- Email: [email protected]
Comparison of total electron content calculated using the IRI with observations in China
Journnl of Atmospheric
and Terresfrd
Physics.
Vol. 56, NO. 3. pp. 411422,
0021-9169194
1994
i-’1993Pergamon
Printedin GreatBritain.
P6.00+ .oO Press Ltd
Comparison of total electron content calculated using the IRI with observations in China DAI KAILIANC and MA JIANMING* China
Research
Institute
of Radiowave
Propagation,
P.O. Box 138-23.453003,
(Received in,finalform 11 March 1993 ; uccepted
Xinxiang,
Henan,
P.R.C.
I 1 March 1993)
Abstract-Values of total electron content (TEC) calculated using the International Reference Ionosphere (IRI-86 and IRI-90) are compared with the observations at Xinxiang based on the Faraday rotation measurement. It is found that the IRI gives acceptable values with respect to the observations during low solar activity. Generally the IRI-90 is better than the IRI-86 and the URSI coefficients are better than the CCIR coefficients in the calculation of TEC. Making use of the.jiiF2 and M(3000)F, calculated using the
Asia Oceania Region F,-layer mapping (AOR) instead of using the CCIR or the URSI coefficients, the IRI gives more accurate TEC values. In October-April during high solar activity, however, the IRI obviously underestimates TEC in the daytime, which could be due to an improper topside electron density profile
INTRODUCTION Electron content and other integral parameters of the ionosphere from the ground to the heights of around 1000 km are important for both geophysical and engineering applications. They impose propagation delay as well as refraction on radio signals traversing the ionosphere. Since measured values are not always available, models must be relied on for many purposes. One of the ionospheric global models is the International Reference Ionosphere (BILITZA, 1990), which is the standard ionospheric model recommended for international use by URSI and COSPAR. The IRI-90 has options to use the standard CCIR coefficients (CCIR, 1986) or to use the new URSI coefficients (RUSH et al., 1989) while the IRI86 provides only the CCIR coefficients. In China the method of predicting the ionospheric F,-layer in the Asia Oceania Region (the AOR mapping) (SUN XIANRU, 1987) is commonly used, which could also act as an option in the IRI. By integrating the electron concentration profiles given by the IRI, the calculated TEC is obtained. A number of papers have been published that deal with the comparison of measured content with the IRI (e.g. MCNAMARA and WILKINSON, 1983 ; MCNAMARA, 1984, 1985; DAI YUE-QIN, 1986; BILITZA et al., 1988; LEITINGER, 1990). Though the IRI gives generally accurate TEC values in some periods, discrepancies do exist. This paper presents a systematic comparison
* Present address: Polar 200129. Shanghai, P.R.C.
Research
Institute
of China,
417
of the calculated TEC, using the various with the observations made in China.
IRI models,
MEASUREMENTTECHNIQUE AND DATA Measured data used in this study were gained by means of Faraday rotation measurements at Xinxiang. The 136 MHz beacon signals were transmitted from the geostationary satellite ETS-II. The TEC values were obtained by using the Faraday factor M’ for a height of 400 km and are regarded as specifying the total content up to a height of about 2000 km (TITHERIDGE,1972). The locations concerning the TEC measurement are as follows : Xinxiang : ETS-II : 400 km sub-ionospheric W uchang : The Faraday
rotation
point:
35.30 N, 113.85’E OO.OO”N, 130.00’ E 32.40-N, 115.58”E 30.57 N, 114.35 E.
angle can be expressed
R = (k/f’)M’
as :
N7
where f is the beacon frequency in MHz, k = 2.36 x lo4 in the MKS system, R is in radians, NT is the electron content in mm2, M’ is the mean magnetic factor. M’ = B’ cos0 secx B’ is the magnetic strength at a point 400 km high above the ground along the ray path, 0 is the angle between the ray path and the magnetic field strength and x is the zenith angle of the satellite. Substituting values of the parameters of the ray path in the above formulas, the electron content may be expressed as :
DAI KAILIANG and
41x N T = 4.50 x IO’hn
where n is the rotation angle of the wave in 7~ Here the observed rotation angle is a relative value; the absolute value is obtained by regressive analysis with fi)F2 observed at Wuchang, the nearest ionosonde station from the sub-ionospheric point (about 200 km away). The Faraday rotation angles were continuously recorded throughout the observational period from 1982 to 1989. except for the time when the satellite transmitter was switched off. Then monthly medians of the observed TEC were deduced for each hour. In this paper 1872 monthly medians were used for comparisons.
