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INTERNATIONAL REFERENCE IONOSPHERE (IRI) IN CHINA
Cao Chong Dai Kailiang Quan Kunhai
China Research Institute of Radiowave Propagation Xinxiang, Henan, 453003, China
Department of Physics, Beijing University, Beijing, 100871, China
There are four Chinese groups working on the International Reference Ionosphere (IRI). Their main works are comparisons of the IRI with ionospheric data observed in China and checking of the IRI available for the Chinese region. The compared results are shown in this paper. The CRIRP group has used the ionospheric data of nine ionosonde stations (Manzhouli, Changchun, Wulumuqi, Beijing, Lanzhou, Lhasa, Chongqing, Guangzhou, and Haikou) during the 21st solar cycle. The compared parameters are foF2, M(3000)F2, foF1, hmE, and bottomside profile, and TEC (Xinxiang). It is indicated that there is a considerable systematic error when the IRI-86 is used in Southern China, especially during the solar maximum. The IRI is improving the use of ionospheric data observed in the Asia-Oceania region. The revised model is called the China Reference Ionosphere (CRI) and it is more accurate than the IRI for use in and around China.
The four Chinese groups working on the International Reference Ionosphere (IRI) belong to the China Research Institute of Radiowave Propagation (CRIRP)[1-9]; Beijing University [10-12]; Wuhan Institute of Physics, Academia Sinica [13-14]; and the Center for Space Science and Applied Research, Academia Sinica , respectively. They have mainly compared the ionospheric data observed in China during the 21st and 22nd solar cycles with the IRI and checked the IRI available for the China region. Tables 1 and 2 list the associated ionospheric observatories, and the duration of observed data, comparative parameters and associated ionosonde stations.
Based on the statistical analyses of a large number of observed data and understanding of the IRI, the CRIRP group puts forward an improved IRI for use in China and the Asia-Oceania region. The revised model is called China Reference Ionosphere (CRI).
Comparisons of foE, foF1 and foF2
The major task has been comparisons of the monthly median foE, foF1 and foF2 measured in China with results calculated from the IRI [3,5,6,8,9,10,11,13,14]. Results show that for any time, including both solar maximum and minimum activity periods, there is a good agreement between the measured and calculated foE and foF1. The relative deviations are less than 5% in most cases [5,10,11,13,14]. Therefore, the foE and foF1 predicted by the IRI is applicable to the China region.
The situation with foF2 and M(3000)F2 is very different from that for foE and foF1. Quan et. al.  have analysed the ionospheric data observed at nine Chinese ionosonde stations during the 21st solar activity cycle, and compared results with others predicted from the IRI. They have found that there are considerable systematic deviations in the predictions for low latitude regions (for example, Guangzhou and Haikou in Southern China) and some mid latitude regions (for example, Lanzhou) when using the IRI model.
Here we define the absolute deviation of foF2 monthly median as:
E = D foF2 = foF2( meas ) - foF2( cal )
Table1. Geographic positions of associated ionosonde stations
*Faraday rotation measurement
Here we define the absolute deviation of foF2 monthly median to be
DfoF2 = E,
E = D foF2 = ( foF2 (meas) - foF2 (cal) ) (1)
The (cal) represents the values calculated from the IRI model, and (meas) represents the values measured by ionosonde. The relative deviation is
The average deviation is
and the standard deviation is
Table 2. Details of comparisons of observed data with the IRI
Duration of data
MZ, BJ, FZ, MZ, XX
March,1976,1978,1980, Feb, 1982, March, 1983
MZ, XX, HK
MZ , CC, ML, BJ, LZ, LS, CQ, GZ, HK
MZ, CC, WL, BJ, LZ, LS, CQ, GZ, HK
foF2, M(3000)F2, foE, foF1, hmE, profile, TEC
 / 
MZ, CC, WL, BJ, LZ, CQ, GZ, HK
foF2 , M(3000)F2
MZ, WL, WC, CQ, HK
foE, foF1, foF2
MZ, WL , CC, BJ, CQ, GZ, HK
foE, foF1, foF2
 / 
MZ, BJ, WC, CQ ,GZ
foE, foF1, foF2
Apr 1978-, Dec.1979
Results for 1977-1986  show that the maximum foF2 average deviation occurred at Haikou, Guangzhou and Lanzhou, with values of +0.6MHz,+0.5MHz and -0.5MHz respectively, 0.2MHz at other stations. Maximum values appeared at Haikou and Guangzhou, and they are 1.5MHz and 1.2MHz respectively, with 0.9MHz at other stations. Quan etal  have also noted that the relative deviations are in the limits15% for the typical mid latitude stations (except Lanzhou station), but for low latitude stations Guangzhou and Haikou, they are larger. The maximum relative deviation reached 77% at Haikou. During sunset and sunrise periods the relative deviations were D>50% for most Chinese stations.
