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PUBLICATIONS Journal of Geophysical Research: Space Physics RESEARCH ARTICLE 10.1002/2015JA021557 Key Points: • Vertical drift and ionospheric F2 parameter observation during solar eclipse • Vertical drift was derived from hmF2, calculated from propagation factor • Influence of magnetic activity to solar eclipse

Correspondence to: B. J. Adekoya, [email protected]; [email protected]

Citation: Adekoya, B. J., V. U. Chukwuma, and B. W. Reinisch (2015), Ionospheric vertical plasma drift and electron density response during total solar eclipses at equatorial/low latitude, J. Geophys. Res. Space Physics, 120, 8066–8084, doi:10.1002/2015JA021557. Received 11 JUN 2015 Accepted 10 AUG 2015 Accepted article online 24 AUG 2015 Published online 26 SEP 2015

Ionospheric vertical plasma drift and electron density response during total solar eclipses at equatorial/low latitude B. J. Adekoya1, V. U. Chukwuma1, and B. W. Reinisch2 1

Department of Physics, Olabisi Onabanjo University, Ago Iwoye, Nigeria, 2Center for Atmospheric Research, University of Massachusetts Lowell, Lowell, Massachusetts, USA

Abstract The response of the vertical plasma drift (Vz) and the electron density (NmF2) during different solar eclipses was investigated. The diurnal values of the direct scaled measurement of F2 peak height and the one derived from M(3000) F2 data, acquired over an equatorial/low-latitude stations, have been used to determine the vertical plasma drift. The ionosphere during a solar eclipse is significantly affected by the E × B vertical drift; the large depletion of electron density at low altitudes can be transported to high altitudes through the plasma vertical drift. The loss in ionization density during the eclipse phase decreases the electron density, which was accompanied by rapid increase in hmF2. This deviation in the NmF2 during eclipse compared to control days can be related to the increase in the loss rate due to recombination, as a result of reduction in thermal energy. However, the maximum reduction in NmF2 is not synchronous with the time of maximum totality but some minutes later. The differences in the solar epochs may contribute to the observed relative changes in the ionospheric F2 region behavior during the eclipse window. Lastly, it is very difficult to separate the influence of magnetic disturbances from solar eclipse. The deviation in NmF2 is higher during magnetic disturbed days than the quiet day. The reverse is the case for hmF2 observation. However, the NmF2 variation increases with an increase in solar activity. 1. Introduction The appearance of solar eclipse on the ionosphere makes valuable contribution to study the transient properties of ionizing radiation from the Sun and explore the chemical and transport processes in the ionosphere. Ionospheric phenomenon occurs naturally and as such, all processes usually act concurrently, but with varying degree of significance. It therefore becomes very difficult, most of the time, to categorize the causes and effects from the observed experimental data without some degree of ambiguity. The ionization process undergoes rapid, predictable changes during the solar eclipse, a rare event. Most studies on ionospheric eclipse had been focused on the midlatitude ionosphere, though on different measurements and methodology. Among these are Müller-Wodarg et al. [1998], Afraimovich et al. [2002], Le et al. [2008a, 2009], Tomás et al. [2009], Wang et al. [2010], Chuo [2013] and Adekoya and Chukwuma [2012]. Some of these studies discuss observations exclusively on the assumption that solar eclipse at midlatitude is largely controlled by plasma diffusion. Jakowski et al. [2008] reported that the solar eclipse through the atmosphere may generate atmospheric gravity wave that propagates upward and is detectable as traveling ionospheric disturbances (TID) at ionospheric height where molecular oxygen heating begins toward the equator. A gravity wave is generated in the neutral atmosphere and propagates into the opposite hemisphere at around 300 m/s [Müller-Wodarg et al., 1998]. Tomás et al. [2009] observed the influence of midlatitude solar eclipse on ionospheric current systems; they address in particular the effects of the solar eclipses on the interhemispheric field-aligned currents and on the solar quiet current (Sq) system. They found that the eclipses might affect the direction and intensity of the interhemispheric currents and possibly influence the direction of zonal winds, therefore changing the direction of the prevailing F region dynamo currents. The eclipse in the southern hemisphere during September equinox caused interhemispheric currents similar to those observed in northern summer.

©2015. American Geophysical Union. All Rights Reserved.

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Adeniyi et al. [2007], Paul et al. [2011], Nayak et al. [2012], and Kumar et al. [2013] are among the few researchers who studied the equatorial /low-latitude ionosphere during eclipse. The variation in electron density with height at F2 region began at lower heights and extended progressively toward the peak of electron density height of the layer, and the maximum effect can be felt not at the maximum phase of the eclipse but somewhat

IONOSPHERIC EFFECTS OF SOLAR ECLIPSE

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later [Adeniyi et al., 2007]. Ionospheric response at low altitudes is governed mainly by photochemical processes so that the decrease of solar radiation during an eclipse is expected to decrease the electron production rate and hence the electron concentration [Nayak et al., 2012]. However, the F2 layer behavior may be quite different as it is governed by photochemical processes as well as by electrodynamical and neutral forcing. It is well established that vertical transport process in the low-latitude ionosphere is controlled by equatorial electric fields, which drives the equatorial electrojet and F region plasma drifts that control the development of the equatorial ionization anomaly (EIA) and the generation of ionospheric plasma instabilities [Fejer, 1997, 2011]. Equatorial electric fields and plasma drifts (electrodynamics uplifting) play fundamental roles on the morphology of the low-latitude ionosphere. The quiet and disturbed vertical E × B drifts (driven by zonal electric fields) are small, but they determine the daytime distribution of ionization over a large area of the Earth through the fountain effect. If solar eclipse period is categorized as geomagnetically quiet period, the quiet time equatorial/low-latitude F region electrodynamic plasma drifts are driven by neutral wind generated E region dynamo and F region polarization electric fields. The low-latitude thermospheric winds and plasma drifts are highly variable as a result of large changes in the global tidal forcing and effects of irregular winds, planetary, and gravity waves. They can also be affected by the dynamic conditions at the base of the thermosphere and by long- and short-term changes in the efficiencies of the E and F region dynamos. Most eclipse events have not been followed by magnetic storm, but it is common that electron density perturbation is related to magnetic activity. Cohen [1984] had pointed out that just a few eclipses occurred during severe magnetic disturbances. However, it is difficult to separate the influence on ionosphere of magnetic disturbances from solar eclipse [An et al., 2010]. Abidin et al. [2006] observed traveling ionospheric disturbances (TIDs) during a solar eclipse of 23 November 2003 and were traced to have resulted from the severe magnetic storm that took place 2 days before the solar eclipse. Bhargava and Subrahmanyam [1960] had earlier showed during their investigation into the movements in the F region of the ionosphere during solar eclipses that the eclipse effects are not confined to simple changes in electron densities. Also, that the large vertical drifts caused at equatorial/low latitudes by the electrostatics polarization field associated with the production of the currents in the dynamo region appear to be considerably modified during solar eclipses. Since the evening upward velocity enhancement is responsible for the rapid rise of the equatorial/low-latitude F2 layer after sunset, which plays an important role in the generation of E and F region plasma instabilities. Vertical drifts had also been recorded as of specific significance in the nighttime sector, as they are the major drivers for the generation of equatorial spread F [Martinis et al., 2005]. Therefore, observing the diurnal behavior of vertical drift at low latitude during a total solar eclipse remains an important task. Le et al. [2009] had shown that the equatorial ionosphere responses to a solar eclipse would be significantly affected by the E × B vertical drift because the large depletion of electron at low altitudes can be transmitted to high altitudes through the plasma vertical drift. Furthermore, the depression in Ne in the equatorial region also will be transmitted to the EIA region through the fountain effect and influence the eclipse effect in this region. In this regards, we investigates the diurnal variation of the vertical drift E × B over equatorial/low-latitude ionospheric F2 layer when the path of the Moon’s shadow progressively passed through the Earth’s. This elucidates the dynamic response of ionospheric F2 electron density (NmF2) and its solar activity dependence during total solar eclipses at the equatorial/low latitude.