COMPARISON
The calculated monthly mean values of the TEC of the IRI were obtained by means of numerical integration from 60 to 1000 km using a step size of IO km, since the IRI is applicable only for altitudes below 1000 km. It could be recognized approximately as identical to that from the ground to 2000 km, because electron content from 1000 to 2000 km contributes only up to 5% of the TEC and would not be noticeable in the present context (MCNAMARA, 1984). Two IRI versions. the IRI-86 and the IRI-90 have been used in the comparison. The IRI-90 uses both the URSI coefficients and the CCIR coefficients in the calculation of,fi>F2. whereas the IRI-X6 only uses the CCIR coefficients. In China, Professor Sun Xianru has analyzed monthly median ,filF2 observed at 39 ionosonde locations in the region of Asia and Oceania, 65 N-40 S and 6&l 50’E. and developed a procedure to predict the monthly median /iIF (SUN XIANRU. 1987). To get satisfactory predictions, Sun Xianru
MA JIANMING introduced an index, namely Ic, which is deduced from the ionospheric parameter directly, and the relation between Ic and,foF2 is assumed to be nonlinear. Having compared the calculated ,foF2 and M(3000)Fz values with the data measured at eight ionosphere observatories in China (DAI KAILIANG rt ul., 1992), we found that the Asia Oceania Region Fz-layer mapping procedure gives the most accurate prediction in China. Therefore we made use of the AOR mapping in the calculation of the IRI electron content as well as the original CCIR and URSI models in the IRI. Therefore five empirical models have been used in the comparison of the TEC. They are the IRI-86 with the CCIR coefficients, the IRI-86 with the AOR mapping, the IRI-90 with the CCIR coefficients, the IRI-90 with the URSI coefficients and the IRI-90 with the AOR mapping. In the statistical analysis, the definition of model error ii. arithmetic mean error and standard deviation are as follows :
6 = calculated A.M.E.
= ;;B
value-observed
S.D. =
value
&
&’
where N is the number of data points. Statistical errors of the calculated TEC values with respect to the observations at Xinxiang are listed in Table 1. In order to show the relationship between the behavior of the TEC and foF2, the observed,foF:! medians at Wuchang, the nearest ionosonde from the sub-ionospheric point, have been selected. Statistical errors of calculated foF2 values with respect to the observations at Wuchang are listed in Table 2 for comparison. From these tables we have the following considerations.
Table I. Arithmetic mean error (left columns) and standard deviation (right columns) of calculated TEC (IO” m ‘). The numbers X6 and 90 stand for IRI-86 and IRI-90; the letters following them stand for the mapping coefficients from which the foF2 were deduced
1982 1983 1984 19x5 I986 1987 198X 1989 1982-m1989 1983%1988 1982and 1989
86CCIR
86AOR
27 56 43 4x 43 61 80 43 51 58 36
6 43 23 I6 9 22 26 -38 I3 25 -18
90CCIR I5 49 41 48 41 58 67 I9 42 52 I7
POURS1
90AOR
86CCIR
86AOR
IO 43 34 43 32 51 62
-7 38 21 I6 7 I9 I7 -58 5 21 -34
62 72 60 57 60 70 92 107 77 72 89
57 57 40 23 26 30
15 37 46 13
51 115 61 42 93
YOCCIR 90URSI 58 62 55 59 57 68 81 IO1 72 66 84
66 53 47 55 49 62 77 108 70 60 91
90AOR
Sample size
59 51 36 24 25 28 52 127 64 40 102
240 240 240 144 144 288 288 288 1872 1344 528
419
TEC and observations in China
Table 2. Arithmetic mean error (left columns) and standard (0.1 MHz, Wuchang)
1981 I YX3 1984 lYX5 1986 IYX? 19XX 19x9 1982 -1989 1983 1988 1982and
1989
deviation
(right columns)
ofji,F?.
