Prereversal Enhancement Research Articles

Formation of maximum electric potential at the geomagnetic equator by the disturbance dynamo

The effect of a disturbance dynamo during geomagnetic activity on the equatorial ionospheric electric fields is investigated, using model results from the NACR/TIEGCM (National Center for Atmospheric Research Thermosphere Ionosphere Electrodynamics General Circulation Model). Model runs are made for different time‐lengths of geomagnetic activity, for different seasons, and for different solar activities to investigate how and where the maximum electric potential forms. Model results show that the maximum electric potential is located at around 300 km altitude and at local time after the pre‐reversal enhancement at equinox for high solar activity. For the case at solstice, without pre‐reversal enhancement, the location moves to around 110 km altitude and to the local time close to midnight. Giving various parameters in the model runs to simulate different background conditions, many important output quantities are used to study the distributions of perturbed electric potential at the geomagnetic equator. Model investigation indicates that normal quiet time electrodynamics, at different seasons with different solar activities, significantly affect the distribution of perturbed current density associated with geomagnetic activity. Furthermore, model results also reveal that significant perturbed zonal electric fields tend to build up six hours after the onset of geomagnetic activity, except at regions close to sunset and sunrise, and the perturbed vertical electric fields increase with the time length of geomagnetic activity.

Equatorial spread F and sporadic E-layer connections during the Brazilian Conjugate Point Equatorial Experiment (COPEX)

The Conjugate Point Equatorial Experiment (COPEX) was conducted in Brazil from October to December 2002. The observational instruments were placed at three sites nearly along a magnetic meridian, one at the magnetic equator and the other two at magnetically conjugate points. We analyzed the spread F (SF) occurrence and sporadic E (Es) layer characteristics in the simultaneous data to look for any possible relation between these two phenomena. The present study does not detect any significant correlation between the SF occurrence/generation at the magnetic equator and the presence of Es layers at the conjugate E regions along the same field line. The integrated field line conductivity is not significantly affected by the occurrence of Es at the conjugate E regions and its presence is not sufficient to inhibit the vertical drift and SF generation at the magnetic equator. Model results show that the presence of a vertical equatorial electric field (associated with the eastward electric field pre-reversal enhancement) is too week to affect the Es development at the conjugate stations.

A comparison of ground and satellite observations of F region vertical velocity near the dip equator

Nighttime F region vertical electrodynamic drifts were made at the magnetic equatorial stations in Africa, Ibadan (7.4°N, 3.9°E; 6°S dip), and Ouagadougou (12°N, 1.5°W; 5.9°N dip) using ionosondes hmF2 data during 1957/1958 International Geophysical Year (IGY) and 1990 periods, respectively; for high solar flux and geomagnetic quiet time conditions. We compare the seasonal averages of vertical drifts with observations made by Incoherent Scatter Radar (ISR) at Jicamarca (11.95°S, 76.87°W; 2°N dip) and Atmospheric Explorer E (AE‐E) satellite for equatorial F layer vertical drifts reported by Fejer et al. (1995). The results indicate good accord between the three techniques at periods when convection dominates other factors (e.g., around prereversal enhancement) except for June solstice drifts. However, when the drifts are completely downward (negative) between 2000 and 0500 LT sector, the mean discrepancies between ionosondes and ISR drifts are of 8–11 m/s (December solstice), 14–17 m/s (equinoxes) but comparable during June solstice with a value of about 14 m/s. Conversely, the typical values of the differences among ionosondes and AE‐E downward velocities are 7–10 m/s (December solstice), 9–12 m/s (equinoxes), yet again similar in winter with a significantly smaller value of about 3 m/s. The evening reversal times are in excellent agreement, apart from June solstice drifts, which exhibits large fluctuations. The morning reversal times show small variations. Equinoctial prereversal enhancement velocities have amplitudes of approximately 17–35 m/s between the methods. Our data are useful for global ionospheric modeling and for the predictions of development of nighttime equatorial F region irregularities at the African region where there is paucity of data.

