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An empirical model of electron density in low latitude at 600 km obtained by Hinotori satellite
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Advances in Space Research 41 (2008) 1495–1499 www.elsevier.com/locate/asr
An empirical model of electron density in low latitude at 600 km obtained by Hinotori satellite Y. Kakinami b
a,*
, S. Watanabe b, K.-I. Oyama
a
a Institute of Space Science, National Central University, Taiwan Department of Cosmoscience, Hokkaido University, Hokkaido University, Japan
Received 26 February 2007; received in revised form 26 September 2007; accepted 26 September 2007
Abstract An empirical model of electron density (Ne) was constructed by using the data obtained with an impedance probe on board Japanese Hinotori satellite. The satellite was in circular orbit of the height of 600 km with the inclination of 31 degrees from February 1981 to June 1982. The constructed model gives Ne at any local time with the time resolution of 90 min and between 25 and 25 degrees in magnetic latitude with its resolution of 5 degrees in the range of F10.7 from 150 to 250 under the condition of Kp < 4. Spline interpolations are applied to the functions of day of year, geomagnetic latitude and solar local time, and linear interpolation is applied to the function of F10.7. Longitude dependence of Ne is not taken into account. Our density model can reproduce solar local time variation of electron density at 600 km altitude better than current International Reference Ionosphere (IRI2001) model which overestimates Ne in night time and underestimates Ne in day time. Our density model together with electron temperature model which has been constructed before will enable more understanding of upper ionospheric phenomenon in the equatorial region. 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Ionosphere; Electron density; Empirical model; IRI
1. Introduction International Reference Ionosphere (IRI) model is the most standard empirical model and has a wide range of applications, such as reference of computer simulation. IRI has been developed since 1978 (Rawer et al., 1978). First IRI-78 was constructed based on ionograms from ground based ionosondes and from topside sounder for topside ionosphere. However electron densities of topside ionosphere were mainly estimated from a peak density of ionosphere and therefore IRI-78 had large deviation from observed density in topside ionosphere. IRI-95 adapted several improvements such as auroral oval and D-region option (Bilitza, 1997). New IRI model *
Corresponding author. Tel.: +886 3 4227151x65775; fax: +886 3 4224394. E-mail address: [email protected] (Y. Kakinami).
(IRI2000) has been developed with many improvements (Bilitza, 2001). IRI2000 predicts more realistic electron density below F1 layer and electron temperature in the topside ionosphere. Storm-time option is also included in IRI2000. However electron density in the topside model still has shortcomings: current IRI model gives higher density in the density region lower than 105 cm3 (Bilitza and Williamson, 2000). These problems have been pointed out by Iwamoto et al. (2002) and Bilitza (2004). Trˇı’skova’ et al. (2006) proposed new topside ionosphere model based on measurements of Intercosmos and Atmospheric Explorer satellite. They compared electron density observed by other satellites to prediction of their model. It has been shown that their model is more improved than IRI model. However shortcomings of the representation of the topside electron density profile in IRI model still remain as was discussed by Bilitza et al. (2006). Japanese sun observation satellite Hinotori was put into a circular orbit of 600 km with an inclination of 31 degrees
0273-1177/$34 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2007.09.031
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Y. Kakinami et al. / Advances in Space Research 41 (2008) 1495–1499
