2014 THEMIS SCIENCE NUGGETS
Are Kelvin-Helmholtz vortices present under southward Interplanetary Magnetic Field?
THEMIS observations say “Yes”
by Guangqing Yan, State Key Laboratory of Space Science,
Center for Space Science and Applied Research, Chinese Academy of Sciences
Introduction
The Kelvin-Helmholtz (K-H) vortex is the nonlinear stage of one surface wave sandwiched between the two different regions with velocity shear. At the flanks of the magnetopause, because the solar wind flows tailward and produces a shear with the stagnant magnetospheric plasma, it is amenable to excite the K-H instability and K-H waves. The nonlinear stage of the K-H surface wave could possibly cause the transport of solar wind into magnetosphere through the low-latitude boundary layer (LLBL).
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| Figure 1. The periodical crossings of the flank side of the magnetopause by the 3 spacecraft of TH-A, TH-D, TH-E, and the vorticity normal to the 3-point triangle plane. |
K-H vortices at the flank of the magnetopause were generally thought to occur preferentially under interplanetary magnetic field (IMF). Almost all the reported of such vortices were observed under northward IMF. Can the K-H vortices occur under southward IMF? Recently, THEMIS observations said “Yes” with clear evidence and regularity, detecting the induced electric field caused by the magnetic field compression at the edges of the vortices.
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| Figure 2. The vortex features in the velocity and electric field observation in LMN coordinates. |
Results
During the interval from UT 10:30 to UT 11:20 on October 12, 2012, THEMIS observed several magnetopause crossings periodically at the duskside of magnetopause under southward IMF, with significant sunward returning flows inside the magnetopause. The TH-B at lunar orbit (in the upstream solar wind with a ten-minute time lag) monitored the steady southward IMF. The vortex features of the flows and the periodic enhancements in the calculated vorticity normal to the spacecraft plane could be found in the observation, and the propagating features could be seen in the time lags from one spacecraft to another. The electric field observation at the magnetopause indicated the distortion of the magnetopause caused by the K-H vortices. Another important finding is the circular-induced electric field at the edges of the K-H vortices caused by the magnetic field compression, which can be estimated to be about several mV/m according to Faraday’s Law, consistent with the observations from spacecraft. This is the first observational report of such induced electric field at the edges of K-H vortices. This is another report of the K-H vortices under southward IMF after the first one by Hwang et al (2011), further indicating that the vortices can also occur under southward IMF, which is contrary to the previous understanding. This new observational evidence could help to understand the K-H vortex and its role in solar wind transport into magnetosphere.
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| Figure 3. The comparison of the observed electric field with convective electric field in measurements of TH-E. |
Conclusion
Under southward IMF, THEMIS observed the K-H vortices with periodicity and regularity at the duskside of magnetopause, with significantly larger returning flows inside the magnetopause. The distortion of the magnetopause, the periodic features of the vortex flows, the tailward propagation, and the evaluation of unstable condition imply that the vortices can be formed, with a tailward propagation of about 292 km/s. The observed electric field mainly comes from the convective electric field at a large scale, and induced electric field of about several mV/m was deduced at both leading and trailing edges of vortices when the magnetic field compression occurred. The bipolar signatures in the induced electric field imply that the induced electric field is circular around the compressed magnetic field, as predicted by Faraday’s Law.
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| Figure 4. The model to estimate the induced electric field at the edge of the K-H vortex. |
Reference
Yan, G. Q, F. S. Mozer, C. Shen, T. Chen, G. K. Parks, C. L. Cai, J. P. Mcfadden (2014), Kelvin-Helmholtz vortices observed by THEMIS at the duskside of magnetopause under southward IMF, Geophys. Res. Lett., 41, 4427-4434, doi:10.1002/2014GL060589.Biographical Note
Guangqing Yan is a faculty at the State Key Laboratory of Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences. His research interest includes magnetopause boundary layers and solar wind transport into the magnetosphere.
Forrest Mozer is an American experimental physicist, inventor, professor and associate director at the Space Science Laboratory, University of California, Berkeley. He is well known best for his pioneering work on electric field measurements in space plasma and for development of solid state electronic speech synthesizers and speech recognizers.
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