Solar and Heliospheric ObservatoryObservations of a Helical Coronal Mass Ejection

The EUV Imaging Telescope (EIT), Large Angle Spectrometric Coronagraph (LASCO), and Ultraviolet Coronagraph Spectrometer (UVCS) instruments aboard the SOHO satellite observed a prominence eruption (coronal mass ejection) on 1997 December 12. Ejected plasma moved at about 130 km s -1 in the plane of the sky and showed Doppler shifts between -350 and +30 km s -1 . The eruption appeared as a strongly curved arch in EIT images low in the corona. Emission in ions ranging from Si III to O VI in the UVCS spectra indicates a temperature range between 10 4.5 and 10 5.5 K. The morphology of the bright emission regions seen by all three instruments suggests several strands of a helical structure of moderate pitch angle. A reasonable fit to the spatial structure and the velocity evolution measured by UVCS is provided by a left-handed helix untwisting at a rate of about 9 × 10 -4 radians s -1 .