An analysis of the scale height at the F2-layer peak over three middle-latitude stations in the European sector
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2012
Received: 11 June 2010
Accepted: 11 April 2011
Published: 27 July 2012
This paper presents the results of an analysis of the variations of the scale height at the F2-layer peak (Hm) under different seasonal and solar-activity conditions. The database includes hourly Hm values derived from ionograms recorded at three middle-latitude stations in the European sector: El Arenosillo (37.1°N; 353.3°E), Ebro (40.8°N, 0.5°E) and Pruhonice (50.0°N; 15.0°E). The results show that, in general: (1) Hm exhibits diurnal variation with higher values during daytime than during night-time and secondary peaks around sunrise and sunset; (2) during winter time the scale height is lower than in summer time; (3) the scale heights increase with increasing solar activity; (4) Hm decreases when the latitude increases; (5) Hm shows a low correlation with the F2-region peak parameters NmF2 and hmF2 and a high correlation with the thickness parameter B0 and the equivalent slab thickness EST; (6) the day-to-day variability is greater at low solar activity than at high solar activity—it reaches maximum values around sunrise or sunset and it is lower around midnight than around noon at low solar activity. The results of this study are similar to those reported by other authors and can be useful for estimating the topside ionosphere from bottomside measurements and modelling.
Key wordsMiddle-latitude ionosphere bottom density profile α-Chapman scale height
Ground-based ionograms recorded during the last decades provide ample available data for studying the bottom side electron-density profile (up to the F2 layer peak, hmF2). However, information concerning the topside electron-density profile, usually derived from topside sounder and incoherent scatter radar measurements, is limited in comparison. Different analytical functions, such as Chapman, exponential, parabolic, or Epstein, functions among others, have been proposed to estimate ionospheric height profiles (e.g., Davies, 1996). The ionospheric scale height is a key parameter of the above-mentioned profile functions which measure the shape of the electron-density profile and indicates the gradient of electron density (e.g. Huang and Reinisch, 1996; Belehaki et al., 2006; Liu et al., 2008; Stankov et al., 2011).
The effective scale height at the hmF2, Hm, deduced from ground-based ionosondes, assuming an α-Chapman profile function (Huang and Reinisch, 2001), is frequently used in various practical applications (e.g., Reinisch and Huang, 2001; Reinisch et al., 2004). The α-Chapman function has also been applied to represent measured topside profiles (Reinisch et al., 2007) and to estimate topside values of the Hm. Moreover, Hm values produced routinely by digisondes can be helpful for constructing the topside electron-density profile when using an appropriate correction factor to estimate the topside scale height (Kutiev et al., 2009). Recent results clearly show that the ratio of Hm values deduced from topside and bottom side measurements depend on the local time and latitude (Nsumei et al., 2010). Thus, a better knowledge of the behavior of Hm enables a better estimation of vertical profiles to be obtained. Since Hm is relatively easy to deduce from ground-based ionosondes, this may provide information of the topside ionosphere, and there is plenty of digisonde data available from more than several solar cycles (Galkin et al., 2006). Many studies have dealt with an analysis of the variations of Hm in recent years (e.g. Belehaki et al., 2006; Zhang et al., 2006; Lee and Reinisch, 2007; Mosert et al., 2007; Nambala et al., 2008). However, the databases used in most cases also enable an analysis of the diurnal and seasonal variations of Hm at particular locations.
The aim of this paper is to extend a previous study (Mosert et al., 2007) in order to analyze the behavior of the digisonde-derived scale heights Hm using data obtained at three European stations: Ebro (40.8°N, 0.5°E), El Arenosillo (37.1°N; 353.2°E) and Pruhonice (50.0°N; 15.0°E), under different temporal conditions, in order to examine the diurnal, seasonal, solar-activity and latitudinal, variations. The analysis of the day-to-day variability of Hm has been also carried out. The correlations of Hm with the main F2-region characteristics—the electron-density maximum (NmF2), the layer peak height (hmF2), the IRI thickness parameter (B0) and the ionospheric slab thickness (EST)—are also analyzed.
