergence. We suggest that this is related to the saturation of the MHD turbulence via parasitic modes of the magnetorotational instability. Even in the case of zero-net magnetic flux, we conclude that our highest resolution simulations (with 32 zones and 64 zones per vertical scale height) have achieved convergence. Our global simulations reach resolutions comparable to those used in local, shearing-box models of MHD disk turbulence. We find that the saturation predictors derived from local simulations correspond well to the instantaneous correlations between local flux and stress found in our global simulations. However, the conservation of magnetic flux implicit in local models is not realized in our global disks. Thus, the magnetic connectivity of an accretion disk represents physics that is truly global and cannot be captured in any ab initio local model.
During geomagnetic storms the intensities of the outer radiation belt electron population can exhibit dramatic variability. Deep depletions in intensity during the main phase are followed by ...increases during the recovery phase, often to levels that significantly exceed their prestorm values. To study these processes, we simulate the evolution of the outer radiation belt during the 17 March 2013 geomagnetic storm using our newly developed radiation belt model (Conservative Hamiltonian Integrator for Magnetospheric Particles) based on test particle and coupled 3‐D ring current and global magnetohydrodynamic (MHD) simulations, and driven solely with solar wind and F10.7 flux data. Our approach differs from previous work in that we use MHD information to identify regions of strong, bursty, and azimuthally localized earthward convection in the magnetotail where test particles are then seeded. We validate our model using in situ Van Allen Probe electron intensities over a multiday period and show that our model is able to reproduce meaningful qualitative and quantitative agreement. Analysis of our model enables us to study the processes that govern the transition from the prestorm to poststorm outer belt. Our analysis demonstrates that during the early main phase of the storm the preexisting outer belt is largely wiped out via magnetopause losses, and subsequently, a new outer belt is created during a handful of discrete, mesoscale injections. Finally, we demonstrate the potential importance of magnetic gradient trapping in the transport and energization of outer belt electrons using a controlled numerical experiment.
Plain Language Summary
During geomagnetic storms the intensity of the outer radiation belt, a population of high‐energy electrons trapped within the near‐Earth space environment, can vary by a factor of 1,000 over a day. Often observed is the near disappearance of the belt at the onset of the storm and its intensification in the aftermath. Not yet understood is the relationship between the prestorm and poststorm belt and the mechanisms that create the more energetic poststorm electrons or equivalently: are the electrons that disappeared the ones that came back? and where did this new energy come from? We introduce our new radiation belt model, Conservative Hamiltonian Integrator for Magnetospheric Particles, and study the 2013 St. Patrick's Day Storm to address these questions. We find that at the onset of the storm the existing belt is annihilated and swept into interplanetary space and that during the course of the storm a new, stronger belt is created. Traditional thinking on belt energization has focused on processes acting on either individual electrons or the global magnetosphere. We show that there is room in the middle by demonstrating, using our high‐resolution simulations, that electrons can become trapped and energized within bursty, confined flow channels that erupt near Earth during the course of the storm.
Key Points
Our radiation belt model broadly reproduces observed electron intensity throughout the 17 March 2013 storm
During the storm the existing belt is lost and rebuilt via a handful of intense injections
Mesoscale structure in the inner magnetosphere can play a role in electron transport and energization
We show the possible existence of a significant axial asymmetry in the reconnection separator at the Jovian magnetopause using first‐principle, physics‐based global simulations. Under eastward ...interplanetary magnetic field (IMF) conditions near Jupiter's orbit, reconnection occurs at the southern‐dusk and northern magnetopause with large shear angles between the magnetospheric and magnetosheath magnetic fields. When driven by the westward IMF, the reconnection position switches to the northern‐dusk and southern magnetopause. Component reconnection at the southern‐dusk/northern‐dusk magnetopause is associated with the interaction of the IMF with the nearly‐dipolar background fields. Nearly‐antiparallel reconnection near the noon‐midnight plane at the northern/southern magnetopause is related to the dawn‐dusk asymmetric, helical, closed lobe magnetic fields, which is a consequence of significant planetary corotation effects and not expected at Earth. Such configuration is testable as Juno has proceeded its orbit to the high‐altitude cusps and provides new insight into the interpretation of measurements from other rotationally‐driven systems.
