Conjugate Stations Research Articles

25-26 Ağustos 2018 Jeomanyetik Fırtına Sırasında Afrika Bölgesi Üzerindeki Manyetik Eşlenik Çiftleri Üzerindeki TEC Değişikliklerinin İncelenmesi

In this study, the electron transport process resulting from electromagnetic drift between two magnetic conjugate pairs over the African region during the August 25-26 2018 geomagnetic storm was investigated. The effects of geomagnetic conditions presented with Dst index and IMF Bz values on Total Electron Content (TEC) values at conjugate stations were compared separately for stormy and quiet periods. During the storm period, the effect of TEC values at stations in the northern hemisphere (Haifa and Djibouti) on the TEC values at stations in the southern hemisphere (Ambalavao and Malindi) is greater than the effect of TEC values in the southern hemisphere on TEC values in the northern hemisphere. According to this result, it can be said that the south-directed electromagnetic convection was more than the north-directed convection in the examined dates. When the coefficients are examined, it can be said that the interaction is more in the magnetic conjugate pair that is closer to the equator during the storm period, and the interaction is more in the magnetic conjugate pair that is far from the equator during the silent period. Considering the coefficients calculated for Dst and IMF Bz, it is seen that the TEC values are very small compared to their coefficients. From this it can be concluded that the effect of Dst and IMF Bz is much smaller than the effect of TEC values at a station on TEC values at its magnetic conjugates.

The investigation on daytime conjugate hemispheric asymmetry along 100°E longitude using observations and model simulations: New insights

The hemispherical asymmetry of the low latitude region along 100°E ± 5°E is scrutinized for the year 2015 at magnetically conjugate points on seasonal and intra-seasonal time scales. Two conjugate Ionosonde station pairs are selected- one pair in the inner valley (from SEALION) and the other in the outer edges of the EIA region. The anomaly in the stations is estimated using the difference of low latitude NmF2 from the dip equatorial NmF2 in the same meridian. A monthly average scheme is used instead of a seasonal mean, as the month-to-month variations are found to provide intricate details. The anomaly at the conjugate stations is highly asymmetric even during the equinoctial months of March and October, whereas it is nearly symmetric during April. During June/July, the morning time hemispheric asymmetry (larger on the winter side) temporarily reduces in the midday period and then reverses sign (larger in summer) in the afternoon. The NmF2 observations suggest a close relation of hemispheric symmetry to the position of the subsolar point with respect to the dip equator and a shift/expansion of the trough region of the EIA towards the summer hemisphere. The inter-hemispheric comparison of the hmF2 suggests a strong modulating influence of meridional winds at both the inner and outer stations which depend strongly on the relative position of the subsolar point with respect to the field line geometry. Theoretical (SAMI3/SAMI2) and empirical model (IRI) simulations show a meridional movement of the EIA region with the subsolar point. The winter to summer hemisphere movement of the EIA trough and crest region is also reproduced in the GIM-TEC along 100°E for 2015. This shifting or tailoring of the trough and the crest region is attributed primarily to the meridional wind field, which varies with the shifting position of subsolar point relative to the field line geometry. The seasonal and intra-seasonal difference in the NmF2 hemispheric asymmetry is attributed to the misalignment of the two centers of power viz., the thermospheric/neutral processes and the electromagnetic forces, due to the geographic-geomagnetic offset in this longitude.

A case study based on ground observations of the conjugate ionospheric response to interplanetary shock in polar regions

