Dipolarization and turbulence in the plasma sheet during a substorm: THEMIS observations and global MHD simulations

Abstract

Spacecraft in the magnetotail have observed fluctuations in velocity and magnetic field that have the characteristics of fluid turbulence. We investigated the properties of these fluctuations during a substorm on 7 February 2009. During this event several spacecraft were observing the magnetotail. By using upstream observations performed by Wind, we conducted a global magnetohydrodynamic (MHD) simulation of the substorm. The simulation results were compared to time series of Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations in the magnetotail, and it performed well as the spacecraft encountered dipolarizations and strong flows. Further, the power spectral densities and probability distribution functions observed in the magnetotail by THEMIS and in the MHD simulations were similar. Notable features of the event include a large dipolarization growing by accreting smaller earthward moving dipolarization fronts. The dipolarizations were associated with a strong channel of earthward flow and a large vortex. In the MHD simulation results, which are supported by the observations, strong narrow flow channels are a clear and persistent feature of magnetotail convection which exist at the driving scales for the turbulent spectra. Key Points Simulated and observed fluctuations have similar spectral indices Fluctuations have the characteristic properties of fluid turbulence Dipolarizations and high-speed flows drive turbulence ©2013. American Geophysical Union. All Rights Reserved.