Topside ionospheric Vary-Chap scale height retrieved from the COSMIC/FORMOSAT-3 data at midlatitudes
Introduction
It is now well known that the Earth’s ionosphere is the most variable component of the atmosphere. Ionospheric electron densities and distributions vary with local time, latitude, season and solar cycle by as much as a factor of ten. Spatial variations, due to thermospheric neutral winds, atmospheric composition and so on, occur on global, regional, and local scales (Rishbeth, 1998). One motivation for studying the ionosphere is to improve techniques to predict ionospheric weather that affects space-borne and ground-based technological systems used for communication, navigation and basic research (Singh et al., 2010). To improve prediction capabilities, better understanding of the ionospheric parameters variability is necessary.
During the past decades, several techniques, such as satellite in site measurements, ground and topside ionosondes, incoherent scatter radars (ISRs) and ionospheric radio occultation (IRO) measurements, have been used for probing the structure of the ionosphere. The electron density profiles (EDPs) retrieved from IRO measurements onboard on LEO satellites, such as COSMIC/FORMOSAT-3 (COSMIC for short), CHAMP, GRACE, SAC-C, Metop-A, and other missions, significantly broaden the data resource for understanding global ionospheric structures and their variations (e.g., Hajj et al., 2004, Jakowski et al., 2002, Liu et al., 2007, Schreiner et al., 2007, Yue et al., 2012).
One major challenge for topside vertical electron density Ne(h) modeling is finding a suitable mathematical representation of the topside Ne(h) profiles. For many years, several mathematical functions, such as Chapman, exponential, parabolic, Epstein, Sech-squared and hyperbolic tangent functions, have been proposed to estimate the ionospheric height profiles (e.g., Depuev and Pulinets, 2004, Gulyaeva, 2011, Kutiev et al., 2006, Kutiev and Marinov, 2007, Nsumei et al., 2012, Rawer, 1988, Reinisch et al., 2004). An important and inherent parameter for these profile functions is the ionospheric scale height. The ionospheric scale height measures the shape of EDPs, indicates the gradient of electron density, and intrinsically connects to ionospheric dynamics, plasma temperature and compositions.
The scale height is frequently used in various practical applications and is one of the important ionospheric parameters. EDPs measured from the techniques mentioned above have been used to retrieve topside scale heights in the literature, which have provided significant contributions to the knowledge of ionospheric scale heights. For example, Stankov and Jakowski, 2006a, Stankov and Jakowski, 2006b retrieved the topside plasma scale height from IRO measurements onboard the CHAMP satellite; Kutiev et al. (2006) used topside ionosondes observation between 1962 and 1978 from Alouoette and ISIS satellites to develop a model of the topside ionospheric scale height; Belehaki et al. (2006) compared the ionosphere scale heights retrieved from topside sounder profiles with those from a Digisonde at Athens; Lei et al. (2005) and Liu et al. (2007) investigated the seasonal and solar activity features of the Chapman scale height using the Millstone Hill and Arecibo ISR observations; Liu et al. (2006a) conducted a statistical analysis on the variations of the ionogram derived scale height (Hm) around the F2 peak, showing that Hm is highest in summer and lowest in winter during the daytime, while it exhibits a much smaller seasonal variation at night; Liu et al. (2008) collected the COSMIC data during the interval from day of year (DOY) 194 in 2006 to DOY 60 in 2008 to investigate the diurnal, seasonal, latitudinal, and longitudinal variations of the vertical scale height at an altitude of 400 km; Potula et al. (2011) and Xu et al. (2013) analyzed the variations of the topside Chapman scale height in global and specified regions derived from COSMIC data; Liu et al. (2014) derived the effective scale height in the topside ionosphere based on satellite in site observations and ionosondes at Wuhan.
Reinisch et al. (2007) showed that the good representation of the topside profile up to plasmaspheric heights can be obtained by using a Chapman function with continuously varying scale height and suggested that Vary-Chap functions might be a possible choice for a topside profile model. Bilitza et al. (2011) considered the Vary-Chap scale height (VCSH) as the efforts toward future improvements to the most famous and widely used International Reference Ionosphere (IRI) model, which specifies the monthly average distributions of the ionosphere. In this report, we collect the post-processed COSMIC EDPs in 2007 to investigate the features of topside VCSH at midlatitudes and to compare the results with those extracted from the empirical IRI-2012 model.
Section snippets
Data and method
COSMIC is a joint Taiwan-US satellite mission that is administered by the National Space Organization in Taiwan and the University Corporation for Atmospheric Research (UCAR) in Boulder, Colorado. Direct comparisons have been reported between COSMIC occultation-based EDPs and those obtained with ionosondes, ISRs, and various ionospheric models such as the Thermosphere-Ionosphere Electrodynamic General Circulation Model (TIEGCM), NeQuick and IRI model (Chu et al., 2010, Lei et al., 2007,
Local time and seasonal variation of VCSH
The VCSHs were obtained by binning electron density data every seasons in each hour, and taking the mean value of observations located in the given latitude grid. Fig. 3 plots the VCSHs at geographic coordinates 42.8°N, 71.5°W (near the Millstone Hill ISR site) as a function of local time in four seasons. The upper panel in Fig. 3 derived from the Millstone Hill ISR data, the middle panel is from the COSMIC data, and the lower panel is from the IRI-2012 model. The color scale is identical in
Conclusions
An α-Chapman function with a continuously varying scale height can be used to describe the topside F2 vertical electron density profile that seamlessly connects the ionosphere with the plasmasphere. This paper investigates the behaviors of the VCSHs at 400–550 km retrieved from the COSMIC data in 2007. The main results are summarized as follows:
The VCSHs are not tightly correlated with the neutral or plasma-scale heights, and have prominent altitudinal dependence, diurnal and seasonal
Acknowledgements
The authors are thankful to two anonymous reviewers for their valuable suggestions to improve the manuscript. We are also grateful to the COSMIC Data Analysis and Archive Center (CDAAC) for the COSMIC IRO data, and to the GSFC, NASA for providing online version of the IRI-2012 model (http://omniweb.gsfc.nasa.gov/vitmo/iri2012_vitmo.html). This work was supported by the National Natural Science Foundation of China (Grant No. 41175025).
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