June Solstice Months Research Articles

The source of tropospheric tides

With the method of Hough mode decomposition (HMD), the tidal sources of the three main tidal components, namely, the migrating components DW1 (diurnal westward propagating wavenumber 1) and SW2 (semidiurnal westward propagating wavenumber 2) and the non-migrating component DE3 (diurnal eastward propagating wavenumber 3), at the tropospheric altitudes (1–12 km) and in the latitude range of ±60°, were obtained from National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) data during the interval from 1988 to 2011. We analyzed these sources in detail at 6 km and obtained the main properties of their yearly variations. The DW1 source was found to present a weak seasonal variation in the lower latitudes (about ±10°–15°). That is, the amplitudes of the DW1 sources were larger in the summer months than in the winter months, and DW1 presented semi-annual variation near the equator (±10°) such that the DW1 source was larger at the equinoxes than at the solstices. In addition, the SW2 source was symmetric and was stronger in the southern hemisphere than in the northern hemisphere. The SW2 source presented remarkable annual and semi-annual variation such that the amplitudes were largest during the March equinox months and larger during the June solstice months. In contrast, DE3 appeared mainly around the equatorial latitudes within about ±30°. The DE3 source presented remarkable semi-annual variation that was larger around the solstices than the equinoxes in the southern hemisphere, and it was opposite in the northern hemisphere. By HMD, we found that the tropospheric tides were primarily dominated by some leading propagating Hough modes, specifically, the (1, 1), (2, 3), and (3, 3) modes; the influences of the other Hough modes were negligible. The consequences of an El Nino–Southern Oscillation modulation of tidal amplitudes for the energy and momentum budgets of the troposphere may now be expected to attract attention. In summary, the above yearly variations of the main tidal sources and the Hough coefficients demonstrate that an HMD analysis can be used to investigate the tropospheric tides.

Evaluation of the SSN and F10.7 indices as a cause for performance variation of the IRI 2016 and IRI-Plas 2017 models

This paper mainly focuses on the evaluation of efficiency of the Sunspot Number (SSN) and the 10.7 cm Solar Radio Flux (F10.7) indices as a cause for the variation of the performance of the latest versions of the International Reference Ionosphere model (IRI 2016 with NeQuick, IRI01-corr and IRI2001 options for the topside electron density) and the IRI Extended to the Plasmasphere (IRI-Plas 2017) for the modeling of the Total Electron Content (TEC). The Global Positioning System (GPS)-derived TEC data obtained from the dual frequency GPS receivers located at Observation Rock, OBSR (geog 46.90°N, 238.18°W, Geom. 52.46°N) and Husband, HUSB (44.12°N, 238.15°W, Geom. 49.73°N) in the West Pacific region during the recent solar maximum (2012-2014) years have been considered for the validation of the performance of the models. The results show that both the GPS-derived TEC (GPS VTEC) and modelled (IRI 2016 and IRI-Plas 2017 VTEC) seasonal diurnal values tend to peak at 00:00 UT (16:00 LT) and 20:00 (12:00 LT) with the highest being observed mostly at 20:00 (12:00 LT); while, their minima are mostly observed at about 13:00 UT (05:00 LT). In addition, in utilizing the SSN, the best performance is generally observed in the June solstice months, especially by the IRI-Plas 2017 model. However, for the equinoctial and December solstice months, the best performance is generally observed by the IRI 2016 model with NeQuick and IRI01 options. It has also been shown that the root-mean-square deviations between the GPS-derived and modelled VTEC diurnal variation in using the F10.7 index are generally less than those of the SSN option in all months, revealing that both the IRI 2016 and IRI-Plas 2017 models generally show better performance using F10.7 index than the SSN. Hence, the F10.7 option is recommended for better TEC modeling employing the IRI 2016 and IRI-Plas 2017 models during the recent solar maximum years over the West Pacific region. In addition, both models cannot effectively estimate the geomagnetic storm time TEC variation.