2. Data Used
The analysis presented in this paper uses data deduced from manually-scaled ionograms recorded by digisondes of three European stations: Pruhonice (50.0°N; 15.0°E), Ebro (40.8°N, 0.5°E) and El Arenosillo (37.1°N; 353.2°E). The database covers the four seasons: summer (July), winter (January), fall (October) and spring (April), and two periods of different levels of solar activity: 2006 (Rz12 = 16) and 2007 (Rz12 = 12) representing low solar activity (LSA), and 2000 (Rz12 = 117) and 2001 (Rz12 = 111) representing high solar activity (HSA). Due to data being unavailable, the station at Pruhonice has furnished only a period of low solar activity: 2005 (Rz12 = 29), 2006 (Rz12 = 16) and 2007 (Rz12 = 12). The scale heights (Hm), F2-region thickness parameter (B0) used in IRI (Bilitza, 2001; Bilitza and Reinisch, 2008) and the F2 peak parameters (peak density NmF2 and its height hmF2) were derived from electron-density profiles obtained from the ionograms using the ARTIST program (Huang and Reinisch, 1996; Reinisch and Huang, 2001). The monthly median values of Hm, B0, NmF2 and hmF2 were calculated for a given month and a given hour for the years, considered and the three ionospheric stations, considered. The equivalent slab thickness (EST) defined as: EST = TEC/NmF2 was calculated from the TEC values derived by integration of the electron-density profiles (ITEC) using the technique proposed by Huang and Reinisch (2001) and the corresponding NmF2 values.
3. Analysis of the Results
3.1 Diurnal and seasonal variations of the scale height
Figure 1 also shows clearly the seasonal variations of Hm. The daytime values are greater in summer and spring than those observed in winter and fall, again for both levels of solar activity. The Hm values for the HSA period range between 62 and 80 km in summer, 56 and 72 km in spring, 30 and 50 km in winter and between 40 and 57 km in fall. The Hm values for LSA vary from 60 to 71 km in summer, from 40 to 69 in spring, from 32 to 53 km in winter and from 39 to 61 km in fall. The seasonal differences are less evident during night-time than during daytime.
3.2 Solar-activity variations
Most of the diurnal and seasonal variations of the scale-height values can be explained taking into account the definition of the scale height. The Hm scale height in the α-Chapman formulation relates to the neutral scale height H = kT/mg (Rishbeth and Garriott, 1969) which is positively correlated to the neutral temperature (T). The secondary peak observed around sunrise might be produced not by an increasing temperature but by the shape change of the electron-density profile (Lee and Reinisch, 2006) and the post sunset peak might be caused by the pre-reversal enhancement of EXB drift velocity (Farley et al., 1986) that make the post sunset peaks of hmF2 and B0 (Lee and Reinisch, 2006).
3.3 Latitudinal variation
In spring, exceptions are found between 6 and 9 UT with greater values at Pruhonice than at Ebro. Another feature of the latitudinal variation of Hm is that, in general, the latitudinal differences are more pronounced during daytime than during night-time. During daytime, the Hm values range between 20 and 70 km and during nighttime between 30 and 52 km.
Nsumei et al. (2010) have evaluated the daily variations of the bottomside-derived Hm at different latitudes, and have reported a latitude dependence in agreement with our current results. Moreover, the results reported by Zhang et al. (2006), Lee and Reinisch (2007), and Nambala et al. (2008), using data from Hainan (19.4°N; 109.0°E), Jicamarca (12.0°S, 223.1°E) and Grahamstown (33.3°S, 26.5°E), respectively, follow well the latitudinal variation of the Hm as reported here.
