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Assessment of improvement of the IRI model for foF2 variability over three latitudes in different hemispheres during low and high solar activities
(2021-03-01)
Timoçin, Erdinç
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Temuçin, Hüseyin
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Inyurt, Samed
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Shah, Munawar
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Jamjareegulgarn, Punyawi
This paper discusses the diurnal and seasonal variations of the F2 layer critical frequency (foF2) and the improvement of performance of the IRI-2016 model in predicting foF2 over three latitudes in different hemispheres during low and high solar activities. We extracted the foF2 data from six ionosonde stations which are Manila (14.7oN, 121.1oE), Yamagawa (31.2oN, 130.6oE), Yakutsk (62.0oN,129.6oE), Townsville (19.6oS, 146.8oE), Hobart (42.9oS, 147.3oE) and Terre Adelie (66.6oS, 140.0oE). The data of both low solar activity (LSA) period and high solar activity (HSA) periods were divided into three seasons as Northern Summer (May, June, July and August), Equinoxes (March, April, September and October) and Northern Winter (November, December, January and February). The present study showed that the IRI-2016 performance is strongly dependent on the solar activity, latitude, season, local time and hemisphere. For both hemispheres, the foF2 values at low latitude station are larger than those at middle latitude station, whereas the foF2 values at middle latitude station are larger than those at high latitude station. The agreement between IRI2016-modelled foF2 and foF2 measurements on all stations selected in the northern hemisphere is best for North Summer and worst for North Winter. For northern hemisphere, the values of relative deviations during both solar activities are largest in high latitudes and smallest in middle latitudes. As for southern hemisphere, the values of relative deviations during LSA are largest in middle latitudes and smallest in high latitudes, whereas the values of relative deviations during HSA are largest in low latitudes and smallest in high latitudes. It is thought that the relative deviations in the observed foF2 values are caused by solar activity that strongly alter chemical and electromagnetic processes in the ionosphere. These results are important for future improvements depending on solar activity and seasons in the IRI model for foF2 values over three latitudes in different hemispheres.
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The occurrence of equatorial spread-F at conjugate stations in Southeast Asia
(2015-04-15)
Klinngam, Somjai
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Supnithi, Pornchai
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Rungraengwajiake, Sarawoot
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Tsugawa, Takuya
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Ishii, Mamoru
In this study, the probability of equatorial spread-F (ESF) occurrence at the conjugate stations in Southeast Asia: Chiangmai station (CMU), Thailand, and Kototabang station (KTB), Indonesia, and near the magnetic equator, Chumphon station (CPN), Thailand, is presented. We analyze the ionogram data recorded by the Frequency Modulated Continuous Wave (FM/CW) ionosondes for the periods of minimum solar activity from September 2008 to April 2009 and in the equinoctial months (March and April) from 2006 to 2013. The spread-F signatures are manually categorized into three types: the frequency spread-F (FSF), the range spread-F (RSF) and the mixed spread-F (MSF) and the monthly average percentage of the occurrence of each ESF type is presented. The results show that the percentage of RSF occurrence at CPN, which is located around the magnetic equator, is higher than at other stations and the RSF mostly occurs during the equinoctial months. On the other hand, the FSF occurrence at CMU and KTB, that are located in the northern and southern hemispheres, respectively, are higher than at CPN. The RSF occurrence typically has the peaks before midnight, while the maximum occurrence rate of FSF is after midnight. Furthermore, the RSF onsets normally precede the FSF onsets by about 1-2 h. As the solar activity levels go up, the percentages of RSF occurrence increase, but the percentages of FSF tends to decrease. In addition, we compare the statistics of observed RSF occurrence with the prediction of the IRI-2012 model. The results show that the IRI model overestimates the observed RSF occurrence at all stations during most of the solar activity levels and seasons, except in June 2008 (21:00-03:00 LT), March 2011 (23:30-01:00 LT) and March 2013 (01:00-02:30 LT) when the IRI model underestimates our observations. However, the IRI model gives closer probability of RSF occurrence to our observed values at CPN, especially in equinoctial months and during the periods of medium solar activity. This work is important for an improvement of the IRI model in the prediction of the spread F occurrence probability in the low-latitude region.
