We connect the time sequence of changes in the IMF-Bz to the development of spread-F at an equatorial station Kwajalein on three different nights in November 2004, one during a geomagnetic quiet period and other two during geomagnetic disturbed periods. The chosen days show clear and smooth variations of IMF-Bz without any large fluctuations thereby enabling one to correlate changes in equatorial spread-F with corresponding changes in IMF-Bz. It is shown that a slow and continuous increase in the IMF-Bz over a duration of few hours has a similar effect on the equatorial ionosphere as of a sudden northward turning of the IMF-Bz in causing an electric field through the polar region and then to the equator. We conclude that the Spread-F at equatorial and low latitudes are due to echoes from ionization irregularities that arise due to the plasma instabilities generated by an eastward electric field on the large plasma density gradient in or below the base of the F-layer during any period of the night time along with the gravity driven Rayleigh-Taylor instability.

The Stratospheric Sudden Warming SSW is one of the most spectacular phenomena in the atmosphere and has impacts on the Earth’s lower, middle, and upper atmospheres. In this study, two major SSW episodes associated with vortex displacement and vortex splitting that occurred in the years 1998 and 1999, respectively, are investigated for the first time over Mt. Abu using lidar observations. Analyses show that ground-based lidar and satellite observations from the Halogen occultation experiment HALOE on board the upper atmospheric research satellite UARS can capture the effect of SSW events. Lidar measurements are able to capture SSW warming and its decay very accurately. The impact of SSW is further investigated in the European Centre for Medium-Range Weather Forecasts ECMWF Interim-reanalysed potential vorticity. Moreover, a detailed study has been presented to understand the latitudinal variation of SSW warming and the associated mesospheric cooling over the Indian region. The results showed that warming is higher over the northern Indian region 35° N, 77° E compared with the southern Indian region 5° N, 77° E.

The effect of solar flare, sudden commencement of magnetic storm and of the disturbances ring current on the equatorial electrojet in the Eastern Brazil region, where the ground magnetic declination is as large as \(20^{^{\circ }}\hbox {W}\) is studied based on geomagnetic data with one minute resolution from Bacabal during November–December 1990. It is shown that the mean diurnal vector of the horizontal field was aligned along \(2{^{\circ }}\hbox {E}\) of north at Huancayo and \(30{^{\circ }}\hbox {W}\) of north at Bacabal during the month of December 1990. Number of solar flares that occurred on 30 December 1990 indicated the direction of solar flare related \(\Delta H\) vector to be aligned along \(5{^{\circ }}\hbox {E}\) of north at Huancayo and \(28{^{\circ }}\hbox {W}\) of north at Bacabal. This is expected as the solar flare effects are due to the enhanced conductivity in the ionosphere. The SC at 2230 UT on 26 November 1990 produced a positive impulse in \(\Delta X\) and negative impulse in \(\Delta Y\) at Bacabal with \(\Delta H\) vector aligned along \(27{^{\circ }}\hbox {W}\) of north. At Huancayo the \(\Delta H\) vector associated with SC is aligned along \(8{^{\circ }}\hbox {E}\) of north, few degrees east to the alignment of the diurnal vector of H. The magnetic storm that followed the SC had a minimum Dst index of –150 nT. The corresponding storm time disturbance in \(\Delta X\) at Huancayo as well as at Bacabal were about –250 nT but \(\Delta Y\) at Bacabal was about +70 nT and very small at Huancayo, that give the alignment of the H vector due to ring current about \(16{^{\circ }}\hbox {W}\) of north at Bacabal and almost along N–S at Huancayo. Thus alignment of the \(\Delta H\) vector due to ring current at Bacabal is \(14{^{\circ }}\hbox {E}\) of the mean direction of \(\Delta H\) vector during December 1990. This is consistent with the direction of ring current dependent on the dipole declination at the ring current altitude which is about \(5{^{\circ }}\hbox {W}\) of north over Bacabal and the deviation of declination due to the ring current during disturbed period given by the angle (\(\psi \)-D).

