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NmF2 and hmF2 measurements at 95° E and 127° E around the EIA northern crest during 2010–2014
© Kalita et al. 2015
- Received: 14 May 2015
- Accepted: 5 November 2015
- Published: 19 November 2015
The characteristics of the F2 layer parameters NmF2 and hmF2 over Dibrugarh (27.5° N, 95° E, 17° N geomagnetic, 43° dip) measured by a Canadian Advanced Digital Ionosonde (CADI) for the period of August 2010 to July 2014 are reported for the first time from this low mid-latitude station lying within the daytime peak of the longitudinal wave number 4 structure of equatorial anomaly (EIA) around the northern edge of anomaly crest. Equinoctial asymmetry is clearly observed at all solar activity levels whereas the midday winter anomaly is observed only during high solar activity years and disappears during the temporary dip in solar activity in 2013 but forenoon winter anomaly can be observed even at moderate solar activity. The NmF2/hmF2 variations over Dibrugarh are compared with that of Okinawa (26.5° N, 127° E, 17° N geomagnetic), and the eastward propagation speed of the wave number 4 longitudinal structure from 95° E to 127° E is estimated. The speed is found to be close to the theoretical speed of the wave number 4 (WN4) structure. The correlation of daily NmF2 over Dibrugarh and Okinawa with solar activity exhibits diurnal and seasonal variations. The highest correlation in daytime is observed during the forenoon hours in equinox. The correlation of daily NmF2 (linear or non-linear) with solar activity exhibits diurnal variation. A tendency for amplification with solar activity is observed in the forenoon and late evening period of March equinox and the postsunset period of December solstice. NmF2 saturation effect is observed only in the midday period of equinox. Non-linear variation of neutral composition at higher altitudes and variation of recombination rates with solar activity via temperature dependence may be related to the non-linear trend. The noon time maximum NmF2 over Dibrugarh exhibits better correlation with equatorial electrojet (EEJ) than with solar activity and, therefore, new low-latitude NmF2 index is proposed taking both solar activity and EEJ strength into account.
- WN4 structure
- Solar activity
The earth’s ionosphere is formed due to photoionization of neutral atmosphere by solar radiation and exhibits latitudinal, longitudinal, altitudinal as well as diurnal and seasonal variations. The behavior of the ionosphere depends on the structure of geomagnetic field and geomagnetic activity in addition to the solar flux (Field and Rishbeth 1997; Buonsanto 1999; Danilov and Lastovicka 2001; Simi et al. 2013, etc). The distribution of ionization in low-latitude ionosphere particularly the F2 layer is affected by the equatorial ionization anomaly (Appleton 1946). The characteristics of the ionosphere also changes with longitude (Bailey 1948 ; Thomas 1968), and there are important differences in Asian, African, and American zones (Rao 1963a). Walker (1981) reported significant longitudinal variation using only ionosonde data. Deminova (1993, 1995) first reported a wavelike longitudinal structure of the critical frequency of the F2 layer both along the equator and the anomaly crest using data from the Intercosmos II satellite. Later, Sagawa et al. (2005) reported wavelike structure in the development of EIA during March–June of 2002, at nighttime by analyzing the OI 135.6-nm nightglow from far ultraviolet imager (FUV) on board the IMAGE satellites and thereby suggested a longitudinal structure with wave number 4 (WN4) or 90° periodicity. England et al. (2006) have shown from CHAMP, Ørsted, and SAC-C satellite observation the existence of longitudinal structure in noon time equatorial electrojet (EEJ) similar to nighttime EIA structure. Immel et al. (2006) and Lin et al. (2007a) have postulated that atmospheric tides are the source of the WN4 phenomenon. Using TOPEX Total Electron Content (TEC) data, Scherliess et al. (2008) have found that the WN4 structure is created in low-latitude TEC in equinox and June solstice with enhancement along the 100° E, 190° E, 270° E, and 10° E. Lühr et al. (2007) have reported a four-peaked longitudinal structure in electron density and zonal wind measured by the CHAMP satellite at 400 km. Kil et al. (2007) and Oh et al. (2008) reported the existence of a structure of four peaks of enhanced plasma density along the equatorial anomaly even at the topside of F region near 10° E, 100° E, 200° E, and 280°E and showed its relation to the vertical E × B drift. Liu and Watanabe (2008) from CHAMP satellite data have observed significant seasonal variation of the longitudinal plasma density structure at fixed solar activity levels. It has also been shown (Sagawa et al. 2005; Immel et al. 2006; Oh et al. 2008) that along these four high-density regions, the anomaly crest moves poleward to higher magnetic latitude. Using FORMOSAT-3/COSMIC (F3/C) satellite constellation, Lin et al. (2007b) have shown that the four-peaked EIA longitudinal structure evolve with local time and tend to move eastward with velocity of several tens of meters per second. The eastward shifting of the location of the daytime peak of the WN4 structure by around 20° was also reported by Liu and Watanabe (2008), Scherliess et al. (2008) and Fang et al. (2009). In particular, the daytime peak along 90–100° E moves to around 120° E by nighttime. The longitudinal WN4 structure also exhibits some altitudinal difference (Kil et al. 2007) particularly at nighttime. The longitudinal structure of plasma density has been detected by different techniques and in different altitude. The optical observations (Sagawa et al. 2005; Henderson et al. 2005; Immel et al. 2006) provide information of the density near the F region. The measurements by ROCSAT-1 (Kil et al. 2007; Kil et al. 2008; Oh et al. 2008; Fang et al. 2009) basically give a picture of the topside density structure. The TEC measurements (Scherliess et al. 2008) and the radio occultation measurements (Lin et al. 2007a, 2007b) are altitude independent. Recently, Bhuyan and Hazarika (2012) and Watthanasangmechai et al. (2015) have reported the characteristics of TEC along 95–100° E which is within the most prominent peak of the WN4 structure at 100° E during the ascending