SH22A-01 10:20h
3D MHD simulation and the role of null points in coronal reconnection
STEREO represents a quantum leap from the perspective of the simulations. By providing a 2-satellite view it will be able to determine the true 3D stereoscopic view of the structure of the magnetic field lines and to investigate the motions of the field lines. A crucial issue among the key mission objectives is the study of reconnection and coronal mass ejections. The theory of 3D magnetic reconnection is still far from being fully developed. But a pivotal aspect is the role played by null points in the topology changes induced by reconnection [1]. STEREO will provide a unique opportunity to look directly at this issue. We will investigate the role of null points using our simulation tool, FLIP-3D [2], and our diagnostic tools to pick up null points and to study Poincare plots. We will provide evidence of the type of theoretical predictions [3] that will be best compared with STEREO observations. [1] Y. Lau, J. Finn, ApJ, 350, 672 (1990) [2] J.U. Brackbill, JCP, 96, 163 (1991) [3] G. Lapenta, D.A. Knoll, Solar Physics, 214, 107-129 (2003); ApJ, submitted.
SH22A-02 10:35h
STEREO/SECCHI Simulations of CMEs and Flares using TRACE Images
We simulate 3D models of EUV images of flare and CME events, using TRACE EUV movies. TRACE movies show 2D images in projection along a particular line-of-sight. We simulate 3D models of erupting filaments, flare loops, and postflare loops using: (1) a ``finger printing'' technique to trace linear structures in 2D images; (2) geometric 3D models based on force-free fields and curvature radius maximization of flare loop and flux rope structures; (3) conservation of velocity and acceleration parameters; (4) multi-temperature plasma filling according to hydrodyamic scaling laws; and (5) 2D projections from secondary line-of-sights that correspond to viewpoints of the secondary STEREO spacecraft. From such simulations we envision to illustrat 3D time-dependent models, what would be observed at the two STEREO spacecraft positions as well as from a near-Earth spacecraft such as SoHO. These simulations are used to test STEREO analysis software and to investigate what physical parameters and geometric 3D reconstructions can be retrieved from STEREO/SECCHI data.
http://secchi.lmsal.com/Science/
SH22A-03 10:50h
The 3D structure of CMEs from LASCO polarization measurements
The polarization of Compton scattered light from a coronal plasma is a function of the distance of that plasma from the plane of the sky. From an image of the polarization of the corona it is possible to construct a 3D cube of intensity which can be visualized at any angle to study it's three dimensional distribution. For a one month period during July and August 2002, the LASCO C2 coronagraph performed polarization measurements with a one hour cadence. During this period, several CMEs were well observed. Two CMEs give the appearance of an ejecting loop arcade. Another CME is not as easily described but may indicate the presence of a flux tube. There are two main ambiguities in the analysis. First, a given polarization can be produced by scattering from in front of or behind the plane of the sky. This ambiguity can be overcome by selecting CMEs that are completely out of the plane of the sky. Second, in reconstructing the 3D distribution of intensity, it is assumed that the intensity can be visualized as coming from a single point in 3D space. The filamentary structures commonly seen in the 3D visualizations suggest that this is not a significant problem.
SH22A-04 11:05h
Morphology Indicators of the Three-Dimensional Size of Flux Rope CMEs: A Prediction for STEREO
We provide a new estimate of the three-dimensional sizes of flux rope coronal mass ejections (CMEs). We base our estimate on the interpretation of two CME morphologies that have previously been considered distinct. We believe these morphologies represent two perspectives of the same large scale cylindrical structure (a magnetic flux rope) seen axially versus broadside. This distinction has not been previously recognized because both morphologies have been classified as "three-part structures". Our preliminary study based on 21 events (13 axial and 8 broadside) indicated an average diameter of 46$\deg$ and an average length of 76$\deg$ for these flux rope CMEs. We compare our statistical results to those obtained using an elliptical flux rope model, and we present the current status of this work-in-progress. This result will be tested in the future by observations of individual CMEs from different locations by NASA's STEREO mission.
SH22A-05 11:20h
The Geometric Localization of STEREO CMEs
We describe a straightforward methodology for determining the location and other gross properties of CMEs within the coronagraph field of view in the upcoming STEREO mission observations. We use geometric triangulation upon a series of lines-of-sight taken from two spacecraft views that are locally tangent to the apparent edges of a CME. From the intersections of these lines-of-sight, we construct a set of stacked quadrilaterals that fully bound the structure and convey something of its location, shape, and size; a time sequence of such determinations can be used to determine the velocity. The technique is relatively robust and promises a substantial improvement in our capability to locate and characterize CMEs for research as well as forecasting purposes.
SH22A-06 11:35h
STEREO HI - From CMEs to Near Earth Objects: a Unique View of the Heliosphere
The two STEREO Heliospheric Imagers (HI) will have a unique view of the Heliosphere, with a field of view encompassing the entire Sun-Earth line. While the primary objective for HI is to detect and obtain information about the size, velocity and direction of Coronal Mass Ejections (CMEs), the sensitivity and field of view of these instruments can be exploited to extend the scientific scope of the mission. For the first time it will be possible to observe objects in this region of space in three dimensions. Simulated images are being used to determine the problems associated with observing objects from comets and asteroids through to the distribution of interplanetary dust.
http://www.stereo.rl.ac.uk
SH22A-07 11:50h
Magnetic Flux Ropes from the Sun to 1 AU*
Any practical model of the dynamics of a coronal mass ejection (CME) and its interplanetary counterpart (ICME) must conform to available observational constraints from sun and to the earth; the upcoming STEREO mission will add significantly to those constraints. We present model/data comparisons for specific CME/ICME events near the sun (using coronagraph image data) and in the heliosphere (using in situ measurements) to show that the flux rope model of Chen and Krall[1-2] provides an accurate physics-based characterization of flux-rope CMEs over this range. We further show that quantitative results, such as the field energy required for eruption, depend on specific aspects of the flux rope geometry, such as the ratio (length/width) of the elliptical shape traced out by the flux-rope axis. It is this geometry that will be determined, for the first time, by STEREO. [1] Chen, J. 1996, JGR, 101, 27499 [2] Krall, J. et al., 2000, ApJ, 539, 964 *Work supported by ONR, NASA and NSF
SH22A-08 12:05h
Magnetic Geometry of Accelerating CMEs
It has been known that the main acceleration phase of a coronal mass ejection (CME) is localized to within 3--4 Rs of the Sun. Recently, it has been theoretically shown [1] that if the initial structure is a magnetic flux rope, the main acceleration peaks at apex height $Z_* \approx S_f/2$, where $S_f$ is the separation distance between the two footpoints anchored in the photosphere. This scaling property is therefore an observational signature of the magnetic geometry. The $S_f$-scaling law is universal in that it only depends on the pre-eruptive flux-rope geometry in 3-D. The initial comparisons with representative LASCO CMEs (C1-C2-C3 data) showed that this prediction is consistent with observations [1]. We will present further model-data comparisons using the archived LASCO data as well as eruptive prominence data. We discuss the significant improvement in theory-data comparison that will be possible with the expected STEREO data. Particularly, the two observing perspectives will allow one to better constrain the flux-rope geometry during the main acceleration phase, especially with respect to the footpoint separation $S_f$. In addition, the near-Sun observing capabilities (down to $\sim$0.5 $R_s$) and high cadence of COR1 will provide much better resolved observations of the initial acceleration of CMEs. \medskip 1.~Chen, J., and J. Krall, JGR, 108, 1410, doi:10.1029/2003JA009849, 2003. \medskip Work supported by ONR and NASA