Global ULF waves generated by a hot flow anomaly

Abstract

Hot flow anomalies (HFAs), which are frequently observed near Earth's bow shock, are phenomena resulting from the interaction between interplanetary discontinuities and Earth's bow shock. Such transient phenomena upstream the bow shock can cause significant deformation of the bow shock and the magnetosphere, generating traveling convection vortices, field-aligned currents, and ULF waves in the Earth's magnetosphere. A large HFA (4.9 RE) was observed by Cluster on 27 April 2008. Nearly monochromatic Pc 3 ULF waves generated by the HFA were observed by multiple spacecrafts and ground stations. The ULF waves show characteristics of standing Alfvén waves. The wave power of the poloidal mode of the generated ULF waves is stronger than that of the toroidal mode. Polarization rotation from the poloidal mode to the toroidal mode is also observed. The Pc 3 ULF waves generated by the HFA were observed at dawn, noon, dusk sectors, and nightside, indicating that the Pc 3 ULF wave response of the magnetosphere to the HFA is global. Our results show that the impact of HFAs to the magnetosphere is much stronger than what we thought before.