Multifluid Magnetohydrodynamics Research Articles

Effects of an Intrinsic Magnetic Field on Ion Escape From Ancient Mars Based on MAESTRO Multifluid MHD Simulations

AbstractIon escape has played a key role in atmospheric loss on ancient Mars due to intense solar activity. Under the existence of a strong global intrinsic magnetic field, as expected on ancient Mars, differential flows between different ion species can be important for ion escape. To assess effects of differential flows, we here developed a new global multifluid magnetohydrodynamics model with a cubed sphere grid named Multifluid Atmospheric Escape Simulations Toward Real elucidatiOn (MAESTRO). A test simulation under present‐day Mars conditions showed solar wind‐Mars interactions, for example, plasma boundaries, ionospheric profiles, and tailward outflows, consistent with observations and simulation studies. We then conducted multifluid and multispecies simulations with six different dipole field strengths under ancient Mars conditions. The multifluid cases show asymmetric outflow distributions with respect to the solar wind convection electric field, as pointed out by previous studies on present‐day Mars. Compared with multispecies cases, the multifluid representation increases the escape rates of O2+ and CO2+ by more than two orders of magnitude in the strong dipole field cases where the cusp outflow is dominant. The O+ escape rate is slightly lower in the multifluid cases with no or weak dipole field due to suppression of ion pickup in the −E hemisphere, while it is reduced by one order of magnitude in the strongest dipole field case. The existence of a dipole field can reduce the total escape rate by a factor of six. The impact on atmospheric evolution is diminished but still significant.

Magnetic Field Enhancements in the Solar Wind: Diverse Processes Manifesting a Uniform Observation Type?

AbstractWithin the solar wind throughout the inner heliosphere, observations reveal the presence of magnetic field enhancements accompanied by thin current sheets at varying distances from the sun and across different longitudes and latitudes. Two primary explanations have been proposed to elucidate these phenomena: Solar wind‐dust interaction and interlacing flux ropes. In this study, we employ multi‐fluid Magnetohydrodynamics (MHD) and Hall MHD models to simulate these hypotheses, respectively. Our findings indicate a concurrence between both models and the observed phenomena, suggesting that both processes may result in the same kind of enhancement. Furthermore, both models make predictions pointing to additional types of observational data, occurring at distinct spatial or temporal stages of the interaction. This convergence of model predictions with empirical data underscores the need for further observational and modeling studies to comprehensively test these models. This research enhances our knowledge of the inner heliosphere's dynamics and the influence of the solar wind on the Earth's magnetosphere, thereby shedding light on critical aspects of space weather and its potential impact on our planet.

Momentum Sources in Multifluid MHD and Their Relation to the Geopauses

AbstractIn Trung et al. (2019b, https://doi.org/10.1029/2019ja026636), we compared the geopause, a surface where the ionospheric plasma and solar wind plasma has equal parameters (by mass density, number density, or pressure), to the magnetopause to show that there was a transition in physics governing the motion of the plasmas between the different boundaries. In this study, we use multifluid magnetohydrodynamics simulation data to examine the relation of the source terms in the momentum equation and to the geopauses. We find that the geopauses do represent a transition in the physics governing the motion of the plasmas. While friction does not play a dominant role in the motion of the heliospheric and ionospheric plasmas, we find that friction shows indirectly that the ionospheric plasma can travel faster than the heliospheric plasma. Further satellite data is needed to verify the possibility of ionospheric plasma reaching speeds exceeding the solar wind.

A New Tool for Understanding the Solar Wind–Venus Interaction: Three-dimensional Multifluid MHD Model

In this paper, we present a new tool to investigate the interaction of the solar wind with Venus with the approach of a global multifluid magnetohydrodynamics (MHD) model. The continuity, momentum, and energy equations for H+, O+, , and are solved self-consistently together with Faraday’s law. The photochemistry of ionospheric ions are considered as the source term in the density, momentum, and energy equations for each ion. We found that the simulated ionospheric density, temperature, and the bow shock location are consistent with previous observations and simulations for both the solar maximum and minimum. The simulated magnetic fields also agree well with the Venus Express observations. Meanwhile, the high-resolving power and low numerical diffusion makes the model capable of capturing the fine structures of the Venusian-induced magnetosphere, such as the Kelvin–Helmholtz instability and the nightside wake. The escape rates have also been estimated and the results are similar to previous estimations. The high-resolution model could be an efficient tool for the exploration of the fine structures of the Venusian space environment system, and also for the application to other unmagnetized planets.

