Bibliography

On the impact of meridional wind circulation changes in the electron density distribution over the Indian equatorial and low latitude ionospheric region during a severe geomagnetic storm

Using a suite of instruments, which included a chain of ground-based dual-frequency GPS receivers, and magnetometers, we have studied the importance of thermospheric meridional wind circulation in controlling the distribution of plasma over the Indian low latitude ionospheric regions during the period of a severe geomagnetic storm. The storm on 15 May 2005, which had its onset coinciding with the local noon time sector for the Indian ionospheric zone, was a severe geomagnetic storm with symH ∼ - 305 nT. A steep increase in the Total Electron Content (TEC) of the ionosphere over the entire Indian ionospheric region was observed on May 15. The enhancement in the TEC was well correlated with the increase in ΔH at the dip-equator due to the prompt penetration of the convection electric field associated with the storm. However, contrary to the previous studies on the storm impact over low latitude regions, a clear signature of disturbance dynamo was absent on the day after the storm. Enhancements in the TEC were observed on May 16, a day after the storm, as well, though the ΔH at the dip-equator was quite below the quite-time mean. The TEC remained well above its monthly mean over the entire Indian ionospheric region during the storm recovery period. We suggest that the TEC enhancement on May 16, even though it looked like due to a prompt penetration effect, was directly related to the compositional disturbances as given by the O/N2 ratio. We conclude that the meridional wind circulation plays an important role in the distribution of electron density over the equatorial and low latitudinal region during the period of a geomagnetic storm.

Ambili, K.; Choudhary, R.;

Published by: Advances in Space Research      Published on: oct

YEAR: 2022     DOI: 10.1016/j.asr.2022.06.027

Compositional disturbances; Equatorial ionosphere; geomagnetic storm; total electron content

Geomagnetic storm-induced plasma density enhancements in the southern polar ionospheric region: A comparative study using St. Patrick s Day storms of 2013 and 2015

order to examine if the variations in the TEC were caused by thermospheric composition changes in the southern high-latitude regions, we present O/N 2 maps obtained from the GUVI

Shreedevi, PR; Choudhary, RK; Thampi, Smitha; Yadav, Sneha; Pant, TK; Yu, Yiqun; McGranaghan, Ryan; Thomas, Evan; Bhardwaj, Anil; Sinha, AK;

Published by: Space Weather      Published on:

YEAR: 2020     DOI: 10.1029/2019SW002383

Morphological study on the ionospheric variability at Bharati, a polar cusp station in the southern hemisphere

Morphological features of the quiet/disturbed time variations in the Total Electron Content (TEC) at the polar cusp station Bharati (76.69\textdegreeS MLAT) during a period of 5 years starting from February 2013 to December 2017 has been studied using GPS TEC measurements. The TEC at Bharati follows a diurnal pattern with its peak appearing close to local noon/magnetic noon during the summer/winter months. A nighttime enhancement in the TEC is seen around the magnetic midnight during winter. The plasma density at Bharati also exhibits semi-annual variation and a strong dependence on solar activity. A comparison of the IRI 2016 model derived TEC and the GPS TEC at Bharati shows significant differences with large underestimation of TEC especially during the nighttime period of the winter months. A two fold difference in magnitude between the GPS and modeled TEC is also observed in the summer months of the high solar activity period of 2013\textendash2015. The response of the TEC to geomagnetic storms is found to depend on the onset time of the storm. We show that the morphological features in the temporal evolution of the plasma density at Bharati vary as the location of Bharati changes from being inside the polar cap, to the auroral region, and to the polar cusp in quick succession in a day. Our results highlight the fact that the dynamic nature of the location of Bharati with respect to the position of the polar cap plays an important role in deciding the plasma distribution at the polar cusp station.

Shreedevi, P.R.; Choudhary, R.K.; Yu, Yiqun; Thomas, Evan;

Published by: Journal of Atmospheric and Solar-Terrestrial Physics      Published on: 10/2019

YEAR: 2019     DOI: 10.1016/j.jastp.2019.105058

Signatures of the Solar Transient Disturbances Over the Low Latitude Ionosphere During 6 to 8 September 2017

Low latitude ionospheric behavior during solar transient disturbances of solar flares and storm time penetrating electric fields comprises an important part of the Earth\textquoterights space weather. The flares enhance the electron density of the sunlit ionosphere by supplying excess solar radiation. However, the degree of these density changes is subjective if a geomagnetic storm persists simultaneously. The present case study addresses the ionospheric variations over the Indian longitudes under the combined effects of the solar flares and a geomagnetic storm during 6 to 8 September 2017 and probably the first of its kind in delineating the effects of these two over the low latitude ionosphere. The X9.3 class flare of 6 September, which occurred during non-storm conditions, produced an intense E region ionization (~500\% over the ambient). However, the total electron content response to this flare was comparatively weak. The flares on 7 and 8 September occurred during the 7\textendash8 September geomagnetic storm. Though the 8 September flare occurred with higher intensity (M8.1) and early in local time compared to the flare of 7 September (M7.3), the equatorial electrojet current enhancement was lesser on 8 September (~75\% over the ambient) than that of 7 September (~110\% over the ambient). This aspect is discussed in view of the storm time convection effects over the low latitudes during 7\textendash8 September storm. The total electron content did not respond to the flares of 7 and 8 September. This behavior is attributed to the varying center-to-limb distance of the solar active region 12673 during this period.

