Thailand low and equatorial F2-layer peak electron density and comparison with IRI-2007 model
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Ionosonde measurements obtained at two Thailand ionospheric stations, namely Chumphon (10.72°N, 99.37°E, dip 3.0°N) and Chiang Mai (18.76°N, 98.93°E, dip 12.7°N) are used to examine the variation of the F2-layer peak electron density (NmF2) which is derived from the F2-layer critical frequency, fof2. Measured data from September 2004 to August 2005 (a period of low solar activity) are analyzed based on the diurnal and seasonal variation and then compared with IRI-2007 model predictions. Our results show that, in general, the diurnal and seasonal variations of the NmF2 predicted by the IRI (URSI and CCIR options) model show a feature generally similar to the observed NmF2. Underestimation mostly occurs in all seasons except during the September equinox and the December solstice at Chumphon, and the September equinox and the March equinox at Chiang Mai, when they overestimate those measured. The best agreement between observation and prediction occurs during the pre-sunrise to post-sunrise hours. The best agreement of the %PD values of both the options occurs during the March equinox, while the agreement is the worst during the September equinox. The NmF2 values predicted by the CCIR option show a smaller range of deviation than the NmF2 values predicted by the URSI option. During post-sunset to morning hours (around 21:00–09:00 LT), the observed NmF2 at both stations are almost identical for the periods of low solar activity. However, during daytime, the observed NmF2 at Chumphon is lower than that at Chiang Mai. The difference between these two stations can be explained by the equatorial ionospheric anomaly (EIA). These results are important for future improvements of the IRI model for NmF2 over Southeast Asia, especially for the areas covered by Chumphon and Chiang Mai stations.
Key wordsEquatorial latitude ionosonde ionogram IRI model NmF2 solar activity
The ionosonde is one of the most widely-used instruments for studying ionospheric variability, which is important for a better understanding of the ionosphere and the design of HF, VHF and UHF communication systems. The F2-layer peak electron density (NmF2) is an important parameter which is derived from the F2-layer critical frequency ( foF2) measured by ionosondes. This parameter is used for the development and improvement of ionospheric models, such as the International Reference Ionosphere (IRI) (Bilitza, 2001). The IRI is a widely-used global empirical ionospheric model, which describes the electron density, electron temperature, ion temperature and ion composition in the altitude range of approximately 50 to 1,500 kilometers, for a given location, time and sunspot number. Many improvements have been made to this model (IRI-80, IRI-90, IRI-95, IRI-2000 and IRI-2001). The most recent update was released in 2007, known as IRI-2007 (Bilitza and Reinisch, 2008). The most important changes in IRI-2007 are: (1) two new options for the topside electron density, (2) a new model for the topside ion composition, (3) the first-time inclusions of a model for the spread F occurrence probability, (4) a Neural Net model for the auroral D-region electron densities, (5) a model for the plasmasphere electron temperature and (6) the latest International Geomagnetic Reference Field (IGRF) model for the computation of magnetic coordinates, including their changes due to the secular variation of the magnetic field. The IRI model has two options for the prediction of the NmF2: one is the model developed by the International Radio Consultative Committee, namely CCIR (CCIR, 1966) and the other is the model developed by the International Union of Radio Science, namely URSI (Rush et al., 1989). The CCIR options are based on monthly median values obtained by a worldwide network of ionosondes (about 150 stations). The URSI options are based on both ionosonde data (about 180 stations) and the values obtained by aeronomic theory for filling the data gaps above the oceans and in the southern hemisphere.
The observed ionospheric data in many parts of the world have been analyzed by investigating the diurnal and seasonal variations, and then compared with the IRI model. In Africa, the variations of the F2 peak parameters obtained by the ionosonde at Ouagadougou (12.4°N, 1.5°W, dip 5.9°N), Burkina Faso and Korhogo (9.3°N, 5.4°W, dip 0.67°S), Cote-d’Ivoire, were compared with IRI model predictions (Adeniyi et al., 2003; Obrou et al., 2003; Bilitza et al., 2004). In South America, a comparison was made between the results from the IRI model and the ionospheric data collected by digisondes located at Sao Luis (2.6°S, 44.2°W, dip 0.5°S), Cachoeira Paulista (22.5°S, 45°W, dip 28°S), Palmas (10.7°S, 45.20°W, dip 10.8°S), Sao Jose dos Campos (23.20°S, 45.86°W, dip 38.41°S), Brazil, and Jicamarca (12.0°S, 76.9°W, dip 1.0°N), Peru, and the data measured by the ionosonde at Tucuman (26.9°S, 294.6°E), Argentina (Batista and Abdu, 2004; Bertoni et al., 2006; Lee and Reinisch, 2006; Lee et al., 2008; Ezquer et al., 2008). In Europe, Ratovsky et al. (2009) compared the observed ionospheric data with the IRI model at Irkutsk (52.3°N, 104.3°E), Russia. In Asia, Zhang et al. (2007), Sethi et al. (2007), Chuo and Lee (2008), Ayub et al. (2009), Wichaipanich et al. (2010) compared the IRI model results with experimental data at Hainan (19.4°N, 109.0°E, dip 22.8°N), China, New Delhi (28.6°N, 77.2°E, dip 42.4°N), India, Chung-Li (24.9°N, 121.1°E, dip 35°N), Taiwan, Karachi (24.95°N, 67.14°E), Islamabad (33.75°N, 72.87°E), Pakistan, and Chumphon (10.72°N, 99.37°E, dip 3.0°N), Thailand, respectively. Although studies of the observed ionospheric data are common in many parts of the world, only a few studies of ionospheric conditions have been carried out for the region over Southeast Asia.
