Global Magnetohydrodynamic Model Research Articles

Dependence of coronal mass ejections on the morphology and toroidal flux of their source magnetic flux ropes

Coronal mass ejections (CMEs) stand as intense eruptions of magnetized plasma from the Sun, and they play a pivotal role in driving significant changes of the heliospheric environment. Deducing the properties of CMEs from their progenitors in solar source regions is crucial for space weather forecasting. The primary objective of this paper is to establish a connection between CMEs and their progenitors in solar source regions, enabling us to infer the magnetic structures of CMEs before their full development. We created a dataset comprising a magnetic flux rope series with varying projection shapes (S-, Z-, and toroid-shaped), sizes, and toroidal fluxes using the Regularized Biot-Savart Laws (RBSL). These flux ropes were inserted into solar quiet regions with the aim of imitating the eruptions of quiescent filaments. Thereafter, we simulated the propagation of these flux ropes from the solar surface to a distance of 25$R_ odot $ with our global coronal magnetohydrodynamic (MHD) model COCONUT. Our parametric survey revealed significant impacts of source flux ropes on the consequent CMEs. Regarding the flux-rope morphology, we find that the projection shape (e.g., sigmoid or torus) can influence the magnetic structures of CMEs at 20$R_ odot $, albeit with minimal impacts on the propagation speed. However, these impacts diminish as source flux ropes become fat. In terms of toroidal flux, our simulation results demonstrate a pronounced correlation with the propagation speed of CMEs as well as the successfulness in erupting. This work builds the bridge between the CMEs in the outer corona and their progenitors in solar source regions. Our parametric survey suggests that the projection shape, cross-section radius, and toroidal flux of source flux ropes are crucial parameters in predicting magnetic structures and the propagation speed of CMEs, providing valuable insights for space weather prediction. On the one hand, the conclusion drawn here could be instructive in identifying the high-risk eruptions with the potential to induce stronger geomagnetic effects ($B_ z $ and propagation speed). On the other hand, our findings hold practical significance for refining the parameter settings of launched CMEs at 21.5$R_ odot $ in heliospheric simulations, such as with EUHFORIA, based on observations for their progenitors in solar source regions.

Filament eruption by multiple reconnections

Context. Filament eruption is a common phenomenon in solar activity, but the triggering mechanism is not well understood. Aims. We focus our study on a filament eruption located in a complex nest of three active regions close to a coronal hole. Methods. The filament eruption is observed at multiple wavelengths: by the Global Oscillation Network Group (GONG), the Solar Terrestrial Relations Observatory (STEREO), the Solar Upper Transition Region Imager (SUTRI), and the Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamic Observatory (SDO). Thanks to high-temporal-resolution observations, we were able to analyze the evolution of the fine structure of the filament in detail. The filament changes direction during the eruption, which is followed by a halo coronal mass ejection detected by the Large Angle Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO). A Type III radio burst was also registered at the time of the eruption. To investigate the process of the eruption, we analyzed the magnetic topology of the filament region adopting a nonlinear force-free-field (NLFFF) extrapolation method and the polytropic global magnetohydrodynamic (MHD) modeling. We modeled the filament by embedding a twisted flux rope with the regularized Biot-Savart Laws (RBSL) method in the ambient magnetic field. Results. The extrapolation results show that magnetic reconnection occurs in a fan-spine configuration resulting in a circular flare ribbon. The global modeling of the corona demonstrates that there was an interaction between the filament and open field lines, causing a deflection of the filament in the direction of the observed CME eruption and dimming area. Conclusions. The modeling supports the following scenario: magnetic reconnection not only occurs with the filament itself (the flux rope) but also with the background magnetic field lines and open field lines of the coronal hole located to the east of the flux rope. This multiwavelength analysis indicates that the filament undergoes multiple magnetic reconnections on small and large scales with a drifting of the flux rope.

The Dynamic Response of Jovian Magnetotail Reconnection to Enhanced Solar Wind Ram Pressure

