A0332 ANNALS OF GEOPHYSICS, 60, 3, 2017, A0332; doi: 10.4401/ag-7065 Analysis of the Probable Signature of Mid-Latitude Electron Density Trough at the Ionospheric Critical Frequencies over Europe Erdinç Timoçіn1,*, İbrahim Ünal2 1 İnönü University, Faculty of Science and Art, Department of Physics, Malatya, Turkey 2 İnönü University, Faculty of Education, Department of Science Teaching, Turkey Article history Received May 29, 2016; accepted March 21, 2017. Subject classification: Ionosphere, magnetosphere, critical frequency, mid-latitude electron density trough. ABSTRACT In this study, probable signature of the mid-latitude electron density trough at the ionospheric critical frequencies ( foF2) is investigated. For this purpose, hourly ionospheric critical frequencies data obtai- ned from ionosonde stations for the year 1972 are used. These stations are situated between 45o-65o invariant magnetic latitudes (Λ) of the Northern hemisphere. The foF2 data are examined depending on local time (LT), seasons, Λ and geomagnetic activity (Kp) variation. The re- sults show that the foF2 troughs occur in the nighttime hours between dusk and dawn sector for all situations. The troughs under geomagne- tic active (Kp>2 +) conditions are more prominent and apparent than that under geomagnetic quiet (Kp≤2 +) conditions. Under geomagnetic active conditions, the troughs and their minimum positions tend to appear at lower Λ for four seasons. Also, the average foF2 values of the trough minimum under geomagnetic active conditions have lower va- lues than that under geomagnetic quiet conditions. It is observed that the seasonal variations have important effects on the structure of the foF2 trough, especially for December Solstice. The results of this study will contribute a better description and prediction of the mid-latitude electron density trough features for the ionospheric F2 peak heights and better understanding the role of the mid-latitude electron density trough on the ionospheric variability. 1. Introduction Solar flares and solar wind streams affect the Ear- th’s magnetic field significantly and this interaction causes large and sudden changes in the electron den- sity of the ionosphere. Especially, under geomagne- tic active conditions, change in the electron density between 40° and 70° Λ creates a negative effect on communication and navigation systems. Approxima- tely at 550 km height, the ambient electron densities trough observed in magnetic mid-latitudes is called mid-latitude electron density trough [Muldrew 1965, Calvert and Van Zandt 1966, Sharp 1966, Carlson 1968, Rishbeth and Garriott 1969, Tulunay and Sayers 1971, Kelley 1989]. As a result of subsequent studies were performed using electron density data measured by Ariel 3 and Ariel 4 satellites about 550 km between the years 1960- 1972, the qualitative identification of the mid-latitude electron density trough has been made and the quanti- tative criteria has been developed [Tulunay and Sayers 1971, Tulunay 1973, Tulunay 1974, Tulunay and Gre- bowsky 1975b, Tulunay and Grebowsky 1978]. The electron density trough is the ionospheric projection of the magnetospheric plasmapause. The trough marks the region where the electron density va- riation is abrupt over a narrow range of geomagnetic latitudes. The unpredictable variability greatly limits the efficiency of the operations of communication, radar and navigation systems which employ high frequency (HF) radio waves. The electron density depletion in the trough region reduces the maximum usable frequency that can be reflected by the ionosphere along the great circle. Therefore, it is essential to understand the role of the trough in the ionospheric variability [Tulunay 1973, Grebowsky et al. 1974, Tulunay and Grebowsky 1975a, Tulunay and Grebowsky 1975b, Kersley et al. 1997, Tulu- nay et al. 1998, Rothkaehl et al. 2000, Tulunay et al. 2003, Pryse et al. 2006, Mikhailov and Leschinskaya 2011]. Earlier studies [Rothkaehl et al. 2000, Tulunay et al. 2003] concluded that (i) It would be advisable to adopt a practical, simple convection model which changes in steps with the planetary geomagnetic activity