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Latitudinal variations of solar flux dependence in the topside plasma density: comparison between IRI model and observations
Ad\,. Spncc Krs. Vol. 29, No. 6, pp. 877-882, 2002 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177/02 $22.00 + 0.00
Pergamon
Pll: SO273.1 177(02)00054-6
LATITUDINAL VARIATIONS OF SOLAR FLUX DEPENDENCE IN THE TOPSIDE PLASMA DENSITY: COMPARISON BETWEEN IRI MODEL AND OBSERVATIONS I. Iwamoto’,
’ Communications
H. Katoh’, T. Maruyama’.
Research
H. Minakoshi’,
S. Watari’,
Laboratory, Independent Administrative Institution, Koganei-shi, Tokyo, 184-879.5, JAPAN
and K. Igarashi’
4-2-l Nukuikita-machi,
ABSTRACT The solar flux variations of plasma density in the topside ionosphere around 1000 km altitude at lower and higher latitudes are compared using the satellite data and the International Reference Ionosphere (IRI) model. The modeled profiles show rather stronger dependence on the solar activity at higher latitudes than at lower latitudes. These strong latitudinal variations are not seen in the observed data. Comparison with ISS-b data has shown that the IRI model gives systematically greater topside electron density at higher latitude. In an average sense the IRI model overestimates the high latitude electron density at 1100 km altitude by about a factor of 5 than the observations during high solar activity periods. 0 2002 COSPAR. Published by Elsevicr Science Ltd. All rights reserved.
INTRODUCTION The International Reference Ionosphere (IRI) has been widely used as the de facto standard of the ionosphere model in the world. The starting point of the model is the maximum electron number density of F2 peak (NmF2). NmF2 is calculated from the CCIR maps or given by observation at any particular location. From that NmF2 value the bottom side and the topside electron density distributions are extended. Because of this fact the NmF2 is especially important for the IRI model. And also because the CCIR maps were based rather older data, we will examine in this paper validity of the IRI NmF2 predictions by comparing them with the long term NmF2 data observed at the Japanese stations, which include most recent observations (1965 - 1999). As for topside electron density distribution, the IRI model relies only on the Bent model, in which the Alouette 1 satellite observations during low to medium solar activity periods were used. Because of this limitation, some authors have pointed out severe discrepancy between the IRI topside electron density distributions and observations. Particularly, using more comprehensive data set from the Alouette 1, 2, and ISIS 1 and 2 topside sounder instruments, Bilitza and Williamson (2000) have shown that the IRJ model values at the topside ionosphere exceed observations by a factor of 3 to 11. The ISS-b satellite observations were done during the high solar activity periods (1978-1981) of the solar cycle 21 at fixed altitude of 1100 km like the ISIS 2 satellite (1400 km). Many maps of the ionospheric parameters were published from those observations (Matuura et al., 1981; Iwamoto 1997). The electron densities at satellite altitude were also obtained from the topside sounder instrument (Iwamoto,
x17
I. 1wam010 Pf trl.
878
1985). Using these data, we will compare the latitudinal variations of the solar activity dependence in the topside electron density between the IRI model and observations. This study will be useful for the current effort to improve the topside electron density model using Alouette and ISIS data base (Bilitza et al., 1998).