phase of solar cycle 2009–2012, but no bottom side (ionosonde) measurements have been reported from this region yet. Satellite- and ground-based methods are complementary in many respects (Booker and Smith 1970) and ground-based ionosondes provide valuable information of vertical ionospheric structure. Lin et al. (2007a) have shown that the wave number 4 longitudinal structure mainly exists in the F region of the ionosphere or above 250–300 km, and therefore, measurements of the F layer by the ionosonde in these sectors should be useful addition to the existing information. In the Indian sector, Kolkata (22.56° N, 88.36° E) was the easternmost ionosonde station nearest to the region of enhanced plasma density which operated till 1976. In the NGDC data base (http://spidr.ngdc.noaa.gov/) between Kolkata and Hainan (19.4° N, 109° E) (Singapore at 103° E, 1.3° N is at the equator), there is no ionosonde station in low-latitude region. Wichaipanich et al. (2012) reported the diurnal and seasonal variations of NmF2 in the low-latitude region over Southeast Asia. Therefore, an ionosonde in the low-latitude region of 90–100° E longitude would provide valuable measurements characterizing the most prominent peak of the longitudinal WN4 structure. The solar activity variation of low-latitude F2 layer density may get modulated by the solar activity variation of the mechanism that creates the WN4 structure. The solar cycle variation of the monthly median NmF2 has been studied extensively (Kane 1992; Sethi et al. 2002; Liu et al. 2003, 2004; Yadav et al. 2011; Liu et al. 2012, etc.) at different longitudes. Solar activity variations of daily averaged noontime NmF2 or NmF2 for multiple stations (Liu et al. 2006) or single station (Cardoso et al. 2011) have also been reported. Chen and Liu (2010) found that the rate of linear increase of monthly mean NmF2 with F10.7 cm solar flux shows remarkable dependency on latitude, season, and local time at low solar activity levels. Walker and Ma (1972) have studied the influence of the solar flux and the equatorial electrojet on the diurnal development of the latitude distribution of the total electron content in the equatorial anomaly and reported positive correlation of electron content under the crest with electrojet. The strength of the EEJ influences the equatorial fountain and the amount and latitudinal extent of ionization transported from the equator to the lower latitudes. Rush and Richmond (1973) have studied the relationship between equatorial anomaly and the strength of the EEJ. The eastward equatorial electric field is related to the strength of EEJ (Alken et al. 2013). Deshpande et al. (1977) have reported the effect of electrojet on the total electron content of the ionosphere over the Indian subcontinent and demonstrated the association between the diurnal development of the equatorial anomaly in TEC and the electrojet strength. Earlier workers have reported positive correlation of NmF2 with EEJ in low-latitude stations in the Indian zone (Dabas et al. 1984; Dabas et al. 2006). Liu et al (2011) studied the relation of equatorial mass anomaly and EIA in the postmidnight period. Adebesin et al. (2013) studied NmF2 and hmF2 variations with electrojet over an African equatorial station. The four peaked wavelike structure in noon time electrojet (England et al. 2006) may cause longitudinal difference in the response of the low-latitude ionosphere to electrojet. Hence, the diurnal variation of solar activity effect on daily NmF2 near the crests of the WN4 structure and the variation of the same with longitude in the context of the movement of the WN4 structure needs to be investigated. The solar cycle 24 is different from previous solar cycles like 23 or 22 in terms of level of solar activity or sunspot number and the representation in terms of solar proxies (Chen et al. 2011). During the deep solar minimum of 2007–2008, no sunspots were recorded for 266 consecutive days. The solar activity picked up very slowly in 2009–2010 and seemed to peak in June solstice of 2012 (F10.7 ~ 183). The solar activity again picked up in December solstice of 2013 to form the maximum (F10.7 ~ 253 sfu) in January 2014. The double-peak structure observed in cycle 23 (Kane 2006) is repeated in cycle 24 with much lower solar activity levels. Even at the maximum, the solar activity of cycle 24 is much less than previous cycles like cycle 23 when the highest F10.7 flux was 325 sfu. Therefore, the investigation of solar activity variation of F2 layer parameters in solar cycle 24 may reveal new features.
In this work, we report the first time ionosonde measurements from Dibrugarh (27.5° N, 95° E, 17° N geomagnetic, 43° dip) which lies in the same longitude as the strongest daytime longitudinal enhancement (90–100° E) in density, EEJ, and vertical E × B drift, etc. (Kil et al. 2008; Lühr et al. 2008) on the northern outer edge of the equatorial anomaly. A comparison of Dibrugarh NmF2 and hmF2 data is made with that of Okinawa (26.8° N, 127° E, 17° N geomagnetic) which lies in the same geomagnetic latitude but close to the longitude of the nighttime peak in WN4 (120° E). The propagation velocity of the WN4 structure is inferred from the local time difference in the magnitude of NmF2 over Dibrugarh and Okinawa. The diurnal variation of solar activity effect on daily NmF2 and hmF2 measured over Dibrugarh and Okinawa from August 2010 till July 2014 and its seasonal variation is studied using solar activity proxies. The widely reported non-linear variation of NmF2 with solar activity (Bhuyan et al. 1983; Balan et al. 1993, 1994; Liu et al. 2006, 2007a) is shown to exhibit a diurnal variation. The correlation of Dibrugarh NmF2 and hmF2 with solar activity as well as EEJ is compared with that of Okinawa. The effect of EEJ on daily NmF2/hmF2 variations and its importance relative to solar activity is investigated. A new composite index of solar and electrodynamical activity for the low-latitude region is proposed. This index is a weighted mean of daily F10.7P and the maximum EEJ strength. The midday maximum NmF2 over Dibrugarh and Okinawa are shown to exhibit better correlation with this index than with F10.7P or EEJ. This work and the data should provide a better understanding of the physics of the low mid-latitude F2 region ionosphere along 90°E–120° E longitude.
