This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

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1 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

2 Available online at Advances in Space Research 43 (2009) Comparison of ionospheric F2 peak parameters fof2 and hmf2 with IRI2001 at Hainan X. Wang *, J.K. Shi, G.J. Wang, Y. Gong State Key Laboratory of Space Weather (Center for Space Science and Applied Research, Chinese Academy of Sciences), Beijing, China Received 1 November 2007; received in revised form 22 September 2008; accepted 22 September 2008 Abstract Monthly median values of fof2, hmf2 and M(3000)F2 parameters, with quarter-hourly time interval resolution for the diurnal variation, obtained with DPS4 digisonde at Hainan (19.5 N, E; Geomagnetic coordinates: E, 8.1 N) are used to investigate the low-latitude ionospheric variations and comparisons with the International Reference Ionosphere (IRI) model predictions. The data used for the present study covers the period from February 2002 to April 2007, which is characterized by a wide range of solar activity, ranging from high solar activity (2002) to low solar activity (2007). The results show that (1) Generally, IRI predictions follow well the diurnal and seasonal variation patterns of the experimental values of fof2, especially in the summer of However, there are systematic deviation between experimental values and IRI predictions with either CCIR or URSI coefficients. Generally IRI model greatly underestimate the values of fof2 from about noon to sunrise of next day, especially in the afternoon, and slightly overestimate them from sunrise to about noon. It seems that there are bigger deviations between IRI Model predictions and the experimental observations for the moderate solar activity. (2) Generally the IRI-predicted hmf2 values using CCIR M(3000)F2 option shows a poor agreement with the experimental results, but there is a relatively good agreement in summer at low solar activity. The deviation between the IRI-predicted hmf2 using CCIR M(3000)F2 and observed hmf2 is bigger from noon to sunset and around sunrise especially at high solar activity. The occurrence time of hmf2 peak (about 1200 LT) of the IRI model predictions is earlier than that of observations (around 1500 LT). The agreement between the IRI hmf2 obtained with the measured M(3000)F2 and the observed hmf2 is very good except that IRI overestimates slightly hmf2 in the daytime in summer at high solar activity and underestimates it in the nighttime with lower values near sunrise at low solar activity. Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Low-latitude ionosphere; F2 peak parameters; IRI model 1. Introduction The International Reference Ionosphere (IRI) is a widely used global empirical model of the ionosphere. It was developed as a joint URSI/COSPAR project since Since its first release in 1978, testing and modification of IRI have permanently continued resulting in essential improvements made available to the international research community through several versions (Rawer et al., 1978; Bilitza, 1990, 1997, 2001; Radicella et al., 1998; Bilitza * Corresponding author. address: wangx@cssar.ac.cn (X. Wang). et al., 2000). IRI uses an analytic formula (Ramakrishnan and Rawer, 1972) to calculate the F2 layer electron density profile based on four important parameters: fof2, hmf2, B0 and B1. The F2 peak parameters (fof2 and hmf2) are two key parameters when producing the electron density profile or TEC using IRI model or other ionospheric models, such as the NeQuick model (Radicella and Zhang, 1995). Their accuracy is the base of the accuracy of the profiles or TEC. The IRI model uses either CCIR or URSI coefficients to predict the fof2 and hmf2 based on the 12-month running average sunspot number. However, the ionospheric data from the Chinese continent were not used when producing both CCIR and URSI coefficients. Further more, the ionosphere over most of the China region /$36.00 Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. doi: /j.asr

