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

2020 ◽  
Vol 125 (8) ◽  
Author(s):  
Rafael L. A. Mesquita ◽  
Miguel F. Larsen ◽  
Irfan Azeem ◽  
Michael H. Stevens ◽  
Bifford P. Williams ◽  
...  
Keyword(s):  
High Latitude ◽  
Lower Thermosphere ◽  
Shear Instability ◽  
In Situ Observations ◽  
Mesosphere And Lower Thermosphere

The Momentum Budget in the Stratosphere, Mesosphere, and Lower Thermosphere. Part II: The In Situ Generation of Gravity Waves

2018 ◽  
Vol 75 (10) ◽  
pp. 3635-3651 ◽  
Author(s):  
Ryosuke Yasui ◽  
Kaoru Sato ◽  
Yasunobu Miyoshi
Keyword(s):  
Gravity Wave ◽  
Gravity Waves ◽  
Lower Thermosphere ◽  
Shear Instability ◽  
Wave Forcing ◽  
Momentum Budget ◽  
Mesosphere And Lower Thermosphere ◽  
In Situ Generation ◽  
Mlt Region

The contributions of gravity waves to the momentum budget in the mesosphere and lower thermosphere (MLT) is examined using simulation data from the Ground-to-Topside Model of Atmosphere and Ionosphere for Aeronomy (GAIA) whole-atmosphere model. Regardless of the relatively coarse model resolution, gravity waves appear in the MLT region. The resolved gravity waves largely contribute to the MLT momentum budget. A pair of positive and negative Eliassen–Palm flux divergences of the resolved gravity waves are observed in the summer MLT region, suggesting that the resolved gravity waves are likely in situ generated in the MLT region. In the summer MLT region, the mean zonal winds have a strong vertical shear that is likely formed by parameterized gravity wave forcing. The Richardson number sometimes becomes less than a quarter in the strong-shear region, suggesting that the resolved gravity waves are generated by shear instability. In addition, shear instability occurs in the low (middle) latitudes of the summer (winter) MLT region and is associated with diurnal (semidiurnal) migrating tides. Resolved gravity waves are also radiated from these regions. In Part I of this paper, it was shown that Rossby waves in the MLT region are also radiated by the barotropic and/or baroclinic instability formed by parameterized gravity wave forcing. These results strongly suggest that the forcing by gravity waves originating from the lower atmosphere causes the barotropic/baroclinic and shear instabilities in the mesosphere that, respectively, generate Rossby and gravity waves and suggest that the in situ generation and dissipation of these waves play important roles in the momentum budget of the MLT region.

Solar Orbiter observations of magnetic Kelvin-Helmholtz waves in the solar wind

2021 ◽  
Author(s):  
Rungployphan Kieokaew ◽  
Benoit Lavraud ◽  
David Ruffolo ◽  
William Matthaeus ◽  
Yan Yang ◽  
...  
Keyword(s):  
Solar Wind ◽  
Heliospheric Current Sheet ◽  
Shear Instability ◽  
Slow Solar Wind ◽  
Flow Shear ◽  
In Situ Observations ◽  
Set Up ◽  
Nonlinear Shear ◽  
Wind Source

<p>The Kelvin-Helmholtz instability (KHI) is a nonlinear shear-driven instability that develops at the interfaces between shear flows in plasmas. KHI is ubiquitous in plasmas and has been observed in situ at planetary interfaces and at the boundaries of coronal mass ejections in remote-sensing observations. KHI is also expected to develop at flow shear interfaces in the solar wind, but while it was hypothesized to play an important role in the mixing of plasmas and exciting solar wind fluctuations, its direct observation in the solar wind was still lacking. We report first in-situ observations of ongoing KHI in the solar wind using Solar Orbiter during its cruise phase. The KHI is found in a shear layer in the slow solar wind near the Heliospheric Current Sheet. We find that the observed conditions satisfy the KHI onset criterion from linear theory and the steepening of the shear boundary layer is consistent with the development of KH vortices. We further investigate the solar wind source of this event to understand the conditions that support KH growth. In addition, we set up a local MHD simulation using the empirical values to reproduce the observed KHI. This observed KHI in the solar wind provides robust evidence that shear instability develops in the solar wind, with obvious implications in the driving of solar wind fluctuations and turbulence. The reasons for the lack of previous such measurements are also discussed.</p>

Winds and tides of the Antarctic mesosphere and lower thermosphere: One year of meteor-radar observations over Rothera (68°S, 68°W) and comparisons with WACCM and eCMAM