In this study, we employ the Grid Agnostic Magnetohydrodynamic (MHD) for Extended Research Applications (GAMERA), a high-resolving-power, three-dimensional global MHD model, to simulate magnetotail reconnection in Jupiter's magnetosphere. While previous satellite observations have provided initial statistics on magnetotail reconnection properties at Jupiter, they have been limited in spacetime coverage, leaving the dynamic process of Jovian magnetotail reconnection and its response to the solar wind (SW) poorly understood. Using MHD simulations, we quantitatively analyze the temporal evolution and spatial dependence of nightside reconnection in Jupiter's magnetotail under ideal quiet and enhanced SW conditions. Our results demonstrate that magnetotail reconnection tends to occur in the midnight and postmidnight sectors, with a low occurrence in the premidnight sector, consistent with both Galileo and Juno observations and predictions by Delamere & Bagenal. The magnetic local time (MLT)–radial distribution of magnetotail reconnection is broad, indicating that Jovian magnetotail reconnection is always dynamic rather than steady state. Enhanced SW ram pressure can decrease the MLT coverage of magnetotail reconnection by compressing Jupiter's magnetosphere. However, the occurrence of magnetotail reconnection near the midnight and postmidnight sectors is enhanced by SW compression beyond 60 R J, but is not significantly impacted by SW compression within 60 R J. Conversely, SW compression suppresses reconnection in the premidnight sector, leading to a stronger dawn–dusk asymmetry in the occurrence and location of magnetotail reconnection. This study validates the applicability of the GAMERA code for simulating Jupiter’s magnetosphere and provides complementary insights into the dynamic structure and the SW response of Jupiter’s magnetosphere.

The Dynamic Response of Jovian Magnetotail Reconnection to Enhanced Solar Wind Ram Pressure

In this study, we employ the Grid Agnostic Magnetohydrodynamic (MHD) for Extended Research Applications (GAMERA), a high-resolving-power, three-dimensional global MHD model, to simulate magnetotail reconnection in Jupiter's magnetosphere. While previous satellite observations have provided initial statistics on magnetotail reconnection properties at Jupiter, they have been limited in spacetime coverage, leaving the dynamic process of Jovian magnetotail reconnection and its response to the solar wind (SW) poorly understood. Using MHD simulations, we quantitatively analyze the temporal evolution and spatial dependence of nightside reconnection in Jupiter's magnetotail under ideal quiet and enhanced SW conditions. Our results demonstrate that magnetotail reconnection tends to occur in the midnight and postmidnight sectors, with a low occurrence in the premidnight sector, consistent with both Galileo and Juno observations and predictions by Delamere & Bagenal. The magnetic local time (MLT)–radial distribution of magnetotail reconnection is broad, indicating that Jovian magnetotail reconnection is always dynamic rather than steady state. Enhanced SW ram pressure can decrease the MLT coverage of magnetotail reconnection by compressing Jupiter's magnetosphere. However, the occurrence of magnetotail reconnection near the midnight and postmidnight sectors is enhanced by SW compression beyond 60 R J, but is not significantly impacted by SW compression within 60 R J. Conversely, SW compression suppresses reconnection in the premidnight sector, leading to a stronger dawn–dusk asymmetry in the occurrence and location of magnetotail reconnection. This study validates the applicability of the GAMERA code for simulating Jupiter’s magnetosphere and provides complementary insights into the dynamic structure and the SW response of Jupiter’s magnetosphere.

The Alfvén transition zone observed by the Parker Solar Probe in young solar wind – global properties and model comparisons

ABSTRACT The transition from subAlfvénic to superAlfvénic flow in the solar atmosphere is examined by means of Parker Solar Probe (PSP) measurements during solar encounters 8 to 14. Around 220 subAlfvénic periods with a duration ≥10 min are identified. The distribution of their durations, heliocentric distances, and Alfvén Mach number are analysed and compared with a global magnetohydrodynamic model of the solar corona and wind which includes turbulence effects. The results are consistent with a patchy and fragmented morphology, and suggestive of a turbulent Alfvén zone within which the transition from subAlfvénic to superAlfvénic flow occurs over an extended range of helioradii. These results inform and establish context for detailed analyses of subAlfvénic coronal plasma that are expected to emerge from PSP’s final mission phase, as well as for NASA’s planned PUNCH mission.

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.

Tracking the Subsolar Bow Shock and Magnetopause: Applying the Magnetosheath Velocity Gradient Method

AbstractBoth the bow shock and magnetopause move in response to varying solar wind and magnetospheric conditions. Tracking their locations can provide important clues to the state of the solar wind‐magnetosphere interaction, but is difficult with single spacecraft observations. This paper employs multipoint THEMIS observations of velocity gradients in the subsolar magnetosheath to remotely sense boundary locations on a continual basis for various solar wind conditions. We present three cases: (a) continuous northward IMF and no magnetopause motion; (b) southward IMF and no magnetopause motion with evidence of nightside activity; and (c) southward IMF and pressure increase with inward motion. When observing spacecraft are located near the Sun‐Earth line, inferred boundary locations agree well with the predictions of the BATS‐R‐US global magnetohydrodynamic model, confirming the utility of both the new method and the models. Results show that boundaries often lie nearly at rest with amplitudes less than 0.5 RE. They provide evidence indicating that nightside reconnection and a strong sunward convection in the outer magnetosphere can counteract the magnetopause erosion expected when a southward interplanetary magnetic field (IMF) initiates reconnection on the dayside magnetopause.

