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Dependence of the topside ion composition on the solar flux and its implication to IRI model
0 IYYY COSPAR.
Pergamon www.elsevier.nl/locate/asr
Adv. Space Res. Vol. 25, No. I, pp. 197-200, 2000 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 027% I 177/00 $20.00 + 0.00
PII: SO273-1177(99)00918-7
DEPENDENCE OF THE TOPSIDE ION COMPOSITION SOLAR FLUX AND ITS IMPLICATION TO IRI MODEL
ON THE
I. Iwamoto’, E. Sagawa’, and S. Watanabe’
‘Communications
Research Laboratory, 4-2-l Nukui-kita-machi,
Koganei-shi,
Tokyo 184-8795,
Japan.
ABSTRACT Statistical analyses of the ion density data at 1100 km altitude during the solar maximum of cycle 21 have revealed that the H’ density has negative correlation with the solar flux F10.7 while 0’ has positive correlation. He’ has generally positive correlation with solar flux in the nighttime but it has negative correlation in the daytime, resulting in almost no correlation if globally averaged. The implication of these characteristics to the International Reference Ionosphere model is discussed 01999 COSPAR. Published by Elsevier Science Ltd.
INTRODUCTION A number of satellites measured the ion composition in 1970’s and 1980’s (Bilitza, 1990), however these results have not comprehensively applied to the International Reference Ionosphere (IRI) modeling (Bilitza, 1993) in the topside ionosphere. Difficulties arise due to the fact that the composition varies according to the solar cycle, altitude, latitude, longitude or local time. Any observation by one satellite cannot fill such a vast space and time. The ISS-b satellite moderated this situation by selecting the orbit nearly circular at 1100 km altitude (Wakai and Matuura, 1980). Using the ISS-b data, the global maps of various ionospheric parameters were obtained (Matuura et al., 1981) corresponding to the solar maximum period of cycle 21 and also the diurnal and seasonal variations of He’ density were studied (Iwamoto, 1993). In order to contribute to the IRI modeling efforts, we will statistically analyze the ion density data obtained from ISS-b to obtain their solar flux and seasonal dependence. DATA ANALYSIS Figure 1 shows the running average ion densities of H’ and 0’ at 1100 km altitude during two years from August 1978. The density data in this figure are produced by accumulating the observed data into bins of 5-degrees in the geomagnetic latitude and 4-months in observation duration (Iwamoto, 1997). Note that the standard deviation for each data point ranges from 10 to 50% due to many varying factors. We can see quite regular seasonal structure in these plots. For O’, summer-winter alternation is evident for both hemispheres and regularly changing structure is clearer at night( 18-06 LT) than in daytime(06-18 LT). In contrast to O’, the contour plots for H’ show very symmetric patterns for northern and southern hemispheres particularly at night. Although these plots show regular seasonal structure, these variations 197
198
I. Iwamoto el d. ISS-b
ii’
AUG 1978
1960
1979
1978 M-b
ISS-b
LTw06-16H
H’
ISS-b
LT-16-06H
0’
LT=O6-16H
DEC
APR
AUG
DEC
APR
1979 0’
At.32
1980
LT.=18-06H
73
‘iuG
APR
DEC.
AUG 1979
1976
DEC
APR
AUG
1980
iUG 1978
DEC
APR
AUG 1979
DEC
APR
AUG
1980
Fig. 1 Contour plots of log ion number density (in cm”) at 1100 km altitude during two years from August 1978 obtained by running average method. To separate the seasonal include the contribution from the solar flux changes. contributions, the averaged ion densities are fitted to the following equation. Lo&N,)
=
and solar
A(l+B.F10.7)[1+Csin(k.M-D)]
flus
(1)
Where, IV,:Average ion density in a bin of 30-dgerees in dip latitude, 6-hours in local time, and 4-months in observation duration. M: Month count from August 1978. icI=O: Au g. - Nov. 1978, M=25: Aug. - Nov. 1980. F10.7: Averaged F10.7 corresponding to M. A, B. C, and D: Coefficients determined by the least squares method, “SALS” developed by the Computer Centre, the University of Tokyo (Nakagawa and Oyanagi, 1982). k : -4 constant
(=2x/12)
This approximate form is assumed to easily see the relative importance of major controlling factors The left column shows the Figure 2 shows the result of the fitting analysis. neglecting other effects. regression coefficients between the ion density and F10.7 (term B) and the right column shows the The uncertainty sigmas for C are generally below 20%, but coefficients of seasonal variation (term 0. As for the they are larger for B so that data points of large fitting errors are excluded from the plots. solar flus dependence, following characteristics are seen from this figure. (1) The H’ density is negatively correlated with F10.7 in most of the latitude-local time domains. (2) 0’ has generally positive coefficients but ir has also negative ones mainly at higher latitude in the southern hemisphere.