Seasonal and solar cycle variability in F‐region vertical plasma drifts over Ouagadougou

F‐region vertical drifts were made using ionosonde for the Africa equatorial station Ouagadougou (12°N, 1.5°W; 5.9°N dip) from 3 a of data during January 1987 to December 1989 for solar cycle minimum, medium, and maximum conditions (F10.7 = 85, 141, and 214, respectively) under geomagnetic quiet‐time. The variations are found to be dominated by the characteristics morning peak and evening prereversal enhancement (PRE) velocities. Seasonal and solar cycle effects are prominent near the dusk sector with an increase of PRE from solar minimum to maximum. The average equinoctial evening prereversal enhancement increases by almost a factor of three from low to high flux. On the average, the values of daytime and nighttime ionosonde‐derived vertical drifts are smaller by about a factor of four than the magnitude often mentioned in publication for equatorial regions from other experimental techniques. In addition, the morning peak velocities maximize between about 0830–0930 LT with typical values of 12–18 m/s. In contrast, PRE have largest amplitude between about 1700–2200 LT with typical values of 5–10 m/s. The morning reversal times do not reveal any dependence on season and solar cycle; but the most probable time of occurrence is around 0530 LT. The evening reversal times are in excellent agreements for the three levels of solar activity periods apart from June solstice that exhibit considerable variations. The most likely time of occurrence is near 2000 LT. Of importance, the link between the onset / inhibition parameters of postsunset irregularities over Ouagadougou indicate anti‐correlations with solar variability.

Large‐scale longitudinal variation in ionospheric height and equatorial spread F occurrences observed by ionosondes

Variations in ionospheric height associated with the prereversal enhancement (PRE) at two equatorial ionosonde stations separated by 6.34° in longitude were studied for the first time. The stations used were in Chumphon (10.7°N, 99.4°E, 3.3 Mag. Lat.), Thailand and Bac Lieu (9.3°N, 105.7°E, 1.6 Mag. Lat.), Vietnam. Variations in the virtual height of the bottomside of the F region (h′F) at 2.5 MHz observed at these two stations were analyzed for a period in March and April 2006. When the equatorial spread F (ESF) was not observed, h′F variations at the two stations were very similar, with a local time shift corresponding to the longitude separation. However, when ESF was observed, they were often significantly different. Our results show that h′F enhancement, which could be interpreted as being due to the eastward electric field enhancement, is quite localized in longitude when ESF occurs.

Longitudinal structure of the vertical E × B drift and ion density seen from ROCSAT‐1

We investigate the longitudinal distribution of the vertical E × B drift velocity and ion density in the low‐latitude ionosphere using the first Republic of China Satellite (ROCSAT‐1) data acquired during 1999–2004. The ROCSAT‐1 observations during daytime demonstrate the presence of the longitudinally periodic patterns of the vertical E × B drift and plasma density on the topside F region (600 km). The four longitude sectors where the peaks in the plasma density are found are coincident with the peaks in the E × B drift. This observation may indicate the association of the large‐scale longitudinal density structure with the daytime E‐region dynamo electric field. The density structure exists before the occurrence of the pre‐reversal enhancement (PRE) and therefore the PRE is not directly related to this phenomenon.

Low‐latitude total electron content enhancement at low geomagnetic activity observed over Japan

Numerous total electron content (TEC) values derived from GPS signals are averaged within 1.5° × 1.5° cells in a 1‐hour time frame, and the relative deviations of these average values from corresponding monthly medians are used to produce latitude‐time plots over Japan. The paper analyzes the appearance and development of enhancements of TEC of equatorial origin (ETEs), occurring outside initial and main phases of geomagnetic storms. ETE structures appear mainly as single‐crest structures in the evening hours local time, with TEC peak around 1900 LT. TEC usually decreases with latitude, and the structures disappear below 40°N. In some cases the TEC peak is found above the plot boundary of 24°N, as depletions toward the equator are also observed. The observed enhanced structures are linked to the well‐known evening prereversal enhancement of ion drift in the equatorial F region. Double‐crest ETEs are also observed, with the second peak occurring in early morning hours. Most of the ETE events appear in periods of low geomagnetic activity, 1–3 days after the main phase of the storms. In some cases the time of rising of ETE structures coincides with the increase of interplanetary electric field (IEF), a fact showing the importance of directly penetrating electric field in formation of ETEs. Often, ETEs appear repeatedly in 2 or 3 consecutive days. It is supposed that planetary atmospheric waves are responsible for this phenomenon. Most of the observed features of ETEs can be explained by the published results of simulations of the coupled thermosphere‐ionosphere‐plasmasphere (CTIP) model and the thermosphere/ionosphere/electrodynamic general circulation model (TIEGCM). It is suggested that ETE structures are produced mainly by a disturbance winds dynamo electric field, built up after the main phase of the storms. Some ETE events, appearing at the end of a prolonged period of low geomagnetic activity, can be linked to directly penetrating IEF in equatorial ionosphere. The intensity of these electric fields is much smaller than is needed to explain the observations.

