Joint Observations of Plasmapause Surface Waves and Giant Undulations in the Electron Aurora

Abstract

After the onset of an intense storm-time substorm (the minimum AL $\sim$−2,400 nT) on 8 September 2017, both Van Allen Probes and ground stations observed large-amplitude plasmasphere surface waves (PSWs) in the duskside, which lasted for more than 3 hr. Van Allen Probe B observations show that PSWs caused spatial and temporal modulations of sub-keV electrons with bidirectional pitch-angle distributions and higher-frequency plasma waves outside the plasmapause at the L-shells of 3.5–5.5. These kinetic-scale waves include kinetic Alfvén waves (KAWs) with frequencies below 10 Hz, time domain structures with frequencies from tens to hundreds of Hz, and electrostatic electron cyclotron harmonic waves (ECHs) with frequencies of tens of kHz. The bidirectional pitch angles and flux enhancement of sub-keV electrons indicate that they originate from the ionospheric outflow and are susceptible to pitch-angle scattering by kinetic-scale plasma waves. Following Probe B with a time delay of about 2 hr, Probe A observed PSW-associated ULF waves inside the plasmasphere at L-shells of 5–6, which can cause the drift-bounce resonance of cold (50 eV) electrons. Corresponding to the duration and location of PSWs, DMSP-F17 observed intense sub-keV electron precipitation and weak ion precipitation, which coincides with the observations of giant undulations (GUs) in the electron aurora. These space-ground joint observations provide new sights into understanding the complicated middle processes between magnetospheric PSWs and ionospheric GUs.