P. Pavan Chaitanya

First results on low‐latitude E and F region irregularities obtained using the Gadanki Ionospheric Radar Interferometer

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 Earths 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−1 during 20–24 LT. Upward velocity as large as 500 m s−1 has been observed in the initial phase of the plume structures. Intriguingly, downward velocity as large as 60 m s−1 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.

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 150 km 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 150 km 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.

Ionospheric variability over Indian low latitude linked with the 2009 sudden stratospheric warming

In this paper, we analyze radar observations of E × B drift and plasma irregularities, ionosonde observations of E and F layer parameters including spread F, and magnetic field observations made from Indian low latitudes linked with the 2009 sudden stratospheric warming (SSW) event. E × B drift variations presented here are the first of their kind from the Indian sector as far as the effect of SSW is concerned. Difference of magnetic fields observed from the equator and low-latitude (∆H) and E × B drift show linear relation, and both show remarkably large positive values in the morning and negative values in the afternoon exhibiting semidiurnal behavior. Remarkable changing patterns in the critical frequency of F2 layer (foF2) and F3 layer (foF3) were observed after the occurrence of SSW. Large variations with quasi 16 day periodicity were observed in ∆H, foF2, and foF3. Both semidiurnal and quasi 16 day wave modulation observed after the 2009 SSW event are consistent with those reported earlier. We also noted quasi 6 day variations in ∆H and foF2 soon after the SSW commencement, not much reported before. During the counterelectrojet events linked with the SSW event, while equatorial Es (Esq) disappeared as expected, there were no blanketing Es (Esb), a finding not reported and discussed earlier. Esb was also not formed at the off-equatorial location, indicating the absence of required vertical wind shear, but E region plasma irregularities were observed by the ionosonde and radar with a close relationship between the two. Weak F region irregularities were observed in the postmidnight hours, and case studies suggest the possible role of SSW-related background electric field in the manifestation of postmidnight F region irregularities.

Highly localized unique electrodynamics and plasma irregularities linked with the 17 March 2015 severe magnetic storm observed using multitechnique common-volume observations from Gadanki, India

In this paper we study equatorial electrodynamics and plasma irregularities linked with the 17 March 2015 severe magnetic storm in the Indian sector using common volume observations made by the Gadanki Ionospheric Radar Interferometer, airglow imager, digisonde and GPS receiver established at Gadanki (13.5o N, 79.2o E). Observations show that with the initiation of the storm at ~6 UT on 17 March, which happened to be midday in the Indian sector, the low latitude ionosphere responded in tune with the storm induced electric field and by the sunset time the base of the F layer ascended to an altitude of 470 km with a peak upward velocity of 50 m s-1 eventually manifesting equatorial plasma bubble and irregularities causing strong GPS scintillation. The most important finding found in this study is the confinement of plasma bubble and irregularities in a narrow longitude zone of 69o-98o E. Results also show reversal of zonal drift of the irregularities from ~120 m s-1 eastward drift to ~120 m s-1 westward drift in a time span of ~30 min. Both observations are shown to be linked with very special electrodynamical conditions induced by the magnetic storm related electric field in the dusk sector. Intriguing details of the longitudinally localized electrodynamics and plasma irregularities are discussed in terms of prompt penetration and disturbed dynamo electric field effects.

Unexpected characteristics of the 150 km echoes observed over Gadanki and their implications

Recent discovery of two distinct types of 150 km echoes, namely type-A and type-B, and subsequent progress in the large-scale kinetic simulation of photoelectron induced plasma waves have begun a new era in resolving the five decades long 150 km echoing riddle. In this paper, we present hitherto unrevealed three important and unexpected findings on the two distinct types of 150 km echoes based on Gadanki radar observations. Our observations show unexpected predominance of type-A echoes, strong seasonal dependence of both type-A and type-B echoes, and a surprising connection of the type-B echoes to the unusually deep solar minimum of 2008-2009. We discuss how these results provide important new clues in tethering the competing processes involved in the daytime 150 km echoes and have significance in the recently proposed photoelectron-induced plasma fluctuations as a potential mechanism for the 150 km echoes.

Quiet time short-period and day-to-day variations in E × B drift studied using 150 km radar echoes from Gadanki

In this paper we present short-period and day-to-day variations in E × B drift during low solar and magnetically quiet conditions, based on radar observations of daytime 150 km echoes from Gadanki, India. Short-period (<100 min) variations in E × B drift show amplitude as large as 7 m s−1 and display large day-to-day variation. Spectral analysis reveals that drift velocity fluctuations consist of several components of short periods in the range of 4–100 min. Among these periods, the most frequently occurring periods are 6–60 min. Observations also show that amplitudes of these periods increase with increasing period. Interestingly, signal-to-noise ratio (SNR) variations also show periods of 3–19 min, which have also been observed in velocity variations. In addition, amplitudes of the periodic variations in SNR tend to increase with increasing period, which is also similar to those observed in drift velocity fluctuations. No correlation between SNR and velocity variations, however, has been found. Noticeable day-to-day variations in drift velocity are found in all seasons, and the variation is as large as ±25 m s−1. Day-to-day variations also show wave-like features with period of 2–4 days. Observed E × B drift variations of 7 m s−1 at short time scale and 25 m s−1 on a day-to-day basis indicate zonal electric field variations of 0.25 mV/m and 0.9 mV/m, respectively. We surmise that quiet time E × B drift variations with periods <100 min and 2–4 days are likely to be the manifestations of gravity wave and planetary wave wind-induced electric fields, respectively, consistent with those reported earlier.

Unusual behavior of the low-latitude ionosphere in the Indian sector during the deep solar minimum in 2009

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 10 day 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.

Daytime zonal drifts in the ionospheric 150 km and E regions estimated using EAR observations

Multi-beam observations of the 150-km echoes made using the Equatorial Atmosphere Radar (EAR), located at Kototabang, Indonesia provide unique opportunity to study both vertical and zonal ExB plasma drifts in the equatorial ionosphere. In this paper, we focus on estimating daytime zonal drifts at the 150 km (140-160 km) and E (100-110 km) regions using multi-beam observations of 150-km- and E-region echoes made using the EAR and study the daytime zonal drifts covering all seasons, not studied before from Kototabang. Zonal drifts in the 150-km and E regions are found to be westward and mostly below -80 m s-1 and -60 m s-1, respectively. While the zonal drifts in the 150-km and E regions do not go hand in hand on a case by case basis, the seasonal mean drifts in the two height regions are found to be in good agreement with each other. Zonal drifts at the 150 km region show seasonal variations with three maxima peaking around May, September and January. The zonal drifts at the 150 km region are found to be smaller than the F region drifts obtained from CINDI onboard C/NOFS by about 25 m s-1 consistent with the height variations of F region zonal drifts observed by the Jicamarca radar. These results constitute the first comprehensive study of zonal drifts at the 150 km and E regions from Kototabang, Indonesia and the results are discussed in the light of current understanding on the low latitude electrodynamics and coupling.