Ilia I. Roussev

Space Weather Modeling Framework: A new tool for the space science community

[1] The Space Weather Modeling Framework (SWMF) provides a high-performance flexible framework for physics-based space weather simulations, as well as for various space physics applications. The SWMF integrates numerical models of the Solar Corona, Eruptive Event Generator, Inner Heliosphere, Solar Energetic Particles, Global Magnetosphere, Inner Magnetosphere, Radiation Belt, Ionosphere Electrodynamics, and Upper Atmosphere into a high-performance coupled model. The components can be represented with alternative physics models, and any physically meaningful subset of the components can be used. The components are coupled to the control module via standardized interfaces, and an efficient parallel coupling toolkit is used for the pairwise coupling of the components. The execution and parallel layout of the components is controlled by the SWMF. Both sequential and concurrent execution models are supported. The SWMF enables simulations that were not possible with the individual physics models. Using reasonably high spatial and temporal resolutions in all of the coupled components, the SWMF runs significantly faster than real time on massively parallel supercomputers. This paper presents the design and implementation of the SWMF and some demonstrative tests. Future papers will describe validation (comparison of model results with measurements) and applications to challenging space weather events. The SWMF is publicly available to the scientific community for doing geophysical research. We also intend to expand the SWMF in collaboration with other model developers.

A THREE-DIMENSIONAL FLUX ROPE MODEL FOR CORONAL MASS EJECTIONS BASED ON A LOSS OF EQUILIBRIUM

A series of simulation runs are carried out to investigate the loss of equilibrium of the three-dimensional flux rope configuration of Titov & Demoulin as a suitable mechanism for the initiation of coronal mass ejections. By means of these simulations, we are able to determine the conditions for which stable equilibria no longer exist. Our results imply that it is possible to achieve a loss of equilibrium even though the ends of the flux rope are anchored to the solar surface. However, in order to have the flux rope escape, it is necessary to modify the configuration by eliminating the arcade field.

A Semiempirical Magnetohydrodynamical Model of the Solar Wind

We present a new MHD model for simulating the large-scale structure of the solar corona and solar wind under “steady state” conditions stemming from the Wang-Sheeley-Arge empirical model. The processes of turbulent heating in the solar wind are parameterized using a phenomenological, thermodynamical model with a varied polytropic index. We employ the Bernoulli integral to bridge the asymptotic solar wind speed with the assumed distribution of the polytropic index on the solar surface. We successfully reproduce the mass flux from Sun to Earth, the temperature structure, and the large-scale structure of the magnetic field. We reproduce the solar wind speed bimodal structure in the inner heliosphere. However, the solar wind speed is in a quantitative agreement with observations at 1 AU for solar maximum conditions only. The magnetic field comparison demonstrates that the input magnetogram needs to be multiplied by a scaling factor in order to obtain the correct magnitude at 1 AU.

DETERMINING THE AZIMUTHAL PROPERTIES OF CORONAL MASS EJECTIONS FROM MULTI-SPACECRAFT REMOTE-SENSING OBSERVATIONS WITH STEREO SECCHI

We discuss how simultaneous observations by multiple heliospheric imagers (HIs) can provide some important information about the azimuthal properties of coronal mass ejections (CMEs) in the heliosphere. We propose two simple models of CME geometry that can be used to derive information about the azimuthal deflection and the azimuthal expansion of CMEs from SECCHI/HI observations. We apply these two models to four CMEs well observed by both STEREO spacecraft during the year 2008. We find that in three cases, the joint STEREO-A and B observations are consistent with CMEs moving radially outward. In some cases, we are able to derive the azimuthal cross section of the CME fronts, and we are able to measure the deviation from self-similar evolution. The results from this analysis show the importance of having multiple satellites dedicated to space weather forecasting, for example, in orbits at the Lagrangian L4 and L5 points.

Three-dimensional MHD Simulation of the 2003 October 28 Coronal Mass Ejection: Comparison with LASCO Coronagraph Observations

We numerically model the coronal mass ejection (CME) event of 2003 October 28 that erupted from AR 10486 and propagated to Earth in less than 20 hr, causing severe geomagnetic storms. The magnetohydrodynamic (MHD) model is formulated by first arriving at a steady state corona and solar wind employing synoptic magnetograms. We initiate two CMEs from the same active region, one approximately a day earlier that preconditions the solar wind for the much faster CME on the 28th. This second CME travels through the corona at a rate of over 2500 km s−1, driving a strong forward shock. We clearly identify this shock in an image produced by the Large Angle Spectrometric Coronagraph (LASCO) C3 and reproduce the shock and its appearance in synthetic white-light images from the simulation. We find excellent agreement with both the general morphology and the quantitative brightness of the model CME with LASCO observations. These results demonstrate that the CME shape is largely determined by its interaction with the ambient solar wind and may not be sensitive to the initiation process. We then show how the CME would appear as observed by wide-angle coronagraphs on board the Solar Terrestrial Relations Observatory (STEREO) spacecraft. We find complex time evolution of the white-light images as a result of the way in which the density structures pass through the Thomson sphere. The simulation is performed with the Space Weather Modeling Framework (SWMF).