on February 1981 and was terminated its mission on June 1982. Although Hinotori satellite was a sun observation satellite, two unique plasma probes, which were developed in Japan were installed. One of them is an electron temperature probe (Oyama, 1994) and the other is an impedance probe (Oya et al., 1986). The orbits covered every longitude within ±42 degrees geomagnetic latitude (Fig. 1). Hinotori satellite observations have achieved many results in study of equatorial ionosphere, such as plasma bubble (Oya et al., 1986; Watanabe and Oya, 1986). Watanabe and Oyama (1995) pointed out large difference of electron temperature between observations of Hinotori satellite and prediction of IRI model. Oyama et al. (2004) has constructed an empirical model of electron temperature based on Hinotori satellite. The electron density observation of Hinotori satellite allows us to construct an empirical model of electron density at 600 km altitude. 2. Model formation We constructed an empirical model based on Hinotori satellite (Hinotori model) as we discuss here. Hinotori satellite observed electron density (Ne) around 600 km with
gðmLat; LT; monthÞ ¼
time (LT), geomagnetic latitude and daily solar flux F10.7. Longitude variation is not included. We set bins as functions of LT and geomagnetic latitude to construct tables every month in two sets of solar activity; one was low solar activity where F10.7 was below 200 and the other was high activity where F10.7 was over 200. Monthly averages of F10.7 were calculated in each data set. Each table was a running mean of 3 months, in which the center month was double weighted because the data sets of a month were not enough to construct the tables. Averages of F10.7 were calculated in each data set. The range of geomagnetic latitude was from 25 degrees to 25 degrees and the range of LT was from 0 to 24 h. A bin for geomagnetic latitude is set every 5 degrees (11 nodes) and a bin for LT is set every 30 min with the resolution of 90 min (48 nodes). The data were averaged in each bin. Averages of F10.7 were between 150 and 180 for low solar activity and between 230 and 260 for high solar activity. Nonetheless, there were blank bins because the paths of Hinotori satellite did not cover every LT and geomagnetic latitude (Fig. 1). Hence blank bins were filled by values interpolated by spline interpolation of geomagnetic latitude and LT. Tables of Ne gradient g in F10.7 were calculated from two tables for the same month but different solar flux.
log10 NeðmLat; LT; monthÞhigh log10 NeðmLat; LT; monthÞlow ; F 10:7 ðmonthÞhigh F 10:7 ðmonthÞlow
time resolution of 10 s. Total number of the data is composed by 4.2 · 105 measurements. We selected the data which were observed in the period of Kp < 4 to construct the model. Ne of the empirical model is calculated as function for four parameters, which are day of year, solar local
ð1Þ
where mLat is geomagnetic latitude, LT is solar local time and F10.7 is averaged solar flux value of each data set. Subscripts of ‘‘high’’ and ‘‘low’’ mean the data sets of high solar flux and low solar flux. We obtained the table ‘‘g’’ for each month. The Ne tables were interpolated by using Ne gradient tables, and then, the Ne tables were created so that F10.7 becomes 200. The Ne tables were created for F10.7 = 200 (Ne200) for each month as follows: log10 NeðmLat;LT; monthÞ200 ¼ gðmLat;LT; monthÞ 200 F 10:7 ðmonthÞlow þ log10 NeðmLat;LT; monthÞlow : ð2Þ
Fig. 1. Orbits of Hinotori satellite in geographic longitude–geomagnetic latitude space.
The model density is reproduced by using these tables. First of all, the gradient g and Ne200 in day of year (doy) are calculated from monthly tables by using spline function. After that, the density is interpolated by spline function by using g and Ne200 calculated in doy as function of geomagnetic latitude and LT. The range of F10.7 is adapted between 150 and 250. Variation of F10.7 is adapted as follows:
Y. Kakinami et al. / Advances in Space Research 41 (2008) 1495–1499
log10 NeðmLat; LT; doyÞ ¼ gðmLat; LT; doyÞ f200 F 10:7 ðdoyÞg þ log10 NeðmLat; LT; doyÞ200 : ð3Þ 3. Discussion Fig. 2 shows all Hinotori satellite data versus those calculated from Hinotori model. Ne calculated from Hinotori model are in good agreement with observed Ne in density region higher than 105 cm3. Whilst Hinotori model give higher density in the density region lower than 105 cm3. The reason for high density to be estimated from Hinotori model in the low density region is that we adapted linear interpolation for Ne dependency on F10.7. Fig. 3 shows comparison of Hinotori observations and IRI2001 model predictions. Gray dots show the ratio of Ne observed by Hinotori satellite to Ne predicted by interquartile range. Black dots show median of ratio and error
Fig. 2. Comparison of Hinotori observations and Hinotori model predictions.
Fig. 3. Ratios of electron density observed by Hinotori satellite to electron density predicted by IRI2001 model in solar local time. Black dots show 2h median of the ratio and error bars show its interquartile range.
