Jozsef Kota

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.

Velocity Correlation and the Spatial Diffusion Coefficients of Cosmic Rays: Compound Diffusion

We consider the transport of charged particles in a stochastic magnetic field using a method based on the velocity correlation function vi(0)vj(t) developed by R. Kubo. This can be used under very general conditions to evaluate the corresponding spatial diffusion coefficients, if the fluctuations are statistically homogeneous in space and time. Although Kubos theory is quite general, it is not obvious how it can be applied to describe compound diffusion when particles are strictly tied to the magnetic field lines and perpendicular transport results solely from the random walk of the field lines. This motion is non-Markovian and leads to a slower Δx2 ∝ t1/2 diffusion in contrast to the Δx2 ∝ t dependence of the standard diffusion. We demonstrate how compound diffusion fits into Kubos formalism. As intuitively as can be anticipated, the non-Markovian nature of the motion results in a long-term anticorrelation in vj(0)vi(t), which causes the ordinary spatial diffusion coefficient to vanish identically. The Δx2 ∝ t1/2 dependence of the compound diffusion can also be recovered from the Laplace transform of the velocity correlation function. Some implications of the long-term anticorrelation are discussed.

Perpendicular transport in 1‐ and 2‐dimensional shock simulations

We consider the foundations of 1- and 2-dimensional shock simulations in which the physical quantities are independent of a coordinate which is not parallel to the magnetic field. We show analytically that in such simulations the ions are effectively tied to the convected magnetic lines of force because of the presence of an ignorable spatial coordinate. This conclusion has important consequences. In particular we conclude that the acceleration of energetic charged particles at quasi-perpendicular shocks cannot be properly studied in such simulations because the role of perpendicular diffusion cannot be properly evaluated.

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)

Interpretation and consequences of large‐scale magnetic variances observed at high heliographic latitude

We consider recent Ulysses observations of the large-scale variances in the transverse components of the interplanetary magnetic field. A previously-suggested theory is shown to provide a good fit to the observed spatial variation and level of the fluctuations. The transport of cosmic rays in the heliosphere will be significantly affected by these fluctuations. In addition to impeding the inward, radial diffusive and drift access of cosmic rays over the poles, the magnetic fluctuations imply a large latitudinal diffusion, caused primarily by the field-line mixing, or random walk.

Particle Acceleration in Solar Wind Compression Regions

We present the results of a theoretical investigation of the acceleration of charged particles in regions of gradual solar wind compression. The mechanism we describe is similar to diffusive shock acceleration, except that it invokes a gradual compression of the plasma over many gyroradii rather than a shock. Recent observations of energetic particles associated with corotating interaction regions (CIRs) at 1 AU suggest that the particles were accelerated within the compression region between the fast and slow solar wind rather than at the associated forward and reverse shocks, which are at larger heliocentric distances. We show that nondiffusive effects such as magnetic mirroring are important in the inner heliosphere, particularly the injection of low-energy particles (e.g., pickup ions and suprathermal solar wind ions). We integrate the trajectories of an ensemble of test particles moving in synthesized electromagnetic fields, which are similar to what is currently known about corotating interaction regions in the inner heliosphere, prior to the formation of the forward and reverse shocks. We show that compression regions associated with CIRs at 1 AU with widths ~0.03 AU can accelerate particles up to ~10 MeV and produce energy spectra which are remarkably similar to recent observations.

The role of corotating interaction regions in cosmic‐ray modulation

We present the first global simulations of the modulation of galactic cosmic rays by a three-dimensional solar wind with corotating interaction regions. The cosmic-ray transport equation is solved in a computed wind and magnetic-field model. The results show both the previously-neglected small-scale response to CIRs and global, drift-dominated, effects which are similar to previous models. Our model predicts a correlation between the local magnetic field and the rate of decrease of the cosmic-ray intensity which is similar to that observed. This is found to be due to inhibited diffusion. We suggest that such small-scale variations are local and that they do not change significantly the global cosmic-ray structure except, perhaps, near sunspot maximum.

EVIDENCE OF A NORTH-SOUTH ASYMMETRY IN THE HELIOSPHERE ASSOCIATED WITH A SOUTHWARD DISPLACEMENT OF THE HELIOSPHERIC CURRENT SHEET

Evidence of a north-south asymmetry in the global heliosphere, first inferred from Ulysses cosmic-ray observations, is investigated using simultaneous Ulysses and Wind magnetic field observations. Such an asymmetry, presumably associated with a southward displacement of the heliospheric current sheet (HCS), is expected to produce significantly different magnitudes of the radial field component, |BR|, in the Suns north and south magnetic hemispheres or, alternatively, in the positive and negative magnetic sectors. Ulysses, while at high latitudes, spends time predominantly in first one and then the other hemisphere. As a consequence, measurements in the positive sector are obtained several months later than measurements made in the negative sector, making comparisons susceptible to temporal changes. To address this ambiguity, the fields in both sectors observed by the Wind spacecraft in the ecliptic were compared. A large difference in |BR| of ≈30% was observed at Wind between 1994 December and 1995 April, with |BR| larger in the south than in the north. Subsequent measurements show a gradual increase in the north (outward) radial component and a decrease in the south (inward) component, ending in only a small difference by 1995 June. Thus, the Wind observations are consistent with a southward displacement of the HCS of ≈10° and with the energetic particle observations. The secular time variation, which occurred as the spacecraft transited from the south to the north hemisphere, explains why a significant north-south difference in |BR| was not evident in the Ulysses measurements. The current sheet configuration and various questions and implications associated with these results are also discussed.

The gradient of galactic cosmic rays at the solar-wind termination shock

We report on a study of the expected spatial variation of the galactic cosmic-ray intensity in the outer heliosphere, in the vicinity of the solar-wind termination shock. Model simulations which contain all of the transport effects, including drifts, predict that the radial gradients change abruptly at the shock, and that the nature of the effect varies significantly with particle energy. At low energies, the radial gradient changes abruptly from a lower value inside the shock to a higher value outside, whereas at high energies, the higher value of the gradient is inside the shock. This effect, which is a consequence of the matching conditions at the shock and is closely related to diffusive shock acceleration, is qualitatively the same for both helisopheric magnetic polarity states and remains much the same in one-dimensional, two-dimensional, and three-dimensional models. Hence drifts do not change the nature of this phenomenon, although they change it quantitatively. The effect is, of course, not present in the absence of a terminal shock and may prove to be an important diagnostic tool for the study of the termination of the solar wind.

Ion injection and acceleration at quasi-perpendicular shocks

We present and discuss results of a new model for ion injection and acceleration at quasi-perpendicular collisionless shocks. We use the one-dimensional hybrid simulation (kinetic ions/fluid electrons) and impose an assumption on the ion motion so that diffusion across the magnetic field (normal to the shock front) is possible. These motions are otherwise suppressed by both one- and two-dimensional simulations. We find that, even in strictly perpendicular shocks, when scattering normal to the field is included, a fraction of the incident ions are accelerated to suprathermal energies. When reasonable scattering times are considered, only pickup ions are injected, whereas thermal solar wind ions are not. The acceleration of these ions is very rapid. We have found that a few of the initially low-energy pickup ions can reach many tens to a few hundred times the plasma ramming energy in less than 100 gyroperiods. Furthermore, highly field aligned energetic ions are found to exist upstream of the slightly oblique shock. The most direct application of this study is toward the interpretation of observations of both solar wind and interstellar pickup ion distributions in the vicinity of interplanetary shocks which are most often quasi-perpendicular. This work also directly addresses a fundamental issue with regard to our current understanding of the anomalous component of cosmic rays.