Solar Flux and ion Composition H’ - F10.7 CORRELATION
+LT:OO-O6 -a- LT:O6-12 +LT:12-18
-90
-60
-30
0
30
60
DIP LATITUDE
He* - F10.7 CORRELATION +
LT100-06
-I-
LT:06-12
H%’
,
I
\
w-w
1
1
E2.5 -90
-60
-30
0
30
60
90
DIP LATITUDE 0’ - F10.7 CORRELATtON 2.5
I
I
z
LT
0* 0.12 I
-
00-06
M
w-12
o--012-18 c-am-24 I
I
+LT:OO-O6 -n-
J -90
-60
LT:06-12
-A-LT:12-18
-e
-30
LT:18-24
0
30
60
90
DIP LATITUDE
Fig. 2 Regression coefficients for correlation between ion density and F10.7 (left column) and coefficients of seasonal variations (right column).
200
I. lwamoto et rrl
(3) Generally, He’ has negative coefficients in daytime, but it has positive ones in nighttime, resulting in almost no correlation with F10.7 if averaged. As for the seasonal dependence, following characteristics are evident. (1) The amplitudes of the seasonal variation are larger in higher latitudes than those in lower latitudes. (2) H’ has the largest amplitudes among the three species. (3) H’ variations in the northern and southern hemispheres are nearly in-phase while 0’ shows regular expected winter - summer alternation. DISCUSSION AND CONCLUSION The above analysis has revealed rather complex solar flux and seasonal variations in ion density structure at 1100 km altitude. H’ density is strongly anti-correlated with F10.7 in agreement with the AE-E observations (Gonzalez et al., 1992). Gonzalez et al. (1992) indicates that during solar minimum He’ is always minor in the topside ionosphere. DE 2 observations (Heelis et al., 1990) have revealed that He’ sometimes becomes dominant at 900 km altitude during solar maximum period. Combination of these two observations might give an impression that He+ density is positively correlated with F10.7. Our analysis at 1lOOkm during solar maximum has indicated that He’ behavior is more complex. According to our analysis, He’ - F10.7 correlation is negative in daytime while it is positive at night and overall 0’ is generally positively correlated with F10.7 similarly to NmF2 and correlation is very little. becomes to be dominant constituent at 1100 km altitude during the solar maximum. The present IRI model shows that the dominant species around 1000 km altitude is H’ and its density at higher latitude tends to become too high values during higher solar flux. We expect that the characteristics described above should be taken into consideration for the future improvements of the present IRI. REFERENCES Bilitza, D., Empirical modeling of ion composition in the middle and topside ionosphere, Adv. Space Res., 10,47 (1990). Bilitza, D., International reference ionosphere - past, present, and future: II. Plasma temperatures, ion composition and ion drift, Adv. Space Res., 13, 1.5 (1993). Gonzalez, S. A., B. G Fejer, R. A. Heelis, and W. B. Hanson, Ion composition of the topside equatorial ionosphere during solar minimum, 1. Geophys. Res., 97,4299 (1992). Heelis, R. A., W. B. Hanson, and G J. Bailey, Distribution of He’ at middle and equatorial latitudes during solar maximum, J. Geophys. Res., 95, 10313 (1990). Iwamoto, I., Diurnal behavior of the equatorial He+ trough at an altitude of 1100 km, 1. Geomag. Geoelectr., 45, 29 (1993). Iwamoto, I., A study on the ion composition of the topside ionosphere by satellite-borne mass spectrometers, J. Comm. Res., 44, 11 (1997). Matuura, N., M. Kotaki, S. Miyazaki, E. Sagawa, and I. Iwamoto, ISS-b experimental results on global distributions of ionospheric parameters and thunderstorm activity, Acta Astronautica, 8, 527 (198 1). Nakagawa, T. and Y. Oyanagi, Experimental data analysis by the least squares fitting (in Japanese), University of Tokyo Publishing Co., Tokyo, 1982. Wakai, N., and N. Matuura, Operation and experimental results of the Ionosphere Sounding Satellite-b, Acta Astronautica, 7, 999 (1980).