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bars show its interquartile range (IQR). Medians of the ratio of observed Ne to predicted Ne have variation on LT. The median of the ratio is the smallest (about 0.34) and its IQR is 0.38 at 5 LT. The median of the ratio is the largest (2.20) at 13 LT and its IQR is 2.06. IQRs around local noon are larger than at other LT. These results indicate that IRI2001 model overestimates Ne in night time and underestimates Ne in day time. Fig. 4 shows the ratio of Hinotori observations to Hinotori model regarding LT. The medians of the ratio are between 0.9 and 1 in the LT range of 6–24. The smallest median of the ratio (0.86) is seen at 5 LT. The largest IQR is 0.61 at 21 LT and the smallest IQR is 0.36 at 7 LT. IQRs of Hinotori model in day time are smaller than that in night time. The IQRs are less than 0.4 from 8 to 18 LT but they become large in night time where observed Ne values are low. They are about 0.55 in night time except 21 LT. Fig. 4 shows Hinotori model can reconstruct Ne better than IRI2001 model. Fig. 5 shows the ratio of Hinotori observations to Hinotori model regarding longitude. The medians of the ratio of observation to Ne predicted by Hinotori model are about 1 and their IQRs are about 0.5 between 40 and 320 degrees longitude (Fig. 5). Many of small ratios occur in the longitude region of 320–360 and 0–40 degrees. The median of ratio is the smallest (0.79) at 350 degrees longitude and its IQR is 0.44. Hinotori model cannot predict longitudinal variation because the data were averaged from all longitude. This shortcomings should be improved in the near future. The effect of neutral wind cannot be neglected because tilted geomagnetic meridian (geomagnetic declination is about 20 degrees westward). The meridional wind effect is on Ne distribution comparable to the zonal wind effect. Fig. 6 represents global contour plots for F10.7 = 200 which is calculated from the Hinotori model for four seasons. The model can reproduce seasonal variation. As well known, Ne is larger in March and September equinox than
Fig. 4. Ratios of electron density observed by Hinotori satellite to electron density predicted by Hinotori model in solar local time. Black dots show 2-h median of the ratio and error bars show its interquartile range.
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Y. Kakinami et al. / Advances in Space Research 41 (2008) 1495–1499
(Fig. 6d). These structures are not seen in equinox clearly (Fig. 6a and c). Equatorial ionization anomaly (EIA) produced by fountain effect was often observed in each Hinotori orbit. But its effect was not found clearly in Hinotori model because the location of EIA is variable and we averaged the data along longitudinal direction. This explains non-existence of latitudinal structure around 13 LT. The second peak can be explained as due to the prereversal enhancement (Watanabe et al., 1995). Second peak in September equinox has strong latitudinal structure. This feature should be studied in the near future.
4. Summary Fig. 5. Ratios of electron density observed by Hinotori satellite to electron density predicted by Hinotori model in geographic longitude. Black dots show 20 degrees median of the ratio and error bars show its interquartile range.
Ne in June and December solstice. Especially Ne in June solstice is the lowest. Two density peaks are visible in all seasons except June solstice. First peak and second peak appear around 13 and 19 LT, respectively. Ne minimum occurs at 5 LT (namely pre-dawn). Ne at 5 LT shows strong seasonal variation. Especially in June solstice, Ne minimum becomes highest around geomagnetic latitude of 20 degrees (Fig. 6b). On the other hand, the maximum of Ne minimum appears around geomagnetic latitude of 20 degrees in December solstice
We constructed an empirical model of electron density at 600 km altitude based on observations by Hinotori satellite. The Hinotori model provides Ne as a function of day of year, geomagnetic latitude, solar local time and daily solar flux F10.7. The medians of ratio of Ne observed by Hinotori satellite to Ne reproduced by Hinotori model are between 0.9 and 1 in the LT range of 6–24. Its IQRs of the ratio are small in the day side where Ne is high and large in the night side where Ne is low. The medians of ratios are small in the longitude region of 320–360 and 0–20 degrees as compared to other longitude. Hence Hinotori model has a shortcoming in longitudinal variation. Compared with Figs. 3 and 4, the medians of the ratio of Hinotori model are better than that of IRI2001 predictions
Fig. 6. Model predictions in the solar local time–geomagnetic latitude space at 600 km altitude for the case of F10.7 = 200. (a) March equinox, (b) June solstice, (c) September equinox and (d) December solstice.