Oxygen Ion Escape at Venus Associated With Three‐Dimensional Kelvin‐Helmholtz Instability

AbstractHow oxygens escape from Venus has long been a fundamental but controversial topic in the planetary research. Among various key mechanisms, the Kelvin‐Helmholtz instability (KHI) has been suggested to play an important role in the oxygen ion escape from Venus. Limited by either scarce in‐situ observations or simplified theoretical estimations, the mystery of oxygen ion escape process associated with KHI is still unsettled. Here we present the first three‐dimensional configuration of KHI at Venus with a global multifluid magnetohydrodynamics model, showing a significantly fine structure and evolution of the KHI. KHI mainly occurred at the low latitude boundary layer if defining the interplanetary magnetic field‐perpendicular plane as the equatorial plane, resulting in escaping oxygen ions through mixing with the solar wind at the Venusian boundary layer, with an escape rate around 4 × 1024 s−1. The results provide new insights into the basic physical process of atmospheric loss at other unmagnetized planet.

Global Ambipolar Potentials and Electric Fields at Mars Inferred From MAVEN Observations

AbstractThe motion of charged particles is governed by electromagnetic forces at high altitudes at Mars and thus the characterization of electrostatic potential and electric fields is important for understanding ion escape at Mars. In this study, we utilize measurements from the Mars Atmosphere and Volatile EvolutioN mission to derive electrostatic potentials above the collisional atmosphere at Mars. We find averaged potentials to be up to ∼100 V in the magnetosheath and down to ∼−70 V in the tail, with respect to the upstream. We then derive electric fields based on averaged potential maps, ranging V/km. These data‐derived electric fields are in good agreement with ambipolar electric fields from a multi‐fluid magnetohydrodynamic (MHD) model. MHD results also reveal that these large electric fields mainly originate from the electron pressure gradient in the magnetosheath and in the transition region from the hot solar wind flow to the cold ionospheric flow. This work provides the first data‐based characterization of global ambipolar electric fields at Mars (outside of the main ionosphere).

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.

Effects of a dipole-like crustal field on solar wind interaction with Mars

A three-dimensional four species multi-fluid magnetohydrodynamic (MHD) model was constructed to simulate the solar wind global interaction with Mars. The model was augmented to consider production and loss of the significant ion species in the Martian ionosphere, i.e., H+, O2+, O+, CO2+, associated with chemical reactions among all species. An ideal dipole-like local crustal field model was used to simplify the empirically measured Martian crustal field. Results of this simulation suggest that the magnetic pile-up region (MPR) and the velocity profile in the meridian plane are asymmetric, which is due to the nature of the multi-fluid model to decouple individual ion velocity resulting in occurrence of plume flow in the northern Martian magnetotail. In the presence of dipole magnetic field model, boundary layers, such as bow shock (BS) and magnetic pile-up boundary (MPB), become protuberant. Moreover, the crustal field has an inhibiting effect on the flux of ions escaping from Mars, an effect that occurs primarily in the region between the terminator (SZA 90°) and the Sun–Mars line of the magnetotail (SZA 180°), partially around the terminator region. In contrast, near the tailward central line the crustal field has no significant impact on the escaping flux.

An Extended MHD Study of the 16 October 2015 MMS Diffusion Region Crossing

AbstractThe Magnetospheric Multiscale (MMS) mission has given us unprecedented access to high cadence particle and field data of magnetic reconnection at Earth's magnetopause. MMS first passed very near an X‐line on 16 October 2015, the Burch event, and has since observed multiple X‐line crossings. Subsequent 3‐D particle‐in‐cell (PIC) modeling efforts of and comparison with the Burch event have revealed a host of novel physical insights concerning magnetic reconnection, turbulence‐induced particle mixing, and secondary instabilities. In this study, we employ the Gkeyll simulation framework to study the Burch event with different classes of extended, multifluid magnetohydrodynamics (MHD), including models that incorporate important kinetic effects, such as the electron pressure tensor, with physics‐based closure relations designed to capture linear Landau damping. Such fluid modeling approaches are able to capture different levels of kinetic physics in global simulations and are generally less costly than fully kinetic PIC. We focus on the additional physics one can capture with increasing levels of fluid closure refinement via comparison with MMS data and existing PIC simulations. In particular, we find that the ten‐moment model well captures the agyrotropic structure of the pressure tensor in the vicinity of the X‐line and the magnitude of anisotropic electron heating observed in MMS and PIC simulations. However, the ten‐moment model is found to have difficulty resolving the lower hybrid drift instability, which plays a fundamental role in heating and mixing electrons in the current layer.

