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    Sporadic E critical frequency detection using three EIA region ionosonde stations over Southeast Asia
    (2025-03-01)
    Wichaipanich, Noraset
    ;
    Nishioka, Michi
    ;
    Min Myint, Lin Min
    ;
    Supnithi, Pornchai
    This paper presents the occurrence of the sporadic E layer critical frequency (foEs) measured from three ionosonde stations in the Southeast Asia equatorial ionization anomaly (EIA) regions. These three ionosonde stations include two in Thailand: Chiang Mai (18.76°N, 98.93°E, Dip 12.7°) and Chumphon (10.72°N, 99.37°E, Dip 3.0°), and one in Indonesia: Kototabang (0.2°S, 100.32°E, Dip −10.1°). The daily hourly foEs values observed during 2010 and 2015 were statistically analyzed for foEs occurrence during low and high solar activity periods. Additionally, the number of foEs occurrences was analyzed in terms of the percentage of occurrence (%foEs). The results show that the occurrences of foEs from all three stations were similar, with the monthly hourly occurrence of foEs peaking in the June solstice season (May, June, July, August). Meanwhile, foEs appeared relatively low during the September equinox (September, October) and the December solstice (November, December, January, February) seasons. Furthermore, the frequency of foEs occurrence peaks around 16–20 LT, except in 2015 at Chiang Mai and Chumphon, where peaks were observed at 10 LT and 15 LT, respectively. Additionally, comparing the three stations reveals that in 2010, the maximum number of foEs occurrences was at Chiang Mai (≈21 %), followed by Kototabang (≈19 %) and Chumphon (≈16 %). In 2015, the highest number was observed at Kototabang (≈17 %), followed by Chumphon (≈14 %) and Chiang Mai (≈8%). Furthermore, the maximum frequency of foEs was highest at Chiang Mai (20–25 MHz), followed by Chumphon (15–20 MHz) and Kototabang (10–15 MHz). Additionally, foEs occurrences during low solar activity (2010) were higher than those during high solar activity (2015). It was assumed that the occurrence of foEs in the Southeast Asian sector was anti-correlated with the solar cycle and asymmetric characteristics. We hope that this analytical information will be useful for future HF and VHF communications design in the Southeast Asia region.
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    Assessing the potential of ionosonde for forecasting post-sunset equatorial spread F: an observational experiment in Southeast Asia
    (2023-12-01)
    Abadi, Prayitno
    ;
    Ali Ahmad, Umar
    ;
    Otsuka, Yuichi
    ;
    Jamjareegulgarn, Punyawi
    ;
    Almahi, Alif
    The occurrence of equatorial spread F (ESF) has the potential to detrimentally impact space-based technological systems. This study investigates the utility of ionosondes in forecasting the incidence of post-sunset ESF in the zonal direction, utilizing observational data obtained from four ionosondes located near the magnetic Equator in Southeast Asia. Data were collected during the equinox seasons (March–April and September–October) between 2003 and 2020. To establish a relationship between the probability of post-sunset ESF occurrence and the evening vertical plasma drift (v), a logistic regression model was employed. Post-sunset ESF occurrence is defined as the presence of ESF during the time window between 19:00 and 21:00 LT, while v is derived from the average time derivative of virtual heights during the interval from 18:30 to 19:00 LT. Results indicate that the probability of post-sunset ESF occurrence approaches zero, signifying that ESF is unlikely to develop when v is negative. Conversely, when v exceeds 30 m/s, the probability of post-sunset ESF occurrence surpasses 0.87, indicating that ESF occurs almost invariably. The likelihood of post-sunset ESF occurrence reaches 1 when v equals or exceeds 40 m/s. Utilizing this model, the study determined that a single ionosonde positioned at the Equator can effectively forecast the incidence of post-sunset ESF up to a longitudinal distance of 30° from its location. The accuracy of ionosondes in predicting post-sunset ESF occurrence above their respective locations is approximately 0.80, with a 10% decrease in accuracy when forecasting ESF occurrence at longitudinal distances of 30°. In conclusion, this study enhances our understanding of the link between the evening vertical plasma drift and the manifestation of post-sunset ESF by leveraging ionosonde data. Furthermore, it provides valuable insights into the recommended coverage range of ionosondes for predicting post-sunset ESF occurrence in the zonal direction, which can be employed to fortify regional space weather services. Graphical Abstract: [Figure not available: see fulltext.].
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    Thailand low and equatorial F2-layer peak electron density and comparison with IRI-2007 model
    (2012-01-01)
    Wichaipanich, N.
    ;
    Supnithi, P.
    ;
    Tsugawa, T.
    ;
    Maruyama, T.
    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 F<inf>2</inf>-layer peak electron density (N<inf>m</inf>F<inf>2</inf>) which is derived from the F<inf>2</inf>-layer critical frequency, f<inf>0</inf>F <inf>2</inf>. 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 N<inf>m</inf>F<inf>2</inf> predicted by the IRI (URSI and CCIR options) model show a feature generally similar to the observed N<inf>m</inf>F<inf>2</inf>. 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 N<inf>m</inf>F<inf>2</inf> values predicted by the CCIR option show a smaller range of deviation than the N<inf>m</inf>F<inf>2</inf> values predicted by the URSI option. During post-sunset to morning hours (around 21:00-09:00 LT), the observed N<inf>m</inf>F<inf>2</inf> at both stations are almost identical for the periods of low solar activity. However, during daytime, the observed N <inf>m</inf>F<inf>2</inf> 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 N<inf>m</inf>F<inf>2</inf> over Southeast Asia, especially for the areas covered by Chumphon and Chiang Mai stations. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS).
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    Thailand equatorial F2-layer peak height and comparison with the IRI-2001 model
    (2009-12-01)
    Wichaipanich, Noraset
    ;
    Supnithi, Pornchai
    In this paper, the F2-layer peak height (hmF2) are calculated from the observed M(3000)F2 and then compared with IRI-2001 model. The data used for this study are obtained from bottomside ionogram recorded by the FM/CW ionosonde at Chumphon campus of King Mongkut's institute of Technology Ladkrabang (latitude 10.72°N and longitude 99.37°E), Thailand, located near the magnetic equator. The measurement data including March, June, September and December in 2004, 2005 and 2006 for the period of low solar activity, are analyzed and then compared with IRI-2001 model predictions. Our study shows that the agreement between the data and the model at Southeast Asia will be better if the Shimazaki's formula is used. The results are important for the future improvements of the IRI model for the hmF2 at Thailand equatorial latitude. ©2009 IEEE.