Y. Kakinami et al. / Advances in Space Research 41 (2008) 1495–1499
in every LT. It means that Hinotori model can reproduce the observations better than IRI2001 model. Hinotori model can reproduce evening density enhancement and its seasonal variation. We believe our Hinotori density model together with electron temperature model will enable more understanding of a variety of upper ionospheric phenomenon. Acknowledgements We are grateful to Editor Dr. Dieter K. Bilitza and the reviewers for their critical reading and suggestions. References Bilitza, D. International ionosphere – status 1995/96. Adv. Space Res. 20 (9), 1751–1754, 1997. Bilitza, D., Williamson, R. Towards a better representation of the IRI topside based on ISIS and alouette data. Adv. Space Res. 25 (1), 149– 152, 2000. Bilitza, D. International reference ionosphere 2000. Radio Sci. 36 (2), 261– 275, 2001. Bilitza, D. A correction for the IRI topside electron density model based on Aloutte/ISIS topside sounder data. Adv. Space Res. 33, 838–843, 2004. Bilitza, D., Reinsch, B.W., Radicella, S., Pulinets, S., Gulyaeva, T., Triskova, L. Improvements of the international reference ionosphere
1499
model for the topside electron density profile. Radio Sci. 41, RS5S15, doi:10.1029/2005RS003370, 2006. Iwamoto, I., Katoh, H., Maruyama, T., Minakoshi, H., Watari, S., Igarashi, K. Latitudinal variations of solar flux dependence in the topside plasma density: comparison between IRI model and observations. Adv. Space Res. 29, 877–882, 2002. Oya, H., Takahashi, T., Watanabe, S. Observation of low latitude ionosphere by the impedance probe on board the Hinotori satellite. J. Geomag. Geoelectr. 38, 111–123, 1986. Oyama, K.-I. Verification of IRI plasma temperature at great altitude by satellite data. Adv. Space Res. 14 (12), 105–113, 1994. Oyama, K.-I., Marinov, P., Kutiev, I., Watanabe, S. Low latitude model of Te at 600 km based on Hinotori satellite data. Adv. Space Res. 34, 2004–2009, 2004. Rawer, K., Bilitza, D., Ramakrishnan, S. Goals and status of international reference ionosphere. Rev. Geophys. 16, 177–181, 1978. Trˇı’skova’, L., Truhlı’k, V., Sˇmilauer, J. An empirical topside electron density model for calculation of absolute ion densities in IRI. Adv. Space Res. 37, 928–934, 2006. Watanabe, S., Oya, H. Occurrence characteristics of low latitude ionosphere irregularities observed by impedance probe on board the Hinotori satellite. J. Geomag. Geoelectr. 38, 125– 149, 1986. Watanabe, S., Oyama, K.-I. Dynamic model and observation of the equatorial ionosphere. Adv. Space Res. 15, 109–112, 1995. Watanabe, S., Oyama, K.-I., Abdu, M.A. Computer simulation of electron and ion densities and temperatures in the equatorial F region and comparison with Hinotori results. J. Geophys. Res. 100, 14581– 14590, 1995.
