Abstract
The MMS satellites encountered a Kelvin-Helmholtz instability (KHI) period in the early non-linear phase at the post-noon flank magnetopause on 8 Sep 2015. The adjacent magnetosheath was characterized by a pre-dominantly northward Bz > 0 magnetic field with weakly positive in-plane components in a GSM coordinate system. Ion velocity distribution functions indicate at least 17 KH vortex intervals with two typically D-shaped ion beam distributions, commonly associated with reconnection exhausts, that stream in both directions along a mostly northward magnetic field at 350–775 km/s with a median 525 km/s ion beam speed. The counter-streaming ion beams are superposed on a core population of slowly drifting magnetosheath ions with a field-aligned 50–200 km/s speed. Each interval lasted no more than 5.25 s with a median duration of 1.95 s corresponding to in-plane spatial scales 3 < ΔS < 22 di assuming a constant 1 di = 61 km ion inertial scale and a tailward VKH∼258 km/s KH vortex propagation speed along the MMS trajectory. The counter-streaming ions are predominantly observed in the warm KH vortex region between the cold magnetosheath proper and the hot isotropic ion temperature of a low-latitude boundary layer as the MMS constellation traverses a KH vortex. The in-plane spatial scales and the locations of the observed counter-streaming ion beams generally agree with the predictions of twice-reconnected magnetic fields at two mid-latitude reconnection (MLR) regions in a two-fluid three-dimensional numerical simulation previously reported for this KH event. MMS typically recorded a higher phase space density of the fast parallel ion beam that we associate with a tailward reconnection exhaust from the southern MLR (SMLR) and a lower phase space density of the fast anti-parallel ion beam that we associate with a tailward reconnection exhaust from the northern MLR (NMLR) of similar speed. This is either consistent with MMS being closer to the SMLR region than the NMLR region, or that the KHI conditions may have favored reconnection in the SMLR region for the observed in-plane magnetosheath magnetic field as predicted by a two-fluid three-dimensional numerical simulation.
Highlights
A fast magnetosheath flow around the Earth’s magnetosphere can trigger a Kelvin-Helmholtz (KH) instability (Chandrasekhar, 1961; Miura and Pritchett, 1982) when the stabilizing interplanetary magnetic field (IMF) is mostly perpendicular to the shear flow across the magnetopause surface (Kokubun et al, 1994)
The commonly observed D-shaped nature of the two ion beams (Cowley, 1982), which are superposed on a slowly drifting magnetosheath population and typically without a hot low-latitude boundary layer (LLBL) population, suggests that the beams are associated with magnetosheath ion particle transmission at a more distant magnetopause reconnection region from the MMS satellites along the magnetic field due to the absence of local reconnection exhausts
The typical T < TKH ion travel times for a field-aligned VB 525 km/s ion speed and a field-aligned distance L∼λKH from an assumed nearby mid-latitude reconnection (MLR) region, as compared with T > 3TKH for ions at this same speed between a northern high-latitude region and MMS at a ∼18 RE field-aligned distance, suggest that the two D-shaped ion beams observed by MMS are probably associated with two nearby MLR magnetopause reconnection regions, or else MMS would have likely measured a more common banana-shaped counter-streaming ion beam distribution (Fuselier et al, 2014) associated with the closed magnetic fields of a double high-latitude reconnection (HLR) region, and across most of the closed field regions of the entire KH vortex
Summary
A fast magnetosheath flow around the Earth’s magnetosphere can trigger a Kelvin-Helmholtz (KH) instability (Chandrasekhar, 1961; Miura and Pritchett, 1982) when the stabilizing interplanetary magnetic field (IMF) is mostly perpendicular to the shear flow across the magnetopause surface (Kokubun et al, 1994). We performed a thorough survey of burst-mode ion velocity distribution functions (VDFs) that the MMS satellites obtained on 8 Sept 2015 for distinct evidence of sustained periods of two counter-streaming ion beams during an early non-linear phase of the KH waves on the dusk flank magnetopause (Nakamura et al, 2017) and relatively close in latitude to a region of maximum KHI growth (Vernisse et al, 2020). The current density measured across the leading edge of the subsequent KH vortex period, as shown, is rather benign and mostly weaker than ∼0.5 μA/m2 when the two distinct ion beams are observed streaming in opposite directions along the magnetic field. When one of the two ion beams, with observed speeds in a range 350 < VB < 775 km/s (Table 2), is absent or displaying a highly suppressed PSD before or after the times listed in Table 1, resulting in one single ion beam, so too does the energy flux of the bi-directional electron signature tend to change, and often in such a way that more electron energy flux will be observed in the same direction along the magnetic field as the remaining field-aligned ion beam
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