In summary, for the relative deviations of foF2 at all stations, night values are larger than day values, winter values are larger than summer values, and low latitude region values are larger than at mid latitudes. Other authors [10-14] provide similar conclusions.
Comparisons of TEC and density profiles
Dai  has compared the TEC measured at Xi'an during 1978-1979 with that predicted by the IRI-79. It is shown that the predicted values are much lower than measured in Spring.
Dai and Ma  have compared the predicted TEC monthly median values by IRI-86 and IRI-90 with the monthly median values observed at Xinxiang by receiving VHF signals from the ETS-II satellite from 1982 to 1989. The compared results indicate that the model predicted TEC values fit the measured values quite well. Normally, the predicted values are a little larger than the measured values. The predicted TEC accuracy of the IRI-90 is better than the IRI-86. During the high solar activity period, the IRI TEC appears to have a different behaviour from that observed. In the Winter and Spring seasons, the IRI TEC is much less (by nearly 50%) than the observed daytime TEC , especially near noon.
A comparison between bottomside electron density profiles deduced from Wuchang ionograms and from the IRI-86 has shown that the IRI hmE is usually lower than measured values, and the sub peak half-thickness is larger than the measured values. Both profiles fit very well in the daytime, but are not good at night [4,7].
Shen  has introduced the atmospheric and ionospheric models MSIS-86 and IRI-86, and deduced the thermospheric wind system, the spatial distribution and temporal variation of collision frequency and ionospheric conductivities. The empirical parameters of MSIS-86 and IRI-86 have been used to deduce the three dimensional distribution of ionospheric conductivity and thermospheric wind system by Shen et al.. The thermospheric wind system, height-integrated conductivity, electrostatic potential and ionospheric current density are compared with observed data and the results deduced from model calculation. There is a good agreement between them.
China Reference Ionosphere
Based on the statistical results of observed data from 39 ionosonde stations (sited in regions from 65oN to 40oS and from 60oE to 150oE) over many years, Sun  has produced a method of predicting the ionospheric F2 layer in the Asia-Oceania Region (AOR). When the IRI-90 applies to region of China, using the AOR mapping instead of the CCIR coefficients or the URSI coefficients, the revised IRI model, called the China Reference Ionosphere, can provide more accurate predicted values of ionospheric parameters for the China region and around China [6,7,8,9]. The CRI is different from the IRI on the following two points:
(1) hmE equals 115km;
(2) When R1260 for latitudes40o, the occurrence of the F1 layer is considered in all seasons, and not as the IRI stipulates with no F1 appearance in Winter.
The values of TEC calculated using both the CRI and IRI have been compared with observations at Xinxiang based on Faraday rotation measurements from 1983 to 1989 [5,9], also with foF2 values predicted with observations at Wuchang based on ionograms [5,9]. The CRI give more accurate TEC and foF2 values than the IRI in the China region.
The IRI available for the China region is more realistic in Northern China (mid latitude region) than in Southern China (low latitude region). The prediction accuracy of foE and foF1 is much better than for foF2. For foF2, in the low latitude region, there are considerable systematic deviations. The CRI is a computerised model, revised from the IRI-90 with substitution of some empirical formulas and assumptions based on the observations made in the China region. The CRI gives more accurate representation of the ionosphere for China and around China than the IRI.
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