Toward understanding the early stages of an impulsively accelerated coronal mass ejection

Context. The expanding magnetic flux in coronal mass ejections (CMEs) often forms a cavity. Studies of CME cavities have so far been limited to the pre-event configuration to evolved CMEs at great heights, and to two-dimensional imaging data. Aims. Quantitative analysis of three-dimensional cavity evolution at CME onset can reveal information that is relevant to the genesis of the eruption. Methods. A spherical model was simultaneously fit to Solar Terrestrial Relations Observatory (STEREO) Extreme Ultraviolet Imager (EUVI) and Inner Coronagraph (COR1) data of an impulsively accelerated CME on 25 March 2008, which displays a well-defined extreme ultraviolet (EUV) and white-light cavity of nearly circular shape already at low heights h ≈ 0.2R⊙. The center height h(t) and radial expansion r(t) of the cavity were obtained in the whole height range of the main acceleration. We interpret them as the axis height and as a quantity proportional to the minor radius of a flux rope. Results. The three-dimensional expansion of the CME exhibits two phases in the course of its main upward acceleration. From the first h and r data points, taken shortly after the onset of the main acceleration, the erupting flux shows an overexpansion compared to its rise, as expressed by the decrease in the aspect ratio from κ = h/r ≈ 3 to κ ≈ (1.5–2). This phase is approximately coincident with the impulsive rise in the acceleration and is followed by a phase of very gradual change in the aspect ratio (a nearly self-similar expansion) toward κ ~ 2.5 at h ~ 10R⊙. The initial overexpansion of the CME cavity can be caused by flux conservation around a rising flux rope of decreasing axial current and by the addition of flux to a growing, or by even newly formed, flux rope by magnetic reconnection. Further analysis will be required to decide which of these contributions is dominant. The data also suggest that the horizontal component of the impulsive cavity expansion (parallel to the solar surface) triggers the associated EUV wave, which subsequently detaches from the CME volume