The geomagnetic storm of November 1998 is a unique event where IMF-Bz remained southward with values exceeding –15 nT for more than a day. The SYM/H index decreased from about 07 hr on 13 November 1998 reaching a minimum of about –120 nT around midnight of 13–14 November 1998. Features of the equatorial electrojet in the Indian region are studied during the geomagnetic storm event of 13–14 November 1998, based on the geomagnetic data from the chain of observatories in India. Sudden northward turning of IMF-Bz for a very short duration around 08 hr on 13 November 1998 resulted in a small and very short duration counter electrojet. A strong (–50 nT) and a long duration counter electrojet, right from 08 to 13 hr on 14 November 1998 was observed resulting in the absence of equatorial Es at Thumba. Absence of the equatorial ionization anomaly was also observed as seen from the ionograms over Thumba and ionspheric data from Ahmedabad. The delayed effect on 14 November 1998 is due to the disturbance dynamo effect.

Characteristics of Mesospheric Temperature Inversions (MTIs) are studied using ~290 nights of Rayleigh Lidar data collected over Mt. Abu (24.5°N, 72.7°E) during the period of 1997 to 2004 for the first time from an Indian sub-tropical location. Three MTI events have been investigated and statistical analysis has also been performed. A strong MTI event, with amplitude of about 30K, at altitudes between 60 and 70km was observed on 30 December 2003. It was accompanied by strongly perturbed mesospheric zonal winds as observed by TIMED Doppler Interferometer (TIDI) onboard Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED). Two MTI episodes during March and December 2000 have been investigated, which showed that MTI can persist for few days and its amplitude varies gradually. Ozone observed by Halogen Occultation Experiment (HALOE) onboard Upper Atmospheric Research Satellite (UARS) also showed significantly higher variability during strong MTI events. A statistical analysis of MTI occurrence and their characteristics have been done using Lidar data (1997–2004), HALOE on board UARS (1997–2004) and SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) on board TIMED (2002–2004). The frequency of occurrence of MTIs is maximum during winter and minimum during summer and average magnitude of MTIs is ~20K with prominent seasonal variability. SABER and Lidar observed occurrence statistics is showing very similar pattern and SABER is able to capture stronger MTIs (~50K). The average bottom height of MTIs from Lidar is having strong seasonality (lower height during winter) in contrast to satellites observed heights of MTIs.

Increased concentration of greenhouse gases due to anthropogenic activities warm the troposphere and have a cooling effect in the middle and upper atmosphere. Ionospheric densities and heights are affected due to cooling. Carbon dioxide is one of the most dominant gases for the cause of long term ionospheric trends along with other radiatively active greenhouse gases. Regular ionospheric soundings are made over Ahmedabad (23.1°N, 72.7°E), since 1953. Long term changes in the ionosphere as a consequence of the cooling of the mesosphere and thermosphere due to the increased concentration of greenhouse gases have been studied. Ionospheric observations over Ahmedabad, a low latitude station in the anomaly crest region, for the years 1955–2003 are examined to study the long term changes in the critical frequencies of the various ionospheric layers and the height of the maximum ionization as characterized by hPF2. A decrease in foF2 (1.9MHz for midday, 1.4MHz for midnight) and hPF2 (18km for midday, 17km for midnight) during about five decades are noted. An increase is noted in foF1 (0.4MHz). The foF2 data are also examined over an equatorial station Kodaikanal (10.2°N, 77.5°E), situated near the magnetic equator for the years 1960–1995 and a decrease of 0.5MHz for midday and 0.7MHz for midnight are noted in ~35 years.