Plasma bubble phenomenon in the topside ionosphere

There are the indications that plasma bubbles/flux tube aligned plasma density depletions, produced by Rayleigh–Taylor instability at the bottomside of ionosphere, could rise up to the topside ionosphere and plasmasphere. Maruyama and Matuura [Maruyama, T., Matuura, N. Longitudinal variability of annual changes in activity of equatorial spread-F and plasma bubbles. J. Geophys. Res. 89(A12), 10903–10912, 1984.], using ISS-b satellite data for the high solar activity period, 1978–1979, have seen the plasma bubbles over equator at 1100 km altitudes in 46 cases in 1700 passes. That is ∼3% only. However, there is distinctly another picture in He + density depletions (subtroughs) according to the ISS-b data for the same period. He + density subtroughs were observed in the topside ionosphere over equatorial and low-latitudinal regions ( L ∼ 1.3–3) in 11% of the cases [Karpachev, A.T., Sidorova, L.N. Occurrence probability of the light ion trough and subtrough in He + density on season and local time. Adv. Space Res. 29, 999–108, 2002; Sidorova, L.N., He + density topside modeling based on ISS-b satellite data. Adv. Space Res. 33, 850–854, 2004.]. We have carried out a statistical study of the He + density subtrough characteristics. The subtrough depth (depletion value) as function of local time (evening–night hours) was compared with the vertical plasma drift velocity variations, obtained for the same periods from the AE-E satellite and IS radar (Jicamarca) data. Striking similarity in development dynamics is revealed for the different seasons. It is noted also that the He + density subtroughs are mostly observed in the evening–night sector (18–05 LT) from October till May, which is very similar to the peculiarities of the equatorial spread-F (ESF), usually associated with plasma bubbles. The monthly mean He + density subtrough occurrence probability, plotted in local time versus month, was compared with the similar plots for ESF occurrence probability derived by Abdu et al. [Abdu, M.A., Sobral, J.H.A., Batista, I.S. Equatorial spread-F statistics in the american longitudes: some problems relevant to ESF description in the IRI scheme. Adv. Space Res. 25, 113–124, 2000.] from ground-based ionograms obtained over Brazilian region for the same years. The comparison shows good enough correlation ( R ∼ 0.67). It is concluded that: (a) He + density subtroughs like ESF are controlled by pre-reversal enhancement electric field (vertical drift); (b) He + density subtroughs and ESF/bubble irregularities may be considered as phenomena of the same plasma bubble origin; (c) it seems, plasma bubbles, reaching the topside ionosphere altitudes, are most easily observable in He + density as depletions.

Geostationary L-band signal scintillation observations near the crest of equatorial anomaly in the Indian zone

