A. K. Patra

Mean thermal structure of the low‐latitude middle atmosphere studied using Gadanki Rayleigh lidar, Rocket, and SABER/TIMED observations

[1]xa0The present study delineates the low-latitude thermal structure in the altitude range of 30 to 110 km using Gadanki (13.5°N, 79.2°E) Rayleigh lidar (1998–2007), Thumba (8.5°N,77°E) rocketsondes (1970–1991), and SABER/TIMED satellite (2002–2007) observations. This study particularly addresses whether (1) the lidar data available only during nighttime is sufficient to study the background mean thermal structure in 30–80 km altitude region, (2) the nonavailability of the lidar data during cloudy seasons (monsoon) will affect the derived background mean thermal structure, and (3) any alternate satellite observations can be used for getting the thermal structure of the middle atmosphere. The comparisons between temperatures measured by Rayleigh lidar and SABER show good agreement, suggesting that SABER data can be used effectively to study the mean thermal structure. The nocturnal average and diurnal average of temperature from SABER show similar features, suggesting that data available from lidar only during nighttime can be effectively used to study the mean background thermal structure between 30 and 80 km. Large difference between SABER and lidar observations during monsoon suggests that low data rate available from the lidar is not sufficient to obtain the mean thermal structure during cloudy seasons. Beside this, variations in stratopause (mesopause) height and temperature are also studied. The stratopause and mesopause lie in the height region of 47–49 km and 97–99 km, with peak temperature of 265 K and 170 K, respectively. Stratopause height and temperature show clear semiannual oscillation. No significant seasonal variation is observed either in mesopause height or in temperature at this low latitude.

Daytime 150-km echoes observed with the Equatorial Atmosphere Radar in Indonesia: First results

[1]xa0Results of the daytime 150-km echoes from 10.36°S magnetic latitude observed using the Equatorial Atmosphere Radar (EAR), which are first of its kind from magnetic southern hemisphere, are presented. The echoing region of 145–165 km shows forenoon descent and afternoon ascent with signal intensity modulation with time resembling a necklace and the Doppler spectra are narrow, which are quite similar to those reported earlier from other locations. Westward and upward/southward irregularity drifts are found to be consistent with daytime background electric fields, providing a tool to investigate low latitude daytime electrodynamics. Detectability of these echoes suggests the absence of strong latitudinal dependence in contrast to the earlier belief, suggesting a possibility of their detection at mid-latitudes. Also the echoes show zonal anisotropy that is opposite to that observed over Pohnpei. The implications of these observations are discussed in the light of present understanding of 150-km echoing phenomenon.

Climatology of low-latitude mesospheric echo characteristics observed by Indian mesosphere, stratosphere, and troposphere radar

[1] Low-latitude mesospheric echo characteristics are investigated using data collected during June 1994 to July 2005 (11 years) by the Indian mesosphere, stratosphere, and troposphere radar located at Gadanki (13.5°N, 79.2°E). Mesospheric echoes are frequently observed during 1000-1530 hrs (local time) in the height range of 68-78 km and are found to be highly intermittent in both space and time, consistent with those reported elsewhere. Although echoes are observed throughout the year, strong seasonal dependence has been observed in both echo occurrence and signal-to-noise ratio (SNR). Percentage occurrence (PO) of mesospheric echoes shows two maxima, one during late March equinox and early summer, and another during September. However, corresponding SNR suggests that strong echoes occur in both equinoxes with a minimum during winter. A clear semiannual variation is observed in PO of echoes with a peak occurring during the months of May and October. Similar variation is observed in SNR with peaks in March and September-November. These features are quite different from those observed at midlatitudes and high latitudes. Annual oscillation seems to fit well above 78 km and below 68 km, although on many occasions, occurrence of echo is poor at these heights. The ratio of vertical to off-vertical beam SNR (which could be taken as a measure of aspect sensitivity) was close to unity at these heights, indicating that scattering is due to turbulence-generated refractive index fluctuations. A positive correlation (R = 0.37) between PO and solar activity is observed, whereas a negative correlation (R = -0.55) is found between SNR and solar activity. The echo characteristics observed have been compared in detail with those reported from midlatitudes and high latitudes. The mechanisms behind the observed features are discussed in the light of mesospheric temperature inversions (MTIs), which are often noticed at this location, and wave breaking at these altitudes.