Canadian Advanced Digital Ionosonde (CADI) was installed in Dibrugarh in July 2010. CADI is a modern low-power digital ionosonde of peak power of around 650 W and frequency range of 1–20 MHz. The transmitter antenna is an inverted delta type. The length of the antenna is around 59 m, and it is supported on a 15-m-high tower. The receiver antenna system consists of four dipole receivers of 30 m each arranged in rectangular geometry. The operation of the ionosonde is automated and the ionograms every 10/15 min are recorded in 24 × 7 mode.
where H eq is the horizontal component of the earth’s magnetic field at the equatorial station and H off equator is the H component at an off-equatorial station. The station pair for the Indian zone is Tirunelveli (0°)-Alibag (23° N), and the data period is August 2010–December 2012. The station pair for Okinawa is Davo (1° S)-Muntinlupa (7° N), and the data period is August 2010–April 2014. The neutral parameters and ion temperature are obtained from the NRLMSISE and IRI models, respectively.
Diurnal and seasonal variations of NmF2
Diurnal and seasonal variations of NmF2 over Okinawa
The longitudinal WN4 structure peak in electron density maximizes around 90°–100° E during daytime and around 120° E at nighttime. The WN4 structure is postulated to be related to the non-migrating eastward propagating diurnal tides (Immel et al. 2006) as it evolves with local time and moves eastward (Lin et al 2007b; Scherliess et al. 2008; Fang et al. 2009). Therefore, comparison of simultaneous ground measurements (NmF2/hmF2) in these longitude sectors (90–120° E) might complement the earlier observation from satellites and reveal new features. The pair of ionosonde stations at Dibrugarh (27.5° N, 95° E, 17° N geomagnetic) and Okinawa (26.5° N, 127° E, 17° N geomagnetic) provide the opportunity of comparing the NmF2 at these longitudes as they are virtually at the daytime and nighttime peak of the WN4 structure and also at the same geomagnetic latitude. Simultaneous measurements at the two stations are available from 2010 to 2014.
The variation of NmF2 with solar activity
Here, N is hourly NmF2, x is solar EUV flux, and a 2 and a 1 are the first- and second-order coefficients. Improvement in correlation is observed depending on local time and season. The correlation of maximum daytime NmF2 over Dibrugarh with F10.7P is poor (r < 0.5) even in the case of second-order fitting, and the saturation effect is very weak (not shown). Bhuyan and Hazarika (2012) observed the saturation effect in maximum daytime TEC measured over Dibrugarh with daily F10.7 cm flux. The apparent difference in the response of TEC and NmF2 could be due to three factors: (1) different solar activity index used, (2) different data period, and (3) different physical mechanism around the F2 layer peak and the topside. Bhuyan and Hazarika (2012) studied the TEC variation with daily F10.7 cm flux for the period 2009–2012 whereas in this study, we have used the modified index F10.7P and solar EUV flux measured by SOHO/SEM for the period 2010–2014. Therefore, the saturation effect observed by Bhuyan and Hazarika (2012) with F10.7 may be due to the non-linear variation of F10.7 cm flux with solar EUV flux in the high solar activity period as pointed out by Balan et al. (1994). Additionally, the TEC represents the vertically integrated density of the ionosphere which include the topside as well as the bottom side density whereas NmF2 is obtained from the measurement of the bottom side of the F2 layer. The TEC is affected by the physics of the topside where diffusion and the electrodynamics and in particular the transport due to equatorial fountain effect via the E × B drift are dominant factors in low-latitude region. The effect of solar activity on electron density varies with altitude (Su and Bailey 1999), and depending on altitude and season, the topside density can exhibit linear, saturation, and amplification trend with solar activity (Liu et al. 2007; Chen et al. 2009). NmF2 is the highest density of the F2 layer, and the altitude of the highest density hmF2 varies with local time, season, solar cycle, and magnetic activity. The variations of hmF2 affect the solar activity variations of NmF2 (Liu et al. 2006). Therefore, the response of the TEC and the NmF2 to solar activity may be different, particularly in terms of the threshold levels for onset of the saturation effect.
