Long‐term tendencies in the mesosphere/lower thermosphere mean winds and tides as observed by medium‐frequency radar at Tirunelveli (8.7°N, 77.8°E)

Sridharan, S.; Tsuda, Toshitaka; Gurubaran, S.

doi:10.1029/2008jd011609

Cited by 34 publications

(39 citation statements)

References 44 publications

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“…Moreover, the O abundance is too low and its lifetime is shorter (few seconds) in the upper stratosphere for the collisional excitation of CO 2 molecule, and thus, the SC response is comparatively smaller in the upper stratosphere than in the upper mesosphere (above 80 km). In addition, since the QBO and ENSO are the prominent interannual variabilities in tropical middle atmosphere (e.g., Sridharan et al, ; Lee et al, , and references therein; Li et al, ), the present study also showed their responses to CO 2 cooling in the SMLT region. The positive responses are larger in the upper mesosphere than at lower heights in association with the solar insolation and dynamical effects.…”

Section: Summary and Discussionsupporting

confidence: 67%

Long‐Term Trends in Tropical (10°N–15°N) Middle Atmosphere (40–110 km) CO₂ Cooling

Ramesh

Sridharan

2018

JGR Space Physics

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Long‐term trends in middle atmosphere (~40–100 km; here extended up to 110 km), 15 μm carbon dioxide (CO2) cooling rates, and their responses to solar cycle (SC), quasi‐biennial oscillation, and El Niño‐Southern Oscillation are investigated over Indian tropical region (10°N–15°N) using TIMED‐SABER (Thermosphere Ionosphere Mesosphere Energetics and Dynamics‐Sounding of the Atmosphere using Broadband Emission Radiometry) observations during 2002–2015 (23–24 SCs). The CO2 cooling directly depends on temperature so that higher temperature causes more infrared emission from CO2 at 15 μm band and thus provides larger CO2 cooling. The 15 μm CO2 cooling trends are relatively smaller below ~70–80 km, and their amplitude increases above this height. The trends are positive between ~40 and ~62 km peaking at ~47 km (0.06 ± 0.014 K/day/decade) and negative at ~63–74 and ~79–84 km. Further the positive trends increase sharply above ~84 km at the rate of ~0.1–3.0 K/day/decade between 85 and 110 km. The CO2 cooling response to SC, El Niño‐Southern Oscillation, and quasi‐biennial oscillation is smaller below ~70–75 and stronger above this height. The 15 μm CO2 heating exhibits semiannual variation with ~0.1–0.8 K/day during equinoxes between ~70 and 77 km. In addition, the increase in CO2 cooling significantly influences the background temperature and thereby ozone concentration, neutral number density and layer thickness (width) in the stratosphere and mesosphere.

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Section: Summary and Discussionsupporting

confidence: 67%

Long‐Term Trends in Tropical (10°N–15°N) Middle Atmosphere (40–110 km) CO₂ Cooling

Ramesh

Sridharan

2018

JGR Space Physics

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“…Rather, it appears that the dynamo electric field slightly decreases when the solar activity is high. It may be unnatural that the neutral wind velocity is slower under higher solar activity condition, but in fact, it is not surprising because some observations [e.g., Liu et al, 2004;Sridharan et al, 2010] showed that the thermospheric tidal wind velocity is smaller during high solar activity period. This effect is likely attributed to the larger ion drag force owing to larger ion density.…”

Section: Resultsmentioning

confidence: 99%

Contribution of wind, conductivity, and geomagnetic main field to the variation in the geomagnetic Sq field

Takeda

2013

JGR Space Physics

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[1] Long-term variation in the geomagnetic Sq field and the cause of the variation were examined. The amplitude of the geomagnetic Y component (Sq(Y)) in equinox was averaged for each year and adopted as a proxy of the Sq field. Sq(Y) was combined with the ionospheric conductivity estimated by the International Reference Ionosphere model to determine the dynamo electric field and neutral wind velocity by using the geomagnetic main field strength. It was found that the solar activity dependence of the Sq field could be almost completely attributed to the conductivity variation, and neutral winds tend to decrease when the solar activity increases. Although the long-term variation in the dynamo field differed among observatories, these differences were mostly attributed to the locality of the geomagnetic secular variation, whereas the variations in neutral wind amplitude were nearly the same in all regions. On the other hand, no clear long-term variation in neutral wind was detected other than that by solar activity.Citation: Takeda, M. (2013), Contribution of wind, conductivity, and geomagnetic main field to the variation in the geomagnetic Sq field,

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“…The establishment of ground-based radars at both equatorial and polar latitudes enhanced the spatial/temporal coverage originally available from the predominately mid-latitude stations that prevailed before 2000 (e.g. Mitchell et al 2002;Sridharan et al 2010;Nozawa et al 2012;Davis et al 2013). New instruments, notably lidars that provide daytime observations, also came on line (e.g She et al 2002).…”

Section: Atmospheric Tides In the Mltmentioning

confidence: 96%

The dynamics of the mesosphere and lower thermosphere: a brief review

Vincent

2015

Prog. in Earth and Planet. Sci.

