The August 11th, 1999 CME

Abstract

We present here a set of observations, space borne and ground based, at different wavelengths, of the solar corona at and after the time of the total solar eclipse of August 11th. It is used to consider some unusual features of the coronal dynamics related to a limb Coronal Mass Ejection (CME) observed after the total eclipse. The complementary aspect of simultaneous ground-based and space-borne observations of the corona is used to produce an accurate composite image of the White Light (W-L) corona before the CME. A high arch system (possibly a dome-like structure, with large cavities inside but without a cusp further out) which appeared on the eclipse W-L images, is suggested to be a large-scale precursor of the CME, well preceding the eruption of the top part of the brightest prominence recorded in W-L. This bright prominence is shown as a filament in absorption using the Transition Region And Corona Explorer (TRACE) images taken in different coronal lines. The analysis of the images of the Large Angle and Spectrometric Coronograph (LASCO) on board the Solar and Heliospheric Observatory (SoHO), showing the progression of the CME, is discussed in an attempt to make a connection with the surface event. A SoHO-EIT (Extreme UV Imager Telescope) image sequence details the prominence eruption and shows the sudden heating processes of the ejected parts. We found that there is no reason to assume that the huge cavity is significantly destabilised well before the eruption of the upper part of the low-lying bright twisted filament which coincides with the position of one of the legs of the high arch. Observations are still compatible with the assumption of both the break-out model and of the flux rope erupting model as a result of a shear or of an increasing poloidal magnetic flux from below. We stress the possible role of buoyancy of the giant cavity as a destabilizing factor leading to the CME, noticing that some motion of coronal material back toward the surface can be seen during at least the first phase of the CME, from both EIT and LASCO observations.