Performance evaluation of the IRI 2016 and IRI-Plas 2017 models over central Asian mid-latitude regions in the descending phase of solar cycle 24

This paper scrutinizes the performance of the latest versions of the IRI model (IRI 2016 with NeQuick, IRI01-corr and IRI2001 options for the topside electron density) and the IRI extended to the plasmasphere (IRI-Plas 2017) models in the estimation of the Vertical TEC (VTEC) variation over the central Asian mid-latitude regions in the descending phase of solar cycle 24 (2014–2016). The GPS dual frequency receivers installed at Kurchatov, KRTV (geog 50.71°N, 78.62°E, Geom. 41.84°N), Khantau, SUMK (44.21°N, 74.00°E, Geom. 35.73°N), Talas, TALA (42.45°N, 72.21°E, Geom.34.13°N) and Kazarman, KAZA (41.38°N, 73.94°E, Geom.32.92°N) have been used to derive the estimate of the vertical TEC (GPS-VTEC) for the comparison of the monthly and seasonal performance of the models. The modelled VTEC values generally tend to be larger than the GPS-VTEC values during periods of high solar irradiance (daytime hours), with the highest overestimation being observed by IRI-Plas 2017 model followed by IRI 2016 model with the IRI2001 topside option. However, the differences between the models and between the models and the GPS-VTEC values become diminished as the solar irradiance decreases, with the highest underestimation being observed by the IRI 2016 model with NeQuick topside option. It has also been shown that the smallest root-mean-square deviations between the GPS-VTEC and modelled VTEC are observed generally in the June solstice months, showing that the models perform best during local summer. On the contrary, the largest root-mean-square deviations between the modelled VTEC and GPS-VTEC are observed during high solar irradiance on the surface of the Earth (especially in the time interval between 05:00 and 10:00 UT which corresponds to the daytime hours 10:00 and 15:00 LT), showing that the models perform poorly during high solar irradiance. In addition, both the IRI 2016 and IRI-Plas 2017 models show a progressive decline in VTEC during a negative storm, but do not adequately estimate the storm time VTEC variation.

Validation of the IRI 2016, IRI-Plas 2017 and NeQuick 2 models over the West Pacific regions using the SSN and F10.7 solar indices as proxy

This paper investigates the validation of the performance of the latest versions of the International Reference Ionosphere (IRI 2016), the IRI extended to the plasmasphere (IRI-Plas 2017) and NeQuick 2 models in the estimation of the variation of the Total Electron Content (TEC) over the West Pacific regions during the 2015–2017 years. This has been performed employing the GPS-derived TEC data obtained from stations located at Observation Rock, OBSR (geog 46.90°N, 238.18°W, Geom. 52.46°N), Husband, HUSB (44.12°N, 238.15°W, Geom. 49.73°N). It has been revealed that both the GPS-derived VTEC and modelled (IRI 2016, IRI-Plas 2017 and NeQuick 2 VTEC) variations attain their minima at about 13:00 UT (05:00 LT) and maxima at about 20:00 UT (12:00 LT). Moreover, because of the enhancement of photo-ionization process in the region as a result of exposure of the ionosphere for direct radiation in the summer months, large measured VTEC values are seen in the June solstice months during 2015–2017. It has also been shown that, because of the variation of the Sunspot number (SSN) and solar radio flux 10.7 cm (F10.7), the VTEC variations for both the GPS-derived and models show decrease at transition from 2015 to 2017, with some exceptions observed in the June solstice months. In addition, the root-mean-square deviations (RMSD) between the GPS-derived and modelled diurnal VTEC variations are generally less than 0.5TECU in using both the SSN and F10.7 indices. This shows that all the three models are good in TEC estimation with the IRI-Plas 2017 and NeQuick 2 perform the best on most of the hours, in comparison with the IRI 2016 model with IRI2001 option, especially in using the F10.7 index when the solar activity increases. However, when the solar activity decreases, utilizing the SSN for all the three models (especially for the IRI-Plas 2017 and NeQuick 2) shows the better performance. Moreover, during the geomagnetic storm condition, both the IRI 2016 and IRI-Plas 2017 models with the storm option “on” do not adequately reflect to the sharp increase or decrease of the GPS-derived VTEC values.

