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Possibilities of the usage of the total electron content in a low-latitude zone
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ScienceDirect Advances in Space Research 64 (2019) 2114–2124 www.elsevier.com/locate/asr
Possibilities of the usage of the total electron content in a low-latitude zone O.A. Maltseva a,⇑, N.S. Mozhaeva b a
Institute for Physics, Southern Federal University, 344090 Rostov-on-Don, Russia b OKTB uˆVektory´, 346880 Bataysk, Russia
Received 8 February 2019; received in revised form 28 June 2019; accepted 10 July 2019 Available online 19 July 2019
Abstract Last years, the small quantity of ionosondes at low latitudes could not give a sufficient picture of behavior of the ionosphere in these areas. Now, reception of experimental data, their analysis and detection of total electron content (TEC) variations at low latitudes are an actual problem. When moving from describing the state of the ionosphere using vertical sounding parameters to describing using TEC, it is important to identify similarities and differences in their behavior in order to assess the possibilities of using TEC. This paper carries out research on four ways of TEC usage in low-latitude zone: (1) research of climatological peculiarities of the TEC behavior, (2) validation of up-to-date TEC models, (3) foF2 calculation according to observational TEC, and (4) peculiarities of TEC behavior during geomagnetic disturbances. Results for station Hainan show the following. 1. In addition to previous investigation it is shown, that the empirical global and local models of TEC provide the values close to observational ones, however climatological features of TEC behavior described by various models may still differ from each other and from the experimental data showing in this case TEC maxima of diurnal variation in the afternoons, seasonal variations in March and October, some slight winter anomaly. 2. In contrast to previous investigation it is shown that the closest to the experimental values of TEC are provided by the IRI-Plas model, while this model overestimates values, and the model NeQuick underestimates values compared to the observational ones. 3. For the first time, the ionospheric equivalent slab thickness s, which plays the role of the coefficient of proportionality between TEC and NmF2, was compared for the IRI, IRI-Plas, NeQuick models with the experimental median s(med) and it was shown, that the usage of observational TEC data and s(med) for calculation of critical frequencies foF2 allows improving correspondence with experimental data by 1.2–2 times compared to the models. 4. Comparison of foF2 and TEC behavior has a particular importance during disturbances. Using the example of 15 strong magnetic storms (Dst < 50 nT), it was shown that, despite its global nature, the details of the TEC response strongly depend on the region. So, the peculiarities of dTEC in comparison with the mid-latitude region are the predominance of the positive phase, and a smaller value of response. The synchronism of variations dTEC and dfoF2 was very high, although there are cases of mismatch. Using the median s (med) allows determining foF2 during disturbances and filling data gaps. Ó 2019 COSPAR. Published by Elsevier Ltd. All rights reserved.
Keywords: Low-latitude ionosphere; Ionospheric models; Total Electron Content (TEC); Equivalent slab thickness; Geomagnetic disturbances
1. Introduction Low-latitude and equatorial regions attract constant attention due to the large variability of the ionospheric ⇑ Corresponding author.
E-mail addresses: [email protected] (O.A. Maltseva), [email protected] (N.S. Mozhaeva). https://doi.org/10.1016/j.asr.2019.07.010 0273-1177/Ó 2019 COSPAR. Published by Elsevier Ltd. All rights reserved.
parameters. In the past, the critical frequency foF2 was the basic characteristic of the ionosphere, but the small number of ionosondes limited possibilities to study the ionosphere in this zone. Now, measurements of TEC by means of satellites of systems GPS, GLONASS and others allow to carry out more detailed researches due to a large number of stations.
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However, despite a considerable number of publications, and special symposia it is impossible to solve all the problems. This paper focuses on the modeling of the TEC parameter and the relationship between TEC and foF2 in the low-latitude zone. In the work (Liu et al., 2014), the monthly medians of TEC and NmF2 were compared over China during 2004 using 34 GPS stations and 10 ionosondes. The example of Guangzhou station (23.1° N, 113.4° E) revealed the following peculiarities of daily and seasonal changes: (1) the TEC and NmF2 maxima fall at the time of afternoon, with the maximum NmF2 delaying by 1–2 h compared to the TEC maximum, and (2) seasonal dependence is characterized by maxima in March and October and with a semi-annual anomaly at which winter values are greater than summer ones; it is clearly expressed for NmF2 and almost not expressed for TEC. This station is the southernmost, but similar results were obtained for higher-latitude stations. The given paper presents the results for an even more southern Hainan station. For representation of TEC, global maps JPL, CODE, UPC, ESA are very widely used (Herna´ndez-Pajares et al., 2009) and also empirical models, global (e.g., Jakowski et al., 2011), and local (Kakinami et al., 2009). Other class of models includes the models based on integration of N(h) -profile (e.g., IRI (Bilitza et al., 2017), NeQuick-2 (Nava et al., 2008)) and others. The important property of these models is long-term prediction of TEC. In the given paper, the special attention is paid to the model IRI-Plas (Gulyaeva, 2011). Its advantages are the assimilation of TEC in the model, presence of very convenient site (http://www.ionolab.org/index.php?language= en), providing great opportunities to study TEC (e.g., Sezen et al., 2018), development of a global forecast system on its basis (Gulyaeva et al., 2013). However this model has not been sufficiently tested, although some works can be picked out. In works (Zakharenkova et al., 2015, Gordiyenko et al., 2018; Atıcı, 2018), results of the model IRI-Plas are used for comparison with experimental data in a zone of the middle latitudes. Authors of the paper (Bolaji et al., 2017) compare several models in an equatorial zone of Africa. The work (Ezquer et al., 2018) examines the ability of the NeQuick-2 and IRI-Plas models to determine TEC values in the US sector from 18.4° N to 64.7° N and the longitude ranges from 281.3° E to 295.9° E. The largest divergences are obtained for low-latitude stations near an equatorial anomaly. It is described, where and when each model works better. In work (Adebiyi, et al., 2017), data analysis of two South American equatorial stations has shown that the usage of TEC alone is not enough to accurately determine the instantaneous parameters of the ionosphere. The most detailed comparison with experimental data was carried out for the NeQuick-2 and IRIPlas models in the work (Okoh et al., 2018). This comparison was performed globally for 36 stations of northern and southern hemispheres from January 2006 to July 2017. It is shown that the trends in the behavior of TECs are fairly accurately reflected by both models, but the