To interpret these local time and seasonal variation of solar activity trends, we discuss various factors influencing the electron density in low- and mid-latitude region. The daytime electron density is determined by the balance of production due to photoionization, recombination loss, and transport (Rishbeth and Garriot 1969). In addition to the effect of neutral dynamics, the transport due to equatorial fountain effect influences the midday electron density in low-latitude region near the anomaly crest. The equatorial anomaly starts to develop in the morning around 0900 LT but the crest in this period is closer to the equator (Sastri 1990). The anomaly reaches its peak strength during the postnoon period (1300–1600 LT) after a time lag of around 2–3 h from the time of the maximum electrojet strength (Rush and Richmond 1973). Therefore, the contribution of the transport process via fountain effect to the enhancement of NmF2 in the low-latitude region is less significant in the forenoon hours than in the midday hours. The E × B vertical drift which peaks around 1100 LT (Fejer et al. 1979; 2008) in the daytime low-latitude region can affect the density at a particular altitude but the daytime E × B vertical drift is almost the same for low and high solar flux conditions (Scherliess and Fejer 1999; Fejer et al. 2008). The variation of F2 layer height (hmF2) may also affect the variation trends of NmF2 with solar activity through the altitudinal dependency of solar activity effect (Su and Bailey 1999). In the forenoon period, the electron density in low mid-latitude stations like Dibrugarh/Okinawa would be mainly affected by the production-loss mechanism and the neutral dynamics. Therefore, the diurnal, seasonal, altitudinal, and solar activity variations of neutral composition, chemical reactions rates, and dynamics would affect the observed diurnal and seasonal variations of NmF2 solar activity effect.
O++N2 → NO++N
O++O2 → O2 ++O
The variation of NmF2 with equatorial electrojet strength
The diurnal and seasonal variations of hmF2 at Dibrugarh and Okinawa
The variation of hmF2 with solar activity and electrojet
NmF2 and hmF2 measured with a CADI over Dibrugarh at the northern edge of the EIA and within the daytime highest peak of the WN4 structure are studied and compared with that of Okinawa where the nighttime WN4 peak is observed. The simultaneous measurements from Dibrugarh and Okinawa are used to study the local time evolution and eastward propagation of the WN4 structure between 95° E and 127° E. The diurnal and seasonal variations of the correlation of NmF2 are suggestive of the physical mechanisms controlling the F2 layer in the low mid-latitude region which change with local time and from season to season. In the morning to forenoon period, the solar flux and neutral composition/thermospheric dynamics is more important than EEJ. Equatorial anomaly and the WN4 structure start to develop in the forenoon, and from then on, the contribution of electrodynamical process also becomes significant. The F2 layer characteristics over the two stations which are in the same geomagnetic latitude but different longitude respond differently to the solar flux and EEJ variations. The longitudinal WN4 structures play a crucial role in shaping the F2 layer characteristics in these stations.
The winter anomaly in NmF2 over Dibrugarh is absent in low solar activity levels and is manifested in moderate to high solar activity levels only.
The eastward propagation speed of the WN4 structure estimated from local time difference of NmF2 is found to be about 4.3°/h in low solar activity period and about 3.2°/h in high solar activity period.
The correlation of NmF2 with solar EUV flux/F10.7P exhibits diurnal variation with maximum in the forenoon hours. The degree of correlation of NmF2 with solar activity and its diurnal variation exhibit seasonal dependence. Correlation is highest in equinox, especially in March equinox. The forenoon NmF2 is more sensitive to solar flux variations than the midday NmF2 in low mid-latitude region.
The NmF2 in March equinox exhibits a tendency for amplification with solar activity in the forenoon and postsunset periods. The NmF2 in the midday period of equinox is more likely to saturate with solar activity. Amplification is also observed in the postsunset period of December solstice. The forenoon amplification effect may be a manifestation of unique location of the two stations and the moderate level of solar activity in the maximum of solar cycle 24. Further study involving a larger data set will probably validate these results.
The influence of EEJ in determining the midday NmF2 is relatively stronger over Dibrugarh than over Okinawa, and the result may be attributed to the longitudinal wave structure in EEJ.
A new composite weighted mean index of F10.7P and EEJ is proposed for low-latitude region. The midday maximum NmF2 is shown to exhibit better correlation with this new index.
The CADI ionosonde in Dibrugarh was installed and operated as part of Indian Space Research Organization’s Space Science Promotion Scheme. The authors would like to thank IIG, Mumbai for the magnetic observatory data in the Indian zone. The MAGDAS/CPMN magnetic data of DAV and MUT were provided by the PI of MAGDAS/CPMN project (http://magdas.serc.kyushu-u.ac.jp/). This work was supported in part by JSPS Core-to-Core Program, B. Asia-Africa Science Platforms. The authors would like to thank Teiji Uozumi for the processing of the MAGDAS/CPMN magnetic data. The F10.7 cm flux data and magnetic data (K p) can be downloaded freely from http://spidr.ngdc.noaa.gov/spidr/. The daily values of the SEM/SOHO EUV fluxes can be obtained from http://www.usc.edu/dept/space_science/semdatafolder/long/.