3 X. Wang et al. / Advances in Space Research 43 (2009) is affected greatly by the Equator Ionospheric Anomaly and has typical low-latitude ionospheric behaviors whose variations are very complicated. A validation study of the model compared with observational results in China region (especially in the low latitude) is therefore necessary. In this paper, the ionospheric data from Hainan (19.5 N, E, dip angle at sub-ionospheric points), China, are used to investigate the ionospheric characteristics and comparisons with the corresponding IRI2001 model results in the low latitude. This is a continuation of previous work (Zhang et al., 2004, 2007) in which ionospheric fof2 and hmf2 in Hainan were used to make comparison study with IRI2000. However, the data used in those two papers cover short period (only 6 months (March August 2002) and about 3 years (March 2002 February 2005) respectively) and have low time interval (half an hour and one hour, respectively). The data used for the present study cover much longer time period (5 years, February 2002 April 2007) which was characterized by a wide range of solar activity, from high solar activity (2002) to low solar activity (2007), and we have used shorter time interval (quarter hour). This will give more and detailed information on ionospheric behavior in the low latitude. The results should be helpful in the future improvement of IRI in the low latitude. 2. Data used In the present study, quarter-hourly values of fof2, hmf2 and M(3000)F2 parameters scaled from the ionograms recorded routinely by the DPS-4 (Digital Portable Sounder) at Hainan (109.1 E, 19.5 N; Geomagnetic coordinates: E, 8.1 N), China, were used. All recorded ionograms used for the present study were manually edited using the SAOExplorer software (SAO Explorer, Interactive Ionogram Scaling Technologies, Reinisch et al., 2004) developed by the University of Lowell Massachusetts, Center for Atmospheric Research for quality control before obtaining the daily quarter-hourly data from ionograms. For the fof2 and hmf2, monthly median values of each parameter are calculated with daily available quiet-time (Kp <3 + ) data. M(3000)F2 is used as IRI model input to calculate the prediction of hmf2. The time coverage of the data used is from February 2002 to April 2007, which was characterized by a wide range of solar activity, from high solar activity (2002) to low solar activity (2007) and their yearly average values of Rz12 are 104, 63.7, 40.4, 29.8, 15.2 and 11.4, respectively. There are no data in September and October 2005 due to instrument failure. In the following, all days in each year were classified into four seasons, namely, equinox (spring: March, April; autumn: September and October), summer (May, June, July and August) and winter (November, December, January and February). 3. Results 3.1. fof2 Fig. 1 shows the contour plots of the monthly median values of observed and IRI-predicted fof2 parameters versus the local time (LT) for the months during the period from February 2002 to April 2007 at Hainan. The left panel of Fig. 1 shows the observed results, and it can be seen that fof2 shows an obvious diurnal, seasonal and solar cycle variation pattern. For the diurnal variation, the fof2 has the highest values in the afternoon and the lowest values at pre-sunrise (about 0600 LT) generally and also there is another peak value at midnight in equinox at high solar activity (2002) which is consistent with the previous results (Zhang et al., 2007). For the daytime values, it has an evident semi-annual seasonal variation pattern with the highest peaks in equinox (especially in spring) and the lowest peaks in solstice (especially in summer), i.e. it reaches its maximum in spring and minimum in summer each year during The rate of increase of fof2 after sunrise in the morning is much slower in summer than that in other seasons and the occurrence time and duration of the daytime peak zone in summer are also the latest and shortest respectively in all seasons. The time of occurrence and duration of daytime fof2 peaks have Fig. 1. Contour plots of observed (left) fof2- and IRI-predicted fof2 with URSI coefficients (right) during February 2002 April 2007 at Hainan.