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quantitative characteristics are different. The average annual TEC prediction errors range from 3 TECU for high-latitude stations to 12 TECU for low-latitude stations for the IRI-Plas model. For the model NeQuick-2, these estimates are 2 TECU and 7 TECU, respectively. The main disadvantage of the IRI-Plas model is an overestimation of observational values. Fig. 8 of the paper (Okoh et al., 2018) shows that an electron concentration of the model NeQuick-2 at heights h > hmF2 is lower than for the model IRI-Plas. It should be noted that the TEC values themselves are strongly dependent on the calculation method (e.g., Arikan et al., 2003). It underlines necessity of further comparison and study of TEC behavior in local regions. This paper explores the possibility of using TEC to determine ionospheric conditions in Southeast Asia. Special attention is paid to using the equivalent slab thickness s of the ionosphere. This parameter is known to have independent significance (Stankov and Warnant, 2009), however, in this paper, its role as a coefficient of proportionality between TEC and NmF2 is investigated (e.g., Gulyaeva et al., 2013; Muslim et al., 2015). The most natural is the use of s of various models, but there is no comparison of these models with the observational s and their efficiency. In this paper, the observational median s (med) is compared with the s of the most common IRI, IRI-Plas, NeQuick models and their effectiveness at calculation of foF2, including during disturbances, is investigated. 2. Used data and models Study is performed using data of station Hainan (19.4° N, 109°E) and some other stations with higher latitudes. Observational data of three types are used: (1) foF2(obs), (2) TEC(obs), (3) data characterizing geomagnetic activity. In several papers, long-term data of vertical sounding are used, for example (Zhang et al., 2007; Hu et al., 2014), however in the now decommissioned SPIDR database, which was available at (http://spidr.ngdc.noaa.gov/spidr/ index.jsp) through the Internet, the number of such data was limited. Data of only 11 months for 2004 were accessible. As for values of TEC, global maps GIM (ftp://cddis. gsfc.nasa.gov/pub/gps/products/ionex/) were used. For data characterizing geomagnetic activity, values of a Dstindex (http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3.html) were used. Models IRI (Bilitza et al., 2017), IRI-Plas (Gulyaeva, 2011), NeQuick-2 (Nava et al., 2008) were used online. As is known, calculation of critical frequencies using TEC needs knowledge of proportionality factor which is the equivalent slab thickness of the ionosphere s. It is defined from relationship s = TEC/NmF2. If s and the observational value TEC(obs) are known it is possible to obtain an instantaneous value NmF2(eff) = TEC(obs)/s. The symbol ‘eff’ was entered in the paper (Gulyaeva, 2003). In this paper, four types of s are used: s(IRI), s(IRI-Plas), s (NeQuick), s(med). The factor s(IRI) is defined by the model IRI: s(IRI) = TEC(IRI)/NmF2(IRI). It was
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traditionally used in researches (e.g., McNamara, 1985; Houminer and Soicher, 1996). The factor s(IRI-Plas) is defined by the model IRI-Plas: s(IRI-Plas) = TEC(IRIPlas)/NmF2(IRI-Plas). It is used in (Gulyaeva, 2003; Gulyaeva, 2011; Gulyaeva et al., 2013; Adebiyi et al., 2017). The factor s(NeQuick) will be used in addition in this paper with calculation under the formula s (NeQuick) = TEC(NeQuick)/NmF2(NeQuick). Values of TEC(NeQuick) and Nmf2(NeQuick) are calculated on a site https://t-ict4d.ictp.it/nequick2/nequick-2-web-model. The factor s(med) is a median of the equivalent slab thickness of the ionosphere s(med) = TEC(obs)/NmF2(obs) (Maltseva and Mozhaeva, 2015). It was used also to fill gaps of ionosonde data. 3. Morphological features of behavior of foF2 and TEC and comparison with models Fig. 1 presents observational monthly medians of foF2, TEC and corresponding values for the models IRI, IRIPlas and NeQuick (foF2 values of three empirical models practically coincide therefore only two curves are shown). It is necessary to make an explanation about the presentation of pictures. Labels of the vertical axes of the graphs in all the Figures are placed in the header inside the image to increase the field of the image.
The maximum values of foF2 fall in March-April and September-November which does not coincide with behavior of foF2 at station Guangzhou (Liu et al., 2014), but it is close to results for station Hainan, obtained on large array of the data, accessible to authors (Wang et al., 2017). The behavior of TEC shows the more expressed seasonal dependence that coincides with peculiarities at station Guangzhou (Liu et al., 2014). For the models IRI-Plas and NeQuick, the spring maximum was displaced for April according to behavior of foF2. A weak manifestation of the winter anomaly, when summer values (in July) are lower than winter ones (in January), is visible. The model IRI-Plas overestimated TEC(obs) values, while the model NeQuick underestimated them. One of the main problems of ionospheric modeling is the development of TEC models. Earlier, the basic attention was paid to global models (Bilitza, 2002). Now, the large interest is addressed to the local models focused on local features, which are important for the theory and applications. Especially intensively such models were developed in Southeast region (Aa et al., 2015). In the paper (Luo et al., 2014), five models GIM (JPL), IRI, PIM, NeQuick, Klobuchar were compared. Here in Fig. 2, results for station Hainan with lower latitude are given for July 2011 to compare with TEC behavior for four various global maps JPL, CODE, UPC, ESA for stations HKTU (22.3°
Fig. 1. A seasonal variations of foF2 and TEC for station Hainan in 2004.
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Fig. 2. Diurnal changes of TEC for 3 stations with various latitudes.
114.18°) and WUHN (30.53°, 114.36°), considered in (Luo et al., 2014). Some trend of increasing the maximum values and decreasing the minimum values with decreasing latitude can be seen. It should be noted, that the difference between values of various global maps is lower than in other regions. Results of the paper (Luo et al., 2014) are supplemented in Fig. 3 by comparison with such global models, as IRI (Bilitza et al., 2017), NGM (Jakowski et al., 2011; Hoque and Jakowski, 2011), IRI-Plas, NeQuick and the local model Taiwan (Kakinami et al., 2009). The model IRI is the international standard, the model NGM (German space centre Neistrelitz) is differed by the detailed account of an equatorial anomaly and its testing for lowlatitude stations has given encouraging results in several regions (Maltseva et al., 2014). The model (Kakinami et al., 2009) is local, so for it the results are given for Guangzhou station, which has the closest coordinates to Chung Li station, for which this model is developed. It can be seen that the results of the IRI and Taiwan models coincide well with the experimental data, presented
by the map CODE, while the NGM model in this case overestimates the values, the IRI-Plas model overestimates them even more, and the NeQuick model overestimates the values for Hainan station and greatly underestimates TEC (obs) for Guangzhou station.
4. The ability to determine NmF2 (foF2) using TEC Among the large number of TEC applications, its use for determining NmF2 and, accordingly, foF2 can be distinguished. For such use, it is necessary to know the proportionality coefficient s between TEC and NmF2, i.e. an equivalent slab thickness of the ionosphere. The purpose of this section is to show the role of the correct determination of s when using TEC to calculate foF2. The traditional used thickness is s(IRI). In the paper (Maltseva and Mozhaeva, 2015), it is proposed to use s(med) and is shown that s(IRI) may differ from s(med) in many regions. Parameter s(IRI-Plas) is used in (Gulyaeva, 2003). In this work, s(NeQuick) is additionally used. Fig. 4 shows the
Fig. 3. Comparison of various foF2 and TEC models for low-latitude stations.