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- Adebesin BO, Adeniyi JO, Adimula IA, Reinisch BW, Yumoto K (2013) F2 layer characteristics and electrojet strength over an equatorial station. Adv Space Res 52(5):791–800View ArticleGoogle Scholar
- Alken P et al (2013) Swarm SCARF equatorial electric field inversion chain. Earth, Planets and Space 65(11):1309–1317View ArticleGoogle Scholar
- Appleton EV (1946) Two anomalies in the ionosphere. Nature 157:691View ArticleGoogle Scholar
- Bailey DK (1948) The geomagnetic nature of the F2-layer longitude-effect. Terr Mag Atmo Elec 53(1):35–39. doi:10.1029/TE053i001p00035 View ArticleGoogle Scholar
- Balan N, Bailey GJ, Jayachandran B (1993) Ionospheric evidence for a nonlinear relationship between the solar e.u.v. and 10.7 cm fluxes during an intense solar cycle. Planet Space Sci 41:141–145. doi:10.1016/0032-0633(93)90043-2 View ArticleGoogle Scholar
- Balan N, Bailey GJ, Jenkins B, Rao PB, Moffet J (1994) Variations of ionospheric ionization and related solar fluxes during an intense solar cycle. J Geophys Res 99:2243–2253. doi:10.1029/93JA02099 View ArticleGoogle Scholar
- Balan N, Otsuka Y (1998) Equinoctial asymmetries in the ionosphere and thermosphere observed by the MU radar. J Geophys Res: 103(A5):9481–9495View ArticleGoogle Scholar
- Bhuyan PK, Tyagi TR, Singh L, Somayajulu YV (1983) Ionospheric electron content measurements at northern low midlatitude station through half solar cycle. Ind JRad Spac Phy 12:84–93Google Scholar
- Bhuyan PK, Chamua M, Bhuyan K, Subrahmanyam P, Garg SC (2003) Diurnal, seasonal and latitudinal variation of electron density in the topside F-region of the Indian zone ionosphere at solar minimum and comparison with IRI. J Atmos Sol-Terr Phys 65:359–368View ArticleGoogle Scholar
- Bhuyan PK, Hazarika R (2012) GPS TEC near the crest of the EIA at 95°E during the ascending half of solar cycle 24 and comparison with IRI simulations. Adv Space Res 52:1247–1260View ArticleGoogle Scholar
- Booker HG, Smith EK (1970) A comparative study of ionospheric measurement techniques. J Atmos Terr Phys: 32(4):467–497View ArticleGoogle Scholar
- Bramley EN, Ruster R (1971) The effect of electric fields and ion drag in the middle latitude F-region. J Atmos Terr Phy 33(2):269–274View ArticleGoogle Scholar
- Buonsanto MJ (1999) Ionospheric storms: a review. Space Science Review 88:563–601View ArticleGoogle Scholar
- Cardoso FA, Sahai Y, Guarnieri FL, Fagundes PR, Pillat VG, da Silva JVPR (2011) Dependence of the F-region peak electron density (NmF2) on solar activity observed in the equatorial ionospheric anomaly region in the Brazilian sector. Adv Space Res 48:837–841View ArticleGoogle Scholar
- Chakrabarty SK, Hajra R (2009) Electrojet control of ambient ionization near the crest of the equatorial anomaly in the Indian zone. Ann Geophy: 27:93–105View ArticleGoogle Scholar
- Chandra H, Rastogi RG (1974) Geomagnetic storm effects on ionospheric drifts and the equatorial Es over the magnetic equator. Ind J Radio Space Phys 3:332–336Google Scholar
- Chen Y, Liu L, Le H (2008) Solar activity variations of nighttime ionospheric peak electron density. J Geophys Res 113, A11306. doi:10.1029/2008JA013114 View ArticleGoogle Scholar
- Chen Y, Liu L, Wan W, Yue X, Su S-Y (2009) Solar activity dependence of the topside ionosphere at low latitudes. J Geophys Res 114, A08306. doi:10.1029/2008JA013957 Google Scholar
- Chen Y, Liu L (2010) Further study on the solar activity variation of daytime NmF2. J Geophys Res: 115, A12337. doi:10.1029/2010JA015847 View ArticleGoogle Scholar
- Chen Y, Liu L, Wan W (2011) Does the F 10.7 index correctly describe solar EUV flux during the deep solar minimum of 2007–2009? J Geophys Res 116:A4. doi:10.1029/2010JA016301 Google Scholar
- Chen Y, Liu L, Wan W, Ren Z (2012) Equinoctial asymmetry in solar activity variations of NmF2 and TEC. Ann Geophys 30:613–622, http://dx.doi.org/10.5194/angeo-30-613-2012 View ArticleGoogle Scholar
- Dabas RS, Bhuyan PK, Tyagi TR, Bhardwaj RK (1984) Day-to-day changes in ionospheric electron content at low latitudes. Radio Science 19(3):749–756View ArticleGoogle Scholar
- Dabas RS, Sharma N, Pillai MGK, Gwal AK (2006) Day-to-day variability of equatorial and low latitude F-region ionosphere in the Indian zone. J Atmos and Sol-Terr Phys: 68(11):1269–1277View ArticleGoogle Scholar
- Danilov AD, Lastovicka J (2001) Effects of geomagnetic storms on the ionosphere and atmosphere. International Journal of Geomagnetism and Aeronomy 2:3Google Scholar
- Da Rosa AV, Waldman H, Bendito J, Garriott OK (1973) Response of the ionospheric electron content to fluctuations in solar activity. J Atmos Terr Phys 35:1429View ArticleGoogle Scholar
- Deminova GF (1993) Wave structure of longitudinal variations in the nighttime equatorial ionosphere. Geomagn Aeron 33(5):167–169Google Scholar
- Deminova GF (1995) Wave structure of longitudinal variations in the nighttime equatorial anomaly. Geomag Aeron 35(4):169–173Google Scholar
- Deshpande MR, Rastogi RG, Vats HO, Kulbtchar J, Sethi G (1977) Effect of electrojet on the total electron content of the ionosphere over the Indian subcontinent. Nature 267:599–600, doi:10.1038/267599a0.