index (Kp) or interplanetary magnetic field (IMF) reversals or any other effective event in order to reflect the influence of the trough phenomena on the ionospheric foF2 predi- ction or forecasting algorithms. (ii) Since the foF2 peak can extend above 500 km, the signature of the trough is TİMOÇİN AND ÜNAL 2 expected to fall in the foF2 variations all over the world. From this point of view it must be essential to include the influence of the trough in foF2 predictions or forecasts for practical applications and modeling. To the knowledge of the authors there has been no final model exists which simulates the trough, based on physical principals to be employed for radio communica- tion applications. In practical applications of the HF ra- dio communications in any model that does not include the trough are not a complete model. The most of previous studies about the mid-latitude electron density trough were performed over the coope- ration in scientific and technology-251 (COST-251) area of the Europe. COST-251 is area of the Europe between latitudes of 35°-70°N and longitude 10° W-60°E. But these studies were limited to only satellite data and were not exactly investigated effect on the electron densities in the ionospheric F2 peak height of the mid-latitude electron density trough considering seasonal, local time and geomagnetic activity changes by using foF2 data. The purpose of our study is to investigate possible signature on foF2 of the structure called the mid-lati- tude electron density trough depending on local time, seasons, Λ and geomagnetic activity changes and is to compare obtained the results from this study with obtai- ned the results from previous studies which were perfor- med for different altitudes. In this way it will establish a better description and prediction of the mid-latitude trough features for foF2. In addition, the results of this work will contribute to better understanding the effect at the ionospheric F2 heights of the mid-latitude electron density trough and the role of the trough on ionospheric variability. 2. Data and methods of analysis In this study, hourly foF2 data of year 1972 are used to investigate probable signature of the mid-la- titude electron density trough at the ionospheric F2 peak heights over the Europe. foF2 data are obtained from ionosonde stations which are situated between 40° and 70° Λ. The geographic latitudes, geographic longitudes and Λ values of these stations are given in the Table 1. We also used the Kp index to investigate the effect of geomagnetic activity changes on foF2 values. This data were taken from Space Physics Interactive Data Resource (SPIDR) (http://spidr.ngdc.noaa.gov/spidr/). Data are analyzed depending on seasons, geoma- gnetic activity and local time as follows: (1) foF2 data are separated into four seasons as March and September Equinoxes and June and De- cember Solstices. (2) The hourly Kp values are calculated from 3-hr Kp values by using linear interpolation method. Thus, the effect of geomagnetic activity changes on foF2 is investigated more in detail for each local time. (3) To investigate the geomagnetic activity effect on the foF2 trough, foF2 data are separated into two groups as geomagnetic quiet hours and geomagnetic active hours. (4) Because Ariel satellite returns around the ear- th about 1344 times during a season, the satellite mea- sures many times electron density for each local time. The average electron density values for each local time are calculated from this data. Then the mid-latitude electron density troughs for each local time are investi- gated considering variations according to Λ of average electron density [Tulunay and Sayers 1971, Tulunay 1973]. Therefore we calculated the average foF2 values for severally each local time of all seasons and then we investigated variations according to Λ of the average foF2 values for different seasons and different geoma- gnetic activity conditions. (5). The contours plots of the average foF2 values Station Names Geographic Latitude (°N) Geographic Longitude (°E) Invariant Magnetic Latitude (°N) Station Names Geographic Latitude (°N) Geographic Longitude (°E) Invariant Magnetic Latitude (°N) Bekescsaba 46.7 21.2 45.0 Uppsala 59.8 17.6 57.0 Kiev 50.5 30.5 48.0 Lycksele 64.7 18.8 61.0 Miedzeszyn 52.2 21.2 50.0 Sodankyla 67.4 26.6 64.0 Kaliningrad 54.7 20.6 52.0 Kiruna 67.8 20.4 65.0 Table 1. Geographic latitude, geographic longitude and Λ values of the ionosonde stations. PROBABLE SIGNATURE OF MID-LATITUDE ELECTRON DENSITY TROUGH 3 are drawn according to Λ and local time for different seasons and geomagnetic activity conditions. 