Ultraviolet Coronagraph Spectrometer Observation of the 1996 December 23 Coronal Mass Ejection

The Ultraviolet Coronagraph Spectrometer (UVCS) aboard the Solar and Heliospheric Observatory ( SOHO ) observed a spectacular coronal mass ejection (CME) caused by a prominence eruption on 1996 December 23. The evolution of the ejected prominence material was followed for 1 hr and 50 minutes. The observation consists of a series of 5 minute exposures, at a fixed heliocentric distance of 1.5 R in several spectral ranges. The Lyman lines of hydrogen brighten more than 2 orders of magnitude during the CME. The C III 977.02 Å line is very bright, and many other low-temperature lines have been detected. Line intensities and profiles provide important diagnostics for the physical and dynamical parameters of the ejected plasma. Lines widths show nonthermal line broadening due to a plasma expansion with velocity larger than 50 km s -1 . The Lyα inside the CME region shows red and blue shifts, up to 0.2 Å (50 km s -1 ) and 0.8 Å (200 km s -1 ), respectively. A preliminary estimate shows a flat emission measure distribution 3 or 4 orders of magnitude smaller than typical prominence emission measures. Essentially the same structure in space and velocity is seen in the Lyman lines, in C III (10 5 K) and in O VI (3 × 10 5 K).