Data acquired by imaging relative ionospheric opacity meters (riometers), ionospheric total electron content (TEC) monitors, and three-wavelength auroral imagers at the conjugate Zhongshan station (ZHS) in Antarctica and Yellow River station (YRS) in the Arctic were analyzed to investigate the response of the polar ionosphere to an interplanetary shock event induced by solar flare activity on July 12, 2012. After the arrival of the interplanetary shock wave at the magnetosphere at approximately 18:10 UT, significantly enhanced auroral activity was observed by the auroral imagers at the ZHS. Additionally, the polar conjugate observation stations in both hemispheres recorded notable evolution in the two-dimensional movement of cosmic noise absorption. Comparison of the ionospheric TEC data acquired by the conjugate pair showed that the TEC at both sites increased considerably after the interplanetary shock wave arrived, although the two stations featured different sunlight conditions (polar night in July in the Antarctic region and polar day in the Arctic region). However, the high-frequency (HF) coherent radar data demonstrated that different sources might be responsible for the electron density enhancement in the ionosphere. During the Arctic polar day period in July, the increased electron density over YRS might have been caused by anti-sunward convection of the plasma irregularity, whereas in Antarctica during the polar night, the increased electron density over ZHS might have been caused by energetic particle precipitation from the magnetotail. These different physical processes might be responsible for the different responses of the ionosphere at the two conjugate stations in response to the same interplanetary shock event. Citation : He F, Hu Z J, Hu H Q, et al. A case study based on ground observations of the conjugate ionospheric response to interplanetary shock in polar regions. Adv Polar Sci, 2021, 32(2): 139-158, doi: 10.13679/j.advps.2021.0012

Characterization of African Equatorial Ionization Anomaly During the Maximum Phase of Solar Cycle 24

AbstractThis study characterizes the African equatorial ionization anomaly (EIA) during the maximum phase of Solar Cycle 24 (2012–2015). Total electron content (TEC) data were obtained from a chain of 13 African Global Navigation Satellite System (GNSS) receivers (within 36–42°E geographic longitude; 29°N to 10°S geographic latitude; ±20°N magnetic latitude) to study quiet time variations of TEC and thereafter construct the EIA profiles. The correlations of TEC for pairs of conjugate stations within the African EIA were determined. TEC station‐to‐trough ratio (TEC‐STR) was used as an index to determine the strength of the EIA crests. Furthermore, the variability of the EIA crests on a 3‐hourly basis over each month was investigated. Overall, equinoxes recorded the highest values of TEC, with the widest latitudinal location of the EIA crests, and June solstice recorded the least, with the characteristic equatorward collapse of the EIA crests. TEC and the location of the crests of the EIA also increased with solar activity. TEC showed good correlations for conjugate pairs of stations, especially for stations closer to the trough. The African EIA morphology showed asymmetry in the locations of the Northern Hemisphere (NH) and Southern Hemisphere (SH) crests. The formation of EIA starts around 0600–0800 LT and decays around 2100–2300 LT. Generally, SH crest forms first before NH crest and also decays first; we attributed this to the effects of the meridional neutral winds. Comparing TEC at African EIA crests with other longitudinal sectors, the Asian sector recorded the highest TEC, while the American sector recorded the least.

Interhemispheric Asymmetries in the Ground Magnetic Response to Interplanetary Shocks: The Role of Shock Impact Angle

AbstractInterplanetary (IP) shocks drive magnetosphere‐ionosphere (MI) current systems that in turn are associated with ground magnetic perturbations. Recent work has shown that IP shock impact angle plays a significant role in controlling the subsequent geomagnetic activity and magnetic perturbations; for example, highly inclined shocks drive asymmetric MI responses due to interhemispherical asymmetric magnetospheric compressions, while almost head‐on shocks drive more symmetric MI responses. However, there are few observations confirming that inclined shocks drive such asymmetries in the high‐latitude ground magnetic response. We use data from a chain of Antarctic magnetometers, combined with magnetically conjugate stations on the west coast of Greenland, to test these model predictions (Oliveira & Raeder, 2015, https://doi.org/10.1002/2015JA021147; Oliveira, 2017, https://doi.org/10.1007/s13538-016-0472-x). We calculate the time derivative of the magnetic field ( ) in each hemisphere separately. Next, we examine the ratio of Northern to Southern Hemisphere intensities and the time differences between the maximum immediately following the impact of IP shocks. We order these results according to shock impact angles obtained from a recently published database with over 500 events and discuss how shock impact angles affect north‐south hemisphere asymmetries in the ground magnetic response. We find that the hemisphere the shock strikes first usually has (1) the first response in and (2) the most intense response in . Additionally, we show that highly inclined shocks can generate high‐latitude ground magnetic responses that differ significantly from predictions based on models that assume symmetric driving conditions.

