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Selvaraj D.,SRM University | Patra A.K.,National Atmospheric Research Laboratory Gadanki India | Sathishkumar S.,Equatorial Geophysical Research Laboratory IIG Tirunelveli India | Kishore Kumar K.,Vikram Sarabhai Space Center | Narayana Rao D.,SRM University
Journal of Geophysical Research: Space Physics | Year: 2017

In the present communication, the role of neutral dynamical processes in governing the morphology and seasonal variations of daytime VHF radar echoes from the mesosphere and the collision-dominated lower E region is discussed. While the seasonal variations of mesospheric echo occurrence show semiannual variation with their maxima in the equinoxes, the occurrence of E region echoes shows strong annual variation peaking in the summer with two secondary maxima identical to semiannual variation of the mesospheric echoes. The occurrence rates of the E region echoes, however, display year-to-year variability. Semiannual variations in the occurrence of low-latitude mesospheric echoes are closely linked with similar variations in the occurrences of temperature inversion, large wind shear, tides, and gravity wave activities in the same height region. In the case of E region echoes, the summer maximum and equinoctial secondary maxima are found to be linked with similar variations in wind and wind shear, Es activity, tidal, and gravity wave activities. Further, the summer maximum is found to be strongly correlated with that of meteor flux and nonmigrating diurnal tide. This finding is significant given the fact that such wind field in the presence of metallic ions could form both Es and field-aligned irregularities. The present results show that tides and gravity waves are the most important dynamical factors governing the morphology and seasonal variations of the mesospheric as well as lower E region echoes observed over Gadanki. These results are discussed in the light of current understanding on the dynamical coupling of the mesosphere and lower E region through tides and gravity waves. © 2017. American Geophysical Union. All Rights Reserved.


Ramesh K.,National Atmospheric Research Laboratory Gadanki India | Sridharan S.,National Atmospheric Research Laboratory Gadanki India | Raghunath K.,National Atmospheric Research Laboratory Gadanki India | Rao S.V.B.,Sri Venkateswara University
Journal of Geophysical Research: Space Physics | Year: 2017

The various occurrence characteristics of day and night tropical (10°N-15°N, 60°E-90°E) mesospheric inversion layers (MILs) are studied by using TIMED Sounding of the Atmosphere using Broadband Emission Radiometry satellite data products of kinetic temperature; volume mixing ratios of O, H, and O3; volume emission rates of O2 (1Δ) and OH (1.6μm channel), and chemical heating rates due to seven dominant exothermic reactions among H, O, O2, O3, OH, HO2, and CO2 cooling rates for the year 2011. Although both dynamics and chemistry play important roles, the present study mainly focuses on the chemical processes involved in the formation of day and night MILs. It is found that the upper level height of daytime (nighttime) MIL descends (ascends) from ~88km (~80km) in winter to ~72km (~90km) in summer. The day and night inversion amplitudes are correlated with total chemical heating rates and CO2 cooling rates, and they show semi annual variation with larger (smaller) values during equinoxes (solstices). The daytime (nighttime) inversion layers are predominantly due to the exothermic reaction, R5: O+O+M→O2+M and R6: O+O2+M→O3+M (R3: H+O3→OH+O2). In addition, the CO2 causes large cooling at the top and small heating at the bottom levels of both day and night MILs. In the absence of dynamical effects, the chemical heating and CO2 cooling jointly contribute for the occurrence of day and night MILs. © 2017. American Geophysical Union. All Rights Reserved.


Venkateswara Rao N.,National Atmospheric Research Laboratory Gadanki India | Jayaraman A.,National Atmospheric Research Laboratory Gadanki India
Journal of Geophysical Research A: Space Physics | Year: 2016

The close encounter of comet Siding Spring with Mars resulted in the formation of a dense transient ionization layer in the Martian upper atmosphere at altitudes between 80 and 120km. Instruments on three spacecraft orbiting Mars detected the presence of this layer, as reported in previous publications. In this study, we reanalyzed the ionograms of the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on Mars Express to get further insight about the recurrence of the layer. For this purpose, data from three periapsis passes of MARSIS that took place 5h, 12h, and 19h after peak dust deposition are used. We found that the transient ionization layer was sustained at least for 19h on the nightside and 12h on the dayside. While the peak density of the layer on the nightside gradually decreases from orbit to orbit, it does not change much on the dayside. Some ionograms in all three orbits show two transient ionization layers that are separated by ~60km in apparent altitude. These double layers occur preferentially in regions of strong vertical magnetic fields. The bottom layer of the double structure is probably an oblique echo due to reflections from ionization bulges (formed in regions of vertical magnetic fields) at altitudes of the transient ionization layer. Horizontal bifurcation of the original layer is considered as another plausible mechanism for explaining the double-layer structure. © 2016. American Geophysical Union. All Rights Reserved.


