2018 ARTEMIS SCIENCE NUGGETS
ARTEMIS observations of Kelvin-Helmholtz waves in the Earth’s magnetotail
by Quanqi Shi & Yiming Ling
Shandong University
Introduction
The instability that arises where there is a difference in velocity at the boundary between two fluids, like winds blowing across water, can create Kelvin-Helmholtz waves (KHWs). This looks like a series of rolling breakers hitting the beach. KHWs have been widely observed at the magnetopause in the region near the Earth, and play an essential role in the transport of solar wind plasma and energy into the magnetosphere under dominantly northward interplanetary magnetic field (IMF) conditions. Yet the conditions under which these waves form and how they evolve over time are still poorly understood, since the previous observations are mainly reported in the near-Earth region. It is necessary to cover both terrestrial space and the magnetotail with more observations in order to provide more detailed benchmarks for theoretical modeling of nonlinear KH dynamics and plasma transport processes along the magnetopause. Here, we report and analyze an event observed by the ARTEMIS spacecraft in the magnetotail on 13 and 14 March 2014, and we also present simultaneous observations by the Geotail spacecraft in the region near the Earth.
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| Figure 1. Time series data in GSM coordinates taken by Geotail (GT, in red), ARTEMIS-P1 (P1, in black) and P2 (blue) from 19:00 UT, 13 March to 02:00 UT, 14 March 2014.Panels show three components of the magnetic field, ion number density, average ion temperature, three components of the bulk velocity, total pressure, energy flux of ions at P1 and P2, respectively. The yellow shaded region indicates the interval of interest. |
Results
An overview of simultaneous observations by the ARTEMIS (P1 and P2) and Geotail spacecraft from opposite sides of Earth’s magnetopause during an instability event that occurred between 13 and 14 March 2014 is shown in Figure 1. From 21:10 UT on 13 March to about 00:48 UT on 14 March, P1 and P2 entered a magnetospheric boundary layer region and observed the obvious quasi-periodic fluctuations of plasma parameters and magnetic fields, which are identified as KHW signals. A Schematic view (Figure 2) shows how Kelvin-Helmholtz instability waves in Earth's magnetic tail might propagate through time and space.
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| Table 1: The period, phase velocity, and spatial scale calculated from ARTEMIS and Geotail data. |
The KHW properties are characterized by their frequency, phase velocity and the wavelength. We calculated these parameters from ARTEMIS and Geotail data (shown in Table 1). Comparing the results gained via data from the two satellites at different locations, the results offer evidence that KHWs develop in Earth’s magnetic tail and that their wavelengths increase as they move tailward along the boundary layer, a finding that agrees with previously published simulations. The observations also suggest that vortices created by the instability develop at roughly the same time on the tail’s dawn and dusk sides, although slight differences between the two sets of satellite observations prevented we from completely ruling out dusk-dawn asymmetry.
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| Figure 2. Schematic view showing how Kelvin-Helmholtz instability waves in Earth's magnetic tail might propagate through time and space. (https://eos.org/research-spotlights/measurements-of-kelvin-helmholtz-waves-in-earths-magnetic-field) |
Conclusion
We present simultaneous observations of KHWs by the ARTEMIS spacecraft (P1 and P2) and Geotail at different locations when the IMF is dominantly northward and provide observational evidence for solar wind-magnetosphere interactions in the tail region near the lunar orbit. By a comparison of the results between these two satellites, we can infer that either (i) the phase velocity and the spatial scale of the KHWs tend to increase along the boundary layer when the waves propagate tailward or (ii) the difference between dawnside and duskside observations is due to the influence of dawn-dusk asymmetry. The time evolution of KHWs and processes within rolled-up vortices that facilitate plasma transport and mixing still remain unclear. Therefore, more observational studies to clarify these unsolved questions will be required.
References
Ling, Y., Shi, Q., Shen, X.-C., Tian, A., Li, W., Tang, B., et al. (2018), Observations of Kelvin-Helmholtz waves in the Earth’s magnetotail near the lunar orbit, Journal of Geophysical Research: Space Physics, 123, 3836–3847, doi:10.1029/2018JA025183Cook, T. (2018), Measurements of Kelvin-Helmholtz waves in Earth’s magnetic field, Eos, 99, https://doi.org/10.1029/2018EO101199. Published on 20 July 2018.
Biographical Note
Quanqi Shi is on the faculty of the Space Sciences Institute, Shandong University, China. His current research involves studying solar wind-magnetosphere interactions. Yiming Ling was an undergraduate student at Shandong University studying space physics, and now is a graduate student in University of Science and Technology of China (USTC)
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu