Equatorial vertical plasma drifts and the measured and IRI model-predicted F2-layer parameters above Ouagadougou during solar minimum
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2012
Received: 27 July 2011
Accepted: 20 October 2011
Published: 27 July 2012
In this paper, hourly median value of ionosonde measurements: peak height F2-layer (hmF2), F2-layer critical frequency ( foF2) and propagation factor M(3000)F2 made at near-equatorial dip latitude, Ouagadougou, Burkina Faso (12°N, 1.5°W; dip: 1.5°N) and relevant F2-layer parameters: thickness parameter (Bo), electron temperature (Te), ion temperature (Ti), total electron content (TEC) and electron density (Ne) (at the fixed altitude of 300 km) provided by the International Reference Ionosphere (IRI) model for the longitude of Ouagadougou are contrasted with the IRI vertical drift model to explore in detail the monthly climatological behavior of equatorial ionosphere and the effects of equatorial electrodynamics on the diurnal structure of F2-layer parameters. The analysis period covers four months representative of solstitial and equinoctial seasonal periods during solar minimum year of 1987 for geomagnetically quiet-day. It is demonstrated that the month-by-month morphological patterns between vertical E × B drifts and F2-layer parameters range from worst to reasonably good and are largely seasonally dependent. A cross-correlation analysis conducted between equatorial drift and F2-layer characteristics yield statistically significant correlations for equatorial vertical drift and IRI-Bo, IRI-Te and IRI-TEC, whereas little or no acceptable correlation is obtained with observational evidence. Examination of the association between measured foF2, hmF2 and M(3000)F2 illustrates consistent much more smaller correlation coefficients with no systematic linkage.
Key wordsEquatorial-ionosphere vertical-drift ionospheric-parameter correlation-analysis solar-minimum IRI
Equatorial vertical plasma drift is an important consequence of the E and F region dynamos (Alken, 2009). Equatorial zonal electric fields control the vertical plasma transport in the low-latitude ionosphere (Kelley,; Fejer, 1997). It drives the equatorial electrojet and the equatorial ionization anomalies (Rush and Richmond, 1973; Kelley, 1989). In the evening hours, it drives plasma to higher heights increasing the linear instability growth rate of spread F plasma instabilities (Kelley, 1989; Sultan, 1996). Fejer et al. (1991) discussed extensively the large day-to-day variability in vertical drift velocities as determined by Jicamarca Incoherent Scatter Radar (JISR), located at the magnetic equator in Peru in the West Coast of South American. Scherliess and Fejer (1999) presented the first comprehensive study of global empirical model of the quiet-time F region equatorial vertical drifts based on combined incoherent scatter radar (ISR) observations at Jicamarca and ion drift meter (IDM) measurements on board the Atmospheric Explorer E (AE-E) satellite. Their study presents local time, season, solar cycle and longitude effects on quiet-time equatorial F region vertical plasma drifts. The physical principles underlying the electrodynamic properties of the equatorial and mid-latitude ionosphere is discussed in a tutorial given by Heelis (2004).
Observations of vertical plasma drifts in the equatorial region of the African ionosphere have also been inferred from ionosonde observations mostly during nighttime solar maximum magnetospherically quiet conditions (e.g., Oyekola et al., 2007, 2008; Oyekola and Oluwafemi, 2007, 2008; Oyekola, 2009a, b; Oyekola and Kolawole, 2010). By and large, except for magnitude the characteristic features of equatorial electromagnetic drifts in the African ionosphere are in good agreement with those obtained for the equatorial region in the America sector using ISR observations (e.g., Fejer et al., 1979, 1991). The general diurnal features are as follows: The F region electrodynamic plasma drifts are upward and westward during the day and downward and eastward at night. The daytime drifts have largest amplitudes between about 0900 and 1100 LT with typical values of 25–30 m/s. The drift exhibits morning and evening reversals from downward nighttime to upward daytime and from upward daytime to downward nighttime, respectively. The morning reversal times and the daytime drifts indicate just little variations with the phase of the solar cycle. Around dusk hours, there is an enhancement in the vertical drift of the F layer plasma, which is produced by an enhancement in the zonal electric field, known as the evening prerever-sal electric field enhancement (PRE). In particular, during solar minimum the prominent feature of equatorial ionosphere, the evening prereversal velocity enhancement reveals small longitudinal changes with largest usually occur in equinoxes, intermediate value in December solstice, and absent at all longitudes during June solstice season.