Observation of the north-south asymmetry in the equatorial spread F intensity at the Brazilian longitude

This study presents an analysis of the observed north-south asymmetry of the range spread F (RSF) intensity at the low latitude region during an equinoctial month of different solar epochs (2002, 2015 and 2017). The ionospheric parameters were obtained during geomagnetic quiet days from four digisonde stations located along the Brazilian longitude, which include a dip equator station (Sao Luiz (SL: 2.33 S, 44.2 W)), conjugate stations (Campo Grande (CG: 20.5°S, 55°W) and Boa Vista (BV: 2.8°N, 60.7°W)) and another low latitude station (Cachoeira Paulista (CP: 22.7°S, 45°W)). The results highlight the competing effect of the post-sunset electric field strength and the trans-equatorial wind on the latitudinal distribution of the irregularity intensity at both hemispheres under varying background ionospheric condition. The RSF intensity was seen to reduce as the solar flux index decreased and the latitudinal peak shifted closer to the dip equator. This was dependent on the variation of the field line mapped irregularity spectrum and the density gradient. Likewise, the north-south asymmetry in the irregularity occurrence was seen to become more significant as a denser ionosphere was observed at the hemisphere with the equatorward meridional wind. This has further proven that the non-linear cascading of the plasma irregularity across the low latitude region is strongly influenced by the local electric field.

Geomagnetically Conjugate Observations of Equatorial Plasma Irregularities From Swarm Constellation and Ground‐Based GPS Stations

AbstractThe near‐polar orbit satellites of Swarm mission provide a good opportunity to investigate the conjugacy of equatorial plasma irregularities (EPIs) since their trajectories at low latitudes are basically aligned with fixed geographical longitude. However, the Swarm in situ electron density occasionally shows EPIs at only one hemisphere at this longitude. In this study, we provide detailed analysis of such EPI events from the in situ electron densities and onboard global positioning system (GPS) measurements of Swarm low pair satellites, and simultaneous GPS data from two geomagnetically conjugate ground stations at the Africa longitudes. The result indicates that when Swam in situ electron density sometime shows EPIs at only one hemisphere, the GPS scintillations are still observed from the Swarm onboard receiver and by the two conjugate ground stations. It implies that the EPIs should generally elongate along the geomagnetic flux tube. More than two‐year statistic results show that the onset time of scintillation in the northern station is on average 16 and 18 min earlier than that in the southern station for September equinox and December solstice in 2015, while for March equinox in 2016 the onset time of scintillation of northern station is about 11 min later than that of southern station, which indicates the asymmetry features of EPIs along the flux tube. Further analysis of nearly three‐year GPS data from two conjugate stations at the Asia longitudes, we find that during solar maximum years the local sunset time plays an important role for causing the difference of onset time of scintillation between two conjugate stations.

Geomagnetic and Ionospheric Responses to the Interplanetary Shock Wave of March 17, 2015

The propagation of perturbation caused by the interplanetary shock wave of March 17, 2015 from the solar wind through the magnetosheath, magnetosphere, and ionosphere down to the Earth’s surface is analyzed. The onboard satellite measurements, global magnetometer network data, and records by the receivers of the global positioning system (GPS) providing the information about the total electron content (TEC) of the ionosphere are used for the analysis. By the example of this event, various aspects of the influence of the interplanetary shock wave on the near-Earth environment and ground-based engineering systems are considered. It is shown which effects of this influence are well described by the existing theoretical models and which ones need additional research. The formation of the fine structure of the magnetic impulse of the storm sudden commencement (SC)—the preliminary impulse (PI) and main impulse (MI)—is considered. The MI and compression of the magnetospheric magnetic field is observed by the GOES and RBSP satellites and on the geomagnetically conjugate stations; however, the PI was only noted on the Earth. The PI was detected in the afternoon sector practically simultaneously (within 1 min) with the shock wave impact on the magnetopause. The wave’s response to the SC includes the strongly decaying resonant oscillations of the magnetic shells and the magnetoacoustic cavity mode. This study supports the possibility of detecting the ionospheric response to the SC by the GPS method. The TEC response to the MI was detected in the auroral latitudes although not on every radio path. The TEC modulation can be associated with the precipitation of superthermal electrons into the lower ionosphere which is undetectable by riometers. The burst in the intensity of the geomagnetically induced currents caused by an interplanetary shock wave turns out to be higher than the currents during the storm’s commencement, although the SC’s amplitude is noticeably lower than the amplitude of the magnetic bay related to the substorm.

