Global MHD Model Research Articles

MHD analysis of propagation of an interplanetary shock across magnetospheric boundaries

An important problem of the Space Weather Program is the interaction of interplanetary (IP) shocks with Earth's magnetosphere because their interaction often (but not always) leads to major geomagnetic storms. Since the huge interaction region can be covered by simultaneous spacecraft observations only sporadically, global MHD modeling can help in our understanding of the interaction process. We have developed a procedure that clearly distinguished magnetospheric boundaries in an output of the global MHD model and compare its results to spacecraft observations. Using one IP shock observed by Geotail, we compare its passage through the magnetosphere with predictions of two modifications of the global BATS-R-US MHD code. We demonstrate the complexity of the shock interaction with the bow shock, magnetopause, and oscillations of the whole system toward a new equilibrium state with a duration of 10–12 min. Moreover, based on the changes of the magnetopause and bow shock locations in the nightside region, we suggest that the information about the IP shock hitting the subsolar magnetopause reaches the nightside magnetopause earlier than the IP shock can arrive. This information is, in our view, mediated by fast magnetosonic waves in the inner magnetosphere. Although we have found generally a good agreement, we discuss possible sources of deviations of modeled and observed locations of the boundaries.

Modeling radiation belt radial diffusion in ULF wave fields: 1. Quantifying ULF wave power at geosynchronous orbit in observations and in global MHD model

To provide critical ULF wave field information for radial diffusion studies in the radiation belts, we quantify ULF wave power (f = 0.5–8.3 mHz) in GOES observations and magnetic field predictions from a global magnetospheric model. A statistical study of 9 years of GOES data reveals the wave local time distribution and power at geosynchronous orbit in field‐aligned coordinates as functions of wave frequency, solar wind conditions (Vx, ΔPd and IMF Bz) and geomagnetic activity levels (Kp, Dst and AE). ULF wave power grows monotonically with increasing solar wind Vx, dynamic pressure variations ΔPd and geomagnetic indices in a highly correlated way. During intervals of northward and southward IMF Bz, wave activity concentrates on the dayside and nightside sectors, respectively, due to different wave generation mechanisms in primarily open and closed magnetospheric configurations. Since global magnetospheric models have recently been used to trace particles in radiation belt studies, it is important to quantify the wave predictions of these models at frequencies relevant to electron dynamics (mHz range). Using 27 days of real interplanetary conditions as model inputs, we examine the ULF wave predictions modeled by the Lyon‐Fedder‐Mobarry magnetohydrodynamic code. The LFM code does well at reproducing, in a statistical sense, the ULF waves observed by GOES. This suggests that the LFM code is capable of modeling variability in the magnetosphere on ULF time scales during typical conditions. The code provides a long‐missing wave field model needed to quantify the interaction of radiation belt electrons with realistic, global ULF waves throughout the inner magnetosphere.

Dynamics of ring current and electric fields in the inner magnetosphere during disturbed periods: CRCM–BATS‐R‐US coupled model

We present simulation results from a one‐way coupled global MHD model (Block‐Adaptive‐Tree Solar‐Wind Roe‐Type Upwind Scheme, BATS‐R‐US) and kinetic ring current models (Comprehensive Ring Current Model, CRCM, and Fok Ring Current, FokRC). The BATS‐R‐US provides the CRCM/FokRC with magnetic field information and plasma density/temperature at the polar CRCM/FokRC boundary. The CRCM uses an electric potential from the BATS‐R‐US ionospheric solver at the polar CRCM boundary in order to calculate the electric field pattern consistent with the CRCM pressure distribution. The FokRC electric field potential is taken from BATS‐R‐US ionospheric solver everywhere in the modeled region, and the effect of Region II currents is neglected. We show that for an idealized case with southward‐northward‐southward Bz IMF turning, CRCM–BATS‐R‐US reproduces well known features of inner magnetosphere electrodynamics: strong/weak convection under the southward/northward Bz; electric field shielding/overshielding/penetration effects; an injection during the substorm development; Subauroral Ion Drift or Polarization Jet (SAID/PJ) signature in the dusk sector. Furthermore, we find for the idealized case that SAID/PJ forms during the substorm growth phase, and that substorm injection has its own structure of field‐aligned currents which resembles a substorm current wedge. For an actual event (12 August 2000 storm), we calculate ENA emissions and compare with Imager for Magnetopause‐to‐Aurora Global Exploration/High Energy Neutral Atom data. The CRCM–BATS‐R‐US reproduces both the global morphology of ring current and the fine structure of ring current injection. The FokRC‐BATS‐R‐US shows the effect of a realistic description of Region II currents in ring current–MHD coupled models.