Multispecies and Multifluid MHD Approaches for the Study of Ionospheric Escape at Mars

AbstractA detailed model‐model comparison between the results provided by a multispecies and a multifluid magnetohydrodynamic (MHD) code for the escape of heavy ions in the Martian‐induced magnetosphere is presented. The results from the simulations are analyzed and compared against a statistical analysis of the outflow of heavy ions obtained by the Mars Atmosphere and Volatile EvolutioN/Suprathermal and Thermal Ion Composition instrument over an extended period of time in order to estimate the influence of magnetic forces in the ion escape. Both MHD models are run with the same chemical reactions and ion species in a steady state mode under idealized solar conditions. Apart from being able to reproduce the asymmetries observed in the ion escape, it is found that the multifluid approach provides results that are closer to those inferred from the ion data. It is also found that the j × B force term is less effective in accelerating the ions in the models when compared with the Mars Atmosphere and Volatile EvolutioN results. Finally, by looking at the contribution of the plume and the ion escape rates at different distances along the tail with the multifluid model, it is also found that the escape of heavy ions has important variabilities along the tail, meaning that the apoapsis of a spacecraft studying atmospheric escape can affect the estimates obtained.

Interaction of magnetospheric Alfvén waves with the ionosphere in the Pc1 frequency band

AbstractA characteristic feature of the upper ionosphere is the occurrence of the ionospheric Alfvén resonator (IAR) and MHD waveguide, which can trap electromagnetic wave energy in the range from fractions of a Hz to a few Hz. This wave trapping ensures the strong dependence of the ionospheric transmission/reflective properties on frequency. We have developed a numerical model of the magnetospheric Alfvén wave interaction with the ionosphere and transmission to the ground based on the solution of multifluid magnetohydrodynamic (MHD) full wave equations in a realistic ionosphere, whose parameters were reconstructed from the International Reference Ionosphere model. The MHD modes are coupled owing to the frequency‐dependent Hall conductivity and geomagnetic field line inclination. This model can be applied to the interpretation of the spectral structure of electromagnetic emissions of the magnetospheric origin in the band 0.1–10 Hz observed at various latitudes. The model predicts that the upper part of the ULF spectrum (f > 1 Hz) will be severely absorbed upon wave transmission through the daytime ionosphere to the ground. At nighttime the transmission coefficient of Alfvén waves has an oscillatory dependence on frequency, with “transmission windows” at the lowest IAR eigenfrequencies (<3 Hz). At higher frequencies (3–10 Hz) the reflection/transmission coefficients are dominated by periodic scale‐dependent modulation owing to the waveguide modes. Broad maxima/minima of the transmission coefficient are determined by the phasing between Alfvén and fast magnetosonic waves at the bottom of the ionosphere.

Magnetic field evolution in superconducting neutron stars

The presence of superconducting and superfluid components in the core of mature neutron stars calls for the rethinking of a number of key magnetohydrodynamical notions like resistivity, the induction equation, magnetic energy and flux-freezing. Using a multi-fluid magnetohydrodynamics formalism, we investigate how the magnetic field evolution is modified when neutron star matter is composed of superfluid neutrons, type-II superconducting protons and relativistic electrons. As an application of this framework, we derive an induction equation where the resistive coupling originates from the mutual friction between the electrons and the vortex/fluxtube arrays of the neutron and proton condensates. The resulting induction equation allows the identification of two timescales that are significantly different from those of standard magnetohydrodynamics. The astrophysical implications of these results are briefly discussed.