CCIR
URSI
AOR
CCIR
URSI
AOR
Sample size
PO.2 3.6 3.X 6.3 6.X x.4 11.0 13.9 7.0 6.9
PI.5 1.X 1.5 4.2 3.4 5.9 Y.5 15.6 5,s 4.7
-4.2 I.0 ~ 1.2 -3.0 -3.5 -2.5 0.0 0.7 - 1.4 -1.3
X.4 9.3 Y.0 9.0 10.5 13.6 19.0 11.x 10.4
10.1 7.X 7.X 6.5 6.1 X.6 19.2 Il.0 8.X
9.X 5.9 7.1 5.9 6.2 5.X 5.9 7.9 7.0 6.1
240 240 240 144 144 288 28X 264 1X4X I344
-1.6
14.9
15.5
X.9
504
7.2
7.5
1. The CCIR coefficients. the URSI coefficients and the AOR mapping. The above three sets of coefficients have been used in the calculation of,foF2 in the IRI90. For the whole data sets (line 1982-1989 in the tables), both TEC and joF2, the URSI coefficients have smaller A.M.E. and S.D. than the CCIR coefficients, but the AOR mapping has the smallest. This means that the URSI coefficients are better than the CCIR coefficients in the calculation of both,fijF? and TEC in the Chinese region but still have large A.M.E. and SD. Because few data from China were used in the dcvelopmcnt of the maps used with the IRI (CCIRIURSI coefficients), this has resulted in a poor empirical representation of the ionosphere in this region. The AOR mapping, developed using more data from China. is the best. 2. The IRI-86 and IRI-90. Comparing the errors in the column 86CCIR with the ones in column 90CCIR in Table I, it is found that the IRI-90 always has smaller A.M.E. and S.D. than the IRI-86 has for all observation periods. This means that the IRI-90. which has taken up earlier comments on the poor topside model and has gone some way to improving this aspect of the model, is better than the IRI-86 in the calculation of TEC in the Chinese region. 3. Low solar activity and high solar activity. In Fig. 1, the diurnal variations of TEC and,foF2 in 1987 (in which the yearly average R,: is 32) and in 1989 (the yearly avcragc R ,2 is 154) arc displayed for the four seasons, representing the behavior during low solar activity and high solar activity, respectively. It is shown that the IRI TEC fits the observations quite well in 1987. During high solar activity, 1989 for example, the IRI TEC appears a little strange compared with the observations. In October-April the IRI TEC is much smaller than the observed TEC during daytime. especially at noon. Similar observations were reported for data from Xi’an for 1979 (DAI YUE-QIN. 1986).
7.x
11.9
Statistical errors of TEC for individual years from 1982 to 1989 arc shown in Table 1. According to their different manner the data have been divided into two sets, the one from 1983 to 1988 which stands for low solar activity, and the other includes the years of 1982 (the yearly average R,, is 114) and 1989 (the yearly average R,? is 154), which stands for high solar activity. The statistical results are listed on the last two lines in Table 1. All standard deviations for the years of 1983-1988 are much smaller than the ones for the years of 1982 and 1989, indicating that the IRI gives more accurate TEC values during low solar activity than during high solar activity. The present models are acceptable during low solar activity, but are clearly inaccurate during some periods in high solar activity. Diurnal and seasonal variations of the errors in TEC for 1983 through 1988 are shown in Figs 2 and 3, respectively. In the figures it is shown that when using the AOR mapping instead of the CCIR the IRI TEC errors are reduced coellicicnts. obviously. The overall A.M.E. is reduced from 5.2.x lO’“to2.1 x 10’“/m’andtheS.D.fron~6.6x IO’” to 4.0 x lO’“/m’ (see also Table I). Statistical results of the errors in,fi)F2 are also shown in Figs 2 and 3. The behavior of the TEC errors look similar to those of the fbF2 errors indicating that the improvement in the calculation of TEC is mainly due to an improvement in theJbF2 calculation during low solar activity. Furthermore, it is shown from the figures that the IRI gives smaller TEC errors in summer than it does in the other seasons, and in the early morning than at other times. 4. The topside profile. It is mentioned above that in October-April during high solar activity the IRI obviously underestimates TEC in daytime. In March 1989. the IRI TEC values are much smaller than the observations, while the observed and the calculated ,f’F2 are in good agreement. Obviously the TEC error
420
DAI KAILIANG and MA JIANMING ._
^
TEC(ld’/d
Jtulc
0
Fig.