The International Reference Ionosphere (IRI) is a global empirical model, which describes the average behavior of ionospheric electron density (Ne) at 300 km height, electron and ion temperatures (Te and Ti), equatorial E × B drift velocity (V z ), total electron content (TEC) and ion composition (Bilitza, 1990, 2001, 2003; Bilitza et al., 2004a, and references therein). As an empirical model based on mixed data, the IRI is obviously only as good as the available observations. The data sources used in the IRI model comprise mid-latitude ionosonde, ISR, rocket as well as satellite measurements. Moreover, the IRI is a six-segment model (Bilitza, 2001, 2003) covering height ranges from D region to the topside. For an update on the considerable progress of IRI-prediction model see Bilitza and Reinisch (2008).
However, a number of prior publications have pursued the correlations between individual values of F2 -layer. Bradley et al. (2004) correlated daily changes in foF2 with M(3000)F2 using data collected at the European mid-latitude stations of Slough, Julisruh and Rome over the years 1986–1994. Zhang and Holt (2004) presented correlation between electron density and electron temperature over Millstone Hill for daytime and nighttime periods for different seasons. Zhang et al. (2008) presented results from correlative analyses between Bo and hmF2, Bo and M(3000)F2, B1 and M3000F2, B1 and hmF2, and B1 and Bo using observations recorded at the Asian low-latitude station of Hainan (19.4°N, 109°E), China over the periods from March 2002 to February 2005. The correlation of equatorial vertical drift with ionospheric F2-layer parameter has not been given serious consideration except those of Obrou et al. (2003). The differences between Obrou et al. (2003) correlation relationship between hmF2 and vertical plasma drift and the current study will be shown later. Using a relatively simple and approximate theoretical model calculation of an upward drift of the equatorial ionospheric plasma, based on the analytic solutions of the continuity equation for daytime solar maximum conditions, Oyekola and Akin Ojo (2008) have examined interrelationship between equatorial ionospheric F2-region parameters. Again, Oyekola (2011) indicated that drift velocity predicted for Ibadan longitude sector by the latest version of IRI (IRI-2007) could be used to explain the observed daily features of foF2 at the African equatorial site of Ibadan for some selected months during the year 1958 (IGY-era) under sunspot maximum conditions. Furthermore, most of the observed features and behavior of equatorial ionospheric F2-layer parameters in the African sector and other equatorial stations around the world have been attributed mostly to variations of vertical V z = E × B plasma drift (e.g., Sambou et al., 1998; Radicella and Adeniyi, 1999).
By comparing the behavior of global equatorial F region vertical drift model of Scherliess and Fejer (1999) incorporated in the IRI model (IRI-2007; Bilitza and Reinisch, 2008) with measured and IRI model-predicted F2-layer parameters, the primary goal of this work is seen as threefold: (1) to describe and quantify the association between vertical drift velocity and F2-region parameters (using available observational data and IRI model values) at equatorial region. Thus, allows significant insight into the role played by electrodynamics in the equatorial ionosphere; (2) to explore if the diurnal variation of vertical drift could enable us to explain many characteristics features of the diurnal changes in the ionospheric F2-region characteristics; (3) to examine diurnal and seasonal structures of foF2-hmF2 and foF2-M(3000)F2 in order to see if the knowledge of the variation of one can predict the rough variation of the other during low solar flux and magnetic quiet conditions.
2. Observational Data and IRI Model-Predicted Values
In view of the fact that IRI model represents an average ionosphere, we use the monthly hourly median value of foF2 and M(3000)F2 obtained from the ionograms recorded by the ionospheric prediction sounder (IPS42) located at Ouagadougou (12.4°N, 1.5°W; dip latitude 1.5°N) Burkina Faso, in the West African sector during the 1987 solar minimum magnetically quiet periods. The hmF2 values were deduced using empirical formula given by Bilitza et al. (1979) from the monthly ionosonde parameters, namely foF2, M(3000)F2 and foE measured over Ouagadougou station. The standard error in deriving hmF2 from the M(3000)F2 factor is estimated to be within about ±(6−9) km over the four seasons.
We used the 2007 version of IRI in this study. IRI outputs were generated for the same geophysical conditions of our ionosonde data using the available model input variables: year, month, day, local time, and the Ouagadougou Observatory geographic coordinates. IRI runs were obtained from IRI website for the equatorial E × B drift velocities (V z ). Table option for thickness parameter (Bo), electron and ion temperatures (Te, Ti), total electron content (TEC) and electron density (Ne) were obtained from the IRI website. Some parameters such as plasma density, electron temperature, ion temperature, and total electron content were considered for 300-km altitude. One of the reasons for chosen electron density, in particular, at the height of 300 km is that ionospheric plasma at the F-region (altitude 300 km) is basically a low collision region (Ridley et al., 2006). Another reason is that the height centered around 300 km incidentally happens to be transition region from chemically controlled to transport dominated F-region (Oyekola, 2009b and references therein). Hence, it is easy to correlate the effect of vertical drifts with the electron density at a specific height and time.