Pergamon www.elsevier.nl/locate/asr
Adv. Space Res. Vol. 25, No. I, pp. 197-200, 2000 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 027% I 177/00 $20.00 + 0.00
PII: SO273-1177(99)00918-7
DEPENDENCE OF THE TOPSIDE ION COMPOSITION SOLAR FLUX AND ITS IMPLICATION TO IRI MODEL
ON THE
I. Iwamoto’, E. Sagawa’, and S. Watanabe’
‘Communications
Research Laboratory, 4-2-l Nukui-kita-machi,
Koganei-shi,
Tokyo 184-8795,
Japan.
ABSTRACT Statistical analyses of the ion density data at 1100 km altitude during the solar maximum of cycle 21 have revealed that the H’ density has negative correlation with the solar flux F10.7 while 0’ has positive correlation. He’ has generally positive correlation with solar flux in the nighttime but it has negative correlation in the daytime, resulting in almost no correlation if globally averaged. The implication of these characteristics to the International Reference Ionosphere model is discussed 01999 COSPAR. Published by Elsevier Science Ltd.
INTRODUCTION A number of satellites measured the ion composition in 1970’s and 1980’s (Bilitza, 1990), however these results have not comprehensively applied to the International Reference Ionosphere (IRI) modeling (Bilitza, 1993) in the topside ionosphere. Difficulties arise due to the fact that the composition varies according to the solar cycle, altitude, latitude, longitude or local time. Any observation by one satellite cannot fill such a vast space and time. The ISS-b satellite moderated this situation by selecting the orbit nearly circular at 1100 km altitude (Wakai and Matuura, 1980). Using the ISS-b data, the global maps of various ionospheric parameters were obtained (Matuura et al., 1981) corresponding to the solar maximum period of cycle 21 and also the diurnal and seasonal variations of He’ density were studied (Iwamoto, 1993). In order to contribute to the IRI modeling efforts, we will statistically analyze the ion density data obtained from ISS-b to obtain their solar flux and seasonal dependence. DATA ANALYSIS Figure 1 shows the running average ion densities of H’ and 0’ at 1100 km altitude during two years from August 1978. The density data in this figure are produced by accumulating the observed data into bins of 5-degrees in the geomagnetic latitude and 4-months in observation duration (Iwamoto, 1997). Note that the standard deviation for each data point ranges from 10 to 50% due to many varying factors. We can see quite regular seasonal structure in these plots. For O’, summer-winter alternation is evident for both hemispheres and regularly changing structure is clearer at night( 18-06 LT) than in daytime(06-18 LT). In contrast to O’, the contour plots for H’ show very symmetric patterns for northern and southern hemispheres particularly at night. Although these plots show regular seasonal structure, these variations 197
198
I. Iwamoto el d. ISS-b
ii’
AUG 1978
1960
1979
1978 M-b
ISS-b
LTw06-16H
H’
ISS-b
LT-16-06H
0’
LT=O6-16H
DEC
APR
AUG
DEC
APR
1979 0’
At.32
1980
LT.=18-06H
73
‘iuG
APR
DEC.
AUG 1979
1976
DEC
APR
AUG
1980
iUG 1978
DEC
APR
AUG 1979
DEC
APR
AUG
1980
Fig. 1 Contour plots of log ion number density (in cm”) at 1100 km altitude during two years from August 1978 obtained by running average method. To separate the seasonal include the contribution from the solar flux changes. contributions, the averaged ion densities are fitted to the following equation. Lo&N,)
=
and solar
A(l+B.F10.7)[1+Csin(k.M-D)]
flus
(1)
Where, IV,:Average ion density in a bin of 30-dgerees in dip latitude, 6-hours in local time, and 4-months in observation duration. M: Month count from August 1978. icI=O: Au g. - Nov. 1978, M=25: Aug. - Nov. 1980. F10.7: Averaged F10.7 corresponding to M. A, B. C, and D: Coefficients determined by the least squares method, “SALS” developed by the Computer Centre, the University of Tokyo (Nakagawa and Oyanagi, 1982). k : -4 constant
(=2x/12)
This approximate form is assumed to easily see the relative importance of major controlling factors The left column shows the Figure 2 shows the result of the fitting analysis. neglecting other effects. regression coefficients between the ion density and F10.7 (term B) and the right column shows the The uncertainty sigmas for C are generally below 20%, but coefficients of seasonal variation (term 0. As for the they are larger for B so that data points of large fitting errors are excluded from the plots. solar flus dependence, following characteristics are seen from this figure. (1) The H’ density is negatively correlated with F10.7 in most of the latitude-local time domains. (2) 0’ has generally positive coefficients but ir has also negative ones mainly at higher latitude in the southern hemisphere.