3. Results and discussions Figures 1-4 show the variations according to Λ of the average foF2 values for different local times and different geomagnetic active conditions during March and September Equinoxes, and June and December Solstices of the year 1972. As seen in Figures 1 and 3, the average foF2 values during the daytime hours between 05-19 LT gradual- ly decrease with increasing Λ under both the geoma- gnetic quiet and geomagnetic active conditions. The boundaries of this local time region where the average foF2 values gradually decrease with increasing Λ is dif- ferent for June and December Solstices. The average foF2 values of local times between 04-00 LT for June Solstice gradually decrease with increasing Λ, while the average foF2 values of local times between 06- 15 LT for December Solstice gradually decrease with increasing Λ. That is, the effect of the mid-latitude electron density trough on foF2 is not observed at all of these local times. In addition, as seen in the Figure 2a, the average foF2 values of local times between 00- 04 LT for June Solstice under geomagnetic quite condi- tions gradually decrease with increasing Λ similarly to latitudinal behavior of the average foF2 values of local times between 04-00 LT. That is, under geomagnetic quiet conditions, the effect of the mid-latitude electron density trough on foF2 was not exactly observed for any local times of June Solstice. As seen in the Figures 1a and 3a, the foF2 troughs Figure 1. Variations according to invariant magnetic latitudes of the average foF2 values for different local times during 21 March Equinox period of 1972 year (a) for geomagnetic quiet condition, (b) for geomagnetic active condition. Figure 2. Variations according to invariant magnetic latitudes of the average foF2 values for different local times during 21 June Solstice period of the year 1972 (a) for geomagnetic quiet condition, (b) for geomagnetic active condition. TİMOÇİN AND ÜNAL 4 for March and September Equinoxes under geomagne- tic quite conditions are observed between about 55°- 62° Λ and for the hours between 19-05 LT. The posi- tion of the trough minimum are located at about 58° Λ for March Equinox and about 61° Λ for September Equinox. For these seasons, the average foF2 value of the trough minimum is about 2.5 MHz. These values are observed for the hours between 03-01 LT for both March Equinox and September Equinox. As seen in the Figure 4a, the foF2 trough for De- cember Solstice under geomagnetic quite conditions is observed between about 53°-62° Λ and for the hours between 05-18 LT. The position of the trough mini- mum for this season is located at about 61° Λ. The average foF2 value of the trough minimum is about 1.3 MHz for the hours between 20-03 LT. This results show that the effect of the mid-latitude electron den- sity trough on foF2 becomes more prominent during December Solstice, but disappear during June Solstice. Figures 1b-4b show the variations according to Λ of the average foF2 values for different local times un- der geomagnetic active conditions during March and September Equinoxes and June and December Sol- stices of the year 1972. As seen in the Figure 2a, not exactly determined the foF2 trough for geomagnetic quite conditions of June Solstice is clearly observed for 01-03 LT under geomagnetic active conditions. Note that the position of the trough minimum shifts equa- torward about 2°-5° Λ for all seasons under geoma- gnetic active conditions. That is, the foF2 trough tends to occur at lower Λ for all seasons under geomagnetic active conditions. The average foF2 value of the trou- gh minimum reduces about 0.3 MHz with increasing geomagnetic activity for all seasons. In addition, the lo- Figure 3. Variations according to invariant magnetic latitudes of the average foF2 values for different local times during 23 September Equi- nox period of the year 1972 (a) for geomagnetic quiet condition, (b) for geomagnetic active condition. Figure 4. Variations according to invariant magnetic latitudes of the average foF2 values for different local times during 21 December Sol- stice period of the year 1972 (a) for geomagnetic quiet condition, (b) for geomagnetic active condition. 