Iku SHINOHARA

Abstract Field line resonance (FLR) is an important source of Ultra‐low‐frequency (ULF) waves observed in the inner magnetosphere. In this study, we present multi‐spacecraft observations of a toroidal ULF wave, detected by the Arase, Van Allen Probe (Radiation Belt Storm Probe, RBSP) ‐A, and GOES‐14. During the event, Arase and RBSP‐A were positioned on two closely spaced magnetic field lines in the midnight sector (magnetic local time ≈23) and on opposite sides of the magnetic equator, providing a rare opportunity to examine the latitudinal structure of FLR. The temporal profiles of these toroidal components exhibited distinct, separated wave packets, with each packet persisting for ∼10 min and one‐to‐one correspondence in timing and amplitude across all three spacecraft. Based on the phase relationships and harmonic eigenfrequency analysis, the observed waves are identified as the fundamental toroidal mode. The observed waveforms resemble transient toroidal waves associated with impulsive disturbances in the near‐Earth magnetotail. This interpretation is further supported by four corresponding enhancements in the Wp index and Pi2‐type pulsations recorded by ground magnetometers. The observed proton phase space density oscillations may result from ULF‐wave‐induced E  ×  B convection of a background phase‐space‐density gradient, as suggested by their phase relation and pitch‐angle distribution.

Abstract At subauroral latitudes, discrete auroras, such as strong thermal emission velocity enhancement and stable auroral red arcs, have been extensively studied. On the other hand, diffuse emissions at subauroral latitudes have not been studied well. In this paper, we report eight events of conjugate observations of faint diffuse 557.7‐nm emissions with intensities of 300–1,000 rayleighs without discrete structures equatorward of the auroral oval, using the Arase satellite and high‐sensitivity all‐sky airglow imagers in North America. Ground‐magnetosphere conjugate observations of such diffuse emissions have not been previously reported. Diffuse 557.7‐nm emissions were observed during both geomagnetically disturbed and quiet conditions. Preference for magnetic local time was not found in these eight events. Arase observed plasma‐sheet electrons with energies from several hundred eV to beyond 10 keV in the source magnetosphere, which is located inside the plasmapause. Electromagnetic waves, such as plasmaspheric hiss and lower‐band chorus waves, were also observed there by the Arase satellite. For five events, the resonance energies estimated from the observed wave frequencies ranged from 30 eV to 30 keV, suitable for generating the 557.7‐nm emission, though the pitch‐angle diffusion coefficient estimated for one event was not so large. We discuss the possibility that plasma‐sheet electrons in the plasmasphere precipitated into the ionosphere through wave‐particle interaction, forming faint diffuse 557.7‐nm emission equatorward of the auroral oval.

Abstract Electron and proton precipitation into the nightside ionosphere is a key process in magnetosphere–ionosphere coupling. Although auroral electron precipitation at –10 keV is traditionally considered the main driver of auroral emissions, recent studies show that keV electrons and protons can significantly contribute to ionospheric energy deposition, especially during storms. While energetic electron precipitation has been studied extensively, the role of energetic proton precipitation remains poorly quantified. Here, we use coordinated ELFIN and DMSP nightside observations during two substorm events, resolving electron and proton precipitation spectra from eV to ∼MeV. We show that keV proton precipitation, consistent with curvature scattering, can have an impact on E‐region ionization comparable to that of energetic electrons. These analyses are further supported by magnetospheric observations from the Arase and THEMIS spacecraft and ground‐based incoherent scatter radar measurements from PFISR.