An empirical model of electron density in low latitude at 600 km obtained by Hinotori satellite

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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.

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 (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 cm−3 (Bilitza and Williamson, 2000). These problems have been pointed out by Iwamoto et al., 2002, 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 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.

Section snippets

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 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 time (LT), geomagnetic latitude and daily solar

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 cm−3. Whilst Hinotori model give higher density in the density region lower than 105 cm−3. 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

Summary

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

Acknowledgements

We are grateful to Editor Dr. Dieter K. Bilitza and the reviewers for their critical reading and suggestions.

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