2. Method of Data Analysis To analyze the equatorial/low-latitude F2 layer response to solar eclipses, we selected ionosonde stations during five eclipse events when the path of the Moon shadow passed through equatorial/low-latitude (Figure 1). The path of totality across the Earth with time of commencement and duration of eclipse was highlighted in Table 1 for each event considered. Total solar eclipses are rare event at any particular location because totality exists only along a narrow path on the Earth’s surface traced by the Moon’s umbra. Figure 2 present the geographic location of each equatorial/low-latitude station along the path of the eclipse used on the map. All the paths of totality were found from National Aeronautics and Space Administration (NASA) Eclipse service (http://eclipse.gsfc.nasa.gov). To ensure that the changes observed in the ionospheric variation due to the total solar eclipse are more obvious with the background ionospheric variability, we only considered those observatories with percentage obscuration larger than 50%. This restricted our choice of stations along the path of the solar eclipse at different solar epoch.

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Figure 1. Map showing the total eclipse path, the period of the eclipse, and geographic distribution of ionospheric F2 region observatories along the eclipse progression path. The map was extracted from the original source at Sun-Earth. gsfc. nasa.gov/eclipse/eclipse.html.

The two sets of ionospheric data used in this study consists of 15 min regular values of foF2 obtained from Space Physics Interactive Data Resource (SPIDR’s) network (http://spidr.ngdc.noaa.gov) and the Global Ionospheric Radio Observatory (GIRO) network of ionosonde stations located in the equatorial/ low-latitudes paths as well as eclipse progression time and percentage of maximum obscuration are presented in Table 2. The data from SPIDR are automatically scaled, and the data were validated for accuracy by comparing the diurnal morphology of the derived NmF2 data from critical frequency (foF2) obtained from some of the stations under study against local time with the electron density profile at other stations [Chuo, 2013; Nayak et al., 2012; Adeniyi et al., 2007]. The results agree well with the known electron density profile. Furthermore, several works have been carried out using SPIDR data, and their results are well documented and published [e.g., Adebesin et al., 2013b]. The SPIDR is designed to allow researchers in the field of solar-terrestrial physics to intelligently annex historical space physics data for knowledge generation, space weather forecast, and incorporation with environment models. The direct scaled measurement data from GIRO network was manually validated. Depicted in Figures 2a–2c is the ionogram display ionospheric parameters for Ascension Island during the start time, totality, and end time of the total solar eclipse of 21 June 2001. On the ionograms are the measured ionospheric parameters of the electron density, the peak heights, and the maximum usable frequency of ionospheric reflection for a hop distance 3000 km. The critical frequency at the start time of eclipse is 10.65 MHz at ~09:45 UT (NmF2 is 1.5 × 1012 e/m3 at 08:45 LT), at the maximum obscuration of eclipse is 10.25 MHz at ~11:15 UT (i.e., NmF2

Table 1. Showing the Date and the Path of Total Solar Eclipse, the Time of Eclipse Commencement and Visibility, and the Geographical Region of Visibility Total Eclipse Date ddmmyy

Time of Commencement of Eclipse hh/min/ss (UT)

Time Length at Which Eclipse Can Be Seen

Geographical Region of Eclipse Visibility

Path of Totality at the Geographical Region

18031988

01:58:56

03 min 49 s

East Asia, east Indies, Australia, and Alaska

21062001

12:04:46

04 min 57 s

East of South America and Africa

29032006 22072009

10:12:22 02:36:25

04 min 07 s 06 min 39s

Africa, Europe, and west Asia East Asia, Pacific Ocean, and Hawaii

13112012

22:12:55

04 min 02 s

Australia, NZ, South Pacific, and south S. America

Malaysia, Indonesia, Philippines, and Pacific South Atlantic, south Africa, and Madagascar Central Africa, Turkey, and Russia India, Nepal, China, and central Pacific North Australia and south Pacific

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b

c

Figure 2. (a) The ionogram displays of the ionospheric parameters data procedure around the time of commencement of total solar eclipse of 21 June 2001. (b) Same as Figure 2a but around the totality (i.e., at maximum obscuration) period. (c) Same as Figure 2a but around the end time of eclipse.

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Table 2. List of Ionosonde Stations With Code, Geographic Coordinates, Eclipse Progression Time, and Percentage of Maximum Obscuration Geographic Coordinate Date ddmmyy 18031988 21062001 29032006 22072009 13112012

Station

Lat

Long

Eclipse Start Time (UT) hh:min:ss

Eclipse Max Time (UT) hh:min:ss

Eclipse End Time (UT) hh:mm:ss

% of Max Obscuration

UT to LT difference

Manila(MN) Ascension Island (ASC) Ascension Island (ASC) Kwajalein (KWJ) Townsville (TV)

14.7 7.90 7.90 9.0 19.7

121.1 14.4 14.4 167.2 146.9

00:03:59.3 09:54:07.8 07:39:35.0 02:23:33.5 19:47:30.0

01:15:12.0 11:16:21.3 08:40:00.7 03:41:40.4 20:42:57.9

02:33:11.6 12:50:11.1 09:46:59.5 04:50:28.4 21:44:30.5

72.25 70.96 81.93 99.86 94.49

+8 1 1 +11 +10

is 1.4 × 1012 e/m3 at 10:15 LT), and at the end time of the eclipse is 11.05 MHz at ~12:45 UT (NmF2 is 1.1 × 1012 e/m3 at 11:45 LT). And, the corresponding F2 layer peak heights (hmF2) of the each electron density are 273.9, 275.0, and 245.8 km, respectively. This is an indicative that at F2 region the minimum level of electron density does not coincides with the time of maximum obscuration and the decrease in the electron density through the window of eclipse shows that electrons are loss due to reduction in photoionization (i.e., electron production rate q is reduced as the linear loss coefficient β increases, N = q/β). The changes observed in electron density are drifted to the altitudes greater than 240 km. According to Reinisch and Galkin [2011] GIRO data with minimal latency allow for the assimilation of the ionogramderived data in real-time models such as the real-time extension planned for the International Reference Ionosphere. The basis of GIRO operations are the Digital Ionogram Data Base (DIDBase) with a Web portal access at http://ulcar.uml.edu/DIDBase/ and the expert-level platform-independent software client “SAO Explorer” with read/write access to DIDBase over the Internet [Reinisch and Galkin, 2011]. Furthermore, to examine the variation of the eclipse effects with the eclipse magnitude, which is defined as the fraction of the Sun’s diameter occulted by the Moon, we performed an analysis of the critical frequency of the ionospheric foF2 for the solar eclipses. These available ionosonde stations that have recorded data for foF2 during the eclipse events are listed in Table 2 with their geographical coordinates and percentage of maximum obscuration (http://xjubier.free.fr/en/site_pages/SolarEclipseCalc_Diagram.html). In this scheme, the F2 region response to total solar eclipse can also be most conveniently describe in terms of NmF2, which were therefore obtained from their corresponding foF2 quarter-hourly peak value using the expression in equation (1) Nm F 2 ¼ ðf o F 2 Þ2 =80:5

(1)

where the unit for NmF2 is m3 and that for foF2 is hertz. Furthermore, the NmF2 provides a measure of the F2 maximum electron density. Thus, during a solar eclipse, NmF2 provides information about the total ionization loss of the ionosphere as a consequence of the reduced electron production process. The decrease in value of NmF2 is regarded as the background ionospheric eclipse effect. However, effective change in the electron density of the F2 region can be regarded as the ionospheric disturbances. The hmF2 values used were derived from the M(3000)F2 values with the help of proposed formula by Shimazaki [1955] and Bilitza et al. [1979]. According to the “Shimazaki formula,” which assumes that the idealized case of radio waves reflected from a parabolic F2 layer above a spherical Earth is given in equation (2): hm F 2 ¼

1490 176 Mð3000ÞF 2

(2)

where ionospheric propagation factor, M(3000)F2, is routinely scaled from ionograms and described the maximum usable frequency (MUF) that refracted in the ionosphere, can be received at a distance of 3000 km, M(3000)F2 = MUF/foF2; foF2 is the critical frequency at which a wave just penetrates a layer of ionization. This technique has been used by Batista et al. [1991], Obrou et al. [2003], Rishbeth et al. [2000], Oyekola and Kolawole [2010], Oyekola and Fagundes [2012], and Mielich and Bremer [2013] to derived hmF2 values, but not during solar eclipse.