Microwave L-band signal at 1.5 GHz from geostationary satellite INMARSAT (64°E) has been recorded on a continuous basis since 1990 at Calcutta (22.58°N, 88.38°E geographic; 32°N magnetic dip). The location of Calcutta, virtually under the northern crest of the equatorial anomaly in the Indian longitude sector, provides an excellent platform for studying equatorial scintillations. This paper presents a long-term study of scintillations at 1.5 GHz observed from Calcutta extending from minimum to high sunspot number (1996–2000). The variations of scintillations with local time, season, solar and magnetic activity are presented. L-band scintillations have been found to be essentially a pre-midnight phenomenon during the equinoctial months of high sunspot number years. Both the occurrence and intensity increase dramatically with solar activity level. During the solar maximum phase of 1998–2000, scintillations in excess of 20 dB occurred for 20% of total time in the pre-midnight hours of March 2000. Considering individual hours, a maximum percentage occurrence of 37 was recorded in the interval 19–20 LT. Although the percentage occurrence appears to be low, scintillations with Scintillation Index (SI)>20 dB were recorded on 23 out of 31 nights during the same month, indicating that scintillations do not occur continuously over a long interval of time, but in small patches with clear gaps in between. The distribution of patch duration during years 1998–2000 has a maximum in the time interval of 1–3 min. From the cumulative distributions of patch duration with SI⩾10 and 20 dB, the 99th percentile values of patch duration have been found to be 49.3 and 39.6 min, respectively. This information may be used to obtain an estimate of longitudinal separation of geostationary satellites to be used in Satellite Based Augmentation System (SBAS). During the equinoctial months of August–October and February–April, occurrence of scintillations is less during magnetically active (Kp=3+–9) conditions than during quiet (Kp=0−3). In the local summer months of May–July, the probability of occurrence of scintillations is more under magnetically active condition than in quiet condition. Equatorial scintillation is essentially controlled by the pre-reversal enhancement of the eastward electric field near sunset, which in turn has season and solar activity dependences. During magnetic activity, the generation of equatorial irregularities resulting in scintillations may be ascribed to the prompt penetration of magnetospheric electric field and disturbance dynamo effects.

Comparison between nighttime ionosonde, incoherent scatter radar, AE‐E satellite, and HF Doppler observations ofFregion vertical electrodynamic plasma drifts in the vicinity of the magnetic equator

NighttimeFregion vertical drifts were made using an ionosonde for the equatorial station Ibadan (7.4°N, 3.9°E, 6°S dip) from 1 year of data during 1957–1958 International Geophysical Year (IGY) that corresponds to a period of solar maximum for undisturbed condition. We compare the seasonal vertical drifts with measurements made by incoherent scatter radar, AE‐E satellite, and HF Doppler for equatorialFregion vertical drifts. We find a comparable variability pattern during periods of highFlayer heights during equinox and the December solstice, and the opposite behavior occurs during June solstice. The drifts are predominantly downward between 2000 and 0500 LT intervals. Ionosonde drifts are smaller in values by either a factor of two or three than other methods, except for consistent June solstice ionosonde and satellite magnitudes. The equinoctial average prereversal enhancements measured by the four techniques are roughly comparable (about 36 m/s) and occur at the same local time (1900 LT) for all the seasons. The evening reversal times are similar, apart from June solstice that exhibits large variations. The morning reversal times are also in accord except for the equinoctial Jicamarca drift. Our observations indicate that ionosonde drifts measurements are in better agreement with vertical drifts results at other equatorial stations.

Eastward traverse of equatorial plasma plumes observed with the Equatorial Atmosphere Radar in Indonesia

Abstract. The zonal structure of radar backscatter plumes associated with Equatorial Spread F (ESF), probably modulated by atmospheric gravity waves, has been investigated with the Equatorial Atmosphere Radar (EAR) in West Sumatra, Indonesia (0.20° S, 100.32° E; dip latitude 10.1° S) and the FM-CW ionospheric sounders on the same magnetic meridian as the EAR. The occurrence locations and zonal distances of the ESF plumes were determined with multi-beam observations with the EAR. The ESF plumes drifted eastward while keeping distances of several hundred to a thousand kilometers. Comparing the occurrence of the plumes and the F-layer uplift measured by the FM-CW sounders, plumes were initiated within the scanned area around sunset only, when the F-layer altitude rapidly increased. Therefore, the PreReversal Enhancement (PRE) is considered as having a zonal variation with the scales mentioned above, and this variation causes day-to-day variability, which has been studied for a long time. Modulation of the underlying E-region conductivity by gravity waves, which causes inhomogeneous sporadic-E layers, for example, is a likely mechanism to determine the scale of the PRE.