5 PROBABLE SIGNATURE OF MID-LATITUDE ELECTRON DENSITY TROUGH cal time interval which the trough is observed extends with increasing geomagnetic activity for all seasons, especially for December Solstice. That is, under geo- magnetic active conditions, the trough has a deeper structure and it is observed for more local times, espe- cially for December Solstice. Many studies were performed about mid-latitude electron density trough for different altitudes by using satellites, tomography and GPS-TEC (total electron content) since its discovery at about 550 km altitude in the 1960s. These studies reveal that the mid-latitu- de electron density trough is a persistent large-scale electron density depletion structure and it forms at the between the mid-latitude ionosphere and the high-lati- tude auroral. That is, the troughs are located between 40°-70° latitudes of both hemispheres for all seasons. The trough extends from dusk sector to dawn sector, so it can be seen explicitly for the nighttime hours whe- reas it cannot be seen clearly for the daytime hours of all seasons. The studies also show that geomagnetic activity and seasons have important effect on forma- tion and structure of the trough. The mid-latitude electron density trough becomes prominent during the winter solstice, but weak during the summer solsti- ce for the Northern Hemisphere. The electron density of the trough minimum reduces with increasing geo- magnetic activity, so trough is more evident during the higher geomagnetic activity conditions. The trough minimum position shifts equatorward with increasing geomagnetic activity and this motion is proportional to the strength of geomagnetic activity. In addition, the local time interval which was observed the mid-la- titude electron density trough expands with increasing geomagnetic activity [Muldrew 1965, Tulunay and Sayers 1971, Tulunay 1972a, Tulunay 1972b, Tulunay 1973, Tulunay 1974, Grebowsky et al. 1974, Tulunay and Grebowsky 1975a, Tulunay and Grebowsky 1975b, Grebowsky et al. 1976, Tulunay and Grebowsky 1978, Kersley et al. 1997, Tulunay et al. 1998, Rothkaehl et al. 2000, Tulunay et al. 2003, Pryse et al. 2006, Lee et al. 2011, Mikhailov and Leschinskaya 2011, He et al. 2011]. Our results agree with these results from previous studies. We find that the foF2 trough appears betwe- en 40°-70° Λ for all seasons under both geomagnetic activity conditions. The foF2 trough was not observed during the daytime hours for all seasons under both geomagnetic activity conditions, while it was expli- citly observed during night hours for March and Sep- tember Equinoxes and December Solstice under both geomagnetic activity conditions. We also find that the seasonal and geomagnetic activity variations have important effects on the structure of the foF2 trough. The foF2 trough is not clearly observed during June Solstice for geomagnetic quiet conditions. Otherwise December Solstice is season to be the most effect of the foF2 trough for both geomagnetic quiet conditions and geomagnetic activity conditions. Under geoma- gnetic active conditions, the minimum position of the foF2 trough is observed at lower Λ for all seasons. That is, the foF2 trough tends to occur at lower Λ for all sea- sons under geomagnetic active conditions. In addition, the foF2 value of the trough minimum reduces with increasing geomagnetic activity for all seasons. The local time interval which is observed the foF2 trough extends with increasing geomagnetic activity for all seasons, especially for December Solstice. That is, the foF2 trough has a deeper structure and is observed for more local times under geomagnetic active conditions. Also the foF2 trough is clearly observed for June Solsti- ce with increasing geomagnetic activity. These results demonstrate that the latitudinal behavior of the electron density at the ionospheric F2 peak heights exhibit a similar structure with the latitu- dinal behavior of the electron density in higher regions than F2 peak. That is, the mid-latitude electron den- sity trough has an important effect on the change of electron density in the ionospheric F2 region. 