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The “Bilitza formula,” which allows for the effect of ionization below the F2-layer, takes the form [Bilitza et al., 1979] hm F 2 ¼

1490 176 Mð3000ÞF 2 þ ΔMðX Þ

ΔMðX Þ ¼

f 1 f 2 þ f4 ðf o F 2 =f o E Þ f 3

(2.1) (2.2a)

f 1 ¼ 0:00232R12 þ 0:222 f 2 ¼ 1 R12=150exp ðϕ=40° Þ2

(2.2b)

f 3 ¼ 1:2 0:0116expðR12=41:84Þ

(2.2d)

f 4 ¼ 0:096ðR12 25Þ=150:

(2.2e)

(2.2c)

where ΔM(X) is an empirical function of the critical frequency ratio X = foF2/foE, taking account of 12 month running mean of solar sunspot number and geomagnetic latitude, Φ. Since total solar eclipses represent a short nighttime, the E region does disappear, the correction is important during noneclipse period by day, but is sufficiently small at night for the simple Shimazaki formula (equation (2)) to be used. The formulas are unreliable if M3000 is small; i.e., the layer is high, or if foF2 is too close to foE [e.g., Rishbeth et al., 2000; Dudeney, 1983]. Although, equations (2.1) and (2.2a) above is more complicated and even more accurate formulas for the derivation of the F2-peak height using additional information about the underlying ionization [e.g., Bilitza et al., 1979]. But due to the reason highlighted above and such data are not available in the used databank. Therefore, we had to use the simple equation (2). In addition, analysis of the error relative to International Reference Ionosphere(IRI) by McNamara [2008] shows that equation (2.1) is useful, since the errors observed is small in the seasonal and solar cycle variations of hmF2. The 15 min values were then computed, ensuring that there is no data loss in the process. From these diurnal quarterhourly value, apparent vertical E × B plasma drift was determined by measuring the time rate change of F2 real height (Vz = [d(hmF2/dt]). The concept of local time is used throughout the analysis. The distinctive way to identify eclipse-induced effect on the ionosphere is to examine the standard deviation of the NmF2 and hmF2 on the eclipse day from that of the respective adjacent 10 days. In this regards, we have employed the definition of standard deviation as emphasized in the work of Adebesin et al. [2014]. The concept of the standard deviation (σ) and the mean (μ) from the adjacent 10 day are used, assuming that the variation represents real changes in electron density and not just a redistribution of existing plasma. The standard deviation (σ) quantifies the exactness of an analysis and hence is a measure of how NmF2 and hmF2 on the eclipse day differ from the average value of the noneclipse/control days. Standard deviation is given by the expression in equation (3) vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N uX u ðx i μÞ2 u t i¼1 (3) σ¼ N1 where xi is the data point of individual observations, μ is the arithmetic mean value of the adjacent 10 days taken, N is the number of observational points, and quantity (xi μ) is the deviation of each data point away from the average.

3. Results In this section we present the diurnal vertical plasma drift (Vz), peak electron density, NmF2, and F2 peak height (hmF2) observed over equatorial/low-latitude region stations along the path of total solar eclipses in the years 1988 and 2012 (years of moderate solar activity (MSA)), 2001 (high solar activity (HSA)), and low solar activity (LSA) years of 2006 and 2009, respectively; see Table 5 for classification solar activity using solar flux F10.7. To observe the eclipse related effect on the ionosphere, we plotted and compared the event day of eclipse and the average of the respective day before and after the eclipse (i.e., control days). The ionospheric parameters for these studied periods are tagged with suffix “c” for the control days and “e” for the eclipse day, ADEKOYA ET AL.


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Figure 3. The diurnal F2 region vertical plasma drift and the corresponding peak height values obtained over the ionosphere of the equatorial/low-latitude stations during total solar eclipse (Vze and hmF2e) and the control days (Vzc and hmF2c). The vertical straight lines showing the eclipse start time (S), maximum magnitude of eclipse (M), and the end of eclipse (E). (a–e) The 15 min obtained from SPIDR network. (b–d) The 15 min obtained from GIRO network.

respectively. The stations under study are the respective equatorial/low latitude, with the percentage of eclipse obscuration above 50%. Figure 3 depicts the diurnal vertical plasma drift of F2 region over equatorial/low-latitude ionosphere during eclipse progression, with the corresponding plots of F2 peak

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Figure 4. Diurnal NmF2 variation over the ionosphere of the equatorial/low-latitude stations during eclipse event day (NmF2e) and noneclipse/control days. The vertical straight lines showing the eclipse start time (S), maximum magnitude of eclipse (M), and the end of eclipse (E).

height (hmF2), while Figure 4 presents their diurnal electron density, NmF2, variation in that order. The vertical lines labeled S, M, and E are used to indicate the time of the start, occurrence of maximum obscuration, and end of the solar eclipse. Generally, there seems to be noticeable changes in the electron density and height of the peak electron density. Figure 3 depicts the 15 min daily variation of F2 region vertical plasma drift and the corresponding F2-peak height during/on the eclipse day (Vze and Vzc) and control days (hmF2e and hmF2c). The control days considered are the respective average of the noneclipse days before and after the eclipse events. This variation on noneclipse period is compared with the eclipse day in order to affirm the eclipse contribution to the dynamic of electron density at the peak height. This is done using data collected from Global

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Ionospheric Radio Observatory (GIRO) network. Its accurate and fine detail by the ionosonde measurements has inspired a number of studies of the ionospheric response to space weather events [Reinisch and Galkin, 2011]. Since it is only Ascension Island and Kwajalein that has a direct measurement of ionograms data from digisondes, in order to have clear picture of ionospheric eclipse related effect, we decided to consider other equatorial/low-latitude stations (i.e., Manila and Townsville) that has a recorded data with Space Physics Interactive Data Resource (SPIDR). Although, the SPIDR does not have in their databank the F2 peak height data therefore we used the alternate in equation (2). The diurnal variation of these parameters is observed on eclipse day and compared with the control days during the eclipse progression, the buildup interval (05:00–09:00 LT) and unobscured period of the daytime (09:00–18:00 LT) and nighttime (18:00–05:00 LT), respectively. The responses of each of these diurnal intervals of vertical drift and corresponding hmF2 are observed and reported accordingly. 3.1. Diurnal Plasma Vertical Drift and Eclipse Related Effect Presented in Figure 3a is the rate at which F2 layer oscillate up and down during the total solar eclipse of 18 March 1988 over Manila ionosphere. At the first contact of the eclipse, the vertical drift decreases from an upward drift value of 37 m/s through the maximum phase of the eclipse downward to 48 m/s around 11:00 LT after the eclipse ends time; however, a sudden and significant upward drift was observed before the decrease compared with Vzc. The decrease of the vertical drift through the eclipse phase was corresponding with a regular increase in hmF2, which are the normal features of the quiet daytime ionospheric phenomena. The noticeable fact is that the plasma drift on the eclipse day (Vze) is more increase than it appeared on the control days (Vzc). Thereafter, a downward drift was observed during the daytime, from where it joins the regular variation trend before it recorded an upward drift at sunset period. The nighttime resurgence of equatorial ionization anomaly (EIA) was observed, when the drift sprung-up to maximum peak at around 19:00 LT before a considerably decrease close to zero drift later at nighttime. This trend of vertical drift was similar to that of associated hmF2, but the maximum Vz was reached before the peak height. The daytime hmF2 peaks are 501 and 455 km around 10:00 LT, and the 347 and 409 km for hmF2e and hmF2c correspond to the prereversal enhancement (PRE) period. The PRE is basically responsible for the larger F2 layer uplift and the evening resurgence of EIA. Figure 3b depicts the observed plasma vertical drift during the eclipse of 21 June 2001 and noneclipse period and their corresponding peak height. The eclipse progression was in the daytime period. However, the Vze variation on the eclipse day was more decrease than it appeared on the control days during the eclipse window. At the beginning of the eclipse window the vertical plasma drift was downward, reaching a minimum value (7 m/s) at the elapsed time of totality before registered an upward drift at the Moon shadow is progressively moving away from the Sun illumination. Similarly, Vzc was downward, close to zero drift around this period, and thereafter increased upward around local noontime. The maximum upward drifts recorded for both Vzc and Vze are 6 and 7 m/s around 12:30 and 14:00 LT, daytime. Thereafter, it decreases through the daytime below zero drift, before peak plasma drift of 16 m/s around 20:15 LT for Vze, and the EIA resurgence period and 10 m/s around 18:30 LT for Vzc. The oscillation is quite significant, especially after the first and final contacts of the eclipse. Toward this end, one can therefore suggest that the daytime vertical drift is not really affected by solar eclipse, but at the eclipse window. Also, the short nighttime during eclipse is apparently similar to the nighttime period. The time shift in PRE occurrence on eclipse day may be connected with some kind of background quiet time ionospheric effect. The prereversal enhancement is a phenomenon strongly related to dynamic processes at the higher altitudes. Embedded in the plot is the variation of hmF2 as a function of local time on eclipse and control days. The peak heights that were associated with the Vzc and Vze are 297 and 269 km around 10:15 LT, 309 and 306 km around 12:30 and 14:00 LT, and nighttime peaks of 345 and 340 km around 20:00 and 23:00 LT, respectively. The rate at which the F2 layer moves up and down over Ascension Island ionosphere during the total solar eclipse of 23 March 2006 and the respective control days is computed and presented in Figure 3c. The Vze and Vzc during the eclipse phase show an intermittent variation; at first contact the Vze was downward below zero drift (3 m/s) before increases upward to 6 m/s at the maximum phase; thereafter, the plasma drift attained a downward drift of 6 m/s after the maximum phase. However, the vertical drift variation on the control days is different; the downward drift observed on the eclipse day corresponds to the upward drift on the control days. Afterward, the Vze slightly increased from zero drift with the corresponding hmF2 to ADEKOYA ET AL.


