Nonlinear Development of ULF waves in the Upstream of Earth’s Bow Shock

Abstract:

In the upstream region of Earth's bow shock ULF waves are frequently observed. These waves are usually associated with backstreaming ions from the bow shock. In this study we investigated nonlinear development of the ULF waves using multi-point measurements from the Cluster spacecraft. Small amplitude waves observed at spacecraft 3 (C3) rapidly grew and became nonlinear as they were observed at spacecraft 1 (C1) located downstream from C3. The nonlinear growth occurred within a few gyro-period of protons. Ion beams with Tperp/Tpara>1 were observed with the small amplitude waves at C3. As the waves grew nonlinearly, the core of the ion beams was decelerated, but they were also scattered both in energy and pitch angle producing more energetic, diffuse particles. This results in more diffuse distributions for the ULF waves with larger amplitude. During the interaction the original solar wind ions were not much disturbed.