Further investigations on 150‐km echoing riddle using simultaneous observations of 150‐km and E region echoes from off‐electrojet location Gadanki

[1]xa0In this paper we present a detailed study of the off-electrojet daytime 150-km echoes based on simultaneous observations of daytime 150-km and E region echoes made on a few days per month during July 2005 to August 2006 using the Gadanki radar. Daytime 150-km echoes are found to occur in all seasons over Gadanki. Although echo occurrence is found to be maximum in equinoxes and minimum in summer, no clear seasonal preference over Gadanki has been observed. In contrast, E region echo occurrence made simultaneously shows a strong seasonal dependence with remarkably low occurrence in winter as compared to other seasons. Interestingly, the 150-km echoes show a distinctly different morphology in winter as compared to other seasons. Echoes are observed in two distinct height regions in winter as compared to one broad region in other seasons. Also the strongest echo and the lowest altitude of echo occurrence are found to be associated with winter. Further, a significant share of Doppler velocities is found to be downward in winter as compared to predominantly upward velocities observed in other seasons. No seasonal dependence in the spectral width of 150-km echoes, however, has been observed. Seasonally averaged Doppler velocities are quite similar to those of the F region drifts reported from Jicamarca. Again seasonally averaged drifts are found to be downward in winter afternoon as compared to upward velocities observed in other seasons. Notwithstanding the seasonal differences observed in the 150-km echoes, echo occurrence is found to have a common feature of strong forenoon-afternoon asymmetry with forenoon occurrence being greater. These observations are discussed in the light of present understanding on the daytime 150-km echoing phenomenon.

Some new aspects of low‐latitude E‐region QP echoes revealed by Gadanki radar: Are they due to Kelvin‐Helmholtz instability or gravity waves?

[1]xa0We present and analyze some interesting new aspects of low-latitude quasi-periodic (QP) echoes observed using the Gadanki radar, not reported before, to address the origin of these echoes. We report QP echoes, embedded in a descending echoing region, with periods in the range of 2–12 min decreasing with time. In one event, we show a deeply modulated echoing region, resembling very closely to Kelvin-Helmholtz billow structure. Associated with this event, we find that another echoing region already ongoing at altitudes higher up, which was not quasi-periodic, turned quasi-periodic when the undulated echoing region came in existence. Interestingly, both the echoing regions displayed identical time history in their periodicity. We have also shown QP event lasting for 5 h or even more. We also observe a clear velocity reversal with height having vertical wavelength of ∼20 km in association with these echoes. On the basis of the observed features and related analysis we have shown that the QP echoes reported here are difficult to be accounted for by the KH instability. We provided a framework and mechanism considering the gravity wave winds in zonal direction as a potential source in forming quasi-periodic plasma blob structures, which could be unstable to plasma instability, to account for the low-latitude QP echoes.

Investigation of low‐latitude E and valley region irregularities: Their relationship to equatorial plasma bubble bifurcation

[1]xa0The low-latitude E, valley and F region 3 m scale irregularities are studied with the Sanya (18.4°N, 109.6°E, dip latitude 12.8°N) VHF coherent scatter radar. The observations show that the E region irregularities (ERIs) often weaken or disappear during the development of postsunset equatorial plasma bubbles (EPBs) in equinoctial months. However, the valley region irregularities (VRIs) are found to occur during the EPB development and show structures with close relation to those of EPBs. The interesting aspect is that the ERI disruption and VRI generation are simultaneously detected. In terms of the electric field coupling from the equatorial F region down to low-latitude E and valley regions, the polarization electric fields (PEFs) associated with the EPB bifurcation are suggested to play key roles in the evolution of ERIs and VRIs. It is shown that the mapping of upward and eastward PEFs generated within the equatorial west tilted bubble would inhibit the occurrence of low-latitude ERIs. However, for the east tilted bubble structure, the associated downward PEFs might map to the low-latitude valley region and play an active role for the development of 3 m scale irregularities through gradient drift instability.

