Prereversal Enhancement Research Articles

Nighttime morphology of vertical plasma drifts at Ouagadougou during different seasons and phases of sunspot cycles 20–22

AbstractThe nighttime morphology of vertical plasma drift (Vd) inferred from ground‐based measurements of the F layer height at Ouagadougou (12.4°N, 358.6°E) in the African Equatorial Ionization Anomaly trough was investigated. The observation covers four seasons, four sunspot cycle phases, annual, and 11 year sunspot cycle (SC) variations of the SCs 20–22 spanning 1966–1998 and a first attempt of such study. The annual mean peak magnitudes of Vd during the prereversal enhancement (PRE) and minimum reversal periods exhibit the 11 year sunspot cycle evolution with the sunspot number (Rz). The PRE peak/Rz and reversal peak/Rz relationships are 98.7% and 84.8%, respectively. PRE peak in June solstice appears 1 h later than for other seasons and is attributed to a decrease in the equatorial zonal wind/conductivity gradient. The highest PRE magnitude and downward perturbation drifts near dusk appear during the equinoxes and lowest in June solstice for all cycles. There is semiannual asymmetry in the variation of Vd during all cycles of the PRE event with peaks in March and September/October. A remarkable feature is the consistent local presunrise drift enhancement for two SCs 20 and 21, which is not a regular feature of the equatorial ionosphere. The rate of inhibition of scintillation effect increases with decreasing phase of sunspot activity and maximizes during the solstices. Both the PRE and minimum reversal peak magnitudes are influenced by the phase of sunspot cycle. Ouagadougou data in this study had shown reliable drift characteristics and can be integrated into the African regional empirical drift model.

Signatures of solar event at middle and low latitudes in the Europe-African sector, during geomagnetic storms, October 2013

This paper presents the variability of the total electron content, VTEC, the ROTI index (proxy of the scintillation index) and the transient variations of the Earth’s magnetic field associated to the impacts of solar events during October 2013. The observations are from middle and low latitudes in European African longitude sector. During October 2013, there are four solar events reaching the Earth. The two first events, on October 2 and October 8 are CME, the third event on October 14, is a jet of fast solar wind flowing from a solar coronal hole, and the last event on October 30 is a slow solar wind with southward excursions of the Bz component of the interplanetary magnetic field, associated to CME passing near the Earth. For the four events, the variation of VTEC at middle latitudes is the same and presents an increase of VTEC at the time of the impact followed by a decrease of VTEC, lasting one or several days. At low latitudes, no clear common pattern for all the events appears. For the four events the variation of the ROTI index over Africa is different showing the asymmetry between West and East Africa. For the first event, on October 2, the scintillations are not inhibited, for the second and the fourth events on October 8 and 30, the scintillations are inhibited on East Africa and for the third event (high speed solar wind stream), on October 14, the scintillations are inhibited over the whole Africa. The available data allow the full explanation of the observations of October 14, indeed, on this day, there is no post sunset increase of the virtual height h′F2 at Ascension Island. There is no Pre Reversal Enhancement (PRE) of the eastward electric field; it is this electric field which moves up the F layer, the necessary condition for the existence of scintillation. The analysis of the variations of the Earth’s magnetic field at low latitudes highlights the presence of the ionospheric disturbance dynamo on October 14, which produces a decrease of the Equatorial Electrojet, several hours after the impact of the high speed solar wind. The disturbance dynamo electric field (DDEF) is westward during the day and till after sunset and turns eastward around 22.30LT. So, on October 14, the westward DDEF inhibits the eastward regular electric field.

Mechanisms underlying the prereversal enhancement of the vertical plasma drift in the low‐latitude ionosphere

AbstractThe evening prereversal enhancement (PRE) of the vertical plasma drift has important consequences for the Appleton density anomaly and the stability of the nighttime ionosphere. Simplified simulations were used to review the three competing theories of the PRE origin, to explore their relative importance, and to indentify their interdependence. The mechanisms involved in the generation and climatology of the PRE are, first, a curl‐free electric field response to rapid changes in the vertical electric field associated with the nighttime F region dynamo; second, a divergence of Hall currents in the E region away from the magnetic equator; and, third, the moderating effect of the large Cowling conductivities in the equatorial E region. The simulations indicate that the equatorial Cowling conductivity creates an important current path that limits the other two mechanisms prior to equatorial sunset and releases them after equatorial sunset. The curl‐free mechanism is the dominant mechanism when the terminator and magnetic meridian are aligned in part due to the accelerating F region zonal wind. When the solar terminator is not aligned with the magnetic meridian, there is an interaction involving all three mechanisms contributing to the magnitude and timing of the PRE. Finally, the altitude profile of the PRE decays more quickly with altitude when the curl‐free mechanism dominates as compared to when the Hall current mechanism dominates.