CORONAL MASS EJECTION SHOCK AND SHEATH STRUCTURES RELEVANT TO PARTICLE ACCELERATION

Most high-energy solar energetic particles are believed to be accelerated at shock waves driven by coronal mass ejections (CMEs). The acceleration process strongly depends on the shock geometry and the structure of the sheath that forms behind the shock. In an effort to understand the structure and time evolution of such CME-driven shocks andtheirrelevancetoparticleacceleration,weinvestigatetheinteractionofafastCMEwiththeambientsolarwind by means of a three-dimensional numerical ideal MHD model. Our global steady state coronal model possesses high-latitudecoronalholesandahelmetstreamerstructurewithacurrentsheetneartheequator,reminiscentofnear solar minimum conditions. Fast and slow solar winds flow at high and low latitude, respectively, and the Archimedean spiral geometry of the interplanetary magnetic field is reproduced by solar rotation. Within this model system, we drive a CME to erupt by introducing a Gibson-Low magnetic flux rope that is embedded in the helmet streamer in an initial state of force imbalance. The flux rope rapidly expands and is ejected from the corona with maximum speeds in excess of 1000 km s � 1 , driving a fast-mode shock from the inner corona to a distance of 1 AU. We find that the ambient solar wind structure strongly affects the evolution of the CME-driven shocks, causing deviations of the fast-mode shocks from their expected global configuration. These deflections lead to substantial compressions of the plasma and magnetic field in their associated sheath region. The sudden postshock increase in magneticfieldstrengthonlow-latitudefieldlinesisfoundtobeeffectiveforacceleratingparticlestotheGeVrange. Subject heading gs: acceleration of particles — MHD — shock waves — Sun: coronal mass ejections (CMEs)

A NUMERICAL MODEL OF A CORONAL MASS EJECTION: SHOCK DEVELOPMENT WITH IMPLICATIONS FOR THE ACCELERATION OF GeV PROTONS

The initiation and evolution of the coronal mass ejection, which occurred on 1998 May 2 in NOAA Active Region 8210, are modeled using a fully three-dimensional, global MHD code. The initial magnetic field for the model is based on magnetogram data from the Wilcox Solar Observatory, and the solar eruption is initiated by slowly evolving the boundary conditions until a critical point is reached where the configuration loses equilibrium. At this time, the field erupts, and a flux rope is ejected that achieves a maximum speed in excess of 1000 km s-1. The shock that forms in front of the rope reaches a fast-mode Mach number in excess of 4 and a compression ratio greater than 3 by the time it has traveled a distance of 5 R☉ from the surface. For such values, diffusive shock acceleration theory predicts a distribution of solar energetic protons with a cutoff energy of about 10 GeV. For this event, there appears to be no need to introduce an additional acceleration mechanism to account for solar energetic protons with energies below 10 GeV.

Studying extreme ultraviolet wave transients with a digital laboratory: Direct comparison of extreme ultraviolet wave observations to global magnetohydrodynamic simulations

In this work, we describe our effort to explore the signatures of large-scale extreme ultraviolet (EUV) transients in the solar corona (EUV waves) using a three-dimensional thermodynamic magnetohydrodynamic model. We conduct multiple simulations of the 2008 March 25 EUV wave (~18:40 UT), observed both on and off of the solar disk by the STEREO-A and B spacecraft. By independently varying fundamental parameters thought to govern the physical mechanisms behind EUV waves in each model, such as the ambient magneto-sonic speed, eruption free energy, and eruption handedness, we are able to assess their respective contributions to the transient signature. A key feature of this work is the ability to synthesize the multi-filter response of the STEREO Extreme UltraViolet Imagers directly from model data, which gives a means for direct interpretation of EUV observations with full knowledge of the three-dimensional magnetic and thermodynamic structures in the simulations. We discuss the implications of our results with respect to some commonly held interpretations of EUV waves (e.g., fast-mode magnetosonic wave, plasma compression, reconnection front, etc.) and present a unified scenario which includes both a wave-like component moving at the fast magnetosonic speed and a coherent driven compression front related to the eruptive event itself.

Numerical investigation of the homologous coronal mass ejection events from active region 9236

We present a three-dimensional compressible magneto-hydrodynamics (MHD) simulation of the three coronal mass ejections (CMEs) of 2000 November 24, originating from NOAA active region 9236. These three ejections, with velocities around 1200 km s-1 and associated with X-class flares, erupted from the Sun in a period of about 16.5 hr. In our simulation, the coronal magnetic field is reconstructed from MDI magnetogram data, the steady-state solar wind is based on a varying polytropic index model, and the ejections are initiated using out-of-equilibrium semicylindrical flux ropes with a size smaller than the active region. The simulations are carried out with the Space Weather Modeling Framework. We are able to reproduce the shape and velocity of the CMEs as observed by the LASCO C3 coronograph. The complex ejecta resulting from the interaction of the three CMEs is preceded at Earth by a single shock wave, which, in our simulation, arrives at Earth 10 hr later than the shock observed by the Wind spacecraft. This article discusses the three-dimensional aspects of the propagation, interaction, and merging of the forward shock waves associated with the three ejections. Synthetic images from the Heliospheric Imagers onboard the STEREO spacecraft are produced, and we predict that the large density jump associated with the interaction of the shocks should be observed by those coronographs in the near future.

Solution-adaptive magnetohydrodynamics for space plasmas: Sun-to-Earth simulations

Space-environment simulations, particularly those involving space plasma, present significant computational challenges. Global computational models based on magnetohydrodynamics equations are essential to understanding the solar systems plasma phenomena, including the large-scale solar corona, the solar winds interaction with planetary magnetospheres, comets, and interstellar medium, and the initiation, structure, and evolution of solar eruptive events.