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11 m/s around 09:45 LT, some minutes after end of eclipse. The daytime upward drift was 18 m/s around 12:15 LT noon, and later downward before a sunset peak of 11 m/s at around 21:00 LT, the PRE. The daytime upward drift could be explained in view of eastward electric field. The strength of daytime drift is very important for the building up of EIA that follows thereafter and shows that the equatorial/low-latitude wind is very important for daytime electrodynamics. The sunset upward peak of Vzc was reached 45 min after with velocity of 21 m/s. The hmF2 uplift during the eclipse window corresponds to the downward rate at which F2 layer moves. Although, during the daytime the lifting of the F2 layer occurs through the ionization of atom/molecule in the ionosphere around 200 km, but the hmF2 lifting after local sunset used to occur through recombination of molecular ion in the lower F region and the electrodynamics lifting of the F region. Therefore, the decrease in the hmF2 during the eclipse phase may be partially related to the nighttime features. The diurnal variation of vertical plasma drift (Vz) during/on the total solar eclipse of 22 July 2009 and the control periods over the ionosphere of Kwajalein are presented in Figure 3d. The eclipse progression occurs during the noontime period, between intervals 13:23–15:50 LT of the day. The transient of vertical drift and the peak height on eclipse day were compared with noneclipse days. The data gap during the eclipse window on eclipse day restricted the observation and comparison of plasma vertical drift around the period as well as the plasma peak height (hmF2), but the Vzc was recorded with a slow oscillatory drift around this period. However, the existing data show an upward drift value at presunset with maximum velocity of 48 m/s around 17:30 LT, hence the PRE. Thereafter, the drift returned downward, close to zero drift, and decreased throughout the nighttime. Also, the associated peak height (hmF2e) at the eclipse phase decreased with a constant value and later increased corresponding to PRE with ~438 km altitude. The ionosphere over Townsville (Figure 3e) did show a similar diurnal trend of plasma vertical drift and peak height compared to Manila. The eclipse occurs during a moderate solar activity, and the progression was at the period of EIA buildup. The drift was considerably enhanced as well as the associated hmF2. The Vze and hmF2e maximum enhancements are within the eclipse progression while Vzc and hmF2c are after the eclipse end time. However, the most important is that the ionization of electron was drifted to the height above 300 km at the sunrise periods. The Vze increased before the eclipse start time and maximized just before the mideclipse with magnitude of 35 m/s at a peak height (hmF2e) of 418 km. The Vzc upward drift was recorded around 10:00 LT with velocity of 10 m/s, which correspond with the hmF2c peak value of 351 km. Thereafter, the drift decreased and was close to zero drift, resulting in the reduction (buildup) around the dip equator producing a double humped latitudinal distribution (EIA) on either side of the magnetic equator. Thereafter, the drift was transported from the lower height to the peak height above 349 km at noontime periods. After that, an upward drift of 19 m/s and 10 m/s (Vze and Vzc) was recorded at sunset around 18:00 LT which later decreases throughout the nighttime. 3.2. Peak Electron Density Observation Figure 4 illustrates the variation of ionospheric F2 region peak electron density over equatorial/low-latitude stations on the eclipse day (NmF2e and hmF2e) and controls days (i.e., NmF2c and hmF2c). The NmF2 variation shows a discernible change during the eclipse phase, daytime, and nighttime periods, respectively. Likewise, the solar activity difference is apparent and homogeneous. The NmF2 variation is higher during high solar activity (HSA), followed by the moderate solar activity (MSA). The corresponding height of peak electron density is vice versa. However, the electron density variation is more reduced on the eclipse day than it appears on the control days. The changes observed especially during the eclipse window can be explained in view of the partial nighttime ionosphere (e.g., the sunset period). The loss coefficient increases as the rate of production reduces, which give the molecular ion (N2+ and O2+) (i.e., the atomic oxygen concentration decreases as the thermal energy decreases) a short-time dominant in the ionosphere. The decrease in the electron densities is synchronous with the increase in hmF2. It is hemispheric symmetry, but increased with an increase in solar activity. The stations along the path of the eclipse in the northern hemisphere are Manila and Kwajalein during 1988 and 2009 total solar eclipses, while Ascension Island and Townsville are in the southern hemisphere, during total solar eclipses of 2001, 2006, and 2012, respectively. Depicted in Figure 4a is the NmF2 and hmF2 variations for Manila; the ionosphere recorded a gradual decrease in NmF2 both on the control days and eclipse day from postmidnight which truncated at around 06:00 LT, sunrise. Afterward, a steady increase in the electron density was observed through the eclipse phase on

