AJP	ISSN : 0971 – 3093 Vol  1 9,  No. 4 , October-December,  2011

 

Vol. 19, No. 4 (2010) 303-314

Current knowledge on low-latitude mesospheric thermal structure

 

M Venkat Ratnam¹ , P Kishore² , A K Patra¹ and B V Krishna Murthy ³

¹National Atmospheric Research Laboratory, Gadanki, Tirupati-517 502, India

²Department of Earth System Science, University of California, Irvine, CA 92697, USA

In the present manuscript, we report the current knowledge on low-latitude mesospheric thermal structure based on long-term (2002-2009) temperature records obtained from SABER/TIMED observations. We also compare them with the ground based lidar and rocket observations wherever necessary. Mean stratopause (mesopause) altitude and temperature are observed to be 48.7 ± 0.7 km (96.2 ± 0.6 km) and 265.8 ± 2.6 K (165.1 ± 1.9 K), respectively, over tropical latitudes (20°N and 20°S). This reveals that, in general, mesosphere extends up to ~100 km, in contrast to the usual belief that mesopause is located at ~85 km. There exists a mesospheric temperature inversion (MTI) between 70 and 75 km with its occurrence probability being 60%. These MTI shows strong semi-annual oscillation in its occurrence with peak around equinoxes. Further, a secondary minimum in temperature is observed in between 75 and 80 km with semi-annual variation again peaking around equinoxes. No significant seasonal variation is observed in the mesopause altitude in the tropical regions. This feature is quite different from that reported over mid- and polar latitudes, where two-level mesopause structure is noticed, which is season dependent. The secondary minimum is smeared out in the long-term averaging revealing that it is due to transient effects of waves. Mesosphere extending up to ~100 km implies that mixing of neutral constituents may extend up to that height. This demands full-fledged investigations on the physical parameters such as the collision frequency and diffusion coefficient, and the altitude of the turbopause. Finally, long-term monthly mean temperature values are presented, which provide a reference temperature for tropical latitudes.

Keywords: Mesosphere, temperature structure, stratopause and mesopause.

 Vol. 19, No. 4 (2010) 315-320

Near simultaneous lidar observations of upper mesospheric inversion layers and sporadic sodium layers over Gadanki (13.5°N, 79.2°E)

 

S Sridharan ¹ *, K Ramesh² , S Sathishkumar³ , S Gurubaran³ , K Raghunath¹ ,

Y Bhavani Kumar1, and S Vijaya Bhaskara Rao2

  ¹National Atmospheric Research Laboratory, Gadanki-517 112, India

²Sri Venkateswara University, Tirupati-517 502, India

The lower power (11 W) laser source of the Indo-Japanese lidar system at Gadanki (13.5°N, 79.2°E) has been replaced by the one with higher power (30 W) during mid-January 2007. As the power of the laser is increased, the molecular scatter signal is also increased. The top height from which the temperature was derived is increased from 90 km to 105 km. The time-altitude cross section of upper mesospheric temperature presented for four nights (20-23 January 2007) show that the upper mesospheric inversion layers (MIL) are present in all the nights and they are strongly influenced by gravity waves, tides and planetary waves. The nightly variation in the amplitude of inversion is modulated by gravity waves of period 2-2.5 h. The MILs are observed to descend at the rate of ~1 km/h indicating the influence of diurnal tides. The clear downward descending structure of MIL is observed only for 20 and 22 January 2007 and is not observed for 21 and 23 January 2007. The day-to day variability of the structure of MIL event is explained as due to the presence of strong 2-day wave activity, which could modulate the diurnal tide. The peak in sodium (Na) density occurs at slight lower height, at which temperature shows maxima. The vertical structure of Na density and temperature appear to be modulated by waves of similar vertical scale. The nightly mean zonal and meridional winds over Tirunelveli (8.7°N, 77.8°E) show strong shear above the inversion layer.