Plain Language Summary
Magnetic reconnection is a fundamental plasma process for understanding the dynamic evolution of planetary magnetospheres. It is related to the breaking and reconnecting of magnetic fields from two different sources. Depending on the orientation of the interplanetary magnetic field (IMF), reconnection at a planetary magnetopause is a combination of antiparallel and component reconnection of magnetic fields in the outer magnetosphere and the inner magnetosheath. In the terrestrial magnetosphere, magnetopause reconnection that is dominated by solar wind‐magnetosphere interactions is generally symmetric about the Sun‐Earth axis in solar‐magnetospheric coordinates regardless of the IMF orientation. However, this axial symmetry may not exist in planetary magnetospheres where the time scale of planetary rotation is much shorter than the solar wind transition, especially at Jupiter that has a faster planetary rotation and a significantly larger magnetosphere than Earth's. This study focuses on an unusual asymmetry in magnetospheric configuration and magnetopause reconnection at Jupiter and its relationship with fast planetary rotation.
Key Points
Unlike at Earth, magnetopause reconnection at Jupiter may exhibit a significant axial asymmetry under east‐west interplanetary magnetic field (IMF) conditions
This asymmetry is caused by the helical, closed lobe magnetic field with dawn‐dusk asymmetry due to fast Jovian rotation
Reconnection may occur at northern and southern‐dusk magnetopause under eastward IMF and has a north‐south reversal for westward IMF
When matter orbits around a central mass obliquely with respect to the mass's spin axis, the Lense-Thirring effect causes it to precess at a rate declining sharply with radius. Ever since the work of ...Bardeen & Petterson, it has been expected that when a fluid fills an orbiting disk, the orbital angular momentum at small radii should then align with the mass's spin. Nearly all previous work has studied this alignment under the assumption that a phenomenological "viscosity" isotropically degrades fluid shears in accretion disks, even though it is now understood that internal stress in flat disks is due to anisotropic MHD turbulence. In this paper we report a pair of matched simulations, one in MHD and one in pure (non-viscous) HD in order to clarify the specific mechanisms of alignment. As in the previous work, we find that disk warps induce radial flows that mix angular momentum of different orientation; however, we also show that the speeds of these flows are generically transonic and are only very weakly influenced by internal stresses other than pressure. In particular, MHD turbulence does not act in a manner consistent with an isotropic viscosity. When MHD effects are present, the disk aligns, first at small radii and then at large; alignment is only partial in the HD case. We identify the specific angular momentum transport mechanisms causing alignment and show how MHD effects permit them to operate more efficiently. Last, we relate the speed at which an alignment front propagates outward (in the MHD case) to the rate at which Lense-Thirring torques deliver angular momentum at smaller radii.
Formation of Alfvén Wave Ducts by Magnetotail Flow Bursts Yang, Ziyi; Zhang, Binzheng; Lotko, William ...
Journal of geophysical research. Space physics,
September 2022, 2022-09-00, 20220901, Letnik:
127, Številka:
9
Journal Article
Recenzirano
Geomagnetic 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.
Plain Language Summary
Geomagnetic activities usually produce Alfvén waves in Earth's outer magnetosphere. In simplest form, an Alfvén wave behaves as a wave on a transmission line with magnetic field lines serving as the lines. Injections of Alfvénic power into Earth's ionosphere cause spectacular dynamic aurora. In this study, we present the discovery of a new phenomenon revealed in global geospace simulations—formation of Alfvén wave ducts. Their occurrence explains the efficient transmission of electromagnetic power from Earth's magnetotail to its ionosphere and atmosphere to produce dynamic aurora as observed in the past. More importantly, this study serves as a new foundation of powering terrestrial (and outer planetary) aurora with global magnetic field, from the view of large‐scale electromagnetic wave energy transmission.
