Behavior of the scale height at the F2 layer peak derived from Digisonde measurements at two European stations

Behavior of the scale height at the F2 layer peak derived from Digisonde measurements at two European stations M. Mosert a,* , R. Ezquer b,c , B. de la Morena d , D. Altadill e , G. Mansilla b , G. Miro Amarante f a CASLEO-CONICET, Avda. Espan ˜a 1512 Sur, 5400 San Juan, Argentina b Lab. de Iono ´sfera, UNT, CONICET, Avda Independencia 1800, 4000 Tucuma ´n, Argentina c GASuR, Fac. Reg. Tucuma ´n, UTN, Rivadavia 1050, 4000 Tucuma ´n, Argentina d INTA, Ctra. San Juan del Puerto-Matalascan ˜as Km. 33, 21130, Huelva, Spain e Observatori de l’Ebre, (CSIC-URL), Carretera de Lo ´bservatori 8 E43896 Roquetes, Spain f ICTP, Strada Costiera 11, 34100 Trieste, Italy Abstract A new technique is presented to estimate the electron density topside profile from information derived from ground-based ionograms. The electron density above the F2 peak is approximated by an a-Chapman function with constant scale height (H m ). The scale height is derived from the shape of the bottomside profile near the F2 peak. Digisonde data obtained at two European stations: Ebro (40.4°N, 0.3°E) and El Arenosillo (37.1°N; 353.2°E) for different times of day, seasons, and periods of the solar cycle to study the variations of H m . The results of the analysis are in good agreement with those reported by other authors. A table of values is presented for typical hours of the day and different seasons and solar activity conditions. This study, once extended to a larger database, will support modeling H m as a function of time, season, latitude, solar activity and magnetic activity, and contribute to the formulation of the topside electron density in the IRI model. Keywords: Middle latitude ionosphere; Topside electron density profile; Scale height 1. Introduction Electron density profiles derived from ground-based ionograms provide information up to the F2 layer peak, hmF2. Different true-height inversion programs are available that calculate the bottomside profiles from the ionogram echo traces (e.g., Huang and Reinisch, 1996; Titheridge, 1985). The information on the topside electron density distribution, usually derived from topside sounder and incoherent scatter radar (ISR) measurements, is scarce, compared with the bottomside. Reinisch and Huang (2001) introduced a new technique of estimating the topside profile from ground-based ionograms, assuming an a-Chapman function with a constant scale height H m . The scale height is derived from the shape of the bottomside profile near the F2 peak and provides a good estimate for the electron density profile up to around 700 km. Recently Reinisch et al. (2004) have shown that the H m values, produced routinely by Digisondes, can be helpful in improving the formulation of the IRI topside electron density profile (Bilitza, 2001). The objective of the present paper is to analyze the behavior of the scale heights using the F2 layer Digisonde data at two European stations: Ebro (40.4°N, 0.3°E) and * Corresponding author. Tel.: +54 264 421 3653; fax: +54 264 421 3693. E-mail addresses: [email protected] (M. Mosert), rezquer@ herrera.unt.edu.ar,[email protected]tn.edu.ar (R. Ezquer), [email protected] (B. de la Morena), [email protected] (D. Altadill), amarante@ictp. trieste.it (G. Miro Amarante). El Arenosillo (37.1°N; 353.2°E) during different times of day, seasons. and periods of the solar cycle. 2. Data used The analysis presented in this paper uses ionograms (manually scaled) recorded by two DGS 256 from the European stations: Ebro (40.4°N, 0.3°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 levels of solar activity: low solar activity, LSA (1995, Rz12 = 17.2) and high solar activity, HSA (1999, Rz12 = 95.3, and 2000, Rz12 = 116.8). The individual electron density profiles with their corresponding scale heights H m at the F2 layer were obtained from the ionograms using the ARTIST program (Huang and Reinisch, 1996; Reinisch and Huang, 2001). The monthly median values of H m (n= 25–30 days) were calculated for a given month and a given hour for the 3 years (1995, 1999, and 2000) and the two ionospheric stations. 