1.Diurnal
6
12
IX
LT
0
)
1989
6
I
18 LT
12
--t
TEC chulatcd
*
TEC obscrvcd a~ Xinsinng
-+
foF2 calculalcd for Wucl~~~g using lhc AOR mapping
-
foF2 observed at Wrichan~
variation
of the medians of
using the IRI-90 with 111~AOR mapping
TEC and fiJF2. The values of /i)F2 have been scaled to,foF2/3 MHz+?!.
during high solar activity cannot mainly be caused by the error in the calculation offoF2. Though the bottomside of the IRI profile does not completely agree with the observations, they could not cause so much discrepancy in the TEC (LUO FACENG et ul., 1992). Moreover, the bottomside electron content consists only a quarter to a third of the total electron
content. Therefore, it should be the topside profile of the IRI that is in error. CONCLUSION
The IRI gives TEC values that agree fairly well with the TEC Faraday measurements made in
TEC and observations -+
in China
statistical error of foF2 calcnlatcd using lhc CCIR cocflicicnts
---.w statistical error of foF2 calcnlatcd using the AOR mapping .-+
stalistical error of TEC of the N-90
with lhc CCIR cocfkients
...e... statistical error of TEC of the N-90
with the AOR mapping
120 Mean error
0
6
12
18
L’f
‘. 0
6
12
IS
LT
Fig. 2. Diurnal variation of the arithmetic mean error and the standard deviation of TEC (I.OEl5/m**2) and fi~F2 (0.01 MHz) for the years of 1983 1988. The sample sizes are approximately 56 for each hour.
Jan.
Apr.
Jan.
July
Apr.
July
Oct.
Fig. 3. Seasonal variation of the arithmetic mean error and the standard deviation of TEC ( I .OE I5!m**2) and /i/F2 (0.01 MHz) for the years of 1983 1988. The sample sizes arc approximately 120 for each month.
China during low solar activity. Generally the IRI-90 is better than the IRI-86. and the URSI coefficients are better than the CCIR coefficients in the calculation of TEC. Making use of the Asia Oceania Region F2-layer mapping instead of the CCIR coefficients or the URSI coefficients, the IRI provides more accurate TEC values. In OctoberApril during high solar activity, however, the IRI obviously underestimates TEC in daytime, which
could he due to an improper profile.
topside electron
density
Ark,zot~,k~4yml~nt,s~This research was supported by grants from the National Natural Science Foundation of China and the Science Foundation of Electronics of China. The authors are indebted to Dr Bilitza for provision of the IRI programs and the URSI mapping procedures. The authors also acknowledge the valuable discussions with Professor Liu Ruiyuan,
Professor
Sun Xianru.
Dr Wu Jian and Wen Bo.
REFERENCES
BILITZA D., RAWEK K. and PALLASCHKE S.
I988
CCIR
I986
DAI KAILIANG. Luo FAGENG. QUAN KIJNHAI and Lru RUIYUAN
1992
DAI YUE-OIN
I986
Study of ionospheric models for satellite orbit determination. Radio Sci. 23, 223-232. Report 340-5, CCIR Atlas of Ionospheric Characteristics, ITU. Geneva. Comparison of the Asia Oceania region F2 layer prediction with the CCIR method. Chinese J. Spaw Sci. 12, 153-156. A test of IRI IRI-79 using ionospheric election content data. Chinex J. Spare Sci. 6, 143-146.
DAI KAILIANGand
422
MA JIANMING
LUO FAGENG,DAI KAILIANG,QUAWKUNHAI, LIANGLIPINGand LIU RUIYUAN
1992
Comparison of subpeak deduced from ionograms
MCNAMAKAL. F.
1984
Prediction
electron density profiles with the IRI. Chirlesc J.
Rudio Sri. 7,42118.
of total electron
content
using the IRI. Adz.
Space Rex. 4, 25 50.
MCNAMAKAL. F.
1985
The use of total electron content measurements to validate empirical models of the ionosphere. Ad?. Spaw
M(,N,XMAKA L. F. and WILKINON P. J.
1983
M.. BILITZAD., DAVIS K.. M~NAMAKA L.. S~WAKT F. and POKEMPNEK M. SUY XIANRU
I989
Prediction of total electron content using the IRI. .I. utm0.r. terr. Ph~,s. 45, 169 114. Ionospheric mapping and update of /oF2 coetficients.
TITHI,KIDGE J. E.
1972
RES. 5, 81 90.
RIJSH C.. Fox
R~ftirencu
is also
nmk
10
the following
unpublished
T~~l~~c~omr,lunici,)mf~~uni~~tion J. 56, I79-
19x7
I X2.
A method of predicting the ionospheric 1;? layer in the Asia Oceania region. J. Chinrr Inst. C’ommun. S(6), 37 45. Determination of ionospheric electron content from the Faraday rotation of geostationary satellite signals. Phrt. Spuw Sci. 20, 353 -369.
mutrriol:
BILITZAD.
1990
LEITINGEKR.