Climatology of GPS signal loss observed by Swarm satellites

Abstract. By using 3-year global positioning system (GPS) measurements from December 2013 to November 2016, we provide in this study a detailed survey on the climatology of the GPS signal loss of Swarm onboard receivers. Our results show that the GPS signal losses prefer to occur at both low latitudes between ±5 and ±20∘ magnetic latitude (MLAT) and high latitudes above 60∘ MLAT in both hemispheres. These events at all latitudes are observed mainly during equinoxes and December solstice months, while totally absent during June solstice months. At low latitudes the GPS signal losses are caused by the equatorial plasma irregularities shortly after sunset, and at high latitude they are also highly related to the large density gradients associated with ionospheric irregularities. Additionally, the high-latitude events are more often observed in the Southern Hemisphere, occurring mainly at the cusp region and along nightside auroral latitudes. The signal losses mainly happen for those GPS rays with elevation angles less than 20∘, and more commonly occur when the line of sight between GPS and Swarm satellites is aligned with the shell structure of plasma irregularities. Our results also confirm that the capability of the Swarm receiver has been improved after the bandwidth of the phase-locked loop (PLL) widened, but the updates cannot radically avoid the interruption in tracking GPS satellites caused by the ionospheric plasma irregularities. Additionally, after the PLL bandwidth increased larger than 0.5 Hz, some unexpected signal losses are observed even at middle latitudes, which are not related to the ionospheric plasma irregularities. Our results suggest that rather than 1.0 Hz, a PLL bandwidth of 0.5 Hz is a more suitable value for the Swarm receiver. Keywords. Ionosphere (equatorial ionosphere; ionospheric irregularities) – radio science (radio wave propagation)

Plasma density undulations correlated with thermospheric neutral mass density in the daytime low‐latitude to midlatitude topside ionosphere

AbstractPlasma density undulations in the dayside low‐latitude/midlatitude ionospheric F region were often attributed to thermospheric gravity waves (TGWs). However, the relationship between the former and the latter has been at best indirectly evidenced. In this study we investigate daytime fluctuations in neutral mass density (ρ) and plasma density (ne) measured onboard CHAMP from 2001 to 2010. A significant amount of daytime fluctuations in ne is strongly correlated with in situ fluctuations of ρ, which we term “TGW‐related ne fluctuations.” The TGW‐related ne fluctuations are (1) stronger in the winter hemisphere than in the summer hemisphere and (2) strongest in the South American sector during June solstice months. These climatological features are in general agreement with those of TGWs reported previously, especially at midlatitudes. On the other hand, the relative amplitude of TGW‐related ne fluctuations does not depend strongly on solar activity.

Comparison of GPS-TEC with IRI-2012 TEC over African equatorial and low latitude regions during the period of 2012–2013

This paper presents the performance of the latest version of the International Reference Ionosphere (IRI-2012) model in estimating the Vertical Total Electron Content (VTEC) variation over equatorial and low latitude East Africa regions during the period of 2012–2013. This has been conducted by comparing the ground-based Global Positioning System (GPS) VTEC inferred from nine dual frequency GPS receivers installed at different regions in the continent. In this work, the diurnal, monthly and seasonal variation in the measured VTEC have been analyzed and compared with the VTEC obtained from IRI-2012 model. It is depicted that the lowest diurnal peaks of the modeled and measured VTEC values are observed in the June solstice months, whereas the highest values are observed in the equinoctial months. The variability of the diurnal GPS-VTEC is also found to be minimal nearly at 03:00 UT and maximal mostly between 09:00 and 14:00 UT. Moreover, the maximum and minimum monthly mean hourly measured VTEC values inferred from both the Arba Minch and Entebbe stations are observed in October and July, respectively. Similarly, the highest and the lowest seasonal mean hourly measured VTEC values are observed in the September equinox and in the June solstice, respectively. The model predictions generally follow the diurnal variations of the measured VTEC, with minimum value at predawn hours and maximum at noontime hours (10:00–13:00 UT). It has been shown that the model better estimates the diurnal VTEC values mostly just after midnight hours (00:00–03:00 UT). Good agreements between the modeled and measured monthly and seasonal mean hourly VTEC values obtained from Arba Minch station are also observed in the equinoctial months. But, for the Entebbe station, the modeled monthly and seasonal VTEC values are larger than the corresponding measured VTEC values by about 75% and 60%, respectively. In addition, there are large discrepancies observed between the diurnal measured and modeled VTEC values (especially in the equinoctial months) in the time interval between 09:00 and 14:00 UT.