3. Results 3.1. Behavior of the scale height at El Arenosillo Figs. 1a and b illustrate the variations of the median values of H m at El Arenosillo for the four seasons and two years of different solar activity: 1995 (Rz12 = 17.2) and 1999 (Rz12 = 95.3). The figures show the following characteristics: (1) H m does not exhibit a clear diurnal variation in winter (January) and fall (October). The values of H m for these two seasons vary from 30 to 42 km at LSA and from 30 to 59 km at HSA. However, in the year 1999, H m presents a slight trend. A peak occurred at sunrise during the two seasons with a value of 52 km in winter and 59 km during fall. (2) For July (summer) and April (spring) a more defined diurnal variation is observed: the parameter H m is greater during the daytime period (06 to 18 UT) than during the night time period (19 to 05 UT), reaching maximum values generally around noon. This behavior is better observed during low solar activity. The nighttime values range from 29 to 46 km at LSA, and from 36 to 50 km at HSA. The daytime values of H m vary between 29 and 68 km at LSA and between 43 and 76 km at HSA. (3) During daytime, H m is generally greater in summer (43–76 km) and spring (37–66 km) than in winter (29– 52 km) and fall (29–59 km). This seasonal behavior is clearer at LSA. The seasonal differences are less evident during night time at both levels of solar activity. (4) Although some exceptions have been found, in general, H m increase with solar activity, ranging between 28 and 68 km at LSA and between 36 and 76 km at HSA. 3.2. Behavior of the scale height at Ebro Figs. 2a and b depict the diurnal variations of H m for Ebro for the four seasons and two different levels of solar activity: 1995 (Rz12 = 17.2) and 2000 (Rz12 = 116.8). It shows that, in general, the variations of H m with time day, season, and level of the solar activity are similar to those observed at El Arenosillo. In order to summarize the results for both stations, we present in Table 1 the values of H m over El Arenosillo and Ebro for four typical hours of the day (00, 06, 12, and 18 UT), the four seasons of the year (summer, winter, fall, and spring) and two levels of solar activity: LSA and HSA. The analysis of the table confirms the main features mentioned above, that is: H m exhibits a clear diurnal variation in summer and spring (with maximum values at noon); the seasonal differences are more evident during daytime than during nighttime, and H m increases with solar activity. 0 2 4 6 8 1012141618202224 0 10 20 30 40 50 60 70 80 El Arenosillo - 1995 (LSA) Hm [km] UT 0 2 4 6 8 1012141618202224 0 10 20 30 40 50 60 70 80 El Arenosillo - 1999 (HSA) Hm [km] UT Jan (winter) Ap (spring) Jl (summer) Oct (fall) b a Fig. 1. Average variation of the scale height H m in km, at El arenosillo (37.1°N; 353.2°E) for the four seasons (winter, summer, fall, and spring) and two years of different level of solar activity: (a) 1995 (Rz12 = 17.2) and (b) 1999 (Rz12 = 95.3) as indicated in the top of each plot. It is important to point out that the diurnal and seasonal variations of the scale heights observed at Ebro and El Arenosillo are similar to those reported by Reinisch et al. (2004) using data from Hainan Island (19.4N; 109.0E) and Millstone Hill (42.0N; 288.5E) during the years 2002 (Rz12 = 101.7) and 2003 (Rz12 = 65.6). 