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the control days, but on eclipse day the electron density decreased before a peak fixed magnitude that spans 15:00–15:45 LT. This discernable change in the electron density on the eclipse day compared to the control days may be traced down to eclipse related effect [Adeniyi et al., 2007]. Followed the nighttime resurgence of EIA the NmF2 decreased throughout the nighttime periods. A look at hmF2 on the control days indicates normal steady rise in hmF2 from 0515 and get to a peak height above 400 km within 09:00–10:45 LT. A decrease of about 90–200 km occurs after this peak, and hmF2 enters a low range of variation. However, on the eclipse day the low response of NmF2 was uplifted to the greater peak height during the eclipse phase and the highest value of electron density that follows thereafter was observed at the lower height. Similarly, the ionosphere over Townsville in Figure 4e emerged with a decrease in electron density from sunrise period increases through the eclipse phase and maximized around postnoon periods. Although, both eclipse events of 18 March 1988 and 13 November 2012 occur during MSA, but the electron density and the height of peak electron density are higher over Townsville (Figure 4e) than it appears over Manila. This may be related to the fact that both stations are from different hemisphere and the contribution of geomagnetic storm to the total solar eclipse of 13 November 2012 may influence the increased in the electron density. On the other hand, eclipse event of 21 June 2001 occur during HSA (Figure 5b) and the ionospheric F2 response was different to the one observed during MSAs. The electron density increases gradually from around 05:30 LT and get to a peak around 11:00 LT. On the eclipse day the electron density decreases during the eclipse phase compared to the continuous increase on the control days before a daytime bite-out around postnoontime period. The reduction in electron concentration that was observed on the eclipse day during the eclipse phase could be induced by the reduction in the electron production rate. These observed discernible changes in the NmF2 and the associated hmF2 during the eclipse day and the control days could be considered to be induced by the eclipse. In Figure 4b, the NmF2 increased during the eclipse period, but the magnitude is more reduced compared to that observed in Figure 4c. These changes in magnitude can be attributed to the effect of solar activity, since solar radiation is lower at low solar activity [Adeniyi et al., 2009]. The most obvious is between an interval of 10:00 LT and 18:00 LT, when the electron density increases around 16:00 LT presunset. But it is higher on control days than on the eclipse day. A critical observation also shows that a diurnal increase in NmF2 corresponds with the decrease in hmF2. The variation of electron density over the ionosphere of Kwajalein during eclipse of the Sun on 22 July 2009 was presented in Figure 4d. On the eclipse day, during the eclipse progression, there was a paucity of data. However, the existing data show a comparable variation in electron density with control days. The electron density is more reduced than it appears on the control days. From sunrise to around 12:00 LT noon the ionosphere showed an uplift of hmF2 with an enhancement of NmF2. The decrease in NmF2 coincides with increase in hmF2 at nighttime and vice versa during the respective daytime and eclipse phase. Observed the nighttime periods, the NmF2 increased from around 17:00 LT to 20:00 h LT before a considerable decrease to around 05:45 LT sunrise. These significant changes in the NmF2 and the associated hmF2 before and after an eclipse could be considered to be induced by the eclipse. Also, the ionospheric phenomena during the short nighttime are different from the normal nighttime period. 3.3. Variability in NmF2e and the Standard Deviation To add credence to the eclipse-induced effect on the ionosphere, additional plots were plotted and are shown in Figure 5. From the figure, the control days are the adjacent 10 days of noneclipse period. The control days are selected in such a way that the geomagnetic activity corresponds with the eclipse day. That is, if the observed eclipse day is geomagnetic quiet as explained in the next section below, the respective adjacent control days must devoid of geomagnetic activity, if otherwise, so be the control days. This is to ascertain that the real changes in NmF2 and hmF2 are related to eclipse during quiet condition or influenced by random perturbation and disturbances of magnetic activity. The plot revealed the standard deviation of the electron density, NmF2, and the plasma peak height, hmF2, variation on the control days (i.e., the adjacent 10 days of the eclipse) from the eclipse day which was represented by the error bar in the figure. The vertical error bar indicates the hourly standard deviation of the control days from the NmF2 and hmF2 variations of eclipse day. The standard deviations (vertical bars) reveal the significance of ionospheric eclipse-induced effect during the solar eclipse. While the vertical lines above and below each plot represent the deviation in electron density on the control days from the eclipse day. Therefore, if the error bar is very close to zero (i.e., there is no deviation) that indicates that there are no significant ionospheric eclipse effects; if otherwise, it is related to eclipse-induced effect.

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From Figure 5, the effect of eclipse is generally evident during the eclipse window and the remaining period of the day. During the eclipse window, one can see that the deviation is smaller at the totality period, which indicates that the large decrease in electron density does not coincide with the totality period but somewhere within the eclipse window. The observed deviation during the nighttime period is larger and smallest during the sunrise period than in the eclipse window. The reverse is the case with the hmF2. During the eclipse window, the hmF2 show a large deviation of hmF2e from the hmF2c as presented by the standard deviation plots below the electron density plot and overturned at nighttime. In other words, although same thermal and chemical processes controlled the chemistry of the ionospheric F2 layer during solar eclipse and the nighttime period, but the rate of electron density distribution is different. Since the deviation observed during the eclipse window is smaller compared to the nighttime period and varies with solar activities. And may be resulted from the transfer rate of molecular ion and loss of recombination by dissociative which solely depends on the rate at which energy is removed. 3.4. Influence of Geomagnetic Activity to Solar Eclipse It is pertinent to confirm the influence of geomagnetic activity and solar activity during eclipse to the coupling of the ionosphere and Sun-Earth. This was achieved using the interplanetary index, Ap. The Ap index is a median value of the geomagnetic activity derived from the last eight 3 h daily index of the geomagnetic activity (i.e., Ak) indexes recorded by various observatories at the end of the day. This value depends also on local conditions. With Kp they constitute the planetary indices. In a nutshell, Ap index is the linearized version of Kp index. The classification of geomagnetic activity to quiet and disturbed day using Ap and Kp indices is presented in Table 3, obtained from the Web page http://www.astrosurf.com/luxorion/qsl-perturbation5.htm. As shown in Table 4, the eclipse period is classified to geomagnetic active and quiet day and the F10.7 is the yearly average solar flux. The calculated daily average and yearly average of Ap index and solar flux index were obtained from International Service of Geomagnetic Indices (ISGI) network http://swdcwww.kugi. kyoto-u.ac.jp/. It is very evident from the table that the eclipse events of 22 July 2009, 12 November 2012, and 21 June 2001 are influenced by geomagnetic activity, which can interfere with the chemical constitution

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Table 3. Classification of Geomagnetic Activity

of the atmosphere. Together, these combine conditions can favor the increase in the electron density. The 0–7 0–1 Quiet low Ap index values for 18 March 1988 8–15 2–3 Unsettled 16–29 4 Active and 2 March 2006 rather indicate that 30–49 5 Major storm the periods are devoid of any geomag50–99 6 Major storm netic activity and that the changes in 100–400 7–9 Severe storm the chemical and thermal processes of the F2 layer during the events are induced by the eclipse related turbulence [e.g., Nayak et al., 2012]. Also, the contribution of solar activity to the ionospheric F2 layer turbulence during eclipse cannot be ruled out. During geomagnetic disturbed conditions the NmF2e was largely deviated from the control days; the deviation was smaller during geomagnetic quiet conditions. The corresponding hmF2 variation was vice versa. This follows the fact An et al. [2010] had reported that magnetic storm has a strong correlation with the solar eclipse and affected baseline solution during solar eclipse. However, the level of increase in NmF2 during high solar activity is higher than it appeared during low solar activity. This was consistent with the work of Adeniyi et al. [2007]; they compared their results, during low solar activity (LSA) year, with the one previously studied at a very close station (i.e., Ibadan) during a high solar activity (HSA) year, and affirmed the solar activity effects. Ap Index

Kp Index

Activity

4. Discussions The data present in the preceding section gave an insight description of the F2 region over the ionosphere of equatorial/low-latitude stations, the vertical plasma drift as well as the electron density variation during the day of total solar eclipses and corresponding control days of the aforementioned years at different solar epochs. Because of the combined control of the photochemical and the plasma transport processes, the eclipse effect in the F2 region is more complicated, which might be related to background peak height of the F2 layer, local time, solar activity, and magnetic dip [Le et al., 2009]. A total solar eclipse is similar to the short nighttime; therefore, the accompanying effects in both cases are normally expected to be similar. However, the observed nighttime and eclipse phase show dissimilarity in their ionospheric F2 variation. The dynamic processes during a specific solar eclipse depend substantially on the local time, geophysical, and solar variations. Furthermore, the observed different variations in the plasma drift and peak electron density parameters are suggested to be as a consequence of ionospheric photochemistry [Ambili et al., 2012], which differs from one latitude to the another. Aside these, changes in the dynamic conditions at the base of the thermosphere [Adebesin et al., 2014] and solar ionizing radiation due to the solar eclipse could also be a good factor [Müller-Wodarg et al., 1998; Le et al., 2008b; Adebesin et al., 2014]. Local time effect had also been reported to be necessary condition for such variation [e.g., Le et al., 2008b]. This study considered plasma vertical drift as one of the essential factors responsible for ionospheric F2 redistribution on the day of eclipse and noneclipse days and occurrence of equatorial spread F (ESF) in particular. Over equatorial/low-latitude ionosphere, the plasma vertical drift and electric fields are controlled by the complex E and F region electrodynamics processes (the vertical drift is upward during the day if the equatorial electric fields are eastward and downward during the nighttime as its movement is westward on the ionosphere). In fact, the electron density distribution over Table 4. Geomagnetic and Solar Activity of the Day and Year of the the entire low-latitude region is cona Total Solar Eclipse trolled by such processes (EIA and ESF) Eclipse Period over the dip equator. As a result of high DD/MM/YY Ap (nT) F10.7 (sfu) electrical conductivity of the equatorial ionosphere, the electric field variations 18/03/1988 7 116.1 06/21/2001 13 188.4 near the magnetic equator are very sen29/03/2006 5 81.5 sitive to the changes in the electric field 22/07/2009 24 143.1 imposed on it and any change in electric 12/11/2012 19 113.1 field of the equatorial ionosphere will a 22 2 1 Note that 1 solar flux unit (sfu) = 10 Wm Hz . have an immediate effect on the plasma

