North‐South Asymmetric Kelvin‐Helmholtz Instability and Induced Reconnection at the Earth's Magnetospheric Flanks

Abstract

We present a three-dimensional study of the plasma dynamics at the flank magnetopause of the Earth's magnetosphere during mainly northward interplanetary magnetic field periods. Two-fluid simulations show that the initial magnetic shear at the magnetopause and the field line bending caused by the dynamics itself (in a configuration taken as representative of the properties of the flank magnetopause) influence both the location where the Kelvin-Helmholtz (KH) instability and the induced magnetic reconnection take place and their nonlinear development. The KH vortices develop asymmetrically with respect to the Earth's equatorial plane where the local KH linear growth rate is maximal. Vortex-driven reconnection processes take place at different latitudes, ranging from the equatorial plane to midlatitude regions but only in the hemisphere that turns out to be the less KH unstable. These results suggest that KH-induced reconnection is not limited to specific regions around the vortices (inside, below, or above) but may be triggered over a broad and continuous range of locations in the vicinity of the vortices.

Latitudinal Dependence of the Kelvin‐Helmholtz Instability and Beta Dependence of Vortex‐Induced High‐Guide Field Magnetic Reconnection

“…In summary, global simulations of the whole magnetosphere as well as a local simulation of the KH instability on the dusk flank magnetopause indicate that the MMS satellites are in the proper location to detect well developed KH structures and, as a consequence, magnetic reconnection induced by the vortices. Furthermore, the local simulation suggests that reconnection proceeds on a wide latitude band as already observed in numerical simulations starting from a configuration similar to the present one (Fadanelli et al, 2018).…”

“…Well‐formed rolled‐up structures are present from z = − λ KH ≈ − 12,000 km to z = +3 λ KH ≈ 36,000 km, as shown by the folded magnetopause, while the magnetopause at z = ± 4 λ KH ≈ ± 48,000 km is nearly unperturbed. The KH development is asymmetric with respect to the equatorial plane, as expected when a flow‐aligned component of the IMF is present (Fadanelli et al, 2018), even if all the other fields are symmetric. In the present case with B IMF · U SW < 0 the vortices develop more vigorously for z > 0, as expected.…”

“…We now analyze a local 3‐D two‐fluid (Hall‐MHD) simulation of the dusk flank to further demonstrate the KH development and vortex rolling‐up during this event, confirming the fact that MMS satellites are well located and consistent with the occurrence of induced reconnection. This simulation starts from a modeled equilibrium, as in Fadanelli et al (2018) that takes as asymptotic values (far away from the magnetopause) the plasma quantities measured during the event (Eriksson et al, 2016) in the boundary layer (outer magnetosphere) and in the magnetosheath plasma depletion layer, respectively. A three‐dimensional rendering of the simulation is given in Figure 2b.…”

Finite-Larmor-radius equilibrium and currents of the Earth’s flank magnetopause

“…where B 0 = B 2 G + ∆B 2 ⊥ and ϑ is the angle between the z-axis and the magnetic field at x → −∞ †. The above magnetic profile accounts both for variations that are purely in magnitude, through ∆B , and for rotations (magnetic shear) of the magnetic-field direction, through ∆B ⊥ (see, e.g., Fadanelli et al 2018, for the effects of ∆B ⊥ on KHI at the Earth's magnetospheric flanks). Note that usually x u,0 = x B,0 and L u = L B are assumed in numerical simulations (see, e.g., Miura 1987;Fujimoto & Terasawa 1995;Otto & Fairfield 2000;Nykyri & Otto 2004;Nakamura & Fujimoto 2005;Faganello et al 2008Faganello et al , 2012Palermo et al 2011;Tenerani et al 2011).…”

Interplay between Kelvin–Helmholtz and lower-hybrid drift instabilities

“…These vortices may in turn feed secondary instabilities, developing on the shoulder of the primary KHI. A typical example is provided by the vortex-induced magnetic reconnection in various forms (e.g., Nakamura & Fujimoto 2008;Faganello et al 2009;Nakamura et al 2013;Nakamura & Daughton 2014;Fadanelli et al 2018), or by the development of pressure anisotropies able to trigger kinetic instabilities (e.g., De Camillis et al 2016). Moreover, when the two plasma flows have different densities, the Rayleigh-Taylor instability (RTI) may be triggered by the large-scale vortex motion (Matsumoto & Hoshino 2004;Faganello et al 2008a,b).…”