The relationship between equatorial ionization anomaly and nighttime equatorial spread F in East Africa

We study the relationship of equatorial ionization anomaly (EIA) to the occurrence of nighttime equatorial spread F (ESF) using data from ground-based GPS stations and a horizontal meridional neutral wind model for the East African sector. Anomaly strength, symmetry of anomaly crests, horizontal meridional wind magnitude and direction, and magnitude of rate of TEC index (ROTI) are computed and analyzed. We find mean crest to trough ratio (CTR) greater than or equal to 1.4 and mean north crest to south crest ratio (CCR) within 0.9 and 1.1 in the time between 16:00 UT (19:00 LT) and 18:00 UT (21:00 LT) as good conditions for the occurrence of ESF. EIA strength and symmetry of anomaly crests are better related with ESF occurrence between conjugate stations (≈ ± 11.5o magnetic latitudes) than between other north-south stations. ROTI is found to be relatively large when CCR of EIA are symmetrical (CCR between 0.9 and 1.1) and CTR is greater than or equal to 1.4, but decreases as the CCR of EIA shows asymmetry and/or CTR of EIA is less than 1.4. EIA strength and symmetry of anomaly crests between 18:00 UT to 20:45 UT are not well related with spread F occurrences. Post-sunset spread F occurrence close to anomaly crest locations are found to be greater than spread F occurrence at magnetic equatorial station. Shifting of EIA crest locations on either side of the magnetic equator is found to depend on season and meridional wind magnitude and direction. The relative difference in spread F occurrence among the four seasons is also discussed.

Temporal evolution of the EIA along 95°E as obtained from GNSS TEC measurements and SAMI3 model

The total electron content (TEC) derived from GNSS measurements at a trans-hemispheric meridional chain of ground stations around 95°E longitude are used to study the quiet time inter-hemispheric structure and dynamics of the equatorial ionization anomaly (EIA) during the period March 2015 to February 2016. The stations are Dibrugarh (27.5°N, 95°E, 43° dip), Kohima (25.6°N, 94.1°E, 39° dip), Aizawl (23.7°N, 92.8°E, 36° dip), Port Blair (11.63°N, 92.71°E, 9° dip) and Cocos Islands (12.2°S, 96.8°E, 43° dip). The observation shows that the northern crest of the EIA lies in the south of 23°N (Aizawl) in all seasons but recedes further south towards the equator during December solstice. The largest poleward expansion of the northern (southern) EIA is observed in the March equinox (December solstice). The equinoctial and hemispherical asymmetry of TEC is noted. The winter anomaly is observed in the northern hemisphere but not in the southern hemisphere. The highest midday TEC over any station is observed in the March equinox. The TEC in southern summer (December solstice) is significantly higher than that in the northern summer (June solstice). The observed northern EIA contracts equatorward in the postsunset period of solstice but the southern EIA persists late into the midnight in the December solstice. The asymmetry may be attributed to the different geographic location of the magnetically conjugate stations. The SAMI3 simulations broadly capture the EIA structure and the inter-hemispheric asymmetry during solstices. The difference between observations and the SAMI3 is higher in March equinox and December solstice. The higher E × B vertical drift in the 90–100°E sector and the large geographic-geomagnetic offset in observing stations may have contributed to the observed differences.

