Overturning instability in the mesosphere and lower thermosphere: analysis of instability conditions in lidar data

Hurd, L.; Larsen, M. F.; Liu, Alan

doi:10.5194/angeo-27-2937-2009

Cited by 5 publications

(7 citation statements)

References 11 publications

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“…Ground-based lidar observations show characteristic periods of 1-3 h for the large-scale overturning in sodium layers. A more extensive discussion of the theoretical predictions and the observations can be found in the articles by Larsen et al (2004) and Hurd et al (2009). Since the Ekman-type intability and associated overturning structures occur in the same altitude range as the strong sporadic E layers and coherent scatter structures, it is reasonable to expect that such instabilities can modulate the longer-term variation in the scattering layers when they are present, as suggested by Chkhetiani and Shalimov (2013).…”

Section: Discussionmentioning

confidence: 94%

“…(2004) and Hurd et al. (2009). Since the Ekman‐type intability and associated overturning structures occur in the same altitude range as the strong sporadic E layers and coherent scatter structures, it is reasonable to expect that such instabilities can modulate the longer‐term variation in the scattering layers when they are present, as suggested by Chkhetiani and Shalimov (2013).…”

Section: Discussionmentioning

confidence: 96%

“…There are other instabilities in the region, however, that can lead to a significant neutral upwelling of the type required to produce coherent radar echoes. Larsen et al (2004) and Hurd et al (2009) pointed out the similarities between conditions in the atmospheric boundary layer and the MLT region, including strong rotational shears, wind profile inflection points, and a region of higher static stability overlying a region of lower static stability, which are all conditions required for convective roll, that is, an Ekman-type instability to form. More recently, Chkhetiani and Shalimov (2013) have suggested specifically that the convective rolls may be responsible for the frontal structures in sporadic E layers underlying the QP echoes.…”

Section: Discussionmentioning

confidence: 99%

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

Hysell

Larsen

2021

JGR Space Physics

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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.

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Section: Discussionmentioning

confidence: 94%

Section: Discussionmentioning

confidence: 96%

Section: Discussionmentioning

confidence: 99%

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

Hysell

Larsen

2021

JGR Space Physics

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“…The region can also be subjected to other types of long‐lived instabilities, such as the convective roll instability first discussed by Larsen (2004) and Hurd et al. (2009) in the context of the mesopause/lower thermosphere interface. Mondal et al.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2020

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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.

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“…However, the speed and turning shears that occur there are conducive to other neutral instabilities as well. Larsen et al ( 2004) and Hurd et al (2009) pointed out the similarities between the atmospheric boundary layer and the MLT transition region and presented lidar observations that were consistent with the expected characteristics for convective rolls associated with Ekman-type inflection point instability. Observations from the MLT region are extremely limited in general in spite of the critical importance of the region.…”

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confidence: 67%

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

2023

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Observations of 30‐MHz coherent backscatter from sporadic‐E ionization layers were obtained with a VHF imaging radar located in Ithaca, New York. The volume probed by the radar lies at relatively high magnetic latitudes, on the northern edge of the mid‐latitude region and underneath the ionospheric trough. Banded, quasi‐periodic (QP) echoes observed from Ithaca are similar to those found in lower midlatitude regions. The Doppler shifts observed are smaller and, so far, do not appear to reach the threshold for Farley‐Buneman instability. However, many of the echoes exhibit fine‐scale structure, with secondary bands or braids oriented obliquely to the primary bands. Secondary bands have been seen only rarely at lower middle latitudes. In previous observations, the QP scattering has been linked to unstable neutral wind shears. Neutral wind shear commonly found in the lower thermosphere could play a key role in the formation of these irregularities and explain some morphological features of the resulting plasma density irregularities and the radar echoes. We consider whether neutral instability and turbulence in the lower thermosphere is the likely cause for some of the structuring in the sporadic‐ E layers. Results of 3D numerical simulations of atmospheric dynamics in the mesosphere to lower thermosphere support the proposition. In particular, we focus on Ekman‐type instabilities that, like the more common Kelvin‐Helmholtz instabilities, are inflection point instabilities, although specifically associated with turning shears, and result in convective rolls aligned close to the mean wind direction, with smaller‐scale secondary waves aligned normal to the primary structures.

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Overturning instability in the mesosphere and lower thermosphere: analysis of instability conditions in lidar data

Cited by 5 publications

References 11 publications

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

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

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

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

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