An Empirical Reconstruction of the 2008 April 26 Coronal Mass Ejection

Abstract

We present a three-dimensional model of the density distribution of a coronal mass ejection (CME) from 2008 April 26. This CME was observed by the two spacecraft composing the Solar Terrestrial Relations Observatory (STEREO), which tracked the CME from near the Sun, into the interplanetary medium (IPM), and all the way to 1 AU. The CME was directed toward STEREO-B and hit that spacecraft on 2008 April 29. The STEREO images of the CME show an internal structure that can be interpreted as having a flux rope shape. The two different perspectives on the event provided by the two STEREO spacecraft allow us to make a particularly strong argument for the flux rope interpretation, and the STEREO data also allow us to study the evolution of the flux rope in the IPM. The flux rope is oriented close to the ecliptic plane, but with the western leg tilted northwards by about 20 • . This implies an orientation roughly perpendicular to the neutral line of the active region at the event's point of origin, apparently an unusual geometry given that previous analyses have found that CME flux ropes are usually, but not always, oriented parallel to the neutral lines of their source regions. The CME model also consists of a front out ahead of the flux rope, possibly a shock launched by the flux rope driver. The model density distribution is reasonably successful at reproducing the CME appearance both close to the Sun in coronagraphic images, and far from the Sun in images of the IPM from STEREO's heliospheric imagers. This suggests that self-similar expansion is a reasonable first-order approximation for this particular CME, and also indicates that the flux rope's orientation does not change much during its journey through the IPM.

ARRIVAL TIME CALCULATION FOR INTERPLANETARY CORONAL MASS EJECTIONS WITH CIRCULAR FRONTS AND APPLICATION TOSTEREOOBSERVATIONS OF THE 2009 FEBRUARY 13 ERUPTION

“…It is actually so faint that it cannot be discerned in STEREO-B HI movies; thus, the observations limit our study to using single-spacecraft HI methods and it is not possible to use any triangulation approaches. As seen in the first image, the ICME clearly has a ring-like shape with a small angular width of roughly 20 • , which is consistent with a view of a croissantshaped ICME edge-on (Thernisien et al 2009;Wood & Howard 2009), i.e., we look along the axis of the ICME, which is pointing out of the paper plane. The second density enhancement at its sunward end disappears around −15 • elongation, leaving only the leading edge visible.…”

“…There exist two practical realizations of this concept: the original technique (Rouillard et al 2008), which assumes a point-like or negligibly narrow shape of the CME front (also called the fixed-Φ approximation; we call this fixed-Φ fitting or FPF), and the method by Lugaz (2010), which assumes a circular CME front, attached to the Sun at all times at one end (harmonic mean fitting or HMF). Note that there also exist more complex approaches to the same problem, such as the Tappin-Howard (T-H) model (Howard & Tappin 2009aTappin & Howard 2009), in which a large set of pre-existing solutions is fitted to the observations, which is also useful for real-time application, and the white-light rendering method by Wood & Howard (2009). In this study we use the FPF and HMF methods, and we present the formulae for converting elongation to distance with FP/HM and the inverted method useful for predicting ICME parameters with FPF/HMF.…”

Combining STEREO SECCHI COR2 and HI1 images for automatic CME front edge tracking

“…Wood & Howard (2009) have performed a detailed kinematic analysis of this event. We compared their outcomes (Wood & Howard 2009) against the reference and our automatic estimates. As shown in Table 3, the estimated speeds using the COR2 images have larger disagreement, whereas HI1 estimates agree very well.…”

“…Coronal mass ejections (CMEs) are large-scale expulsions of plasma and magnetic field from the solar atmosphere (Tousey 1973;Brueckner 1974;MacQueen et al 1974). It is well known that they play a crucial role in disturbing the space environment as they propagate into the interplanetary medium and interact with the Earth's magnetic field producing strong geomagnetic storms (Allen 1944;Burlaga & Lepping 1977;Gosling et al 1990;Crooker & McAllister 1997;Srivastava & Venkatakrishnan 2002, 2004Srivastava et al 2009).…”

Regarding the detectability and measurement of coronal mass ejections

“…As with those developed for coronagraphs, techniques of varying complexity and sophistication have been developed. Some noteworthy examples include , Wood & Howard (2009), Lugaz et al (2010, Byrne et al (2010), and Möstl & Davies (2013).…”