Indian Institute of Geomagnetism Navi Mumbai India

Navi Mumbai, India

Indian Institute of Geomagnetism Navi Mumbai India

Navi Mumbai, India

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Bhattacharyya A.,Indian Institute of Geomagnetism Navi Mumbai India | Kakad B.,Indian Institute of Geomagnetism Navi Mumbai India | Gurram P.,Indian Institute of Geomagnetism Navi Mumbai India | Sripathi S.,Indian Institute of Geomagnetism Navi Mumbai India | Sunda S.,Space Application Center Ahmedabad India
Journal of Geophysical Research: Space Physics | Year: 2017

An important aspect of the development of intermediate-scale length (approximately hundred meters to few kilometers) irregularities in an equatorial plasma bubble (EPB) that has not been considered in the schemes to predict the occurrence pattern of L-band scintillations in low-latitude regions is how these structures develop at different heights within an EPB as it rises in the postsunset equatorial ionosphere due to the growth of the Rayleigh-Taylor instability. Irregularities at different heights over the dip equator map to different latitudes, and their spectrum as well as the background electron density determine the strength of L-band scintillations at different latitudes. In this paper, VHF and L-band scintillations recorded at different latitudes together with theoretical modeling of the scintillations are used to study the implications of this structuring of EPBs on the occurrence and strength of L-band scintillations at different latitudes. Theoretical modeling shows that while S4 index for scintillations on a VHF signal recorded at an equatorial station may be >1, S4 index for scintillations on a VHF signal recorded near the crest of the equatorial ionization anomaly (EIA) generally does not exceed the value of 1 because the intermediate-scale irregularity spectrum at F layer peak near the EIA crest is shallower than that found in the equatorial F layer peak. This also explains the latitudinal distribution of L-band scintillations. Thus, it is concluded that there is greater structuring of an EPB on the topside of the equatorial F region than near the equatorial F layer peak. © 2017. American Geophysical Union. All Rights Reserved.


Joshi L.M.,Indian Institute of Geomagnetism Navi Mumbai India
Journal of Geophysical Research A: Space Physics | Year: 2016

A comprehensive investigation of spread F irregularities over the Indian sector has been carried out using VHF radar and ionosonde observations. Two different categories of spread F observations, one where the onset of the range spread F (RSF) was concurrent with the peak h′F (category 1) and another where the RSF onset happened ~90min after the peak h′F time (category 2), are presented. RSF in category 2 was preceded by the presence of oblique echoes in ionograms, indicating the irregularity genesis westward of Sriharikota. The average peak h′F in category 1 was ~30km higher than that in category 2 indicating the presence of standing large-scale wave structure (LSWS). Occurrence of the blanketing Es during 19:30 to 20:30Indian Standard Time in category 1 (category 2) was 0% (>50%). Model computation is also carried out to further substantiate the observational results. Model computation indicates that zonal variation of low-latitude Es can generate zonal modulation in the F layer height rise. It is found that the modulation of the F layer height, linked with the low-latitude Es, assists the equatorial spread F onset by modifying both the growth rate of the collisional Rayleigh-Taylor (R-T) instability and also its efficiency. A predominant presence of low-latitude Es has been observed, but the increase in the F layer height and the R-T instability growth in the evening hours will maximize with complete absence of low-latitude Es. A new mechanism for the generation of LSWS and its implications on R-T instability is discussed. © 2016. American Geophysical Union. All Rights Reserved.


Joshi L.M.,Indian Institute of Geomagnetism Navi Mumbai India | Sripathi S.,Indian Institute of Geomagnetism Navi Mumbai India
Journal of Geophysical Research A: Space Physics | Year: 2016

Vertical EXB drift measured using the ionosonde Doppler sounding during the daytime suffers from an underestimation of the actual EXB drift because the reflection height of the ionosonde signals is also affected by the photochemistry of the ionosphere. Systematic investigations have indicated a fair/good correlation to exist between the C/NOFS and ionosonde Doppler-measured vertical EXB drift during the daytime over magnetic equator. A detailed analysis, however, indicated that the linear relation between the ionosonde Doppler drift and C/NOFS EXB drift varied with seasons. Thus, solar, seasonal, and also geomagnetic variables were included in the Doppler drift correction, using the artificial neural network-based approach. The RMS error in the neural network was found to be smaller than that in the linear regression analysis. Daytime EXB drift was derived using the neural network which was also used to model the ionospheric redistribution in the SAMI2 model. SAMI2 model reproduced strong (weak) equatorial ionization anomaly (EIA) for cases when neural network corrected daytime vertical EXB drift was high (low). Similar features were also observed in GIM TEC maps. Thus, the results indicate that the neural network can be utilized to derive the vertical EXB drift from its proxies, like the ionosonde Doppler drift. These results indicate that the daytime ionosonde measured vertical EXB drift can be relied upon, provided that adequate corrections are applied to it. © 2016. American Geophysical Union. All Rights Reserved.


