A medium-scale traveling ionospheric disturbance (MS-TID) is the signature in the ionosphere of the passage of atmospheric gravity waves, the ions being forced along the field lines by the neutral air winds driven by the pressure wave [e.g., Hocke and Schlegel, 1996]. The generation process is related with meteorological phenomena like neutral winds and the solar terminator [Somsikov, 1995].
 A statistical study regarding the occurrence and seasonal variations of MS-TIDs at low and middle latitudes by Kotake et al.  leads to the following picture: The MS-TIDs activity is highest at daytime during the winter season, when the gradient of the neutral temperature near the mesopause is so low, that the atmospheric gravity waves can propagate. This gradient is steep in the summer season, preventing wave propagation. The nighttime activity of the MS-TIDs is also higher during winter. In general, the production of TIDs at midlatitudes increases with the solar activity. Geomagnetic storms can lead to the production of so-called large-scale TIDs due to heating in the thermosphere at high latitudes by the Joule effect [e.g., Jacobson et al., 1995]. This leads to an energy transfer toward lower latitudes in the form of thermospheric waves interacting with ionospheric ions.
 At high latitudes, the TIDs are also called polar cap patches due to their location of production in the polar cap and auroral oval. Crowley  defined polar cap patches as plasma structures with a horizontal extent of at least 100 km and a plasma density of at least twice the density of the surrounding background plasma. Such enhancements of the ionospheric plasma density were proposed to be produced at the dayside at auroral and subauroral latitudes [e.g., Weber et al., 1984]. In general, the patches are observed to convect antisunward to higher latitudes in the polar region [e.g., Carlson et al., 2002, 2004] and can reach the nightside auroral oval [e.g., Lockwood and Carlson, 1992]. As the geomagnetic field lines connected to high latitudes reach farther out in the space than field lines at midlatitudes, the polar ionosphere is strongly impacted by geomagnetic perturbations due to increased solar activity [e.g., Moen et al., 2007]. The number and lifetime of polar cap patches are expected to increase during time periods with increased solar activity [Schunk and Sojka, 1987].
 It was first observed by Foster , using the Chatinka incoherent scatter radar, that the polar cap plasma exhibits seasonal variations. This has been modeled by Schunk and Sojka . The outcome of these studies is that the plasma densities drawn into the polar cap are much larger and have longer lifetime in winter than in summer. The decreased lifetime in summer was explained by the maintenance of background densities due to solar EUV radiation and the increased recombination rates resulting from increased ion temperatures and differences in the thermosphere composition. The seasonal effect on the polar cap patches in the high-latitude nightside ionosphere has been confirmed by observations and modeling efforts [e.g., Wood and Pryse, 2010].
 Free electrons play an important role in affecting Global Navigation Satellite Systems (GNSS) measurements when the signal propagates through the ionosphere [e.g., Hoffman-Wellenhof et al., 1994]. The total integrated electron density along a propagation path could be on the order of hundreds of total electron content unit (TECU), 1 TECU = 1016 el m−2. In high-precision GNSS usage, this effect is compensated by using observations at different frequencies. However, several GNSS applications are sensitive to atmospheric spatial variations. Relative GNSS measurements, for example, network Real-Time Kinematic (RTK), are very sensitive to such effects [Emardson et al., 2010]. Thus, despite that MS-TIDs amplitudes are typically less than 1 TECU [Hernandez-Pajares et al., 2006], they can have a negative impact on results obtained from relative GNSS measurements. Due to their spatial extension, they affect GNSS measurements using receivers separated with distances up to ~1000 km, thus including typical sizes of RTK networks.
 Based on historical GPS data, it is possible to characterize the variability of the ionosphere. In this study, we have used 13 years of GPS data from European networks to cover a complete solar cycle. The geographical location covers both high and middle latitudes. In this paper we have chosen to focus on spatial scales from 500 km and downward. Characterization is performed by using observations of the ionospheric delay from three GNSS sites forming a triangle in order to interpolate the ionospheric delay for the site in the middle of the triangle. By then comparing the interpolated time series with those actually measured, the small-scale spatial variability of the ionosphere may be characterized.