1990
International Reference Ionosphere 1990. NSSDC! WDC-A-R&S, 90-22. Electron Content Measurements and IRI. International Reference Ionosphere 90. edited by D. Bilitza, NSSDC:WDC-A-R&S., 90-22.
and Terresfrd
Physics.
Vol. 56, NO. 3. pp. 411422,
0021-9169194
1994
i-’1993Pergamon
Printedin GreatBritain.
P6.00+ .oO Press Ltd
Comparison of total electron content calculated using the IRI with observations in China DAI KAILIANC and MA JIANMING* China
Research
Institute
of Radiowave
Propagation,
P.O. Box 138-23.453003,
(Received in,finalform 11 March 1993 ; uccepted
Xinxiang,
Henan,
P.R.C.
I 1 March 1993)
Abstract-Values of total electron content (TEC) calculated using the International Reference Ionosphere (IRI-86 and IRI-90) are compared with the observations at Xinxiang based on the Faraday rotation measurement. It is found that the IRI gives acceptable values with respect to the observations during low solar activity. Generally the IRI-90 is better than the IRI-86 and the URSI coefficients are better than the CCIR coefficients in the calculation of TEC. Making use of the.jiiF2 and M(3000)F, calculated using the
Asia Oceania Region F,-layer mapping (AOR) instead of using the CCIR or the URSI coefficients, the IRI gives more accurate TEC values. In October-April during high solar activity, however, the IRI obviously underestimates TEC in the daytime, which could be due to an improper topside electron density profile
INTRODUCTION Electron content and other integral parameters of the ionosphere from the ground to the heights of around 1000 km are important for both geophysical and engineering applications. They impose propagation delay as well as refraction on radio signals traversing the ionosphere. Since measured values are not always available, models must be relied on for many purposes. One of the ionospheric global models is the International Reference Ionosphere (BILITZA, 1990), which is the standard ionospheric model recommended for international use by URSI and COSPAR. The IRI-90 has options to use the standard CCIR coefficients (CCIR, 1986) or to use the new URSI coefficients (RUSH et al., 1989) while the IRI86 provides only the CCIR coefficients. In China the method of predicting the ionospheric F,-layer in the Asia Oceania Region (the AOR mapping) (SUN XIANRU, 1987) is commonly used, which could also act as an option in the IRI. By integrating the electron concentration profiles given by the IRI, the calculated TEC is obtained. A number of papers have been published that deal with the comparison of measured content with the IRI (e.g. MCNAMARA and WILKINSON, 1983 ; MCNAMARA, 1984, 1985; DAI YUE-QIN, 1986; BILITZA et al., 1988; LEITINGER, 1990). Though the IRI gives generally accurate TEC values in some periods, discrepancies do exist. This paper presents a systematic comparison
* Present address: Polar 200129. Shanghai, P.R.C.
Research
Institute
of China,
417
of the calculated TEC, using the various with the observations made in China.
IRI models,
MEASUREMENTTECHNIQUE AND DATA Measured data used in this study were gained by means of Faraday rotation measurements at Xinxiang. The 136 MHz beacon signals were transmitted from the geostationary satellite ETS-II. The TEC values were obtained by using the Faraday factor M’ for a height of 400 km and are regarded as specifying the total content up to a height of about 2000 km (TITHERIDGE,1972). The locations concerning the TEC measurement are as follows : Xinxiang : ETS-II : 400 km sub-ionospheric W uchang : The Faraday
rotation
point:
35.30 N, 113.85’E OO.OO”N, 130.00’ E 32.40-N, 115.58”E 30.57 N, 114.35 E.
angle can be expressed
R = (k/f’)M’
as :
N7
where f is the beacon frequency in MHz, k = 2.36 x lo4 in the MKS system, R is in radians, NT is the electron content in mm2, M’ is the mean magnetic factor. M’ = B’ cos0 secx B’ is the magnetic strength at a point 400 km high above the ground along the ray path, 0 is the angle between the ray path and the magnetic field strength and x is the zenith angle of the satellite. Substituting values of the parameters of the ray path in the above formulas, the electron content may be expressed as :
DAI KAILIANG and
41x N T = 4.50 x IO’hn
where n is the rotation angle of the wave in 7~ Here the observed rotation angle is a relative value; the absolute value is obtained by regressive analysis with fi)F2 observed at Wuchang, the nearest ionosonde station from the sub-ionospheric point (about 200 km away). The Faraday rotation angles were continuously recorded throughout the observational period from 1982 to 1989. except for the time when the satellite transmitter was switched off. Then monthly medians of the observed TEC were deduced for each hour. In this paper 1872 monthly medians were used for comparisons.