4. Conclusions The results obtained from this study are presented at three main headings as follows: (1) The different behaviors according to Λ of ave- rage foF2 values for the daytime hours and the night- time hours are substantially similar to the structure of magnetosphere and plasmosphere in the nightti- me hours and the daytime hours. Because the region between magnetosphere boundary and plasmosphere boundary is wider during the nighttime hours, the number of magnetic field lines decreases between 40°- 70° Λ. Therefore, the plasma density of this region decreases suddenly and sharply during the nighttime hours. This explains why the trough is usually obser- ved during the nighttime hours. In addition to this, be- cause the ionizing effect of ultraviolet and X-rays fill the trough shortly after dawn sector, the trough is not observed during the daytime hours [Tulunay 1973, Tu- lunay 1974, Tulunay and Grebowsky 1978]. (2) The geomagnetic activity increase caused by solar storms occur a magnetospheric electric field. This electric field occurs a drift velocity that represen- ted by . It is oriented towards to the Earth in the nighttime hours, while it is oriented outward from ExB B2 TİMOÇİN AND ÜNAL 6 the Earth in the daytime hours. This drift velocity is one of the most important physical parameters that determine the width of the plasmosphere borders at night and day sides. Because the drift velocity under geomagnetic active hours is greater than that under geomagnetic quiet hour, it causes to more narrowing of plasmosphere border in the nighttime hours and more expansion of plasmosphere border in the dayti- me hours. This narrowing causes to more decrease of magnetic field lines and electron density between 40°-70° Λ. Therefore, under the higher geomagnetic activity conditions, the troughs are more prominent and are observed more clearly for all seasons [Tulunay 1971, Tulunay 1973, Tulunay 1974, Tulunay and Gre- bowsky 1978]. (3) The formation and structure of trough vary depending to seasonal changes. It is determined that June Solstice is season to be least effect on foF2 of the mid-latitude electron density trough, while December solstice is season to be the most effect on foF2 of the mid-latitude electron density trough. This difference is a result of changing at magnetic equator position of the Earth according to Sun for different seasons and is associated with the relative positions of sub-solar point to the magnetic equator varies considerably for December Solstice [Tulunay 1971, Tulunay 1973, Tu- lunay 1974, Tulunay and Grebowsky 1978]. The fundamental similar behaviors between the foF2 trough and the mid-latitude electron density trough that are observed at different altitudes can be explained with the effect of diffusion created by the magnetic field gradient and drift. The results also indi- cate that magnetic structure called as magnetospheric plasma boundary on the mid-latitudes has a significant effect on changes of electron densities at lower altitu- des. In addition to this, in order to establish a better description and prediction of the mid-latitude trough features, it is essential to combine the in situ satellite observations with those of the ground based measu- rements. Acknowledgements. The authors are grateful to Prof. Dr. Yurdanur Kabasakal Tulunay for making this research possible by providing the facilities needed for the study. References Calvert, W. and T. E. Van Zandt (1966). Fixed-frequen- cy observation of plasma resonances in the topside ionosphere, J. Geophys. Res., 71, 1799-1813. Carlson, H. C. (1966). Ionospheric heating by magne- tic conjugate point photoelectrons. J. Geophys. Res., 