We studied the response of the ionosphere (F region) in the Brazilian sector during extreme space weather event of 17 March 2015 using a large network of 102 GPS‐ total electron content (TEC) stations. It is observed that the vertical total electron content (VTEC) was severely disturbed during the storm main and recovery phases. A wavelike oscillation with three peaks was observed in the TEC diurnal variation from equator to low latitudes during the storm main phase on 17–18 March 2015. The latitudinal extent of the wavelike oscillation peaks decreased from the beginning of the main phase toward the recovery phase. The first peak extended from beyond 0°S to 30°S, the second occurred from 6°S to 25°S, whereas the third diurnal peaks was confined from 13°S to 25°S. In addition, a strong negative phase in VTEC variations was observed during the recovery phase on 18–19 March 2015. This ionospheric negative phase was stronger at low latitudes than in the equatorial region. Also, two latitudinal chains of GPS‐TEC stations from equatorial region to low latitudes in the east and west Brazilian sectors are used to investigate the storm time behavior of the equatorial ionization anomaly (EIA) in the east and west Brazilian sectors. We observed an anomalous behavior in EIA caused by the wavelike oscillations during the storm main phase on 17 March, and suppression of the EIA, resulting from the negative phase in VTEC, in the storm recovery phase.

The present study investigates the ionospheric total electron content (TEC) and F-layer response in the Southern Hemisphere equatorial, low, and middle latitudes due to major sudden stratospheric warming (SSW) event, which took place during January-February 2009 in the Northern Hemisphere. In this study, using 17 ground-based dual frequency GPS stations and two ionosonde stations spanning latitudes from 2.8°N to 53.8°S, longitudes from 36.7°W to 67.8°W over the South American sector, it is observed that the ionosphere was significantly disturbed by the SSW event from the equator to the midlatitudes. During day of year 26 and 27 at 14:00 UT, the TEC was two times larger than that observed during average quiet days. The vertical TEC at all 17 GPS and two ionosonde stations shows significant deviations lasting for several days after the SSW temperature peak. Using one GPS station located at Rio Grande (53.8°S, 67.8°W, midlatitude South America sector), it is reported for the first time that the midlatitude in the Southern Hemisphere was disturbed by the SSW event in the Northern Hemisphere.

The equatorial electrojet (EEJ) is a narrow band of current flowing eastward at the ionospheric E region altitudes along the dayside dip equator. Mutually perpendicular electric and magnetic fields over the equator results in the formation of equatorial ionization anomaly (EIA), which in turn generates large electron density variabilities. Simultaneous study on the characteristics of EEJ and EIA is necessary to understand the role of EEJ on the EIA variabilities. This is helpful for the improved estimation of total electron content (TEC) and range delays required for satellite‐based communication and navigation applications. Present study reports simultaneous variations of EEJ and GPS‐TEC over Indian and Brazilian sectors to understand the role of EEJ on the day‐to‐day characteristics of the EIA. Magnetometer measurements during the low solar activity year 2004 are used to derive the EEJ values over the two different sectors. The characteristics of EIA are studied using two different chains of GPS receivers along the common meridian of 77°E (India) and 45°W (Brazil). The diurnal, seasonal, and day‐to‐day variations of EEJ and TEC are described simultaneously. Variations of EIA during different seasons are presented along with the variations of the EEJ in the two hemispheres. The role of EEJ variations on the characteristic features of the EIA such as the strength and temporal extent of the EIA crest has also been reported. Further, the time delay between the occurrences of the day maximum EEJ and the well‐developed EIA is studied and corresponding results are presented in this paper.

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.

Abstract. Understanding the vertical electron density profile, which is the altitudinal variation of ionospheric electron density distribution is an important aspect for the ionospheric investigations. In this paper, the bottom-side electron density profiles derived from ground based ionosonde data and the ROCSAT-1 in-situ electron density data were used to determine the estimates of the topside electron density profiles using α-Chapman function over an equatorial station Trivandrum (8.47 • N, 76.91 • E) and a low latitude station Waltair (17.7 • N, 83.3 • E) in the Indian region. The reconstructed electron density profiles are compared with IRI (2007) model derived vertical electron density profiles which resulted in significant deviations between the two different profiles. Both the reconstructed electron density profiles and the IRI model derived profiles are integrated independently to derive the Total Electron Content (TEC) values which are compared with GPS derived TEC values. TEC values derived from the reconstructed electron density profiles give better estimates with the GPS-TEC compared to those of IRI model derived TEC values. Compared to the GPS-TEC, the IRI model is underestimating the TEC values during day-time and is overestimating during night-time at both the stations. The percentage deviations of IRI derived TEC from GPS-TEC are larger compared to those between reconstructed profile derived TEC and GPS-TEC.F2-layer peak electron density, peak height and electron density at ROCSAT altitudes (≈600 km) are used to derive the effective scale heights (H T ) of the topside ionosphere during the period from July 2003 to June 2004. The diurnal and seasonal variations of H T and E × B drift velocities are presented in this paper. The diurnal variation of the effective scale height (H T ) shows peak values around noon hours with higher values during day-time and lower values during Correspondence to: P. V. S. Rama Rao (palurivsrao@gmail.com) night-time both at Trivandrum and Waltair. The E × B drift velocities at both the places also have shown a clear diurnal variation with a negative peak around 04:00 LT and maximum during day-time hours. The higher and lower values of H T seem to be associated with positive and negative phases of the E × B drift velocities, respectively.