TEC prediction performance of IRI‐2012 model during a very low and a high solar activity phase over equatorial regions, Uganda

AbstractThis paper examines the capacity of the latest version of the International Reference Ionosphere (IRI‐2012) model in predicting the vertical total electron content (VTEC) variation over Uganda during a very low solar activity phase (2009) and a high solar activity (2012) phase. This has been carried out by comparing the ground‐based Global Positioning System (GPS) VTEC inferred from dual‐frequency GPS receivers installed at Entebbe (geographic latitude 0.038°N and longitude 32.44°E; geomagnetic latitude −9.53°N and longitude 104.06°E) and Mbarara (geographic latitude −0.60°N and longitude 30.74°E; geomagnetic latitude −10.02°N and longitude 102.36°E). In this work, the diurnal, monthly, and seasonal variations in the measured VTEC have been analyzed and compared with the VTEC derived from IRI‐2012 model. It has been shown that the lowest diurnal peak GPS‐VTEC values are observed in the June solstice months during both the low and the high solar activity phases. Similarly, the highest diurnal peak IRI‐2012 VTEC values are observed in equinoctial months during both phases. The variability of the VTEC in both the experimental and model is minimal nearly at 03:00 UT (06:00 LT) and maximal mostly between 10:00 and 13:00 UT (13:00–16:00 LT) during both phases. The diurnal highest peak modeled VTEC value observed during the high solar activity phase is almost twice larger than the diurnal highest peak modeled VTEC value depicted during the low solar activity phase. Moreover, the highest monthly mean hourly measured VTEC value observed in October during the high solar activity phase is larger by more than twice the corresponding highest monthly mean hourly measured VTEC value observed in November during the low solar activity phase. Similarly, the lowest monthly measured VTEC value observed in July during the high solar activity phase is larger by more than twice the corresponding lowest measured monthly VTEC value observed in the same month during the low solar activity phase. It has also been shown that the highest measured seasonal mean hourly VTEC values are observed in the December solstice and the March equinox, respectively, during the low and the high solar activity phases, while the corresponding lowest measured VTEC values are observed in the June solstice during both phases. The model prediction generally follows the monthly and seasonal variations of measured VTEC, with the highest and the lowest monthly values being observed in equinoctial and solstice months, respectively, during both phases. The overall results show that the modeled diurnal, monthly, and seasonal VTEC values are generally larger than those corresponding measured VTEC values observed during both the low and the high solar activity phases, with the largest deviations being observed during the low solar activity phase. Moreover, the model does not respond to the effects resulting from the storm. Hence, unlike the measured VTEC values, the modeled VTEC values are unaffected by the storm.

Ionospheric slab thickness at the equatorial anomaly region after the deep solar minimum of cycle 23/24

The ionospheric slab thickness is a measure of the shape of the ionospheric profile and can be obtained from the ratio of total electron content (TEC) to the F-region peak electron density (NmF2). In this study the ionospheric slab thickness was obtained during one year of extremely low solar activity (from March 2009 to February 2010). The period analyzed were separated in three seasonal groups: equinoxes, June solstice (winter) and December solstice (summer) months. The ionospheric slab thickness was studied at two stations located around the southern crest of the equatorial ionization anomaly (EIA). One station located in the inner edge of the anomaly region, named Palmas (10.12°S, 48.21°O, 7.73°S dip lat), and another station located under the southern crest of the anomaly, situated at São José dos Campos (23.07°S, 45.52°O, 19.61°S dip lat). The TEC data have been obtained from dual-frequency Global Navigation Satellite System (GNSS) receivers and the NmF2 data were calculated from the foF2 parameter scaled from ionograms recorded by simultaneous measurements of digital ionosondes. The diurnal and seasonal variations of the ionospheric slab thickness at the EIA region were then analyzed. The minimum values of TEC and NmF2 were observed in both stations during the early morning, and the maximum values during afternoon hours. During the summer and equinoctial periods the slab thickness reached their highest values. The nearly equatorial site of Palmas showed values of daytime slab thickness larger than those observed at the low latitude station of São José dos Campos, except during the June solstice months. At São José dos Campos, a pronounced pre-dusk increase in the equivalent slab thickness was observed during the winter solstice months. The atmospheric neutral temperature (Tn) was also estimated from the correlation analysis between the MSIS modeled Tn and the calculated ionospheric slab thickness over the two observations sites. Other relevant aspects of the ionospheric slab thickness behavior and its comparison with International Reference Ionosphere (IRI) model results will be presented and discussed.