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drift in the ionosphere [Adebesin et al., 2013a]. According to Rishbeth [1968] the electron density changes during an eclipse must modify the electrical conductivity of the ionosphere. This may affect the distribution of electric field and must certainly alter the electric currents which flow mainly in the E layer. With the help of the magnetic field lines away from the equator pass through the E region, then arch up into the F region, the dynamo E region field in these off-equator regions maps up along the magnetic field into the F region. Considering the continuity equation, electron density is a function of the balance between the production and loss processes as well as the transport process. Therefore, when there is no radiation influx for ionization, the transport process, which is mainly governed by the E × B drift, becomes the dominant process [e.g., Adeniyi et al., 2007]. As observed in the last section, the equatorial/low-latitude ionosphere during a solar eclipse is significantly affected by the E × B vertical drift, because the large depletion of electron density at low altitudes can be transported to high altitudes through the plasma vertical drift. The short nighttime during the daytime cannot change the morphology of vertical plasma drift. However, the bottomside ionospheric eclipse response was associated with the change in ionospheric profile that was driven by the magnetic equatorial E × B vertical drift and redistribution. That is, the variation in F2 layer is dominated by the vertical E × B and is affected by production and recombination [e.g., Chuo, 2013]. Then the large depression in number density of electrons in the equatorial region would reduce the plasma diffusion flux reaching the equatorial ionization anomaly (EIA) region along magnetic field line and hence affect the ionosphere [Le et al., 2009]. Observation shows that prereversal enhancement (PRE) is one of the major features of nighttime ionospheric F2 phenomenon that is basically responsible for the large uplift of the F layer and evening time resurgence of the EIA. This is a phenomenon that was measured with an enhancement of vertical drift at sunset and the activation of F region dynamo in the evening hour [Martinis et al., 2005; Sreeja et al., 2009; Adebesin et al., 2013a]. The vertical drift increases, reached a maximum peak at the start of eclipse, and decreases downward through the eclipse window close to zero drift thereafter. This is normal feature of vertical drift, since the eclipse occurs during the daytime period; the vertical plasma drift is expected to be upward around the period. The plasma instability was uplifted to the peak height at the PRE. The notable point is that the vertical drift is higher on the eclipse day than the control days during the eclipse window and overturned at PRE period. This morphology was feat to the hmF2 variation. During the eclipse window, the electron drift was higher on eclipse day than control days and overturned at sunset period. This result was consistent with the work of Le et al. [2009], since the low variation of electron density was transported to the greater peak height through the plasma vertical drift. Furthermore, since the rate of loss of electrons by recombination increases during the eclipse, the apparent upward movement of the equatorial F layer during the eclipse window can be linked to the contributions of eastward electric field and recombination of molecular species at the bottom side of the F layer, whereas apparent downward movement is solely controlled by electric field. The quantitative magnitude of the variation between plasma drift and electron density is highlighted in Table 5. Here the peak values of the parameters few minutes before the eclipse maximum magnitude for both eclipse days and control days were presented. The table revealed that the drift response around this period was higher on the eclipse day than during the control day. However, in Figure 3, during the eclipse window the vertical drift increased and reached peak values before the appearance of the maximum magnitude of the eclipses and thereafter reverses downward below the zero line marks prior to the maximum decrease in the electron density. That is, the decrease in electron density during the eclipse window was due to downward (westward movement of electric field) movement of plasma Table 5. The Peak Response of NmF2 and Vertical Drift During the vertical drift. This compliment the fact Eclipse Window for Both Eclipse and Control Days that ionospheric behavior at this region 12 2 NmF2 × 10 (e/m ) Vertical Drift, Vz (m/s) during solar eclipse window is controlled by plasma vertical drift and transport Station e c e c process, which was similar to the sunset Manila (18031988) 0.93 1.20 28.94 20.86 period (PRE period). Ascen Is. (21062001) Ascen. Is. (29032006) Kwajalein (22072009) Townsville (12112012)

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It is worth mentioning that conditions during a solar eclipse are almost similar to conditions during the sunset period,

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and in most occasions more turbulent. In this view the drift calculated from the time rate of change of the height of the peak electron density (hmF2) in this work do have some limitations (or error) in inferring it. Adeniyi et al. [2014] and Adebesin et al. [2015] had analyzed and reported some of the limitations attached to plasma drifts inferred from F2 layer height profile. Bittencourt and Abdu [1981] had reported that the vertical plasma drifts inferred from ground-based observations of hmF2 can only be equivalent to the E × B drift when hmF2 is well above the 300 km threshold value. The hmF2 height profile recorded during the eclipse period in this work has hmF2 greater than 300 km during the eclipse window for Manila (in 18 March 1988) and Townsville in (13 November 2012) and hmF2 less than 300 km for Ascension Island (in 21 June 2001 and 29 March 2006) total solar eclipses. However, Adebesin et al. [2013c] and Risbeth [1981] had suggested that although the 300 km threshold value suggested by Bittencourt and Abdu [1981] can be necessary condition for equating the vertical plasma drift with the equatorial E × B drift, but is not a sufficient condition. Consequently, the drift velocities obtained in the work are a near accurate representation of the E × B force and can be regarded as an apparent value for representing the E × B. The only issue to the drift inferred from hmF2 < 300 km condition is that the plasma drift may be underestimated [e.g., Bittencourt and Abdu, 1981], but this does not mean that it cannot be used or relevant. The diurnal variations of NmF2 and hmF2 are presented in Figures 3 and 4, during the equatorial ionization anomaly, EIA, buildup (05:00–09:00 LT), daytime (09:00–18:00 LT), nighttime (18:00–06:00 LT), and eclipse phase, respectively. The eclipse effect is apparent in variation of electron density, whose exhibits difference peaks magnitude at difference solar epoch. Followed the concurrent decrease in plasma vertical drift after the eclipse start time during the eclipse phase, the NmF2 decreased and reach a minimum peak few minutes later. The decrease in electron density is corresponding to the increase in the peak height. The NmF2 on the eclipse day is more decrease than it appears during the control days. This signified the induced effect of eclipse, which occurs as a result of reduction in photochemical processes. However, the ionospheric F2 eclipse effect is solar epoch dependent. During low solar activity (LSA), the NmF2 variation is lower, followed by MSA and highest during HSA year. However, after the end of the eclipse the electron densities at peak heights in the EIA region recover with time and recorded a peak NmF2 values at presunset periods with maximum magnitude on control days. Note the concurrent decrease in hmF2 around sunset period, this corresponds with the observed PRE peak. Followed the presunset increase of NmF2, the nighttime was considerably decreased in both hemispheres except for Manila during 1988 total solar eclipse. For clarity of the eclipse effect on the ionosphere, we highlighted in Table 6 the NmF2 and the hmF2 variations during the eclipse phase, nighttime and daytime. The minimum peak values of response of the parameters during different periods were recorded for the respective eclipse day, and the control days and the daytime are the respective period of the day that were unobscured by the Moon. As observed from the table, the NmF2 variation is more reduced on the eclipse day than on the control days and their associated hmF2 were vice versa. On the eclipse day, although, a solar eclipse represents a short nighttime, but the NmF2 variations are not similar. The low value of NmF2 during the eclipse phase was uplift to the greater peak height than the nighttime during the MSA years and was overturned during HSA. The decrease observed in NmF2 during the window of eclipse will be produced if the hmF2 is higher and the delay in the minimum electron density response relative to maximum phase of eclipse depends on the local time. Also, the electron density during the eclipse window increases with an increase in the delay time. The observed delay time of minimum

Table 6. Comparisons of the NmF2 and hmF2 Variations on the Eclipse Day, e and the Average Noneclipse Days, c Eclipse Phase Station and Date of Eclipse Event (ddmmyy) Manila (18031988) Townsville (13112012) Ascen. Island (21062001) Ascen. Island (29032006) Kwajalein (22072009)