The effects of sudden stratospheric warming (SSW) on ionosphere have been investigated by several scientists, using different observational techniques and model simulations. However, the minor SSW event during January 2012 is one of those that are less studied. Influences of several types of possible drivers—minor SSW event, changing solar flux, moderate geomagnetic storm on 22–25 January, and one of the largest solar proton events on 23–30 January—make it a challenging period to interpret. In the present study, the GPS‐total electron content (TEC) measurements from a network of 72 receivers over the Brazilian region are considered. This network of 72 GPS‐TEC locations lies between 5°N and 30°S (35°) latitudes and 35°W and 65°W (30°) longitudes. Further, two chains of GPS receivers are used to study the response of the equatorial ionization anomaly (EIA) in the Brazilian eastern and western sectors, as well as its day‐to‐day variability before and during the SSW‐2012. It was noted that the TEC is depleted to the order of 30% all over the Brazilian region, from equator to beyond the EIA regions and from east to west sectors. It is also noticed that the EIA strengths at the east and west sectors were weakened during the SSW‐2012. However, the Brazilian eastern sector was found to be more disturbed compared to the western sector during this SSW‐2012 event.

The present study investigates the ionospheric F region response in the Brazilian sector due to sudden stratospheric warming (SSW) events of 2014. The data used for this work are obtained from GPS receivers and magnetometer measurements during day of year (DOY) 01 to 120, 2014 at different stations in the equatorial and low‐latitude regions in the Brazilian sector. In addition, the data obtained from Communication/Navigation Outage Forecasting System satellites during DOY 01 to 75 of 2014 are used. The main novelty of this research is that, during the 2014 SSW events, daytime vertical total electron content (VTEC) shows a strong increase on the order of about 23% and 11% over the equatorial and low‐latitude regions, respectively. We also observed that the nighttime VTEC (SSW days) is increased by 8% and 33% over equatorial and low‐latitude regions, respectively. The magnetometer measurements show a strong counterelectrojet during the SSW days. The results show an amplification of the 0.5 day and ~2–16 day periods in the VTEC and equatorial electrojet during the SSWs. The occurrences of ionospheric irregularities during the SSW events are around 84% and 53% in the equatorial and low‐latitude regions, respectively, which is less frequent when compared with those during the pre‐SSW periods.

Abstract. The ionospheric equivalent slab-thickness is an important parameter which measures the skewness of the electron density profile of the ionosphere. In this paper, the diurnal, seasonal, day-to-day and latitudinal variations of ionospheric parameters namely total electron content (TEC), the peak ionization density of F-layer (NmF2), equivalent slab-thickness (τ ) and neutral temperature (T n ) are presented. The simultaneous data of GPS-TEC and NmF2 from Trivandrum (8.47 • N, 76.91 • E), Waltair (17.7 • N, 83.3 • E) and Delhi (28.58 • N, 77.21 • E) are used to compute the slabthickness (τ = TEC/NmF2) of the low sunspot period, [2004][2005]. The day-time TEC values at Waltair are found to be greater than those at Trivandrum, while at Delhi the day-time TEC values are much lower compared to those at Trivandrum and Waltair. The trends of variation in the monthly mean diurnal variation of TEC and NmF2 are similar at Delhi, while they are different at Trivandrum and Waltair during the daytime. The slab-thickness (τ ) has shown a pre-sunrise peak around 05:00 LT at all the three stations, except during the summer months over Delhi. A consistent secondary peak in slab-thickness around noon hours has also been observed at Trivandrum and Waltair. During equinox and winter months a large night-time enhancement in the slab-thickness (comparable to the early morning peak in slab-thickness) is observed at Delhi. The latitudinal variation of slab-thickness has shown a decrease from the equatorial station, Trivandrum to the low-mid latitude station, Delhi. The neutral temperatures (T n ) computed from the slab-thickness (τ ) has shown a sharp increase around 05:00 LT over Trivandrum and Waltair. Whereas at Delhi, a double peaking around 05:00 and 23:00 LT is observed during winter and equinoctial months. The neutral temperatures computed are compare well with those of the MSIS-90 model derived temperatures.