THEMIS survey of the frequency of Kelvin-Helmholtz waves at Earth's magnetopause

It has been long suggested that Kelvin-Helmholtz (KH) waves at Earth's low-latitude magnetopause play an important role in the entry of solar wind plasma into the magnetosphere, in particular under northward interplanetary magnetic field (IMF) conditions. In the past it remained unknown, due to sparse data ,how frequently KH waves occurs; however, with THEMIS we now have a much richer data set. Thus, to investigate the frequency of KH waves at the magnetopause, we have conducted a survey of THEMIS data to create a database of magnetopause crossings throughout the entire mission. We have classified the crossings according to their characteristics, with particular emphasis on crossings that exhibit waves. We find that about half of the crossings show waves or quasi-periodic variations, but not all of them are KH waves. Often, such periodic or quasi-periodic structures seem to be caused by FTEs or by solar wind fluctuations. Preliminary analysis, using very conservative criteria to positively identify KH waves, suggests that KH waves are present at the magnetopause approximately 10-20% of the time. That frequency is much higher than previous estimates and suggests that KH waves may play a much larger role transporting plasma across the magnetopause than previously thought. We also find that KH waves are not restricted to periods of large solar wind velocity and/or nearly northward IMF. We have found cases with KH waves for solar wind speeds as low as 350 km/s.