Observations of ICMEs and ICME-like Solar Wind Structures from 2007-2010 Using Near-Earth and STEREO Observations

The generally low interplanetary magnetic field magnitude around the minimum between Solar Cycles 23 and 24 (SC 23/24 minimum) allows us to identify weak and small solar wind structures. We use observations from near-Earth and twin STEREO spacecraft to study solar wind conditions from January 2007 through December 2010. In addition to 84 clear interplanetary coronal mass ejections (ICMEs), we identified 58 ICME-like transients, which exhibit some classical ICME signatures but have weak magnetic fields (< 7 nT) and/or short durations (< 10 hours). The number of ICME-like transients peaked during the SC 23/24 minimum, while the ICME rate increased with increasing solar activity. The magnetic structures of flux rope type ICMEs and transients show similar solar cycle variation trends, suggesting that ICMEs and transients originate from similar polarity regions at the Sun. We observed a gradual transition from ICME-like structures to ICMEs. The identified events display continuous distributions in duration and magnetic field magnitude ranging from a few hours to several days and from a few nanoteslas to a few tens of nanoteslas, respectively. Our ICME-like transient rate (less than one event/month) is considerably smaller than that suggested by solar observations of narrow CMEs. This implies that the majority of small coronal ejections are merged as a part of the solar wind by the time they reach 1 AU. We found that ICME-like transients generally occur closer to stream interaction regions (SIRs) than ICMEs, and the majority of the events we identified in declining parts of fast solar wind streams were ICME-like structures. This suggests that ICME-like transients tend to arise close to coronal hole boundaries and thus may have an important role in coronal hole dynamics. Diverse solar wind transients presumably manifest the variation of solar eruptions from small-scale blobs to wide CMEs.