Simulation of the interchange instability in a magnetospheric substorm site

Abstract

Abstract. We perform modeling of the interchange instability driven by longitudinal pressure asymmetry in the region of the pressure buildup that forms in the inner magnetosphere at the substorm growth phase. The simulation refers to the dawnward side of the Harang discontinuity and times after B z IMF turning northward. The solution for the equilibrium state indicates tailward flows associated with vortices, which is in agreement with a previous finding of Ashour-Abdalla et al. (1999, 2002). We show that in the regions of equilibrium field-aligned currents (FACs), small initial perturbations in pV γ (p is the isotropic plasma pressure, V is the unit magnetic flux tube volume, γ =5/3 the adiabatic exponent), set up as ripples inclined to azimuth, grow in time. For the background FAC of ∼10 −6 A/m 2 , the linear growth rate of the instability is ∼ 6 min. Starting from the 12th min of evolution, the perturbations exhibit nonlinear deformations, develop undulations and front steepening. An interesting peculiarity in the distribution of the associated small-scale FACs is that they become asymmetric with time. Specifically, the downward currents are more localised, reaching densities up to 15×10 −6 A/m 2 at the nonlinear stage. The upward FACs are more dispersed. When large enough, these currents are likely to produce the aurora. We also run our simulation for the initial perturbations of large transverse scales in order to demonstrate that the interchange instability can be responsible for pressure and cross-tail current spatial variations of great extent.

Criterion for interchange instability in a plasma connected to a conducting ionosphere

“…[60] We remark that, although this early work of Volkov and Mal'tsev [1986] did not include the shear effect, recent work by the same group has done so. Mingalev et al [2006] present numerical solutions to illustrate how changes in IMF B z can affect interchange in the plasma sheet and thus affect the substorm phenomenon. Our paper focuses on the basic physics involved in interchange with shear, using a highly idealized geometry.…”

“…[2] Interchange instability in Earth's magnetosphere has been discussed by many authors in various contexts [e.g., Gold, 1959;Cheng, 1985;Southwood and Kivelson, 1987;Hameiri et al, 1991]. It has been advanced as an explanation for auroral [Golovchanskaya and Maltsev, 2003] and substorm features [Mingalev et al, 2006] as well as bursty bulk flows [Pontius and Wolf, 1990;Chen and Wolf, 1993] and auroral undulations [Sazykin et al, 2002]. Lemaire [1975] maintains that centrifugal-force-driven interchange governs the shape and size of the plasmapause.…”

“…[11] It should be noted that Mingalev et al [2006] included shear flow in calculations of interchange instability in substorms. The present paper focuses on the basic physics of interchange instability in a magnetospheric system with velocity shear, not on implications for magnetospheric phenomena.…”

Periodic black auroral patches at the dawnside dipolarization front during a substorm

“…It should be noted that the downward current shown as black color does not necessarily dominate over the upward current in our model setting. However, previous simulation study of interchange instability by Mingalev et al [2006] have already show that, at the nonlinear stage, downward field‐aligned currents is more localized and has lager local densities compared to the upward field‐aligned currents.…”

To the Interchange Instability Criterion in the Magnetosphere in the Presence of Velocity Shear

“…Later, Mal'tsev and Mingalev (2000) generalized the results of Volkov and Mal'tsev (1986) for the arbitrary β plasma configuration, but plasma velocity shear was still neglected in that study. The velocity shear was included in the simulation of the interchange instability in a magnetospheric substorm by Mingalev et al (2006).…”