Climatology of GPS amplitude scintillations over equatorial Africa during the minimum and ascending phases of solar cycle 24

This study characterizes African equatorial scintillations at L-band frequency during the minimum and ascending phases of solar cycle 24. Three years’ (2009–2011) of amplitude scintillation data from three African equatorial GPS stations, namely; Lagos (6.48°N, 3.27°E, dip: 4.95°S), Nigeria; Nairobi (1.16°S, 36.80°E, dip: 10.65°S), Kenya; and Kampala (0.30°N, 32.50°E, dip: 11.12°S), Uganda were used for this investigation. We grouped the data into daily, monthly, seasonal, and yearly scales, at elevation angles greater than or equal to 30°. Scintillations exhibit daily trend of occurrences during the hours of 1900 LT–0200 LT, with higher occurrence levels being localized within the hours of 2000–2300 LT. Generally, highest scintillation occurrences were recorded during equinoxes and the least during June solstice. Intriguingly, over equatorial Africa, January was observed to be a non-scintillation month, and post-midnight scintillations were observed during June solstice months, although at weak intensities. Scintillations were also observed to increase with solar and geomagnetic activities. These results would support the development of future African equatorial scintillation models, which could also be of support to the implementation of global navigation satellite system (GNSS)-based navigation in Africa.

Statistical characteristics of low-latitude ionospheric scintillation over China

The Global Positioning System (GPS) L-band ionospheric scintillation produced by electron density irregularities in the ionospheric E- and F-regions, is mainly a low- and high-latitude phenomenon. In this study, the statistical behavior of GPS ionospheric scintillation over a Chinese low-latitude station Sanya (18.3°N, 109.6°E; dip lat: 12.8°N) has been investigated. A detailed study on the seasonal and solar activity dependence of scintillation occurrence during July 2004–December 2012 show that the amplitude scintillation pattern, with a maximum occurrence during equinox of solar maximum, agrees with plasma bubble observations by in situ satellites in this longitude. A few daytime periodic scintillation events are found during June solstice months of solar minimum. Interestingly, a significant equinoctial asymmetry of scintillation onset time is found in 2011–2012. The initiation of scintillation during September–October is on average earlier than that of March–April about 25min. Meanwhile, the zonal drifts of irregularities estimated using two spatially separated GPS receivers over Sanya show a similar behavior during the two equinoxes, slowly decreasing from 150m/s at post-sunset to 50m/s near midnight. The possible mechanisms responsible for the occurrence characteristics of GPS scintillation over Sanya, and relevant aspects of the zonal drifts of the irregularities are discussed.

Nonmigrating tidal signatures in the magnitude and the inter-hemispheric asymmetry of the equatorial ionization anomaly

Abstract. Based on nine years of observations from the satellites CHAMP and GRACE the tidal signatures in the magnitude and the inter-hemisphere asymmetry of the equatorial ionization anomaly (EIA) have been investigated in this study. The EIA magnitude parameters show longitudinal wavenumber 4 and 3 (WN4/WN3) patterns during the months around August and December, respectively, while for different EIA parameters the contributions of the various tidal parameters are different. For the crest-to-trough ratio (CTR) the dominating nonmigrating tidal component contributing to WN4 is DE3 during the months around August, while during the months around December solstice the stationary planetary wave, SPW3, takes a comparable role to DE2 in contributing to WN3. For the apex height index (ApexHC) of the EIA fluxtube the stationary planetary waves, SPW4/SPW3, exceed the amplitudes of DE3/DE2 taking the leading role in causing the longitudinal WN4/WN3 patterns. During the months around December solstice the SW3 tide is prominent in both CTR and ApexHC. SW3 shows a strong dependence on the solar flux level, while it is hardly dependent on magnetic activity. For the EIA inter-hemispheric asymmetry only WN1 and WN2 longitudinal patterns can be seen. During June solstice months the pattern can be explained by stationary planetary waves SPW1 and SPW2. Conversely, around December solstice months longitudinal features exhibit some local time evolution, in particular the diurnal nonmigrating tide D0 takes the leading role.