Interaction of plasma cloud with external electric field in lower ionosphere

Abstract. In the auroral lower-E and upper-D region of the ionosphere, plasma clouds, such as sporadic-E layers and meteor plasma trails, occur daily. Large-scale electric fields, created by the magnetospheric dynamo, will polarize these highly conducting clouds, redistributing the electrostatic potential and generating anisotropic currents both within and around the cloud. Using a simplified model of the cloud and the background ionosphere, we develop the first self-consistent three-dimensional analytical theory of these phenomena. For dense clouds, this theory predicts highly amplified electric fields around the cloud, along with strong currents collected from the ionosphere and circulated through the cloud. This has implications for the generation of plasma instabilities, electron heating, and global MHD modeling of magnetosphere-ionosphere coupling via modifications of conductances induced by sporadic-E clouds.

Global MHD Modeling of the Solar Corona and Inner Heliosphere for the Whole Heliosphere Interval

AbstractWith the goal of understanding the three-dimensional structure of the solar corona and inner heliosphere during the “Whole Heliosphere Interval” (WHI), we have developed a global MHD solution for Carrington rotation (CR) 2068. Our model, which includes energy transport processes, such as coronal heating, conduction of heat parallel to the magnetic field, radiative losses, and the effects of Alfvén waves, is capable of producing significantly better estimates of the plasma temperature and density in the corona than have been possible in the past. With such a model, we can compute emission in extreme ultraviolet (EUV) and X-ray wavelengths, as well as scattering in polarized white light. Additionally, from our heliospheric solutions, we can deduce magnetic field and plasma parameters along specific spacecraft trajectories. We have made detailed comparisons of both remote solar and in situ observations with the model results, allowing us to: (1) Connect these disparate sets of observations; (2) Infer the global structure of the inner heliosphere; and (3) Provide support for (or against) assumptions in the MHD model, such as the empirically-based coronal heating profiles.

A novel metric for coronal MHD models

In the interest of quantitatively assessing the capabilities of coronal MHD models, we have developed a metric that compares the structures of the white light corona observed with SOHO LASCO C2 to model predictions. The MAS model is compared to C2 observations from two Carrington rotations during solar cycle 23, CR1913 and CR1984, which were near the minimum and maximum of solar activity, respectively, for three radial heights, 2.5 R⊙, 3.0 R⊙, and 4.5 R⊙. In addition to simulated polarization brightness images, we create a synthetic image based on the field topology along the line of sight in the model. This open‐closed brightness is also compared to LASCO C2 after renormalization. In general, the model's magnetic structure is a closer match to observed coronal structures than the model's density structure. This is expected from the simplified energy equations used in current global corona MHD models.

Time‐dependent global MHD simulations of Cassini T32 flyby: From magnetosphere to magnetosheath

When the Cassini spacecraft flew by Titan on 13 June 2007, at 13.6 Saturn local time, Titan was directly observed to be outside Saturn′s magnetopause. Cassini observations showed dramatic changes of magnetic field orientation as well as other plasma flow parameters during the inbound and outbound segments. In this paper, we study Titan's ionospheric responses to such a sudden change in the upstream plasma conditions using a sophisticated multispecies global MHD model. Simulation results of three different cases (steady state, simple current sheet crossing, and magnetopause crossing) are presented and compared against Cassini Magnetometer, Langmuir Probe, and Cassini Plasma Spectrometer observations. The simulation results provide clear evidence for the existence of a fossil field that was induced in the ionosphere. The main interaction features, as observed by the Cassini spacecraft, are well reproduced by the time‐dependent simulation cases. Simulation also reveals how the fossil field was trapped during the interaction and shows the coexistence of two pileup regions with opposite magnetic orientation, as well as the formation of a pair of new Alfven wings and tail disconnection during the magnetopause crossing process.