- Essex EA (1977) Equinoctial variations in the total electron content of the ionosphere at northern and southern hemisphere stations. J Atmos Terr Phys 39:645View ArticleGoogle Scholar
- England SL, Maus S, Immel TJ, Mende SB (2006) Longitudinal variation of the E-region electric fields caused by atmospheric tides. Geophys Res Lett 33, L21105. doi:10.1029/2006GL027465 View ArticleGoogle Scholar
- Fang TW, Kil H, Millward G, Richmond AD, Liu JY, Oh SJ (2009) Causal link of wave-4 structure in plasma density and vertical plasma drift in low latitude ionosphere. J Geophys Res 114:A10315. doi:10.1029/2009JA014460 View ArticleGoogle Scholar
- Fejer BG, Farley DT, Woodman RF, Calderon C (1979) Dependence of equatorial F-region vertical drifts on season and solar cycle. J Geophys Res 84:5792View ArticleGoogle Scholar
- Fejer BG, Jensen JW, Su S-Y (2008) Quiet time equatorial F region vertical plasma drift model derived from ROCSAT-1 observations. J Geophys Res 113, A05304. doi:10.1029/2007JA012801 Google Scholar
- Field P, Rishbeth RH (1997) The response of the ionospheric F2-layer to geomagnetic activity (1997) an analysis of worldwide data. J Atmos Terr PhysPhysics 59(2):163–180, http://dx.doi.org/10.1016/S1364-6826(96)00085-5 Google Scholar
- Gong Y, Zhou Q, Zhang S, Aponte N, Sulzer M, Gonzalez S (2012) Midnight ionosphere collapse at Arecibo and its relationship to the neutral wind, electric field, and ambipolar diffusion. J Geophys Res 117, A08332. doi:10.1029/2012JA017530 Google Scholar
- Hazarika R, Bhuyan PK (2014) Spatial distribution of TEC across India in 2005: seasonal asymmetries and IRI predictions. Adv Space Res 54(9):1751–1767. doi:10.1016/j.asr.2014.07.011 View ArticleGoogle Scholar
- He M, Liu L, Wan W, Lei J, Zhao B (2010) Longitudinal modulation of the O/N2 column density retrieved from TIMED/GUVI measurement. Geophys Res Lett 37, L20108. doi:10.1029/2010GL045105 Google Scholar
- Henderson SB, Swenson CM, Christensen AB, Paxton LJ (2005) Morphology of the equatorial anomaly and equatorial plasma bubbles using image subspace analysis of global ultraviolet imager data. J Geophys Res 110:A11306, http://dx.doi.org/10.1029/2005JA011080 View ArticleGoogle Scholar
- Hierl PM, Dotan I, Seeley JV, Van Doren JM, Morris RA, Vigiano AA (1997) Rate constants for the reactions of O + with N2 and O2 as a function of temperature (300-1800 K). J Chem Phys 106:3540–3544View ArticleGoogle Scholar
- Ikubanni SO, Adeniyi JO (2013) Variation of saturation effect in the ionospheric F2 critical frequency at low latitude. J Atmos Terr Phys 100–101, 24–33, ISSN 1364-6826. http://dx.doi.org/10.1016/j.jastp.2013.03.012.
- Immel TJ, Sagawa E, England SL, Henderson SB, Hagan ME, Mende SB, Frey HU, Swenson CM, Paxton LJ (2006) The control of equatorial ionospheric morphology by atmospheric tides. Geophys Res Lett 33, L15108. doi:10.1029/2006GL026161 View ArticleGoogle Scholar
- Jin H, Miyoshi Y, Fujiwara H, Shinagawa H (2008) Electrodynamics of the formation of ionospheric wave number 4 longitudinal structure. JGeophysRes 113:A09307Google Scholar
- Kane RP (1992) Sunspots, solar radio noise, solar EUV and ionospheric NmF2. J Atmos Terr Phys 54:463–466View ArticleGoogle Scholar
- Kane RP (2006) Are the double-peaks in solar indices during solar maxima of cycle 23 reflected in ionospheric NmF2? J Atmos Sol-Terr Phys 68(8):877–880View ArticleGoogle Scholar
- Kil H, Oh S-J, Kelley MC, Paxton LJ, England SL, Talaat E, Min K-W, Su S-Y (2007) Longitudinal structure of the vertical E × B drift and ion density seen from ROCSAT-1. Geophysical Res Lett 34. doi:10.1029/2007GL030018. issn: 0094-8276.
