Evidence of unusually cold material inside Coronal Mass Ejections

Coronal mass ejections (CMEs) are explosive events which escape the Sun's corona carrying solar material and energy into interplanetary space. The classic picture of a CME observed in the corona presents a "three-part structure", including a bright front at the leading edge indicating dense plasma, a low density cavity which may include an embedded flux rope, and the so-called core which is often observed to be associated with the erupting prominence. Although there are experimental analogs to the first two parts of the CME when observed in situ, there are very few (~5) observations of cold, prominence type plasma. Several previous studies have identified cold material inside ICMEs based on the presence of He+, thought to be evidence of the erupting prominence, including work in the 1980s by Schwenn (1980), Bame (1983), and Zwickl et al. (1982) where over a period of 8 years only 3 events with cold charge states were reported. A similar event occurred in 1998 (Skoug et al., 1999) for which Gloeckler et al. (1999) reported the first high-resolution observations of extremely low charge states of C, O and Fe observed in an ICME. We present the first long-time survey of such low charge states observed onboard the Advanced Composition Explorer (ACE) Solar Wind Ion Composition Spectrometer (SWICS). We find that these cold events are much more common than previously assumed: in a survey of 10 years of SWICS data we find 12 events, 10 of which are "cold ICME" events, roughly 5% of all ICMEs observed during this time.