Statistical Study of ICMEs with Low Mean Carbon Charge State Plasmas Detected from 1998 to 2011

We present a statistical study of 219 ICMEs measured by both ACE and WIND from 1998 to 2011. ICME plasmas are defined as possessing cold materials if the carbon average charge states are lower than those of the preceding solar wind by three standard deviations and the carbon ionic temperature is lower than 10^6.05 K. A total of 69 ICMEs were identified as containing cold materials. These ICMEs tend to have speeds in the range of 300-600 km s^-1, with durations between 2 and 6 hr. Cold materials tend to be present once or twice per ICME. We further identify two special types of cold materials: the ionic-cold type (IC) shows simultaneous lower average charge states of O, Mg, Si, and Fe ions than those of the preceding solar wind, while the carbon-only cold type (COC) shows a totally opposite trend in that these ions show higher average charge than in the preceding solar wind. We found that the IC has a higher proton temperature than the ICME mean value, whereas the COC has a lower proton temperature than the ICME mean value, and the COC is most often measured in magnetic cloud. A detailed examination of the IC and the COC material suggests that they are related to solar filaments. Their special mean charge indicates that the filaments are a mixture of coronal and choromospheric materials. Heating and collision processes beyond the carbon freeze-in height are crucial in generating the two distinct types.