Comparison of GPS TEC variations with IRI-2007 TEC prediction at equatorial latitudes during a low solar activity (2009–2011) phase over the Kenyan region

This work presents an analysis of the Total Electron Content (TEC) derived from the International GNSS Service (IGS) receivers at Malindi (mal2: 2.9oS, 40.1oE, dip −26.813o), Kasarani (rcmn: 36.89oE, 1.2oS, dip −23.970o), Eldoret (moiu: 35.3oE, 0.3oN, dip −21.037o) and GPS-SCINDA (36.8oE, 1.3oS, dip −24.117o) receiver located in Nairobi for the period 2009–2011. The diurnal, monthly and seasonal variations of the GPS derived TEC (GPS-TEC) and effects of space weather on TEC are compared with TEC from the 2007 International Reference Ionosphere model (IRI-TEC) using the NeQuick option for the topside electron density. The diurnal peaks in GPS-TEC is maximum during equinoctial months (March, April, October) and in December and minimum in June solstice months (May, June, July). The variability in GPS-TEC is minimal in all seasons between 0:00 and 04:00 UT and maximum near noon between 10:00 and 14:00 UT. Significant variability in TEC at post sunset hours after 16:00 UT (19:00 LT) has been noted in all the seasons except in June solstice. The TEC variability of the post sunset hours is associated with the occurrence of the ionization anomaly crest which enhances nighttime TEC over this region. A comparison between the GPS-TEC and IRI-TEC indicates that both the model and observation depicts a similar trend in the monthly and seasonal variations. However seasonal averages show that IRI-TEC values are higher than the GPS-TEC. The IRI-TEC also depicts a double peak in diurnal values unlike the GPS-TEC. This overestimation which is primarily during daytime hours could be due to the model overestimation of the equatorial anomaly effect on levels of ionospheric ionization over the low latitude regions. The IRI-TEC also does not show any response to geomagnetic activity, despite the STORM option being selected in the model; the IRI model generally remains smooth and underestimates TEC during a storm. The GPS-TEC variability indicated by standard deviation seasonal averages has been presented as a basis for extending the IRI-model to accommodate TEC-variability.

Longitudinal development of low‐latitude ionospheric irregularities during the geomagnetic storms of July 2004

During the period 22–28 July 2004, three geomagnetic storms occurred due to a sequence of coronal mass ejections. In this paper we present and discuss the ionospheric observations from a set of in situ satellites and ground‐based GPS total electron content and scintillation receivers, a VHF radar, and two chains of ionosondes (∼300°E and ∼120°E, respectively) that provide the evolutionary characteristics of equatorial and low‐latitude ionospheric irregularities versus longitude during these storm periods. It is found that the irregularities occurred over a wide longitudinal range, extending from around 300°E to 120°E on storm days 25 and 27 July 2004. On 25 July plasma bubbles (PBs) began premidnight in America and postmidnight in Southeast Asia. On 27 July the occurrence of irregularities followed the sunset terminator and was observed sequentially after sunset from American to Southeast Asian longitudes. Past studies have reported that storm‐time low‐latitude ionospheric irregularities are mostly confined to a narrower longitude range, <90°, after sunset hours and are associated with the prompt penetration of eastward electric fields (PPEFs) into low latitudes. In June solstice months the occurrence of range‐type spread F or PBs is very low in Southeast Asian and South American sectors. In contrast, the present results indicate that geomagnetic storms triggered the wide longitudinal development of PBs. In the American sector this was probably due to the effects of PPEFs on both storm days. However, in the Southeast Asian sector the PBs on the 2 days probably arose from disturbance dynamo electric field (DDEF), PPEF, and gravity wave seeding effects. This study further shows that under complex storm conditions, besides the long duration or multiple penetrations, the combined effects of PPEFs and DDEFs could result in a wide longitude extent of ionospheric irregularities at times.

