THEMIS observations of plasma transport via eddy diffusion

Abstract. We provide an event study of THEMIS observations of the low-latitude boundary layer in the dayside magnetosphere. Simultaneous multipoint observations obtained on 5 December 2008 show that the magnetosheath-like plasma in the low-latitude boundary layer is transferred earthward from the magnetopause. This earthward transport is accompanied by decrease in the density and fluctuating bulk flow. We calculate the eddy diffusion coefficients, which can be estimated from the observed velocity data, and found that the numbers are in good quantitative agreement with the spatial and time scales of the observed earthward transport signatures. It is shown that other possible plasma transport processes such as convection or diffusion induced by plasma wave turbulence are inconsistent with the observations. Our study strongly suggests that the observed transport is due to diffusive transport via turbulent eddy motions as is the case of an ordinary (Navier–Stokes) fluid.

Download Full-text

Plasma transport into the duskside magnetopause caused by Kelvin–Helmholtz vortices in response to the northward turning of the interplanetary magnetic field observed by THEMIS

10.5194/angeo-2019-103 ◽

2019 ◽

Author(s):

Guang Qing Yan ◽

George K. Parks ◽

Chun Lin Cai ◽

Tao Chen ◽

James P. McFadden ◽

...

Keyword(s):

Magnetic Field ◽

Boundary Layer ◽

Interplanetary Magnetic Field ◽

Time History ◽

Plasma Transport ◽

Low Latitude ◽

Unique Event ◽

History Of ◽

The Magnetic Field ◽

Low Latitude Boundary Layer

Abstract. A train of Kelvin–Helmholtz (K–H) vortices with plasma transport across the magnetopause has been observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) when the interplanetary magnetic field (IMF) abruptly turns northward. This unique event occurred without pre-existing denser boundary layer to facilitate the instability. Two THEMIS spacecraft, TH-A and TH-E, separated by 3 Re, periodically encountered the duskside magnetopause and the low-latitude boundary layer (LLBL) with a period of 2 minutes and tailward propagation of 194 km/s. There was no high-velocity low-density feature, but the rotations in the bulk velocity observation, distorted magnetopause with plasma parameter fluctuations and the magnetic field line stretching, indicate the formation of rolled-up K–H vortices at the duskside magnetopause. A mixture of magnetosheath ions with magnetospheric ions and enhanced energy flux of hot electrons is identified in the K–H vortices. This mixture region appears more periodic at the upstream spacecraft and more dispersive at the downstream location, indicating a significant transport can occur and evolve during the tailward propagation of the K–H waves. There is still much work to fully understand the Kelvin–Helmholtz mechanism. The observations of direct response to the northward turning of the IMF, the unambiguous plasma transport within the vortices, involving both ion and electron fluxes can provide additional clues to the K–H mechanism.

Download Full-text