Geomagnetically conjugate observation of plasma bubbles and thermospheric neutral winds at low latitudes

AbstractThis is the first paper that reports simultaneous observations of zonal drift of plasma bubbles and the thermospheric neutral winds at geomagnetically conjugate points in both hemispheres. The plasma bubbles were observed in the 630 nm nighttime airglow images taken by using highly sensitive all‐sky airglow imagers at Kototabang, Indonesia (geomagnetic latitude (MLAT): 10.0°S), and Chiang Mai, Thailand (MLAT: 8.9°N), which are nearly geomagnetically conjugate stations, for 7 h from 13 to 20 UT (from 20 to 03 LT) on 5 April 2011. The bubbles continuously propagated eastward with velocities of 100–125 m/s. The 630 nm images at Chiang Mai and those mapped to the conjugate point of Kototabang fit very well, which indicates that the observed plasma bubbles were geomagnetically connected. The eastward thermospheric neutral winds measured by two Fabry‐Perot interferometers were 70–130 m/s at Kototabang and 50–90 m/s at Chiang Mai. We compared the observed plasma bubble drift velocity with the velocity calculated from the observed neutral winds and the model conductivity, to investigate the F region dynamo contribution to the bubble drift velocity. The estimated drift velocities were 60–90% of the observed velocities of the plasma bubbles, suggesting that most of the plasma bubble velocity can be explained by the F region dynamo effect.

The occurrence of equatorial spread-F at conjugate stations in Southeast Asia

In this study, the probability of equatorial spread-F (ESF) occurrence at the conjugate stations in Southeast Asia: Chiangmai station (CMU), Thailand, and Kototabang station (KTB), Indonesia, and near the magnetic equator, Chumphon station (CPN), Thailand, is presented. We analyze the ionogram data recorded by the Frequency Modulated Continuous Wave (FM/CW) ionosondes for the periods of minimum solar activity from September 2008 to April 2009 and in the equinoctial months (March and April) from 2006 to 2013. The spread-F signatures are manually categorized into three types: the frequency spread-F (FSF), the range spread-F (RSF) and the mixed spread-F (MSF) and the monthly average percentage of the occurrence of each ESF type is presented. The results show that the percentage of RSF occurrence at CPN, which is located around the magnetic equator, is higher than at other stations and the RSF mostly occurs during the equinoctial months. On the other hand, the FSF occurrence at CMU and KTB, that are located in the northern and southern hemispheres, respectively, are higher than at CPN. The RSF occurrence typically has the peaks before midnight, while the maximum occurrence rate of FSF is after midnight. Furthermore, the RSF onsets normally precede the FSF onsets by about 1–2h. As the solar activity levels go up, the percentages of RSF occurrence increase, but the percentages of FSF tends to decrease. In addition, we compare the statistics of observed RSF occurrence with the prediction of the IRI-2012 model. The results show that the IRI model overestimates the observed RSF occurrence at all stations during most of the solar activity levels and seasons, except in June 2008 (21:00–03:00LT), March 2011 (23:30–01:00LT) and March 2013 (01:00–02:30LT) when the IRI model underestimates our observations. However, the IRI model gives closer probability of RSF occurrence to our observed values at CPN, especially in equinoctial months and during the periods of medium solar activity. This work is important for an improvement of the IRI model in the prediction of the spread F occurrence probability in the low-latitude region.

Geomagnetic response to solar wind dynamic pressure impulse events at high‐latitude conjugate points

It is commonly assumed that geomagnetic activity is symmetrical between interhemispheric conjugate locations. However, in many cases, such an assumption proved to be wrong. Especially in high‐latitude regions where the magnetosphere and the ionosphere are coupled in a more complex and dynamic fashion, asymmetrical features in geomagnetic phenomena are often observed. This paper presents investigations of geomagnetic responses to sudden change in solar wind pressure to examine interhemispheric conjugate behavior of magnetic field variations, which have rarely been made mainly due to the difficulty of facilitating conjugate‐point measurements. In this study, using magnetometer data from three conjugate stations in Greenland and Antarctica, solar wind pressure impulse events (>5 nPa in <16 min) and their geomagnetic responses, typically seen as magnetic impulse events, have been examined. Our results suggest that asymmetry in ground response patterns between the conjugate locations often shows little correlation with interplanetary magnetic field orientation, season, and ionospheric conductivity, indicating that much more complex mechanism might be involved in creating interhemispheric conjugate behavior.