COMPARISON BETWEEN THE IRI MODEL AND OBSERVATIONS Figure 1 shows the latitudinal distributions of the electron density at 1100 km altitude during (a) the solar minimum (1996) and (b) the solar maximum (1979) conditions calculated by RI-95 model for Lon = 139 degrees (the longitude of the Kokubunji station) and LT = 12. Other parameters are set to default values of the IRI-95 program. It is immediately noticed that the electron densities at higher latitude during the solar maximum period increase severely compared to those of the solar minimum period while the densities at low latitudes are not much different to each other. The increase amounts over 1 order of magnitude beyond about 50 degrees latitudes. Calculations at other local times and longitudes produce very similar profiles. It is primary question of this paper whether this strong tendency is real one or some artifact of the model. Before proceeding to the comparison in the topside ionosphere, we would like to confirm validity of NmF2 prediction by the RI model. The monthly median values of the NmF2 observed during 1965 - 1999 periods at three stations in Japan, namely, Wakkanai (45.23”N, 141.41”E), Kokubunji (35.42”N, 139.29”E), and Okinawa (26.16”N, 127.48”E), are compared with corresponding IRI-95 calculations. Figure 2 shows an example of the correlation. The slope of the linear regression is 1.22 and the correlation coefficient is 0.971 for this particular example. The scatter of the points in the figure represents variability of the density mainly due to the solar activity. The Such analyses are done for all local times and for variation is near one order of magnitude for this example. the three stations, and the result is shown in Fig. 3. From this figure it is seen that in an average sense the This result gives confidence to predictions of NmF2 by the IRI-95 model are good within about 30% error. the starting point of the IRI model, however, we must be aware that it could include up to about 30% error in
(a) Ne at ,,E+()6
I 1OOkm during 1996 (Solar Minimum)
Lon=139, L-12 _.-.-.. “““. ._I_I . “... “. I.. ..
1 i 2
(b) Ne at
1lOO!mduring 1979 (Solar Maximum) Lon=139, LT=lZ
I .E+06
1.E+04 -70
0
Geographic
70
Latitude
-70
0
70
Geographic Latitude
Fig. 1. Latitudinal distributions of the IRI-95 topside electron density at 1100 km altitude during (a) the solar minimum (1996) and (b) the solar maximum (1979) periods for Len= 139 and LT=12. Thin line: March, thick line: June, dashed thick line: September, and dashed thin line: December.
879
NmF2
at KOKUBUNJI
OOLT (1965-l
999)
2.E+06
.z z z 5 n
1 .E+06
id g i%
O.E+OO O.E+OO
1 .E+06
2.E+06
OBSERVED DENSITY (cm^-3) Fig. 2.
An
example
of correlation
NmFI? at the Kokubur~_ji station
hetwecn
the ohscr\;ecl
for 00 LT during
and calculated
monthly
median
values
of
1965 - I9W.
In Figure 3 the local time dependencies at the Kokubunji and Wakkanai stations are similar, some cases. The reason for but that at Okinawa station exhibits a tendency almost opposite to the other two stations. these differences is not clear. Next, we will compare the electron density observed at the ISS-b satellite altitude with the IRI-95 model The ISS-b data base contains the electron densities at satellite altitude which were obtained calculation. For every observation from the AGC voltage of the topside sounder (see Iwamoto, 1985, for details). position and time of ISS-b orbit we calculated corresponding IRI electron density value and then we obtained monthly average values in the bins of 10 degrees in latitude for day (06-18 LT) and night (18-06 LT) The average starts from August The northern and southern hemispheres are not discriminated. conditions. 1978 and ends December 1980, that is. monthly average values are obtained for 29 months.
+ --C
00
Okinawa Kokubunji
06
12 Local Time
Fig. 3. Slopes of linear regression between during 19651999 for three stations in Japan
the observed
18
and calculated
24
monthly
median
NmF2
I. Iwamoto
All Latitude-Local
I
‘f
.
time
l
s %
- . .
$ I.E+05 9” x .z f” B
et al
.
PI:::
1.E+04 l.E+04
Hti
1.E+05
1.E+06
Density Observed by ISS-b (cm*-3)
Fig. 4. Correlation between averaged ISS-b observations corresponding IRI-95 calculations.
in bins of 10 degrees in latitude and
The correlation of all the 29-months data is shown in Fig. 4 as a scatter plot. There are 29 x 7 x 2 = 406 points in the figure. If the IRI predictions are correct, then the points should align along the line that is inclined 45 degrees to x-axis. This-figure clearly demonstrates that the IRI-95 model overestimates the electron density at the ISS-b altitude at the high density end which corresponds to high solar activity. To look at this more closely, Figure 5 shows examples of scatter plots between the average ISS-b observations and corresponding MI-95 calculations in the individual bins.