Three-dimensional Magnetohydrodynamic Simulations of Periodic Variations of Ganymede’s Footprint

Alfvénic power is generated through the interaction between the mini-magnetosphere of Ganymede and the corotating flow of the Jovian plasma, contributing to the formation of the Ganymede auroral footprint at Jupiter’s ionosphere. Using a three-dimensional, high-resolution global magnetohydrodynamic model of the Ganymede magnetosphere, we investigate the temporal variations of the Alfvénic power generated in the mini-magnetosphere of the moon under steady-state upstream conditions. Results show that (1) the Alfvénic power caused by the intermittent magnetic reconnections and the Kelvin–Helmholtz instabilities is quasiperiodic, varying over time with the periods about 71–333 s, which are consistent with the 100 s timescale periodicities of the Ganymede footprint's emitted power; and (2) the magnitude of the Alfvénic power is smaller when Ganymede is inside the Jovian plasma sheet where the B y component is smaller, and larger when the moon is outside the plasma sheet where the B y component is larger.

Quantifying the ability of magnetohydrodynamic models to reproduce observed Birkeland current and auroral electrojet magnitudes

Although global magnetohydrodynamic (MHD) models have increased in sophistication and are now at the forefront of modeling Space Weather, there is still no clear understanding of how well these models replicate the observed ionospheric current systems. Without a full understanding and treatment of the ionospheric current systems, global models will have significant shortcomings that will limit their use. In this study we focus on reproducing observed seasonal interhemispheric asymmetry in ionospheric currents using the Space Weather Modeling Framework (SWMF). We find that SWMF does reproduce the linear relationship between the electrojets and the FACs, despite the underestimation of the currents’ magnitudes. Quantitatively, we find that at best SWMF is only capturing approximately 60% of the observed current. We also investigate how varying F10.7 effects the ionospheric potential and currents during the summer and winter. We find that simulations ran with higher F10.7 result in lower ionospheric potentials. Additionally, we find that the models do not always replicate the expected behavior of the currents with varying F10.7. This work points to a needed improvement in ionospheric conductance models.

The role of plasma β in global coronal models: Bringing balance back to the force

Context. COolfluid COrona uNstrUcTured (COCONUT) is a global coronal magnetohydrodynamic (MHD) model that was recently developed and will soon be integrated into the ESA Virtual Space Weather Modelling Centre (VSWMC). In order to achieve robustness and fast convergence to a steady state for numerical simulations with COCONUT, several assumptions and simplifications were made during its development, such as prescribing filtered photospheric magnetic maps to represent the magnetic field conditions in the lower corona. This filtering leads to smoothing and lower magnetic field values at the inner boundary (i.e. the solar surface), resulting in an unrealistically high plasma β (greater than 1 in a large portion of the domain). Aims. We aim to examine the effects of prescribing such filtered magnetograms in global coronal simulations and formulate a method for achieving more realistic plasma β values and improving the resolution of electromagnetic features without losing computational performance. Methods. We made use of the newly developed COCONUT solver to demonstrate the effects of the highly pre-processed magnetic maps set at the inner boundary and the resulting high plasma β on the features in the computational domain. Then, in our new approach, we shifted the inner boundary to 2 R⊙ from the original 1.01 R⊙ and preserved the prescribed highly filtered magnetic map. With the shifted boundary, the boundary density and pressure were also naturally adjusted to better represent the considered physical location. This effectively reduces the prescribed plasma β and leads to a more realistic setup. The method was applied on a magnetic dipole, a minimum (2008) and a maximum (2012) solar activity case, to demonstrate its effects. Results. The results obtained with the proposed approach show significant improvements in the resolved density and radial velocity profiles, and far more realistic values of the plasma β at the boundary and inside the computational domain. This is also demonstrated via synthetic white light imaging (WLI) and with the validation against tomography data. The computational performance comparison shows similar convergence to a limit residual on the same grid when compared to the original setup. Considering that the grid can be further coarsened with this new setup, as its capacity to resolve features or structures is superior, the operational performance can be additionally increased if needed. Conclusions. The newly developed method is thus deemed as a good potential replacement of the original setup for operational purposes, providing higher physical detail of the resolved profiles while preserving a good convergence and robustness of the solver.