Effects of pre-reversal enhancement of E × B drift on the latitudinal extension of plasma bubble in Southeast Asia

We investigated the effects of the F region bottomside altitude (h’F), maximum upward E × B drift velocity, duration of pre-reversal enhancement and the integral of upward E × B drift on the latitudinal extension of equatorial plasma bubbles in the Southeast Asian sector using the observations recorded by three GPS receivers and two ionosondes. The GPS receivers are installed at Kototabang (0.2°S, 100.3°E; 9.9°S magnetic latitude), Pontianak (0.02°S, 109.3°E; 9.8°S magnetic latitude) and Bandung (6.9°S, 107.6°E; 16.7°S magnetic latitude) in Indonesia. The ionosondes are installed at magnetically equatorial stations, Chumphon (10.7°N, 99.4°E; 0.86°N magnetic latitude) in Thailand and Bac Lieu (9.3°N, 105.7°E; 0.62°N magnetic latitude) in Vietnam. We analysed those observations acquired in the equinoctial months (March, April, September and October) in 2010–2012, when the solar activity index F 10.7 was in the range from 75 to 150. Assuming that plasma bubbles are the major source of scintillations, the latitudinal extension of the bubbles was determined according to the S4 index. We have found that the peak of h’F, maximum upward E × B drift and the integral of upward E × B drift during the pre-reversal enhancement period are positively correlated with the maximum latitude extension of plasma bubbles, but that duration of pre-reversal enhancement does not show correlation. The plasma bubbles reached magnetic latitudes of 10°–20° in the following conditions: (1) the peak value of h’F is greater than 250–450 km, (2) the maximum upward E × B drift is greater than 10–70 m/s and (3) the integral of upward E × B drift is greater than 50–250 m/s. These results suggest that the latitudinal extension of plasma bubbles is controlled mainly by the magnitude of pre-reversal enhancement and the peak value of h’F at the initial phase of development of plasma bubbles (or equatorial spread F) rather than by the duration of pre-reversal enhancement.

Nonmigrating tidal signature in the distributions of equatorial plasma bubbles and prereversal enhancement

AbstractSome wave‐like features in the longitudinal distribution of equatorial plasma bubbles understood in association with diurnal eastward propagating zonal wave number 3 nonmigrating tide (DE3) in the dayside. However, whether or not the wave features are the daytime DE3 signature has not yet been rigorously investigated. This study investigates (1) the existence of the DE3 signature in the longitudinal distribution of bubbles by analyzing the first Republic of China (ROCSAT‐1) satellite data acquired in 2000–2002 and (2) the role of daytime DE3 in the creation of bubbles by examining the linear growth rate of the generalized Rayleigh‐Taylor (R‐T) instability. The linear growth rate is derived from the “Sami2 is Another Model of the Ionosphere” model simulation results. In the longitudinal distribution of bubbles derived from ROCSAT‐1 observations, the wave number 4 component, the representative characteristic of DE3, is a weak feature. In addition, the amplitude and phase of the wave number 4 component do not show a consistent behavior in comparison with those of DE3. Our numerical calculation results show that the linear growth rate of the R‐T instability is not sensitive to the variation of the daytime vertical plasma drift. These results indicate that the DE3 signature in the occurrence rate of bubbles is not obvious and the effect of daytime DE3 on the creation of bubbles is negligible.

