S. T. Wu

Direct Detection of a Coronal Mass Ejection-Associated Shock in Large Angle and Spectrometric Coronagraph Experiment White-Light Images

The LASCO C2 and C3 coronagraphs recorded a unique coronal mass ejection on April 2, 1999. The event did not have the typical three-part CME structure and involved a small filament eruption without any visibile overlying streamer ejecta. The event exhibited an unusually clear signature of a wave propagating at the CME flanks. The speed and density of the CME front and flanks were consistent with the existence of a shock. To better establish the nature of the white light wave signature, we employed a simple MHD simulation using the LASCO measurements as constraints. Both the measurements and the simulation strongly suggest that the white light feature is the density enhancement from a fast-mode MHD shock. In addition, the LASCO images clearly show streamers being deflected when the shock impinges on them. It is the first direct imaging of this interaction.The Large Angle and Spectrometric Coronagraph Experiment (LASCO) C2 and C3 coronagraphs recorded a unique coronal mass ejection (CME) on 1999 April 2. The event did not have the typical three-part CME structure and involved a small-filament eruption without any visible overlying streamer ejecta. The event exhibited an unusually clear signature of a wave propagating at the CME flanks. The speed and density of the CME front and flanks were consistent with the existence of a shock. To better establish the nature of the white-light wave signature, we employed a simple MHD simulation using the LASCO measurements as constraints. Both the measurements and the simulation strongly suggest that the white-light feature is the density enhancement from a fast-mode MHD shock. In addition, the LASCO images clearly show streamers being deflected when the shock impinges on them. It is the first direct imaging of this interaction.

Simultaneous SOHO and Ground-Based Observations of a Large Eruptive Prominence and Coronal Mass Ejection

Coronal mass ejections (CMEs) are frequently associated with erupting prominences near the solar surface. A spectacular eruption of the southern polar crown prominence was observed on 2xa0June 1998, accompanied by a CME that was well-observed by the LASCO coronagraphs on SOHO. The prominence was observed in its quiescent state and was followed throughout its eruption by the SOHO EIT and later by LASCO as the bright, twisted core of the CME. Ground-based Hα observations of the prominence were obtained at the Ondřejov Observatory in the Czech Republic. A great deal of fine structure was observed within the prominence as it erupted. The prominence motion was found to rotate about its axis as it moved outward. The CME contained a helical structure that is consistent with the ejection of a magnetic flux rope from the Sun. Similar structures have been observed by LASCO in many other CMEs. The relationship of the flux rope to other structures in the CME is often not clear. In this event, the prominence clearly lies near the trailing edge of the structure identified as a flux rope. This structure can be observed from the onset of the CME in the low corona all the way out to the edge of the LASCO field of view. The initiation and evolution of the CME are modeled using a fully self-consistent, 3D axisymmetric, MHD code.

A NOVEL NUMERICAL IMPLEMENTATION FOR SOLAR WIND MODELING BY THE MODIFIED CONSERVATION ELEMENT/SOLUTION ELEMENT METHOD

In this paper, the spacetime conservation element and solution element (CESE) method is applied to threedimensional magnetohydrodynamics (MHD) equations in Cartesian coordinates for solar wind plasma, with the purpose of modeling the steady state solar atmospheric study. To illustrate this newly developed scheme we have studied two examples: (1) two-dimensional coronal dynamical structure with multipole magneticfields and (2) threedimensional coronal dynamical structure, using measuredsolar surface magnetic fields and the empirical values of the plasma properties on the solar surface as the initial conditions for the set of MHD equations and then the relaxationmethod toachieve aq uasiYsteadystate.From these examples we have shown that the newlydeveloped modified spacetime CESE scheme possesses the ability to model the Sun-Earth environment and other astrophysical flows. Subject headingg methods: numerical — MHD — solar wind

Shear-induced instability and arch filament eruption: A magnetohydrodynamic (MHD) numerical simulation

We investigate, via a two-dimensional (nonplanar) MHD simulation, a situation wherein a bipolar magnetic field embedded in a stratified solar atmosphere (i.e., arch-filament-like structure) undergoes symmetrical shear motion at the footpoints. It was found that the vertical plasma flow velocities grow exponentially leading to a new type of global MHD-instability that could be characterized as a ‘Dynamic Shearing Instability’, with a growth rate of about √8{ovV}Aa, where {ovV}A is the average Alfvén speed and a−1 is the characteristic length scale. The growth rate grows almost linearly until it reaches the same order of magnitude as the Alfvén speed. Then a nonlinear MHD instability occurs beyond this point. This simulation indicates the following physical consequences: the central loops are pinched by opposing Lorentz forces, and the outer closed loops stretch upward with the vertically-rising mass flow. This instability may apply to arch filament eruptions (AFE) and coronal mass ejections (CMEs).To illustrate the nonlinear dynamical shearing instability, a numerical example is given for three different values of the plasma beta that span several orders of magnitude. The numerical results were analyzed using a linearized asymptotic approach in which an analytical approximate solution for velocity growth is presented. Finally, this theoretical model is applied to describe the arch filament eruption as well as CMEs.

