In Situ Observations of Neutral Shear Instability in the Statically Stable High‐Latitude Mesosphere and Lower Thermosphere During Quiet Geomagnetic Conditions

Though the Kelvin‐Helmholtz instability (KHI) has been extensively observed in the mesosphere, where breaking gravity waves produce the conditions required for instability, little has been done to describe quantitatively this phenomenon in detail in the mesopause and lower thermosphere, which are associated with the long‐lived shears at the base of this statically stable region. Using trimethylaluminum (TMA) released from two sounding rockets launched on 26 January 2018, from Poker Flat Research Range in Alaska, the KHI was observed in great detail above 100 km. Two sets of rocket measurements, made 30 min apart, show strong winds (predominantly meridional and up to 150 ms−1) and large total shears (90 ms−1 km−1). The geomagnetic activity was low in the hours before the launches, confirming that the enhanced shears that triggered the KHI are not a result of the E‐region auroral jets. The four‐dimensional (three‐dimensional plus time) estimation of KHI billow features resulted in a wavelength, eddy diameter, and vertical length scale of 9.8, 5.2, and 3.8 km, respectively, centered at 102‐km altitude. The vertical and horizontal root‐mean‐square velocities measured 29.2 and 42.5 ms−1, respectively. Although the wind structure persisted, the KHI structure changed significantly with time over the interval separating the two launches, being present only in the first launch. The rapid dispersal of the TMA cloud in the instability region was evidence of enhanced turbulent mixing. The analysis of the Reynolds and Froude numbers (Re = 7.2 × 103 and Fr = 0.29, respectively) illustrates the presence of turbulence and weak stratification of the flow. Key Points Observations of the neutral shear instability in the high‐latitude mesopause/lower thermosphere region at 102‐km altitude are presented Statically stable region with instability is mainly driven by a large shear, often observed in that altitude range, even in quiet conditions The overturning structure of the instability is expected to produce mixing and vertical transport across the statically stable layer