Small solar wind transients at 1 AU: STEREO observations (2007-2014) and comparison with near-Earth wind results (1995-2014)


This paper discusses small solar wind transients (STs) from 1995 to 2014. Using STEREO data, we have more sites from which to study STs near 1 AU. STEREO measurements are compared with wind observations near the Sun-Earth line. We examined statistically the dependence of ST occurrence frequency on (i) solar cycle phase, as monitored by the sunspot number (SSN), and (ii) solar wind speed. We find dependencies on both: an anticorrelation with SSN, an opposite trend to that of interplanetary coronal mass ejections (ICMEs), and correlation with slow solar wind (over 80% in the slow wind). We compare ST distributions during solar maximum year 2003, which had the lowest percentage of slow wind, and minimum year 2009, which had the highest percentage thereof, and show evidence of both dependencies. We give a statistical overview of ST parameters: field strength, B, Alfvén Mach number, MA, and proton beta, βp. They show the same temporal trends as the ambient solar wind but have twice (B) or one half (MA, βp) of its average values. In STs, the proton temperature is not below the temperature expected from corotating solar wind expansion. Non-force free models should be used in solar minimum years where βplasma≈ 1, while the force free models could be used in solar maximum when βplasma<<1. We find that only ∼5% of STs show enhanced values of iron charge states. Our work further supports the two-source origin of STs, i.e., the solar corona and the interplanetary medium.

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JGR: Space Physics



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