The role of solar wind electrons at the solar wind termination shock

Abstract Voyager 2 plasma observations have recently revealed that, as predicted by theory, there exists a solar wind termination shock at 87 au. However, it is evidently different from classical expectations, for instance revealing the downstream solar wind protons still to be in a supersonic mode. In this paper we show that in order to explain the non-classical structure and facts of this shock, one has to start from a multi-fluid magnetohydrodynamic (MHD) approach describing the shock transition. Different from our earlier attempt, here we consider solar wind electrons as an additional extra fluid and allow for preferential heating of electrons compared to thermal solar wind protons at the shock. As we can then show, with this enlargement of the MHD theory by a separate electron fluid we are able to describe observed features of the solar wind termination shock. We arrive at the conclusion that the downstream solar wind thermal plasma is energetically dominated by the pressure of quasi-mass-less electrons.

Solution of Magnetohydrodynamic Oscillations in Electrolytes with Ion-Neutral Collisions

The single-fluid Magnetohydrodynamic (MHD) equations for electrolytes in the presence of a magnetic field are derived from multi-fluid MHD equations with ion-neutral collisions in partially ionized conductive fluids. The dispersion relationship of MHD waves is also investigated, which is different from that for plasmas or liquid metals. Based on the equations, we find that MHD waves are dispersive in electrolytes, and the critical frequencies for excitation of Alfven waves vary with magnetic field or conductivity, so the exciting of MHD waves is severely restricted in electrolytes with relatively low conductivity except at an extremely low frequency or when it is permeated by a considerably strong ambient magnetic field. These theories are applied to seawater to estimate the magnetic field vibration caused by the large-scale motion of seawater (e.g., ocean currents or tides). It is found that high frequency waves are dampened severely in seawater, while low frequency waves can propagate over a long distance without much attenuation.

Adaptive numerical algorithms in space weather modeling

Space weather describes the various processes in the Sun–Earth system that present danger to human health and technology. The goal of space weather forecasting is to provide an opportunity to mitigate these negative effects. Physics-based space weather modeling is characterized by disparate temporal and spatial scales as well as by different relevant physics in different domains. A multi-physics system can be modeled by a software framework comprising several components. Each component corresponds to a physics domain, and each component is represented by one or more numerical models. The publicly available Space Weather Modeling Framework (SWMF) can execute and couple together several components distributed over a parallel machine in a flexible and efficient manner. The framework also allows resolving disparate spatial and temporal scales with independent spatial and temporal discretizations in the various models. Several of the computationally most expensive domains of the framework are modeled by the Block-Adaptive Tree Solarwind Roe-type Upwind Scheme (BATS-R-US) code that can solve various forms of the magnetohydrodynamic (MHD) equations, including Hall, semi-relativistic, multi-species and multi-fluid MHD, anisotropic pressure, radiative transport and heat conduction. Modeling disparate scales within BATS-R-US is achieved by a block-adaptive mesh both in Cartesian and generalized coordinates. Most recently we have created a new core for BATS-R-US: the Block-Adaptive Tree Library (BATL) that provides a general toolkit for creating, load balancing and message passing in a 1, 2 or 3 dimensional block-adaptive grid. We describe the algorithms of BATL and demonstrate its efficiency and scaling properties for various problems. BATS-R-US uses several time-integration schemes to address multiple time-scales: explicit time stepping with fixed or local time steps, partially steady-state evolution, point-implicit, semi-implicit, explicit/implicit, and fully implicit numerical schemes. Depending on the application, we find that different time stepping methods are optimal. Several of the time integration schemes exploit the block-based granularity of the grid structure. The framework and the adaptive algorithms enable physics-based space weather modeling and even short-term forecasting.