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Comparison of ionosphere characteristic parameters obtained by ionosonde with IRI-2007 model over Southeast Asia
(2013-11-15)
Wichaipanich, N.
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Supnithi, P.
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Tsugawa, T.
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Maruyama, T.
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Nagatsuma, T.
In this work, the foF2 and hmF2 parameters at the conjugate points near the magnetic equator of Southeast Asia are studied and compared with the International Reference Ionosphere (IRI) model. Three ionosondes are installed nearly along the magnetic meridian of 100 E; one at the magnetic equator, namely Chumphon (10.72 N, 99.37 E, dip angle 3.0 N), and the other two at the magnetic conjugate points, namely Chiang Mai (18.76 N, 98.93 E, dip angle 12.7 N) and Kototabang (0.2 S, 100.30 E, dip angle 10.1 S). The monthly hourly medians of the foF2 and hmF2 parameters are calculated and compared with the predictions obtained from the IRI-2007 model from January 2004 to February 2007. Our results show that: the variations of foF2 and hmF2 predicted by the IRI-2007 model generally show the similar feature to the observed data. Both parameters generally show better agreement with the IRI predictions during daytime than during nighttime. For foF2, most of the results show that the IRI model overestimates the observed foF2 at the magnetic equator (Chumphon), underestimates at the northern crest (Chiang Mai) and is close to the measured ones at the southern crest of the EIA (Kototabang). For hmF2, the predicted hmF2 values are close to the hmF2(M3000F2 <inf>OBS</inf> ) during daytime. During nighttime, the IRI model gives the underestimation at the magnetic equator and the overestimation at both EIA crests. The results are important for the future improvements of the IRI model for foF2 and hmF2 over Southeast Asia region. © 2012 COSPAR. Published by Elsevier Ltd. All rights reserved.
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Thailand low and equatorial F2-layer peak electron density and comparison with IRI-2007 model
(2012-01-01)
Wichaipanich, N.
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Supnithi, P.
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Tsugawa, T.
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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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Ionospheric variation at Thailand equatorial latitude station: Comparison between observations and IRI-2001 model predictions
(2010-01-15)
Wichaipanich, Noraset
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Supnithi, Pornchai
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Ishii, Mamoru
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Maruyama, Takashi
In this paper, the F2-layer critical frequency (foF2) and peak height (hmF2) measured by the FM/CW ionosonde at Thailand equatorial latitude station, namely Chumphon (10.72°N, 99.37°E, dip 3.22) are presented. The measurement data during low solar activity from January 2004 to December 2006 are analyzed based on the diurnal, seasonal variation. The results are then compared with IRI-2001 model predictions. Our study shows that: (1) In general, both the URSI and CCIR options of the IRI model give foF2 close to the measured ones, but the CCIR option produces a smaller range of deviation than the URSI option. The agreement during daytime is generally better than during nighttime. Overestimation mostly occurs in 2004 and 2006, while underestimation is during pre-sunrise hours in June solstice in 2005. The peak foF2 around sunset is higher during March equinox and September equinox than the other seasons, with longer duration of maximum levels in March equinox than September equinox. Large coefficients of variability foF2 occur during pre-sunrise hours. Meanwhile, the best agreement between the observed foF2 and the IRI model is obtained in June solstice. (2) In general, The IRI (CCIR) model predicts the observed hmF2 well during daytime in June solstice from 2004-2006, but it overestimates during March equinox, September equinox and December solstice. For nighttime, the model overestimates hmF2 values for all seasons especially during March equinox and September equinox. However, the model underestimates hmF2 values during September equinox and for some cases during June solstice and December solstice at pre-sunrise. The agreement between the IRI model and the hmF2(M3000 <inf>OBS</inf> ) is worst around noontime, post-sunset and pre-sunrise hours. All comparative studies give feedback for new improvements of CCIR and URSI IRI models. © 2009.

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