Advances in Space Research 41 (2008) 1495–1499 www.elsevier.com/locate/asr
An empirical model of electron density in low latitude at 600 km obtained by Hinotori satellite Y. Kakinami b
a,*
, S. Watanabe b, K.-I. Oyama
a
a Institute of Space Science, National Central University, Taiwan Department of Cosmoscience, Hokkaido University, Hokkaido University, Japan
Received 26 February 2007; received in revised form 26 September 2007; accepted 26 September 2007
Abstract An empirical model of electron density (Ne) was constructed by using the data obtained with an impedance probe on board Japanese Hinotori satellite. The satellite was in circular orbit of the height of 600 km with the inclination of 31 degrees from February 1981 to June 1982. The constructed model gives Ne at any local time with the time resolution of 90 min and between 25 and 25 degrees in magnetic latitude with its resolution of 5 degrees in the range of F10.7 from 150 to 250 under the condition of Kp < 4. Spline interpolations are applied to the functions of day of year, geomagnetic latitude and solar local time, and linear interpolation is applied to the function of F10.7. Longitude dependence of Ne is not taken into account. Our density model can reproduce solar local time variation of electron density at 600 km altitude better than current International Reference Ionosphere (IRI2001) model which overestimates Ne in night time and underestimates Ne in day time. Our density model together with electron temperature model which has been constructed before will enable more understanding of upper ionospheric phenomenon in the equatorial region. 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Ionosphere; Electron density; Empirical model; IRI
1. Introduction International Reference Ionosphere (IRI) model is the most standard empirical model and has a wide range of applications, such as reference of computer simulation. IRI has been developed since 1978 (Rawer et al., 1978). First IRI-78 was constructed based on ionograms from ground based ionosondes and from topside sounder for topside ionosphere. However electron densities of topside ionosphere were mainly estimated from a peak density of ionosphere and therefore IRI-78 had large deviation from observed density in topside ionosphere. IRI-95 adapted several improvements such as auroral oval and D-region option (Bilitza, 1997). New IRI model *
Corresponding author. Tel.: +886 3 4227151x65775; fax: +886 3 4224394. E-mail address: [email protected] (Y. Kakinami).
(IRI2000) has been developed with many improvements (Bilitza, 2001). IRI2000 predicts more realistic electron density below F1 layer and electron temperature in the topside ionosphere. Storm-time option is also included in IRI2000. However electron density in the topside model still has shortcomings: current IRI model gives higher density in the density region lower than 105 cm3 (Bilitza and Williamson, 2000). These problems have been pointed out by Iwamoto et al. (2002) and Bilitza (2004). Trˇı’skova’ et al. (2006) proposed new topside ionosphere model based on measurements of Intercosmos and Atmospheric Explorer satellite. They compared electron density observed by other satellites to prediction of their model. It has been shown that their model is more improved than IRI model. However shortcomings of the representation of the topside electron density profile in IRI model still remain as was discussed by Bilitza et al. (2006). Japanese sun observation satellite Hinotori was put into a circular orbit of 600 km with an inclination of 31 degrees
0273-1177/$34 2007 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2007.09.031
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Y. Kakinami et al. / Advances in Space Research 41 (2008) 1495–1499
on February 1981 and was terminated its mission on June 1982. Although Hinotori satellite was a sun observation satellite, two unique plasma probes, which were developed in Japan were installed. One of them is an electron temperature probe (Oyama, 1994) and the other is an impedance probe (Oya et al., 1986). The orbits covered every longitude within ±42 degrees geomagnetic latitude (Fig. 1). Hinotori satellite observations have achieved many results in study of equatorial ionosphere, such as plasma bubble (Oya et al., 1986; Watanabe and Oya, 1986). Watanabe and Oyama (1995) pointed out large difference of electron temperature between observations of Hinotori satellite and prediction of IRI model. Oyama et al. (2004) has constructed an empirical model of electron temperature based on Hinotori satellite. The electron density observation of Hinotori satellite allows us to construct an empirical model of electron density at 600 km altitude. 2. Model formation We constructed an empirical model based on Hinotori satellite (Hinotori model) as we discuss here. Hinotori satellite observed electron density (Ne) around 600 km with
gðmLat; LT; monthÞ ¼
time (LT), geomagnetic latitude and daily solar flux F10.7. Longitude variation is not included. We set bins as functions of LT and geomagnetic latitude to construct tables every month in two sets of solar activity; one was low solar activity where F10.7 was below 200 and the other was high activity where F10.7 was over 200. Monthly averages of F10.7 were calculated in each data set. Each table was a running mean of 3 months, in which the center month was double weighted because the data sets of a month were not enough to construct the tables. Averages of F10.7 were calculated in each data set. The range of geomagnetic latitude was from 25 degrees to 25 degrees and the range of LT was from 0 to 24 h. A bin for geomagnetic latitude is set every 5 degrees (11 nodes) and a bin for LT is set every 30 min with the resolution of 90 min (48 nodes). The data were averaged in each bin. Averages of F10.7 were between 150 and 180 for low solar activity and between 230 and 260 for high solar activity. Nonetheless, there were blank bins because the paths of Hinotori satellite did not cover every LT and geomagnetic latitude (Fig. 1). Hence blank bins were filled by values interpolated by spline interpolation of geomagnetic latitude and LT. Tables of Ne gradient g in F10.7 were calculated from two tables for the same month but different solar flux.