Chaitanya P.P.,National Atmospheric Research Laboratory Gadanki India | Patra A.K.,National Atmospheric Research Laboratory Gadanki India | Balan N.,National Institute for Space Research
Journal of Geophysical Research A: Space Physics | Year: 2016

In this paper we carry out a comparative study of the daytime (7-18 LT) behavior of the near-equatorial ionospheric F region at the end of the long deep solar minimum (2009) with respect to that of the previous normal solar minimum (1995) in the Indian longitude sector using ionosonde observations of F layer parameters, radar observations of E×B drift, and the IRI-2012 (International Reference Ionosphere-2012) model. We investigate the F2 and F3 layer behaviors separately. The results reveal that the peak frequencies of the F layer (fpeak), F2 layer (foF2), and F3 layer (foF3) in 2009 are consistently lower than those in 1995. Maximum difference in fpeak/foF2/foF3 between 2009 and 1995 observations is found in the equinoxes followed by winter and summer. The annual mean, seasonal mean, and 10day mean peak electron density (corresponding to fpeak) in 2009 were lower than those in 1995 by as much as 34%, 46%, and 65%, respectively. Solar rotation effect is less conspicuous in 2009 than in 1995, consistent with the solar rotation signature in F10.7. Observations also show considerable amount of equinoctial asymmetry in electron density, which is found to be closely linked with the corresponding asymmetry in the vertical E×B drift. Seasonal mean peak electron densities of the F layer (corresponding to fpeak) and the F2 layer (corresponding to foF2) observed during the deep solar minimum of 2009 were smaller than those corresponding to IRI-2012 model foF2 by as much as 45% and 50%, respectively, underlining the need to incorporate the data collected during the long deep minimum in the IRI model. The unusually weak ionosphere observed in 2009 is discussed in terms of the direct effect of the low solar EUV flux in 2009 as compared to 1995 and its indirect effects on ionospheric electric field, thermospheric composition (or O/N2 ratio), and thermospheric neutral winds. © 2016. American Geophysical Union. All Rights Reserved.


Sivakandan M.,National Atmospheric Research Laboratory Gadanki India | Taori A.,National Atmospheric Research Laboratory Gadanki India | Sathishkumar S.,EGRL Indian Institute of Geomagnetism Tirunelveli India | Jayaraman A.,National Atmospheric Research Laboratory Gadanki India
Journal of Geophysical Research A: Space Physics | Year: 2015

We investigate a gravity wave event exhibiting dissipation noted in the mesospheric O(1S) airglow emission image measurements, over Gadanki (13.5°N, 79.2°E), on 16 March 2012 (during 16:20-16:45 UT). These gravity waves were found to propagate from south-west to north-east directions at ~65° azimuth in OH as well as in O(1S) images. These waves had horizontal wavelength ~21.5km with apparent horizontal phase speed ~49ms-1 and period ~7.3min. These waves were noted to fizzle out in turbulent patches within 15min. To identify the causative mechanism of this event, we analyze the background wind and temperature data using the medium-frequency radar wind from Tirunelveli (8.7°N, 77.8°E), ground-based Rayleigh lidar temperature data with improved capability over Gadanki, and spaceborne Sounding of the Atmosphere using Broadband Emission Radiometry/Thermosphere Ionosphere Mesosphere Energetics and Dynamics temperature data (20:30 UT) for a latitude-longitude grid of 3-23°N, 60-100°E. Our analysis reveals that convective instability was responsible for the observed event. © 2015 American Geophysical Union.


Sunilkumar K.,National Atmospheric Research Laboratory Gadanki India | Narayana Rao T.,National Atmospheric Research Laboratory Gadanki India | Saikranthi K.,Indian Institute of Science | Purnachandra Rao M.,Andhra University
Journal of Geophysical Research: Atmospheres | Year: 2015