_ 3.E+OS m
5.E+O5
(b) DAY LAT=60-70
5
a
0” l.E+05 h y = 5.5171x R2 = 0.5852 O.E+OO
1.E+05
2.E+05
3 .E+05
Average Density by ISS-b Observations (cm”-3) Fig.
O.E+OO O.OE+OO
II
.
I
.
5 .OE+04
I 1.OE+05
Average Density by ISS-b Observations (cm”-3)
5. Examples of correlation between average ISS-b observations and IRI model. LAT=O-10 bin, (b) DAY, LAT=60-70 bin. Note that x- and y-scales are different for (b).
(a) DAY,
881
-
0
+Daytime
’
&Nighttime
I
I
O-10
10-20
I
20-30
Geographic
variations Fig. 6. Latitudinal corresponding IRI calculations
of linear
I
I
30-40
40-50
I
SO-60
60-70
Latitude (Absolute Value)
regression
slopes between
average
ES-b
observations
and
The y-intercepts are The slopes of linear regression are 1.42 and 5.52 for (a) and (b), respectively. Figure 6 shows latitudinal forced to zero because our intension here is to understand only general trends. For daytime condition the slopes variations of such linear correlation analyses for day and night conditions. of regression in the low latitudes are close to 1.0 and hence it means that the M-95 predictions are generally That good here, however, they become larger for higher latitude, reaching over 5 at 60-70 degrees latitude. is, IRI-95 overestimates the high latitude electron density by about a factor of 5 at the altitude and during It is interesting to note that the overestimation at lower latitudes is larger period of ES-b observations. during nighttime, whereas at high latitudes it is higher during daytime.
DISCUSSION By comparing with the ISS-b topside electron density observations during the high solar activity periods, we have demonstrated in the previous section that the M-95 progressively overestimates the topside electron The overestimations become larger than observations by more than a factor of 5. density at higher latitudes. Using more comprehensive database from the This result is consistent with Bilitza and Williamson (2000). Alouette and ISIS satellites, they showed that IRI overestimates the measurements by a factor of 3 to 11. Similar results were pointed out by IS-radar measurements (Pandey et al., 1996; Pandey et al., 1997). This overestimation at the topside ionosphere is one of the shortcomings of the present IRI model that originates from the Bent model. In the IRI-95 model, the topside density is given by following functional form (Rawer, 1981): M = IVl7zF2* exp(-y.Ah)
(1)
Where dh is normalized height from the reference height and y is the inverse on the F10.7, geomagnetic latitude, and critical frequency foF2. Bilitza et al. (1998) gives the dependence of this parameter as: P=t,,+t,cos’~+~t2R(F10.7)+f~cos~(PR(F10.7)+f,foF2+fjfoF2cos’Q>+t,(foF2)’ R( F10.7) = ( F10.7 - 40) /SO
(3)
scale height that depends
(2)
I.
882
Iwamotoet nl.
Where @ is the geomagnetic latitude. The terms that depend on both the solar activity and latitude are t3 and ts. The dependence of ts term on the solar activity is implicit because foF2 depend on the solar activity. Our analysis above shows that these two terms are problematic. Iwamoto (1997) has shown that the solar activity dependence coefficients of the plasma density at the ISS-b altitude (1100 km) are not much dependent on the latitude. The original Bent model was based on the data that do not include very high solar activity periods and also it gave only three-steps-latitudinal dependence (low, middle and high latitudes). That seems to be the main reason why the coefficients, ts and t5 in the IRI model are not appropriate at higher latitudes and during high solar activity periods. The improvement of the topside model is now underway (Bilitza et al., 1998, Bilitza; Williamson, 2000) using more comprehensive ISIS and Alouette data set. We hope that improvement should appear as soon as possible. CONCLUSIONS Because the NmF2 is the most important basis of the IRI modeling, we have examined the predictions of NmF2 by the IRI-95 model comparing them with the 35 years of observations in Japanese stations. It has been confirmed that the IRI predictions of NmF2 are correct within about 30% in average. Next, we have compared the monthly averaged topside electron densities at 1100 km altitude observed by ISS-b satellite during high solar activity (1978-1981) with corresponding IRI-95 calculations. It has been found that the IRI-95 model gives systematically larger topside electron densities than observations for higher latitude during the high solar activity periods. The model values at 60-70 degrees latitude become larger than observation by a factor of 5, which is consistent with other studies (Bilitza and Williamson, 2000). The need for improvement of the IRI topside profile is quite evident and we hope that improvement will appear very soon. ~ ACKNOWLEDGMENTS The authors wish to acknowledge two referees for their helpful comments and suggestions.
