A statistical analysis of properties of small transients in the solar wind 2007-2009: STEREO andWind observations

摘要

We present a comprehensive statistical analysis of small solar wind transients (STs) in 2007-2009. Extending work on STs by Kilpua et al. (2009) to a 3 year period, we arrive at the following identification criteria: (i) a duration < 12 h, (ii) a low proton temperature and/or a low proton beta, and (iii) enhanced field strength relative to the 3 year average. In addition, it must have at least one of the following: (a) decreased magnetic field variability, (b) large, coherent rotation of the field vector, (c) low Alfven Mach number, and (d) T-e/T-p higher than the 3 year average. These criteria include magnetic flux ropes. We searched for STs using Wind and STEREO data. We exclude Alfvenic fluctuations. Case studies illustrate features of these configurations. In total, we find 126 examples, approximate to 81 of which lie in the slow solar wind ( 450 km s(-1)). Many start or end with sharp field and flow gradients/discontinuities. Year 2009 had the largest number of STs. The average ST duration is approximate to 4.3 h, 75 < 6 h. Comparing with interplanetary coronal mass ejections (ICMEs) in the same solar minimum, we find the major difference to be that T-p in STs is not significantly less than the expected T-p. Thus, whereas a low T-p is generally considered a very reliable signature of ICMEs, it is not a robust signature of STs. Finally, since plasma approximate to 1, force-free modeling of STs having a magnetic flux rope geometry may be inappropriate.