 Vol. 19, No. 4 (2010) 321-328

Short period mesospheric wave measurements by optical means over Gadanki

(13.5o N, 79.2o E) during May 2009

 

Alok Taori¹ , V Kamalakar² and S V B Rao²

¹National Atmospheric Research Laboratory, Gadanki-517 112, India,

We report first mesospheric measurements of short-period gravity wave signatures in temperature data deduced from OH and O2 airglow emissions from Gadanki (13.5° N, 79.2° E). The mesospheric OH and O2 temperature estimates, which represents ~ 85 and 94 km altitude regions, respectively, are in good agreement with the overhead SABER measured temperature values onboard the TIMED satellite. The data show that both the nights were dominated by upward propagation of waves. The wavelet analysis reveals the dominant wave periods to be ~ 0.3-2 hr in both OH and O2 data for both the nights with 3-5 K amplitudes.

 Vol. 19, No. 4 (2010) 329-348

ELF/VLF studies of Mesosphere-the D region ionosphere and magnetosphere at low latitudes

 

Rajesh Singh¹ , B Veenadhari² , Abhay K Singh³  and P Pant⁴

¹KSK Geomagnetic Research Laboratory, Indian Institute of Geomagnetism, Chamanganj, Jhusi, Allahabad – 221 505

²Indian Institute of Geomagnetism, New Panvel, Navi Mumbai – 410 218, India

³Physics Department, Banaras Hindu University, Varanasi – 221 005

ELF/VLF electromagnetic waves have their origin in natural/artificial sources and because of being reflected from lower boundary of the ionosphere finds its application as a diagnostic tool in the studies of the D-region ionosphere, which is the part of the mesosphere. Also on the other hand some of the radiated energy leaks through the ionosphere and propagates through the magnetosphere along the geomagnetic field lines in whistler-mode and are used in the studies of magnetosphere during various geophysical conditions. This paper is the brief review of the ELF/VLF waves as subject, its probing potentiality and studies carried out in Indian low latitude region over the past four decades. Also the latest addition of VLF experiment in India and its usefulness is discussed in detail.

 Vol. 19, No. 4 (2010) 349-365

Mesospheric turbulence over low latitudes from in-situ measurements

 

H S S Sinha and H Chandra

Physical Research Laboratory, Ahmedabad, India

Neutral turbulence is an important phenomenon that significantly controls the energy and momentum budget of the lower atmosphere. Turbulence parameters in mesosphere and lower thermosphere have been determined primarily from the fluctuations in the electron/ion density measured by rocket-borne in-situ measurements or from the ground based radar measurements. In mesosphere the fluctuations in neutral constituents are transferred to ions and electrons because of the large collision frequency between them. The fluctuations in electron density also give rise to fluctuations in the radio refractive index that is responsible for the coherent radar backscatter provided there are plasma density fluctuations with scale size one half of the radar wavelength. The paper reviews the different methods of estimating the turbulence parameters at low latitudes especially from the Indian region with an emphasis on in situ measurements.

 Vol. 19, No. 4 (2010) 367-373

Planetary wave modulations in ionospheric TEC over Gadanki during low solar activity epoch of 2008-09: First results

 

N Dashora

The first evidence of signature of planetary waves in ionospheric total electron content (TEC) over Gadanki (low latitude Indian zone) has been reported. The variations in TEC, estimated using dual frequency GPS receiver at Gadanki, for the period from October 2008 to March 2009, have been analyzed using Lomb-Scargle periodogram. The concerned period falls under the recent lowest solar activity epoch and covers two equinox and one winter season in Indian zone. The analysis reveals multi-period modulation of vertical ionospheric TEC wherein the largest amplitudes have been found for quasi 36-38, 15-16 and 9-10 day planetary waves respectively, apart from other less significant smaller scale waves (3, 6, 8, and quasi -16 hours), over and above the diurnal (24 hour), semi-diurnal (12 hour) and solar rotation related (27 day) ionospheric TEC variations.

 Vol. 19, No. 4 (2010) 375-396

Middle atmospheric temperature investigations over a sub-tropical location using Rayleigh lidar

 

