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.


Falkowski B.J.,Jet Propulsion Laboratory | Tsurutani B.T.,Jet Propulsion Laboratory | Lakhina G.S.,Indian Institute of Geomagnetism Navi Mumbai India | Pickett J.S.,University of Iowa
Journal of Geophysical Research: Space Physics | Year: 2017

A study of dayside plasmaspheric hiss at frequencies from ~22Hz to ~1.0kHz was carried out by using 1year of Polar data. It is shown that intense, dayside plasmaspheric hiss is correlated with solar wind pressure with P>2.5nPa. The dayside effect is most prominent in the ~300 to ~650Hz range. Intense dayside waves are also present during SYM-H<-5nT. The latter is centered at local noon, with the greatest intensities in the L=2 to 3 region. Assuming drift of ~25keV electrons from midnight to the wave magnetic local time, plasmaspheric hiss is shown to be highly correlated with precursor AE* and SYM-H* indices, indicating that the hiss is associated with substorms and small injection events. Our hypothesis is that both sets of waves originate as outer zone (L=6 to 10) chorus and then propagate into the plasmasphere. Fourteen high-intensity dayside plasmaspheric hiss events were analyzed to identify the wave k, polarization, and the degree of coherency. The waves are found to be obliquely propagating, elliptically polarized and quasi-coherent (~0.5 to 0.8 correlation coefficient). It is hypothesized that the dayside plasmaspheric hiss is quasi-coherent because the chorus has been recently generated in the outer magnetosphere and have propagated directly into the plasmasphere. It is possible that the quasi-coherency of the dayside hiss at L=2 to 3 may be an alternate explanation for the generation of the energetic particle slot region. © 2017. American Geophysical Union. All Rights Reserved.


Chakrabarty D.,Physical Research Laboratory Ahmedabad India | Hui D.,Physical Research Laboratory Ahmedabad India | Rout D.,Physical Research Laboratory Ahmedabad India | Sekar R.,Physical Research Laboratory Ahmedabad India | And 3 more authors.
Journal of Geophysical Research: Space Physics | Year: 2017

On 7 January 2005 (Ap=40) prompt penetration electric field perturbations of opposite polarities were observed over Thumba and Jicamarca on a few occasions during 13:45-16:30 UT. However, the electric field was found to be eastward during 14:45-15:30 UT over both Thumba and Jicamarca contrary to the general expectation wherein opposite polarities are expected at nearly antipodal points. On closer scrutiny, three important observational features are noticed during 14:10-15:15 UT. First, during 14:10-14:45 UT, despite increasing southward interplanetary magnetic field (IMF) Bz condition, the already westward electric field over Thumba weakened (less westward) while the eastward electric field over Jicamarca intensified (more eastward). Second, the electric field not only became anomalously eastward over Thumba but also got intensified further during 14:45-15:00 UT similar to Jicamarca. Third, during 15:00-15:15 UT, despite IMF Bz remaining steadily southward, the eastward electric field continued to intensify over Thumba but weakened over Jicamarca. It is suggested that the changes in IMF By component under southward IMF Bz condition are responsible for skewing the ionospheric equipotential patterns over the dip equator in such a way that Thumba came into the same DP2 cell as that of Jicamarca leading to anomalous electric field variations. Magnetic field measurements along the Indian and Jicamarca longitude sectors and changes in high-latitude ionospheric convection patterns provide credence to this proposition. Thus, the present investigation shows that the variations in IMF By are fundamentally important to understand the prompt penetration effects over low latitudes. ©2017. American Geophysical Union.


Venkatesh K.,University of Paraíba Valley | Tulasi Ram S.,Indian Institute of Geomagnetism Navi Mumbai India | Fagundes P.R.,University of Paraíba Valley | Seemala G.K.,Indian Institute of Geomagnetism Navi Mumbai India | Batista I.S.,National Institute for Space Research
Journal of Geophysical Research: Space Physics | Year: 2017

The St. Patrick's Day storm of 17 March 2015 has a long-lasting main phase with the Dst reaching a minimum of -223 nT. During the main phase, two strong prompt penetration electric field (PPEF) phases took place; first with the southward turning of IMF Bz around ~1200 UT and the second with the onset of a substorm around ~1725 UT leading to strong equatorial zonal electric field enhancements. The consequent spatiotemporal disturbances in the ionospheric total electron content and the resultant modifications in the equatorial ionization anomaly (EIA) over the Brazilian longitudinal sector are investigated in detail. The simultaneous measurements from a large network of GPS receivers, ionosonde, and magnetometers over the Brazilian longitudinal sector are used for this study. In the presence of enhanced zonal electric field, the equatorial F2 layer peak (hmF2) experienced a rapid uplift without any significant change in the base height (h'F); while the F2 layer is redistributed into F2 and F3 layers. The enhanced zonal electric filed due to PPEF led to the strong super fountain effect under which the anomaly crest departed poleward to ~40°S latitude. In the presence of westward and equatorward wind surge over Brazil with the coexisting disturbance dynamo fields, strong hemispheric asymmetry is seen in the storm time response of EIA during both the PPEF phases. ©2017. American Geophysical Union.


Sau S.,Equatorial Geophysical Research Laboratory Indian Institute of Geomagnetism Tirunelveli India | Narayanan V.L.,National Atmospheric Research Laboratory Gadanki India | Gurubaran S.,Indian Institute of Geomagnetism Navi Mumbai India | Ghodpage R.N.,Shivaji University | Patil P.T.,Shivaji University
Journal of Geophysical Research: Space Physics | Year: 2017

In this work, we have studied the characteristics of equatorial plasma bubbles (EPBs), such as their zonal drift and tilt, from the low-latitude and dip equatorial region in the Indian longitude sector during the main phase of the 17 March 2015 storm. All-sky airglow imaging observations from Tirunelveli (8.7°N, 77.8°E geographic, 1.7°N dip latitude) and Kolhapur (16.7°N, 74.3°E geographic, 11.5°N dip latitude) are utilized here. On 17 March 2015, EPBs were observed to drift eastward during 14:30-16:30 UT between 3°S and 15°N dip latitudes. A westward drift presumably under the influence of the disturbance dynamo electric field initially appeared at higher dip latitudes almost 10 h after the storm onset, and subsequently, the same was observed at lower dip latitudes. The EPBs attained a peak westward drift at ~17:00 UT followed by a gradual decrease in their speed till ~18:30 UT. After regaining their westward speed, the EPBs continued to drift westward till 22:00 UT. Moreover, a latitudinal gradient in the drift motion of the EPBs was also observed on this night. Another interesting observation made from the images obtained from Tirunelveli was the presence of a large westward tilt of the EPBs. The most intriguing finding of this study, however, was the asymmetry in the tilt of the EPBs at conjugate points during the premidnight hours on 17 March 2015. In this study, the possible mechanisms that can explain these observations are discussed in light of the current understanding of the equatorial electrodynamics and EPBs. ©2017. American Geophysical Union.


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.


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.


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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