The data used for this study cover four different seasons: December solstice, March equinox, June solstice and September equinox, represented by January, April, July and October, respectively. These months had average sunspot numbers of 18, 24, 31 and 44, respectively and the yearly-averaged value was about 32. The corresponding geomagnetic Ap index for January, April, July and October was 9, 10, 17 and 21 units, respectively. The annual mean Ap index was approximately 17.
This section is presented in the following order: In Subsection 3.1, we examine the relation of IRI representation of the vertical drift identified as Scherliess and Fejer (1999) to ionosonde observed F2-layer peak data. Local time variation of IRI vertical drift is compared with diurnal changes of IRI F2-region parameters, predicted for Ouagadougou is our focus in Subsection 3.2. This is followed by a comparison between ionosonde observed F2-layer critical frequency with peak height F2-layer and propagation factor (Subsection 3.3).
3.1 Description of equatorial vertical drift and ionosonde F2-layer peak data
Figure 1 shows typical diurnal and seasonal characteristics behavior of the Vz and hmF2 during low solar activity year. As can be seen, January Vz is upward between 0700–1400 LT and completely downward between 1500–0600 LT. The duration of upward drift velocity is relatively shorter. The morning velocity peak occurs at about 1000 LT with a value near 20 m/s, but the evening velocity peak is absent. The drift indicates an earlier downward reversal compared to other seasons. Conversely, the drift in the month of July is positive between 0700 and 1700 LT and negative between 1800–0600 LT. The early morning velocity peak shows up at about 1000 LT with magnitude of ∼20 m/s, while the evening prereversal velocity peak is again absent. April vertical drift is upward between 0700–1800 LT, but mostly downward from 1900 and 0600 LT. Here the daytime drift has largest amplitude at about 0900 LT with typical value of 23 m/s, while the evening prereversal velocity enhancement occurs at dusk with amplitude of about 18 m/s. In contrast, October vertical plasma drift solar minimum is mainly upward between 0700 and 1800 local time, but downward from 1900–0600 LT. Here the early morning velocity peak is shifted by an hour later compared to vernal equinox. While occurrence time of the evening prereversal enhancement of the upward drift velocity is the same as that of the month of April, the magnitude of the evening velocity peak is approximately 24 m/s, a full 25% larger than that of April PRE in vertical plasma drift. It is important to state that the general characteristics of IRI model-predicted equatorial vertical drift during low solar flux conditions, emphasize above will be used to contrast the behavior of F2-layer parameters in Figs. 1–8.
During the months of January and July, hmF2 decreases quickly at local afternoon and reaches the lowest value of hmF2 at about 2200 LT and 2300 LT for January and July, respectively. While January hmF2 decreases rapidly from local midnight with a value of about 296 km and attains early morning (0700 LT) minimum with a value of ∼228 km, July hmF2 indicates significant fluctuations between 0000 LT and 0700 LT and shows hmF2 minimum of approximately 250 km. After the postsunrise drop, hmF2 increases rapidly to attain daytime maximum, which varies from about 340–380 km during 1000–1100 LT for both January and July. January hmF2 follows closely the trend in vertical drift during 1000–2200 LT sector. The rate of decrease in the magnitude of plasma drift velocity is faster between about midday and 2200 LT sector during June and July months, which agrees well with variation found in the ionosonde hmF2 curves.
In contrast, the morphology of equinoctial hmF2 are somewhat similar to each other, except that April hmF2 decreases gradually after about local noon till about local sunset and then suddenly rise to attain evening peak in hmF2, while hmF2 rises fairly linearly within the time interval 0800–1800 LT in October. It can be seen also that both April and October hmF2 decreases rapidly from local midnight to reach a minimum value of ∼240 km for April and October at about 0700 LT.
Furthermore, postsunset peak in hmF2 occurs between about 1900–2100 LT with typical values of 300–390 km over the four seasons. The evening enhancement in hmF2 during solstices periods is considerably smaller than those of equinoxes probably due to absent of PRE in vertical drift during these periods. Except for time shift, the daytime drift peak corresponds to the early morning F2-layer height peak for all seasons. Equinoctial evening prereversal velocity enhancement also coincides with maximum and minimum in hmF2.