MAPPING THE STRUCTURE OF THE CORONA USING FOURIER BACKPROJECTION TOMOGRAPHY

Estimating the structure, or density distribution, of the solar corona from a set of two-dimensional white-light images made by coronagraphs is a critical challenge in coronal physics. This work describes new data-analysis procedures which are used to create global maps of the coronal structure at heights where the corona becomes approximately radial (≥ 3 R ☉). The technique, which is named Qualitative Solar Rotational Tomography (QSRT), uses total brightness white light observations, processed with a suitable background subtraction and a Normalizing Radial Graded Filter (NRGF). These observations are made with high frequency by the Large Angle and Spectrometric Coronagraph Experiment (LASCO) C2 coronagraph, which allows a standard Fourier-transform-based tomographical reconstruction. In this paper, we first test the technique using a model corona. QSRT is then applied to a set of observations made during Carrington Rotation (CR) 2000-2001 (2003 March 16 to 2003 March 31). Since the maps are constructed from data which are normalized using the NRGF process, QSRT cannot give electron density directly. Nevertheless, the tests using the model corona demonstrate the technique's ability to give a good qualitative reconstruction of the coronal structure at high latitude, with decreasing but acceptable accuracy at the equator. These tests also demonstrate QSRT's insensitivity to noise. For the LASCO C2 observations, good agreement is found between synthetic images calculated from the reconstructed corona and the original observations, and good agreement is found between the distribution of density in a QSRT reconstruction and that found using a global MHD model. Despite their lack of quantitative information on absolute electron density, the resulting maps (which are constructed directly from high-resolution coronal data observed at the appropriate height), contain useful information on the distribution of density in the corona.

Geotail and LFM comparisons of plasma sheet climatology: 1. Average values

We compare the statistics of central plasma sheet properties from 6 years of Geotail observations with 2 months of Lyon‐Fedder‐Mobarry (LFM) global MHD simulations. This statistical validation effort represents an inherently new method of systematically characterizing and quantifying global MHD model performance. For our comparison, we identify the central plasma sheet in the observations and simulation by identical criteria and select the simulation interval to ensure statistically similar distributions of solar wind conditions in both studies. After verifying our plasma sheet selection by inspecting the magnetic signatures of both studies, we compare the resultant number densities, thermal pressures, thermal energies, and bulk flows as functions of position across the equatorial plane. We find that the LFM model successfully reproduces the gross features of the global plasma sheet in a statistical sense. However, our comparison also reveals certain systematic discrepancies between the model and the observations. The LFM predicts a plasma sheet which is too dense, too cool, and exhibits faster globally averaged bulk flows than the observed plasma sheet. By quantifying the LFM overestimate of ionospheric transpolar potential and showing that ΦPC correlates with plasma sheet flow speed, we demonstrate that 15% of the plasma sheet velocity discrepancy is reflected in a ΦPC overestimate. This statistical validation effort represents an essential first step toward the rigorous, quantitative evaluation of a global MHD model in the plasma sheet.

A 3-D global MHD model for the effect of neutral jets during the Deep Space 1 Comet 19P/Borrelly flyby

Abstract The Deep Space 1 (DS1) spacecraft passed the sunward side of Comet 19P/Borrelly in 2001. Along its relatively north–south orbit, a set of plasma density and velocity measurement revealed a northward shift of the plasma boundaries and the mass loading peak. Both onboard and ground based telescopes found evidence for asymmetric distribution of the dust and neutrals. In this paper, five mass-loading patterns are studied to present the first study of the effect of non-spherical neutral distribution profiles on the solar wind-cometary plasma interaction environment. Using magnetohydrodynamic simulations, it is found that a combination of Gaussian and cosine neutral jet distribution, with cosine being the major part, can fit the DS1 general plasma measurement well, with a total gas production rate of around 5 × 10 28 s −1 . These model-data comparisons indicate that the general plasma distribution around Comet Borrelly can be explained with its aspherical neutral jet distribution. However, such neutral jets by themselves are insufficient to produce the density offset in the central peak. Kinetic effects, such as finite gyroradius may be required to create the offset plasma peak.

The critical solar wind pressure for IMF penetration into the Venus ionosphere

Early observations and simulations have revealed that the occurrence of IMF penetration into the Venus ionosphere depends on the upstream solar wind pressure, and that IMF is transported into the ionosphere by the downward convection when the solar wind dynamic pressure is relatively large. In this paper, we investigated the critical solar wind pressure for the IMF penetration, by using a two‐dimensional global MHD model. The simulations showed that variations in the upstream solar wind pressure (PSW) affect plasma dynamics and chemistry in the subsolar ionosphere. The chemical plasma loss rate increases as PSW increases, and the amount of plasma loss exceeds that of plasma production when PSW exceeds a certain critical level (Pcrtc). On this occasion, the plasma convection in the ionosphere connects to the solar wind flow, and as a result the connected downward convection transports IMF into the ionosphere. The simulations also showed that IMF penetration occurs only when PSW exceeds Pcrtc, even if the solar wind pressure changes abruptly. The critical pressure is not necessarily equal to the ionospheric peak thermal pressure (Ppeak). Our simulations suggested that Pcrtc is smaller than Ppeak.