FORMOSAT‐3/COSMIC observations of seasonal and longitudinal variations of equatorial ionization anomaly and its interhemispheric asymmetry during the solar minimum period

Using an extremely valuable global data set from Formosa Satellite (FORMOSAT‐3)/Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation experiment, a comprehensive study has been carried out on the seasonal and longitudinal variations of equatorial ionization anomaly (EIA) and the temporal variation in the hemispheric asymmetry of EIA during the low solar activity period from November 2006 to October 2007. The interesting result observed from this investigation is the local‐time‐dependent variation in the hemispheric asymmetry of EIA. During the solstices, it has been consistently observed that the EIA crest in the winter hemisphere appears stronger than that in the summer hemisphere during morning to noon hours. In contrast to this, during noon to early afternoon hours, the ionization in the winter EIA crest decreases rapidly, and the crest in the summer hemisphere becomes more intensified than that in the winter hemisphere. Further, this transition of stronger EIA crest from winter hemisphere to summer hemisphere occurs around 1200–1300 LT during the December solstice months and is delayed by a couple of hours (seen around ∼1400 LT) during June solstice months. The causative neutral and electrodynamical mechanisms are discussed in light of relative contributions from the field‐aligned plasma transport due to transequatorial interhemispheric neutral wind, the strength of the equatorial fountain process, and the ion drag effects during different local times and seasons. The results from the Sami2 is Another Model of the Ionosphere (SAMI2) model simulation also exhibit similar local‐time‐dependent variation in the hemispheric asymmetry of EIA, which further supports our argument. Also, it was observed that the large magnetic declination of the field lines and the four‐peaked longitudinal structure of EIA can significantly modulate the interhemispheric asymmetry of EIA even during the equinoxes.

Occurrence and zonal drifts of small-scale ionospheric irregularities over an equatorial station during solar maximum – Magnetic quiet and disturbed conditions

A statistical study of L-band amplitude scintillations and zonal drift velocity of Fresnel-scale ionospheric irregularities is presented. Ground-based global positioning system (GPS) data acquired at the equatorial station of São Luís (2.33°S, 44.21°W, dip latitude 1.3°S), Brazil, during the solar maximum period from March 2001 to February 2002 are used in the analysis. The variation of scintillations and irregularity drift velocities with local time, season and magnetic activity are reported. The results reveal that for the near overhead ionosphere (satellite elevation angle >45°) a broad maximum in the occurrence of scintillation is seen from October to February. In general, weak scintillations (S 4 < 0.4) dominated (>90%) during equinox (March–April; September–October) and December solstice (November–February) quiet time conditions and, many of the scintillations, occurred during pre-midnight hours. The mean zonal velocities of the irregularities are seen to be ∼30 m s −1 larger near December solstice, while during the equinoctial period the velocities decay faster and the scintillations tend to cease earlier. On geomagnetically disturbed nights, scintillation activity seems to be strongly affected by the prompt penetration of magnetospheric electric fields and disturbance dynamo effects. On disturbed days, during the equinox and December solstice seasons, the scintillations tend to be suppressed in the pre-midnight hours, whereas during June solstice months (May–August) the effect is opposite. In the post-midnight period, the mostly marked increase in the scintillation occurrence is observed during the equinox months. The results show that during disturbed conditions only one type of storm (when the main phase maximum takes place during the daytime hours) agrees with the Aarons’ description, that is the suppression of L-band scintillations in the first recovery phase night. The results also reveal that the storm-time irregularity drifts become more spread in velocity and occasionally may present westward drift. The present work is important to evaluate the behavior of the ionospheric irregularities at equatorial latitudes under geomagnetically quiet and disturbed conditions, which is one of the most relevant themes in the space weather studies.

Equatorial F-region vertical ion drifts during quiet solar maximum

Median values of ionosonde h′ F data acquired at Ibadan (Geographic:7.4°N, 3.9°E, Magnetic: dip 6°S, and magnetic declination, 3°W), Nigeria, West Africa, have been used to determine vertical ion drift (electric field) characteristics in the postsunset ionosphere in the African region during a time of high solar activity (average F 10.7 −208). The database spans from January and December 1958 during the era of International Geophysical Year (IGY) for geomagnetic quiet conditions. Bimonthly averaged diurnal variations patterns are very similar, but differ significantly in magnitude and in the evening reversal times. Also, monthly variations of F-region vertical ion drift reversal times inferred from the time of h′ F maximum indicates early reversal during equinoxes and December solstice months except for the month of April. Late reversal is observed during the June solstice months. The equatorial evening prereversal enhancement in vertical ion drift ( V zp ) occurs largely near 1900 LT with typical values ∼20–45 m/s. Comparison of Ibadan ionosonde V zp with the values of prereversal peak velocity reported for Jicamarca (South America), Kodaikanal (India), and Scherliess and Fejer global model show considerable disparity. The changes of postsunset peak in virtual height of F-layer ( h′ F P ) with prereversal velocity peak V zp are anti-correlated. Investigation of solar effects on monthly values of V zp and h′ F P revealed that these parameters are independent of monthly averaged solar flux intensity during quiet-time sunspot maximum conditions.

