A simple MHD model for the formation of multiple dipolarization fronts

Abstract

A simplified MHD model is proposed that explains characteristic features of dipolarization fronts observed by the five‐probe THEMIS mission, and in particular the recurrent or multiple fronts, as structures arising from the nonlinear evolution of the interchange instability of the initial reconnection ejecta in the terrestrial magnetotail. Modeling the effects of the magnetic field curvature and plasma braking by an effective gravity and imposing an initial seed perturbation consistent with the observed dawn‐dusk scale of fronts is shown to reproduce the observed variations of the north‐south magnetic field, bulk flow plasma velocity, number density and pressure.

Dipolarization fronts in the near-Earth space and substorm dynamics

“…To the contrary, in our model low entropy bubbles/dipolarization fronts are formed from the reconnection of the stretched geomagnetic field with the earthward moving flux ropes leading to the generation of reconnected flux tubes of increased volume V and low plasma density. The pressure balance inconsistency stems from the fact that the ratio β of particle pressure to magnetic pressure obtains unduly high values in the near-Earth equatorial plane when a flux tube shortens considerably as it convects earthward from −60 to −10 R E (Erickson and Wolf, 1980), assuming adiabatic compression of plasma during the earthward magnetoplasma transport.…”

“…The phenomenological model introduced in this study provides a simple explanation of the so-called "pressure balance inconsistency" or "pressure crisis" problem first noted by Erickson and Wolf (1980), since it does not involve the arrival of flux tubes from the distant tail to the near-Earth region. To the contrary, in our model low entropy bubbles/dipolarization fronts are formed from the reconnection of the stretched geomagnetic field with the earthward moving flux ropes leading to the generation of reconnected flux tubes of increased volume V and low plasma density.…”

“…γ is the ratio of specific heats, P is the plasma pressure within the flux tube, and V = (1/B)ds is the volume of the flux tube containing 1 Wb of magnetic flux, where s is the length of the flux tube. They reflect the properties of reconnection regions they originate from and thus are of key importance for our understanding of magnetospheric substorm dynamics, and this is the reason why there is a plethora of studies investigating these plasma structures both theoretically/numerically (Sitnov and Swisdak, 2011;Nakamura et al, 2002;Birn et al, 2011;Sitnov et al, 2009;Guzdar et al, 2010) and observationally (Nakamura et al, 2001(Nakamura et al, , 2002Runov et al, 2009;Sergeev et al, 2009;Slavin et al, 2003a, b), just to mention but a few.…”

Reconnection and Waves: A Review with a Perspective

“…Possible explanations are that these recurrent appearances reflect multiple jet ejection or that they are due to the spacecraft's traversal of a highly structured front. Guzdar et al (2010) showed that the depolarization front could deform itself into a mushroom shape and proposed that the multiple sharp magnetic jumps seen in the time-series data were reproduced in a virtual observation along a cut through the mushroom-shaped front. 5 Magnetic field data from four THEMIS spacecraft aligned along the tail-axis observing a depolarization front sequentially at X = −20.1 R e , −16.7 R e and at −11.0 R e .…”

Three-Dimensional Simulations of Magnetic Reconnection with or Without Velocity Shears

“…Another example can be processes like Earthward and tailward depolarization front evolution connected to substorm events which are unstable to the interchange instability. Recent MHD numerical simulations of magnetotail current sheet and consequent depolarization front dynamics show different instability regimes (unsteady reconnection, kinking instability, interchange instability) leading to a final very turbulent state (Lapenta and Bettarini 2011a;Guzdar et al 2010) as reported in Fig. 11 displaying a two-dimensional clip of the density at the center of three-dimensional box.…”