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Fig. 4. Comparison of the behavior of equivalent slab thicknesses s of various models used to obtain foF2.
diurnal dependences of s of various models for several months of 2004 (January, March, June, September). Large differences in behavior and in values of s are visible. The greatest difference relates to the morning maximum: values of s(med), s(IRI), s(IRI-Plas) differ, but for the NeQuick model this maximum is absent. For the first three s models, some seasonal variation is visible, while for the NeQuick model, it is significant only near noon. A more visual representation of the differences between s can be obtained from Fig. 5 (left side). And namely these s values are used to calculate the instantaneous foF2 values in accordance with TEC(obs). The results are shown in Fig. 5 (right side) in the form of absolute deviations |DfoF2| of calculated values from experimental ones. Values foF2 (IRI) of the IRI model are monthly medians and do not reflect day to day variations of foF2 which are defined by instantaneous values. However in many cases, values foF2(IRI) are used as diurnal (i.e., instantaneous). In this case, the difference |DfoF2| (mark IRIins) is calculated as a difference between the instantaneous observational value foF2(obs) and value foF2(IRI) at each hour (in this case at each 2 h because global maps have the permission of 2 h). These differences are not connected with the usage of observational TEC(obs). Other curves are calculated with the usage of TEC(obs) and a corresponding thickness s. Diamonds show deviations when using the model values as instantaneous. Of course, the use of values TEC(obs) should reduce this difference, but this is not always the case. In this example, the use of s(IRI) leads to an increase in the difference (the curve shown in squares) in comparison with IRIins, i.e. it is
better to just use the model values. The use of TEC(obs) and s(IRI-Plas) leads to an improvement in the summer months. Using TEC(obs) and s(NeQuick) improves the closeness of the calculated values to the experimental values throughout the year (curve with triangles). The best results shown by dots are given by using TEC(obs) and s (med). The quantitative estimate is presented in Table 1 as average annual deviations. In other areas, the results may be different in determining the advantages of a particular model, because no model can work equally well in all regions, but in any case the best fit with observational foF2 will provide s(med). As noted in section 1, s together with TEC(obs) can be used to fill gaps in ionosonde data. Obviously, the use of s(med) will give the best results in this case. 5. Features of TEC behavior during disturbances A large number of articles are devoted to the behavior of the ionosphere during disturbances, but it turned out that a new look at the relationship between magnetosphere-iono sphere-thermospheric effects is possible when using TEC to assess the state of the environment. It is presented in the paper (Cander, 2016) on the example of the behavior of the relative deviations dTEC during 15 magnetic storms from January 2009 to May 2015 according to HERS GPS data (0.33° E, 50.86° N). The seasonal dependence of the effect of magnetic storms on local characteristics known from earlier ionospheric studies was confirmed for the TEC and the relative role of various processes was revealed. At the latitude of the HERS station, the typical
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Fig. 5. Comparison of the effectiveness of using s of different models.
Table 1 Comparison of the effectiveness of using s of different models. |DfoF2|, MHz
ins
med
IRI
Plas
NeQuick
Mean
1.43
0.89
1.77
1.51
1.21
morphology of the ionospheric storm is as follows: (1) the sequence of phases has common features, (2) TEC tends to increase in the first 24 h (positive phase), then negative phase and 1–2 days recovery, (3) during negative phase of changes in the TEC, amplitude of disturbances shows a significant decrease in the summer compared to the winter, and (4) on average, there are a large number of positive disturbances followed by negative ones and an insignificant number of negative disturbances followed by positive ones. In the present article, the results are obtained for the Hainan station for all cases considered in (Cander, 2016). Figs. 6–8 illustrate the results on the example of three cases from (Cander, 2016) relating to different seasons. The graphs of variations showed ±25% levels as an indicator of changes from day to day under quiet conditions (Cander, 2016). The case of November 12–16, 2012, presented in Fig. 6, was considered by first in (Cander, 2016) and refers to winter conditions. SSC appeared on 12 November 2012 at 2311 UT, the minimum Dst = 108 nT appeared at 0800 UT on 14 November 2012. At the HERS station, a positive disturbance appeared immediately after the SSC and the next day turned into a negative
disturbance. At Hainan station, positive disturbance lasted 3 days. The summer case of July 14–18, 2012 is presented in Fig. 7. SSC appeared in 1809 UT 14 July 2012, the minimum Dst = 127 nT appeared at 1900 UT on 15 July. The situation in the HERS is characterized by a negative perturbation of 16 July, followed by a long negative phase. At Hainan station, a positive disturbance is seen with dTEC = 60%, a clear negative phase on 16 July and quiet conditions on the following days. In case of equinox March 16–19, 2015 SSC appeared at 0445 UT 03/17/2015, the minimum Dst = 223 nT appeared at 2300 UT on March 17. The results are shown in Fig. 8. Since the Sanya ionosonde data were available, the results are also given for foF2. As in the HERS, there is a negative phase with a sufficiently high synchronism of variations dfoF2 and dTEC. It should be noted that the graph for foF2 shows not only the observational values (curve, shown by dots) and medians (curve with squares), but also the model values of foF2 (curve with rhombus), foF2(rec) values calculated using TEC(obs) and s(med). These foF2(rec) values are closest to the observational ones. The main results are presented in Table 2 as a comparison of TEC variations for two stations. The first column shows the date of the case in question (the number of the Figure from the article (Cander, 2016) is shown in brackets). The second column shows the results from (Cander, 2016): the maximum in
Fig. 6. TEC values (the left plot) and their percentage deviations dTEC from the monthly median (the right plot) on 12–16 November 2012.
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Fig. 7. TEC values (the left plot) and their percentage deviations dTEC from the monthly median (the right plot) on 14–18 July 2012.
Fig. 8. TEC values (the left plot), foF2 (the middle plot), their percentage deviations dTEC and dfoF2 from the corresponding monthly medians (the right plot) for March 2015.
