7 research outputs found
The occurrence altitudes of middle atmospheric temperature inversions and mesopause over low-latitude Indian sector
We study the occurrence characteristics of mesospheric inversion layers
(MILs) in the 60–105 km altitude region over the low-latitude Indian sector. We
note that lower inversions in the mesospheric temperatures occur in the
70–75 km altitude regions while the upper inversions occur in 90–95 km
altitude regions. The mesopause altitude is mostly noted to be ~ 98 km
with the night-time mesopause (particularly in the year 2002) showing a small
peak in the mesopause occurrence at ~ 75 km altitude. We note higher
occurrence rate of MILs during high solar activity year compared to low
solar activity year. It is also observed that night time MILs show a
systematic seasonal variability, with higher occurrence of single and double
temperature inversions during equinoxes
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Evidence for the In‐Situ Generation of Plasma Depletion Structures Over the Transition Region of Geomagnetic Low‐Mid Latitude
On a geomagnetic quiet night of October 29, 2018, we captured an observational evidence of the onset of dark band structures within the field-of-view of an all-sky airglow imager operating at 630.0 nm over a geomagnetic low-mid latitude transition region, Hanle, Leh Ladakh. Simultaneous ionosonde observations over New Delhi shows the occurrence of spread-F in the ionograms. Additionally, virtual and peak height indicate vertical upliftment in the F layer altitude and reduction in the ionospheric peak frequency were also observed when the dark band pass through the ionosonde location. All these results confirmed that the observed depletions are indeed associated with ionospheric F region plasma irregularities. The rate of total electron content index (ROTI) indicates the absence of plasma bubble activities over the equatorial/low latitude region which confirms that the observed event is a mid-latitude plasma depletion. Our calculations reveal that the growth time of the plasma depletion is ∼2 h if one considers only the Perkins instability mechanism. This is not consistent with the present observations as the plasma depletion developed within ∼25 min. By invoking possible Es layer instabilities and associated E-F region coupling, we show that the growth rate increases roughly by an order of magnitude. This strongly suggests that the Cosgrove and Tsunoda mechanism may be simultaneously operational in this case. Furthermore, it is also suggested that reduced F region flux-tube integrated conductivity in the southern part of onset region created conducive background conditions for the growth of the plasma depletion on this night
Impacts of acoustic and gravity waves on the ionosphere
The impact of regional-scale neutral atmospheric waves has been demonstrated to have profound effects on the ionosphere, but the circumstances under which they generate ionospheric disturbances and seed plasma instabilities are not well understood. Neutral atmospheric waves vary from infrasonic waves of <20 Hz to gravity waves with periods on the order of 10 min, for simplicity, hereafter they are combined under the common term Acoustic and Gravity Waves (AGWs). There are other longer period waves like planetary waves from the lower and middle atmosphere, whose effects are important globally, but they are not considered here. The most ubiquitous and frequently observed impact of AGWs on the ionosphere are Traveling Ionospheric Disturbances (TIDs), but AGWs also affect the global ionosphere/thermosphere circulation and can trigger ionospheric instabilities (e.g., Perkins, Equatorial Spread F). The purpose of this white paper is to outline additional studies and observations that are required in the coming decade to improve our understanding of the impact of AGWs on the ionosphere
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Impacts of acoustic and gravity waves on the ionosphere
The impact of regional-scale neutral atmospheric waves has been demonstrated to have profound effects on the ionosphere, but the circumstances under which they generate ionospheric disturbances and seed plasma instabilities are not well understood. Neutral atmospheric waves vary from infrasonic waves of <20 Hz to gravity waves with periods on the order of 10 min, for simplicity, hereafter they are combined under the common term Acoustic and Gravity Waves (AGWs). There are other longer period waves like planetary waves from the lower and middle atmosphere, whose effects are important globally, but they are not considered here. The most ubiquitous and frequently observed impact of AGWs on the ionosphere are Traveling Ionospheric Disturbances (TIDs), but AGWs also affect the global ionosphere/thermosphere circulation and can trigger ionospheric instabilities (e.g., Perkins, Equatorial Spread F). The purpose of this white paper is to outline additional studies and observations that are required in the coming decade to improve our understanding of the impact of AGWs on the ionosphere
Mesospheric gravity wave characteristics and identification of their sources around spring equinox over Indian low latitudes
We report OI557.7 nm night airglow observations with the help of a charged-couple device (CCD)-based
all-sky camera from a low-latitude station, Gadanki (13.5° N;
79.2° E). Based on the data collected during March and April over 3
years, from 2012 to 2014 (except March 2013), we characterize the small-scale
gravity wave properties. During this period, 50 gravity wave events
were detected. The horizontal wavelengths of the gravity waves are found to
ranging from 12 to 42 km with the phase velocity 20–90 m s<sup>−1</sup>. In most
cases, these waves were propagating northward with only a few
occurrences of southward propagation. In the present novel investigation
from the Indian sector, each of the wave events was reverse-ray-traced to
its source. The outgoing longwave radiation (OLR) suggested that
tropospheric convection was a possible source for generation of the observed
waves. It was found that approximately 66 % of the events were triggered
directly by the convection
Investigation of a Dissipating Mesospheric Bore Using Airglow Imager and Direct Numerical Simulation
AbstractAtmospheric gravity waves play an important role in driving the dynamics of the Mesosphere and Lower Thermosphere and the basic structure of this region is determined by momentum deposition of these waves. Mesospheric bores are a type of non‐linear response that cause the amplification of gravity wave, due to trapping, that is characterized by a propagating step‐like jump followed by undulating waves. They require a stable layer or duct to travel horizontally with little attenuation thereby capable of transporting wave energy and momentum over larger distances. We present a prominent bright undular bore event observed in the mesospheric O(1S), O2, and OH emission layers on 16 March 2021 over Germany. A striking feature of this observation is the capture of bore's rapid dissipation around the center of the imager's field of view. The vertical temperature profile obtained from the satellite data indicates the presence of temperature inversion layer which acted as a thermal duct for the bore propagation. In addition, we have performed idealized two dimensional direct numerical simulations (DNS) of Navier‐Stokes equations under Boussinesq approximation. The DNS results reproduce many important characteristics of the observed airglow event like the nonlinear wave‐steepening, number of trailing waves, and its dissipation by implementing a thermal duct and a wave‐like perturbation. Furthermore, the DNS results also indicate that the duct width and amplitude of the initial perturbation have a considerable effect on the bore morphology.Key Points:
Observation of a mesospheric bright bore event that dissipated within the field of view
The duct that enabled the bore propagation was near the O(1S) emission layer based on the observational data
The majority of the observed features are reproduced with idealized 2D direct numerical simulations using Boussinesq approximation
Deutsche Forschungsgemeinschaft
http://dx.doi.org/10.13039/501100001659Alexander von Humboldt‐Stiftung
http://dx.doi.org/10.13039/100005156https://doi.org/10.22000/809http://sirius.bu.edu/data/http://saber.gats-inc.com/coin.ph