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decrease in electron density ranges between 5 and 60 min. This time-lag difference is consistent with the work of Adeniyi et al. [2007]. They observed a time delay of 1 h 20 min of minimum decrease of foF2 after the totality over Ilorin, a low-latitude station. This time lag was also reported to result from the sudden reduction in temperature, which subsequently reduces the rate of transport process [e.g., Chen et al., 1999], as it is sensitive to the temperature [see Kumar et al., 2013]. Whereas, Le et al. [2008a, 2008b] and Chen et al. [2011] had reported that the time lag increases with an increase in the F2 altitude. However, the time delay in minimum decrease observed by Kumar et al. [2013] during solar eclipse window is more reduced (i.e., 2–15 min) compared to the present work. This may result from the fact that the peak electron density during a solar eclipse is under the influence of production, loss, and transport process, while TEC is dominated by the electron density profile above the peak, this make both NmF2 and TEC vary with different speeds that resulted in the time lag difference. During a solar eclipse, the loss rate of electron obviously depends on the number of gas particle encountered as a result of loss in photoionization. The inflow photoionization in the region of eclipse is reduced at totality; this as well reduces the production rate and subsequently causes a large decrease in electron temperature in this region at various heights in the ionosphere. This consequently resulted in the decrease in electron density at the eclipse region. The difference in nighttime/eclipses period variation of NmF2 may be attributed to the differences in solar epoch [e.g., Le et al., 2008a, 2008b]. The eclipse effect in the F2 region is larger at high solar activity than it appeared at low solar activity [e.g., Nayak et al., 2012]. The NmF2 decrease during the eclipse may be connected with the loss in ionization processes which perhaps resulted from the decrease in the rate of production of electron density. The decrease in NmF2 during the eclipse period was accompanied by a rapid increase in hmF2. This result was consistent with the work of Adeniyi et al. [2007] and Le et al. [2009], who reported that considerable enhancement in the hmF2 is expected on the eclipse day (during the maximum magnitude) as compared to the control days. Moreover, it is very difficult to separate the influence of magnetic storm from solar eclipse [e.g., An et al., 2010]. The eclipse events of 21 June 2001, 22 July 2009, and 13 November 2012 occur during magnetic disturbed days, which consequently enhanced the ionospheric response significantly during the eclipse phase, on the eclipse day. During magnetic quiet days the ionospheric F2 layer variation shows a low deviation compared to control days. However, the percentage of NmF2 increase is higher during HSA and lower during LSA, which confirmed the solar epoch dependence.

5. Summary and Conclusion The response of the vertical plasma drift and the electron density, NmF2, during different solar eclipses was investigated. Diurnal values of the direct scaled measurement of F2 peak height and the one derived from the maximum usable frequency that refracted in the ionosphere at an altitude of 3000 km, acquired from an equatorial/low-latitude stations, have been used to determine the vertical plasma drift. Both the direct and the derived hmF2 values represent the hmF2 morphology well. Vertical drift is one of the essential factors responsible for ionospheric F2 redistribution during and before/after solar eclipses and occurrence of ESF in particular. It was suggested that equatorial/low-latitude ionosphere during a solar eclipse is significantly affected by the E × B vertical drift, because larger depletion of electron density at low altitudes can be transported to high altitudes through the plasma vertical drift. However, before the eclipse the uplift of electrodynamics processes is mainly caused by the vertical drift. Also, the variation in the F2 layer is dominated by the vertical E × B and is affected by recombination and production. The ionospheric F2 region processes during eclipse window adjust itself to partial nighttime mode, where molecular ions (N2+ and O2+) predominate. During the daytime, the dynamo electric fields are eastward, which causes an upward E × B plasma drift, while the reverse occurs at night. The loss in ionization processes, which perhaps resulted from the decrease in production rate during an eclipse, causes the decreases in the electron density, NmF2. This decrease was accompanied by rapid increase in hmF2. The NmF2 on the eclipse day is more decrease than it appears during the control days, and the deviation is larger during magnetic disturbed day than magnetic quiet days. Conversely, the associated hmF2 was vice versa. This signified the induced effect of eclipse; the radiation that could ionize the dense neutral particle in the thermosphere is either partly or wholly blocked, consequently reduced the photochemical processes. The rate of reduction is higher during the quiet days than the disturbed days. The difference in

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the NmF2 variation during nighttime/eclipse phase may be attributed to the chemical and thermal processes explained above. Solar eclipse represents a short nighttime, but the NmF2 variations are not similar. The low value of NmF2 during eclipse phase was uplift to the greater peak height than the nighttime during the MSA years and was overturned during HSA. In general, the NmF2/hmF2 increases/decreases as the solar activity increases. The decreased in electron density during eclipse window increases with time lag. However, the maximum eclipse effect did not occur during the maximum obscuration but somewhat later [e.g., Chuo, 2013]. These observed ionospheric effects may be related to the eclipse-caused dynamic. Lastly, the influence of geomagnetic activity and solar activity cannot be ruled out when considering ionospheric related turbulence, which solar eclipse happen to be one. Also, physical mechanism may be responsible for these observed effects, which are related to eclipse-caused dynamics. Acknowledgments The authors acknowledge the management team of NASA Eclipse (National Aeronautics and Space Administration) service (http://eclipse.gsfc.nasa.gov) for making available the information about the eclipse progression on the view of the Earth. We are also grateful to the working team of National Geophysical Data Center’s SPIDR (Space Physics Interactive Data Resource) network’s (http://spidr.ngdc.noaa.gov), the Global ionospheric Radio Observatory (GIRO) network’s with Web portal access at http://ulcar.uml.edu/DIDBase/, and International Service of Geomagnetic Indeces (ISGI) network http://swdcwww. kugi.kyoto-u.ac.jp/. Also, the solar eclipse calculator from the Web address http://xjubier.free.fr/en/site_pages/ SolarEclipseCalc_Diagram.html is very helpful. One of the authors (Adekoya, B.J.) sincerely appreciates Adebesin, Babatunde O. for his contribution, suggestion, and preview of the manuscript. Alan Rodger thanks the reviewers for their assistance in evaluating this paper.