Table 2 Morphological features of dTEC behavior at Hainan station during 15 magnetic storms. Number of issue
Data dTEC of (Cander, 2016)
Results for Hainan
Min Dst
(1) 12–16.11.2012 (2) 6–10.01.2015 (3) 14–18.7.2012 (4) 31.5–4.6. 2013 (5) 27.6–1.7.2013 (9) 4–8.4.2010 (11) 25–29.9.2011 (13) 23–27.10.2011 (6) 8–12.3.2012 (10) 23–27.4.2012 (12) 30.9–4.10.2012 (14) 7–11.10.2012 (7) 16–20.3.2013 (15) 1–5.10.2013 (8) 16–20.3.2015
72%, 176%, None, None, None, 85%, 101%, None, 51%, 37%, None, None, 92%, None, 65%,
72%, 0.5% 36%, 31% 61%, 61% 34%, 24% 19%, 34% 33%, –33% 42%, 16% –33%, 45% 33%, –33% 43%, 58% 61%, 3% 44%, 57% 35%, 7% 34%, 34% 58%, 24%
108 99 127 119 98 81 118 147 118 108 119 105 132 67 –223
87% 57% 71% 49% 59% 55% 54% 55% 71% 65% 65% 68% 66% 84% 77%
positive and negative values of relative deviations, the third column represents the results for the Hainan station. The last column gives the minimum values of the Dst index. It can be seen that all disturbances are global in nature and the morphology in this region is close to the morphology of the mid-latitude station, but there are differences, namely, the positive phases prevail, and the force of disturbances is less in most cases. As in (Cander, 2016), the amplitude of the perturbed change in TEC does not strongly depend on the intensity of the magnetic storm. In the paper (Cander, 2016), a study was conducted to assess the possibility of predicting magnetic storms from
variations of dTEC. We study synchronism of variations of dfoF2 and dTEC. In the paper (Cander, 2016), only one case, 4–8 April 2010, has been presented. In this paper, the synchronism of variations dfoF2 and dTEC is important for the possibility of using TEC to determine foF2 during disturbances. The study was conducted using available 2004 data. Table 3 shows the dTEC estimates, similar to the results of Table 2. The prevalence of positive perturbations and independence from the disturbance force estimated by the Dst index are confirmed. Examples of changes in parameters and their deviations from the medians are shown in
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Table 3 Features of dTEC behavior at Hainan station during 11 magnetic storms 2004. Date of issue
dTEC
21–23.01 10–12.02 9–11.03 3–5.04 16–18.07 22–24.07 25–26.07 27–28.07 29–31.08 12–14.10 6–10.11
95.2; 111.9 91.3; 21.7 39.3; 47.4 42.2; 10.9 97.2; 26.3 52.8; 68.5 38.0; –32.5 74.2; 45.4 34.9; 52.41 33.9; 37.8 60.1; 80.8
Min Dst 149 109 77 112 80 101 148 197 126 63 373;
289
Figs. 9–11 for some cases in January, November and July 2004. In all Figures, graphs for TEC include observational values and monthly medians, graphs for foF2 contain observational values, medians, values for the original
Fig. 10. TEC values (the upper plot), foF2 (the middle plot), their percentage deviations dTEC and dfoF2 from the monthly medians (the lower plot) for November 2004.
Fig. 9. TEC values (the upper plot), foF2 (the middle plot), their percentage deviations dTEC and dfoF2 from the monthly medians (the lower plot) for January 2004.
model (IRI icon), and values recovered (rec icon) using TEC(obs) and s(med). The graphs for deviations dfoF2 and dTEC additionally contain the values of the Dst index, the value of which was changed in order not to greatly increase the size of the graph. In most cases, graphs for TEC show positive disturbances, which can be observed both in the main phase and in the recovery phase of the magnetic storm. Fig. 11 for 13–18 July illustrates the case of positive disturbance in quiet conditions (after two weeks of such conditions). Such bursts are mentioned in (Cander, 2016) and discussed in detail in (Cander and Ciraolo, 2010) for the mid-latitude European region. The plots for foF2 illustrate that the values calculated using TEC(obs) and s(med) are the closest to the observational foF2. Several cases are seen that are quite common when working with ionosonde data. So, in January 2004, apparently, anomalous ionization of the type of the Es layer appears, leading to an artificial overestimation of dfoF2. More frequent is the absence of foF2 values. It can be assumed that in these cases the use of TEC(obs) and s(med) gives values that are quite close to real.
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Fig. 11. TEC values (the upper plot), foF2 (the middle plot), their percentage deviations dTEC and dfoF2 from the monthly medians (the lower plot) for July 2004.
The deviation plots show fairly high synchronism of dfoF2 and dTEC variations. This is consistent with the results of the work (Maltseva, 2018), which investigated the correlation between foF2 and TEC, dfoF2 and dTEC, and the correlation of these parameters with geomagnetic activity indices where was shown that the usage of TEC to determine foF2 during disturbances is possible in wide latitude range. 6. Conclusions In this paper, the possibilities of using TEC in the lowlatitude zone are investigated. It is shown that the behavior of the TEC in this region is peculiar: the TEC maximums in the diurnal course occur at the time of afternoon, in the seasonal course in March and September–November, and there is a weak manifestation of the winter anomaly. TEC is an indispensable parameter in assessing the accuracy of positioning, causing the need to develop a model of this parameter. As is known, the values of different global maps can vary greatly among themselves (Arikan et al.,
2003). In this region, the difference is minimal. Global and local models provide values close to experimental values. Models in which the TEC values are determined by integrating the model N(h)-profile give more significant deviations: the IRI, IRI-Plas models overestimate values, while the NeQuick model underestimates values. To determine the instantaneous foF2 values using observational TEC, it is necessary to know the equivalent slab thickness s of the ionosphere. It is shown that the models s(IRI), s(IRIPlas), s(NeQuick) differ greatly from each other and from the median of the experimental equivalent slab thickness s(med). The best fit with experimental foF2 is provided by using s(med): it is 1.2–2 times better than models. When studying the TEC behavior during disturbances, the idea of re-visit (Cander, 2016) on the possibility of using TEC to assess the state of the environment was confirmed. The peculiarities of dTEC in comparison with the mid-latitude region are revealed: the predominance of the positive phase, and a smaller value of response. The synchronism of variations dTEC and dfoF2 was very high, although there are cases of mismatch. Using the median s(med)
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allows determining foF2 during disturbances and filling data gaps. The results are in the trend of such articles as, for example (Liang et al., 2015; Nayak et al., 2016), and others. In the first paper, GPS data is used to update the IRI model, while, the authors of the second paper examined the possibility of using the IRTAM Global Maps to assess the state of the ionosphere during a disturbance during 17–19 March 2015 and came to a positive conclusion. This is especially important when using TEC to assess the state of the ionosphere in local areas not provided with vertical sounding data. Acknowledgements Authors thank International GNSS Service (IGS) for GPS data and products, groups of scientists providing the data for SPIDR database, the models IRI, IRI-Plas, NeQuick, NGM. This work was supported by Grant under the state task N3.9696.2017/8.9 from Ministry of Science and Higher Education of Russia. References Aa, E., Huang, W., Yu, S., Liu, S., Shi, L., Gong, J., Chen, Y., Shen, H., 2015. A regional ionospheric TEC mapping technique over China and adjacent areas on the basis of data assimilation. J. Geophys. Res. Space Phys. 120, 5049–5061. https://doi.org/10.1002/2015JA021140. Adebiyi, S.J., Adebesin, B.O., Ikubanni, S.O., Joshua, B.W., 2017. Performance evaluation of GIM-TEC assimilation of the IRI-Plas model at two equatorial stations in the American sector. Space Weather 15, 726–736. https://doi.org/10.1002/2017SW001596. Arikan, F., Erol, C.B., Arikan, O., 2003. Regularized estimation of vertical total electron content from Global Positioning System data. J. Geophys. Res. 108 (A12), 1–12. https://doi.org/10.1029/ 2002JA009605. Atıcı, R., 2018. Comparison of GPS TEC with modelled values from IRI 2016 and IRI-PLAS over Istanbul, Turkey. Astrophys. Space Sci. 363, 231. https://doi.org/10.1007/s10509-018-3457-0. Bilitza, D., 2002. Ionospheric Models for Radio Propagation Studies Review of Radio Science: 1999-2002 URSI by W. Ross Stone (Editor), 625–679. Bilitza, D., Altadill, D., Truhlik, V., Shubin, V., Galkin, I., Reinisch, B., Huang, X., 2017. International Reference Ionosphere 2016: from ionospheric climate to real-time weather predictions. Space Weather 15, 418–429. https://doi.org/10.1002/2016SW001593. Bolaji, O.S., Oyeyemi, E.O., Adewale, A.O., Wu, Q., Okoh, D., Doherty, P.H., et al., 2017. Assessment of IRI-2012, NeQuick-2 and IRI-Plas 2015 models with observed equatorial ionization anomaly in Africa during 2009 sudden stratospheric warming event. J. Atmos. Solar. Terr. Phys. 164, 203–214. https://doi.org/10.1016/j.jastp.2017.08.025. Cander, L.R., 2016. Re-visit of ionosphere storm morphology with TEC data in the current solar cycle. J. Atmos. Solar Terr. Phys. 138–139, 187–205. https://doi.org/10.1016/j.jastp.2016.01.008. Cander, L.R., Ciraolo, L., 2010. Ionospheric total electron content and critical frequencies over Europe at solar minimum. Acta Geophys. 58 (3), 468–490. https://doi.org/10.2478/s11600-009-0061-2. Ezquer, R.G., Scida, L.A., Migoya Orue, Y., Nava, B., Cabrera, M.A., Brunini, C., 2018. NeQuick 2 and IRI Plas VTEC predictions for low latitude and South American sector. Adv. Space Res. 61, 1803–1818. https://doi.org/10.1016/j.asr.2017.10.003. Gordiyenko, G.I., Maltseva, O.A., Arikan, F., Yakovets, A.F., 2018. The performance of the IRI-Plas model as compared with Alouette II and GIM-TEC data over the midlatitude station Alma-Ata. J. Atmos.