Low-latitude TEC variability studied from magnetically conjugate locations along 73°E longitude

A comparative study on the variation of total electron content (TEC) was performed by using the observations from two GPS receivers located at magnetically conjugate stations near the equatorial ionization anomaly crests: Udaipur, India in the nortnern hemisphere and Diego Garcia in the southern hemisphere, located at approximately same geographic longitude. Our study reveals that the TECs at both stations shows the semi-annual variability being higher in equinoxes than in solstices. An important finding of the work is that in the northern hemisphere, during the declining phase of solar cycle 23, the amplitude of the winter crest is smaller than that in the summer whereas it is found to be comparable/higher (which is normally termed as “winter anomaly” in the mid latitudes) during the rising phase of solar cycle 24. Thus it is concluded that the so called “winter anomaly” at low latitudes is a phenomenon totally different from the one observed at mid latitudes. In fact its manifestation at low latitudes can be accounted for in terms of variation of the solar flux and/or the strength of the equatorial electrojet during different seasons.

Hemisphere-coupled modeling of nighttime medium-scale traveling ionospheric disturbances

Nighttime medium-scale traveling ionospheric disturbances (MSTIDs), which have tilted frontal structures in the midlatitude ionosphere, are investigated by the midlatitude ionosphere electrodynamics coupling (MIECO) model in this study. It has been proposed that the electrodynamic coupling between the E and F regions plays an important role in generating MSTIDs within a few hours. An intriguing aspect of MSTIDs is that they were simultaneously observed at magnetic conjugate locations in the Northern and Southern Hemispheres. In order to study the hemisphere-coupled electrodynamics, the MIECO model has been upgraded to consist of two simulation domains for both hemispheres in which the electrostatic potential is solved by considering electrodynamics in both hemispheres. The simultaneous occurrence of MSTIDs at the magnetic conjugate stations has clearly been reproduced when the F-region neutral wind satisfies the unstable condition in both hemispheres and a sporadic-E layer is given only at the Northern (summer) Hemisphere. Even if the unstable condition is satisfied in the summer hemisphere, an unfavorable F-region neutral wind in the winter hemisphere largely suppresses the growth of MSTIDs in both hemispheres.

The response of TEC at quasi-conjugate point GPS stations during solar flares

Global Positioning System (GPS) derived total electron content (TEC) measurements were analyzed to investigate the ionospheric response during the X-class solar flare event that occurred on 5-6 December 2006 at geomagnetic conjugate stations: Syowa, Antarctica (SYOG) (GC: 69.00°S, 39.58°E; CGM: 66.08°S, 71.65°E) and Arholt, Iceland (ARHO) (GC: 66.19°N, 342.89°E; CGM: 66.37°N, 71.48°E). Bernese GPS software was used to derive the TEC maps for both stations. The focus of this study is to determine the symmetry or asymmetry of TEC values which is an important parameter in the ionosphere at conjugate stations during these solar flare events. The results showed that during the first flares on 5 December, effects were more pronounced at SYOG than at ARHO. However, on 6 December, the TEC at ARHO showed a sudden spike during the flare with a different TEC variation at SYOG.

Possible association between anomalous geomagnetic variations and the Molise Earthquakes at Central Italy during 2002

The Molise region, on 31 October and 1 November 2002, was struck by two earthquakes of moderate magnitudes (Mw = 5.7) at depth between 12 and 24 km. Geomagnetic data from the Circum-pan Pacific Magnetometer Network (CPMN) were analyzed in order to detect any anomalous geomagnetic variations in association with the Molise earthquakes. Our results indicate the presence of long-term anomalous variations in the geomagnetic components at L’Aquila (LAQ) station (near the epicenter), which started about four months before the occurrence of the Molise earthquakes. Moreover, the geomagnetic components recorded at LAQ are found to show no correlation with those at other conjugate stations in the period between June and October 2002. In addition, anomalous ULF signal (Pc 3:10–45 s) started few weeks before the Molise earthquakes and lasted for about one week after the onset of the seismic activity. The observed anomalous variations may be explained as a result of the crustal stress variations and the enhancement of the lithospheric conductivity in the Molise region during the preparation period of the Molise earthquakes.

