Pc3 pulsations during low density solar wind events

We study the generation of Pc3 geomagnetic pulsations (22-100 mHz) measured at Tihany, Hungary (THY: 42.44° E, 92.39° N; L ≈ 1.84) during intervals of very low solar wind density (low density events - LDE´s), when N_p ≲ 2 cm^-3. The main driver of Pc3 pulsations of the geomagnetic field is ULF waves generated upstream of the bow shock (upstream waves - UW´s) by a plasma instability set up due to the reflection of ion beams from the bow shock. A statistical investigation into the dependence of Pc3´s on solar wind density is made, and it is found that SW density only influences Pc3 generation during LDE´s. Furthermore, we derive the growth rate of the instability responsible for upstream waves, and show that damping occurs for low density, thereby inhibiting the generation of UW´s and Pc3´s.