Low‐latitude mesospheric mean winds observed by Gadanki mesosphere‐stratosphere‐troposphere (MST) radar and comparison with rocket, High Resolution Doppler Imager (HRDI), and MF radar measurements and HWM93

[1]xa0Low-latitude mesospheric winds are investigated using data collected from 1995 to 2006 (11 years) by Indian MST radar located at Gadanki (13.5°N, 79.2°E). Clear eastward and westward flow in zonal wind is noticed during solstices and equinoxes, respectively. The meridional wind shows equatorward flow below 75 km and poleward flow above 75 km quite consistent with that observed with other techniques. The winds show a clear semiannual oscillation (SAO) with maxima during equinoxes. The strength of the SAO during spring is larger than that of fall equinox, and the first peak occurs at higher altitudes than the second peak. The observed features are compared with other techniques, namely, rocket, High Resolution Doppler Imager (HRDI), and medium frequency (MF) radar and also with horizontal wind model HWM93. In general, good comparison is seen among various techniques, with some discrepancy observed in amplitudes. Interestingly, decrease in eastward wind with time during winter months is noticed. The significance of the present results lies in showing the consistency/inconsistency of various experimental techniques to measure the middle atmospheric winds, which are very important to assess the climate variability.

Vertical ExB drifts from radar and C/NOFS observations in the Indian and Indonesian sectors: Consistency of observations and model

In this paper, we analyze vertical ExB drifts obtained from the Doppler shifts of the daytime 150u2009km radar echoes from two radar stations located off the magnetic equator, namely, Gadanki in India and Kototabang in Indonesia, and compare those with corresponding Coupled Ion Neutral Dynamics Investigation (CINDI) observations onboard the C/NOFS satellite and the Scherliess-Fejer model in an effort to understand to what extent the low-latitude vertical ExB drifts of the 150u2009km region represent the F region vertical ExB drifts. The radar observations were made during 9–16 LT in January, June, July, and December 2009. A detailed comparison reveals that vertical ExB drifts observed by the radars at both locations agree well with those of CINDI and differ remarkably from those of the model. Importantly, the model and observed drifts show large disagreement when the observed drifts are either large or downward. Further, while the CINDI as well as the radar observations from the two longitudes are found to agree with each other on the average, they differ remarkably on several occasions when compared on a one-to-one basis. The observed difference in detail is due to measurements made in different volumes linked with latitudinal and/or longitudinal differences and underlines the role of neutral dynamics linked with tides and gravity waves in the two longitude sectors on the respective vertical ExB drifts. The results presented here are the first of their kind and are expected to have wider applications in furthering our understanding on fine-scale longitudinal variabilities in the ionosphere in general and ionospheric electrodynamics in the Indian and Indonesian sectors in particular.

On the linkage of daytime 150 km echoes and abnormal intermediate layer traces over Sanya

[1]xa0The daytime 150 km echoes, which are associated with the upper E region field-aligned irregularities, have been observed around the equatorial electrojet region but never before at magnetic latitudes near the northern anomaly crest region. We present first results of daytime 150 km echo over Sanya, China, a station located far away from the dip equator in the Northern Hemisphere. A layer of weak radar echoes with spectral width less than 10u2009mu2009s−1 was seen in the height range from 145u2009km at 12:15 LT to 152u2009km at 13:45 LT on 21 July 2010. The interesting aspect is that the rare observation of daytime 150 km echoes with the small Sanya VHF radar was preceded by the occurrence of an unusual intermediate layer, which is identified as abnormal traces at the upper E region in corresponding ionograms. The abnormal intermediate layer associated with possible gravity wave activity (that implicates the presence of upper E region density gradients) could make a significant contribution to the growth of irregularities responsible for the rarely detected daytime 150 km echoes over Sanya.

VHF radar echoes in the vicinity of tropopause during the passage of tropical cyclone: first observations from the Gadanki MST radar

First observations on the characteristics of VHF radar echoes in the vicinity of tropopause (VOT) during depression or passage of tropical cyclone made using the Gadanki MST radar are presented. The most significant and new observation is that signal-to-noise ratio (SNR) in zenith and off-zenith beams are nearly equal in the VOT when a cyclone is located close to the radar site. During depression, however, a slight decrease in echo strength with zenith angle was found. Observations made during such disturbed conditions are in contrast to the usual aspect-sensitive characteristics of the radar echoes. Spectral widths during disturbed atmospheric conditions in general are found to be much larger than that of normal condition. Further, spectral widths observed in the vertical beam during cyclone are considerably larger than that during normal condition. These features, however, have not been noticed at other height regions during such atmospheric conditions. All these observations clearly indicate preferential enhancement in turbulence activity in the VOT. These observations constitute the first experimental evidence of such a zenith angle dependence of VHF radar echo characteristics in the VOT during depression and cyclone. These results are presented and discussed in the light of present understanding of turbulence activity in the VOT.