Thermosphere–ionosphere coupling in response to recurrent geomagnetic activity Plamen Mukhtarov n , Dora Pancheva Geophysical Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Street, Bl. 3, 1113 Sofia, Bulgaria article info Article history: Received 16 September 2011 Received in revised form 12 February 2012 Accepted 13 February 2012 Available online 23 February 2012 Keywords: Solar wind Recurrent geomagnetic activity Zonally symmetric oscillations abstract The paper presents the global thermosphere–ionosphere response to the high-speed solar wind streams and the subsequent recurrent geomagnetic variations with a period of 9 d during the period of time 1 October 2007–31 March 2009. The COSMIC electron density at fixed heights, as well as the ionospheric parameters f o F2 and h m F2, and the two coefficients characterizing the top and bottom side vertical gradients of the electron density profile, are used for investigating the ionospheric 9-d (s ¼0) wave response. The SABER temperature data are utilized for studying the response of the lower thermosphere to the recurrent auroral heating. The COSMIC and SABER measurements are analyzed by one and the same method where the atmospheric tides and planetary waves which are present in the temperature and electron density measurements are simultaneously extracted from the data. The use of such data analysis approach brings to light additional features of the ionospheric response to a recurrent geomagnetic activity which have not been found before. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction The solar–terrestrial relationship includes the effect of solar output and its variations, and also includes the propagational effects in the interplanetary medium, which ultimately produces disturbances in the geomagnetic field. The thermosphere– ionosphere system acts as the intermediary region between the plasma-dominated magnetosphere and the bulk of neutral atmo- sphere below. In this way the thermosphere–ionosphere system forms the most variable part of Earth’s atmosphere as it is forced from above and below. Its high sensitivity to the external forcing (e.g. solar EUV radiation, X-ray flux, particle precipitations, etc.) causes significant global variability on different time scales. Additional variations are produced through interactions with the magnetosphere above and the middle atmosphere below. To understand and forecast such variability is one of the main tasks of space weather research. It is known that the ionosphere, where the free electrons are formed mainly by the solar X-rays and EUV radiation, is above all under solar control. Usually during periods of high solar activity through the interaction between coronal mass ejections and Earth’s magnetic field the largest geomagnetic disturbances are excited. The rapid and global response of the ionosphere to these strong geomagnetic disturbances is well studied (Pr¨ olss, 1995; Fuller-Rowell et al., 1994, 1996, 2000; Buonsanto, 1999; Muhtarov and Kutiev, 1998; Kutiev and Muhtarov, 2001, 2003; Mendillo, 2006). In addition to strong geomagnetic activity a number of other processes disturb the ionosphere–thermosphere system from its mean, quiet-time state. There are quasi-periodic variations of the solar indices, such as the 27 d rotation period of the solar EUV radiation, that may generate such variability in the ionosphere (Pancheva et al., 1991; Altadill et al., 2001; Altadill and Apostolov, 2003; Pancheva et al., 2002). A first estimation of the solar forced ionospheric quasi-oscillations in total electron content (TEC) was achieved by Borries and Hoffmann (2010) using wavelet filter algorithms. They allocated 38–42% of the planetary wave type oscillations observed in TEC to quasi-periodic variability of the EUV, solar wind speed and geomagnetic disturbances. Geomagnetic storms are often considered to be isolated events; however, geomagnetic activity has been observed to occur on a periodic basis. Recent studies demonstrated that subharmo- nics of the 27 d solar rotation and particularly the 9 d solar wind variations during 2005 are due to the existence of a triad of solar coronal holes distributed roughly 1201 apart in solar longitude (Temmer et al., 2007; Vrˇ snak et al., 2007). Coronal holes are associated with high speed streams in the solar wind. They are most prevalent during the declining phase of the solar cycle and can persist for many solar rotations (Borovsky and Denton, 2006; Vrˇ snak et al., 2007). Coronal holes of substantial size near the Sun–Earth line can create disturbances in the solar wind, called corotating interaction regions (CIRs) that lead to geomagnetic disturbances (Tsurutani et al., 2006). The latter are moderate but Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jastp Journal of Atmospheric and Solar-Terrestrial Physics 1364-6826/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2012.02.013 n Corresponding author. E-mail address: engpjm@abv.bg (P. Mukhtarov). Journal of Atmospheric and Solar-Terrestrial Physics 90–91 (2012) 132–145