No abstract

No abstract

No abstract

We present systematic observations of daytime 150‐km echoes from Gadanki (13.5° N, 79.2° E, 6.3° dip latitude). The echoing region is confined to 142–162 km altitude and show forenoon descent and afternoon ascent displaying a ‘necklace shape’ quite similar to that of the equatorial 150‐km echoes. Signal‐to‐noise ratio, velocity and spectral width are as high as 13 dB above noise, −5 to 35 m s−1 and 2 to15 m s−1, respectively. The observed ‘necklace shape’ is first of its kind from Gadanki and the present observations have almost all characteristics of equatorial daytime 150‐km echoes. The Gadanki radar observations hence provide first experimental evidence on the occurrence of daytime 150‐km echoes outside the equatorial electrojet belt.

A 30 MHz radar has recently been established at Gadanki (13.5°N, 79.2°E; 6.5°N magnetic latitude) to make unattended observations of the ionospheric field-aligned irregularities (FAI). This radar, called the Gadanki Ionospheric Radar Interferometer (GIRI), has been designed to have scanning capability of 100°in the east-west plane perpendicular to Earth's magnetic field and interferometry/imaging system to study drifts and spatial distribution of plasma irregularities at both large and small scales. In this paper, we present the first results on the E and F region FAI made using the scanning capability of the GIRI. Daytime observations of E region FAI show type 2 echoes with velocities predominantly upward northward (downward-southward) at altitudes >100 km (<100 km) and westward (eastward) in the forenoon (afternoon) with signature of tidal wind field. F region irregularities show bottom-type, bottomside and plume structures with close resemblance to those observed over the magnetic equator. Observations made with the east-west scanning capability have been used to study the origin, evolution, and drift of the FAI for the first time from Gadanki. Eastward drifts are estimated to be 90-210 m s À1 during 20-24 LT. Upward velocity as large as 500 m s À1 has been observed in the initial phase of the plume structures. Intriguingly, downward velocity as large as 60 m s À1 has also been observed in the plumes, displaying descending pattern, observed in the early evening hours. These results are presented and discussed in the light of current understanding of low-latitude plasma irregularities, and future prospects of GIRI are outlined.

[1] Coordinated observations of equatorial plasma bubbles (EPBs) have been made with an all-sky airglow imager, narrow bandwidth photometer, VHF radar, and ionosonde over the Indian sector on the night of 23 March 2009. The prereversal enhancement (PRE) in the vertical plasma drift during the postsunset hours on this day was moderate. Range type spread F was found to occur immediately after the satellite traces were noted in the ionograms. This was well recorded in measurements made by all-sky imager, narrow band photometer, and VHF radar. The airglow emission intensities also revealed the presence of a large-scale wave-like structure (LSWS) together with the plasma bubbles that coincided with plume structures observed in the VHF radar echoes. The periodicity of the occurrence of bubbles (and plumes) and their interdepletion distances suggest the presence of small-scale wave-like structures (SSWS) on this night. The results are compared with the ionosonde observations made on the night of 21 February 2008. The PRE and the maximum height attained by the F layer were very similar to that of 23 March 2009. In addition, the ionograms showed the presence of satellite traces. However, no subsequent evolution of spread F was noticed. Considering the satellite traces to have their origin in LSWS, these observations imply that though the presence of LSWS is important for the triggering of EPBs, they alone are not sufficient. However, the coexistence of both LSWS and SSWS may have the potential to trigger EPBs.

[1] Long term variations of low-latitude mesospheric semi-annual oscillation (MSAO) and quasi-biennial oscillation (MQBO) in the zonal wind are studied using observations available from 1977 to 2006. The MQBO showed large inter-annual variability in the phase difference between 77.5 km and 72.5 km indicating such variability in eddy viscosity. Association of these oscillations with stratospheric quasi-biennial oscillation (SQBO) is investigated. In general, the MQBO at 77.5 km is out-ofphase with SQBO and this relation fails during strong and long SQBO. The MSAO at 72.5 km and 77.5 km generally shows strong westward phase during strong eastward phase of SQBO. The most striking feature observed in the present study is the strong modulation of the MSAO by MQBO revealing a non-linear type of interaction with presence of sidebands of the MSAO. Citation: Ratnam, M.

[1] In this paper we present the first results of F region field-aligned irregularities (FAI) made during the summer of low solar condition using the Gadanki mesosphere-stratosphere-troposphere radar. FAI echoes were observed on all 20 nights of radar observations and were mostly confined to the postmidnight hours. Echo morphology is found to be very different from the equinoctial postsunset features reported earlier from Gadanki. Echo SNRs are lower by 25 dB than their equinoctial postsunset counterparts but are quite comparable to those of the equinoctial decaying FAI during the postmidnight hours. The Doppler velocities, which lie in the range of ±100 m s À1 , are predominantly upward-northward during 0000-0300 LT and downward-southward afterward, in contrast to those observed as predominantly downward-southward associated with the decaying equinoctial postmidnight F region FAI. Spectral widths of the summer echoes, which are well below 50 m s À1 and are very similar to those of the decaying equinoctial irregularities, represent the presence of weak plasma turbulence. Simultaneous observations made using a collocated ionosonde show no ionogram trace during 2200-0530 LT except for a few cases of weak spread F events. Concurrent ionosonde observations made from magnetic equatorial location Trivandrum also show very similar results. The observations are discussed in the light of current understanding on the postmidnight occurrence of F region irregularities in the summer of low solar condition.