log10 NeðmLat; LT; monthÞhigh log10 NeðmLat; LT; monthÞlow ; F 10:7 ðmonthÞhigh F 10:7 ðmonthÞlow
time resolution of 10 s. Total number of the data is composed by 4.2 · 105 measurements. We selected the data which were observed in the period of Kp < 4 to construct the model. Ne of the empirical model is calculated as function for four parameters, which are day of year, solar local
ð1Þ
where mLat is geomagnetic latitude, LT is solar local time and F10.7 is averaged solar flux value of each data set. Subscripts of ‘‘high’’ and ‘‘low’’ mean the data sets of high solar flux and low solar flux. We obtained the table ‘‘g’’ for each month. The Ne tables were interpolated by using Ne gradient tables, and then, the Ne tables were created so that F10.7 becomes 200. The Ne tables were created for F10.7 = 200 (Ne200) for each month as follows: log10 NeðmLat;LT; monthÞ200 ¼ gðmLat;LT; monthÞ 200 F 10:7 ðmonthÞlow þ log10 NeðmLat;LT; monthÞlow : ð2Þ
Fig. 1. Orbits of Hinotori satellite in geographic longitude–geomagnetic latitude space.
The model density is reproduced by using these tables. First of all, the gradient g and Ne200 in day of year (doy) are calculated from monthly tables by using spline function. After that, the density is interpolated by spline function by using g and Ne200 calculated in doy as function of geomagnetic latitude and LT. The range of F10.7 is adapted between 150 and 250. Variation of F10.7 is adapted as follows:
Y. Kakinami et al. / Advances in Space Research 41 (2008) 1495–1499
log10 NeðmLat; LT; doyÞ ¼ gðmLat; LT; doyÞ f200 F 10:7 ðdoyÞg þ log10 NeðmLat; LT; doyÞ200 : ð3Þ 3. Discussion Fig. 2 shows all Hinotori satellite data versus those calculated from Hinotori model. Ne calculated from Hinotori model are in good agreement with observed Ne in density region higher than 105 cm3. Whilst Hinotori model give higher density in the density region lower than 105 cm3. The reason for high density to be estimated from Hinotori model in the low density region is that we adapted linear interpolation for Ne dependency on F10.7. Fig. 3 shows comparison of Hinotori observations and IRI2001 model predictions. Gray dots show the ratio of Ne observed by Hinotori satellite to Ne predicted by interquartile range. Black dots show median of ratio and error
Fig. 2. Comparison of Hinotori observations and Hinotori model predictions.
Fig. 3. Ratios of electron density observed by Hinotori satellite to electron density predicted by IRI2001 model in solar local time. Black dots show 2h median of the ratio and error bars show its interquartile range.
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bars show its interquartile range (IQR). Medians of the ratio of observed Ne to predicted Ne have variation on LT. The median of the ratio is the smallest (about 0.34) and its IQR is 0.38 at 5 LT. The median of the ratio is the largest (2.20) at 13 LT and its IQR is 2.06. IQRs around local noon are larger than at other LT. These results indicate that IRI2001 model overestimates Ne in night time and underestimates Ne in day time. Fig. 4 shows the ratio of Hinotori observations to Hinotori model regarding LT. The medians of the ratio are between 0.9 and 1 in the LT range of 6–24. The smallest median of the ratio (0.86) is seen at 5 LT. The largest IQR is 0.61 at 21 LT and the smallest IQR is 0.36 at 7 LT. IQRs of Hinotori model in day time are smaller than that in night time. The IQRs are less than 0.4 from 8 to 18 LT but they become large in night time where observed Ne values are low. They are about 0.55 in night time except 21 LT. Fig. 4 shows Hinotori model can reconstruct Ne better than IRI2001 model. Fig. 5 shows the ratio of Hinotori observations to Hinotori model regarding longitude. The medians of the ratio of observation to Ne predicted by Hinotori model are about 1 and their IQRs are about 0.5 between 40 and 320 degrees longitude (Fig. 5). Many of small ratios occur in the longitude region of 320–360 and 0–40 degrees. The median of ratio is the smallest (0.79) at 350 degrees longitude and its IQR is 0.44. Hinotori model cannot predict longitudinal variation because the data were averaged from all longitude. This shortcomings should be improved in the near future. The effect of neutral wind cannot be neglected because tilted geomagnetic meridian (geomagnetic declination is about 20 degrees westward). The meridional wind effect is on Ne distribution comparable to the zonal wind effect. Fig. 6 represents global contour plots for F10.7 = 200 which is calculated from the Hinotori model for four seasons. The model can reproduce seasonal variation. As well known, Ne is larger in March and September equinox than
Fig. 4. Ratios of electron density observed by Hinotori satellite to electron density predicted by Hinotori model in solar local time. Black dots show 2-h median of the ratio and error bars show its interquartile range.