Key Points
We discovered a new phenomenon revealed in global geospace simulations—formation of Alfvén wave ducts
The suggested Alfvén ducts promote low‐loss transmission of equatorial Alfvénic power to the near‐Earth space
Alfvén ducts elegantly explain the different propagation efficiencies of Alfvénic power in the magnetosphere
In cylindrical/spherical geometries, MHD solvers using explicit finite-volume methods face restrictive time steps imposed by the CFL condition due to the clustering of cells in the azimuthal ...direction near the pole/axis. We use a conservative averaging-reconstruction method (Ring Average) on structured cylindrical/spherical mesh to remove this severe time step restriction for multi-dimensional curvilinear finite-volume MHD solvers. The Ring Average technique is implemented as a post-processing step and thus requires no changes to the existing data structure, grid definition or numerical methods in the original MHD solver. This paper describes the Ring Average algorithm and presents simulation results using field loop advection and MHD blast waves in cylindrical/spherical geometries for validation. The algorithm is shown to be inexpensive and easily implemented in existing curvilinear finite-volume MHD solvers.
•Ring Average is a “post-processing” module without modifying the original solver.•Ring Average relaxes time-step restrictions in MHD solvers with axis singularities.•Ring Average conserves mass, momentum and energy for proper shock propagations.•Ring Average works for codes with either divergence cleaning or constrained transport.
The role of diffuse electron precipitation in the formation of subauroral polarization streams (SAPS) is investigated with the Multiscale Atmosphere‐Geospace Environment (MAGE) model. Diffuse ...precipitation is derived from the distribution of drifting electrons. SAPS manifest themselves as a separate mesoscale flow channel in the duskside ionosphere, which gradually merges with the primary auroral convection toward dayside as the equatorward auroral boundary approaches the poleward Region‐2 field‐aligned currents (FACs) boundary. SAPS expand to lower latitudes and toward the nightside during the main phase of a geomagnetic storm, associated with magnetotail earthward plasma flows building up the ring current and intensifying Region‐2 FACs and electron precipitation. SAPS shrink poleward and sunward as the interplanetary magnetic field turns northward. When diffuse precipitation is turned off in a controlled MAGE simulation, ring current and duskside Region‐2 FACs become weaker, but subauroral zonal ion drifts are still comparable to auroral convection. However, subauroral and auroral convection manifest as a single broad flow channel without showing any mesoscale structure. SAPS overlap with the downward Region‐2 FACs equatorward of diffuse precipitation, where poleward electric fields are strong due to a low conductance in the subauroral ionosphere. The Region‐2 FACs extend to latitudes lower than the diffuse precipitation because the ring current protons penetrate closer to the Earth than the electrons do. This study reproduces the key physics of SAPS formation and their evolution in the coupled magnetosphere‐ionosphere during a geomagnetic storm. Diffuse electron precipitation is demonstrated to play a critical role in determining SAPS location and structure.
Plain Language Summary
Subauroral polarization streams (SAPS) are a mesoscale (∼100–500 km) plasma flow channel frequently observed in the duskside subauroral ionosphere. This study investigates how diffuse electron precipitation affects the location and structure of SAPS, using the newly developed, state‐of‐the‐art geospace model of Multiscale Atmosphere‐Geospace Environment (MAGE). The MAGE model has the capability to directly simulate diffuse precipitation using particle drift physics. MAGE numerical experiments show that SAPS exhibit as a separate flow channel when diffuse precipitation is included in the simulation, but subauroral and auroral convections become one broad channel when diffuse precipitation is turned off. SAPS are produced in the gap region between the low latitude boundaries of electron aurora and downward field‐aligned current (FAC) on the duskside, where the ionospheric conductance is low due to lack of ionization while downward region‐2 FAC closes through this low conductance region. Strong poleward electric fields are generated to drive large westward ion drifts in the SAPS channel. Tracing back to the magnetosphere, the gap between the inner boundaries is formed because the ring current protons, whose distribution primarily determines the downward FAC, penetrate deeper than the electrons. Thus diffuse aurora, with plasma sheet as the source population, occurs poleward of the lower boundary of the downward Region‐2 current. This study demonstrates the importance of including diffuse precipitation in coupled geospace models to understand the dynamics of SAPS.