Figure 2 compares the diurnal cycle of IRI vertical drift with the local time changes of Ouagadougou ionosonde foF2 for four seasonal periods during solar minimum. Apparently, the local time behavior of vertical drift with foF2 is radically different from each other. Three diurnal peaks of the critical frequency of the F2-layer are clearly shown. The appearance of forenoon maximum of foF2 occurs at about 0700–0900 LT with typical values of 7.7–11.1 MHz. The postnoon maximum of foF2 ranges from 10.4–11.1 during 1400–1500 LT, except for January foF2 that show up at sunset with a value of about 7.8 MHz. The evening foF2 maximum is within 7.8–10.6 MHz during 1800–2100 LT and is least pronounced in July and October. Analysis of V z and foF2 curves indicates that the increase in the critical frequency coincides basically with upward direction of drift, and that the decrease corresponds to downward drift. Thus, prenoon foF2 peak occurs roughly at the same time as that early velocity peak apart from the month of April in which foF2 maximum and the drift peak are differing by about 3 hours. The evening prereversal enhancement of the upward drift velocity matches the evening foF2 maximum during April. The diurnal variation of foF2 also demonstrates minimum, which ranges from 1.6 to 5.0 MHz at about 0500 LT for all seasons, but not for April and October that show up minimum at about 0100 LT. There is a greater fluctuation of foF2 in July, while a deep trough is seen in October.
The theory predicts that the maximum of the critical frequency should occur at local noon. However, a careful inspection of the foF2 plots, one notes that the foF2 maximum is shifted away from midday by about 3–5 hours, occurring at 0700–0900 local time. So at local noon, depletion of ionization occurs (noon bite-out), although is not well formed in April. Noontime foF2 varies from 6.2 to 9.5 MHz and the corresponding midday value of vertical velocity is in the range of ∼10–14 m/s.
Figure 3(a) displays the diurnal variation patterns between IRI Vz and the measured M(3000)F2 during low solar flux periods for four seasonal conditions. Obviously, the morphological pattern of vertical ionospheric motion and M(3000)F2 is completely different from each month. Going by the plots in Fig. 3(a), one may conclude that there is strong anti-correlation between Vz and M(3000)F2 curves for all seasons. In Fig. 3(b), IRI model-predicted vertical drift for Ouagadougou is plotted with the inverse of observed propagation factor, 1/M(3000)F2. The dominant information to come from Fig. 3(b) is that the modeled vertical drift diurnal shape and characteristics appear to match diurnal pattern and features seen in 1/M(3000)F2. In particular, the early morning “spikelike” peak of 1/M(3000)F2 in April occurs almost the same time as that of morning velocity peak, whereas the evening 1/M(3000)F2 maximum is away by about 1–2 hours from PRE of equatorial ionospheric vertical drift.
Correlation coefficient, R, between IRI model equatorial vertical plasma drift and the calculated hmF2 and measured foF2 and M(3000)F2 during daytime, nighttime and daytime-nighttime conditions for four different seasons under solar minimum periods of 1987.
Day and night
3.2 Analysis of equatorial plasma drift and the IRI-simulated F2-layer parameters
In this subsection we analyze the link between IRI equatorial vertical plasma drift and other IRI model predictions of F2-layer parameters. Figure 4 shows the trends in the modeled vertical E × B drift velocity and the simulated F2-layer bottomside thickness parameter for four different seasons. The figure demonstrates typical characteristics of diurnal cycle and seasonal cycle during low solar flux conditions. As can be seen the daytime Bo-Tab curve from 0700–1600 LT remain practically constant at a value of approximately 200 km for all seasons, but shows significant variation at other times (1700–0600 LT). The modeled thickness parameter follows the behavior of the vertical drift from 0400–0800 LT during solstices, but from 0400–0800 LT and 1600–2000 LT during equinoxes. Since it is difficult to get bottomside thickness parameter derived from the experimental data for Ouagadougou station of precisely the same year. We looked for year with similar solar and geophysical conditions. Figure 4(b) depicts the daytime morphology of Bo for Ouagadougou and IRI-90 model-prediction during low solar activity year of 1994 for four seasons. Coincidentally these months correspond to the months/seasons that we analyzed in this work. The average sunspot number for that period was 30 and the corresponding geomagnetic Ap index was about 16. Figure 4(c) presents average diurnal variation of inferred and modeled Bo parameter over Ouagadougou. Figures 4(a) and 4(b) were adopted from the work of Adeniyi and Radicella (1998). Naturally, the observed equatorial Bo present similar structure for all seasons, symmetrical about midday during solstices, but the noontime peak is shifted away by 1–2 hours in equinoxes. The nighttime behavior (Fig. 4(c)) indicates somewhat “wave-like” pattern parallel to longitudinal behavior presented by Liu et al. (2010) and seasonal variations of Bo reported by Chen et al. (2006) and Zhang et al. (2008). Comparisons between Figs. 4(a), (b) and (c) hints that daytime Bo should correspond to daytime upward vertical drift, particularly during the solstices. Nonetheless, the result must be regarded with caution until Bo inferred from the experimental observations and quantified the correlation between the equatorial vertical drift and F2-layer bottomside thickness. Hence, further analysis is necessary.