3.4 Day-to-day variability of Hm
It is generally acknowledged that, for a good description of the variability of ionospheric magnitudes, the performance of ionospheric models, such as the International Reference Ionosphere, IRI (Bilitza and Reinisch, 2008), need to be improved. For many applications, the users of ionospheric models need to know not only the monthly average condition but also the expected deviation from the mean, or median, values. Many authors have studied the variability of ionospheric parameters using different indexes (Aravindan and Iyer, 1990; Jayachandran et al., 1995; Mosert and Radicella, 1995; Bradley, 2000; Gulyaeva and Mahajan, 2001; Radicella and Adeniyi, 2001; Rishbeth and Mendillo, 2001; Ezquer et al., 2002, 2004; Kouris and Fotiadis, 2002; Mosert et al., 2002; Ezquer and Mosert, 2007; among others).
Taking into account that the distribution of ionospheric magnitudes is not a normal distribution and that the median and quartiles have the advantage of being less affected by large deviations which can occur during magnetic storms, we use, in this paper, the variability index proposed by Ezquer et al. (2004) and Ezquer and Mosert (2007): Cup - Clo where Cup = upper quartile/median and Clo = lower quartile/median.
3.5 Correlation between Hm and the F2-region parameters
We have also analyzed the correlation between the scale height Hm and the F2-region parameters such as the F2 peak characteristics NmF2 and hmF2, the IRI F2-region thickness parameter B0, and the equivalent slab thickness EST. This kind of analysis can be helpful due to the fact that if we find good correlations between Hm and parameters established by the IRI model in its formulation (Bilitza, 2001; Bilitza and Reinisch, 2008), it would be possible to estimate the topside profile using parameters derived from bottomside-measurements. The current IRI version (Bilitza and Reinisch, 2008) has adopted the topside formulation of the NeQuick2 (Nava et al., 2008). The topside model of the NeQuick2 is represented by a semi-Epstein layer with a height-dependent thickness parameter H which is analytically obtained from bottomside measurements modeling.
The good correlation between Hm and B0 suggests that it may be possible to construct the topside profile near the F2 peak hmF2, using the parameter B0 provided by the IRI model (Zhang et al., 2006). It is important to point out that these results are comparable with those reported by Zhang et al. (2006) and Nambala et al. (2008).
Many efforts have been made in the last decades to improve ionospheric models, such as the International Reference Ionosphere, IRI (Bilitza, 2001; Bilitza and Reinisch, 2008), using different techniques. The introduction of a new technique for estimating the topside electron-density profile from the information contained in the ground-based ionograms (Reinisch and Huang, 2001) offers a new tool for the study of the topside electron-density profile and a new data resource to improve the topside profiles. Parameters such as total electron content and scale height can be derived from the ionograms using the mentioned technique.
This paper presents an analysis of diurnal, seasonal, solar-activity, and latitudinal, variations of the scale height at the F2-layer peak (Hm) derived from an α-Chapman profile formulation. The database includes hourly Hm values derived from ionograms recorded at three middle-latitude stations in the European sector: El Arenosillo (37.1N; 353.3E), Ebro (40.8°N, 0.5°E) and Pruhonice (50.0°N; 15.0°E). The results show that, in general: (1) Hm exhibits a diurnal variation with higher values during daytime than during night-time with the greatest values around noon, and secondary peaks around sunrise and sunset; (2) during winter time, the scale height is lower than in summer time; (3) the scale heights increase with increasing solar activity; (4) Hm decreases when the latitude increases; (5) Hm shows a low correlation between Hm and the F2-region peak parameters NmF2 and hmF2 and a high correlation with the thickness parameter B0 and the equivalent slab thickness EST; (6) the day-to-day variability is greater at a low solar activity than at a high solar activity—it reaches maximum values around sunrise or sunset and is lower around midnight than around noon at LSA. This behavior is, in general, inverted at HSA.
The results of this study agree with those reported by other authors (e.g. Zhang et al., 2006; Lee and Reinisch, 2007; Nambala et al., 2008, among others) and they can be useful for obtaining information for the topside-profile formulation from bottomside measurements and modelling. However, further studies and analyses are needed to validate analytical functions relating to the topside and bottomside parameters in order to contribute to the IRI modelling.
The work of D. A. has been partly supported by Spanish projects 2009SGR507 and CTM2010-21312-C03-01.
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