Uncovering the Wave Nature of the EIT Wave for the 2010 January 17 Event through Its Correlation to the Background Magnetosonic Speed

An EIT wave, which typically appears as a diffuse brightening that propagates across the solar disk, is one of the major discoveries of the Extreme ultraviolet Imaging Telescope on board the Solar and Heliospheric Observatory. However, the physical nature of the so-called EIT wave continues to be debated. In order to understand the relationship between an EIT wave and its associated coronal wave front, we investigate the morphology and kinematics of the coronal mass ejection (CME)-EIT wave event that occurred on 2010 January 17. Using the observations of the SECCHI EUVI, COR1, and COR2 instruments on board the Solar Terrestrial Relations Observation-B, we track the shape and movements of the CME fronts along different radial directions to a distance of about 15 solar radii (Rs ); for the EIT wave, we determine the propagation of the wave front on the solar surface along different propagating paths. The relation between the EIT wave speed, the CME speed, and the local fast-mode characteristic speed is also investigated. Our results demonstrate that the propagation of the CME front is much faster than that of the EIT wave on the solar surface, and that both the CME front and the EIT wave propagate faster than the fast-mode speed in their local environments. Specifically, we show a significant positive correlation between the EIT wave speed and the local fast-mode wave speed in the propagation paths of the EIT wave. Our findings support that the EIT wave under study is a fast-mode magnetohydrodynamic wave.

Reconstructing Spherical Nonlinear Force-free Field in the Solar Corona

We present a spherical nonlinear force-free field (NFFF) reconstructing method based on the photospheric vector magnetograms. The importance of this method is its ability to reveal the NFFF configurations, which is necessary for understanding the physical mechanisms of the initiation of the large-scale solar eruptions, such as coronal mass ejections and sympathetic flares. Using smooth continuous functions, the basic NFFF-governing partial differential equations in spherical coordinates reduce to a set of tractable ordinary differential equations. The numerical scheme used in this paper is similar to the recent local nonlinear force-free one developed by Song and coworkers. To illustrate this method, we give two test examples. One is to compute a well-known NFFF analytical solution given by Low & Lou. The other is for two active regions NOAA 10486 and NOAA 10488 observed on 2003 October 29. The results show that the transequatorial magnetic loops are revealed and coincided with some EUV Imaging Telescope loops.

Multi-spacecraft Observations of the 2008 January 2 CME in the Inner Heliosphere

We perform a detailed analysis of a coronal mass ejection (CME) on 2008 January 2. The combination of the Solar and Heliospheric Observatory and the twin STEREO spacecraft provides three-point observations of this CME. We track the CME in imaging observations and compare its morphology and kinematics viewed from different vantage points. The shape, angular width, distance, velocity, and acceleration of the CME front are different in the observations of these spacecraft. We also compare the efficiency of several methods, which convert the elongation angles of the CME front in images to radial distances. The results of our kinematic analysis demonstrate that this CME experiences a rapid acceleration at the early stage, which corresponds to the flash phase of the associated solar flare in time. Then, at a height of about 3.7 solar radius, the CME reaches a velocity of 790 km s?1 and propagates outward without an obvious deceleration. Because of its propagation direction away from the observers, the CME is not detected in situ by either ACE or STEREO.

Propagation of MHD body and surface waves in magnetically structured regions of the solar atmosphere

The fact that magnetically structured regions exist in the solar atmosphere has been known for a number of years. It has been suggested that different kinds of magnetohydrodynamic (MHD) waves can be efficiently damped in these regions and that the dissipated wave energy may be responsible for the observed enhancement in radiative losses. From a theoretical point of view, an important task would be to investigate the propagation and dissipation of MHD waves in these highly structured regions of the solar atmosphere. In this paper, we study the behavior of MHD body and surface waves in a medium with either a single or double (slab) magnetic interface by use of a nonlinear, two-dimensional, time-dependent, ideal MHD numerical model constructed on the basis of a Lagrangean grid and semi-implicit scheme. The processes of wave confinement and wave energy leakage are discussed in detail. It is shown that the obtained results depend strongly on the type of perturbations imposed on the interface or slab and on the plasma parameter, β. The relevance of the obtained results to the heating problem of the upper parts of the solar atmosphere is also discussed.

ON THE CAUSES OF PLASMOID ACCELERATION AND CHANGES IN MAGNETIC FLUX IN A RESISTIVE MAGNETOHYDRODYNAMIC PLASMA

Observationally, the change of acceleration of coronal mass ejections is commonly attributed to the change of the reconnection rate. In this study, we use a two-dimensional magnetohydrodynamic simulation with finite resistivity to study: (1) the forces that lead to the acceleration of the plasma and plasmoid and (2) the time evolution of the topological change of the magnetic flux across the current sheet. Our results show that the fast flows are not limited to the direction perpendicular to the local magnetic field. The fast parallel flows are accelerated by the parallel component of the pressure gradient force. The net force perpendicular to the magnetic field can accelerate the plasma and the plasmoid along the current sheet. The acceleration of the plasmoid is also controlled by the mass contained in the plasmoid. We find that the fast ejection of the plasmoid can stretch the current sheet and consequently reduce the magnetic reconnection/reconfiguration rate temporally before a new plasmoid is formed. We show that the topological change of the magnetic flux is due to the non-uniform magnetic annihilation rate along the current sheet. Therefore, the reconnection/reconfiguration site does not necessarily stay at the neutral point. It can move with the Y-line next to the bifurcated current sheets.

Coronal heating through lack of MHD equilibrium

We present an analytical example of a series of magnetostatic equilibria with an endpoint. Numerical simulation demonstrates that oscillatory behavior sets in at the endpoint, with a typical amplitude of 50 km/sec. We suggest this in situ wave generation is an energy source for coronal heating.