# On the Feasibility of Interhemispheric Patch Detection Using Ground-Based GNSS Measurements
## 1 Introduction  
The polar and subpolar areas are characterized by an extremely complicated and dynamic ionospheric structure. It is mainly related to the shape of the geomagnetic field, which allows the transfer of solar wind energy into the ionosphere-magnetosphere system and its dissipation into the ionized part of the atmosphere. Furthermore, these processes are driven by the interplanetary magnetic field (IMF), which is the magnetic field of the Sun frozen into the solar wind flux. It is well-recognized that the particular components of IMF vectors (Bx, By--parallel to the ecliptic, toward the Sun, and east-west directions, respectively, Bz--perpendicular to the ecliptic) have different impacts on the ionospheric conditions in polar and auroral regions. The strongest activity of the ionosphere occurs for the southwardly-oriented Bz. It is related to the magnetic reconnection as well as the two-cell pattern of high-latitude plasma convection for such IMF conditions. The former factor is conducive to the growth of particle precipitation, whereas the latter enables feeding the polar ionosphere by the dense stream of plasma from mid-latitudes. As a consequence of both of these processes, the high-latitude ionosphere is affected by the different-scale irregularities with sizes ranging from decameter to severalhundreds of kilometers [1; 2; 3; 4; 5]. The impact of the remaining IMF components is much less significant. Nevertheless, it is known that variations of By influence the shape of the convection pattern, which in turn modulates the trajectory of polar structures [6]. Polar patches are defined as islands of F-region high-density plasma in a low-density environment with a horizontal size of up to 1000 km. The foreground-to-background ratio for these structures ranges from two to 10, where the lower limit is usually adopted as a detection threshold. The occurrence of polar patches was preliminarily studied with photometer and ionosonde measurements [7; 8]. Subsequently, such analyses were extended, taking advantage of ground-based GNSS data [9; 10], incoherent scatter radar [11], and in situ measurements [12]. The initial results confirmed the propagation of these structures from the dayside ionosphere across the polar cap with velocities of 400-1000 m/s. Furthermore, the studies reported that patches exiting from the polar cap fed the auroral oval (also called auroral blobs) and flow in the sunward direction according to the high-latitude convection pattern [13; 14; 15]. The further studies considered the origin of the patches and mechanism fracturing the plasma. They confirmed that the primary source of high-density plasma is a reservoir at sub-auroral middle latitudes, which are a result of solar EUV radiation [16; 17; 18; 19]. During the storms, the signature of this reservoir, extending from the dusk sector of mid-latitudes to the dayside polar cap, is often referenced as storm-enhanced density (SED) [20; 21]. The patch generation was also associated with particle precipitation [22]. However, this process can be responsible only for weak patches, up to an equivalent of 5 MHz for foF2 [4; 23]. A few explanations have been proposed with regard to the mechanisms responsible for plasma segmentation. The contributions [24; 25] linked the patch generation with the temporal variations in IMF By and IMF Bz. In [22], it was suggested that this process is related to high-speed flow channel events, while [16] explained it by transient magnetopause reconnection. Although patch generation seems to still be an open issue, the recent studies [26; 27] showed good agreement between the latter method and observed patches. The significance of patches on the appropriate modeling of the polar cap ionosphere was also the reason for detailed climatological studies performed with different techniques, such as: in situ data from low Earth orbit (LEO) satellites (Dynamics Explorer 2, DMSP and SWARM) [6; 28], Halley HF radar data [29] and spaceborne GNSS measurements derived from the CHAMP satellite [30].