A Linear Velocity Gradient in the Subsolar Magnetosheath

AbstractTracking the location and motion of the bow shock and magnetopause can reveal many aspects concerning the nature of the solar wind‐magnetosphere interaction. In the absence of missions that image the magnetopause, we propose to detect and track the magnetopause and bow shock motion using multipoint in situ observations of the magnetosheath. Making theoretical assumptions corroborated by numerical simulations, Sibeck et al. (2022, https://doi.org/10.1029/2022JA030704) proposed a method to predict the subsolar magnetopause and the bow shock from the linear variation in magnetosheath plasma velocity expected from global magnetohydrodynamic simulations along the Sun‐Earth line. This paper presents a case study that employs the velocity gradient method using multi‐point THEMIS plasma observations from 21 to 22 June 2007. Using the five THEMIS spacecraft we demonstrate that the radial gradient in plasma velocity remains nearly linear across the subsolar magnetosheath, except when the magnetosheath lies behind the quasi‐parallel bow shock and conditions are very turbulent. The bow shock and magnetopause move outward in response to a gradual decrease in solar wind dynamic pressure. We show that results from the velocity gradient method agree well with predictions of global magnetohydrodynamic and empirical models for this time interval. The velocity gradient method can therefore be used to remotely determine the location of the magnetopause and bow shock, solely based on two‐spacecraft measurements at the subsolar magnetosheath when not downstream of the quasi‐parallel shock.

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.

First Infrared Coronal Spectra from DKIST/Cryo-NIRSP: Comparisons with Global MHD Models

The Cryogenic Near-Infrared Spectropolarimeter (Cryo-NIRSP) is now conducting science operations at the National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST). We report results obtained during instrument commissioning that advance multiwavelength coronagraphic observations in the infrared. This includes the first observations of the Fe xiii 1075 nm, Fe xiii 1079 nm, Si x 1430 nm, and Si ix 3934 nm forbidden lines. The Si ix observations, in particular, significantly improve upon the few previous ground-based detections and open up this diagnostic for more routine study. We analyse long-slit spectra in each line sampling elongations from 1.06 to 1.52 solar radii above the northeast solar limb on 2022 February 14. Line characteristics as a function of radial distance are derived and discussed in comparison to modeled emission emergent from global coronal magnetohydrodynamic models with different heating prescriptions. We find favorable correspondence between the observations and different aspects of these models, and further demonstrate the utility of infrared observations for constraining coronal models. Continued DKIST/Cryo-NIRSP observations including polarimetric data, together with ongoing improvements in data calibrations, promise significant value for coronal studies.

Soft X‐Ray Imaging of Magnetopause Reconnection Outflows Under Low Plasma‐β Solar Wind Conditions

AbstractWe examined soft X‐ray emission by the solar wind charge‐exchange process around the Earth's magnetosphere using a global magnetohydrodynamic simulation model. The dayside magnetopause reconnection heats and accelerates the plasma whereby the X‐ray emission becomes as bright as ∼6 × 10−6 eV cm−3 s−1 under the southward interplanetary magnetic field conditions. In particular, under low plasma‐β solar wind conditions, we found that the X‐ray intensity reflects the bulk motion of outflows from the reconnection region. We propose that this particular solar wind condition would allow visualization of the mesoscale magnetopause reconnection site, as observed in the solar corona.

Simulation of Magnetospheric Sawtooth Oscillations: The Role of Kinetic Reconnection in the Magnetotail

AbstractMagnetospheric sawtooth oscillations are observed during strong and steady solar wind driving conditions. The simulation results of our global magnetohydrodynamics (MHD) model with embedded kinetic physics show that when the total magnetic flux carried by constant solar wind exceeds a threshold, sawtooth‐like magnetospheric oscillations are generated. Different from previous works, this result is obtained without involving time‐varying ionospheric outflow in the model. The oscillation period and amplitude agree well with observations. The simulated oscillations cover a wide range of local times, although the distribution of magnitude as a function of longitude is different from observations. Our comparative simulations using ideal or Hall MHD models do not produce global time‐varying features, which suggests that kinetic reconnection physics in the magnetotail is a major contributing factor to sawtooth oscillations.

