  The expanding magnetic flux in coronal mass ejections (CMEs) often forms a
cavity. A spherical model is simultaneously fit to STEREO EUVI and 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 ~ 0.2 Rs. The center height h(t) and radial expansion
r(t) of the cavity are 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, respectively. 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 of the aspect
ratio from k=h/r ~ 3 to k ~ (1.5-2.0). This phase is approximately coincident
with the impulsive rise of the acceleration and is followed by a phase of very
gradual change of the aspect ratio (a nearly self-similar expansion) toward k ~
1.5 at h ~ 10 Rs. 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 even newly forming,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.