Key Points
A fully coupled geospace model captures the key physical mechanism and distinct spatial structure of subauroral polarization streams
The separation of the flow channel from the main auroral convection is determined by diffuse electron precipitation boundary
The evolution of the SAPS in the ionosphere and magnetosphere is reproduced by the simulation in accordance with observations
Plasma convection in the Earth's magnetosphere from the distant magnetotail to the inner magnetosphere occurs largely in the form of mesoscale flows, i.e., discrete enhancements in the plasma flow ...with sharp dipolarizations of magnetic field. Recent spacecraft observations suggest that the dipolarization flows are associated with a wide range of kinetic processes such as kinetic Alfvén waves, whistler-mode waves, and nonlinear time-domain structures. In this paper we explore how mesoscale dipolarization flows produce suprathermal electron instabilities, thus providing free energy for the generation of the observed kinetic waves and structures. We employ three-dimensional test-particle simulations of electron dynamics one-way coupled to a global magnetospheric model. The simulations show rapid growth of interchanging regions of parallel and perpendicular electron temperature anisotropies distributed along the magnetic terrain formed around the dipolarization flows. Unencumbered in test-particle simulations, a rapid growth of velocity-space anisotropies in the collisionless magnetotail plasma is expected to be curbed by the generation of plasma waves. The results are compared with in situ observations of an isolated dipolarization flow at one of the Magnetospheric Multiscale Mission spacecraft. The observations show strong wave activity alternating between broad-band wave activity and whistler waves. With estimated spatial extent being similar to the characteristic size of the temperature anisotropy patches in our test-particle simulations, the observed bursts of the wave activity are likely to be produced by the parallel and perpendicular electron energy anisotropies driven by the dipolarization flow, as suggested by our modeling results.
When solving hydrodynamic equations in spherical or cylindrical geometry using explicit finite-difference schemes, a major difficulty is that the time step is greatly restricted by the clustering of ...azimuthal cells near the pole due to the Courant–Friedrichs–Lewy condition. This paper adapts the azimuthal averaging–reconstruction (ring average) technique to finite-difference schemes in order to mitigate the time step constraint in spherical and cylindrical coordinates. The finite-difference ring average technique averages physical quantities based on an effective grid and then reconstructs the solution back to the original grid in a piecewise, monotonic way. The algorithm is implemented in a community upper-atmospheric model, the Thermosphere–Ionosphere Electrodynamics General Circulation Model (TIEGCM), with a horizontal resolution up to 0.625∘×0.625∘ in geographic longitude–latitude coordinates, which enables the capability of resolving critical mesoscale structures within the TIEGCM. Numerical experiments have shown that the ring average technique introduces minimal artifacts in the polar region of general circulation model (GCM) solutions, which is a significant improvement compared to commonly used low-pass filtering techniques such as the fast Fourier transform method. Since the finite-difference adaption of the ring average technique is a post-solver type of algorithm, which requires no changes to the original computational grid and numerical algorithms, it has also been implemented in much more complicated models with extended physical–chemical modules such as the Coupled Magnetosphere–Ionosphere–Thermosphere (CMIT) model and the Whole Atmosphere Community Climate Model with thermosphere and ionosphere eXtension (WACCM-X). The implementation of ring average techniques in both models enables CMIT and WACCM-X to perform global simulations with a much higher resolution than that used in the community versions. The new technique is not only a significant improvement in space weather modeling capability, but it can also be adapted to more general finite-difference solvers for hyperbolic equations in spherical and polar geometries.
Highlights.
The ring average technique is adapted to solve the issue of clustered grid cells in polar and spherical coordinates with a finite-difference method. The ring average technique is applied to develop a 0.625∘×0.625∘ high-resolution TIEGCM and more complicated geoscientific models with polar and spherical coordinates as well as finite-difference numerical schemes. The high-resolution TIEGCM shows good capability in resolving mesoscale structures in the ionosphere–thermosphere (I–T) system.
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