Rayleigh lidars at Gadanki (13.5° N, 79.2° E), a tropical site, and at Mt. Abu (24.5° N, 72.7° E), a subtropical site, in India were operated simultaneously during the months of March, April, and May 2004. Significant differences are found in the temperatures over both the locations. Higher temperature, ~10–20 K, in the altitude region of 40–65 km is found during March 2004 over Mt. Abu. The mean stratopause temperature during March 2004 is found ~284 K at an altitude of 48 km over Mt. Abu, which is 18 K higher than the observed stratopause temperature of ~266 K over Gadanki. During April and May 2004, the temperatures over Mt. Abu are higher in the entire altitude range of 30–70 km than over Gadanki. Lidar-observed temperatures, over both the locations, are compared with the temperatures observed by SABER (Sounding of the Atmosphere using Broadband Emission Radiometry; onboard TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics)) and HALOE (Halogen Occultation Experiment; onboard UARS (Upper Atmosphere Research Satellite)). It is found that the lidar-observed temperatures are in qualitative agreement with the temperature observed by satellites, though quantitatively there are significant differences. Wave types of fluctuations have been noted in the upper stratosphere and in the lower mesosphere over both the locations.

Combining the data of in-situ measurements of ionospheric current, Jm by rocket-borne instruments and the ground based geomagnetic H field close to the magnetic equator a linear relation has been found between the peak current density Jm and the daily range of H, (RH). This relationship has been used to convert long series of RH data into Jm. Combining Jm and the E-region peak electron density Nm, the electron velocity in the ionosphere, VE has been calculated. It is shown that after all corrections are made of the solar zenith angle variations, the ionospheric current as well as electron drift in American and Indian sectors show strong equinoctial maxima, the mean values of both the parameters are larger at American than in Indian sector. The solar cycle variation of the electrojet current is primarily due to the variations of NmE, and not due to the variations of electric field. The diurnal variation of the electric field with peak at 09–10 LT interacting with noon peak of NmE making ΔH to peak at an hour earlier than noon. It is stressed to realise the importance of electric field in diurnal, seasonal and longitudinal variations of the equatorial electrojet current.

The paper describes the results of spread-F at low latitude stations around the world during the magnetic storm starting at 0130 UT on 22 January 2004. The storm can be divided into two phases, first phase up to 1000 UT when interplanetary magnetic field IMF-Bz was highly fluctuating around a small positive value and the second phase after a sudden large southward turning of IMF-Bz at 1030 UT. The first phase produced strong spread-F at Jicamarca, Sao Luis, and Ascension Island and caused complete inhibition of spread-F at Thumba and Waltair in India. It generated weak spread-F at Ho Chi Minh City in Vietnam and strong spread-F at Hainan and Chung Li. The strong spread-F at Hainan and Chung Li were caused by the positive IMF-Bz during the first phase of the storm and not by the negative pulse of IMF-Bz at 1000 UT.

We present a comprehensive study on radar scattering cross section of mesospheric echoes and mesospheric turbulence parameters based on several days of observations made during two rocket-radar campaigns, one in July 2004 and another in April 2005, meant for studying mesospheric turbulence. Radar scattering cross section was found to have large local time and day-to-day variability and was found to be as low as 3.1×10−18 m−1 and as high as 1×10−14 m−1 and the median values were in the range of 4.4×10−18–4.7×10−16 m−1. Echoes connected with the low value of scattering cross section could be detected only when a long pulse width was used. Turbulence parameters were found to vary remarkably with time of the day and also from one day to another. In July, the turbulent kinetic energy (TKE) dissipation rate, outer scale and inner scale were in the range of 0.08–150 mW/kg, 33–1500 m, and 1.9–50 m, respectively, and their median values were in the range of 5–52 mW/kg, 293–977 m, and 2–31 m, respectively. In April, these estimates were in the range of 0.9–69 mW/kg, 38–1081 m, and 4–21 m, respectively, and their median values were in the range of 1–12 mW/kg, 140–378 m, and 8–13 m, respectively. These parameters are found to agree quite well with those estimated from rocket-borne observations, which were in the range of 4–117 mW/kg, 220–1475 m, and 15–31 m, respectively, in July and 2–36 mW/kg, 170–680 m, and 17–37 m, respectively, in April. Interestingly, the inner and outer scales estimated using both radar and rocket observations agree exceedingly well with model values. These results are compared in detail with those reported from low, middle and high latitudes including model and discussed in the light of current knowledge of mesospheric turbulence.