The postsunset vertical plasma drift and its effects on the generation of equatorial plasma bubbles observed by the C/NOFS satellite

AbstractThe prereversal enhancement (PRE) of the vertical plasma drift in the postsunset sector is an important factor that controls the generation of equatorial plasma bubbles. In this study, we use the measurements of the ion velocity meter on board the Communication/Navigation Outage Forecasting System satellite during 2008–2014 to identify the PRE and its effects on the occurrence of plasma bubbles. The seasonal and longitudinal distributions of the PRE are derived at different solar flux levels. Large PRE occurs at 240–360° longitudes in equinoctial months and December solstice, and small or downward PRE occurs around ±60° in June solstice. The seasonal and longitudinal distributions of large‐amplitude equatorial spread F (ESF) (ΔN > 5 × 1010 m−3) are similar to that of the PRE, while the occurrence probability of ESF including smaller‐amplitude perturbations (ΔN > 1 × 1010 m−3) can be quite high at any longitude in any season. A quantitative relationship between the PRE and the ESF occurrence probability is derived and well characterized by the cumulative distribution function of a continuous probability distribution. Such a distribution implies that the occurrence of ESF is a probability event. The ESF occurrence probability is small when the PRE is zero or downward and becomes larger than 80% when the PRE is greater than 40 m s−1. Both the ESF occurrence probability and the amplitude increase with the solar radio flux.

Electrodynamics of the equatorial evening ionosphere: 2. Conductivity influences on convection, current, and electrodynamic energy flow

AbstractWe analyze how the evening equatorial plasma vortex and the prereversal enhancement (PRE) of the vertical drift are influenced by the distributions of conductivity in the E and F regions in relation to the wind, through numerical simulations with the thermosphere‐ionosphere‐electrodynamics general circulation model coupled with the global ionosphere‐plasmasphere model. The nightside electric potential satisfies an approximate minimization principle that unifies the connection of the horizontal and vertical components of plasma convection to the wind and conductivity distributions. The relative roles of E and F region conductivities on the convection and current closure are clarified. Evening time F region zonal winds at latitudes that encompass the equatorial ionization anomaly (EIA) region provide the main energy source to drive the convection, including the PRE. The E region helps regulate both the meridional and the zonal convection through drag on the meridional convection associated with Cowling current. For large nighttime E region conductivities, additional drag on the zonal convection comes from the Pedersen conductance. The minimization principle favors meridional plasma inflow to the EIA region from lower rather than higher magnetic apex heights, so long as the E region Cowling conductance is not too large. This upward/poleward inflow maximizes on field lines that traverse the lower F layer near the equatorward edge of the EIA region, producing a PRE with maximum vertical velocity within the equatorial F layer.

Electrodynamics of the equatorial evening ionosphere: 1. Importance of winds in different regions

AbstractThe importance of winds at different altitudes and latitudes for the electrodynamics of the low‐latitude evening ionosphere is examined with a model of the global coupled ionosphere‐thermosphere system. The model reproduces the main observed features of the evening equatorial plasma vortex and the prereversal enhancement (PRE) of the vertical drift. The electrodynamics is driven primarily by the zonal wind forced by the diurnally varying zonal pressure‐gradient force. The zonal wind lags the zonal pressure‐gradient force owing to inertia. When ion drag is important, the time lag of the wind behind the pressure gradient force is shortened, and the high‐altitude evening wind turns eastward earlier than the wind at lower altitudes, where ion drag is less important. Therefore, a vertical shear of the zonal wind tends to develop at altitudes around the transition between small and large ion drag at the bottom of the F region. This wind shear is closely associated with the vertical shear in the zonal convection velocity that is part of the evening plasma vortex. Unlike previous studies, we find that the winds driving the PRE lie mainly on field lines with apexes above the peak of the equatorial F layer, field lines that extend in magnetic latitude out to nearly 30° and encompass the entire evening equatorial ionization anomaly region. Contrary to previous suggestions, the westward convection in the bottomside of the evening plasma vortex is found to weaken, rather than strengthen, the PRE. Daytime winds have relatively little influence on the low‐latitude evening electrodynamics.

Morphology of the postsunset vortex in the equatorial ionospheric plasma drift

The postsunset vortex in the equatorial ionosphere exhibits clockwise plasma motions after sunset in longitude (time) and altitude coordinates when the equatorial ionosphere is viewed looking northward. We describe the typical morphology of the postsunset vortex using incoherent scatter radar observations at Jicamarca in Peru during the previous solar maximum (2000–2002). A pronounced vortical plasma motion appears around 1700 LT along with the onset of the prereversal enhancement (PRE). The center of this vortex is located near an altitude of 270 km. A smaller-scale vortex also appears about 0.5 ~ 1 h later at higher altitudes. However, the morphology and occurrence time of this small vortex depend on the characteristics of the coherent backscatter region. We find that the earlier vortex is the major feature of the postsunset vortices because it is repeatable, associated with the PRE, and independent to the occurrence of the coherent backscatter region.