Equatorial Ionospheric Scintillations and Zonal Irregularity Drifts Observed with Closely-Spaced GPS Receivers in Indonesia

To investigate drift velocities of a few hundred meter-scale irregularities associated with equatorial plasma bubbles, we used three single-frequency GPS receivers at the Equatorial Atmosphere Radar (EAR) site at Kototabang, Indonesia (0.20°S, 100.32°E; geomagnetic latitude 10.6°S), since January 2003. The GPS receivers sampled GPS signal intensity at a rate of 20 Hz. Distances between the receivers were 116, 127, and 152 m. An analysis of scintillation index (S 4 ) in two years (2003-2004) revealed that the scintillations often occurred between 2000-0100 LT at equinoxes and that their occurrence rate was higher during March-April than during September-October. Drift velocities of irregularities were measured using cross-correlation analysis with the time series of the GPS signal intensity obtained from the three receivers. From a statistical analysis of the drift velocities, the eastward component of drift velocity just after sunset is found to be greater during March-April than during September-October. Based on these results, for the first time, we suggest that the east-west component of plasma drift velocity (or vertical electric field) may be related to the evolution of plasma irregularities causing scintillations throughout the mechanism causing the prereversal enhancement of the eastward electric fields. The equinoctial asymmetry of the drift velocity could be attributed to the equinoctial asymmetry of neutral winds in the thermosphere.

The prereversal enhancement in the vertical drift for Fortaleza and the sporadic E layer

Using ionogram-recorded data for the location of Fortaleza ( 3 . 8 ∘ S, 38 . 0 ∘ W; magnetic latitude = - 6 ∘ ), the relation between the evening prereversal enhancement in the vertical F region plasma drift ( E × B ) and the diurnal variation of the sporadic E layer ( Es ) is examined. The database examined corresponds to the period of June 2001. The F region vertical drifts obtained as dh ′ F / dt for quiet days reveal that when the amplitudes of the evening prereversal enhancement (PRE) are higher, an interruption of an ongoing sporadic E layer is observed. A recent investigation by Abdu et al. [2003. Journal of Geophysical Research 108(A6), 1254. doi: 10.1029/2002JA009285.], showed that an upward electric field, intensified during the PRE, can be responsible for the inhibition of the sporadic E layer. In this work, we investigate the possible influence of a modified conductivity on the PRE development. The model developed by Batista et al. [1986. Journal of Geophysical Research 91(A11), 12055–12064.], based on the work of Heelis et al. [1974. Planetary and Space Science 22, 743–756.], was used to test the response of the PRE (its amplitude, time of occurrence and time of inversion) when the electrical conductivity of the E region is modified, specifically around sunset. The real cause of this modified conductivity is not investigated here, but the presence of a sporadic E layer could partially contribute to that. The causes of the agreement or disagreement between the computed and the observed value of vertical drift are discussed.

Simultaneous observations of the main trough using GPS imaging and the EISCAT radar

Abstract. We use a digisonde at Jicamarca and a chain of GPS receivers on the west side of South America to investigate the effects of the pre-reversal enhancement (PRE) in ExB drift, the asymmetry (Ia) of equatorial ionization anomaly (EIA), and the magnetic activity (Kp) on the generation of equatorial spread F (ESF). Results show that the ESF appears frequently in summer (November, December, January, and February) and equinoctial (March, April, September, and October) months, but rarely in winter (May, June, July, and August) months. The seasonal variation in the ESF is associated with those in the PRE ExB drift and Ia. The larger ExB drift (>20m/s) and smaller |Ia| (<0.3) in summer and equinoctial months provide a preferable condition to development the ESF. Conversely, the smaller ExB drift and larger |Ia| are responsible for the lower ESF occurrence in winter months. Regarding the effects of magnetic activity, the ESF occurrence decreases with increasing Kp in the equinoctial and winter months, but not in the summer months. Furthermore, the larger and smaller ExB drifts are presented under the quiet (Kp<3) and disturbed (Kp≥3) conditions, respectively. These results indicate that the suppression in ESF and the decrease in ExB drifts are mainly caused by the decrease in the eastward electric field.

The effects of the pre-reversal ExB drift, the EIA asymmetry, and magnetic activity on the equatorial spread F during solar maximum

Abstract. We use a digisonde at Jicamarca and a chain of GPS receivers on the west side of South America to investigate the effects of the pre-reversal enhancement (PRE) in ExB drift, the asymmetry (Ia) of equatorial ionization anomaly (EIA), and the magnetic activity (Kp) on the generation of equatorial spread F (ESF). Results show that the ESF appears frequently in summer (November, December, January, and February) and equinoctial (March, April, September, and October) months, but rarely in winter (May, June, July, and August) months. The seasonal variation in the ESF is associated with those in the PRE ExB drift and Ia. The larger ExB drift (>20m/s) and smaller |Ia| (<0.3) in summer and equinoctial months provide a preferable condition to development the ESF. Conversely, the smaller ExB drift and larger |Ia| are responsible for the lower ESF occurrence in winter months. Regarding the effects of magnetic activity, the ESF occurrence decreases with increasing Kp in the equinoctial and winter months, but not in the summer months. Furthermore, the larger and smaller ExB drifts are presented under the quiet (Kp<3) and disturbed (Kp≥3) conditions, respectively. These results indicate that the suppression in ESF and the decrease in ExB drifts are mainly caused by the decrease in the eastward electric field.

