Projection effects in coronal mass ejections

Abstract

Context. Basic observational parameters of a coronal mass ejection (CME) are its speed and angular width. Measurements of the CME speed and angular width are severely influenced by projection effects. Aims. The goal of this paper is to investigate a statistical relationship between the plane-of-sky speeds of CMEs and the direction of their propagation, hopefully providing an estimate of the true speeds of CMEs. Methods. We analyze the correlation between the plane-of-sky velocity and the position of the CME source region, employing several non-halo CME samples. The samples are formed applying various restrictions to avoid crosstalk of relevant parameters. For example, we select only CMEs observed to radial distances larger than 10 solar radii; we omit CMEs showing a considerable acceleration in the considered distance range and treat CMEs of different angular widths separately. Finally, we combine these restriction criteria, up to the limits beyond which the statistical significance of the results becomes ambiguous. Results. A distinct anti-correlation is found between the angular width of CMEs and their source-region position, clearly showing an increasing trend towards the disc center. Similarly, all of the considered subsamples show a correlation between the CME projected speed and the distance of the source region from the disc center. On average, velocities of non-halo limb-CMEs are 1.5−2 times higher than in the case of non-halo CMEs launched from regions located close to the disc center. Conclusions. Unfortunately, the established empirical relationships provide only a rough estimate of the velocity correction as a function of the source-region location. To a certain degree, the results can be explained in terms of CME cone models, but only after taking crosstalk of various parameters and observational artifacts into account.

Acceleration in Fast Halo CMEs and Synchronized Flare HXR Bursts

“…Such a feedback relationship is naturally established by magnetic reconnection occurring in a current sheet behind the CME, as envisaged in the standard flare/CME picture. On the one hand, magnetic reconnection adds poloidal flux to the CME sustaining the Lorentz force which drives the CME acceleration (Chen 1996;Chen & Krall 2003;Vršnak et al 2004). On the other hand, the higher the acceleration of the CME, the larger the space that is evacuated per unit time in the coronal region behind.…”

“…This indicates that during the impulsive acceleration phase it is the Lorentz force that mainly drives the CME. As the electric current in the erupting system decreases as the CME expands and propagates out from the Sun, the Lorentz force weakens and the drag force owing to the solar wind becomes important (e.g., Cargill et al 1996;Chen 1996;Vršnak et al 2004).…”

“…2001(Zhang et R , al. , 2004Vršnak et al 2004;Maričić et al 2007). For a substantial fraction of the studied events, a close relationship between both phenomena was found.…”

“…This means that a considerable part of the early CME evolution appears against the solar disk; i.e., it cannot be observed by coronagraphs. Therefore, it is important to identify CME structures in on-disk coronal images (see studies by Hudson et al 1996;Dere et al 1997;Alexander et al 2002;Gallagher et al 2003;Uralov et al 2005;Vršnak et al 2006;Dauphin et al 2006).…”

Effects of Source Position on the Dh-Type Ii Cme Properties

“…Vrsnak et al(2007) examined several non-halo CMEs and found a correlation between plane-of-sky velocity and distance of the CME from the disc center. That is, CMEs with lower longitudes have lower speeds than the CMEs near limb.…”