The variation of simulated equatorial ionospheric drift and IRI electron temperature for quiet magnetic activity and low solar flux conditions is depicted in Fig. 5. Visually, Fig. 5 shows that there is strong positive correlation between ionization temperature and electrodynamic E × B drift velocity for all seasons, but not for July. The electron temperature indicates steep rise in the early morning reaches a peak (morning overshoot) at about 0800 LT (January, April, and October). Afterward decrease with a minimum around 1500 LT (January), at 1100 LT in equinoctial months. A second peak (evening overshoot) occurs after midday just before sunset around 1400 LT in April and October but show up around dusk hour in January. One other important feature found in Fig. 5 is that morning peak in Te occurs ∼1 hour earlier than the morning maximum seen in vertical plasma drift in January and equinoctial months. Generally the morning overshoot is slightly higher than the evening overshoot, except for July, where the two peaks are approximately comparable. Additionally, July electron temperature shows a midday peak that is higher than the morning and evening temperature peaks.
The diurnal variations of IRI vertical drift and IRI model-predicted ion temperature are shown in Fig. 6, respectively, for January, April, July, and October. As can be seen, the changes in ionospheric drift velocities seem not to have link with the variations observe in ion temperature in comparison to the electron temperatures (Fig. 5). It should be noted that the diurnal variation of ion temperature presents fairly “wave-like” patterns in April and October.
Figure 7 displays diurnal morphological structures of modeled drift velocity and IRI total electron content during low solar flux equinoxes and solstitial periods. The total electron content is measured in TECU (1 TECU = 1016 el/m2). The regular characteristic features of the diurnal variation of IRI model-predicted TEC are as follows: The pre-sunrise minimum, which occurs at about 0400 LT for the four seasons and almost coincides with the period when vertical drift reaches predawn minimum. The morning enhancement in TEC varies between about 7–12 TECU and occurs during 0800–0900 LT. There is afternoon and early evening TEC peak, which has a magnitude ranging from 8–10 TECU between about 1500–1700 LT. The morning TEC peak appears to correspond to the daytime drift velocity peak in all seasons, implying likely linkage. The TEC peak also indicates larger equinoctial morning value compared to those of solstice seasons. On the other hand, morning and afternoon TEC enhancement in April are somewhat similar. October morning peak is about 17% higher than the postnoon peak in TEC. There is also a deep midday trough (noon bite-out) seen in all seasons, which has a magnitude varying from about 5 to 8 TECU. Generally the overall variation patterns of vertical drift and ionospheric total electron content are quite impressive.
Figure 8 gives the diurnal changes in electron density for January, April, July and October. We can see that IRI model simulated electron density displays a diurnal cycle low at around 0400 LT, except for the month of April, which indicates minima at about 0300 LT and another at 0600 LT. Morning electron density peak occurs during 0800–0900 LT with characteristic value of 0.6–0.8 × 1012 el/m3 and afternoon and evening peak appears around 1500–1700 LT, but has magnitude varying between approximately 0.8–1.2×1012 el/m3 over the four seasons. In January presunrise Ne peak is noticed with a value near 0.7×1012 el/m3. Notice that the diurnal cycle low is most pronounced in October. There is also a noontime minimum for all seasons, which varies from about 0.5−0.8 × 1012 el/m3. It is quite interesting to see that during vernal equinox, the evening velocity peak and the evening electron density enhancement occurs virtually at around 1800 LT. Immediately after the peak is reached, the modeled electron density follows the behavior of the IRI vertical drift.
Correlation coefficient, R, between IRI model equatorial vertical drift and IRI-Bo, IRI-Te, IRI-Ti, IRI-TEC, and IRI-Ne at 300 km height during daytime, nighttime and daytime-nighttime conditions for four different seasons under low solar flux periods of 1987.
Day and night
3.3 Comparisons among ionosonde measured F2-layer peak parameters
Correlation coefficient, R, between measured foF2 and inferred hmF2, and measured foF2 and M(3000)F2 at Ouagadougou during quiet-day and low solar flux periods of 1987 for daytime, nighttime and daytime-nighttime conditions for four different seasons.