Investigation of height gradient in vertical plasma drift at equatorial ionosphere using multifrequency HF Doppler radar

A multifrequency HF Doppler radar installed at the magnetic equatorial station Trivandrum provides an opportunity to study the height gradient in vertical plasma drift at the bottomside of equatorial F region during evening time. The multifrequency radar gives near‐simultaneous observation of vertical plasma drift at three close by F region heights above the sounding station. The height gradient of the vertical drift shows a negative value during the prereversal enhancement (PRE) period and turns to positive value after the prereversal enhancement. The average height gradient in vertical plasma drift remains negative around PRE and its magnitude decreases with altitude, below F peak. This could be a clear‐cut manifestation of the curl‐free nature of the low‐latitude electric field, and it could also indicate a partial signature of the postsunset velocity vortex at the equatorial F region. The magnitude of the mean height gradient around PRE exhibits a seasonal variation.

Seasonal-longitudinal variability of equatorial plasma bubbles

Abstract. We compare seasonal and longitudinal distributions of more than 8300 equatorial plasma bubbles (EPBs) observed during a full solar cycle from 1989-2000 with predictions of two simple models. Both models are based on considerations of parameters that influence the linear growth rate, γRT, of the generalized Rayleigh-Taylor instability in the context of finite windows of opportunity available during the prereversal enhancement near sunset. These parameters are the strength of the equatorial magnetic field, Beq, and the angle, α, it makes with the dusk terminator line. The independence of α and Beq from the solar cycle phase justifies our comparisons. We have sorted data acquired during more than 75000 equatorial evening-sector passes of polar-orbiting Defense Meteorological Satellite Program (DMSP) satellites into 24 longitude and 12 one-month bins, each containing ~250 samples. We show that: (1) in 44 out of 48 month-longitude bins EPB rates are largest within 30 days of when α=0°; (2) unpredicted phase shifts and asymmetries appear in occurrence rates at the two times per year when α≈0°; (3) While EPB occurrence rates vary inversely with Beq, the relationships are very different in regions where Beq is increasing and decreasing with longitude. Results (2) and (3) indicate that systematic forces not considered by the two models can become important. Damping by interhemispheric winds appears to be responsible for phase shifts in maximum rates of EPB occurrence from days when α=0°. Low EPB occurrence rates found at eastern Pacific longitudes suggest that radiation belt electrons in the drift loss cone reduce γRT by enhancing E-layer Pedersen conductances. Finally, we analyze an EPB event observed during a magnetic storm at a time and place where α≈-27°, to illustrate how electric-field penetration from high latitudes can overwhelm the damping effects of weak gradients in Pedersen conductance near dusk.

The effect of gravity and pressure in the electrodynamics of the low‐latitude ionosphere

The source of the plasma drift structure in the low‐latitude ionosphere during magnetically quiet times is generally understood. Nearly all the quiet time electric field structure has been attributed to divergences in the neutral wind current dynamo. However, there are other current drivers active in the low latitude ionosphere: a gravity‐driven current (g × B drift of O+) and a gradient‐pressure current (∇P × B drift of e and O+). The gravity‐driven current is important in the development of the Rayleigh‐Taylor instability and equatorial spread F (ESF) but is ignored in global dynamo electric field models. The gradient pressure and gravity terms in the ionosphere momentum equation normally oppose each other. This paper examines the magnitude of the large‐scale electric fields generated by divergences in these lesser currents in a coupled ionosphere‐electrodynamics model. The low‐latitude electric fields generated by the neutral wind dynamo are altered only slightly by these additional terms. The resulting vertical electric fields (zonal plasma drifts) do not change significantly but the vertical plasma drifts are affected by ∼10 to 15 m/s in the hours before sunrise and after sunset. These low‐latitude plasma drift differences are due entirely to the inclusion of the gravity‐driven current. The gravity current term produces a downward drift after sunset, which reduces the evening prereversal enhancement. Additionally, the positive vertical drift before sunrise is sometimes large enough to cause a predawn vertical enhancement. The gradient pressure terms can be ignored for large and medium scale ionosphere features.