Exploiting Laboratory and Heliophysics Plasma Synergies

Recent advances in space-based heliospheric observations, laboratory experimentation, and plasma simulation codes are creating an exciting new cross-disciplinary opportunity for understanding fast energy release and transport mechanisms in heliophysics and laboratory plasma dynamics, which had not been previously accessible. This article provides an overview of some new observational, experimental, and computational assets, and discusses current and near-term activities towards exploitation of synergies involving those assets. This overview does not claim to be comprehensive, but instead covers mainly activities closely associated with the authors’ interests and reearch. Heliospheric observations reviewed include the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) on the National Aeronautics and Space Administration (NASA) Solar Terrestrial Relations Observatory (STEREO) mission, the first instrument to provide remote sensing imagery observations with spatial continuity extending from the Sun to the Earth, and the Extreme-ultraviolet Imaging Spectrometer (EIS) on the Japanese Hinode spacecraft that is measuring spectroscopically physical parameters of the solar atmosphere towards obtaining plasma temperatures, densities, and mass motions. The Solar Dynamics Observatory (SDO) and the upcoming Solar Orbiter with the Heliospheric Imager (SoloHI) on-board will also be discussed. Laboratory plasma experiments surveyed include the line-tied magnetic reconnection experiments at University of Wisconsin (relevant to coronal heating magnetic flux tube observations and simulations), and a dynamo facility under construction there; the Space Plasma Simulation Chamber at the Naval Research Laboratory that currently produces plasmas scalable to ionospheric and magnetospheric conditions and in the future also will be suited to study the physics of the solar corona; the Versatile Toroidal Facility at the Massachusetts Institute of Technology that provides direct experimental observation of reconnection dynamics; and the Swarthmore Spheromak Experiment, which provides well-diagnosed data on three-dimensional (3D) null-point magnetic reconnection that is also applicable to solar active regions embedded in pre-existing coronal fields. New computer capabilities highlighted include: HYPERION, a fully compressible 3D magnetohydrodynamics (MHD) code with radiation transport and thermal conduction; ORBIT-RF, a 4D Monte-Carlo code for the study of wave interactions with fast ions embedded in background MHD plasmas; the 3D implicit multi-fluid MHD spectral element code, HiFi; and, the 3D Hall MHD code VooDoo. Research synergies for these new tools are primarily in the areas of magnetic reconnection, plasma charged particle acceleration, plasma wave propagation and turbulence in a diverging magnetic field, plasma atomic processes, and magnetic dynamo behavior.

Tsallis distributions of magnetic field strength variations in the heliosphere: 5 to 90 AU

The Tsallis (q‐exponential) distribution function, derived from the entropy principle of nonextensive statistical mechanics, describes fluctuations in the magnetic field strength on many scales throughout the heliosphere. This paper shows that a one‐dimensional multifluid magnetohydrodynamic (MHD) model, with Advanced Composition Explorer (ACE) observations at 1 AU as input, predicts Tsallis distributions between 5 and 90 AU on scales from 1 to 128 days. At a scale of 1 day, the radial variation of the entropic indexqdecreases fromq≥ 5/3 atR≤ 50 AU toq≤ 5/3 atR≥ 60 AU, corresponding to a change from a divergent to a convergent second moment of the Tsallis distribution, suggesting the possibility of a “phase transition” and/or a relaxation effect at ≈60 AU. The Tsallis distribution derived from the time series of one‐dimensional MHD model is nearly identical to those observed by Voyager 1 at ∼80 AU over the scales from 1 to 64 days during the year 2000. The Tsallis distribution appears over a wide range of scales and distances despite the complex nonlinear dynamical evolution of the heliospheric magnetic field during 1999/2000.

A two-dimensional multifluid MHD model of Titans plasma environment

Titan possesses an extensive neutral atmosphere consisting mainly of molecular nitrogen and methane. Titan also has an ionosphere due to the photoionization of the neutrals by solar extreme ultraviolet photons or due to ionization by energetic electrons associated with Saturns magnetosphere. Saturns magnetospheric plasma and field impinges on Titans atmosphere and ionosphere with a flow speed of about 120 km/s. We are studying Titans plasma environment using a two-dimensional (2-D), quasi-multifluid magnetohydrodynamic (MHD) model which maintains high spatial resolution in Titans ionosphere by employing a grid with cylindrical geometry and non-uniform radial grid spacing. The ion species included in the model are a generic light (e.g., H +, H + 2), medium (e.g., N +, CH + 5), and heavy (e.g., N + 2, H 2CN +) species. The inner boundary is located at an altitude of 725 km and the outer boundary at a radial distance of 1.5×10 6 km. Titan first begins to affect the external magnetospheric plasma flow at a distance of about 10 Titan radii (or 10 R T). The plasma flow is subsonic, although superAlfvénic, and a bow shock does not appear in the model results. The flow for radial distances between about 2 and 10 R T qualitatively resembles potential flow around a hard cylinder. The flow inside 2 R T is much slower due to the build-up of a magnetic barrier and due to the collisional interaction of the plasma with Titans neutral atmosphere. Comparison will be made with the results of an earlier 1-D MHD model of Titans ionosphere (Keller, Cravens and Gan, J. Geophys. Res. 99, 11199, 1994) and with a 3-D single fluid MHD model.