Effects of the Upstream Solar Wind Structures on EMIC Waves in the Earth's Inner Magnetosphere

Upstream solar wind (SW) structures such as high speed streams (HSSs) and coronal mass ejections (CMEs) are the most common drivers of geomagnetic activity and ultimately responsible for the physical processes occurring in the Earth's magnetosphere. Geomagnetic activities driven by CMEs and HSSs differ from each other, and so the spatial distributions, occurrence rates, and different characteristics of electromagnetic ion cyclotron (EMIC) waves generated under different SW drivers are also expected to be different. Here we analyze the statistics of EMIC waves observed in the Earth's inner magnetosphere during CME, HSS, and quiet solar wind (QSW) conditions in the upstream SW. For our analysis we use the EMIC wave observations by the two Van Allen Probes during their first magnetic local time (MLT) revolution. The major results of our analysis are: 1) Quantitative c riteria to identify the HSS, CME, and QSW conditions in the upstream SW are formulated. 2) 54%, 36%, and 10% of EMIC wave events are observed during CME, HSS, and QSW, respectively. 3) 12% of events are closely associated with the fast growth of magnetospheric compression, among which 76%, 24%, and 0% are observed during CME, HSS, and QSW, respectively. 4) A majority of the QSW, HSS-driven, and CME-driven events is observed in the 9-1 2 hr, 12-24 hr, and 8-24 hr MLT sectors, respectively. 5) CME-driven events are distributed along all L shells, whereas the majority of the HSS-driven and QSW events are confined to L > 3.5. 6) The fractions of events during CME and HSS have a maximum in the near equatorial region, whereas the fractions of the QSW events have a minimum there. 7) Independent of the SW driver, no strong events are observed below the local O ⁺ gyrofrequency, whereas 65-70% and 30-35% of events are observed between the O ⁺ and He ⁺ gyrofrequencies and above the He ⁺ gyrofrequency, respectively.

Acknowledgments: This paper is supported by NASA g rant 80NSSC18K1221 .