REFERENCES Bilitza, D., C. Koblinsky, R.Williamson, and S. Bhardwaj, Improving the topside electron density model for IRI, Adv. Space Res., 22, #6,777-787, 1998. Bilitza, D., and R. Williamson, Toward a better representation of the IRI topside based on ISIS and Alouette data, Adv. Space Res., 25, #25, 149-152,200O. Iwamoto, I. (Ed.), Summary Plots of Ionospheric Parameters Obtained from Ionosphere Sounding Satellite-b Vol. 1-4, Published by Radio Res. Lab., Koganei-shi, Tokyo, 1985. Iwamoto, I., A study on the ion composition of the topside ionosphere by satellite-borne mass spectrometers, J. Comm. Res. Lab., 44, #l, 11-186, 1997. Matuura, N., M. Kohtaki, S. Miyazaki, E. Sagawa, and I. Iwamoto, ISS-b experimental results on global distributions of ionospheric parameters and thunderstorm activity, Acta Astronotautica 8,527-548, 1981. Pandey, V. K., N. K. Sethi, and K. K. Mahajan, Comparison of IRI topside electron density profile with Arecibo incoherent scatter measurements, Adv. Space Res., 18, #6,289-292, 1996. Pandey, V. K., N. K. Sethi, and K. K. Mahajan, Topside electron density distribution during sunrise and sunset conditions at Arecibo: comparison with IRI, Adv. Space Res., 20, #9, 1765-1768, 1997. Rawer, K. (Chairman), International Reference Ionosphere - IRI 79, Report UAG-82, Published by World Data Center A for Solar-Terrestrial Physics, NOAA, Boulder, Co., 1981.
Pergamon
Pll: SO273.1 177(02)00054-6
LATITUDINAL VARIATIONS OF SOLAR FLUX DEPENDENCE IN THE TOPSIDE PLASMA DENSITY: COMPARISON BETWEEN IRI MODEL AND OBSERVATIONS I. Iwamoto’,
’ Communications
H. Katoh’, T. Maruyama’.
Research
H. Minakoshi’,
S. Watari’,
Laboratory, Independent Administrative Institution, Koganei-shi, Tokyo, 184-879.5, JAPAN
and K. Igarashi’
4-2-l Nukuikita-machi,
ABSTRACT The solar flux variations of plasma density in the topside ionosphere around 1000 km altitude at lower and higher latitudes are compared using the satellite data and the International Reference Ionosphere (IRI) model. The modeled profiles show rather stronger dependence on the solar activity at higher latitudes than at lower latitudes. These strong latitudinal variations are not seen in the observed data. Comparison with ISS-b data has shown that the IRI model gives systematically greater topside electron density at higher latitude. In an average sense the IRI model overestimates the high latitude electron density at 1100 km altitude by about a factor of 5 than the observations during high solar activity periods. 0 2002 COSPAR. Published by Elsevicr Science Ltd. All rights reserved.