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References Abidin, A. R. Z., M. M. Awad, and S. Sulaiman (2006), GPS ionospheric TEC measurements during the 23rd November 2003 total solar eclipse at Scott Base Antarctica, J. Atmos. Sol. Terr. Phys., 68, 1219–1236. Adebesin, B. O., J. O. Adeniyi, I. A. Adimula, and B. W. Reinisch (2013a), Equatorial vertical plasma drift velocities and electron densities inferred from ground-based ionosonde measurements during low solar activity, J. Atmos. Sol. Terr. Phys., 97, 58–64, doi:10.1016/ j.jastp.2013.02.010. Adebesin, B. O., S. O. Ikubanni, S. J. Adebiyi, and B. W. Joshua (2013b), Multi-station observation of ionospheric disturbance of March 9, 2012 and comparison with IRI-model, Adv. Space Res., 52, 604–613. Adebesin, B. O., J. O. Adeniyi, I. A. Adimula, and B. W. Reinisch (2013c), Low latitude nighttime ionospheric vertical E×B drifts at African region, Adv. Space Res., 52, 2226–2237, doi:10.1016/j.asr.2013.09.033. Adebesin, B. O., B. J. Adekoya, S. O. Ikubanni, S. J. Adebiyi, O. A. Adebesin, B. W. Joshua, and K. O. Olonade (2014), Ionospheric foF2 morphology and response of F2 layer height over Jicamarca during different solar epochs and comparison with IRI-2012 model, J. Earth Syst. Sci., 123(4), 751–765. Adebesin, B. O., J. O. Adeniyi, I. A. Adimula, O. A. Oladipo, A. O. Olawepo, and B. W. Reinisch (2015), Comparative analysis of nocturnal vertical plasma drift velocities inferred from ground-based ionosonde measurements of hmF2 and h′F, J. Atmos. Sol. Terr. Phys., 122, 97–107. Adekoya, B. J., and V. U. Chukwuma (2012), Response of the mid-latitude ionospheric F2 to total solar eclipses of solar cycle 23, Indian J. Radio Sci. Space Phys., 41, 594–605. Adeniyi, J. O., S. M. Radicella, I. A. Adimula, A. A. Willoughby, O. A. Oladipo, and O. Olawepo (2007), Signature of the 29 March 2006 eclipse on the ionosphere over an equatorial station, J. Geophys. Res., 112, A06314, doi:10.1029/2006JA012197. Adeniyi, J. O., O. A. Oladipo, S. M. Radicella, I. A. Adimula, and A. O. Olawepo (2009), Analysis on 29 March 2006 eclipse effect on the ionosphere over Ilorin, Nigeria, J. Geophys. Res., 114, A11303, doi:10.1029/2009JA014416. Adeniyi, J. O., B. O. Adebesin, I. A. Adimula, O. A. Oladipo, A. O. Olawepo, S. O. Ikubanni, and B. W. Reinisch (2014), Comparison between African equatorial station ground-based inferred vertical E×B drift, Jicamarca direct measured drift, and IRI model, Adv. Space Res., 54(8), 1629–1641. Afraimovich, E. L., E. A. Kosogorov, and O. S. Lesyuta (2002), Effects of the August 11, 1999 total solar eclipse as deduced from total electron content measurement at the GPS network, J. Atmos. Solar Terr. Phys., 64, 1933–1941. Ambili, K. M., J.-P. St-Maurice, and R. K. Choudhary (2012), On the sunrise oscillation of the F region in the equatorial ionosphere, Geophys. Res. Lett., 39, L16102, doi:10.1029/2012GL052876. An, J. C., Z. M. Wang, E. Dong-Chen, and W. Sun (2010), Ionospheric behavior during the solar eclipse of 22 July 2009 and its effect on positioning, Chinese J. Geophys., 53(5), 731–739. Batista, I. S., E. R. de Paula, M. A. Abdu, and I. J. Kantor (1991), Comparative study of ionograms F2 peak height from different techniques, Geofis. Int., 30(4), 249–252. Bhargava, B. N., and R. V. Subrahmanyam (1960), Movements in the F region of the ionosphere during solar eclipses, Indian J. Meteorol. Geophys., 11(4), 363–370. Bilitza, D., N. M. Sheikh, and R. Eyfrig (1979), A global model for the height of the F2-peak using M3000 values from the CCIR numerical maps, Telecomunication J., 46(9), 549–553. Bittencourt, J. A., and M. A. Abdu (1981), A theoretical comparison between apparent and real vertical ionization drift velocities in the equatorial F region, J. Geophys. Res., 86, 2451–2454, doi:10.1029/JA086iA04p02451. Chen, A.-H., S.-B. Yu, and J.-S. Xu (1999), Ionospheric response to a total solar eclipse deduced by the GPS beacon observations, Wuhan Univ. J. Nat. Sci., 4, 439–444. Chen, G., Z. Zhao, B. Ning, Z. Deng, G. Yang, C. Zhou, M. Yao, S. Li, and N. Li (2011), Latitudinal dependence of the ionospheric response to solar eclipse of 15 January 2010, J. Geophys. Res., 116, A06301, doi:10.1029/2010JA016305. Chuo, Y. J. (2013), Ionospheric effects on the F region during the sunrise for the annular solar eclipse over Taiwan on 21 May 2012, Ann. Geophys., 31, 1891–1898. Cohen, E. A. (1984), The study of the effect of solar eclipses on the ionosphere based on satellite beacon observations, Radio Sci., 9(3), 769–777, doi:10.1029/RS019i003p00769. Dudeney, J. R. (1983), The accuracy of simple methods for determining the height of the maximum electron concentration of the F2-layer from scaled characteristics, J. Atmos. Terr. Phys., 45, 629–640. Fejer, B. G. (1997), The electrodynamics of the low-latitude ionosphere: Recent results and future challenges, J. Atmos. Sol. Terr. Phys., 59, 1465–1482. Fejer, B. G. (2011), Low latitude ionospheric electrodynamics, Space Sci. Rev., 158, 145–166. Jakowski, N., et al. (2008), Ionospheric behavior over Europe during the solar eclipse of 3 October 2005, J. Atmos. Sol. Terr. Phys., 70, 836–853. Kumar, S., A. K. Singh, and R. P. Singh (2013), Ionospheric response to total solar eclipse of 22 July 2009 in different Indian regions, Ann. Geophys., 31, 1549–1558.


8083


10.1002/2015JA021557

Le, H., L. Liu, X. Yue, and W. Wan (2008a), The ionospheric responses to the 11 August 1999 solar eclipse: Observations and modeling, Ann. Geophys., 26, 107–116. Le, H., L. Liu, X. Yue, and W. Wan (2008b), The midlatitude F2 layer during solar eclipses: Observations and modeling, J. Geophys. Res., 113, A08309, doi:10.1029/2007JA013012. Le, H., L. Liu, X. Yue, W. Wan, and B. Ning (2009), Latitudinal dependence of the ionospheric response to solar eclipse, J. Geophys. Res., 114, A07308, doi:10.1029/2009JA014072. Martinis, C. R., M. J. Mendillo, and J. Aarons (2005), Toward a synthesis of equatorial spread F onset and suppression during geomagnetic storms, J. Geophys. Res., 110, 7306–7319, doi:10.1029/2003JA010362. McNamara, L. F. (2008), Accuracy of models of hmF2 used for long-term trend analyses, Radio Sci., 43, RS2002, doi:10.1029/2007RS003740. Mielich, J., and J. Bremer (2013), Long trends in the ionospheric F2 region with different solar activity indices, Ann. Geophys., 31, 291–303. Müller-Wodarg, I. C. F., A. D. Aylward, and M. Lockwood (1998), Effects of a mid-latitude solar eclipse on the thermosphere and ionosphere-A modelling study, Geophys. Res. Lett., 25(20), 3787–3790, doi:10.1029/1998GL900045. Nayak, C. K., D. Tiwari, K. Emperumal, and A. Bhattacharyya (2012), The equatorial ionospheric response over Tirunelveli to the 15 January 2010 annular solar eclipse: Observations, Ann. Geophys., 30, 1371–1377, doi:10.5194/angeo-30-1371-2012. Obrou, O. K., D. Bilitza, J. O. Adeniyi, and S. M. Radicella (2003), Equatorial F2-layer peak height and correlation with vertical ion drift and M (3000)F2, Adv. Space Res., 31(3), 513–520. Oyekola, O. S., and P. R. Fagundes (2012), Equatorial F2-layer variations: Comparison between F2 peak parameters at Ouagadougou with the IRI-2007 model, Earth Planets Space, 64, 553–566. Oyekola, O. S., and L. B. Kolawole (2010), Equatorial vertical E×B drift velocities inferred from ionosonde measurements over Ouagadougou and the IRI-2007 vertical ion drift model, Adv. Space Res., 46, 604–612. Paul, A. T., S. Das, A. Ray, D. B. Das, and A. DasGupta (2011), Response of the equatorial ionosphere to the total solar eclipse of 22 July 2009 and annular eclipse of 15 January 2010 as observed from a network of stations situated in the Indian longitude sector, Ann. Geophys., 29, 1955–1965. Reinisch, B. W., and I. A. Galkin (2011), Globa Ionospheric Radio Observatory (GIRO), Earth Planets Space, 63, 377–381, doi:10.5047/ eps.2011.03.001. Risbeth, H. (1981), The F-region dynamo, J. Atmos. Terr. Phys., 43, 387–392. Rishbeth, H. (1968), Solar eclipses and ionospheric theory, Space Sci. Rev., 8, 543–554. Rishbeth, H., K. J. F. Sedgemore-Schlthess, and T. Ulich (2000), Semiannual and annual variations in the height of the ionospheric F2-peak, Ann. Geophys., 18, 285–299. Shimazaki, T. (1955), World-wide daily variation in the height of the maximum electron density of the ionospheric F2-layer, Japan Radio Res. Lab., 2, 85–97. Sreeja, V., S. R. Devasia, and K. P. Tarun (2009), Observational evidence for the plausible linkage of equatorial electrojet (EEJ) electric field variations with the post sunset F-region electrodynamics, Ann. Geophys., 27, 4229–4238. Tomás, A. T., H. Lühr, and M. Rother (2009), Mid-latitude solar eclipses and their influence on ionospheric current systems, Ann. Geophys., 27, 4449–4461. [Available at www.ann-geophys.net/27/4449/2009/.] Wang, X., J. J. Berthelier, and J. P. Lebreton (2010), Ionosphere variations at 700 km altitude observed by the DEMETER satellite during the 29 March 2006 solar eclipse, J. Geophys. Res., 115, A11312, doi:10.1029/2010JA015497.

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