A statistical study of satellite traces and evolution of equatorial spread F

The ionosonde observations made at 5-min intervals at the Indian dip equatorial station Tirunelveli (8.7°N, 77.8°E geographic; 1.1°N dip latitude) from March 2008 to February 2009 during the extended solar minimum period are used to study the interlink between equatorial spread F (ESF) and satellite traces (STs) which are assumed to represent tilts in the bottomside iso-electron density surfaces probably caused by large-scale wave-like structures (LSWS). The data show different patterns of ESF onset in the bottomside F region, which are illustrated through examples. In addition, the statistics of occurrence of ST and its relation to the formation of ESF are studied. The results indicate that (1) the zonally drifting ESF irregularities can be differentiated from those forming over the observing station. (2) Nearly half of the ESF events were preceded by ST. (3) In about 30% of the cases of occurrence of ST, ESF was not formed afterwards implying that LSWS may not always lead to ESF. (4) The percentage of ESF following ST was high in summer and increased with the time of the night. (5) Following the first occurrence of ST, the ESF onset was delayed by about 30 min on the average suggesting that ST may be used as a precursor of ESF. (6) Pre-reversal enhancement (PRE) of upward plasma drift was found insignificant during the period of study. The trapping of high-frequency radio waves between the E and F regions during intense sporadic E is also illustrated.

Comparative analysis of nocturnal vertical plasma drift velocities inferred from ground-based ionosonde measurements of hmF2 and h′F

Abstract Variations in the evening/nighttime ionosonde vertical plasma drift velocities inferred from the time rate of change of both the base of the F-layer height (Vz (h′F) ) and height of the peak electron density (Vz (hmF2) ) from an equatorial station were compared for better description of the E×B drifts. For better interpretation, both results were compared with the Incoherent Scatter (IS) radar observations (Vz (ISR) ) which is taken to be the most accurate method of measuring drift, and therefore the data of reference level. An equinoctial maximum and June solstice minimum in post-sunset pre-reversal enhancement (PRE) was observed for Vz (hmF2) , Vz (ISR) , and Vz (h′F) . The percentage correlation between Vz hmF2 and Vz h′F ranges within 55–70%. While PRE for Vz (hmF2) peaked at 19 LT for all seasons, Vz (h′F) peaked at 18 LT for September equinox and December solstice, and start earlier. The nighttime downward reversal peak magnitudes for Vz (hmF2) and Vz (h′F) are respectively within the range of −4 to −14 and −2 to −14 m/s; whereas Vz (ISR) ranges within −12 and −34 m/s; and the peak time was reached earlier with the ionosonde observations than for the ISR. The PRE peak magnitude for Vz (hmF2) , Vz (h′F) and Vz (ISR) varies between 3–14, 2–14, and 4–14 m/s for the entire seasons. Our results revealed higher drift correlation coefficients in both Vz (hmF2) vs. Vz (ISR) (0.983) and Vz (h′F) vs. Vz (ISR) (0.833) relationships during the equinoxes between 16–20 LT, at which time the F-layer altitude is higher than the 300 km threshold value; and lower for solstice period (0.326 and 0.410 in similar order). A better linear relationship between Vz (hmF2) and Vz (h′F2) was observed during the reversal (19–21 LT) phase period. PRE velocity was shown to be seasonal and solar activity dependent. Both Vz hmF2 and Vz h′F compares almost equally with the ISR measurement. However, the PRE peak magnitude for the drift inferred using h′F2 is closer to the corresponding ISR magnitude during the equinoxes; whereas the drift inferred from hmF2 best represent the ISR magnitude for solstices. We established that both Vz hmF2 and Vz h′F are governed by the same mechanism at nighttime, and as such any of them can be used to infer vertical drift as long as the 300 km threshold value condition is considered, otherwise chemical correction may be required for the F-layer uplift.