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Possibilities of the usage of the total electron content in a low-latitude zone O.A. Maltseva a,⇑, N.S. Mozhaeva b a
Institute for Physics, Southern Federal University, 344090 Rostov-on-Don, Russia b OKTB uˆVektory´, 346880 Bataysk, Russia
Received 8 February 2019; received in revised form 28 June 2019; accepted 10 July 2019 Available online 19 July 2019
Abstract Last years, the small quantity of ionosondes at low latitudes could not give a sufficient picture of behavior of the ionosphere in these areas. Now, reception of experimental data, their analysis and detection of total electron content (TEC) variations at low latitudes are an actual problem. When moving from describing the state of the ionosphere using vertical sounding parameters to describing using TEC, it is important to identify similarities and differences in their behavior in order to assess the possibilities of using TEC. This paper carries out research on four ways of TEC usage in low-latitude zone: (1) research of climatological peculiarities of the TEC behavior, (2) validation of up-to-date TEC models, (3) foF2 calculation according to observational TEC, and (4) peculiarities of TEC behavior during geomagnetic disturbances. Results for station Hainan show the following. 1. In addition to previous investigation it is shown, that the empirical global and local models of TEC provide the values close to observational ones, however climatological features of TEC behavior described by various models may still differ from each other and from the experimental data showing in this case TEC maxima of diurnal variation in the afternoons, seasonal variations in March and October, some slight winter anomaly. 2. In contrast to previous investigation it is shown that the closest to the experimental values of TEC are provided by the IRI-Plas model, while this model overestimates values, and the model NeQuick underestimates values compared to the observational ones. 3. For the first time, the ionospheric equivalent slab thickness s, which plays the role of the coefficient of proportionality between TEC and NmF2, was compared for the IRI, IRI-Plas, NeQuick models with the experimental median s(med) and it was shown, that the usage of observational TEC data and s(med) for calculation of critical frequencies foF2 allows improving correspondence with experimental data by 1.2–2 times compared to the models. 4. Comparison of foF2 and TEC behavior has a particular importance during disturbances. Using the example of 15 strong magnetic storms (Dst < 50 nT), it was shown that, despite its global nature, the details of the TEC response strongly depend on the region. So, the peculiarities of dTEC in comparison with the mid-latitude region are the predominance of the positive phase, and a smaller value of response. The synchronism of variations dTEC and dfoF2 was very high, although there are cases of mismatch. Using the median s (med) allows determining foF2 during disturbances and filling data gaps. Ó 2019 COSPAR. Published by Elsevier Ltd. All rights reserved.
Keywords: Low-latitude ionosphere; Ionospheric models; Total Electron Content (TEC); Equivalent slab thickness; Geomagnetic disturbances
1. Introduction Low-latitude and equatorial regions attract constant attention due to the large variability of the ionospheric ⇑ Corresponding author.
E-mail addresses: [email protected] (O.A. Maltseva), [email protected] (N.S. Mozhaeva). https://doi.org/10.1016/j.asr.2019.07.010 0273-1177/Ó 2019 COSPAR. Published by Elsevier Ltd. All rights reserved.
parameters. In the past, the critical frequency foF2 was the basic characteristic of the ionosphere, but the small number of ionosondes limited possibilities to study the ionosphere in this zone. Now, measurements of TEC by means of satellites of systems GPS, GLONASS and others allow to carry out more detailed researches due to a large number of stations.