4 1814 X. Wang et al. / Advances in Space Research 43 (2009) Fig. 2. Contour plots of DfoF2 (top) and DfoF2 (%) (bottom) during February 2002 April 2007 at Hainan. Left results with URSI coefficients; right results with CCIR coefficients. The value of thick black line is zero. obvious seasonal and solar cycle variation. There is the earliest time of occurrence in autumn (about 1200 LT before 2004), then later in spring and the latest in summer (about 1800 LT). For the duration of daytime fof2 peaks, it is longest in autumn (more than 6 h at high solar activity) and shortest in summer (only 2 3 h and even shorter at low solar activity). The minima at pre-sunrise have evident annual variation with minimum in winter and maximum in summer. Fig. 1 shows that fof2 has evident solar activity dependence with its value decreasing from the high solar activity year 2002 to the low solar activity year The right panel of Fig. 1 shows the results obtained by IRI model using URSI option (results with CCIR option is similar, so its plot is not shown here). It shows very similar diurnal and seasonal variation patterns and solar activity dependence to the observed ones. However, the magnitudes of fof2 values given by IRI model with CCIR and URSI options have some differences with the observed ones, especially for the peak values from noon to midnight in the high solar activity years. The duration of daytime peak zone of IRI-predicted fof2 values is shorter than that of observed ones. In order to get the difference of IRI prediction and observation in details, their differences are calculated. Fig. 2 shows the diurnal and seasonal variations of the difference between the observed fof2 value (fof2obs) and that produced by IRI model (fof2iri). The difference DfoF2 and DfoF2 (%) are defined as: DfoF2 = fof2iri fof2obs and DfoF2 (%) = [(fof2iri fof2obs)/ fof2obs] 100%, where fof2iri = fof2ccir for CCIR option and fof2iri = fof2ursi for the URSI one. It can be seen that there are systematical differences between observed fof2 and IRI-predicted fof2 with either URSI or CCIR coefficients. For most of the day time at high and moderate solar activity years ( ), both fof2ccir and fof2ursi underestimate fof2 values with DfoF2 (%) values varying between about 10% and 35%. At low solar activity years ( ), most time, IRI with either URSI or CCIR options overestimates the observed fof2. Generally, there are three periods ( LT, about 2100 LT and 0200 LT) which have strong IRI underestimation. During a few hours around sunrise (about 6 7 o clock), the IRI predictions generally overestimate the observed ones with DfoF2 (%) sometimes reaching as large

5 X. Wang et al. / Advances in Space Research 43 (2009) Fig. 3. Variation of monthly fof2 peak (left) and its occurrence time (right) during February 2002 April 2007 at Hainan. as 54%. The IRI predictions also overestimate the observed ones at about 0900 LT in equinox and at midnight in winter. However, there was an exception for the spring summer period of the year The best agreement is obtained in summer when using the URSI option. It can be seen that the agreement between the model predictions and the observed values is better for the high and low solar activity years (2002, ), but bad for moderate solar activity years ( ). With the decreasing of the solar activity from 2002 to 2007, it becomes more evident that the model results overestimate the observational ones at sunrise and in the morning, even extending to the daytime in In order to investigate with more detail the solar activity dependence of fof2, the fof2 peak in one day and its occurrence time in each month are shown in Fig. 3 and the deviations of the IRI-predicted and observed fof2 in each season during are shown in Fig. 4. It can be seen that the fof2 peak has obvious solar cycle and seasonal variation. The value of fof2 peak decreases with the solar activity decreasing and has two peaks (in spring and autumn) each year. Generally there are similar values of the fof2 peak in autumn and spring of the next year and the variation of the fof2 peak from spring to summer is the biggest. The observed fof2 peaks are bigger than the IRI-predicted ones with either URSI or CCIR options, there are bigger deviations during the moderate solar activity period ( ) and smaller ones during high and low solar activity periods (2002, ). The sunspot number has seasonal variation and it seems that there is the same variation trend for the sunspot number and the fof2 peaks during except for the spring and summer in 2003 and the opposite one during For the occurrence time of the fof2 peak, generally it is during LT except in 2002 which there is another more intensive fof2 peak near the midnight. It has obvious seasonal variation (about LT in winter and LT in equinox and summer) and the seasonal variation increases with the solar activity decreasing. The IRI-predicted occurrence time of fof2 peaks with either URSI or CCIR options show similar variation with observed ones but they are later in winter and earlier in other seasons except in 2002 and have smaller variation amplitude. It is notable that the occurrence time of IRI-predicted fof2 peaks are later than that of observed ones in summer and earlier in other seasons. It can be seen from Fig. 4 that the deviations between observed fof2 and IRI-predicted ones with either URSI or CCIR options show obvious diurnal, seasonal and solar cycle variation. Generally there is the smallest deviation during LT and bigger ones in the afternoon and evening (two peaks at about 1300 LT and 2000 LT); there are the biggest deviations in equinox except in 2002 and it is notable that the deviation in winter is the biggest in 2002 and 2003 but is the smallest in 2006 and It also can be seen that there is bigger deviation for the moderate solar activity which is the same as the foregoing results hmf2 The diurnal and seasonal variations of observed hmf2 and IRI predictions with CCIR M(3000)F2 at Hainan during the period from February 2002 to April 2007 are shown in Fig. 5. The IRI model uses a formula to calculate hmf2 from the propagation factor M(3000)F2 (Bilitza, 1990). The model offers two options in the use of M(3000)F2, namely the CCIR M(3000)F2 and the measured M(3000)F2. For comparison, IRI results with both options are obtained and denoted by hmf2ccir and hmf2(obsm3000), respectively. From the left panel of Fig. 5, it can be seen that the variations are similar to that of fof2. The observed F2 peak height (hmf2obs) shows an obvious solar activity dependency, whose value decreases with decreasing solar activity, and well-defined systematic diurnal and seasonal variation patterns. Generally hmf2obs has 3 peaks (around sunrise, LT and midnight) in one day except that there is another peak at 2000 LT in winter, i.e. there are four peaks one day in winter. For the noontime peak, it shows an annual variation pattern with minimum occurring in winter and maximum zone in summer and equinox. It has bigger values in equinox for the high solar activity years ( ). With solar activity decreasing from 2002 to 2007, the noontime peak in summer increases and becomes one peak zone