Yamazaki et al. (2009) reported two strong negative crochets during midday (solar flares on 8 June 2000, 3 July 2002), along the Circum-pan pacific magnetometers network (CPMN). The association of these with equatorial counter electrojet was doubted and suggestion was made to investigate the cause of these unique events. Present investigations were motivated by their paper. In this paper, detailed examination of geomagnetic data for the two events is made at all stations within 75°E (Indian) and 160°E (western Pacific) longitude sectors. Latitudinal variations of ΔH on 18 June 2000 showed negative gradient towards the dip equator suggesting a partial counter electrojet both in the Indian and CPMN sectors. A partial counter electrojet also existed from morning to 1300 LT in the western Pacific sector on 3 July 2002. There are two current sheets over the equatorial electrojet region, one at higher level flowing eastward associated with global Sq current system and another intense current layer at 107 km, flowing eastward during normal and westward during partial/full counter electrojet periods. Solar flares are likely to affect the electrojet current more strongly as a result of the absorption, in the lower E-region, of the shorter wavelength solar X-rays flare spectrum.

The paper describes the characteristics of the equatorial electrojet at Huancayo (HUA, 12.1oS, 75.3oW, inclination 1.5oN, declination 1.0oE) in western side of South America, where the geomagnetic field is aligned almost along the geographic meridian, and at Itinga (ITI, 4.3oS, 47.oW, inclination 1.4oN, declination 19.3oW) in eastern part of South America, where the geomagnetic field is aligned about 19o west of the geographic meridian; although the mean intensity of the magnetic field in the two regions are almost of the same order. Further comparisons are made of the current at Itinga and at Tatuoca (TTB, 1.2oS, 48.5oW, inclination 7.8o N, declination 18.7oW), a low latitude station in the same longitude sector. The daily range of horizontal component of the geomagnetic field, H, is shown to be almost 16% higher at HUA compared to that at ITI. The daily variation of the eastward field, Y, showed a strong minimum of -40 nT around 13-14 hr LT at ITI whereas very low values were observed at HUA with a positive peak of about 4 nT around 11- 12 hr LT. The vertical field, Z, showed abnormally large negative values of -70 nT at TTB around 13 hr LT. The day-today fluctuations of midday and midnight values of X field were positively correlated between HUA and ITI with a high correlation coefficient of 0.78 and 0.88 respectively. Values of Y field were also significantly positively correlated between HUA and ITI for midnight hours (0.72), while no correlation was observed for the midday hours. The midnight values of X field at HUA, ITI and TTB showed significant (0.90 or greater) correlation with Dst index. Correlation values of about 0.7 were observed between Dst and midday values of X at ITI and TTB and to a lesser degree (0.4) at HUA.

[1] In this paper we present and discuss observational results on low-latitude mesospheric turbulence obtained from a coordinated experiment made using a Langmuir probe (LP) onboard a rocket launched from Sriharikota (13.6°N, 80.2°E) and the mesosphere-stratosphere-troposphere (MST) radar from Gadanki (13.5°N, 79.2°E) on 8 April 2005. The LP detected electron density irregularities, with scale sizes in the range of about 1 m to 1 km, in three height regions: one region in between 69.6 and 72 km and the other two around 75 km and 78 km each having thickness of less than 1 km. The MST radar observations, however, showed two distinct scattering layers, one around 66 km and another around 75 km. The wave number spectra of the in situ observations, except for those of 69.6–72 km, and radar observed spectral parameters clearly suggest that the electron density fluctuations detected by the LP and those responsible for the radar echoes are of turbulence origin. Energy dissipation rates estimated from both rocket-borne in situ measurements and radar observed spectral width are found to be in the range of 1–70 mWkg−1. The RMS turbulent velocities estimated from the two observations are found to be <3 m s−1. Further, the energy dissipation rates and RMS turbulent velocities estimated for the height region of ∼75 km which is common in the in situ and radar data and not reported earlier, are found to be in good agreement with each other. The energy dissipation rates are compared critically with those reported earlier, and the radar-rocket observations are discussed in the light of current understanding of the low-latitude mesospheric echoes.

