Date of Award
Program or Major
Doctor of Philosophy
Charles J. Farrugia
Charles J. Farrugia
Using case studies and statistical analysis on a large data sample we investigate properties of small solar wind transients (STs) and discuss their modeling. The observations are from the Wind and STEREO spacecraft. By "small" we mean a duration of 0.5-12 hours. We do not restrict ourselves to magnetic flux ropes. Having arrived at a definition based on an extension of previous work, we apply an automated algorithm to search for STs. In one chapter we focus on the solar activity minimum years 2007-2009. We find an average ST duration of ~4.3 hours, with 75% lasting less than 6 hours. A major difference from large-scale transients (i.e. ICMEs) in the same solar minimum is that the low proton temperature (Tp) is not a robust signature of STs, which is opposite to the trend in ICMEs. Further, the plasma beta (electrons + protons) ~ 1 and thus force free modeling of flux rope STs may not be appropriate. We then examine a much wider sample covering almost 2 solar cycles (1995-2014). After Alfve'nic fluctuations are removed, we obtain about 2000 STs. We find that their occurrence frequency has a two-fold dependence: it is (i) correlated strongly with slow solar wind speeds, and (ii) anti-correlated with solar activity, as monitored by the sunspot number. As regards (i) we find that over 80% of STs occur in the slow wind (< 450 km/s). The anti-correlation with solar cycle activity is contrary to what is observed with ICMEs. Most of the STs convect with the ambient solar wind. Studying the normalized expansion parameter, we conclude that many STs do not expand at all, i.e. they are static structures. Only ~ 5% of STs show enhanced values of iron charge states.
We also find that the plasma beta of STs depends on solar activity level, being << 1 for maximum and of order 1 or more for solar minimum. Thus non-force free models should be used in solar minimum years while the force free models could be used in solar maximum. Motivated by these results, we then explore ST modeling with static, non-force free methods, using two analytical (with circular and elliptical cross-sections, respectively) and one numerical model (Grad-Shafranov reconstruction). We illustrate and compare results for 8 examples of flux rope STs. The two analytical models give fairly similar results. We show that our non-force free models can also fit the data as well, or even better, than the force free model.
Grad-Shafranov reconstruction shows that the small flux ropes tend to have elliptical cross-section. Finally, we address some aspects of the disturbances in the Earth's magnetosphere, focusing on substorms and storms. We find substorm occurrence to be relatively common during passages of STs at Earth: ~47% of STs of duration 1-5 hours were associated with substorms, a conclusion reached in other studies but here valid over a much larger data set. Further, about 3% of these STs were associated with geomagnetic storms.
Yu, Wenyuan, "Small Solar Wind Transients 1995 - 2014: Properties, Modeling, and Effects on the Magnetosphere" (2016). Doctoral Dissertations. 2265.