INTRODUCTION The International Reference Ionosphere (IRI) has been widely used as the de facto standard of the ionosphere model in the world. The starting point of the model is the maximum electron number density of F2 peak (NmF2). NmF2 is calculated from the CCIR maps or given by observation at any particular location. From that NmF2 value the bottom side and the topside electron density distributions are extended. Because of this fact the NmF2 is especially important for the IRI model. And also because the CCIR maps were based rather older data, we will examine in this paper validity of the IRI NmF2 predictions by comparing them with the long term NmF2 data observed at the Japanese stations, which include most recent observations (1965 - 1999). As for topside electron density distribution, the IRI model relies only on the Bent model, in which the Alouette 1 satellite observations during low to medium solar activity periods were used. Because of this limitation, some authors have pointed out severe discrepancy between the IRI topside electron density distributions and observations. Particularly, using more comprehensive data set from the Alouette 1, 2, and ISIS 1 and 2 topside sounder instruments, Bilitza and Williamson (2000) have shown that the IRJ model values at the topside ionosphere exceed observations by a factor of 3 to 11. The ISS-b satellite observations were done during the high solar activity periods (1978-1981) of the solar cycle 21 at fixed altitude of 1100 km like the ISIS 2 satellite (1400 km). Many maps of the ionospheric parameters were published from those observations (Matuura et al., 1981; Iwamoto 1997). The electron densities at satellite altitude were also obtained from the topside sounder instrument (Iwamoto,
x17
I. 1wam010 Pf trl.
878
1985). Using these data, we will compare the latitudinal variations of the solar activity dependence in the topside electron density between the IRI model and observations. This study will be useful for the current effort to improve the topside electron density model using Alouette and ISIS data base (Bilitza et al., 1998).
COMPARISON BETWEEN THE IRI MODEL AND OBSERVATIONS Figure 1 shows the latitudinal distributions of the electron density at 1100 km altitude during (a) the solar minimum (1996) and (b) the solar maximum (1979) conditions calculated by RI-95 model for Lon = 139 degrees (the longitude of the Kokubunji station) and LT = 12. Other parameters are set to default values of the IRI-95 program. It is immediately noticed that the electron densities at higher latitude during the solar maximum period increase severely compared to those of the solar minimum period while the densities at low latitudes are not much different to each other. The increase amounts over 1 order of magnitude beyond about 50 degrees latitudes. Calculations at other local times and longitudes produce very similar profiles. It is primary question of this paper whether this strong tendency is real one or some artifact of the model. Before proceeding to the comparison in the topside ionosphere, we would like to confirm validity of NmF2 prediction by the RI model. The monthly median values of the NmF2 observed during 1965 - 1999 periods at three stations in Japan, namely, Wakkanai (45.23”N, 141.41”E), Kokubunji (35.42”N, 139.29”E), and Okinawa (26.16”N, 127.48”E), are compared with corresponding IRI-95 calculations. Figure 2 shows an example of the correlation. The slope of the linear regression is 1.22 and the correlation coefficient is 0.971 for this particular example. The scatter of the points in the figure represents variability of the density mainly due to the solar activity. The Such analyses are done for all local times and for variation is near one order of magnitude for this example. the three stations, and the result is shown in Fig. 3. From this figure it is seen that in an average sense the This result gives confidence to predictions of NmF2 by the IRI-95 model are good within about 30% error. the starting point of the IRI model, however, we must be aware that it could include up to about 30% error in
(a) Ne at ,,E+()6
I 1OOkm during 1996 (Solar Minimum)
Lon=139, L-12 _.-.-.. “““. ._I_I . “... “. I.. ..
1 i 2
(b) Ne at
1lOO!mduring 1979 (Solar Maximum) Lon=139, LT=lZ
I .E+06
1.E+04 -70
0
Geographic
70
Latitude
-70
0
70
Geographic Latitude
Fig. 1. Latitudinal distributions of the IRI-95 topside electron density at 1100 km altitude during (a) the solar minimum (1996) and (b) the solar maximum (1979) periods for Len= 139 and LT=12. Thin line: March, thick line: June, dashed thick line: September, and dashed thin line: December.
879
NmF2
at KOKUBUNJI
OOLT (1965-l
999)
2.E+06
.z z z 5 n
1 .E+06
id g i%
O.E+OO O.E+OO
1 .E+06
2.E+06
OBSERVED DENSITY (cm^-3) Fig. 2.
An
example
of correlation
NmFI? at the Kokubur~_ji station
hetwecn
the ohscr\;ecl
for 00 LT during
and calculated
monthly
median
values
of
1965 - I9W.