There was a solar event around 1850 UT on 9th November 2004, associated with an abnormally large solar wind flow pressure and large southward interplanetary magnetic field, causing an abnormally large prompt penetration electric field between 1850 and 2100 UT. Abnormally large vertical F-region drifts by Jicamarca backscatter radar were reported associated with the event. The F-region over Jicamarca, Peru (14–16 LT) and Sao Luis, Brazil (16–18 LT) was lifted upward, broken into two portions and the upper one was blown out of the range of the ionosonde. At Fortaleza, an off-equatorial station in Brazil, the F-region was also lifted up but later the f o F 2 increased due to the flow of ionization from upper layer blown up over the equatorial region. The F-region at Ascension Island (19–21 LT), an off-equatorial station, was lifted up without any deformations till 1915 LT but descended at 1930 LT due to reversal of electric field polarity. At Indian stations, Trivandrum and Waltair (00–02 LT), the F-region was pushed down and later disappeared as a consequence of enhanced westward ionospheric electric field in the night sector. The ionosonde did not receive any echo for a couple of hours till the next sunrise. The F-region at Kototaban (03–05 LT), Indonesia also disappeared after a rapid descend. At Kwajelien (06–08 LT) there was no equatorial type of sporadic-E at 07 to 09 LT due to the westward electric field.

The ionospheric total electron content (TEC) in the northern hemispheric equatorial ionization anomaly (EIA) crest region is investigated by using dual-frequency signals of the Global Positioning System (GPS) acquired from Rajkot (Geog. Lat. 22.29°N, Geog. Long. 70.74°E; Geom. Lat. 14.21°N, Geom. Long. 144.90°E), India. The day-to-day variability of EIA characteristics is examined during low solar activity period (F10.7∼83sfu). It is found that the daily maximum TEC at EIA crest exhibits a day-to-day and strong semi-annual variability. The seasonal anomaly and equinoctial asymmetry in TEC at EIA is found non-existent and weaker, respectively. We found a moderate and positive correlation of daily magnitude of crest, Ic with daily F10.7 and EUV fluxes with a correlation coefficient of 0.43 and 0.33, respectively indicating an existence of a short-term relation between TEC at EIA and the solar radiation even during low solar activity period. The correlation of daily Ic with Dst index is also moderate (r=−0.35), whereas no correlation is found with the daily Kp index (r=0.14) respectively. We found that the magnitude of EIA crest is moderately correlated with solar flux in all seasons except winter where it is weakly related (0.27). The magnitude of EIA crest is also found highly related with EEJ strength in spring (r=0.69) and summer (r=0.65) than autumn (0.5) and winter (r=0.47), though EEJ is stronger in autumn than spring.

Earth's middle atmospheric thermal structure is unique, and having imprints of various physical and chemical processes taking place in the Earth's atmosphere. Nd-YAG laser based Rayleigh lidars are operational at Mt. Abu (24.5°N, 72.7°E) and at Gadanki (13.5°N, 79.2°E) to study the middle atmospheric temperature structure in the altitude region of 30–75km. Temperature profiles are derived using Rayleigh lidar measured neutral density profiles. Nightly mean temperature profiles, during 1997–2001, are utilized to obtain average temperature profile for each month over Mt. Abu. Lidar observed temperatures are compared with the temperatures observed by Halogen Occultation Experiment (HALOE), on-board Upper Atmospheric Research Satellite (UARS), the CIRA-86 and MSISE-90. Observed temperature profiles are in qualitative agreement with CIRA-86 and MSISE-90 model below 50km, and the agreement is better during winter months. Quantitatively there are significant differences noted, up to 10K, above 50km. The temperature profiles are also compared with the equatorial model for the Indian region, based on rocket and balloon measurements. Significant day to day variability is found, which is as high as ±10K at ∼70km. The mean values of the stratopause height and temperature are found to be 48km and 271K, respectively. Seasonal variation shows equinoctial and summer maxima below 55km, whereas above 70km winter maximum with equinoctial minima are present. Comparative study of thermal structure with Gadanki, a tropical station, revealed significant differences in the thermal structure over tropical and sub-tropical locations.

