R. Rajaram

The mesospheric quasi-2-day wave over Tirunelveli (8.7°N)

Abstract Observations of the quasi-2-day planetary wave over a period of two years with the medium frequency (MF) radar operated at Tirunelveli (8.7° N , 77.8° E ) , India, are presented. The observational aspects of the characteristics of the wave over the low latitude station and its possible interaction with the mean wind and the diurnal tide are studied in detail. The amplitude of the 2-day wave has been observed to be larger in winter than in summer with noticeable inter-annual variability. The period of the wave varied between 46 and 53 h with a dependence on season. The vertical wavelength during large-amplitude wave bursts has been observed to be in the range of 35– 70 km . The clear anti-correlation between the diurnal tide and the quasi-2-day wave occurrences and the presence of the 16-h wave provide indications for their interaction, which might have extracted energy from the primary diurnal tide. The changes in the mean wind during the passage of the quasi-2-day wave reflect the importance of the wave in determining the mesospheric momentum budget.

Signatures of equatorial electrojet in the mesospheric partial reflection drifts over magnetic equator

The partial reflection (PR) drifts at 98 km measured by the medium frequency (MF) radar operating at Tirunelveli, India, situated close to the magnetic equator, are shown to have the combined effects of mesospheric winds and the equatorial electrojet. The evidences for this behavior of the PR drifts are brought out from a detailed examination of the temporal variation of the dynamical and the geometrical parameters associated with the ground diffraction pattern.

Radar observations of the 3.5-day ultra-fast Kelvin wave in the low-latitude mesopause region

Abstract The medium frequency (MF) radar observations of the 3.5-day ultra-fast Kelvin (UFK) wave in the mesopause region (84– 98 km ) over Tirunelveli ( 8.7° N , 77.8° E ), India for a period of nearly 3 years are reported in the present work. The UFK wave reveals semi-annual variability at heights ( ∼85 km ) where the mesopause semi-annual oscillation (MSAO) in the mean wind peaks. Large-amplitude wave events preferentially occur during the westward flow regimes of the background wind. Mean eastward winds and their shears are qualitatively shown to be associated with bursts of waves with moderate amplitudes. Vertical wavelength estimates agree with earlier estimates based on satellite temperature retrievals for a wavenumber 1 ultra-fast Kelvin wave.

Structural changes in the tidal components in mesospheric winds as observed by the MF radar during afternoon counter electrojet events

In this paper we examine the relationship between the tidal characteristics as observed by the MF radar at Tirunelveli (8.7°N, 77.8°E, geographic; 0.3°N magnetic dip) and the occurrence of afternoon counter electrojet (ACEJ) events. A reduction in the diurnal tide activity and/or an enhancement of semi-diurnal tide amplitude is observed on many of the ACEJ days. A clear anticorrelation is seen between the afternoon electrojet strength and the amplitude of the semi-diurnal tide in the solstitial months of June and July, 1995. The results presented herein provide observational support to the earlier numerical models that consider the interplay of various tidal modes in the evolution of equatorial ionospheric current system whose extreme manifestation in ground-geomagnetic field variations being the reversal of the electrojet in the afternoon hours.

QBO influences on the variability of planetary waves in the equatorial mesopause region

MF radar observations of hourly mean zonal and meridional winds at 86 km over Tirunelveli (8.7°N, 77.8°E), India, over a period of five years (1993–1997) are utilized to study the possible influence of stratospheric quasibiennial oscillation (QBO) on the vertical propagation of planetary-scale waves. The radiosonde winds at Singapore are used to identify the phase of the stratospheric QBO at 30 hPa. The study reflects that the interannual variability of the planetary waves may be explained partly by the direction of the stratospheric QBO winds. Limitations of the present work are: (i) the QBO cycles considered are not many and (ii) the observed waves are presumed to originate from below and propagate through the stratospheric QBO wind system.

Radar observations of the diurnal tide in the tropical mesosphere-lower thermosphere region: Longitudinal variabilities

Significant attention is being paid in recent times by several observational and modeling studies to quantify the spatial and temporal variabilities of diurnal tide in the mesosphere and lower thermosphere (MLT) region. These variabilities are ascribed to spatial and temporal variations in the tidal forcing or interactions between the propagating tides and background wind, planetary waves or gravity waves. The present work makes use of simultaneous ground-based radar wind observations of different durations from five equatorial/low latitude sites in the Indian, Indonesian and Pacific sectors: Tirunelveli (8.7°N, 77.8°E), Jakarta (6.4°S, 106.7°E), Pontianak (0.03°N, 109°E), Kauai (22°N, 160°W) and Christmas Island (2°N, 157°W). This study delineates the longitudinal differences in the tidal characteristics in (i) interannual time scales over Tirunelveli and Kauai during 1993–2002, (ii) seasonal time scales over Christmas Island, Jakarta and Tirunelveli for the years 1993–1997 and (iii) shorter than seasonal time scales over Christmas Island, Pontianak and Tirunelveli during 1996–1997. An important observational feature noticed in this work is the differing behavior of the long-term tidal fields over Tirunelveli and Kauai. The monthly tidal amplitudes over Tirunelveli reveal a strong QBO signature whereas a similar, strong QBO signal could not be traced in the long-term observations from Kauai.

Lower E-region MF radar spaced antenna measurements over magnetic equator

Abstract Medium frequency (MF) radar spaced antenna observations at heights close to the location of the equatorial electrojet over Tirunelveli (8.7°N, 77.8°E, geographic, 0.35°N magnetic dip) are used to examine the influence of the electrodynamical processes in the determination of neutral winds in the lower E-region. On many days, daytime wind and tidal characteristics change at heights above 94 km , when analysis is performed on day-to-day basis. Under these conditions, the spaced antenna measurements need not represent neutral winds at all. On the other hand, on a few other days, the ‘drifts’ at 98 km are opposite to those expected from the electrodynamical considerations leading us to believe that we might be measuring neutral winds around this time. Selected days during the year 1995 bring out this complex nature of the spaced antenna measurements carried out over the magnetic equator. Ground-based geomagnetic field data provide information on the electrodynamical state of the equatorial ionospheric E-region.

A comparative study of atmospheric Maxwell current and electric field from a low latitude station, Tirunelveli

Simultaneous measurements of atmospheric Maxwell current and electric field, using horizontal long wire antenna and passive horizontal wire antenna system at 1 m above the Earth’s surface, were carried out at Tirunelveli (8.7°N, 77.8°E), India, during January/February 2002. The objectives of the present work have been to understand the nature of the measured atmospheric electrical parameters and explore the possibility of detecting the signature associated with the global thunderstorm activity. As the measurements indicate, the atmospheric electrical parameters at Tirunelveli are severely masked by the locally induced current components during disturbed weather conditions, for example, severe convective activity. During the selected fair weather conditions, the hourly averaged diurnal variation curves of Maxwell current and electric field are characterized by two peaks: The first peak is the local “sunrise effect” and the second peak occurs at times close to 1900 UT, the time of maximum global thunderstorm activity as noted in the famous “Carniege curve”. The correlation coefficient between the measured Maxwell current and electric field has a high value (more than 0.8) for all the fair weather days. These results support our view that this site is free from local pollution during the fair weather conditions and is suitable for long-term measurements of atmospheric electrical parameters.