COMPARISON
The calculated monthly mean values of the TEC of the IRI were obtained by means of numerical integration from 60 to 1000 km using a step size of IO km, since the IRI is applicable only for altitudes below 1000 km. It could be recognized approximately as identical to that from the ground to 2000 km, because electron content from 1000 to 2000 km contributes only up to 5% of the TEC and would not be noticeable in the present context (MCNAMARA, 1984). Two IRI versions. the IRI-86 and the IRI-90 have been used in the comparison. The IRI-90 uses both the URSI coefficients and the CCIR coefficients in the calculation of,fi>F2. whereas the IRI-X6 only uses the CCIR coefficients. In China, Professor Sun Xianru has analyzed monthly median ,filF2 observed at 39 ionosonde locations in the region of Asia and Oceania, 65 N-40 S and 6&l 50’E. and developed a procedure to predict the monthly median /iIF (SUN XIANRU. 1987). To get satisfactory predictions, Sun Xianru
MA JIANMING introduced an index, namely Ic, which is deduced from the ionospheric parameter directly, and the relation between Ic and,foF2 is assumed to be nonlinear. Having compared the calculated ,foF2 and M(3000)Fz values with the data measured at eight ionosphere observatories in China (DAI KAILIANG rt ul., 1992), we found that the Asia Oceania Region Fz-layer mapping procedure gives the most accurate prediction in China. Therefore we made use of the AOR mapping in the calculation of the IRI electron content as well as the original CCIR and URSI models in the IRI. Therefore five empirical models have been used in the comparison of the TEC. They are the IRI-86 with the CCIR coefficients, the IRI-86 with the AOR mapping, the IRI-90 with the CCIR coefficients, the IRI-90 with the URSI coefficients and the IRI-90 with the AOR mapping. In the statistical analysis, the definition of model error ii. arithmetic mean error and standard deviation are as follows :
6 = calculated A.M.E.
= ;;B
value-observed
S.D. =
value
&
&’
where N is the number of data points. Statistical errors of the calculated TEC values with respect to the observations at Xinxiang are listed in Table 1. In order to show the relationship between the behavior of the TEC and foF2, the observed,foF:! medians at Wuchang, the nearest ionosonde from the sub-ionospheric point, have been selected. Statistical errors of calculated foF2 values with respect to the observations at Wuchang are listed in Table 2 for comparison. From these tables we have the following considerations.
Table I. Arithmetic mean error (left columns) and standard deviation (right columns) of calculated TEC (IO” m ‘). The numbers X6 and 90 stand for IRI-86 and IRI-90; the letters following them stand for the mapping coefficients from which the foF2 were deduced
1982 1983 1984 19x5 I986 1987 198X 1989 1982-m1989 1983%1988 1982and 1989
86CCIR
86AOR
27 56 43 4x 43 61 80 43 51 58 36
6 43 23 I6 9 22 26 -38 I3 25 -18
90CCIR I5 49 41 48 41 58 67 I9 42 52 I7
POURS1
90AOR
86CCIR
86AOR
IO 43 34 43 32 51 62
-7 38 21 I6 7 I9 I7 -58 5 21 -34
62 72 60 57 60 70 92 107 77 72 89
57 57 40 23 26 30
15 37 46 13
51 115 61 42 93
YOCCIR 90URSI 58 62 55 59 57 68 81 IO1 72 66 84
66 53 47 55 49 62 77 108 70 60 91
90AOR
Sample size
59 51 36 24 25 28 52 127 64 40 102
240 240 240 144 144 288 288 288 1872 1344 528
419
TEC and observations in China
Table 2. Arithmetic mean error (left columns) and standard (0.1 MHz, Wuchang)
1981 I YX3 1984 lYX5 1986 IYX? 19XX 19x9 1982 -1989 1983 1988 1982and
1989
deviation
(right columns)
ofji,F?.