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The dynamics of the mesosphere-lower thermosphere (MLT) (60 to 110 km) is dominated by waves and their effects. The basic structure of the MLT is determined by momentum deposition by small-scale gravity waves, which drives a summer-to-winter pole circulation at the mesopause. Atmospheric tides are also an important component of the dynamics of the MLT. Observations from extended ground-based networks, satellites as well as numerical modelling show that non-migrating tidal modes in the MLT are more important than previously thought, with evidence for directly coupling into the thermosphere/ionosphere. Major disturbances lower in the atmosphere, such as wintertime sudden stratospheric warmings, temporarily disrupt the circulation pattern and thermal structure of the MLT. In the equatorial mesosphere, gravity wave driving leads to oscillations in the zonal wind on semiannual time scales, although variability on quasi-biennial time scales is also apparent. Planetary-scale waves such as the quasi-two-day wave temporarily dominate the dynamics of the summertime MLT, especially in the southern hemisphere. Impacts may include short-term changes to the thermal structure and physics of the high-latitude MLT. Here, we briefly review the dynamics of the MLT, with a particular emphasis on developments in the past decade.Keywords: Atmospheric tides; Gravity waves; Planetary waves; Middle atmosphere; Mesosphere; Lower thermosphere; Wave coupling Review IntroductionThe mesosphere-lower thermosphere (MLT) is defined as the region of the atmosphere between about 60 and 110 km in altitude. It constitutes the upper part of what is often referred to as the middle atmosphere (10 to 110 km). The MLT is dominated by the effects of atmospheric waves, including planetary waves, tides and gravity waves. The source regions for these waves are lower in the atmosphere. As the waves propagate upward their amplitudes grow exponentially to compensate for the decrease in atmospheric density (e.g. Andrews et al. 1987). Consequently, wave motions often dominate the wind field in the MLT, so considerable averaging may be required to extract the mean flow, especially that of the mean meridional (NS) motions, which are smaller than the zonal (EW) flow. Zonal-mean zonal winds are eastward (westward) in the winter (summer) middle atmosphere, reaching peak values of approximately 60 to 70 ms −1 near 70 km and Correspondence: robert.vincent@adelaide.edu.au Department of Physics, School of Physical Sciences, University of Adelaide, North Terrace., SA, 5005 Adelaide, Australia then they reduce in magnitude until they reverse sign at heights between 90 and 100 km.Large wave amplitudes lead to wave breaking and hence momentum deposition. This, in turn, produces body forces that drive large scale residual circulations. In the winter stratosphere (approximately 20 to 60 km), planetary wave breaking drives a residual circulation from the equator to the winter pole, while in the mesosphere, gravity wave breaking and dissipation drives a circulation from ...

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Long‐term tendencies in the mesosphere/lower thermosphere mean winds and tides as observed by medium‐frequency radar at Tirunelveli (8.7°N, 77.8°E)

Cited by 34 publications

References 44 publications

Long‐Term Trends in Tropical (10°N–15°N) Middle Atmosphere (40–110 km) CO₂ Cooling

Long‐Term Trends in Tropical (10°N–15°N) Middle Atmosphere (40–110 km) CO₂ Cooling

Contribution of wind, conductivity, and geomagnetic main field to the variation in the geomagnetic Sq field

The dynamics of the mesosphere and lower thermosphere: a brief review

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Long‐term tendencies in the mesosphere/lower thermosphere mean winds and tides as observed by medium‐frequency radar at Tirunelveli (8.7°N, 77.8°E)

Cited by 34 publications

References 44 publications

Long‐Term Trends in Tropical (10°N–15°N) Middle Atmosphere (40–110 km) CO2 Cooling

Long‐Term Trends in Tropical (10°N–15°N) Middle Atmosphere (40–110 km) CO2 Cooling

Contribution of wind, conductivity, and geomagnetic main field to the variation in the geomagnetic Sq field

The dynamics of the mesosphere and lower thermosphere: a brief review

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Long‐Term Trends in Tropical (10°N–15°N) Middle Atmosphere (40–110 km) CO₂ Cooling

Long‐Term Trends in Tropical (10°N–15°N) Middle Atmosphere (40–110 km) CO₂ Cooling