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However, despite a considerable number of publications, and special symposia it is impossible to solve all the problems. This paper focuses on the modeling of the TEC parameter and the relationship between TEC and foF2 in the low-latitude zone. In the work (Liu et al., 2014), the monthly medians of TEC and NmF2 were compared over China during 2004 using 34 GPS stations and 10 ionosondes. The example of Guangzhou station (23.1° N, 113.4° E) revealed the following peculiarities of daily and seasonal changes: (1) the TEC and NmF2 maxima fall at the time of afternoon, with the maximum NmF2 delaying by 1–2 h compared to the TEC maximum, and (2) seasonal dependence is characterized by maxima in March and October and with a semi-annual anomaly at which winter values are greater than summer ones; it is clearly expressed for NmF2 and almost not expressed for TEC. This station is the southernmost, but similar results were obtained for higher-latitude stations. The given paper presents the results for an even more southern Hainan station. For representation of TEC, global maps JPL, CODE, UPC, ESA are very widely used (Herna´ndez-Pajares et al., 2009) and also empirical models, global (e.g., Jakowski et al., 2011), and local (Kakinami et al., 2009). Other class of models includes the models based on integration of N(h) -profile (e.g., IRI (Bilitza et al., 2017), NeQuick-2 (Nava et al., 2008)) and others. The important property of these models is long-term prediction of TEC. In the given paper, the special attention is paid to the model IRI-Plas (Gulyaeva, 2011). Its advantages are the assimilation of TEC in the model, presence of very convenient site (http://www.ionolab.org/index.php?language= en), providing great opportunities to study TEC (e.g., Sezen et al., 2018), development of a global forecast system on its basis (Gulyaeva et al., 2013). However this model has not been sufficiently tested, although some works can be picked out. In works (Zakharenkova et al., 2015, Gordiyenko et al., 2018; Atıcı, 2018), results of the model IRI-Plas are used for comparison with experimental data in a zone of the middle latitudes. Authors of the paper (Bolaji et al., 2017) compare several models in an equatorial zone of Africa. The work (Ezquer et al., 2018) examines the ability of the NeQuick-2 and IRI-Plas models to determine TEC values in the US sector from 18.4° N to 64.7° N and the longitude ranges from 281.3° E to 295.9° E. The largest divergences are obtained for low-latitude stations near an equatorial anomaly. It is described, where and when each model works better. In work (Adebiyi, et al., 2017), data analysis of two South American equatorial stations has shown that the usage of TEC alone is not enough to accurately determine the instantaneous parameters of the ionosphere. The most detailed comparison with experimental data was carried out for the NeQuick-2 and IRIPlas models in the work (Okoh et al., 2018). This comparison was performed globally for 36 stations of northern and southern hemispheres from January 2006 to July 2017. It is shown that the trends in the behavior of TECs are fairly accurately reflected by both models, but the
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quantitative characteristics are different. The average annual TEC prediction errors range from 3 TECU for high-latitude stations to 12 TECU for low-latitude stations for the IRI-Plas model. For the model NeQuick-2, these estimates are 2 TECU and 7 TECU, respectively. The main disadvantage of the IRI-Plas model is an overestimation of observational values. Fig. 8 of the paper (Okoh et al., 2018) shows that an electron concentration of the model NeQuick-2 at heights h > hmF2 is lower than for the model IRI-Plas. It should be noted that the TEC values themselves are strongly dependent on the calculation method (e.g., Arikan et al., 2003). It underlines necessity of further comparison and study of TEC behavior in local regions. This paper explores the possibility of using TEC to determine ionospheric conditions in Southeast Asia. Special attention is paid to using the equivalent slab thickness s of the ionosphere. This parameter is known to have independent significance (Stankov and Warnant, 2009), however, in this paper, its role as a coefficient of proportionality between TEC and NmF2 is investigated (e.g., Gulyaeva et al., 2013; Muslim et al., 2015). The most natural is the use of s of various models, but there is no comparison of these models with the observational s and their efficiency. In this paper, the observational median s (med) is compared with the s of the most common IRI, IRI-Plas, NeQuick models and their effectiveness at calculation of foF2, including during disturbances, is investigated. 2. Used data and models Study is performed using data of station Hainan (19.4° N, 109°E) and some other stations with higher latitudes. Observational data of three types are used: (1) foF2(obs), (2) TEC(obs), (3) data characterizing geomagnetic activity. In several papers, long-term data of vertical sounding are used, for example (Zhang et al., 2007; Hu et al., 2014), however in the now decommissioned SPIDR database, which was available at (http://spidr.ngdc.noaa.gov/spidr/ index.jsp) through the Internet, the number of such data was limited. Data of only 11 months for 2004 were accessible. As for values of TEC, global maps GIM (ftp://cddis. gsfc.nasa.gov/pub/gps/products/ionex/) were used. For data characterizing geomagnetic activity, values of a Dstindex (http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3.html) were used. Models IRI (Bilitza et al., 2017), IRI-Plas (Gulyaeva, 2011), NeQuick-2 (Nava et al., 2008) were used online. As is known, calculation of critical frequencies using TEC needs knowledge of proportionality factor which is the equivalent slab thickness of the ionosphere s. It is defined from relationship s = TEC/NmF2. If s and the observational value TEC(obs) are known it is possible to obtain an instantaneous value NmF2(eff) = TEC(obs)/s. The symbol ‘eff’ was entered in the paper (Gulyaeva, 2003). In this paper, four types of s are used: s(IRI), s(IRI-Plas), s (NeQuick), s(med). The factor s(IRI) is defined by the model IRI: s(IRI) = TEC(IRI)/NmF2(IRI). It was
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traditionally used in researches (e.g., McNamara, 1985; Houminer and Soicher, 1996). The factor s(IRI-Plas) is defined by the model IRI-Plas: s(IRI-Plas) = TEC(IRIPlas)/NmF2(IRI-Plas). It is used in (Gulyaeva, 2003; Gulyaeva, 2011; Gulyaeva et al., 2013; Adebiyi et al., 2017). The factor s(NeQuick) will be used in addition in this paper with calculation under the formula s (NeQuick) = TEC(NeQuick)/NmF2(NeQuick). Values of TEC(NeQuick) and Nmf2(NeQuick) are calculated on a site https://t-ict4d.ictp.it/nequick2/nequick-2-web-model. The factor s(med) is a median of the equivalent slab thickness of the ionosphere s(med) = TEC(obs)/NmF2(obs) (Maltseva and Mozhaeva, 2015). It was used also to fill gaps of ionosonde data. 3. Morphological features of behavior of foF2 and TEC and comparison with models Fig. 1 presents observational monthly medians of foF2, TEC and corresponding values for the models IRI, IRIPlas and NeQuick (foF2 values of three empirical models practically coincide therefore only two curves are shown). It is necessary to make an explanation about the presentation of pictures. Labels of the vertical axes of the graphs in all the Figures are placed in the header inside the image to increase the field of the image.
The maximum values of foF2 fall in March-April and September-November which does not coincide with behavior of foF2 at station Guangzhou (Liu et al., 2014), but it is close to results for station Hainan, obtained on large array of the data, accessible to authors (Wang et al., 2017). The behavior of TEC shows the more expressed seasonal dependence that coincides with peculiarities at station Guangzhou (Liu et al., 2014). For the models IRI-Plas and NeQuick, the spring maximum was displaced for April according to behavior of foF2. A weak manifestation of the winter anomaly, when summer values (in July) are lower than winter ones (in January), is visible. The model IRI-Plas overestimated TEC(obs) values, while the model NeQuick underestimated them. One of the main problems of ionospheric modeling is the development of TEC models. Earlier, the basic attention was paid to global models (Bilitza, 2002). Now, the large interest is addressed to the local models focused on local features, which are important for the theory and applications. Especially intensively such models were developed in Southeast region (Aa et al., 2015). In the paper (Luo et al., 2014), five models GIM (JPL), IRI, PIM, NeQuick, Klobuchar were compared. Here in Fig. 2, results for station Hainan with lower latitude are given for July 2011 to compare with TEC behavior for four various global maps JPL, CODE, UPC, ESA for stations HKTU (22.3°
Fig. 1. A seasonal variations of foF2 and TEC for station Hainan in 2004.
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Fig. 2. Diurnal changes of TEC for 3 stations with various latitudes.
