Dynamically Unstable Strong Wind Shears Observed in the Polar Mesosphere Summer Echo Layer Associated With Geomagnetic Disturbances

Abstract

We report observations of dynamically unstable strong wind shear (Richardson number < 0.25) capable of inducing Kelvin‐Helmholtz instability in the polar mesospheric summer echoes (PMSE) layer (80–90 km) using very high frequency radar measurements in Kiruna (67.8°N, 20.4°E), Sweden, in 2006. The unstable strong wind shear can play an important role in producing PMSE by inducing turbulence and adiabatic cooling. We find that the strong shears take up 64% of the observed wind profiles and are frequently composed of systematically single‐shear/multishear (layer) structures, which gradually or abruptly vary in wind directions, so‐called wind shifts, through heights at intervals of 4–8 km. The strong shear rate normalized by PMSE counts has a good correlation (R = 0.7) in day‐to‐day variation with energetic electron (>30 keV) precipitations that were related to high‐speed solar wind streams. The observations of strong wind shear can be supported by satellite‐measured temperature modulations matching with the peaks of the first three high‐speed solar wind stream events. This study suggests that PMSE production is closely associated with the strong shear that is in turn linked to the effects of energetic electron precipitation.

Study of a Quasi‐27‐Day Wave in the MLT Region During Recurrent Geomagnetic Storms in Autumn 2018

“…The recurrent geomagnetic storms are believed to be triggered by periodic oscillations in solar winds (Lei et al., 2008; Tsurutani et al., 2006; Temmer et al., 2007). Energetic particle precipitations during recurrent geomagnetic activities can modulate the temperature and ozone in the MLT region, which leads to disturbances in mesospheric neutral densities and neutral winds in the polar region (Lee et al., 2020; Meredith et al., 2011; Yi, Reid, Xue, Younger, Murphy, et al., 2017). Using the temperature and ozone measured by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) satellite (version 2.0 level 2A), we further examine the MLT temperature and ozone evolutions over BJ during the recurrent geomagnetic storms in the autumn of 2018.…”

“…(2015) found evidence of neutral temperature enhancements in the MLT region during geomagnetic storms, which is caused by the southward transport of the storm‐time heating zone. Anomalies of neutral winds in the MLT region have been also found during geomagnetic storms (e.g., Lee et al., 2020; Salah & Goncharenko, 2001). Using the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model simulations at mid‐latitudes, Li et al.…”

“…According to the neutral temperature measurement by a mid-latitude sodium lidar, Yuan et al (2015) found evidence of neutral temperature enhancements in the MLT region during geomagnetic storms, which is caused by the southward transport of the storm-time heating zone. Anomalies of neutral winds in the MLT region have been also found during geomagnetic storms (e.g., Lee et al, 2020;Salah & Goncharenko, 2001). Using the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model simulations at mid-latitudes, Li et al (2019) suggested that storm-time temperature variations could reverse the direction of meridional winds in the MLT region through the upward vertical winds.…”

Polar Mesospheric Summer Echo Characteristics in Magnetic Local Time and Height Profiles

“…The shear persisted for more than 6 min. Lee et al (2020) investigated using a hodogram to determine whether the shear is connected to winds propagating from lower altitudes. The findings revealed that the shear was independent of the upward propagation of wind and instead abruptly occurred at local altitudes.…”

“…Recently, energetic electron precipitation induced by geomagnetic disturbance has been recognized as a significant contributor to PMSE production (Zeller & Bremer 2009;Kirkwood et al 2013;Lee et al 2013Lee et al , 2014Lee et al , 2015Lee et al , 2018Lee et al , 2020. During high-speed solar wind streams (HSSs), PMSE exhibits periodicities of 7, 9, and 13.5 days, in phase with solar wind speed and the auroral electrojet (AE) index (Lee et al 2013).…”