Som Sharma¹ , H Chandra¹ , S Lal¹ , Y B Acharya¹  and A Jayaraman¹

¹Space & Atmospheric Sciences Division, Physical Research Laboratory

Ahmedabad-380 009, India

Middle atmosphere is an important region of Earth’s atmosphere and its temperature structure is unique, and having imprints of various atmospheric processes. A powerful Nd-YAG laser based Rayleigh Lidar was set up at Gurushikhar, Mt. Abu (24.5o N, 72.7o E, height ASL 1.7 km) by the Physical Research Laboratory to study the middle atmospheric temperature structure in the altitude region of 30-75 km. Temperature profiles are derived from relative density measurements. Measurements were made for about 5-10 nights, depending on local weather condition, of each month around new moon except the monsoon season (June-August). Temperature profiles obtained during 1997 to November 2001 are used to derive average temperature profile for each month (September to May). Lidar observed temperatures are compared with temperatures observed by Halogen Occultation Experiment (HALOE), on-board Upper Atmospheric Research Satellite (UARS), the CIRA-86 and MSISE-90 Model temperatures. Observed temperature profiles are in qualitative agreement with CIRA-86 and MSISE-90 model below 50 km, and the agreement is better during winter months. Quantitatively there are significant differences noted, up to 10 K, above 50 km. The temperature profiles are also compared with the equatorial model for the Indian region. Significant day to day variability is found, which is as high as ± 10 K at ~70 km. The mean values of the stratopause height and temperature are found to be 48 km and 271 K, respectively.  Seasonal variation shows equinoctial and summer maxima below 55 km, whereas above 70 km winter maximum with equinoctial minima are present. Temperatures are also compared with the observed temperatures over other low and sub-tropical locations in the northern and southern hemisphere.

 Vol. 19, No. 4 (2010) 397-406

Signatures of sudden stratospheric warming (SSW) during 2005-2006 and 2008-2009 winters in the northern hemisphere stratosphere and lower mesosphere

 

S Sathishkumar and S Gurubaran

Equatorial Geophysical Research Laboratory, Indian Institute of Geomagnetism 

This work presents some results on the response of the stratosphere and lower mesosphere to the major stratospheric warming (SSW) that occurred during the winters of 2005-2006 and 2008-2009. The vertical and latitudinal structure and dynamics of the SSW events were examined for the Northern Hemisphere stratosphere and lower mesosphere by using the UKMO (UK Meteorological Office) and ECMWF (European Centre for Medium-Range Weather Forecasts) data sets. The zonal mean zonal winds at 60oN revealed the characteristic reversal to westward direction associated with SSW in the entire stratosphere and lower mesosphere. A comparative study was made with long lasting SSWs during January 2006 and January 2009 and it is reported here that the SSW during January 2009 was a polar vortex  split event as against the polar vortex displacement event observed during January 2006.

 Vol. 19, No. 4 (2010) 407-414

Multi-pattern nocturnal variation in the mesospheric OH and O2 emissions

 

A Guharay¹ , M J Taylor² and R Sekar ³

¹Physical Research Laboratory, Navrangpura, Ahmedabad, India

²Center for Atmospheric and Space Sciences, Utah State University, Logan, Utah, USA

A set of measurements obtained from mesospheric OH and O2 emissions from a low latitude station, Maui (20.8°N, 156.2oW) are utilized to report unusual mesospheric variability in the nocturnal pattern, where five cases are exclusively elucidated from the database during 2001-2003. Our results not only describe most common wave features as noted in the several existing literatures, but also several other uncommon patterns are revealed. Overall, the results can be categorized under five classes, i.e. evident phase propagation, no evident phase propagation, anti-phase variation, uni-active and duo-active variations. The probable causes of such variability are also explained.

 Vol. 19, No. 4 (2010) 415-422

Study of mesospheric temperature inversions over Mt. Abu

 

H Chandra, Som Sharma, S Lal and Y B Acharya

Characteristics of Mesospheric Temperature Inversions (MTI) are studied using about 150 nights of temperature profiles obtained from the Rayleigh lidar data collected over Mt. Abu during 1997-2001. The magnitude of MTI is ranging between 9K and 36 K with most probable value of about 14 K. The occurrence frequency of MTI over Mt. Abu shows prominent seasonal variation with maximum during winter months, which is in contrast to that observed over a low latitude location Gadanki with maximum occurrence during equinoxes. Average height of occurrence of MTI over Mt. Abu is found lower than over a low latitude location Gadanki. Gravity wave breaking is considered as the major cause for the MTI. However, role of mesospheric ozone and chemical heating can not be ruled out in producing MTI’s of larger magnitude.