114.18°) and WUHN (30.53°, 114.36°), considered in (Luo et al., 2014). Some trend of increasing the maximum values and decreasing the minimum values with decreasing latitude can be seen. It should be noted, that the difference between values of various global maps is lower than in other regions. Results of the paper (Luo et al., 2014) are supplemented in Fig. 3 by comparison with such global models, as IRI (Bilitza et al., 2017), NGM (Jakowski et al., 2011; Hoque and Jakowski, 2011), IRI-Plas, NeQuick and the local model Taiwan (Kakinami et al., 2009). The model IRI is the international standard, the model NGM (German space centre Neistrelitz) is differed by the detailed account of an equatorial anomaly and its testing for lowlatitude stations has given encouraging results in several regions (Maltseva et al., 2014). The model (Kakinami et al., 2009) is local, so for it the results are given for Guangzhou station, which has the closest coordinates to Chung Li station, for which this model is developed. It can be seen that the results of the IRI and Taiwan models coincide well with the experimental data, presented
by the map CODE, while the NGM model in this case overestimates the values, the IRI-Plas model overestimates them even more, and the NeQuick model overestimates the values for Hainan station and greatly underestimates TEC (obs) for Guangzhou station.
4. The ability to determine NmF2 (foF2) using TEC Among the large number of TEC applications, its use for determining NmF2 and, accordingly, foF2 can be distinguished. For such use, it is necessary to know the proportionality coefficient s between TEC and NmF2, i.e. an equivalent slab thickness of the ionosphere. The purpose of this section is to show the role of the correct determination of s when using TEC to calculate foF2. The traditional used thickness is s(IRI). In the paper (Maltseva and Mozhaeva, 2015), it is proposed to use s(med) and is shown that s(IRI) may differ from s(med) in many regions. Parameter s(IRI-Plas) is used in (Gulyaeva, 2003). In this work, s(NeQuick) is additionally used. Fig. 4 shows the
Fig. 3. Comparison of various foF2 and TEC models for low-latitude stations.
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Fig. 4. Comparison of the behavior of equivalent slab thicknesses s of various models used to obtain foF2.
diurnal dependences of s of various models for several months of 2004 (January, March, June, September). Large differences in behavior and in values of s are visible. The greatest difference relates to the morning maximum: values of s(med), s(IRI), s(IRI-Plas) differ, but for the NeQuick model this maximum is absent. For the first three s models, some seasonal variation is visible, while for the NeQuick model, it is significant only near noon. A more visual representation of the differences between s can be obtained from Fig. 5 (left side). And namely these s values are used to calculate the instantaneous foF2 values in accordance with TEC(obs). The results are shown in Fig. 5 (right side) in the form of absolute deviations |DfoF2| of calculated values from experimental ones. Values foF2 (IRI) of the IRI model are monthly medians and do not reflect day to day variations of foF2 which are defined by instantaneous values. However in many cases, values foF2(IRI) are used as diurnal (i.e., instantaneous). In this case, the difference |DfoF2| (mark IRIins) is calculated as a difference between the instantaneous observational value foF2(obs) and value foF2(IRI) at each hour (in this case at each 2 h because global maps have the permission of 2 h). These differences are not connected with the usage of observational TEC(obs). Other curves are calculated with the usage of TEC(obs) and a corresponding thickness s. Diamonds show deviations when using the model values as instantaneous. Of course, the use of values TEC(obs) should reduce this difference, but this is not always the case. In this example, the use of s(IRI) leads to an increase in the difference (the curve shown in squares) in comparison with IRIins, i.e. it is
better to just use the model values. The use of TEC(obs) and s(IRI-Plas) leads to an improvement in the summer months. Using TEC(obs) and s(NeQuick) improves the closeness of the calculated values to the experimental values throughout the year (curve with triangles). The best results shown by dots are given by using TEC(obs) and s (med). The quantitative estimate is presented in Table 1 as average annual deviations. In other areas, the results may be different in determining the advantages of a particular model, because no model can work equally well in all regions, but in any case the best fit with observational foF2 will provide s(med). As noted in section 1, s together with TEC(obs) can be used to fill gaps in ionosonde data. Obviously, the use of s(med) will give the best results in this case. 5. Features of TEC behavior during disturbances A large number of articles are devoted to the behavior of the ionosphere during disturbances, but it turned out that a new look at the relationship between magnetosphere-iono sphere-thermospheric effects is possible when using TEC to assess the state of the environment. It is presented in the paper (Cander, 2016) on the example of the behavior of the relative deviations dTEC during 15 magnetic storms from January 2009 to May 2015 according to HERS GPS data (0.33° E, 50.86° N). The seasonal dependence of the effect of magnetic storms on local characteristics known from earlier ionospheric studies was confirmed for the TEC and the relative role of various processes was revealed. At the latitude of the HERS station, the typical
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Fig. 5. Comparison of the effectiveness of using s of different models.
Table 1 Comparison of the effectiveness of using s of different models. |DfoF2|, MHz
ins
med
IRI
Plas
NeQuick
Mean
1.43
0.89
1.77
1.51
1.21
morphology of the ionospheric storm is as follows: (1) the sequence of phases has common features, (2) TEC tends to increase in the first 24 h (positive phase), then negative phase and 1–2 days recovery, (3) during negative phase of changes in the TEC, amplitude of disturbances shows a significant decrease in the summer compared to the winter, and (4) on average, there are a large number of positive disturbances followed by negative ones and an insignificant number of negative disturbances followed by positive ones. In the present article, the results are obtained for the Hainan station for all cases considered in (Cander, 2016). Figs. 6–8 illustrate the results on the example of three cases from (Cander, 2016) relating to different seasons. The graphs of variations showed ±25% levels as an indicator of changes from day to day under quiet conditions (Cander, 2016). The case of November 12–16, 2012, presented in Fig. 6, was considered by first in (Cander, 2016) and refers to winter conditions. SSC appeared on 12 November 2012 at 2311 UT, the minimum Dst = 108 nT appeared at 0800 UT on 14 November 2012. At the HERS station, a positive disturbance appeared immediately after the SSC and the next day turned into a negative
disturbance. At Hainan station, positive disturbance lasted 3 days. The summer case of July 14–18, 2012 is presented in Fig. 7. SSC appeared in 1809 UT 14 July 2012, the minimum Dst = 127 nT appeared at 1900 UT on 15 July. The situation in the HERS is characterized by a negative perturbation of 16 July, followed by a long negative phase. At Hainan station, a positive disturbance is seen with dTEC = 60%, a clear negative phase on 16 July and quiet conditions on the following days. In case of equinox March 16–19, 2015 SSC appeared at 0445 UT 03/17/2015, the minimum Dst = 223 nT appeared at 2300 UT on March 17. The results are shown in Fig. 8. Since the Sanya ionosonde data were available, the results are also given for foF2. As in the HERS, there is a negative phase with a sufficiently high synchronism of variations dfoF2 and dTEC. It should be noted that the graph for foF2 shows not only the observational values (curve, shown by dots) and medians (curve with squares), but also the model values of foF2 (curve with rhombus), foF2(rec) values calculated using TEC(obs) and s(med). These foF2(rec) values are closest to the observational ones. The main results are presented in Table 2 as a comparison of TEC variations for two stations. The first column shows the date of the case in question (the number of the Figure from the article (Cander, 2016) is shown in brackets). The second column shows the results from (Cander, 2016): the maximum in
Fig. 6. TEC values (the left plot) and their percentage deviations dTEC from the monthly median (the right plot) on 12–16 November 2012.