Bagiya, Mala; Thampi, Smitha; Hui, Debrup; Sunil, A.; Chakrabarty, D.; Choudhary, R.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 08/2018

YEAR: 2018     DOI: 10.1029/2018JA025496

Impact of Oscillating IMF B _z During 17 March 2013 Storm on the Distribution of Plasma Over Indian Low-Latitude and Mid-Latitude Ionospheric Regions

We show evidence of a positive ionospheric storm occurring simultaneously at the equatorial, low-latitude and mid-latitude ionospheric regions in the Indian sector in response to an intense geomagnetic storm on 17 March 2013. The storm had its onset time coinciding with the local noon. An on-site digisonde at Trivandrum (dip equator) recorded a sharp decrease in the height of F\ region peak in the afternoon, which is a signature of westward electric field associated with a counter electrojet (CEJ). Coupled with it was a simultaneous increase in total electron content (TEC) in the entire Indian region. The magnitude of increase in TEC decreased slowly northward of the dip equator and had almost no change near the anomaly crest. Farther northward of the anomaly crest, the TEC started increasing again and at Shimla, a mid-latitude station, it had a value close to 2 times its monthly mean. We surmise that the westward electric field resulting from the CEJ pushed the F\ layer at the dip equator down to the altitude regions where recombination and diffusion played minimal roles. No loss of plasma due to diffusion while photoproduction of ions was still taking place, led to an enhancement in the electron density near the equatorial/low-latitude region. The Joule heating of the thermosphere, on the other hand, gave rise to the traveling atmospheric disturbances which pushed the plasma up in altitude at the equatorial anomaly region. It supplemented the loss of plasma at the anomaly crest region resulting in no change in the TEC thereat and a marked increase in the TEC in the mid-latitude ionosphere.

Shreedevi, P.; Choudhary, R.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 10/2017

YEAR: 2017     DOI: 10.1002/2017JA023980

Impact of Oscillating IMF B z During 17 March 2013 Storm on the Distribution of Plasma Over Indian Low-Latitude and Mid-Latitude Ionospheric Regions

We show evidence of a positive ionospheric storm occurring simultaneously at the equatorial, low-latitude and mid-latitude ionospheric regions in the Indian sector in response to an intense geomagnetic storm on 17 March 2013. The storm had its onset time coinciding with the local noon. An on-site digisonde at Trivandrum (dip equator) recorded a sharp decrease in the height of F region peak in the afternoon, which is a signature of westward electric field associated with a counter electrojet (CEJ).

Shreedevi, PR; Choudhary, RK;

Published by: Journal of Geophysical Research: Space Physics      Published on:

YEAR: 2017     DOI: 10.1002/2017JA023980

Direct observational evidence for disturbance dynamo on the daytime low-latitude ionosphere: A case study based on the 28 June 2013 space weather event

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\textendash29 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.

Thampi, Smitha; Shreedevi, P.; Choudhary, R.; Pant, Tarun; Chakrabarty, D.; Sunda, S.; Mukherjee, S.; Bhardwaj, Anil;

Published by: Journal of Geophysical Research: Space Physics      Published on: 09/2016

YEAR: 2016     DOI: 10.1002/2016JA023037

On the latitudinal changes in ionospheric electrodynamics and composition based on observations over the 76--77 E meridian from both hemispheres during a geomagnetic storm

The relative contributions of the composition disturbances and the disturbance electric fields in the redistribution of ionospheric plasma is investigated in detail by taking the case of a long-duration positive ionospheric storm that occurred during 18\textendash21 February 2014. GPS total electron content (TEC) data from the Indian Antarctic station, Bharti (69.4\textdegreeS, 76.2\textdegreeE geographic), the northern midlatitude station Hanle (32.8\textdegreeN, 78.9\textdegreeE geographic), northern low-latitude station lying in the vicinity of the anomaly crest, Ahmedabad (23.04\textdegreeN, 72.54\textdegreeE geographic, dip latitude 17\textdegreeN), and the geomagnetic equatorial station, Trivandrum (8.5\textdegreeN, 77\textdegreeE geographic, dip latitude 0.01\textdegreeS) are used in the study. These are the first simultaneous observations of TEC from Bharti and Hanle during a geomagnetic storm. The impact of the intense geomagnetic storm (Dst\~-130\ nT) on the southern hemisphere high-latitude station was a drastic reduction in the TEC (negative ionospheric storm) starting from around 0330 Indian standard time (IST) on 19 February which continued till 21 February, the maximum reduction in TEC at Bharti being \~35 TEC units on 19 February. In the northern hemisphere midlatitude and equatorial stations, a positive ionospheric storm started on 19 February at around 0900 IST and lasted for 3\ days. The maximum enhancement in TEC at Hanle was about \~25 TECU on 19 February while over Trivandrum it was \~10 TECU. This long-duration positive ionospheric storm provided an opportunity to assess the relative contributions of disturbance electric fields and composition changes latitudinally. The results indicate that the negative ionospheric storm over Bharti and the positive ionospheric storm over Hanle are the effect of the changes in the global wind system and the storm-induced composition changes. At the equatorial latitudes, the positive ionospheric storm was due to the interplay of prompt penetration electric field and disturbance dynamo electric field.

Shreedevi, P.; Thampi, Smitha; Chakrabarty, D.; Choudhary, R.; Pant, Tarun; Bhardwaj, Anil; Mukherjee, S.;

Published by: Journal of Geophysical Research: Space Physics      Published on: 02/2016

YEAR: 2016     DOI: 10.1002/2015JA021841

Geomagnetic storms; High latitude low latitude coupling; Ionosphere; positive ionospheric storm