The influence of Corotating Interaction Region (CIR) driven geomagnetic storms on the development of equatorial plasma bubbles (EPBs) over wide range of longitudes

Recurrent high speed solar wind streams from coronal holes on the Sun are more frequent and Geoeffective during the declining phase of solar cycle which interact with the ambient solar wind leading the formation of Corotating Interaction Regions (CIRs) in the interplanetary medium. These CIR-High Speed Stream (HSS) structures of enhanced density and magnetic fields, when they impinge up on the Earth’s magnetosphere, can cause recurrent geomagnetic storms in the Geospace environment. In this study, we investigate the influence of two CIR-driven recurrent geomagnetic storms on the equatorial and low-latitude ionosphere in the context of the development of equatorial plasma bubbles over Indian and Asian longitudes. The results consistently indicate that prompt penetration of eastward electric fields into equatorial and low-latitudes under southward IMF Bz can occur even during the CIR-driven storms. Further, the penetration of eastward electric fields augments the evening pre-reversal enhancement and triggers the development of EPBs over wide longitudinal sectors where the local post-sunset hours coincide with the main phase of the storm. Similar results that are consistently observed during both the CIR-driven geomagnetic storms are reported and discussed in this paper.

Equatorial ionization anomaly variability over the Brazilian region during boreal sudden stratospheric warming events

AbstractThis study refers to the connection between the stratosphere and ionosphere, investigating, specifically, the behavior of the equatorial ionization anomaly (EIA) and ionospheric effects over the Brazilian region during sudden stratospheric warming (SSW) events. We studied three major warmings that occurred in the Northern Hemisphere winter 2007–2008, 2008–2009, and 2009–2010 and a minor warming that occurred in 2010–2011. The solar activity was low for the first two cases and relatively moderate for the last two. In this study the EIA behavior was investigated using the ΔTEC (total electron content) parameter, which expresses the EIA relative intensity for the Brazilian sector. The results for the Brazilian region show, mainly after SSW temperature peak, an increase in the EIA intensity in the morning, followed by a decrease in the afternoon. As identified through ΔTEC signatures and consistently confirmed through wavelet power spectra analysis, this semidiurnal behavior is preserved for a number of days equal to the polar region thermal stabilization phase and it is very similar to the results obtained in pioneer studies in the Peruvian sector, in which TEC data was also used. In some cases the TEC negative variation is stronger than the positive, being noticeably more intense around the prereversal enhancement time, when the EIA is strongly suppressed in the Brazilian sector.

Geomagnetic control of equatorial plasma bubble activity modeled by the TIEGCM with Kp

AbstractDescribing the day‐to‐day variability of Equatorial Plasma Bubble (EPB) occurrence remains a significant challenge. In this study we use the Thermosphere‐Ionosphere Electrodynamics General Circulation Model (TIEGCM), driven by solar (F10.7) and geomagnetic (Kp) activity indices, to study daily variations of the linear Rayleigh‐Taylor (R‐T) instability growth rate in relation to the measured scintillation strength at five longitudinally distributed stations. For locations characterized by generally favorable conditions for EPB growth (i.e., within the scintillation season for that location), we find that the TIEGCM is capable of identifying days when EPB development, determined from the calculated R‐T growth rate, is suppressed as a result of geomagnetic activity. Both observed and modeled upward plasma drifts indicate that the prereversal enhancement scales linearly with Kp from several hours prior, from which it is concluded that even small Kp changes cause significant variations in daily EPB growth.

Altitudinal dependence of evening equatorial F region vertical plasma drifts

AbstractWe use Jicamarca incoherent scatter radar measurements to study for the first time the altitudinal variations of late afternoon and early night equatorial F region vertical plasma drifts. We also present the initial vertical drift measurements over the altitudinal range from about 200 to 2000 km. These data show that the afternoon drifts decrease weakly with altitude. Near their evening prereversal enhancements, the vertical drifts generally increase with altitude below about the F layer peak, decrease with height near the F layer peak and above, and are nearly height independent in the (solar flux dependent) topside ionosphere. The transition altitudes from height‐decreasing to height‐independent evening upward drifts decrease with altitude from solar maximum to solar minimum. After their reversal to downward, the vertical drifts do not change much with height. The altitudinal dependence of the evening vertical drifts has large day‐to‐day variability and is closely related to the time dependence of the zonal drifts, as expected from the curl‐free electric field condition.