- Kil H, Talaat ER, Oh SJ, Paxton LJ, England SL, Su SY (2008) Wave structures of the plasma density and vertical E × B drift in low-latitude F region. J Geophys Res 113:A09312, http://dx.doi.org/10.1029/2008JA013106 Google Scholar
- Lin CH, Wang HME, Hsiao CC, Immel TJ, Hsu ML, Liu JY, Paxton LJ, Fang TW, Liu CH (2007a) Plausible effect of atmospheric tides on the equatorial ionosphere observed by the FORMOSAT-3/COSMIC: three-dimensional electron density structures. Geophys Res Lett 34, L11112. doi:10.1029/2007GL029265 View ArticleGoogle Scholar
- Lin CH, Hsiao CC, Liu JY, Liu CH (2007b) Longitudinal structure of the equatorial ionosphere: the evolution of the four peaked structure. J Geophys Res 112, A12305. doi:10.1029/2007JA012455 View ArticleGoogle Scholar
- Liu JY, Chen YI, Lin JS (2003) Statistical investigation of the saturation effect in the ionospheric NmF2 versus sunspot, solar radio noise and solar EUV radiation. J Geophys Res 108(A2):1067, http://dx.doi.org/10.1029/2001JA007543 View ArticleGoogle Scholar
- Liu L, Wan W, Ning B (2004) Statistical modelling of ionospheric NmF2 over Wuhan. Radio Sci 39:RS2013. doi:10.1029/2003RS003005 View ArticleGoogle Scholar
- Liu L, Wan W, Ning B, Piog OM, Kurkin VI (2006) Solar activity variations of the ionospheric peak electron density. J Geophys Res 111:A08304, http://dx.doi.org/10.1029/2006JA011598 Google Scholar
- Liu L, Wan W, Yue X, Ning B, Zhang ML (2007a) The dependence of plasma density in the topside ionosphere on the solar activity level. Ann Geophys 25:1337–1343View ArticleGoogle Scholar
- Liu L, Wan W, Zhang ML, Ning B, Zhang SR (2007b) Variations of topside ionospheric scale heights over Millstone Hill during the 30-day incoherent scatter radar experiment. Ann Geophys: 25(9):2019–2027View ArticleGoogle Scholar
- Liu H, Watanabe S (2008) Seasonal variation of the longitudinal structure of the equatorial ionosphere: does it reflect tidal influences from below? J Geophys Res 113, A08315. doi:10.1029/2008JA013027 Google Scholar
- Liu J, Liu L, Zhao B, Lei J, Wan W (2011) On the relationship between the postmidnight thermospheric equatorial mass anomaly and equatorial ionization anomaly under geomagnetic quiet conditions. J Geophys Res 116, A12312. doi:10.1029/2011JA016958 View ArticleGoogle Scholar
- Liu J, Liu LB, Zhao BQ, Wan WX, Chen YD (2012) Empirical modeling of ionospheric F2 layer critical frequency over Wakkanai under geomagnetic quiet and disturbed conditions. Science China Technological Sciences 55(5):1169–1177View ArticleGoogle Scholar
- Lühr H, Hausler K, Stolle C (2007) Longitudinal variation of F region electron density and thermospheric zonal wind caused by atmospheric tides. Geophys Res Lett 34, L16102. doi:10.1029/2007GL030639 Google Scholar
- Lühr H, Rother M, Hausler K, Alken P, Maus S (2008) Influence of nonmigrating tides on the longitudinal variation of the equatorial electrojet. J Geophys Res 113, A08313. doi:10.1029/2008JA013064 Google Scholar
- Mayr HG, Mahajan KK (1971) Seasonal variation in the F 2 region. J Geophys Res 76(4):1017–1027View ArticleGoogle Scholar
- Ma R, Xu J, Wang W, Yuan W (2009) Seasonal and latitudinal differences of the saturation effect between ionospheric NmF2 and solar activity indices. J Geophys Res 114, A10303. doi:10.1029/2009JA014353 View ArticleGoogle Scholar
- Mikhailov AV, Marin D (2001) An interpretation of the NmF2 and hmF2 long-term trends in the framework of the geomagnetic control concept. Ann Geophy 19:733–748View ArticleGoogle Scholar
- Mikhailov AV, Perrone L (2011) On the mechanism of seasonal and solar cycle NmF2 variations: a quantitative estimate of the main parameters contribution using incoherent scatter radar observations. J Geophys Res 116, A03319. doi:10.1029/2010JA016122 Google Scholar
- Oberheide J, Forbes JM (2008) Thermospheric nitric oxide variability induced by nonmigrating tides. Geophys Res Lett 35, L16814. doi:10.1029/2008GL034825 View ArticleGoogle Scholar
- Oh S, Kil H, Kim WT, Paxton LJ, Kim YH (2008) The role of the vertical E × B drift for the formation of the longitudinal plasma density structure in the low-latitude F region. Ann Geophys 26:2061–2067View ArticleGoogle Scholar
- Rao BNC (1963a) Some characteristic features of the equatorial ionosphere and the location of the F-region equator, 1963. J Geophys Res 68(9):2541–2549View ArticleGoogle Scholar
- Rao BNC (1963b) The postsunset rise of f o F 2 in the transition region and its dependence on the postsunset rise of h′F in the equatorial region, 1963. J Geophys Res 68(9, 1): 2551–2557Google Scholar
- Rastogi RG, Sanatani S (1968) Forenoon bite-out of F2 layer ionization at tropical latitudes. Ann Geophys 24:75–80Google Scholar
- Rastogi RG, Chandra H, Sharma RP, Rajaram G (1972) Ground based measurements of ionospheric phenomenon associated with the equatorial electrojet. IndJRadioSpace Phys 1:119–135Google Scholar
- Ratovsky KG, Oinats AV (2009) Medvedev AV Diurnal and seasonal variations of F2 layer characteristics over Irkutsk during the decrease in solar activity in 2003–2006: observations and IRI-2001 model predictions. Advances in Space Research 43:1806–1811View ArticleGoogle Scholar