CCIR
URSI
AOR
CCIR
URSI
AOR
Sample size
PO.2 3.6 3.X 6.3 6.X x.4 11.0 13.9 7.0 6.9
PI.5 1.X 1.5 4.2 3.4 5.9 Y.5 15.6 5,s 4.7
-4.2 I.0 ~ 1.2 -3.0 -3.5 -2.5 0.0 0.7 - 1.4 -1.3
X.4 9.3 Y.0 9.0 10.5 13.6 19.0 11.x 10.4
10.1 7.X 7.X 6.5 6.1 X.6 19.2 Il.0 8.X
9.X 5.9 7.1 5.9 6.2 5.X 5.9 7.9 7.0 6.1
240 240 240 144 144 288 28X 264 1X4X I344
-1.6
14.9
15.5
X.9
504
7.2
7.5
1. The CCIR coefficients. the URSI coefficients and the AOR mapping. The above three sets of coefficients have been used in the calculation of,foF2 in the IRI90. For the whole data sets (line 1982-1989 in the tables), both TEC and joF2, the URSI coefficients have smaller A.M.E. and S.D. than the CCIR coefficients, but the AOR mapping has the smallest. This means that the URSI coefficients are better than the CCIR coefficients in the calculation of both,fijF? and TEC in the Chinese region but still have large A.M.E. and SD. Because few data from China were used in the dcvelopmcnt of the maps used with the IRI (CCIRIURSI coefficients), this has resulted in a poor empirical representation of the ionosphere in this region. The AOR mapping, developed using more data from China. is the best. 2. The IRI-86 and IRI-90. Comparing the errors in the column 86CCIR with the ones in column 90CCIR in Table I, it is found that the IRI-90 always has smaller A.M.E. and S.D. than the IRI-86 has for all observation periods. This means that the IRI-90. which has taken up earlier comments on the poor topside model and has gone some way to improving this aspect of the model, is better than the IRI-86 in the calculation of TEC in the Chinese region. 3. Low solar activity and high solar activity. In Fig. 1, the diurnal variations of TEC and,foF2 in 1987 (in which the yearly average R,: is 32) and in 1989 (the yearly avcragc R ,2 is 154) arc displayed for the four seasons, representing the behavior during low solar activity and high solar activity, respectively. It is shown that the IRI TEC fits the observations quite well in 1987. During high solar activity, 1989 for example, the IRI TEC appears a little strange compared with the observations. In October-April the IRI TEC is much smaller than the observed TEC during daytime. especially at noon. Similar observations were reported for data from Xi’an for 1979 (DAI YUE-QIN. 1986).
7.x
11.9
Statistical errors of TEC for individual years from 1982 to 1989 arc shown in Table 1. According to their different manner the data have been divided into two sets, the one from 1983 to 1988 which stands for low solar activity, and the other includes the years of 1982 (the yearly average R,, is 114) and 1989 (the yearly average R,? is 154), which stands for high solar activity. The statistical results are listed on the last two lines in Table 1. All standard deviations for the years of 1983-1988 are much smaller than the ones for the years of 1982 and 1989, indicating that the IRI gives more accurate TEC values during low solar activity than during high solar activity. The present models are acceptable during low solar activity, but are clearly inaccurate during some periods in high solar activity. Diurnal and seasonal variations of the errors in TEC for 1983 through 1988 are shown in Figs 2 and 3, respectively. In the figures it is shown that when using the AOR mapping instead of the CCIR the IRI TEC errors are reduced coellicicnts. obviously. The overall A.M.E. is reduced from 5.2.x lO’“to2.1 x 10’“/m’andtheS.D.fron~6.6x IO’” to 4.0 x lO’“/m’ (see also Table I). Statistical results of the errors in,fi)F2 are also shown in Figs 2 and 3. The behavior of the TEC errors look similar to those of the fbF2 errors indicating that the improvement in the calculation of TEC is mainly due to an improvement in theJbF2 calculation during low solar activity. Furthermore, it is shown from the figures that the IRI gives smaller TEC errors in summer than it does in the other seasons, and in the early morning than at other times. 4. The topside profile. It is mentioned above that in October-April during high solar activity the IRI obviously underestimates TEC in daytime. In March 1989. the IRI TEC values are much smaller than the observations, while the observed and the calculated ,f’F2 are in good agreement. Obviously the TEC error
420
DAI KAILIANG and MA JIANMING ._
^
TEC(ld’/d
Jtulc
0
Fig.