In Figure 3 the local time dependencies at the Kokubunji and Wakkanai stations are similar, some cases. The reason for but that at Okinawa station exhibits a tendency almost opposite to the other two stations. these differences is not clear. Next, we will compare the electron density observed at the ISS-b satellite altitude with the IRI-95 model The ISS-b data base contains the electron densities at satellite altitude which were obtained calculation. For every observation from the AGC voltage of the topside sounder (see Iwamoto, 1985, for details). position and time of ISS-b orbit we calculated corresponding IRI electron density value and then we obtained monthly average values in the bins of 10 degrees in latitude for day (06-18 LT) and night (18-06 LT) The average starts from August The northern and southern hemispheres are not discriminated. conditions. 1978 and ends December 1980, that is. monthly average values are obtained for 29 months.
+ --C
00
Okinawa Kokubunji
06
12 Local Time
Fig. 3. Slopes of linear regression between during 19651999 for three stations in Japan
the observed
18
and calculated
24
monthly
median
NmF2
I. Iwamoto
All Latitude-Local
I
‘f
.
time
l
s %
- . .
$ I.E+05 9” x .z f” B
et al
.
PI:::
1.E+04 l.E+04
Hti
1.E+05
1.E+06
Density Observed by ISS-b (cm*-3)
Fig. 4. Correlation between averaged ISS-b observations corresponding IRI-95 calculations.
in bins of 10 degrees in latitude and
The correlation of all the 29-months data is shown in Fig. 4 as a scatter plot. There are 29 x 7 x 2 = 406 points in the figure. If the IRI predictions are correct, then the points should align along the line that is inclined 45 degrees to x-axis. This-figure clearly demonstrates that the IRI-95 model overestimates the electron density at the ISS-b altitude at the high density end which corresponds to high solar activity. To look at this more closely, Figure 5 shows examples of scatter plots between the average ISS-b observations and corresponding MI-95 calculations in the individual bins.
_ 3.E+OS m
5.E+O5
(b) DAY LAT=60-70
5
a
0” l.E+05 h y = 5.5171x R2 = 0.5852 O.E+OO
1.E+05
2.E+05
3 .E+05
Average Density by ISS-b Observations (cm”-3) Fig.
O.E+OO O.OE+OO
II
.
I
.
5 .OE+04
I 1.OE+05
Average Density by ISS-b Observations (cm”-3)
5. Examples of correlation between average ISS-b observations and IRI model. LAT=O-10 bin, (b) DAY, LAT=60-70 bin. Note that x- and y-scales are different for (b).
(a) DAY,
881
-
0
+Daytime
’
&Nighttime
I
I
O-10
10-20
I
20-30
Geographic
variations Fig. 6. Latitudinal corresponding IRI calculations
of linear
I
I
30-40
40-50
I
SO-60
60-70
Latitude (Absolute Value)
regression
slopes between
average
ES-b
observations
and
The y-intercepts are The slopes of linear regression are 1.42 and 5.52 for (a) and (b), respectively. Figure 6 shows latitudinal forced to zero because our intension here is to understand only general trends. For daytime condition the slopes variations of such linear correlation analyses for day and night conditions. of regression in the low latitudes are close to 1.0 and hence it means that the M-95 predictions are generally That good here, however, they become larger for higher latitude, reaching over 5 at 60-70 degrees latitude. is, IRI-95 overestimates the high latitude electron density by about a factor of 5 at the altitude and during It is interesting to note that the overestimation at lower latitudes is larger period of ES-b observations. during nighttime, whereas at high latitudes it is higher during daytime.