6 1816 X. Wang et al. / Advances in Space Research 43 (2009) Fig. 4. Seasonal averages for DfoF2 each year during March 2002 February 2007 at Hainan. Left results with URSI coefficients; right results with CCIR coefficients. with that in equinox. The midnight peak has similar values and annual variation pattern with noontime one. For the peak around sunrise, it reaches its maximum in winter and decreases from 2002 to 2007, but the magnitude of its variation is smaller compared with the noontime and midnight peaks. The peak at 2000 LT occurs only in winter. The right panel of Fig. 5 shows the results of IRI results with CCIR M(3000)F2 option. It can be seen that hmf2ccir could reproduce reasonably well the large-scale structures of hmf2obs, but not the detailed fine structures. However, when the observed M(3000)F2 was used as input to calculate the F2 peak height hmf2 (plot not shown here), the results produced by IRI model were able to

7 X. Wang et al. / Advances in Space Research 43 (2009) Fig. 5. Contour plots of observed (left) hmf2- and IRI-predicted hmf2 with CCIR M3000 coefficients (right) during February 2002 April 2007 at Hainan. Fig. 6. Contour plots of DhmF2 (top) and DhmF2 (%) (bottom) during February 2002 April 2007 at Hainan. Left results with CCIR M(3000)F2 coefficients; right results with observed M(3000)F2 coefficients. The value of thick black line is zero. Fig. 7. Variation of monthly hmf2 peak (left) and its occurrence time (right) during February 2002 April 2007 at Hainan.

8 1818 X. Wang et al. / Advances in Space Research 43 (2009) Fig. 8. Seasonal averages for DhmF2 each year during March 2002 February 2007 at Hainan. Left results with CCIR M(3000)F2 coefficients; right results with observed M(3000)F2 coefficients. reproduce the detailed fine structures of hmf2obs. These results are the same to that obtained by Zhang et al. (2007) and similar to those Adeniyi et al. (2003) and Obrou et al. (2003) for other low-latitude stations. The disagreement between the hmf2obs and hmf2ccir stems from the M(3000)F2 values given by CCIR, the CCIR M(3000)F2 does not produce the small scale structures observed in the measured M(3000)F2 (Zhang et al., 2007). In order to investigate the difference of observed hmf2 and IRI predictions in details, their difference are calculated and shown in Fig. 6. From the left panel of Fig. 6, it can be seen that the DhmF2ccir has obvious