Vol. 19, No. 4 (2010) 423-434

Characteristics of gravity waves and associated momentum flux using Indian MST radar at Gadanki (13.2°N, 79.2°E)

 

N Y Pandya, H P Joshi and K N Iyer

Atmospheric Gravity waves have been a subject of intense research activity in recent years because of their myriad effects and their major contributions to atmospheric circulation, structure and variability. The Indian MST radar at Gadanki (13.2oN, 79.2oE) provides excellent opportunity to study gravity waves and their momentum flux with high time and height resolution particularly in the Troposphere and Lower Stratosphere. Study of gravity waves was made from MST radar campaigns during the years 2001 to 2005. Results indicate that the values of variance and momentum flux increase with height and reach a maximum at around 14 to 17 km. The power spectra also show that the power density increases with height and reaches maximum around 14 to 17 km. This clearly shows that sources of the gravity waves are located in the troposphere and that waves propagate upward. The strong gravity wave activity observed in the month of August may be attributed to the convective activity. The variance has been observed to be maximum in the monsoon season at height about 14 to 17 km and very less in the post monsoon season. No activity has been observed during October. From the power spectral analysis maximum power spectral density is observed during all the seasons at altitude of ~ 14 to18 km and the dominant period of wave has been observed to be between 20 to 30 minutes. Vertical wavelength is inferred from the hodograph analysis to be ~5 to 6 km with upward energy propagation. The maximum momentum fluxes are observed during August; the magnitude of the zonal and meridional momentum flux lie between 1.4 m2s–2 and -0.9 m2s–2 and 0.3 and -0.3 m2s–2 respectively and minimum momentum flux during October. The magnitude of both the zonal and meridional momentum flux varies from 0.05 to -0.05 m2 s–2 and from 0.03 to –0.03 m2s–2 respectively.

Key words: Gravity waves, Momentum flux, Variance, Power spectra, Vertical wavelength

 Vol. 19, No. 4 (2010) 435-442

A brief review of neutral atmosphere-ionosphere coupling over the dip equator

 

T K Pant, C Vineeth and M M Hossain

 Vol. 19, No. 4 (2010) 443-456

A Review on the Na Airglow Mechanism using Simultaneous Na Airglow and Na Lidar Measurements over India

 

R Sekar, S Sarkhel, and D Chakrabarty

IIn order to evaluate the dependence of mesospheric sodium (Na) airglow intensity on neutral Na atoms, a narrow band Na airglow photometer was operated in conjunction with the Na lidar from Gadanki (13.5o N, 79.2oE), India for a few nights during March, 2007. The altitude profiles of mesospheric ozone, temperature and pressure available for the near-by locations during the local post midnight hours are obtained from SABER instrument onboard TIMED satellite and used in the present investigation. It is found that variation in the average Na airglow emission intensity level from one night to another is not in accordance with the variation in the Na atom concentration. On another night, the correlation coefficient between the Na airglow intensity variation and Na atoms is not found significant at the peak altitude of Na concentration. Volume emission rate calculations suggest that the Chapman mechanism for Na airglow emission is inadequate to explain the above-mentioned observations. The importance of the modified Chapman mechanism proposed by Slanger et al. [1], where the average Na airglow intensity level depends critically on the ambient molecules, is brought out based on the above observations. In addition to the active role played by collisional quenching in determining the night-to-night variability of average Na airglow intensity level, altitude dependence of collisional quenching is indicated to play a vital role in determining Na airglow intensity variation on a given night.

 Vol. 19, No. 4 (2010) 457-474

Characteristics of winds in the equatorial MLT region-A Review

 

 Geetha Ramkumar, Veena Suresh Babu and K V Subrahmanyam

Space Physics Laboratory

This paper presents a detailed review of observational studies conducted in the Mesospheric Lower Thermospheric (MLT) Region over an equatorial station Thumba. The equatorial region being significantly different from mid and high latitudes, the studies on dynamics of this region especially that of mesospheric lower thermospheric region are very crucial in the dynamical coupling processes from lower atmospheric region and those related to space weather studies. Studies on the various scales of atmospheric oscillations like gravity waves, planetary waves, tidal oscillations, Annual, Semi-annual and Quasi-biennial oscillations making use of wind measurements in the MLT region by means of rocket flights and ground based radars for more than three decades are presented along with interaction between waves and wave-mean flow interaction. 

Total Ref : 80