Day and night
This paper deals with the correlation of the equatorial electrodynamic plasma (E × B) drifts derived from IRI model with a set of important F2-layer ionospheric/upper thermospheric parameters for some chosen months during quiet-day low solar flux periods, in order to determine the electrodynamic dependent of ionospheric characteristics at F-layer heights and to what degree vertical drift component can explain a number of regular characteristic features of the diurnal and seasonal variations of these parameters.
It is of note that the former authors, Obrou et al. (2003) used ionosonde data recorded at Korhogo, Cote-d’Ivoire (9.3°N, 5.4°W, dip latitude: 2°S) in West-African region to investigate ways of improving the representation of equatorial F2 peak height, hmF2 in the IRI. The authors correlated observed hmF2 at Korhogo with the equatorial F region vertical drift given by the model of Scherliess and Fejer (1999) during low solar activity year of 1995. The monthly averaged sunspot number, Rz12 was 18 and corresponding geomagnetic Ap index was 12. The current research utilizes F2 peak height, hmF2 inferred from ionosonde M(3000)F2 measurements from the Ouagadougou Observatory (12°N, 1.5°W; dip: 1.5°N) in West-African sector to study the link between calculated hmF2 and the IRI/Scherliess and Fejer (1999) global equatorial vertical drift model during low solar flux year of 1987. The monthly averaged smoothed sunspot number was about 32 and the yearly-averaged geomagnetic Ap index for that period was approximately 17. The behaviors of estimated equatorial hmF2 and empirical plasma drift are correlated; hence the effects of vertical drift on hmF2 are investigated. Detailed comparisons between the two stations unveil the following points: First, the behavior of observed hmF2 and IRI vertical drift at Korhogo during solstice periods is completely opposite to those seen between calculated hmF2 and IRI vertical drift at Ouagadougou. Specifically, between about 1500 and 2300 LT in January 1995 at Korhogo, the variation of hmF2 and Vz is comparable to the trend noticed in hmF2 and Vz between 1100 and 2300 local time in 1987 in the month of July at Ouagadougou. While that of July 1995 at Korhogo from local time between 0100–2300 LT corresponds to that of January 1987 at Ouagadougou between 1100–2300 LT. This reverse relationship is also evident in equinoctial months (April and October). In addition, the prominent equinoctial feature of equatorial ionosphere, postsunset peak in hmF2 is clearly evident at Ouagadougou, indicating consistent observations of the F2 layer uplifting even at lower solar flux periods. This feature is absolutely absent at Korhogo probably due much smaller value of sunspot index during 1995 compared to that of 1987. Second, at Korhogo, the authors found no clear correlation between measured hmF2 and modeled vertical drift by day, but strong positive correlation by night. At Ouagadougou, the daytime correlation relationship between calculated hmF2 and IRI plasma drift is dramatically low and no correlations exist during the daytime. At night, the derived hmF2 is weakly anti-correlated with Vz in equinoctial months (April and October). For day-night, correlation is still low but not for January where Vz-hmF2 are well connected. Third, the remarkable disparities found at the two sites within West-African sector, Korhogo and Ouagadougou may probably due to the differences in geomagnetic latitudes. This means that drastically opposite behaviors seen at Korhogo and Ouagadougou reveals some differences that are probably consistent with their magnetic northern hemisphere and southern hemisphere locations. In addition, the differences of our results from Obrou et al. (2003) may partly come from the longitudinal structure of hmF2. The observations given above are in accord with the findings reported by Bilitza et al. (2004b) using data collected at the two stations, Korhogo and Ouagadougou during low, moderate and high solar activity years of 1995 and 1985, 1998 and 1993, and 2000 and 1991, respectively; to examine variability of foF2 at the two stations. In a comparative analysis between foF2 observations and IRI model at low latitude station, Ibadan, Nigeria (7.4°N, 3.9°E; 5°S dip angle), Oyekola (2011) found IRI model to be larger than the measured foF2 in June from 1900–0500 LT and September from 1900–2300 LT. In contrast, Bittencourt and Chryssafidis (1994) using data from a low latitude station, Fortaleza, Brazil (3.9°S, 38.45°W, dip angle: 11.5°S) under comparable solar and geophysical conditions analyzed by us, reported that the observed F2 peak electron density (or foF2) were generally higher than those predicted by the IRI