9 X. Wang et al. / Advances in Space Research 43 (2009) diurnal and seasonal variation and also solar cycle dependence. Generally IRI overestimates hmf2 within the limits of 50 km during 3 periods (morning, around 0300 LT and post-midnight) and the difference increases with solar activity decreasing from 2002 to 2007; IRI underestimates hmf2 within the limits of 125 km around sunrise and in the afternoon and the difference decreases with solar activity decreasing. It seems that there are the smallest difference in summer and the biggest one in equinox. For the results with measured M(3000)F2, we can see that generally the IRI underestimates the hmf2 slightly (DhmF2 is less than 30 km or 10%), especially in the nighttime, except for in April 2003 and there are several daytime short-time overestimations (less than 7%) in equinox. The difference between IRI prediction with measured M(3000)F2 and observed hmf2 decreases from 2002 to Fig. 7 shows the hmf2 peak variation and its occurrence time and the deviations of the IRI-predicted and observed hmf2 in each season during are shown in Fig. 8. It can be seen that the hmf2 peak has obvious solar cycle and seasonal variation. The value of hmf2 peak decreases with the solar activity decreasing and has the smallest in winter (December) and higher ones in other seasons. There are small difference between observed hmf2 peaks and IRI-predicted one with measured M(3000)F2 option. The observed hmf2 peak generally is bigger than IRI-predicted one with CCIR M(3000)F2 option in 2002, 2003 and 2004 and they have small difference in 2005, 2006 and The occurrence time of observed hmf2 peak is during the period of LT and the IRI-predicted ones with measured M(3000)F2 option show the very similar variation, but the IRI-predicted ones with CCIR M(3000)F2 option is earlier than the observed ones and the range of variation is also smaller. It seems that the occurrence time of observed hmf2 peak is earlier with the solar activity decreasing and closer to the IRI-predicted one with CCIR M(3000)F2 option. From Fig. 8 it can be seen that the deviation between observed hmf2 and IRI-predicted ones with measured M(3000)F2 option is near the same for all the seasons in each year except for in 2002 and decreases with the solar activity decreasing. Generally there are bigger deviations at nighttime and smaller ones at daytime. For the IRI-predicted hmf2 with CCIR M(3000)F2 option, the deviation between observed ones and IRI prediction are big at sunrise and in the evening for the high solar activity (2002, 2003 and 2004) and it decreases with the solar activity decreasing. It is notable that the deviation in the morning increase with the solar activity decreasing as mentioned before. 4. Conclusions Comparison of the low-latitude ionospheric F2 peak parameters fof2 and hmf2 with IRI2001 were done using the ionospheric parameters obtained with DPS-4 digisonde in Hainan observatory from 2002 to The conclusions can be drawn as follows. For the fof2: (a) Generally IRI predictions follow well the diurnal, seasonal variation and solar cycle patterns of the experimental values of fof2. The occurrence time of observed fof2 peak zone is earlier than that of the IRI-predicted one and it also can last for a longer time; but for the occurrence time of fof2 maximum, the IRIpredicted ones are later than the observed ones in winter and earlier in other seasons except in (b) There are systematical deviation between experimental values and IRI predictions with either CCIR or URSI coefficients. Generally IRI model underestimates the values of fof2 from about noon to sunrise of next day, especially in the period of LT, and slightly overestimates them from about 0600 LT to about 1100 LT and also about midnight in winter for the CCIR option and low solar activity. The underestimation is strongest in equinox and weak in winter. The overestimation is strongest in winter and weak in summer. (c) It seems that there are bigger deviations between IRI model predictions and experimental observations for the moderate solar activity. (d) There is better agreement between observations and IRI predictions with URSI coefficients in summer and at low solar activity. For the hmf2: (a) Generally the IRI-predicted hmf2 values using CCIR M(3000)F2 option shows a poor agreement with the experimental results, but there is a relatively good agreement in summer for moderate and low solar activity. There are no peaks around sunrise and at about 2000 LT for the IRI predictions with CCIR M(3000)F2. (b) There are systematical deviation between hmf2ccir and hmf2obs. The IRI overestimates hmf2 from 0700 LT to 1200 LT and about 0300 LT and underestimates it at sunrise and from noon to sunset. The deviations around sunrise and in the afternoon decrease with decreasing of the solar activity from 2002 to 2007, but the deviations in the morning increases. (c) The deviations at sunrise and from noon to sunset have bigger values in equinox. (d) The agreement between the IRI-predicted hmf2 values with the measured M(3000)F2 and the experimental hmf2 is very good but IRI overestimates slightly hmf2 in the daytime sometimes and underestimates it in the nighttime especially at high solar activity. Acknowledgments This research was supported by National Natural Science Foundation of China (Grant Nos and ), the Specialized Research Fund for State Key

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