2. Data and Methodology
The monthly hourly medians and the seasonally hourly medians of NmF2 at Chumphon and Chiang Mai for four seasons, including the September equinox (September and October in 2004), the December solstice (November, December in 2004 and January and February in 2005), the March equinox (March and April in 2005), and the June solstice (May, June, July and August in 2005), have been plotted and compared with the IRI model predictions.
For the comparison between observation and the model, both the URSI and CCIR options of the IRI-2007 model are used to predict the NmF2 values, which can be downloaded from the site: http://ccmc.gsfc.nasa.gov/modelweb/models/ iri_vitmo.php.
3. Results and Discussions
3.1 Chumphon station
3.2 Chiang Mai station
3.3 Comparing NmF2 at Chumphon and Chiang Mai
Maximum seasonal hourly medians of NmF2 (×1012 electron/m3).
Difference in NmF2 between stations
The observed NmF2 at Chiang Mai are higher than that at Chumphon during the daytime which can be explained by the Equatorial Anomaly (Anderson, 1973). The equatorial and low-latitude regions show some unique behavior when compared with middle and high latitudes. The vertical electromagnetic drift is enhanced and the Equatorial Ionization Anomaly (EIA) is intensified resulting in variations in the F-layer at equatorial and low-latitudes as follows: the F-layer is lifted up at the magnetic equator but the peak density decreases, however, the F-layer peak density increases at the crest of the anomaly (located at approximately 15° north and south of the magnetic latitude, moderated by the meridional wind from the magnetic equator to the crests of the anomaly). While Chumphon is close to the magnetic equator (Geomagnetic dip latitude +3.0°N), Chiang Mai is located at the northern anomaly crest (Geomagnetic dip latitude +12.7°N), causing higher NmF2 median values at Chiang Mai. The location of both stations in relation to the Equatorial Anomaly explains the differences in electron density between the two stations.
When compared with previous studies for periods with low solar activity, our results are similar to the study of Ayub et al. (2009) at two Pakistan low-latitude stations, namely Karachi and Islamabad, in that the IRI model predicts NmF2 values close to the observed NmF2 during pre-sunrise to pre-noon (around 03:00–09:00 LT), but a difference occurred during daytime, when they found that the IRI/URSI model overestimates NmF2obs, while our results show an underestimation. Furthermore, they found that the observed NmF2 values at Karachi are higher than that at Islamabad due to the equatorial anomaly. While Karachi is located in the EIA, Islamabad is outside the anomaly, explaining the higher NmF2 median values and bite-outs at Karachi. The maximum NmF2 values reach a peak during the equinox seasons and a minimum level during the solstice seasons. In addition, our results differ from the study of Ezquer et al. (2008) at Tucuman, Argentina, in that good predictions are provided by the URSI option for night-time and the agreement between prediction and measurement is worst during the June solstice in that the %PD varies between −50 and 80% during the pre-noontime hours (10:00–11:00 LT), while our results show good predictions from the CCIR option, with a disagreement between prediction and measurement occurring during the September equinox where the %PD varies between −70 and +80% during night-time at Chumphon station and between −40 and +50% during night-time at Chiang Mai station. Furthermore, our results differ from the studies of Lee and Reinisch (2006) and Lee et al. (2008) at the equatorial latitude station in Peru, namely Jicamarca, in that both the URSI and CCIR options of the IRI-2007 model are generally close to the observed values, but our results show that both models underestimate observed values during the midnight and pre-sunrise hours, especially in 2005. This underestimation is consistent with the results of Wichaipanich et al. (2010) although, in that work, foF2 is studied from 2004–2006.
The diurnal and seasonal variations of NmF2 predicted by the IRI (URSI and CCIR options) model generally show the same features as the observed NmF2.
In most cases both the URSI and CCIR options underestimate the observed NmF2 except during the September equinox and the December solstice at Chumphon, and the September equinox and the March equinox at Chiang Mai, when they overestimate NmF2obs.
The best agreement between observation and prediction occurs during pre-sunrise to post-sunrise hours (around 03:00–09:00 LT).
The best percentage agreement occurs during the March equinox for Chumphon and Chiang Mai stations.
The worst %PD values are found during night-time during the September equinox for both stations with the highest value observed at Chumphon, where %PD reaches 80% for the CCIR option.
Although both the URSI and CCIR options of the IRI model predict NmF2 close to the NmF2obs especially during daytime, the CCIR option produces a smaller range of deviation than the URSI option.
During post-sunset to morning hours (around 21:00–09:00 LT), the observed NmF2 at both stations are almost identical for the periods of low solar activity. However, during daytime, the observed NmF2 values at Chiang Mai are larger than those at Chumphon due to the higher dip angle related to the Equatorial Anomaly.
A bite-out phenomenon is clearly seen during noontime hours (around 11:00 LT) at Chumphon for all seasons, but it rarely occurs during the equinox seasons at Chiang Mai.
In this research work, the financial support from the Telecommunications Research and Industrial Development Institute (TRIDI), National Telecommunications Commission (NTC) fund (Grant No. PHD/004/2552) to Mr. Noraset Wichaipanich and Assoc. Prof. Dr. Pornchai Supnithi is acknowledged. We would like to thank the Space Environment Group, National Institute of Information and Communications Technology (NICT), Japan, for the equipment and technical support. In addition, we are grateful to the reviewers and the editor for comments and suggestions which greatly helped to improve this manuscript.
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