Kelvin-Helmholtz Instability under southward IMF: THEMIS observations and OpenGGCM simulations

While Kelvin-Helmholtz (KH) waves for southward IMF were long thought to be non-existent, both the Kavosi and Raeder [2015] study, and two other independent studies [Hwang et al., 2011; Yan et al., 2014] have found southward IMF KH events. It still remains a mystery, though, why KH under southward IMF occurs only at one quarter of the rate compared to northward IMF [Kavosi and Raeder, 2015], and whether or not such waves occur only under specific conditions. The previous study [Hwang et al., 2011] suggested that the irregular and temporally intermittent structure of KH waves due to dynamically active sub solar behavior under southward IMF condition may explain the preferential in situ detection of KH waves under northward IMF. This explanation is also consistent with the KH events under southward IMF in our database. The majority of the events during southward IMF are irregular, short and polychromatic in compare to regular, long lasting and monochromatic waves under northward IMF. To effectively isolate these differences, we have used both our extensive THEMIS KH event database and OpenGGCM simulations. Our simulation results show that the KH waves under southward IMF are irregular, higher frequency, lower amplitude, and polychromatic compared to northward IMF. Additionally, our statistical analysis shows that occurrence rate of KH wave as a function of solar wind plasma parameters is different under southward IMF compared to northward IMF.