A climatological study of nocturnal equatorial F-region irregularities at the west Pacific longitudes by using phase fluctuations of the global positioning system

This study investigated a long-term climatology of nocturnal equatorial F-region irregularities by using phase fluctuations of the global positioning system during the solar cycle of 1996–2006 at the west Pacific longitudes. The results showed that the distribution of the occurrence of irregularities is a two-peak pattern, which peaks in equinoxes with a shallow/deep dip in June/December solstice during high solar activity but with two about the same dips in magnitude in both June and December solstices during low solar activity. Moreover, the most interesting longitudinal effect in the area is in solstice occurrences of irregularities during high solar activity that the irregularities in December solstice months develop easier in the west area but those in June solstice months develop easier in the east area.

A study of long-term climatology of ionospheric irregularities by using GPS phase fluctuations at the Brazilian longitudes

We have examined the ionospheric plasma irregularities that were recorded by using three ground-based receivers of the global positioning system (GPS) located at Brazilian longitudes during the period of a complete solar cycle, 1995–2005. The statistic results show that ionospheric irregularities are very easy to occur in December solstice months but rare to occur in June solstice months. Besides, the occurrence rates of irregularities in both December and June solstice months are little dependent on solar activity. However, in equinoctial months, the development of irregularities is obviously dependent on solar activity. There is a new finding in this study that if strong irregularities are distinguished from moderate ones, their occurrence rates would increase with solar activity during the December solstice months.

Response of nighttime equatorial and low latitude F-region to the geomagnetic storm of August 18, 2003, in the Brazilian sector

This paper presents an investigation of geomagnetic storm effects in the equatorial and low latitude F-region in the Brazilian sector during the intense geomagnetic storm on 18 August, 2003 (SSC 14:21 UT on 17/08; ΣKp = 52+; Ap = 108; ∣Dst∣ max = 168 at 1600 UT on 18/08). Simultaneous ionospheric sounding measurements from two stations, viz., Palmas (10.2°S, 48.2°W; dip latitude 5.7°S) and Sao Jose dos Campos (23.2°S, 45.9°W; dip latitude 17.6°S), Brazil, are presented for the nights of 16–17, 17–18 and 18–19 August, 2003 (quiet, disturbed and recovery phases). Both stations are equipped with the Canadian Advanced Digital Ionosonde (CADI). Quiet and disturbed conditions of the F-region ionosphere are compared using data collected from the two stations. The relationship between magnetospheric disturbance and low-latitude ionospheric dynamics, and generation of ionospheric irregularities are discussed. On the disturbed nights (17–18 and 18–19 August), the low latitude station S. J. Campos showed strong enhancements in the F-region critical frequency (foF2), whereas the near equatorial station Palmas showed strong uplifting of the F-layer about 1 h earlier. Normally during the June solstice months (May–August) in the Brazilian sector, large-scale ionospheric irregularities in form of plasma bubbles are rarely observed. On the night of 17–18 August, ionsospheric sounding observations at Palmas showed the presence of bottomside spread-F, whereas on the night of 18–19 August, the observations at Palmas and S. J. Campos showed the presence of plasma bubbles when the storm recovery phase had just started. The complementary GPS data available from several stations in the “Rede Brasileira de Monitoramento Continuo de GPS (Brazilian Network for Continuous GPS Monitoring)” are used to obtain the vertical total electron content (VTEC) and the rate of change of TEC per minute on UT days 18 and 19 August, 2003 and presented. Also, several global ionospheric TEC maps from the worldwide network of GPS receivers are presented, showing widespread latitudinal and longitudinal TEC changes during the different phases of the storm. All the observations (local ionospheric sounding and GPS network measurements, and global GPS measurements) presented in this investigation related to the geomagnetic storm on 18 August indicate that the equatorial and low latitude region in the Brazilian sector had much stronger effect during the recovery phase compared with the main phase. A comparison of the observed disturbance drifts with the Fejer–Scherliess storm-time model drifts indicate that the modeled drifts are not consistent with the present observations.