4. Conclusion Many efforts have been made in the last decades to improve the International Reference Ionosphere, IRI (Bilitza, 1990, 2001) using measurements obtained by different techniques. The ground-based ionosonde data recorded at different locations have been widely used to test the IRI predictions of different ionospheric characteristics such as the critical frequencies of the E, F1, and F2 layers (foE, foF1, foF2), the corresponding peak heights (hmE, hmF1, hmF2) and parameters derived from electron density profiles (B0, B1, D1). The introduction of a new technique of 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 profiles and a new data resource to improve the IRI topside profiles. Parameters such as total electron content and scale heights can be derived from the ionograms using the mentioned technique. This paper analyzes the variations of the scale heights obtained from ionosonde data recorded at El Arenosillo (37.1°N; 353.2°E) and Ebro (40.4°N; 0.3°E) during different seasonal and solar activity conditions. The main results of our study are: (1) H m exhibits a clear diurnal variation in summer and spring with maximum values around noon at both levels of solar activity, while the diurnal variations in winter and fall are small; (2) during daytime H m presents higher values during summer and spring than during winter and fall, and the nighttime values exhibit no clear seasonal dependence; (3) in general, the scale heights increase with increasing solar activity; (4) a table (Table 1) of preliminary H m values is presented for the four seasons, four typical hours of the day and two levels of solar activity (LSA and HSA) capturing the variations of the scale height at the two locations for which the analysis was done. The results of this study are similar to those reported by Reinisch et al. (2004) using data from Hainan Island (19.4N; 109.0E) and Millstone Hill (42.0N; 288.5E) for the years 2002 (Rz12 = 101.7) and 2003 (Rz12 = 65.6). While the analysis presented here is only valid for two locations, it can be combined with results from other stations to 0 2 4 6 8 1012141618202224 0 10 20 30 40 50 60 70 80 Ebro - 2000 (HSA) Hm [km] UT Jan (winter) Ap (spring) Jl (summer) Oct (fall) 0 2 4 6 8 1012141618202224 0 10 20 30 40 50 60 70 80 Ebro - 1995 (LSA) Hm [km] UT a b Fig. 2. Average variation of the scale height H m in km, at Ebro (40.4°N; 0.3°E) for the four seasons (winter, summer, fall and spring), and two years of different level of solar activity: (a) 1995 (Rz12 = 17.2) and (b) 2000 (Rz12 = 116.8) as indicated in the top of each plot. Table 1 Monthly median values of the scale heights H m at (a) El Arenosillo and (b) Ebro during the four seasons (winter, spring, summer, and fall) at four typical hours of the day (00, 06, 12, and 18 UT) for two levels of solar activity: low solar activity (LSA) and high solar activity (HSA) Season Solar Act. 00 UT 06 UT 12 UT 18 UT (a) El Arenosillo Winter LSA 35 39 36 40 HSA 42 52 41 39 Spring LSA 41 38 66 41 HSA 44 42 55 37 Summer LSA 32 45 66 50 HSA 45 68 76 57 Fall LSA 36 36 42 28 HSA 50 59 53 35 (b) Ebro Winter LSA 34 35 36 37 HSA 54 56 50 40 Spring LSA 41 38 63 40 HSA 45 53 67 49 Summer LSA 35 51 68 54 HSA 48 62 78 57 Fall LSA 41 41 44 30 HSA 56 60 54 43 arrive at a global model for H m as a function time of day, season, latitude, and solar and magnetic activities. Such a model would support the development of a better IRI Ne topside model. References Bilitza, D. The International Reference Ionosphere 1990, National Space Science Data Center, NSSDC/WCA-A-R&S Report 90–22, Greenbelt, Maryland, November 1990. Bilitza, D. International reference ionosphere 2000. Radio Sci. 36 (2), 261–275, 2001. Huang, X., Reinisch, B.W. Vertical electron density profiles from Digisonde ionograms the average representative profiles. Annali di Geofisica XXXIX (4), 751–756, 1996. Reinisch, B.W., Huang, X. Deducing topside profiles and total electron content from bottomside ionograms. Adv. Space Res. 27 (1), 23–30, 2001. Reinisch, B.W., Huang, X., Belehaki, A., Shi, J., Zhang, M.L., Ilma, R. Modeling the IRI topside using scale heights from groundbased ionosonde measurements. Adv. Space Res. 34, 2026–2031, 2004. Titheridge, J. Ionogram analysis with the generalized Program POLAN, World Data Center Rep. UAG-93, World Data Center for Sol.-Terr. Phys., Boulder, Colo., 1985.