1.Diurnal
6
12
IX
LT
0
)
1989
6
I
18 LT
12
--t
TEC chulatcd
*
TEC obscrvcd a~ Xinsinng
-+
foF2 calculalcd for Wucl~~~g using lhc AOR mapping
-
foF2 observed at Wrichan~
variation
of the medians of
using the IRI-90 with 111~AOR mapping
TEC and fiJF2. The values of /i)F2 have been scaled to,foF2/3 MHz+?!.
during high solar activity cannot mainly be caused by the error in the calculation offoF2. Though the bottomside of the IRI profile does not completely agree with the observations, they could not cause so much discrepancy in the TEC (LUO FACENG et ul., 1992). Moreover, the bottomside electron content consists only a quarter to a third of the total electron
content. Therefore, it should be the topside profile of the IRI that is in error. CONCLUSION
The IRI gives TEC values that agree fairly well with the TEC Faraday measurements made in
TEC and observations -+
in China
statistical error of foF2 calcnlatcd using lhc CCIR cocflicicnts
---.w statistical error of foF2 calcnlatcd using the AOR mapping .-+
stalistical error of TEC of the N-90
with lhc CCIR cocfkients
...e... statistical error of TEC of the N-90
with the AOR mapping
120 Mean error
0
6
12
18
L’f
‘. 0
6
12
IS
LT
Fig. 2. Diurnal variation of the arithmetic mean error and the standard deviation of TEC (I.OEl5/m**2) and fi~F2 (0.01 MHz) for the years of 1983 1988. The sample sizes are approximately 56 for each hour.
Jan.
Apr.
Jan.
July
Apr.
July
Oct.
Fig. 3. Seasonal variation of the arithmetic mean error and the standard deviation of TEC ( I .OE I5!m**2) and /i/F2 (0.01 MHz) for the years of 1983 1988. The sample sizes arc approximately 120 for each month.
China during low solar activity. Generally the IRI-90 is better than the IRI-86. and the URSI coefficients are better than the CCIR coefficients in the calculation of TEC. Making use of the Asia Oceania Region F2-layer mapping instead of the CCIR coefficients or the URSI coefficients, the IRI provides more accurate TEC values. In OctoberApril during high solar activity, however, the IRI obviously underestimates TEC in daytime, which
could he due to an improper profile.
topside electron
density
Ark,zot~,k~4yml~nt,s~This research was supported by grants from the National Natural Science Foundation of China and the Science Foundation of Electronics of China. The authors are indebted to Dr Bilitza for provision of the IRI programs and the URSI mapping procedures. The authors also acknowledge the valuable discussions with Professor Liu Ruiyuan,
Professor
Sun Xianru.
Dr Wu Jian and Wen Bo.
REFERENCES
BILITZA D., RAWEK K. and PALLASCHKE S.
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CCIR
I986
DAI KAILIANG. Luo FAGENG. QUAN KIJNHAI and Lru RUIYUAN
1992
DAI YUE-OIN
I986
Study of ionospheric models for satellite orbit determination. Radio Sci. 23, 223-232. Report 340-5, CCIR Atlas of Ionospheric Characteristics, ITU. Geneva. Comparison of the Asia Oceania region F2 layer prediction with the CCIR method. Chinese J. Spaw Sci. 12, 153-156. A test of IRI IRI-79 using ionospheric election content data. Chinex J. Spare Sci. 6, 143-146.
DAI KAILIANGand
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MA JIANMING
LUO FAGENG,DAI KAILIANG,QUAWKUNHAI, LIANGLIPINGand LIU RUIYUAN
1992
Comparison of subpeak deduced from ionograms
MCNAMAKAL. F.
1984
Prediction
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Rudio Sri. 7,42118.
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content
using the IRI. Adz.
Space Rex. 4, 25 50.
MCNAMAKAL. F.
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The use of total electron content measurements to validate empirical models of the ionosphere. Ad?. Spaw
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M.. BILITZAD., DAVIS K.. M~NAMAKA L.. S~WAKT F. and POKEMPNEK M. SUY XIANRU
I989
Prediction of total electron content using the IRI. .I. utm0.r. terr. Ph~,s. 45, 169 114. Ionospheric mapping and update of /oF2 coetficients.
TITHI,KIDGE J. E.
1972
RES. 5, 81 90.
RIJSH C.. Fox
R~ftirencu
is also
nmk
10
the following
unpublished
T~~l~~c~omr,lunici,)mf~~uni~~tion J. 56, I79-
19x7
I X2.
A method of predicting the ionospheric 1;? layer in the Asia Oceania region. J. Chinrr Inst. C’ommun. S(6), 37 45. Determination of ionospheric electron content from the Faraday rotation of geostationary satellite signals. Phrt. Spuw Sci. 20, 353 -369.
mutrriol:
BILITZAD.
1990
LEITINGEKR.
1990
International Reference Ionosphere 1990. NSSDC! WDC-A-R&S, 90-22. Electron Content Measurements and IRI. International Reference Ionosphere 90. edited by D. Bilitza, NSSDC:WDC-A-R&S., 90-22.