Constraining Global Solar Models through Helioseismic Analysis

Global hydrodynamic simulations of internal solar dynamics have focused on replicating the conditions for solar-like (equator rotating faster than the poles) differential rotation and meridional circulation using the results of helioseismic inversions as a constraint. Inferences of meridional circulation, however, have provided controversial results showing the possibility of one, two, or multiple cells along the radius. To help address this controversy and develop a more robust understanding of global flow regimes in the solar interior, we apply a “forward-modeling” approach to the analysis of helioseismic signatures of meridional circulation profiles obtained from numerical simulations. We employ the global acoustic modeling code GALE to simulate the propagation of acoustic waves through regimes of mean mass-flows generated by global hydrodynamic and magnetohydrodynamic models: EULAG, the Pencil code, and the Rayleigh code. These models are used to create synthetic Dopplergram data products, used as inputs for local time–distance helioseismology techniques. Helioseismic travel-time signals from solutions obtained through global numerical simulations are compared directly with inferences from solar observations, in order to set additional constraints on global model parameters in a direct way. We show that even though these models are able to replicate solar-like differential rotation, the resulting rotationally constrained convection develops a multicell global meridional circulation profile that is measurably inconsistent with local time–distance inferences of solar observations. However, we find that the development of rotationally unconstrained convection close to the model surface is able to maintain solar-like differential rotation, while having a significant impact on the helioseismic travel-time signal, replicating solar observations within one standard deviation of the error due to noise.

Gravitoturbulent dynamo in global simulations of gaseous disks

Context. The turbulence driven by gravitational instabilities (GIs) can amplify magnetic fields in massive gaseous disks. This GI dynamo may appear in young circumstellar disks, whose weak ionization challenges other amplification routes, as well as in active galactic nuclei. Although regarded as a large-scale dynamo, only local simulations have so far described its kinematic regime. Aims. We study the GI dynamo in global magnetohydrodynamic (MHD) models of accretion disks, focusing on its kinematic phase. Methods. We perform resistive MHD simulations with the PLUTO code for different radiative cooling times and electrical resistivities. A weak magnetic field seeds the dynamo, and we adopt mean-field and heuristic models to capture its essence. Results. We recover the same induction process leading to magnetic field amplification as previously identified in local simulations. The dynamo is, however, global in nature, connecting distant annuli of the disk via a large-scale dynamo mode of a fixed growth rate. This large-scale amplification can be described by a mean-field model that does not rely on conventional α-Ω effects. When varying the disk parameters we find an optimal resistivity that facilitates magnetic amplification, whose magnetic Reynolds number, ℛm ≲ 10, is substantially smaller than in local simulations. Unlike local simulations, we find an optimal cooling rate and the existence of global oscillating dynamo modes. The nonlinear saturation of the dynamo puts the disk in a strongly magnetized turbulent state on the margins of the effective range of GI. In our simulations, the accretion power eventually exceeds the threshold required by local thermal balance against cooling, leaving the long-term nonlinear outcome of the GI dynamo uncertain.

The Dependence of the Venusian Induced Magnetosphere on the Interplanetary Magnetic Field: An MHD Study

The influences of the interplanetary magnetic field (IMF) on the induced magnetosphere of Venus are investigated using a global multispecies magnetohydrodynamics (MHD) model. The simulation results show that the induced magnetosphere is controlled by the IMF components perpendicular to the solar wind velocity (B Y and B Z in the Venus Solar Orbital coordinate), rather than the IMF magnitude (∣B∣). With the increase of , the induced magnetosphere becomes stronger in field strength and thicker in spatial scale, and the bow shock locates farther from the planet. The parallel IMF component (B X ) has relatively small impacts on the magnetic barrier and the magnetotail, regardless of the various IMF magnitudes and orientations caused by different B X . The responses of the Venusian induced magnetosphere to the change of upstream IMF are also studied. The time-dependent MHD calculations show that the dayside magnetosphere responds quickly with a timescale of 10 s–10 minutes, depending on the considered magnetospheric region. For comparison, the timescale required for the adjustment of magnetotail as driven by an IMF rotation is derived to be ∼10–20 minutes.

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.

Explicit IMF B y -Dependence of Energetic Protons and the Ring Current.

The most important parameter driving the solar wind‐magnetosphere interaction is the southward (B z ) component of the interplanetary magnetic field (IMF). While the dawn‐dusk (B y ) component of the IMF is also known to play an important role, its effects are usually assumed to be independent of its sign. Here we demonstrate for the first time a seasonally varying, explicit IMF B y ‐dependence of the ring current and Dst index. Using satellite observations and a global magnetohydrodynamic model coupled with a ring current model, we show that for a fixed level of solar wind driving the flux of energetic magnetospheric protons and the growth‐rate of the ring current are greater for B y < 0 (B y > 0) than for B y > 0 (B y < 0) in Northern Hemisphere summer (winter). While the physical mechanism of this explicit B y ‐effect is not yet fully understood, our results suggest that IMF B y modulates magnetospheric convection and plasma transport in the inner magnetosphere.