Using Global Simulations to Relate the Three‐Part Structure of Coronal Mass Ejections to In Situ Signatures

White-light observations of coronal mass ejections (CMEs) often show the classic three-part structure consisting of (1) a bright front; (2) a dark cavity; and (3) a bright, compact core. It has proven difficult to unambiguously associate these features with in situ measurements of interplanetary CMEs (ICMEs), in all but a few cases. In this study we use a global MHD model to simulate the eruption and evolution of a CME out to 0.25 AU, allowing us to continuously track these features from the Sun and through the solar wind. Our results support the generally held view that the interplanetary flux rope corresponds to the dark cavity. We find that the bright front merges with solar wind material swept up by the ICME. Thus, the sheath material found ahead of fast ejecta is in fact composed from both ambient solar wind material, as well the bright front. We also note that, in this simulation, the bright front is formed from the overlying streamer configuration from within which the CME erupted and is not itself coronal material swept up during the early phase of the eruption. The conclusions reached in this study are undoubtedly sensitive to the initial configuration and mechanism used to initiate the CME, and thus care should be taken when using them to interpret specific observations. On the other hand, they provide a unique, unbroken connection between remote solar and interplanetary observations. Ultimately, detailed comparisons between observations and simulation results may be able to constrain or even rule out some mechanisms of CME initiation.

Continuous reconnection line and pressure‐dependent energy conversion on the magnetopause in a global MHD model

We present results on magnetopause reconnection processes in the Gumics‐4 global MHD simulation model. The reconnection line on the magnetopause is identified using a topological method, and its behavior as a function of the interplanetary magnetic field (IMF) direction is shown to be consistent with the component reconnection hypothesis. Energy conversion associated with reconnection is quantified using integrals of Poynting vector divergence. The efficiency of energy conversion depends on both the IMF direction and the solar wind dynamic pressure. We vary the solar wind speed and density separately and show that while both affect the appearance of magnetopause processes, speed has a much stronger effect on the total reconnection efficiency.

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.

A global MHD simulation of an event with a quasi-steady northward IMF component

Abstract. We show results of the Lyon-Fedder-Mobarry (LFM) global MHD simulations of an event previously examined using Iridium spacecraft observations as well as DMSP and IMAGE FUV data. The event is chosen for the steady northward IMF sustained over a three-hour period during 16 July 2000. The Iridium observations showed very weak or absent Region 2 currents in the ionosphere, which makes the event favorable for global MHD modeling. Here we are interested in examining the model's performace during weak magnetospheric forcing, in particular, its ability to reproduce gross signatures of the ionospheric currents and convection pattern and energy deposition in the ionosphere both due to the Poynting flux and particle precipitation. We compare the ionospheric field-aligned current and electric potential patterns with those recovered from Iridium and DMSP observations, respectively. In addition, DMSP magnetometer data are used for comparisons of ionospheric magnetic perturbations. The electromagnetic energy flux is compared with Iridium-inferred values, while IMAGE FUV observations are utilized to verify the simulated particle energy flux.

Magnetopause energy and mass transfer: results from a global MHD simulation

Abstract. We use the global MHD model GUMICS-4 to investigate the energy and mass transfer through the magnetopause and towards the closed magnetic field as a response to the interplanetary magnetic field (IMF) clock angle θ=arctan (BY/BZ), IMF magnitude, and solar wind dynamic pressure. We find that the mass and energy transfer at the magnetopause are different both in spatial characteristics and in response to changes in the solar wind parameters. The energy transfer follows best the sin2 (θ/2) dependence, although there is more energy transfer after large energy input, and the reconnection line follows the IMF rotation with a delay. There is no clear clock angle dependence in the net mass transfer through the magnetopause, but the mass transfer through the dayside magnetopause and towards the closed field occurs preferably for northward IMF. The energy transfer occurs through areas at the magnetopause that are perpendicular to the subsolar reconnection line. In contrast, the mass transfer occurs consistently along the reconnection line, both through the magnetopause and towards the closed field. Both the energy and mass transfer are enhanced in response to increased solar wind dynamic pressure, while increasing the IMF magnitude does not affect the transfer quantities as much.