Variation of the 0+ ion density during low and high solar activity as measured by the SR0SS-C2 satellite

The O+ ion density measured by the SROSS-C2 satellite during solar minima (year 1995, F10.7=77) and maxima (year 2000, F10.7=181) has been analyzed for studying diurnal, seasonal, latitudinal and geomagnetic variations. The study region covers an area encompassed between 5-3 5° N latitude and 65-95° E longitude over India at a ~500 km altitude (F2 region). The year 2000 shows significant enhancement in the annual average of O+ ion density and attainment of post sunset secondary enhancement compared to 1995. Attributed to photoionization, daytime shows a maximum ionization compared to nighttime. However, attainment of post sunset secondary maxima is attributed to the strong pre-reversal enhancement in the vertical E x B drift. The effect of geomagnetic activity Kp through the E x B drift dynamic movement on O+ ion density distribution studies indicates periodicities of seven and nine days in 1995 and 2000, respectively, and polynomial dependency of the O+ ion density on geomagnetic activity Kp. Further on, stratification of the O+ ion density according to latitude (5-35° N) indicates high density in mid-latitudes (12-24° N) compared to high and low latitudes, except in the winter of 1995, which shows a distinct trend (i.e., density decreases with increasing latitude).

Comparison between African equatorial station ground-based inferred vertical E × B drift, Jicamarca direct measured drift, and IRI model

The Incoherent Scatter Radar measurement over Jicamarca, together with the IRI model-2007 measurements were compared with ground-based digisonde inferred E×B drift over Ilorin in the African region during year of solar minima (F10.7=81). Seasonally, Ilorin pre-reversal enhancement (PRE) had peak drift velocities of 7.2, 3.7 and 7.9m/s for March equinox, September equinox and December solstice respectively, while Jicamarca drifts indicated 13.0, 10.5 and 5.2m/s; as well as the IRI model with 14.3, 8.4 and 0.7m/s in similar order. PRE value was insignificant during June solstice. The PRE magnitude of the IRI-model during the equinoxes is twice the value obtained at Ilorin. The daytime E×B drift peaked over Ilorin 1–2h earlier than both the modeled and Jicamarca observations. This could be due to the difference in sunset time at the conjugate points corresponding to the altitude of the observation. During the evening time PRE, the respective correlation coefficients (R) for Vz–F10.7 relation over Jicamarca, Ilorin and the modeled observations are −0.5559, 0.4796 and −0.4979. Similarly, the Vz–Ap relation exhibit excellent anti-correlation coefficient (R=−0.8637) for the IRI-model, −0.4827 over Jicamarca and 0.3479 for Ilorin. Annual mean drift velocities over Jicamarca, Ilorin and IRI model measurements respectively are 10, 5.6 and 10m/s for the peak PRE observation; 15, 16 and 21m/s for the daytime pre-sunrise peak values; and −21, −9 and −16m/s for the nighttime downward reversals. The root-mean square (RMS) deviation between IRI-model and the Ilorin drift between 2000 and 0500h is 4.37, 2.03, 3.71 and 2.42m/s for March equinox, June solstice, September equinox and December solstice respectively. For Jicamarca–Ilorin drift relation, RMS deviation is 5.48, 2.30, 3.47 and 1.27m/s in the same order respectively. Annual hmF2 inferred drift over Ilorin during daytime is higher by a factor of ≈2 and 3 at Jicamarca and IRI model measurements respectively; and by a factor of ≈5 for both during the night-time period. The limitations in using hmF2 to infer drifts are discussed.

Ionospheric vertical drifts over an Indian low latitude station and its comparison with IRI-2007 vertical drift model