Bulusu J.,Indian Institute of Geomagnetism Navi Mumbai India | Sinha A.K.,Indian Institute of Geomagnetism Navi Mumbai India | Vichare G.,Indian Institute of Geomagnetism Navi Mumbai India
Journal of Geophysical Research A: Space Physics | Year: 2015

The developed analytic model for toroidal oscillations under infinitely conducting ionosphere ("Rigid-end") has been extended to "Free-end" case when the conjugate ionospheres are infinitely resistive. The present direct analytic model (DAM) is the only analytic model that provides the field line structures of electric and magnetic field oscillations associated with the "Free-end" toroidal wave for generalized plasma distribution characterized by the power law ρ = ρo(ro/r)m, where m is the density index and r is the geocentric distance to the position of interest on the field line. This is important because different regions in the magnetosphere are characterized by different m. Significant improvement over standard WKB solution and an excellent agreement with the numerical exact solution (NES) affirms validity and advancement of DAM. In addition, we estimate the equatorial ion number density (assuming H+ atom as the only species) using DAM, NES, and standard WKB for Rigid-end as well as Free-end case and illustrate their respective implications in computing ion number density. It is seen that WKB method overestimates the equatorial ion density under Rigid-end condition and underestimates the same under Free-end condition. The density estimates through DAM are far more accurate than those computed through WKB. The earlier analytic estimates of ion number density were restricted to m = 6, whereas DAM can account for generalized m while reproducing the density for m = 6 as envisaged by earlier models. © 2015 American Geophysical Union.


Ajith K.K.,Indian Institute of Geomagnetism Navi Mumbai India | Ram S.T.,Indian Institute of Geomagnetism Navi Mumbai India | Yamamoto M.,Kyoto University | Yokoyama T.,Japan National Institute of Information and Communications Technology | And 4 more authors.
Journal of Geophysical Research A: Space Physics | Year: 2015

Using the fan sector backscatter maps of 47MHz Equatorial Atmosphere Radar (EAR) at Kototabang (0.2°S geographic latitude, 100.3°E geographic longitude, and 10.4°S geomagnetic latitude), Indonesia, the spatial and temporal evolution of equatorial plasma bubbles (EPBs) were examined to classify the evolutionary-type EPBs from those which formed elsewhere and drifted into the field of view of radar. A total of 535 EPBs were observed during the low to moderate solar activity years 2010-2012, out of which about 210 (~39%) are of evolving type and the remaining 325 (~61%) are drifting-in EPBs. In general, both the evolving-type and drifting-in EPBs exhibit predominance during the postsunset hours of equinoxes and December solstices. Interestingly, a large number of EPBs were found to develop even a few minutes prior to the apex sunset during equinoxes. Further, the occurrence of evolving-type EPBs exhibits a clear secondary peak around midnight (2300-0100 LT), primarily, due to higher rate of occurrence during the postmidnight hours of June solstices. A significant number (~33%) of postmidnight EPBs generated during June solstices did not exhibited any clear zonal drift, while about 14% of EPBs drifted westward. Also, the westward drifting EPBs are confined only to June solstices. The responsible mechanisms for the genesis of fresh EPBs during postmidnight hours were discussed in light of equatorward meridional winds in the presence of weak westward electric fields. © 2015. American Geophysical Union. All Rights Reserved.


Reddy C.D.,Indian Institute of Geomagnetism Navi Mumbai India | Seemala G.K.,Indian Institute of Geomagnetism Navi Mumbai India
Journal of Geophysical Research A: Space Physics | Year: 2015

The coseismic-induced ionospheric total electron content (TEC) perturbations were analyzed following the Mw 7.8 Nepal earthquake (28.147°N, 84.708°E; depth ~15km) that occurred on 25 April 2015 at 06:11:26 UTC. The ionospheric response is due to both the modes, i.e., shock acoustic waves (slow mode) and Rayleigh wave induced (fast mode). The continuous Global Positioning System (GPS) data at about 60 sites from various GPS networks have been used in the present study. All the sites within epicentral distance of ~2400km and 70°-170° azimuth recorded the Rayleigh wave-induced TEC response, while the sites within ~400-2200km in the same azimuth recorded the response from both the modes. The maximum coseismic-induced peak-to-peak TEC amplitude is ~1.2 total electron content unit, 1TECU=1016elm-2. From Hodochron plot, the apparent Rayleigh wave velocity has been determined as ~2400m/s on the average and the acoustic wave velocity as 1180m/s, both these waves being discernible beyond ~1200km of epicentral distance as also evident from Hodochron plot and wavelet spectrographs. We reckoned the Rayleigh wave group velocities using ionospheric response at selected radial pairs of stations and validated. The ionospheric response distribution seen mainly depending on the epicentral distance, satellite geometry, directivity of radiation pattern, and the upper crustal heterogeneity. This study highlights the characteristics of ionospheric response consequent to the 2015 Nepal earthquake. ©2015. American Geophysical Union.


