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.