- Ren Z, Wan W, Liu L, Xiong J (2009) Intra-annual variation of wave number 4 structure of vertical E × B drifts in the equatorial ionosphere seen from ROCSAT-1. J Geophys Res 114, A05308. doi:10.1029/2009JA014060 Google Scholar
- Ren Z, Wan W, Xiong J, Liu L (2014) Influence of DE3 tide on the equinoctial asymmetry of the zonal mean ionospheric electron density. Earth, Planets and Space 66:117. doi:10.1186/1880-5981-66-117
- Rishbeth H (1971) Thermospheric winds and the F-region: a review. J Atmos Terr Phy 34:1–47View ArticleGoogle Scholar
- Rishbeth H, Garriott OK (1969) Introduction to Ionospheric Physics, Elsevier, New York.Google Scholar
- Rishbeth H, Sedgemore-Schulthess KJF, Ulich (2000) The semiannual and annual variations in the height of the ionospheric F2-peak. Ann Geophy 18:285–299View ArticleGoogle Scholar
- Rishbeth H (2004) Questions of the equatorial F2-layer and thermosphere. J Atmos Sol-Terr Phys 66:1669–1674View ArticleGoogle Scholar
- Rush CM, Richmond AD (1973) The relationship between the structure of the equatorial anomaly and the strength of the equatorial electrojet. J Atmos Terr Phy 35(6):1171–1180View ArticleGoogle Scholar
- Sagawa E, Immel TJ, Frey H, Mende SB (2005) Longitudinal structure of the equatorial anomaly in the nighttime ionosphere observed by IMAGE/FUV. J Geophys Res 110:A11302, http://dx.doi.org/10.1029/2004JA010848 View ArticleGoogle Scholar
- Sastri JH (1990) The relationship between the structure of the equatorial anomaly and the strength of the equatorial electrojet. Ind J Radio Space Phys 19:225–240Google Scholar
- Scherliess L, Fejer BG (1999) Radar and satellite global equatorial F region vertical drift model. J Geophys Res 104(A4):6829–6842. doi:10.1029/1999JA900025 View ArticleGoogle Scholar
- Scherliess L, Thompson DC, Schunk RW (2008) Longitudinal variability of low-latitude total electron content: tidal influences. J Geophys Res 113:A01311, http://dx.doi.org/10.1029/2007JA012480 Google Scholar
- Sethi NK, Goel MK, Mahajan KK (2002) Solar cycle variations of NmF2 from IGY to 1990. Ann Geophys 20:1677–1685View ArticleGoogle Scholar
- Sethi NK, Dabas RS, Vohra VK (2004) Diurnal and seasonal variations of hmF2 deduced from digital ionosonde over New Delhi and its comparison with IRI 2001. Ann Geophys 22:453–458View ArticleGoogle Scholar
- Shepherd MG, Shepherd GG, Cho Y-M (2012) Longitudinal variability of thermospheric temperatures from WINDII O(1S) dayglow. J Geophys Res 117, A10302. doi:10.1029/2012JA017777 View ArticleGoogle Scholar
- Simi KG, Manju G, Madhav Haridas MK, Prabhakaran Nayar SR, Tarun Kumar Pant, Alex S (2013) Ionospheric response to a geomagnetic storm during November 8-10, 2004, Earth, Planets and Space, April 2013, 65(4):343–350Google Scholar
- Stubbe P, Chandra S (1970) The effect of electric fields on the F-region behaviour as compared with neutral wind effects. J Atmos Terr Phy 32(12):1909–1919View ArticleGoogle Scholar
- Su YZ, Bailey GJ (1999) Altitude dependencies in the oslar activity variations of the ionospheric electron density. J Geophys Res 104:47,14879–14891Google Scholar
- Titheridge JE (1973) The electron content of the southern midlatitude ionosphere, 1965-1971. J Atmos Terr Phys 981–1001Google Scholar
- Titheridge J (1985) Ionogram analysis with the generalized Program POLAN, World Data Center Rep. UAG-93 World Data Cent. for SolarTerrestrial Physics, Boulder, Colorado, (http://www.sws.bom.gov.au/IPSHosted/INAG/uag_93/uag_93.html)
- Titheridge J (1995) Winds in the ionosphere—a review. J Atmos Terr Phys 57(14):1681–1714View ArticleGoogle Scholar
- Thomas L (1968) The F2-region equatorial anomaly during solstice periods at sunspot maximum. J Atmos Terr Phys 30:1631–1640View ArticleGoogle Scholar
- Walker GO, Ma JHK (1972) Influence of solar flux and the equatorial electrojet on the diurnal development of the latitude distribution of total electron content in the ‘equatorial anomaly’. J Atmos Terr Phys 34(8):1419–1424View ArticleGoogle Scholar
- Walker GO (1981) Longitudinal structure of the F-region equatorial anomaly—a review. J Atmos Terr Phys 43:763View ArticleGoogle Scholar
- Watthanasangmechai K, Yamamoto M, Saito A, Maruyama T, Yokoyama T, Nishioka M, Ishii M (2015) Temporal change of EIA asymmetry revealed by a beacon receiver network in Southeast Asia. Earth, Planets and Space 67:75. doi:10.1186/s40623-015-0252-9
- Wichaipanich N, Supnithi P, Tsugawa T, Maruyama T (2012) Thailand low and equatorial F2-layer peak electron density and comparison with IRI-2007 model. Earth, Planets and Space 64(6):485–491Google Scholar
- Yadav S, Dabas RS, Das RM, Upadhayaya AK, Sharma K (2010) Gwal A (2009) Diurnal and seasonal variation of F2-layer ionospheric parameters at equatorial ionization anomaly crest region and their comparison with IRI-2001. Adv Spac Res 45:361–367View ArticleGoogle Scholar
- Yadav S, Dabas RS, Das RM, Upadhayaya AK, Sarkar SK, Gwal AK (2011) Variation of F-region critical frequency (NmF2) over equatorial and low-latitude region of the Indian zone during 19th and 20th solar cycle. Adv Spac Res 47(1):124–137View ArticleGoogle Scholar