Gokani S.A.,Indian Institute of Geomagnetism Navi Mumbai India | Singh R.,Dr Ks Krishnan Geomagnetic Research Laboratory Indian Institute Of Geomagnetism Allahabad India | Cohen M.B.,Georgia Institute of Technology | Kumar S.,University of The South Pacific | And 3 more authors.
Journal of Geophysical Research A: Space Physics | Year: 2015

We present analysis of more than 2000 lightning-generated whistlers recorded at a low-latitude station, located at Allahabad (geographic latitude, 25.40°N; geographic longitude, 81.93°E; L=1.081), India, during December 2010 to November 2011. The main focus of this work is on the correlation between observed low-latitude whistlers and lightning activity detected by the World-Wide Lightning Location Network near the conjugate point (geography 9.87°S, 83.59°E) of Allahabad. Whistler occurrence is higher in the postmidnight period as compared to the premidnight period. Whistlers were observed in the daytime only on 2days that too before 8:30 LT (morning). Seasonally, occurrence is maximum during winter months, which is due to more lightning activity in the conjugate region and favorable ionospheric conditions. About 63% of whistlers were correlated with lightning strokes in the vicinity of the conjugate point within spatial extent of 1000km (conjugate area). Most (about 53%) whistlers were found to be associated with lightning strokes that were offset to the southeast of the conjugate point. The results indicate that an energy range of 7.5-17.5kJ of lightning strokes generate most of whistlers at this station. The L shell calculations show that propagation paths of the observed whistlers were embedded in the topside ionosphere. Based on these results we suggest a possibility of ducted mode of propagation even for such very low latitude whistlers. ©2015. American Geophysical Union.


Thampi S.V.,Vikram Sarabhai Space Center | Shreedevi P.R.,Vikram Sarabhai Space Center | Choudhary R.K.,Vikram Sarabhai Space Center | Pant T.K.,Vikram Sarabhai Space Center | And 4 more authors.
Journal of Geophysical Research A: Space Physics | Year: 2016

A case of the westward disturbance dynamo (DD) electric field, influencing the daytime equatorial and low-latitude ionosphere, during a geomagnetic storm that occurred on 28-29 June 2013 is presented. The GPS total electron content (TEC) observations from a network of stations in the Indian equatorial, low and middle latitude regions along with the radio beacon TEC, ionosonde, and magnetic field observations are used to study the storm time behavior of the ionosphere. Negative ionospheric storm effects were seen over the low and middle latitudes during the storm time due to the presence of a westward DD electric field. Observations show that the suppression of the equatorial ionization anomaly (EIA) from the morning hours itself on 29 June 2013 took place due to the prevailing westward DD electric field, providing evidence for the model calculations by Balan et al. (2013). Simulations using the GITM model also agree well with our results. The present study gains importance as the direct observational evidences for disturbance dynamo effects on the daytime low-latitude ionosphere and the EIA are sparse, as it has been difficult to delineate it from the compositional disturbances. ©2016. American Geophysical Union.


Thampi S.V.,Vikram Sarabhai Space Center | Bagiya M.S.,Indian Institute of Geomagnetism Navi Mumbai India | Chakrabarty D.,Space and Atmospheric science Division Physical Research Laboratory Ahmedabad India | Acharya Y.B.,Space and Atmospheric science Division Physical Research Laboratory Ahmedabad India | Yamamoto M.,Kyoto University
Radio Science | Year: 2014

A GNU Radio Beacon Receiver (GRBR) system for total electron content (TEC) measurements using 150 and 400MHz transmissions from Low-Earth Orbiting Satellites (LEOS) is fabricated in house and made operational at Ahmedabad (23.04°N, 72.54°E geographic, dip latitude 17°N) since May 2013. This system receives the 150 and 400MHz transmissions from high-inclination LEOS. The first few days of observations are presented in this work to bring out the efficacy of an ensemble average method to convert the relative TECs to absolute TECs. This method is a modified version of the differential Doppler-based method proposed by de Mendonca (1962) and suitable even for ionospheric regions with large spatial gradients. Comparison of TECs derived from a collocated GPS receiver shows that the absolute TECs estimated by this method are reliable estimates over regions with large spatial gradient. This method is useful even when only one receiving station is available. The differences between these observations are discussed to bring out the importance of the spatial differences between the ionospheric pierce points of these satellites. A few examples of the latitudinal variation of TEC during different local times using GRBR measurements are also presented, which demonstrates the potential of radio beacon measurements in capturing the large-scale plasma transport processes in the low-latitude ionosphere. © 2014. American Geophysical Union. All Rights Reserved.


Narayanan V.L.,Indian Institute of Science | Gurubaran S.,Indian Institute of Geomagnetism Navi Mumbai India | Shiokawa K.,Nagoya University
Journal of Geophysical Research A: Space Physics | Year: 2016

In this work we present direct ground-based observational evidence for the merging of individual equatorial plasma bubbles (EPBs) obtained through the imaging of OI 630.0nm airglow. Three potential mechanisms have been identified: (1) One of the EPBs tilts and reaches location of the adjacent growing EPB finally merging with it. (2) Some of the branches of an EPB arising from secondary instabilities reach out to adjacent EPB and merge with it. (3) The eastward zonal drift of the EPB on the eastern side slows down while the adjacent EPB on the western side drifts relatively faster and catches up. In one of the cases, a branch of an EPB was observed to get interchanged with another EPB as a result of merging and consequent pinching off from the parent EPB. © 2016. American Geophysical Union. All Rights Reserved.

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