model in the same months of June and September. The reason for such difference may lie in the fact that the geomagnetic peculiarity of the Brazilian equatorial region characterized by its magnetic declination with weaker magnetic field intensity (Abdu et al., 2010). The foF2 (or NmF2) and hmF2 measurements in Ibadan and Fortaleza indicates that opposite effects are seen in F2 peak parameters as one moves away from magnetic equator and moves toward anomaly peak implying anomaly peak effects in one hand. On the other hand, confirmed the anomalous and difficult behavior of F2-region to predict (Brum et al., 2011). It is observed that the variation of hmF2 in Fig. 1 is very similar to foF2 diurnal variation in Fig. 2 shown also in Fig. 9, where minimum of foF2 is observed at midnight excpt for April. In addition, the behavior of both hmF2 and foF2 differs from typical opposite diurnal variation at midlatitudes where foF2 peak occurs around noon but hmF2 peaks by midnight. This unexpected opposite behavior in ionospheric F2 layer peak parameters at equatorial and mid-latitudes is essentially latitude difference, which seems to be connected to the geomagnetic field configuration. Both foF2 (NmF2) and hmF2 have peculiarities in the equatorial zone. It is well known that the daytime upward plasma drift velocity lifts electrons to higher altitudes in the equatorial region, where recombination loss rate is low. In addition, the fact that the source of electron production is still present during the daytime that will favor the daytime higher values of hmF2 and enhanced foF2 (or NmF2) at low solar activity. At nighttime, the converse is the case. The reversal of the vertical drift to the downward direction occurs shortly after the postsunset increase. Thus in the postsun-set ionosphere, downward drift pushes electrons to lower heights in the equatorial region, where chemical recombination rate is high, coupled with the fact that photoionization of atomic oxygen by solar UV radiation ceases. What is then observed is lowering of the peak height from postsun-set to sunrise sector with reduced plasma density. A close look at Fig. 2, one would therefore expect a greater nighttime downward plasma drift toward the equator in October than in April. This explains why foF2 minimum occurs at midnight in April and not in October. A more reliable experimental comparison of plasma drifts and F2 layer peak parameters at near equatorial region is needed to confirm this conclusion.
Generally the time variations of equatorial ionospheric F2-layer parameters are dominated by several characteristic features such as, diurnal low (depression), morning peak (prenoon peak), midday trough (noon bite-out), post-noon peak and evening enhancements. These observed features and many others were hitherto explained on the basis of equatorial plasma dynamics associated with the Appleton ionization anomaly (EIA—Equatorial Ionization Anomaly).
The correlation relationship between IRI vertical plasma drift and the F2-layer characteristics are typically low, but there is a significant correlation, R exceeding 0.70, 0.80 and 0.90, typically between Vz and Bo, Vz and Te, together with Vz and TEC during the daytime-nighttime (upward and downward drift). There are fewer instances during daylight and nighttime hours where statistically significant correlations are also observed. Inverse correlation prevails for Vz and measured propagation factors relationship, except for the nighttime in April and October, which shows low positive correlation values. It is worth noting that correlation analysis results for vertical drift and derived hmF2 are totally opposite those of Vz and measured M(3000)F2, whereas vertical drift and 1/M(3000)F2 correlation results is fully consistent with the association between Vz and hmF2. These results validate the strong anti-correlations between hmF2 and M(3000)F2, for empirical relationship between hmF2 and M(3000)F2, see Bilitza et al. (1979). It is also worthwhile to point out that while daytime correlation relationship indicates positive value, the nighttime correlation factor, R, present opposite effect in most circumstances. In other words, if daytime correlation is negative, the nighttime correlation may be positive (see Tables 1 and 2).
Due to rather straightforward approach of correlating equatorial electrodynamics with ionospheric F2-layer characteristics, quantifying the correlations between the F2-layer parameters and vertical drift in one hand, and among the F2-layer peak parameters on the other hand, is still a complex and challenging issue, yet we cannot completely rule out the likelihood of other influences on the variation of the ionosphere/thermosphere parameters.