Effects of charge exchange and electron impact ionization on the formation of the magnetic pileup boundary at Mars

A relatively sharp increase in the solar wind magnetic field, Magnetic Pileup Boundary (MPB), has been observed in the inner Martian magnetosheath. We examined effects of exospheric processes, such as charge exchange reactions (CE) and electron impact ionizations (EII), on the formation of the MPB by using a newly developed two‐dimensional global MHD model coupled to Monte Carlo simulations. The CE and EII processes change the energy distribution of the solar wind plasma, causing a depletion of the thermal pressure, and thus an enhancement of magnetic pressure so as to maintain the pressure balance in the radial direction. Beside decreasing the thermal pressure, these processes decelerate the solar wind by adding mass of slow ions (protons and oxygen ions). These processes were treated self‐consistently in the new MHD model. The MHD model implicitly includes the time evolution of ion and electron distributions being deformed from Maxwellian shapes under the influence of CE and EII reactions, by making use of their reaction rate coefficients that were derived as functions of “effective temperatures” from Monte Carlo simulations. The MHD simulation results suggest that an enhancement of magnetic field magnitude occurs in the inner magnetosheath mainly due to the CE effect at the solar minimum and to the EII effect at the solar maximum. EII also decelerates the solar wind significantly through the mass‐loading effect, which lengthens the interaction time between the solar wind plasma and the exospheric atoms, resulting in an additional enhancement of magnetic field magnitude both at the solar minimum and maximum. It was also found that these exospheric processes significantly influence the ion density profiles above the upper ionosphere. The large and nearly constant scale height of total ion density as observed by past measurements can be caused by the EII process. Moreover, EII produces a large number of O+ ions above the ionopause especially at the solar maximum, which is potentially important in terms of atmospheric escape.

Modeling and analysis of solar wind generated contributions to the near-Earth magnetic field

Abstract Solar wind generated magnetic disturbances are currently one of the major obstacles for improving the accuracy in the determination of the magnetic field due to sources internal to the Earth. In the present study a global MHD model of solar wind magnetosphere interaction is used to obtain a physically consistent, divergence-free model of ionospheric, field-aligned and magnetospheric currents in a realistic magnetospheric geometry. The magnetic field near the Earth due to these currents is analyzed by estimating and comparing the contributions from the various parts of the system, with the aim of identifying the most important aspects of the solar wind disturbances in an internal field modeling context. The contribution from the distant magnetospheric currents is found to consist of two, mainly opposing, contributions from respectively the dayside magnetopause currents and the cross-tail current. At high latitudes the field-aligned component is of partidular interest in connection with internal field-modelling. In the altitude regime of 400–800 km (typical for low Earth orbit satellites) the ionospheric currents are found to contribute significantly to the disturbancance, and account for more than 90% of the field-aligned disturbance. The magnetic disturbance field from field-aligned currents (FACs) is basically transverse to the main field, and they therefore contribute with less than 2% to the disturbance in total field intensity. Inhomogeneity in ionospheric conductance is identified as the main cause of main-field disturbance in the field-aligned direction. These disturbances are associated with the ionospheric Pedersen currents, and may introduce systematic errors in internal field models.

Effect of anomalous electron heating on the transpolar potential in the LFM global MHD model

The tendency of global MHD models to overestimate the transpolar potential in simulations of strong geomagnetic storms and evidence of an adverse feedback of the ionospheric conductance on the potential suggest that these models lack important physics leading to the conductance enhancement. Farley‐Buneman instability in the auroral ionosphere provides this lacking physics. This instability is believed to cause strong anomalous electron heating which affects the ionospheric conductivity. We use an earlier developed model of anomalous electron heating to estimate the ionospheric conductance disturbance as a function of the local electric field. This result is used to modify the ionospheric conductance in the LFM model to study its effect on the simulated transpolar potential. An idealized and a real‐case simulations are accomplished. In both cases a considerable drop in the simulated transpolar potential is found. The latter is in a good agreement with AMIE model and DMSP data.

The Effects of Differential Rotation on the Magnetic Structure of the Solar Corona: Magnetohydrodynamic Simulations

Coronal holes are magnetically open regions from which the solar wind streams. Magnetic reconnection has been invoked to reconcile the apparently rigid rotation of coronal holes with the differential rotation of magnetic flux in the photosphere. This mechanism might also be relevant to the formation of the slow solar wind, the properties of which seem to indicate an origin from the opening of closed magnetic field lines. We have developed a global MHD model to study the effect of differential rotation on the coronal magnetic field. Starting from a magnetic flux distribution similar to that of Wang and coworkers, which consists of a bipolar magnetic region added to a background dipole field, we applied differential rotation over a period of 5 solar rotations. The evolution of the magnetic field and of the boundaries of coronal holes are in substantial agreement with the findings of Wang and coworkers. We identified examples of interchange reconnection and other changes of topology of the magnetic field. Possible consequences for the origin of the slow solar wind are also discussed.