VHF Imaging Radar Observations and Theory of Banded Midlatitude Sporadic E Ionization Layers

Abstract

The altitude range in the mesosphere/lower thermosphere where the turbopause occurs, that is, nearly an altitude of 100 km, is particularly important because of the rapid changes in dynamics, electrodynamics, and chemistry that occur there. The transition from the negative lapse rates in the mesosphere to isothermal, and even positive lapse rates in the lower thermosphere is a natural inhibitor to significant vertical transport across that region due to the inherently more stable air overlying the less stable air in the mesosphere. Layers in the mesosphere frequently show evidence of shear instabilities of the Kelvin-Helmholtz type, as shown in the case studies presented by Hecht et al. (2021) and Chau et al. (2020), for example. See also their discussion of other past observations from that region.Observations from higher altitudes near the critical turbopause transition are much more limited, but the available data indicate that the region is characterized by frequent and persistent shears associated with the large winds that occur there. The shears often meet the criteria for shear instability, as shown by Larsen (2002) using an extensive set of rocket-based wind measurements and by Sherman and She (2006) using a long time series of lidar wind measurements that extended to altitudes of 105 km. Liu (2017) was able to reproduce the essential characteristics of the observed winds and shears in that altitude range and discussed their implications for diffusion and transport. Recently Mesquita et al. (2020) presented an example of a K-H billow observed directly with chemical tracer measurements in the transition altitude where the more stable isothermal lapse rate occurs. One conclusion was that this type of instability can contribute significantly to vertical transport into the more stable portions of the atmosphere in the lower thermosphere, which would otherwise inhibit strong vertical motions.Data from the lower thermosphere are much more limited than data from lower altitudes in the mesosphere, but a long series of observations from the Caribbean have shown that there is a strong relationship between the development of coherent scatter radar echoes associated with sporadic E layers and upward displacements of the ionization layers. Furthermore, the quasi-periodic (QP) scattering structures that are now known to be a common characteristic of sporadic E layers have been tied to unstable shears in many of the observations. The vertical displacements associated with the billow structures are, therefore, the likely driver for the plasma instabilities responsible for the QP structure in the radar scatter.

Midlatitude Sporadic E‐Layer Horizontal Structuring Modulated by Neutral Instability and Mixing in the Lower Thermosphere

“…The radar itself uses software‐defined signal synthesis and reception. Additional system hardware and software details and some preliminary observations were presented by Hysell and Larsen (2021).…”

“…In this article, we report on observations of 30-MHz coherent backscatter from patchy sporadic-E ionization layers (E s layers) obtained using a radar in Ithaca, New York, in the summer of 2021. This is a follow-on to the preliminary study presented by Hysell and Larsen (2021). The data set considered here is more extensive and includes 13 events observed between July and September 2021.…”

“…In a series of papers, we have used such observations to analyze both the electrodynamics responsible for the plasma irregularities and the neutral shear instabilities and upwelling that drive the processes. The recent paper by Hysell and Larsen (2021) discusses coherent scatter data showing evidence for neutral shear instabilities of the Kelvin‐Helmholtz type and the evolution of secondary instabilities. References to earlier work can also be found in that article.…”

Simulation of Es Layer Modulated by Nonlinear Kelvin–Helmholtz Instability

“…In this work, the negative vorticity layer and the adjacent positive vorticity layer form a jet flow. The secondary vortices (Hysell and Larsen, 2021) are triggered in the strongly sheared braid regions. The secondary vortices roll up the stratification boundary layer with a higher velocity.…”

“…The negative vorticity layers and the adjacent positive vorticity layers create a jet flow, as shown in the left panel (a) of Figure 6. The jet flow moves and amplifies along with the braids, leading to the generation of the secondary vortices (Hysell & Larsen, 2021) in the braid regions between the weakened primary KH vortices, as shown in the right panel (a) of Figure 6. The thin ion layers between the positive and negative vorticity layers converged, which leads to the accumulation of ions in the middle of the vortex region and the formation of the secondary vortex structure of ions in the braid regions, as shown in right panel (b) of Figure 6.…”