The vertical plasma drift velocity (Vz=dh′Fdt) of the Ionospheric F-Region is calculated using the Ionosonde data over an Indian low latitude station, Waltair (17.7°N, 83.3°E, dip 20°N). The vertical drift velocities thus computed are compared with those of the IRI-2007 vertical drift model during the post sunset hours of the high sunspot period of 2001–2002. The vertical drift values are entirely downward between 2000 and 0700hLT for the IRI-2007 vertical drift model compared to those derived using Ionosonde. The root mean square errors (RMSE) in the vertical drifts during the post sun-set hours computed between the IRI vertical drift model and Ionosonde measured values are found to be 13.54, 21.68 and 22.39m/s for summer, equinox and winter seasons respectively. The average vertical drifts derived using Ionosonde showing pre-reversal enhancements during evening hours (1700–1900hLT) over Waltair are found to be 8, 16 and 12m/s, whereas the IRI-2007 model derived values show 15, 35 and 36m/s during summer, equinox and winter seasons respectively. The correlation analysis of peak drift velocities derived using Ionosonde and those from IRI model with Rz and F10.7 shows more or less similar variation while they are differed in their variation with Ap index. A good correlation is observed between the vertical plasma drift obtained using Ionosonde and Equatorial Electrojet Strength (EEJ) except during winter, while the correlation coefficients computed using IRI model are little higher than those obtained using Ionosonde with maximum values during summer season. The percentage of deviation for IRI-model correlation coefficient and that of Ionosonde-inferred drift for equinoctial and summer months is around 50% and 25% respectively where as it is more than 100% for winter.

Traveling planetary wave ionospheric disturbances and their role in the generation of equatorial spread-F and GPS phase fluctuations during the last extreme low solar activity and comparison with high solar activity

This investigation studies traveling planetary wave ionospheric disturbance (TPWID) type oscillations on the modulation of the F region virtual height rise during the E×B electric field pre-reversal enhancement (PRE), near sunset hours. We also studied their role in the generation of equatorial spread F (ESF) and GPS phase fluctuations during periods of the last extreme low solar activity (LSA) of January 2009 to April 2010 (F10.7¯=73). A comparison is made with periods of high solar activity (HSA) in 2003 and 2004 near equatorial region. The ionospheric irregularities investigated are medium (bottom-side) and large (plasma bubble) scales. Ionospheric F region oscillations with period of days are due to the TPWIDs, which play an important role in producing favorable or unfavorable conditions for equatorial ionospheric irregularities, changing the electron vertical profile and F region height. In this paper, we present simultaneous ionospheric sounding (ionosonde) and GPS vertical total electron content (vTEC) observations carried out near equatorial region (Palmas 10.2°S, 48.2°W) and low latitude region (São José dos Campos 23.2°S, 45.9°W; located under the southern crest of the equatorial ionospheric anomaly), Brazil. Observations show that the occurrence of fresh ESF during LSA and HSA and fresh GPS phase fluctuations at equatorial region follow the trend of day-to-day variations in the F region virtual height, which are due to electric field PRE modulated by TPWID wave like oscillations. During LSA, the altitude of 250km acts as a threshold height for the generation of fresh ionospheric irregularities, whereas during HSA, the threshold height is 300km. The observations also found a strong increase in the generation of fresh ionospheric irregularities from October 2009 to March 2010 during LSA and from September 2003 to March 2004 during the HSA. Furthermore, in LSA, the period of fresh ionospheric irregularities was less than during HSA, though both periods followed a similar seasonal pattern. In the low-latitude, we observed more ESFs than phase fluctuations because ionosonde is more sensitive than GPS. We also observed periods with and without day-to-day oscillations in the F region virtual height. The observations made by GPS stations and ionosondes in the equatorial region, for much of the period analyzed, presented similar results with regard to the generation of equatorial ionospheric irregularities. In the low latitude, some nights of January, February, October, and December 2009 also showed a similarity.

Analysis of post-sunset F-region vertical plasma drifts during Counter Electrojet days using multi frequency HF Doppler Radar

In this paper, through a case study, an attempt has been made to bring out the relationship between post noon E-region electric field and post sunset F-region vertical plasma drift on quiet time Counter Electrojet (CEJ) days. Study carried out using the data from a multi frequency HF Doppler Radar and Digital Ionosonde located over Trivandrum (8.5° N; 77° E; 0.5° N dip lat.) a geomagnetic dip equatorial station in India during quite time CEJ days of the years 2004 and 2006, revealed some interesting aspects of the E region electrodynamics and post sunset F region electrodynamics. It has been observed that, in contrast to the normal electrojet (EEJ) days, the Pre-Reversal Enhancement (PRE) is either weakened or inhibited on CEJ days and the field reversal takes place much earlier than that on a normal day. It is suggested that even after the effects of the field reversal ceases to show up in the ground magnetic data, the reversed field may persist and shows up as a decrease in the PRE experienced by the F-region. In other words, the study indicates that the EEJ associated electrodynamics have a significant role in controlling the PRE.