Ionospheric convection patterns from the Super Dual Auroral Radar Network are used to determine the trajectories, transit times and decay rates of three polar cap patches from their creation in the dayside polar cap ionosphere to their end of life on the nightside. The first two polar cap patches were created within 12 minutes of each other and travelled through the dayside convection throat, before entering the nightside auroral oval after 104 and 92 minutes, respectively. When the patches approached the nightside auroral oval, an intensification in the poleward auroral boundary occurred close to their exit point, followed by a decrease in the transit velocity. The airglow decay rates of patches 1 and 2 were found to be ≈0.6% and ≈0.9% per minute, respectively. The third patch decayed completely within the polar cap and had a lifetime of only 78 minutes. After a change in drift direction, patch 3 had a radar backscatter power half-life of 4.23 minutes, which reduced to 1.80 minutes after a stagnation, indicating a variable decay rate. 28 minutes after the change in direction, and 16 minutes after stagnation, patch 3 completely disintegrated. We relate this rapid decay to increased frictional heating, which speeds up the recombination rate. Therefore, we suggest that the stagnation of a polar cap patch is a main determinant to whether or not a polar cap patch can exit through the nightside auroral oval.

The ionospheric modification experiments are providing the means for understanding mechanisms and physical processes leading to the generation and evolution of ionospheric irregularities. We present experimental results concentrating on the features and evolution, generation conditions and mechanisms of small-scale field-aligned irregularities (FAIs) in the high latitude ionosphere F region induced by the controlled injection of the powerful HF radio waves from the ground into the ionosphere. Experiments have been carried out at the EISCAT HF Heating facility near Tromsø, Norway, at heater frequencies of 4.5 – 8.0 MHz with an effective radiated power of 200 – 750 MW. HF pump wave with ordinary (O-mode) or extraordinary (X-mode) polarization was injected along the magnetic field-aligned direction. Instrument diagnostics included the CUTLASS (Co-operative UK Twin Located Auroral Sounding System) radar, the European Incoherent Scatter (EISCAT) UHF radar at 931 MHz near Tromsø and the EISCAT ionosonde (dynasonde). It was found that the features and physical driving mechanisms of FAIs with the spatial scale across the geomagnetic field of 7.5 – 15 m are significantly different for O- and X-mode HF pumping, presenting challenges for understanding the relevant processes. The main attention is paid to the recently discovered X-mode FAIs. By a contrast to the O-mode FAIs excited by a thermal parametric (resonance) instability at the upper hybrid resonance altitude, the X-mode FAIs are generated via two-step process. In the first step the generation of elongated large-scale irregularities (with the spatial scale across the geomagnetic field of the order of 1 -10 km depending on the background geophysical conditions and the pump frequency) is occurred. As a second step, we suggest that the filamentation instability can be responsible for the generation of small-scale FAIs. As is found from EISCAT UHF radar measurements FAIs greatly impact on the development of strong artificial turbulence such as Langmuir and ion-acoustic plasma waves.