This study presents a comprehensive evaluation of five widely used multisatellite precipitation estimates (MPEs) against 1°×1° gridded rain gauge data set as ground truth over India. One decade observations are used to assess the performance of various MPEs (Climate Prediction Center (CPC)-South Asia data set, CPC Morphing Technique (CMORPH), Precipitation Estimation From Remotely Sensed Information Using Artificial Neural Networks, Tropical Rainfall Measuring Mission's Multisatellite Precipitation Analysis (TMPA-3B42), and Global Precipitation Climatology Project). All MPEs have high detection skills of rain with larger probability of detection (POD) and smaller "missing" values. However, the detection sensitivity differs from one product (and also one region) to the other. While the CMORPH has the lowest sensitivity of detecting rain, CPC shows highest sensitivity and often overdetects rain, as evidenced by large POD and false alarm ratio and small missing values. All MPEs show higher rain sensitivity over eastern India than western India. These differential sensitivities are found to alter the biases in rain amount differently. All MPEs show similar spatial patterns of seasonal rain bias and root-mean-square error, but their spatial variability across India is complex and pronounced. The MPEs overestimate the rainfall over the dry regions (northwest and southeast India) and severely underestimate over mountainous regions (west coast and northeast India), whereas the bias is relatively small over the core monsoon zone. Higher occurrence of virga rain due to subcloud evaporation and possible missing of small-scale convective events by gauges over the dry regions are the main reasons for the observed overestimation of rain by MPEs. The decomposed components of total bias show that the major part of overestimation is due to false precipitation. The severe underestimation of rain along the west coast is attributed to the predominant occurrence of shallow rain and underestimation of moderate to heavy rain by MPEs. The decomposed components suggest that the missed precipitation and hit bias are the leading error sources for the total bias along the west coast. All evaluation metrics are found to be nearly equal in two contrasting monsoon seasons (southwest and northeast), indicating that the performance of MPEs does not change with the season, at least over southeast India. Among various MPEs, the performance of TMPA is found to be better than others, as it reproduced most of the spatial variability exhibited by the reference. © 2015. American Geophysical Union. All Rights Reserved.


Kherani E.A.,National Institute for Space Research | Patra A.K.,National Atmospheric Research Laboratory Gadanki India
Journal of Geophysical Research A: Space Physics | Year: 2015

This paper presents a three-dimensional simulation of the collisional interchange instability generating equatorial plasma bubble (EPB) in the evening ionospheric F region and associated fringe field (FF) in the valley-upper-E (VE) region. This simulation is primarily intended to address hitherto unexplained radar observations of ascending irregularity structures only in the vicinity of the magnetic equator in association with the EPB phenomenon. Novel results of the present simulation are the following: (1) EPB-associated FF penetrating into the E region is found to be confined to a latitude belt of ±5{ring operator}, (2) ascending irregularity structures from the E region is formed only when perturbation in plasma parameters similar to those responsible for forming EPB are present in the VE region, and (3) perturbation in the VE region provide conditions for the formation of ascending irregularity structures on the eastern wall of the plasma bubble. These results are in excellent agreement with radar observations and also account for the presence of metallic ions in the EPB at and above the F region peak. © 2015. American Geophysical Union. All Rights Reserved.


Ramesh K.,Sri Venkateswara University | Sridharan S.,National Atmospheric Research Laboratory Gadanki India | Vijaya Bhaskara Rao S.,Sri Venkateswara University
Journal of Geophysical Research A: Space Physics | Year: 2015

Monthly averaged zonal mean temperature and ozone volume mixing ratios obtained from "Sounding of the Atmosphere using Broadband Emission Radiometry" instrument on board "Thermosphere Ionosphere Mesosphere Energetics and Dynamics" satellite for the years 2002-2012 are used to study the seasonal and solar cycle variabilities of tropical (10°N-15°N) mesopause structure. The mesopause temperature and ozone mixing ratios are positively correlated with solar cycle due to changes in CO2 and O, respectively. Although the seasonal variation in mesopause temperatures is quite small, the mesopause altitudes are comparatively higher (~99-100km) in April and lower (~95km) in September. The factors controlling the tropical mesopause structure are investigated by taking the mesopause variability for the year 2011 as a case study as it resembles the long-term mean seasonal variation. It is found that the radiative cooling due to 15μm CO2 infrared emissions is the only cooling mechanism in the mesopause region. The net heating rates obtained from (i) solar heating by O2 and O3, (ii) chemical heating due to seven major exothermic reactions, (iii) O3 long-wave radiative heating, and (iv) CO2 cooling are smaller (~20K/d) in April and larger (~85K/d) in September at lower thermosphere (~99-101km). The downward heat conduction from the lower thermosphere forces the mesopause to lower heights in September, although no downward heat conduction is observed in April. ©2015. American Geophysical Union.


Ghosh P.,National Atmospheric Research Laboratory Gadanki India | Ramkumar T.K.,National Atmospheric Research Laboratory Gadanki India
Atmospheric Science Letters | Year: 2014

We report that often vertical wave number spectra of the Martian lower atmospheric temperature are distinctly different from that normally observed for the Earth's atmosphere. In many cases, the observed spectra show wave saturation characteristics (log-log slope of -3) even at wave numbers greater than the ones corresponding to dissipative regime (log-log slope of -5/3), a kind of which is not reported so far for the Earth's atmosphere. Also, some times, steep spectral slopes are obtained at smaller wave numbers near the ones associated with dissipation and positive slopes are noticed at the higher end of the wave numbers of spectra. © 2014 Royal Meteorological Society.

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