Numerical study on interchange instability as generation mechanism of dipolarization fronts in the magnetotail

Abstract:

Energy and magnetic flux transports associated with flow bursts and bursty bulk flows (BBFs) are considered to be important during substorm activity in the magnetotail. Dipolarization fronts (DFs) play important roles in transporting energy fluxes and accelerating particles. Although Hall effect and electron pressure gradient effect on the mesoscale of ion inertial length was considered in our previous study, observations indicated that the key features on DFs are on the scale of ion gyro-radius, which means that the ion finite Larmor radius (FLR) effect might have influence on the mesoscale of DF. Resent investigations demonstrate that the gyro-viscous cancellation arising due to the FLR effect would cause the drifts of the structure of interchange instability in the direction of ion diamagnetic drift. Therefore, it is reasonable to speculate that the FLR effect would be the cause for the dawnward drifting movement of DFs. Two dimensional Hall MHD simulation argumented by FLR effects was performed to reproduce the key mesoscale feature of interchange instability as generation mechanism of DFs. Numerical results indicated that the interchange instability is a solid candicate of generation mechanism of DFs. On DFs, Hall effects make the plasma density and magnetic field asymmetric in the dawn-dusk direction, the electric field is mainly produced by Hall term, and the contributions from the convectional and EPG electric fields are very small. The FLR effect becomes important in the regime L>>ρ_i, where L is the characteristic scale length. FLR effect arises due to the gyro-viscous component of the ion stress tensor that appears in the moment equations. The simplified expressions of the gyro-viscous stress can be frequently approximated in the dimensionless form by ▽·π_i≈-d_iρV_*·▽V, where V_* is ion velocity associated with the so-called gyroviscous cancellation via subtracting a significant part from the advective acceleration. Despite the fact that the gyro-motion velocity is composed by ion diamagnetic velocity and the magnetic drift velocity, the gyro-motion velocity is mainly contributed by ion diamagnetic velocity. Therefore, the ion diamagnetic velocity determines the drifting motion of the whole structure of the interchange instability.