@article {420, title = {Effects of prolonged southward interplanetary magnetic field on low-latitude ionospheric electron density}, journal = {Journal of Geophysical Research: Space Physics}, volume = {119}, year = {2014}, month = {07/2014}, pages = {5764 - 5776}, abstract = {

The present work describes the low-latitude ionospheric variability during an unusually prolonged (~33 h) geomagnetically disturbed condition that prevailed during 15{\textendash}16 July 2012. The low-latitude electron density in summer hemisphere, investigated using ground- and satellite-based observations, responded to this by generating strong negative ionospheric storm on 16 July. The maximum electron density on 16 July over Indian low latitudes was reduced by more than 50\% compared to that on a geomagnetically quiet day (14 July 2012). In contrast to the extreme reduction in total electron content (TEC) in the Northern Hemisphere, TEC from a winter hemispheric station revealed substantial (~23 total electron content unit, 1 TECU = 1016 el m-2) enhancements on the same day. This contrasting hemispherical response in TEC is suggested to be due to the combined effects of strong interhemispheric and solar-driven day-night winds. Further, very weak equatorial electrojet (EEJ) strength on 16 July indicated that the westward electric field perturbations in the low-latitude ionosphere were possibly due to the disturbance dynamo effect associated with meridional circulation from polar to equatorial latitudes. Interestingly, despite reduction in the integrated EEJ strength on 15 July, the low-latitude electron density showed substantial enhancement, highlighting the significant effect of the positive ionospheric storm on the low-latitude ionosphere. The roles of electrodynamical/neutral-dynamical and compositional disturbances are discussed in view of these observations to understand low-latitude ionospheric response when geomagnetic disturbance persists for longer duration.

}, keywords = {low-latitude ionosphere, neutral winds;prolonged southward IMF Bz, thermospheric neutral composition}, doi = {10.1002/2014JA020156}, url = {http://doi.wiley.com/10.1002/2014JA020156}, author = {Bagiya, Mala S. and Hazarika, Rumajyoti and Laskar, Fazlul I. and Sunda, Surendra and Gurubaran, S. and Chakrabarty, D. and Bhuyan, P. K. and Sridharan, R. and Veenadhari, B. and Pallamraju, D.} } @article {424, title = {Spatial distribution of TEC across India in 2005: Seasonal asymmetries and IRI prediction}, journal = {Advances in Space Research}, volume = {54}, year = {2014}, month = {11/2014}, pages = {1751 - 1767}, abstract = {

Total electron content measured simultaneously at 10 locations over India during the low solar activity year 2005 is used to examine the temporal and spatial asymmetries and also to assess the predictability of the International Reference Ionosphere in respect of the observed asymmetrical distribution. The stations are distributed in latitude along 77{\textdegree}E and in longitude along 23{\textdegree}N forming a meridional and a zonal chain respectively. A longitudinal gradient positive towards east was observed in the daytime hours of equinox and summer. Equinoctial asymmetry was prevalent across India during this year. Within the crest and equator, winter anomaly has been observed. It is found that IRI 2012 (with Ne Quick option, URSI coefficients) is unable to fully capture the temporal variation and spatial gradients of the ionization density in the Indian sector during 2005. The amount of offset between the model and measurement varies with local time and location.

}, keywords = {Equatorial ionization anomaly, Ionosphere, IRI, solar activity, TEC}, issn = {02731177}, doi = {10.1016/j.asr.2014.07.011}, url = {http://linkinghub.elsevier.com/retrieve/pii/S0273117714004359http://api.elsevier.com/content/article/PII:S0273117714004359?httpAccept=text/xmlhttp://api.elsevier.com/content/article/PII:S0273117714004359?httpAccept=text/plain}, author = {Hazarika, Rumajyoti and Bhuyan, Pradip Kumar} } @article {414, title = {GPS TEC near the crest of the EIA at 95{\textdegree}E during the ascending half of solar cycle 24 and comparison with IRI simulations}, journal = {Advances in Space Research}, volume = {52}, year = {2013}, month = {10/2013}, pages = {1247 - 1260}, abstract = {

Total electron content (TEC) data obtained from GPS dual frequency measurements during the ascending half of the solar cycle 24 from 2009 to 2012 over Dibrugarh (27.5{\textdegree}N, 94.9{\textdegree}E; 17.6{\textdegree}N MLAT) have been used to study the diurnal, seasonal, annual and solar cycle variation of TEC. The measurements reported here are for the first time from the location situated at the poleward edge of the northern equatorial ionization anomaly (EIA) and within the peak region of the longitudinal wave number 4 (WN4) structure in EIA crest TEC. TEC exhibits a minimum around 0600\ LT and diurnal maximum around 1300{\textendash}1600\ LT. In the low and moderate solar activity years 2009{\textendash}2010 and 2010{\textendash}2011, average daytime (1000{\textendash}1600\ LT) TEC in summer was higher (25.4 and 36.6 TECU) compared to that in winter (21.5 and 26.1 TECU). However, at the peak of the solar cycle in 2011{\textendash}2012, reversal in the level of ionization between winter and summer takes place and winter TEC becomes higher (50.6 TECU) than that in summer (45.0 TECU). Further, TEC in spring (34.1, 49.9 and 63.3 TECU respectively in 2009{\textendash}10, 2010{\textendash}11 and 2011{\textendash}12) is higher than that in autumn (24.2, 32.3 and 51.9 TECU respectively) thus showing equinoctial asymmetry in all the years of observation. The winter anomaly in high solar activity years and equinoctial asymmetry all throughout may be largely attributed to changes in the thermospheric O/N2 density ratio. A winter to summer delay of \~{}1\ h in the time of occurrence of the diurnal maximum has also been observed. Daytime maximum TEC bears a nonlinear relationship with F10.7 cm solar flux. TEC increases linearly with F10.7 cm solar flux initially up to about 140\ sfu (1\ sfu\ =\ 10-22\ W\ m-2\ Hz-1) after which it tends to saturate. On the contrary, TEC increases linearly with solar EUV flux (photons cm-2\ s-1, 0.5{\textendash}50\ nm) during the same period. TEC predicted by the IRI 2012 are lower than the measured TEC for nearly 90\% of the time.

}, keywords = {Equatorial ionosphere, GPS, Ionosphere, IRI, TEC}, issn = {02731177}, doi = {10.1016/j.asr.2013.06.029}, url = {http://linkinghub.elsevier.com/retrieve/pii/S0273117713004079}, author = {Bhuyan, Pradip Kumar and Hazarika, Rumajyoti} }