2021 ◽  
Author(s):  
Shaun M Dempsey ◽  
Neil Hindley ◽  
Tracy Moffat-Griffin ◽  
Corwin Wright ◽  
Anne Smith ◽  
...  
Keyword(s):  
Planetary Wave ◽  
Lower Thermosphere ◽  
Model Development ◽  
Small Variation ◽  
Meteor Radar ◽  
Tidal Amplitude ◽  
Mesosphere And Lower Thermosphere ◽  
One Year ◽  
The Antarctic

<p>Tides are crucially important to the dynamics of the MLT. Therefore, models which aim to span the whole atmosphere must be capable of reproducing these tides, making observations of tides vital to constrain model development. Here, we present a novel climatology of 12- and 24-hour tides, measured at heights of 80–100 km by a meteor radar over the Rothera Station, Antarctica (68°S, 68°W). We use these observations to test two GCMs: WACCM and eCMAM (the latter 24-hr only). Our observations reveal large-amplitude tides with strong seasonal variability. The 12-hour tide maximises around the equinoxes and the smaller-amplitude 24-hour tide maximises in summer.<span>  </span>WACCM reproduces 12-hour tidal amplitudes at 80 km well, but not their increase with height or equinoctial maxima, and reproduces the observed small variation in 24-hr tidal amplitude with height well but with anomalously-large amplitudes. eCMAM reproduces observed 24-hr tidal amplitudes and their small variation with height. Our observations also reveal sizeable day-to-day variability in tidal amplitude at planetary wave periods, which we suggest originates from non-linear tidal/planetary-wave coupling. Furthermore, we see notable differences between observed and model background winds which are not reproduced in the models; we propose these differences may arise from the lack of in-situ gravity-wave sources in the models.</p>

scholarly journals In situ observations of waves in Venus’s polar lower thermosphere with Venus Express aerobraking

2016 ◽  
Vol 12 (8) ◽  
pp. 767-771 ◽  
Author(s):  
Ingo C. F. Müller-Wodarg ◽  
Sean Bruinsma ◽  
Jean-Charles Marty ◽  
Håkan Svedhem
Keyword(s):  
Lower Thermosphere ◽  
In Situ Observations ◽  
Venus Express

In situ observations of small scale neutral and plasma dynamics in the mesosphere/lower thermosphere at 79°N

2006 ◽  
Vol 38 (11) ◽  
pp. 2388-2393 ◽  
Author(s):  
B. Strelnikov ◽  
M. Rapp ◽  
T.A. Blix ◽  
N. Engler ◽  
J. Höffner ◽  
...  
Keyword(s):  
Lower Thermosphere ◽  
Small Scale ◽  
Plasma Dynamics ◽  
In Situ Observations

scholarly journals Double Star, Cluster, and ground-based observations of magnetic reconnection during an interval of duskward oriented IMF: preliminary results

2005 ◽  
Vol 23 (8) ◽  
pp. 2903-2907 ◽  
Author(s):  
J. A. Wild ◽  
S. E. Milan ◽  
J. A. Davies ◽  
S. W. H. Cowley ◽  
C. M. Carr ◽  
...  
Keyword(s):  
Magnetic Reconnection ◽  
Northern Hemisphere ◽  
High Latitude ◽  
Double Star ◽  
Flux Tubes ◽  
Flux Transfer Events ◽  
Subsolar Point ◽  
Magnetic Flux Tubes ◽  
In Situ Observations

Abstract. We present a space- and ground-based study exploiting data from the coordinated Cluster and Double Star missions in order to investigate dayside magnetic reconnection under BY+ dominated IMF conditions. In-situ observations of magnetosheath flux transfer events combined with measurements of pulsed poleward and dawnward directed flows in the pre-noon sector high-latitude northern hemisphere ionosphere are interpreted as indications of pulsed magnetic reconnection during an interval in which the IMF remained relatively steady. Observations of newly-reconnected magnetic flux tubes anchored in the northern hemisphere both at mid-latitudes and in the vicinity of the subsolar point suggests that during BY+ dominated IMF, reconnection is not, as proposed previously, limited to the high-latitude magnetopause.

High-latitude tidal behavior in the mesosphere and lower thermosphere

1989 ◽  
Vol 51 (7-8) ◽  
pp. 595-608 ◽  
Author(s):  
S.K. Avery ◽  
R.A. Vincent ◽  
A. Phillips ◽  
A.H. Manson ◽  
G.J. Fraser
Keyword(s):  
High Latitude ◽  
Lower Thermosphere ◽  
Mesosphere And Lower Thermosphere
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