71, 195-199. Grebowsky, J. M., Y. K. Tulunay and A. J. Chen (1974). Temporal variations in the dawn and dusk mid-lati- tude trough position and plasmapause, Planet. Space Sci., 22, 1089-1099. Grebowsky, J. M., N. C. Maynard, Y. K. Tulunay and L. J. Lanzerotti (1976). Coincident observations of iono- spheric troughs and the equatorial plasmapause, Pla- net. Space Sci., 24, 1177-1185. Grebowsky, J. M., N. C. Maynard and Y. K. Tulunay (1976). Coincident plasma pause-troughs observations, EOS T. Am Geophys. Un., 57:4, 299-299. He, M., L. Liu, W. Wan and B. Zhao (2011). A study on the nighttime midlatitude ionospheric trough, J. Geophys. Res., 116, A05315, doi: 10.1029/2010JA016252. Kelley, M. C. (1989). The Earth’s Ionosphere (Plasma Phy- sics and Electrodynamics), Academic Press, 350-354. Kersley, L., S. E. Pryse, L. K. Walker, J. A. T. Heaton, C. N. Mitchell, M. J. Williams and C. A. Willson (1997). Imaging of electron density troughs by tomographic techniques, Radio Science, 32-4, 1607-1621. Lee, I. T., W. Wang, J. Y. Liu, C. Y. Chen and C. H. Lin (2011). The ionospheric mid-latitude trough ob- served by FORMOSAT-3/COSMIC during so- lar minimum. J. Geophys. Res., 116, A06311, doi: 10.1029/2010JA015544. Mikhailov, A. V. and T. Y. Leschinskaya (2011). Ionospheric altitude profiles in the main ionospheric trough as ob- served by field-aligned EISCAT incoherent scatter ra- dar observations, J. Atmosph. Terr. Phys., 73, 488-498. Muldrew, D. B. (1965). F-layer ionisation troughs deduced from Alouette data, J. Geophys. Res., 70, 2635-2650. Pryse, S. E., L. Kersley, D. Malan and G. J. Bishop (2006). Parameterization of the main ionospheric trough in the European sector, Radio Sci., 41, RS5S14, doi: 10.1029/2005RS003364. Rishbeth, H. and O. K. Garriott (1969). Introduction to Io- nospheric Physics, Academic Press, 250-251. Rothkaehl H., I. Stanislawska, R. Leitinger and Y. K. Tu- lunay (2000). Application of a trough model for tele- communication purposes, Physics and Chemistry of the Earth, 25:4, 315-318. Sharp, G. W. (1966). Mid-latitude trough in the night iono- sphere, J. Geophys. Res., 71, 1345-1356. Tulunay, Y. K. (1972a). Some topside electron density mea- surements made by the Ariel 3 satellite during the geo- magnetic storm of May 25-27 1967, Planet. Space Sci., 20, 1299-1307. Tulunay, Y. K. (1972b). Magnetically symmetrical detection of the mid-latitude electron density trough by the Ariel 3 satellite, J. Atmosph. Terr. Phys., 34, 1547-1551. Tulunay, Y. K. (1973). Global electron density distributions from the Ariel 3 satellite at mid-latitudes during quiet magnetic periods, J. Atmosph. Terr. Phys., 35, 233-254. Tulunay, Y. K. (1974). Mid-latitude ionosphere as observed by satellites Ariel 3 and Ariel 4, B. Am. Meteorolog. Soc., 55:6, 650-650. Tulunay Y. K., H. Rothkaehl, G. Juchnikowski, Y. Laletas and I. Stanislawska (1998). A comparison between the Ariel 4 ambient electron density and ionospheric criti- cal frequency over COST 251 area, Proceedings of the 2nd COST 251 Workshop, Side-Antalya, Turkey. Tulunay, Y. K., I. Stanislawska and H. Rothkaehl (2003). Revisiting the Ariel trough work for HF telecommuni- cation purposes, Cosmic Research Journal, 41:4, 1-13. Tulunay, Y. K. and J. Sayers (1971). Characteristics of mid-latitude trough as determined by the electron den- sity experiments on Ariel 3, J. Atmosph. Terr. Phys., 33, 1737-1761. Tulunay, Y. K. and J. M. Grebowsky (1975a). Temporal variations in dawn and dusk mid-latitude trough posi- tions, EOS T. Am. Geophys., 56:3, 172-172. Tulunay, Y. K. and J. M. Grebowsky (1975b). Temporal va- riations in the dawn and dusk mid-latitude trough po- sition-measured (Ariel 3, Ariel 4) and modeling, Ann. Geophys., 31, 29-38. Tulunay, Y. K. and J. M. Grebowsky (1978). The noon and midnight mid-latitude trough as seen by Ariel 4, J. At- mosph. Terr. Phys., 40, 845-855. *Corresponding author: Erdinç Timoçіn İnönü University, Faculty of Science and Art, Department of Phy- sics, 44280 Malatya, Turkey; email: ertim44@hotmail.com. 2017 by Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved PROBABLE SIGNATURE OF MID-LATITUDE ELECTRON DENSITY TROUGH 7