Solar Coronal Mass Ejections in 3D
Jason P. Byrne1, Shane A. Maloney1, R. T. J. McAteer1,2, Jose M. Refojo3, Peter T. Gallagher1
1Astrophysics Research Group, School of Physics, Trinity College Dublin, Dublin 2
2Department of Astronomy, New Mexico State University, Las Cruces, New Mexico 88003-8001, USA
3Trinity Centre for High Performance Computing, Trinity College Dublin, Dublin 2
Coronal mass ejections (CMEs) are spectacular eruptions of plasma and magnetic field from the surface of the Sun into the heliosphere. Travelling at speeds of up to 2,500 km s-1 and with masses of up to 1016 g, they are recognised as drivers of geomagnetic disturbances and adverse space weather on Earth and on other planets in the solar system1,2. Recently, new methods to track CMEs in 3D have been developed for the Solar Terrestrial Relations Observatory (STEREO) mission. Launched by NASA in 2006, STEREO consists of two near-identical spacecrafts in heliocentric orbits ahead (STEREO-A) and behind (STEREO-B) the Earth, which drift away from the Sun-Earth line at a rate of ±22º per year (Figure 1). This provides a unique twin perspective of the Sun and inner heliosphere for studying CMEs in 3D3, with the aim of better understanding their true kinematics and morphology as they propagate through interplanetary space. To achieve this, each spacecraft is equipped with a suite of remote sensing telescopes comprising an EUV imager of the solar disk, two white-light coronagraphs COR1/2 with a field-of-view out to 15 R☉ (1 R☉ = 6.95x108 m), and two wide-angle heliospheric imagers HI1/2 that span a field-of-view out to the orbit of Earth. Physicists in Trinity’s Astrophysics Research Group are using these instruments to study and track CMEs as they erupt from the Sun through the heliosphere4,5.
On the 12 December 2008, an erupting prominence was observed by STEREO while the spacecrafts were in near quadrature at 86.7º separation (Figure 2a). The eruption was visible at 50-55º north of the Sun’s equator (Figure 2b), and the prominence is considered to be the inner material of the CME that was observed to propagate through the solar atmosphere over a number of hours (Figure 2c). The faint and diffuse morphology of CMEs makes them difficult to track using traditional image processing techniques, so we employ a method of multiscale edge detection for accurately determining the front of the CME structure. The CME front is best fit with an ellipse to characterise its propagation through the field-of-view of each instrument.
3D information may be gleaned from two independent viewpoints of a feature using tie-pointing techniques to triangulate lines of sight in space within an epipolar geometry. However, when the object is known to be a curved surface, sight lines will be tangent to it and not necessarily intersect upon it. Consequently, CMEs cannot be reconstructed by tie-pointing alone, but rather their localisation may be constrained by intersecting sight lines tangent to the leading edges of a CME. It is possible to extract the intersection of a given epipolar plane through the ellipse fits in both the STEREO-A and –B images, resulting in a quadrilateral in 3D space (Figure 3a). Inscribing an ellipse within the quadrilateral, such that it is tangent to all four sides, provides a slice through the CME that matches the observations from each spacecraft. A full reconstruction is achieved by stacking ellipses from numerous epipolar slices (Figure 3b). As the positions and curvatures of these inscribed ellipses are constrained by the characterised curvature of the CME front in each stereoscopic image pair, the modelled CME front is considered an optimum reconstruction of the true CME front. This is repeated for every frame of the eruption to build the reconstruction as a function of time and view the changes to the CME front as it propagates in 3D (Figure 3c). The full reconstruction is best explored with the use of the visualisation suite in TCHPC. This leads to detailed studies on the true CME kinematics and morphology, greatly improving the accuracy with which the CME impact at Earth may be predicted4.
This work is supported by the Science Foundation Ireland under Grant no. 07-RFP-PHYF399.
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