Conjugate Sporadic-E measurements

During an equatorial campaign called COPEX (Conjugate Point Equatorial Experiment) ionosondes at a pair of conjugate stations in Brazil were operated for 2 months in 2002. One expected the sporadic-E (E s) layers to be rather similar at the conjugate locations. However, there were significant differences in the E s, particularly in the part of the E s trace that extends above the blanketing frequency. Considerations of the ambient conditions at the conjugate locations indicate that relatively small differences in parameters such as the tidal winds, electric fields, and background density gradients can significantly affect the structuring of the E s layer.

Equinoctial asymmetry of a low-latitude ionosphere-thermosphere system and equatorial irregularities: evidence for meridional wind control

Abstract. Nocturnal ionospheric height variations were analyzed along the meridian of 100° E by using ionosonde data. Two ionosondes were installed near the magnetic conjugate points at low latitudes, and the third station was situated near the magnetic equator. Ionospheric virtual heights were scaled every 15 min and vertical E×B drift velocities were inferred from the equatorial station. By incorporating the inferred equatorial vertical drift velocity, ionospheric bottom heights with the absence of wind were modeled for the two low-latitude conjugate stations, and the deviation in heights from the model outputs was used to infer the transequatorial meridional thermospheric winds. The results obtained for the September and March equinoxes of years 2004 and 2005, respectively, were compared, and a significant difference in the meridional wind was found. An oscillation with a period of approximately 7 h of the meridional wind existed in both the equinoxes, but its amplitude was larger in September as compared to that in March. When the equatorial height reached the maximum level due to the evening enhancement of the zonal electric field, the transequatorial meridional wind velocity reached approximately 10 and 40 m/s for the March and September equinoxes, respectively. This asymmetry of the ionosphere-thermosphere system was found to be associated with the previously reported equinoctial asymmetry of equatorial ionospheric irregularities; the probability for equatorial irregularities to occur is higher in March as compared to that in September at the Indian to Western Pacific longitudes. Numerical simulations of plasma bubble developments were conducted by incorporating the transequatorial neutral wind effect, and the results showed that the growth time (e-folding time) of the bubble was halved when the wind velocity changed from 10 to 40 m/s.

Ionospheric zonal velocities at conjugate points over Brazil during the COPEX campaign: Experimental observations and theoretical validations

We analyze in detail the zonal velocities of large‐scale ionospheric plasma depletions over two conjugate stations inferred from OI 630 nm airglow all‐sky images obtained during the Conjugate Point Equatorial Experiment (COPEX) campaign carried out in Brazil between October and November 2002. The conjugate stations were Boa Vista (BV) (geogr. 2.8N, 60.7W, dip angle 22.0°N) and Campo Grande (CG) (geogr. 20.5S, 54.7W, dip angle 22.32°S). Over Campo Grande, the zonal velocities were measured also by a system of spaced GPS scintillation receivers. The airglow zonal velocities at the conjugate sites were seen to agree very closely, except for a slightly increased velocity over CG which we attribute to the presence of the geomagnetic anomaly. The results show a high degree of alignment of the bubbles along the geomagnetic field lines during the bubble development phase and as the bubbles travel eastward, thereby suggesting that the neutral zonal wind effect in the zonal plasma motion is an integrated effect along the flux tube. The zonal velocities obtained from the GPS technique were always larger than those calculated by the airglow technique, which permitted observation of zonal plasma velocity shear between the altitudes of the airglow emitting layer and of the GPS scintillation. Theoretical ambient plasma zonal velocities calculated using the formulations by Haerendel et al. (1992) and Eccles (1998) are compared with the experimental results. Our results also reveal some degree of dependence of the zonal velocities on the solar flux (F10.7) and magnetic (Kp) indices during the COPEX period.