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Fig. 7. TEC values (the left plot) and their percentage deviations dTEC from the monthly median (the right plot) on 14–18 July 2012.
Fig. 8. TEC values (the left plot), foF2 (the middle plot), their percentage deviations dTEC and dfoF2 from the corresponding monthly medians (the right plot) for March 2015.
Table 2 Morphological features of dTEC behavior at Hainan station during 15 magnetic storms. Number of issue
Data dTEC of (Cander, 2016)
Results for Hainan
Min Dst
(1) 12–16.11.2012 (2) 6–10.01.2015 (3) 14–18.7.2012 (4) 31.5–4.6. 2013 (5) 27.6–1.7.2013 (9) 4–8.4.2010 (11) 25–29.9.2011 (13) 23–27.10.2011 (6) 8–12.3.2012 (10) 23–27.4.2012 (12) 30.9–4.10.2012 (14) 7–11.10.2012 (7) 16–20.3.2013 (15) 1–5.10.2013 (8) 16–20.3.2015
72%, 176%, None, None, None, 85%, 101%, None, 51%, 37%, None, None, 92%, None, 65%,
72%, 0.5% 36%, 31% 61%, 61% 34%, 24% 19%, 34% 33%, –33% 42%, 16% –33%, 45% 33%, –33% 43%, 58% 61%, 3% 44%, 57% 35%, 7% 34%, 34% 58%, 24%
108 99 127 119 98 81 118 147 118 108 119 105 132 67 –223
87% 57% 71% 49% 59% 55% 54% 55% 71% 65% 65% 68% 66% 84% 77%
positive and negative values of relative deviations, the third column represents the results for the Hainan station. The last column gives the minimum values of the Dst index. It can be seen that all disturbances are global in nature and the morphology in this region is close to the morphology of the mid-latitude station, but there are differences, namely, the positive phases prevail, and the force of disturbances is less in most cases. As in (Cander, 2016), the amplitude of the perturbed change in TEC does not strongly depend on the intensity of the magnetic storm. In the paper (Cander, 2016), a study was conducted to assess the possibility of predicting magnetic storms from
variations of dTEC. We study synchronism of variations of dfoF2 and dTEC. In the paper (Cander, 2016), only one case, 4–8 April 2010, has been presented. In this paper, the synchronism of variations dfoF2 and dTEC is important for the possibility of using TEC to determine foF2 during disturbances. The study was conducted using available 2004 data. Table 3 shows the dTEC estimates, similar to the results of Table 2. The prevalence of positive perturbations and independence from the disturbance force estimated by the Dst index are confirmed. Examples of changes in parameters and their deviations from the medians are shown in
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Table 3 Features of dTEC behavior at Hainan station during 11 magnetic storms 2004. Date of issue
dTEC
21–23.01 10–12.02 9–11.03 3–5.04 16–18.07 22–24.07 25–26.07 27–28.07 29–31.08 12–14.10 6–10.11
95.2; 111.9 91.3; 21.7 39.3; 47.4 42.2; 10.9 97.2; 26.3 52.8; 68.5 38.0; –32.5 74.2; 45.4 34.9; 52.41 33.9; 37.8 60.1; 80.8
Min Dst 149 109 77 112 80 101 148 197 126 63 373;
289
Figs. 9–11 for some cases in January, November and July 2004. In all Figures, graphs for TEC include observational values and monthly medians, graphs for foF2 contain observational values, medians, values for the original
Fig. 10. TEC values (the upper plot), foF2 (the middle plot), their percentage deviations dTEC and dfoF2 from the monthly medians (the lower plot) for November 2004.
Fig. 9. TEC values (the upper plot), foF2 (the middle plot), their percentage deviations dTEC and dfoF2 from the monthly medians (the lower plot) for January 2004.
model (IRI icon), and values recovered (rec icon) using TEC(obs) and s(med). The graphs for deviations dfoF2 and dTEC additionally contain the values of the Dst index, the value of which was changed in order not to greatly increase the size of the graph. In most cases, graphs for TEC show positive disturbances, which can be observed both in the main phase and in the recovery phase of the magnetic storm. Fig. 11 for 13–18 July illustrates the case of positive disturbance in quiet conditions (after two weeks of such conditions). Such bursts are mentioned in (Cander, 2016) and discussed in detail in (Cander and Ciraolo, 2010) for the mid-latitude European region. The plots for foF2 illustrate that the values calculated using TEC(obs) and s(med) are the closest to the observational foF2. Several cases are seen that are quite common when working with ionosonde data. So, in January 2004, apparently, anomalous ionization of the type of the Es layer appears, leading to an artificial overestimation of dfoF2. More frequent is the absence of foF2 values. It can be assumed that in these cases the use of TEC(obs) and s(med) gives values that are quite close to real.
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Fig. 11. TEC values (the upper plot), foF2 (the middle plot), their percentage deviations dTEC and dfoF2 from the monthly medians (the lower plot) for July 2004.
The deviation plots show fairly high synchronism of dfoF2 and dTEC variations. This is consistent with the results of the work (Maltseva, 2018), which investigated the correlation between foF2 and TEC, dfoF2 and dTEC, and the correlation of these parameters with geomagnetic activity indices where was shown that the usage of TEC to determine foF2 during disturbances is possible in wide latitude range. 6. Conclusions In this paper, the possibilities of using TEC in the lowlatitude zone are investigated. It is shown that the behavior of the TEC in this region is peculiar: the TEC maximums in the diurnal course occur at the time of afternoon, in the seasonal course in March and September–November, and there is a weak manifestation of the winter anomaly. TEC is an indispensable parameter in assessing the accuracy of positioning, causing the need to develop a model of this parameter. As is known, the values of different global maps can vary greatly among themselves (Arikan et al.,
2003). In this region, the difference is minimal. Global and local models provide values close to experimental values. Models in which the TEC values are determined by integrating the model N(h)-profile give more significant deviations: the IRI, IRI-Plas models overestimate values, while the NeQuick model underestimates values. To determine the instantaneous foF2 values using observational TEC, it is necessary to know the equivalent slab thickness s of the ionosphere. It is shown that the models s(IRI), s(IRIPlas), s(NeQuick) differ greatly from each other and from the median of the experimental equivalent slab thickness s(med). The best fit with experimental foF2 is provided by using s(med): it is 1.2–2 times better than models. When studying the TEC behavior during disturbances, the idea of re-visit (Cander, 2016) on the possibility of using TEC to assess the state of the environment was confirmed. The peculiarities of dTEC in comparison with the mid-latitude region are revealed: the predominance of the positive phase, and a smaller value of response. The synchronism of variations dTEC and dfoF2 was very high, although there are cases of mismatch. Using the median s(med)
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