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(Fig. 6d). These structures are not seen in equinox clearly (Fig. 6a and c). Equatorial ionization anomaly (EIA) produced by fountain effect was often observed in each Hinotori orbit. But its effect was not found clearly in Hinotori model because the location of EIA is variable and we averaged the data along longitudinal direction. This explains non-existence of latitudinal structure around 13 LT. The second peak can be explained as due to the prereversal enhancement (Watanabe et al., 1995). Second peak in September equinox has strong latitudinal structure. This feature should be studied in the near future.
4. Summary Fig. 5. Ratios of electron density observed by Hinotori satellite to electron density predicted by Hinotori model in geographic longitude. Black dots show 20 degrees median of the ratio and error bars show its interquartile range.
Ne in June and December solstice. Especially Ne in June solstice is the lowest. Two density peaks are visible in all seasons except June solstice. First peak and second peak appear around 13 and 19 LT, respectively. Ne minimum occurs at 5 LT (namely pre-dawn). Ne at 5 LT shows strong seasonal variation. Especially in June solstice, Ne minimum becomes highest around geomagnetic latitude of 20 degrees (Fig. 6b). On the other hand, the maximum of Ne minimum appears around geomagnetic latitude of 20 degrees in December solstice
We constructed an empirical model of electron density at 600 km altitude based on observations by Hinotori satellite. The Hinotori model provides Ne as a function of day of year, geomagnetic latitude, solar local time and daily solar flux F10.7. The medians of ratio of Ne observed by Hinotori satellite to Ne reproduced by Hinotori model are between 0.9 and 1 in the LT range of 6–24. Its IQRs of the ratio are small in the day side where Ne is high and large in the night side where Ne is low. The medians of ratios are small in the longitude region of 320–360 and 0–20 degrees as compared to other longitude. Hence Hinotori model has a shortcoming in longitudinal variation. Compared with Figs. 3 and 4, the medians of the ratio of Hinotori model are better than that of IRI2001 predictions
Fig. 6. Model predictions in the solar local time–geomagnetic latitude space at 600 km altitude for the case of F10.7 = 200. (a) March equinox, (b) June solstice, (c) September equinox and (d) December solstice.
Y. Kakinami et al. / Advances in Space Research 41 (2008) 1495–1499
in every LT. It means that Hinotori model can reproduce the observations better than IRI2001 model. Hinotori model can reproduce evening density enhancement and its seasonal variation. We believe our Hinotori density model together with electron temperature model will enable more understanding of a variety of upper ionospheric phenomenon. Acknowledgements We are grateful to Editor Dr. Dieter K. Bilitza and the reviewers for their critical reading and suggestions. References Bilitza, D. International ionosphere – status 1995/96. Adv. Space Res. 20 (9), 1751–1754, 1997. Bilitza, D., Williamson, R. Towards a better representation of the IRI topside based on ISIS and alouette data. Adv. Space Res. 25 (1), 149– 152, 2000. Bilitza, D. International reference ionosphere 2000. Radio Sci. 36 (2), 261– 275, 2001. Bilitza, D. A correction for the IRI topside electron density model based on Aloutte/ISIS topside sounder data. Adv. Space Res. 33, 838–843, 2004. Bilitza, D., Reinsch, B.W., Radicella, S., Pulinets, S., Gulyaeva, T., Triskova, L. Improvements of the international reference ionosphere
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