DISCUSSION By comparing with the ISS-b topside electron density observations during the high solar activity periods, we have demonstrated in the previous section that the M-95 progressively overestimates the topside electron The overestimations become larger than observations by more than a factor of 5. density at higher latitudes. Using more comprehensive database from the This result is consistent with Bilitza and Williamson (2000). Alouette and ISIS satellites, they showed that IRI overestimates the measurements by a factor of 3 to 11. Similar results were pointed out by IS-radar measurements (Pandey et al., 1996; Pandey et al., 1997). This overestimation at the topside ionosphere is one of the shortcomings of the present IRI model that originates from the Bent model. In the IRI-95 model, the topside density is given by following functional form (Rawer, 1981): M = IVl7zF2* exp(-y.Ah)
(1)
Where dh is normalized height from the reference height and y is the inverse on the F10.7, geomagnetic latitude, and critical frequency foF2. Bilitza et al. (1998) gives the dependence of this parameter as: P=t,,+t,cos’~+~t2R(F10.7)+f~cos~(PR(F10.7)+f,foF2+fjfoF2cos’Q>+t,(foF2)’ R( F10.7) = ( F10.7 - 40) /SO
(3)
scale height that depends
(2)
I.
882
Iwamotoet nl.
Where @ is the geomagnetic latitude. The terms that depend on both the solar activity and latitude are t3 and ts. The dependence of ts term on the solar activity is implicit because foF2 depend on the solar activity. Our analysis above shows that these two terms are problematic. Iwamoto (1997) has shown that the solar activity dependence coefficients of the plasma density at the ISS-b altitude (1100 km) are not much dependent on the latitude. The original Bent model was based on the data that do not include very high solar activity periods and also it gave only three-steps-latitudinal dependence (low, middle and high latitudes). That seems to be the main reason why the coefficients, ts and t5 in the IRI model are not appropriate at higher latitudes and during high solar activity periods. The improvement of the topside model is now underway (Bilitza et al., 1998, Bilitza; Williamson, 2000) using more comprehensive ISIS and Alouette data set. We hope that improvement should appear as soon as possible. CONCLUSIONS Because the NmF2 is the most important basis of the IRI modeling, we have examined the predictions of NmF2 by the IRI-95 model comparing them with the 35 years of observations in Japanese stations. It has been confirmed that the IRI predictions of NmF2 are correct within about 30% in average. Next, we have compared the monthly averaged topside electron densities at 1100 km altitude observed by ISS-b satellite during high solar activity (1978-1981) with corresponding IRI-95 calculations. It has been found that the IRI-95 model gives systematically larger topside electron densities than observations for higher latitude during the high solar activity periods. The model values at 60-70 degrees latitude become larger than observation by a factor of 5, which is consistent with other studies (Bilitza and Williamson, 2000). The need for improvement of the IRI topside profile is quite evident and we hope that improvement will appear very soon. ~ ACKNOWLEDGMENTS The authors wish to acknowledge two referees for their helpful comments and suggestions.
REFERENCES Bilitza, D., C. Koblinsky, R.Williamson, and S. Bhardwaj, Improving the topside electron density model for IRI, Adv. Space Res., 22, #6,777-787, 1998. Bilitza, D., and R. Williamson, Toward a better representation of the IRI topside based on ISIS and Alouette data, Adv. Space Res., 25, #25, 149-152,200O. Iwamoto, I. (Ed.), Summary Plots of Ionospheric Parameters Obtained from Ionosphere Sounding Satellite-b Vol. 1-4, Published by Radio Res. Lab., Koganei-shi, Tokyo, 1985. Iwamoto, I., A study on the ion composition of the topside ionosphere by satellite-borne mass spectrometers, J. Comm. Res. Lab., 44, #l, 11-186, 1997. Matuura, N., M. Kohtaki, S. Miyazaki, E. Sagawa, and I. Iwamoto, ISS-b experimental results on global distributions of ionospheric parameters and thunderstorm activity, Acta Astronotautica 8,527-548, 1981. Pandey, V. K., N. K. Sethi, and K. K. Mahajan, Comparison of IRI topside electron density profile with Arecibo incoherent scatter measurements, Adv. Space Res., 18, #6,289-292, 1996. Pandey, V. K., N. K. Sethi, and K. K. Mahajan, Topside electron density distribution during sunrise and sunset conditions at Arecibo: comparison with IRI, Adv. Space Res., 20, #9, 1765-1768, 1997. Rawer, K. (Chairman), International Reference Ionosphere - IRI 79, Report UAG-82, Published by World Data Center A for Solar-Terrestrial Physics, NOAA, Boulder, Co., 1981.