Equatorial F-layer heights, evening prereversal electric field,and night E-layer density in the American sector: IRI validation with observations

The equatorial F-layer height variations resulting from the variabilities in the zonal electric fields and winds and associated variability in ionospheric dynamo strength are important factors in determining the distribution and structuring of the electron density of the equatorial ionization anomaly (EIA) region. Especially, the evening enhancement in the F-layer heights and the associated prereversal enhancement in the zonal electric field due to the F-layer dynamo are believed to provide the most basic precondition for the equatorial spread F/ plasma bubble irregularity (ESF) generation. A realistic description by the International Reference Ionosphere (IRI) of the quiet time equatorial F-layer heights is therefore of fundamental importance for applications related to the studies of the ESF and EIA variabilities. The existing IRI description scheme (that uses the CCIR coefficients) appears to represent the equatorial F-layer peak density ( N mF 2/ƒ oF2) better than the peak height ( h m F2) and the heights of specific densities, the largest disagreement with observations being verified during the evening hours. Digisonde data from the three permanent stations in Brazil: São Luis (2.33°S, 44.2°W, dip angle: −.5°); Fortaleza (3.9°S, 38.45°W, dip angle: −9°); and Cachoeira Paulista (22.6°S, 315°E; dip angle: −28°) and from Jicamarca (12°S, 76.9°W; dip latitude: 1°N) in Peru have been analysed, to determine the quiet time mean behavior of the key F-layer parameters as a function of local time, season, and solar activity. These are complemented by data from the three conjugate point stations: Boa Vista (02.8°N; 60.66°W, dip angle: 22.5°) in the north and Campo Grande (20.45°S; 54.65°W, dip angle: −22.5°) in the south, and an equatorial station, Cachimbo (9.47°S; 54.83°W, dip angle: −3.9°) that were operated during the 2002 COPEX (Conjugate Point Experiment) campaign conducted in Brazil. The data for São Luis and Jicamarca are used to evaluate the longitudinal differences in the prereversal F-layer vertical drift, arising from the large magnetic declination angle difference that characterize these Brazilian and Peruvian longitude sectors. An attempt is made to characterize and quantify any systematic difference that exists between the mean behavioral patterns of the critical parameters as described by the IRI and those observed, with an objective to improve the IRI prediction capability.

Evolution and dynamics of equatorial plasma bubbles: Relationships to ExB drift, postsunset total electron content enhancements, and equatorial electrojet strength

The growth in altitude/latitude of equatorial plasma bubbles was monitored, using simultaneous recordings of VHF scintillations at five locations situated between 3° and 23°N magnetic latitudes along a common meridian (84°E) during February 1980. The onsets of postsunset scintillation were mostly abrupt in character, and their occurrence at higher latitudes was conditional on their prior appearance at lower latitudes, indicating a causal link to irregularities associated with rising equatorial plasma bubbles. The day‐to‐day occurrence and the latitudinal, and effectively altitudinal, growths are examined in relation to the prereversal enhancement in h′F during sunset hours and its rate of rise, the onset of a postsunset secondary maximum (PSSM) in ionospheric electron content (IEC), and equatorial electrojet strength (EEJ) variations. It is observed that the bubble and associated irregularities, after its onset over the magnetic equator, reached the highest altitudes/latitudes only on those days when a prior PSSM in IEC is observed there in addition to high values of h′F, dh′F/dt and bubble rise velocity; otherwise it will be confined to near equatorial latitudes only. Also, the equatorial h′F, dh′F/dt, magnitude of PSSM and intensity of 4 GHz scintillations at low latitude are all showing positive correlation with daytime EEJ strength variations. It is concluded that, after the initial development of a bubble, the ExB drift and the PSSM play an important role in the subsequent growth and evolution, and EEJ is a useful parameter for the prediction of the development.