A reasonable explanation of the behavior highlighted above can be sought in the following speculation. During magnetically quiet daytime at the magnetic equator, the polarization electric field of E-layer dynamo origin is pointing eastwards. It is mapped into the F-layer by the magnetic lines of force. In the F-region, the E × B drift controls the plasma transport across the magnetic field. Charge particles are lifted by E × B = Vz drift velocity, which generally revert downward during the night. Quiet daytime drift carries the plasma at altitudes up to 700 km above the F2 peak (at 300 to 450 km altitudes). The plasma is then displaced by ambipolar diffusion along magnetic lines of force under gravity and plasma pressure gradients (e.g., Sambou et al., 1998, and references therein). Ionization tends to move along magnetic field lines, giving rise to the well known “equatorial fountain effect” that originates in the so-called equatorial anomaly of the geomagnetic low-latitude ionosphere. The latitude profiles of F2 peak electron density remain smooth at daytime hours with a minimum centered at the dip equator, in the general appearance of a “trough”. In local time this trough is modulated along a noon bite-out shape due to stronger upward drift velocity at midday hours. Consequently, fountain effect mechanism explains in part the observed midday minimum of F2-layer critical frequency (equivalently peak F2 density) (Observation 2), total electron content (Observation 7) and electron concentration (Observation 8). Moreover, the enhancement in the vertical drift of the F-layer plasma at sunset is indicative of a sudden removal of plasma from the equatorial ionosphere. The net effect is a rise in peak height F2-layer and concurrent sharp drop in electron density immediately after sunset.
The important interesting features found in the diurnal behavior of the F2-layer can also be explained basically by the interplay between effects of ionization production by solar ultraviolet radiation, loss through charge exchange and transport by diffusion, electrodynamic drift and neutral wind processes that occurs in the upper ionosphere. A regular feature in the equatorial ionosphere, morning enhancements observed virtually in all F2-layer parameters is probably due to persistent photoelectron heating (e.g., Zhang et al., 1999). Thermospheric neutral winds have also been recognized to be an important candidate responsible in part to the nighttime behavior of equatorial ionosphere (e.g., Villa et al., 1998).
Moreover, recent investigations have indicated that direct dynamic forcing, changes in temperature and thermospheric composition and daily changes in E-region dynamo electric fields that produce drifts in the F-region are other factors affecting the electron density in the F2-layer (Mendillo et al., 2002). Zhang and Holt (2004) pointed out that thermospheric temperature could have fundamental effects on the ionospheric electron density by changing the neutral composition and winds and by altering the production and diffusion chemical loss equilibrium of the ionospheric ions. It has been reported that the upper ionosphere is greatly influenced by magnetospheric processes from above and lower atmospheric processes from below even during geomagnetic quiet conditions. Physical sources of ionospheric variability are studied in detail by Forbes et al. (2000) and by Rishbeth and Mendillo (2001). Several former investigations proposed that tides and planetary waves play an important role in the electrodynamic of lower thermosphere which, in turn cause significant variations in the equatorial vertical plasma drifts; for analysis of the propagation mechanism and effects of atmospheric tides and planetary waves, see recent publications by Zou et al. (2000), Muller-Wodarg et al. (2001), Lastovicka (2006), Pancheva et al. (2006), Kil et al. (2007), Lin et al. (2007).
The average diurnal and season-to-season patterns between the time variation of equatorial vertical drift and local time changes of F2-layer parameters are generally satisfactory; implying that the main features found in diurnal variations of F2-layer parameters may be attributed to electrodynamic conditions in the equatorial ionosphere. The relationship between vertical drift and F2-layer parameters is largely seasonally dependent.
IRI model vertical plasma drift is in a good correlation with IRI-Bo, IRI-Te and IRI-TEC, whereas little or no acceptable correlation is obtained with observational evidence.
The daytime (upward drift), nighttime (downward drift) and daytime-nighttime (upward and downward drift) correlations relationship provides some evidence that equatorial ionosphere is largely influenced by other mechanisms controlling at day/night or combined effect of electrodynamic drift plus other mechanisms. The results illustrate complicated behavior of the ionosphere, also.
Statistically significant correlation coefficient that exists between equatorial vertical drift velocity and electron temperature hints that upper ionosphere and upper thermosphere are strongly coupled.
The current research shed more light on the importance of equatorial ionospheric electrodynamics through the plasma fountain effect. Nonetheless, the work essentially recapitulates the success of the IRI simulation in explaining observed ionospheric and other phenomena. Finally, these findings do not necessary apply to latitude regimes outside the West African region.
The author acknowledges benevolence of the United State National Oceanic and Atmospheric Administration (NOAA) for providing equatorial F2-layer parameters from the IRI website at: http://omniweb. gsfc.nasa.gov/vitmo/iri_vitmo.html. The author wish to acknowledge the support of Dr. Rudi Hanbaba, Centre National d’Etudes des Telecommunications, Lannion, France, for providing observational data used in this work. The author is also indebted to three anonymous referees who gave worthwhile evaluation and critical remarks on the draft of this paper. Their comments and suggestions rigorously helped in improving the scientific value of this research paper. Thanks are also due to the guest editor, Dr. Dieter Bilitza, for his impressive and encouraging remarks in carrying out the revised version of this work.
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