An impact of a magnetic cloud on the Earth’s magnetosphere occurred at 1636 UT on 25 June, 1998, associated with a sudden increase of the solar wind density and velocity, as well as a sudden increase of the zenithal component of the interplanetary magnetic field (IMF-Bz). Following the impact of the magnetic cloud, IMF-Bz was northward (10 nT) and remained steadily strong (about 15 nT) for the next six hours. IMF-Bz turned southward at 2330 UT on 25 June, 1998, and remained strongly southward (−15 nT) for the next four hours. During the positive phase of IMF-Bz, both the Auroral index and ring current index SYM/H remained steadily low indicating complete isolation of the Earth’s magnetosphere from the solar wind and no significant changes were observed in the equatorial ionosphere. After the southward turning, the steady southward IMF-Bz permitted solar wind energy to penetrate the magnetosphere and caused the generation of a magnetic storm associated with strong auroral electrojet activity (AE index). Strong southward IMF-Bz corresponds to the dawn-dusk interplanetary electric field (eastward on the dayside and westward on the night side). The ionograms at Jicamarca (night side) showed strong spread-F and at Thumba (dayside) showed an absence of equatorial type of sporadic-E, indicating a dusk-to-dawn electric field. Thus, the observations point to an electric field opposite in direction to that expected by the prompt penetration of the interplanetary electric field. An abnormally-large Auroral index (AE) associated with the start of the storm suggests that the cause of the equatorial electric field changes is due to the disturbance dynamo effect.

Features of the equatorial electrojet are studied at Sao Luiz (2.6°S, 44.2°W, inclination −0.25°) in eastern Brazil and Sikasso (11.3°N, 5.7°W, inclination 0.1°) in the western African sector. The stations are situated on either side of the lowest magnetic field intensity in the region of rapid changes in the declination. The daily variations of Δ X at the two stations are almost similar with the peak around noon with maximum values during equinoxes and minimum values during J-solstices. Daily variations of Δ Y differ with the maximum deviation of about −35 nT around noon at Sao Luiz and much smaller value of about −10 nT around 14 h LT for Sikasso. The direction of the H vector varies from 15°W of north at 08 h to more than 30°W of north at 17 h for Sao Luiz and from 14°E of north to 25°W of north at 18 h for Sikasso. The plot of the deviations in Δ X and Δ Y at different hours for the two stations shows the points along narrow ellipses with major axis aligned along 22°W of north for Sao Luiz and along 3°W of north for Sikasso as compared to declination of 20°W for Sao Luiz and 6°W for Sikasso. The deviations in Δ X at the two stations are fairly well correlated.

Ground geomagnetic data at a chain of stations in India, interplanetary magnetic field, solar wind parameters and the ionospheric data at Thumba are examined to study the great geomagnetic storm of 25 September 1998. The geomagnetic storm is unique because of the strong counter electrojet in the morning, strong electrojet around noon and then counter electrojet in the afternoon hours caused by the enhanced solar wind density and velocity in association with the changes in IMF Bz. Disappearance of Esq in the morning hours, absence of ionization anomaly in the pre-noon hours, strong ionization anomaly near noon and later the absence of the post-sunset height rise and equatorial spread-F on 25 September 1998 are observed as a consequence of the imposition of electric fields associated with the space weather event.