Solar Flux and ion Composition H’ - F10.7 CORRELATION
+LT:OO-O6 -a- LT:O6-12 +LT:12-18
-90
-60
-30
0
30
60
DIP LATITUDE
He* - F10.7 CORRELATION +
LT100-06
-I-
LT:06-12
H%’
,
I
\
w-w
1
1
E2.5 -90
-60
-30
0
30
60
90
DIP LATITUDE 0’ - F10.7 CORRELATtON 2.5
I
I
z
LT
0* 0.12 I
-
00-06
M
w-12
o--012-18 c-am-24 I
I
+LT:OO-O6 -n-
J -90
-60
LT:06-12
-A-LT:12-18
-e
-30
LT:18-24
0
30
60
90
DIP LATITUDE
Fig. 2 Regression coefficients for correlation between ion density and F10.7 (left column) and coefficients of seasonal variations (right column).
200
I. lwamoto et rrl
(3) Generally, He’ has negative coefficients in daytime, but it has positive ones in nighttime, resulting in almost no correlation with F10.7 if averaged. As for the seasonal dependence, following characteristics are evident. (1) The amplitudes of the seasonal variation are larger in higher latitudes than those in lower latitudes. (2) H’ has the largest amplitudes among the three species. (3) H’ variations in the northern and southern hemispheres are nearly in-phase while 0’ shows regular expected winter - summer alternation. DISCUSSION AND CONCLUSION The above analysis has revealed rather complex solar flux and seasonal variations in ion density structure at 1100 km altitude. H’ density is strongly anti-correlated with F10.7 in agreement with the AE-E observations (Gonzalez et al., 1992). Gonzalez et al. (1992) indicates that during solar minimum He’ is always minor in the topside ionosphere. DE 2 observations (Heelis et al., 1990) have revealed that He’ sometimes becomes dominant at 900 km altitude during solar maximum period. Combination of these two observations might give an impression that He+ density is positively correlated with F10.7. Our analysis at 1lOOkm during solar maximum has indicated that He’ behavior is more complex. According to our analysis, He’ - F10.7 correlation is negative in daytime while it is positive at night and overall 0’ is generally positively correlated with F10.7 similarly to NmF2 and correlation is very little. becomes to be dominant constituent at 1100 km altitude during the solar maximum. The present IRI model shows that the dominant species around 1000 km altitude is H’ and its density at higher latitude tends to become too high values during higher solar flux. We expect that the characteristics described above should be taken into consideration for the future improvements of the present IRI. REFERENCES Bilitza, D., Empirical modeling of ion composition in the middle and topside ionosphere, Adv. Space Res., 10,47 (1990). Bilitza, D., International reference ionosphere - past, present, and future: II. Plasma temperatures, ion composition and ion drift, Adv. Space Res., 13, 1.5 (1993). Gonzalez, S. A., B. G Fejer, R. A. Heelis, and W. B. Hanson, Ion composition of the topside equatorial ionosphere during solar minimum, 1. Geophys. Res., 97,4299 (1992). Heelis, R. A., W. B. Hanson, and G J. Bailey, Distribution of He’ at middle and equatorial latitudes during solar maximum, J. Geophys. Res., 95, 10313 (1990). Iwamoto, I., Diurnal behavior of the equatorial He+ trough at an altitude of 1100 km, 1. Geomag. Geoelectr., 45, 29 (1993). Iwamoto, I., A study on the ion composition of the topside ionosphere by satellite-borne mass spectrometers, J. Comm. Res., 44, 11 (1997). Matuura, N., M. Kotaki, S. Miyazaki, E. Sagawa, and I. Iwamoto, ISS-b experimental results on global distributions of ionospheric parameters and thunderstorm activity, Acta Astronautica, 8, 527 (198 1). Nakagawa, T. and Y. Oyanagi, Experimental data analysis by the least squares fitting (in Japanese), University of Tokyo Publishing Co., Tokyo, 1982. Wakai, N., and N. Matuura, Operation and experimental results of the Ionosphere Sounding Satellite-b, Acta Astronautica, 7, 999 (1980).