We performed a comprehensive statistical study of electromagnetic ion cyclotron (EMIC) waves observed by the Van Allen Probes and Exploration of energization and Radiation in Geospace satellite (ERG/Arase). From 2017 to 2018, we identified and categorized EMIC wave events with respect to wavebands (H+ and He+ EMIC waves) and relative locations from the plasmasphere (inside and outside the plasmasphere). We found that H-band EMIC waves in the morning sector at L>8 are predominantly observed with a mixture of linear and right-handed polarity and higher wave normal angles during quiet geomagnetic conditions. Both H+ and He+ EMIC waves observed in the noon sector at L~4-6 have left-handed polarity and lower wave normal angles at |MLAT|< 20˚ during the recovery phase of a storm with moderate solar wind pressure. In the afternoon sector (12-18 MLT), He-band EMIC waves are dominantly observed with strongly enhanced wave power at L~6-8 during the storm main phase, while in the dusk sector (17-21 MLT) they have lower wave normal angles with linear polarity at L>8 during geomagnetic quiet conditions. Based on distinct characteristics at different EMIC wave occurrence regions, we suggest that EMIC waves in the magnetosphere can be generated by different free energy sources. Possible sources include the freshly injected particles from the plasma sheet, adiabatic heating by dayside magnetospheric compressions, suprathermal proton heating by magnetosonic waves, and off-equatorial sources.

ERG/Arase satellite observations show a simultaneous enhancement of whistler-mode chorus emissions and electron flux associated with toroidal mode ultra-low frequency (ULF) waves. The satellite observed the intensification of both chorus emissions and electron flux in the energy range satisfying the cyclotron resonance condition during the westward oscillation phase of the toroidal mode ULF waves. A model for the observed periodic variations is proposed. We consider the modulation of the drift speed of energetic electrons to be the drift due to the radial component of the wave electric field and the ambient magnetic field . Assuming a wave phase variation of the toroidal mode ULF oscillation in the azimuthal direction, we expect the accumulation of energetic electrons at locations corresponding to a specific phase angle, consistent with the observed phase relationship.

Pulsating aurora (PsA) is characterized by quasi-periodic intensity modulations with a period of a few to a few tens of seconds. It is caused by precipitation of energetic electrons of a few to several tens of kilo electronvolts produced by chorus waves in the magnetosphere. Precipitating electron energies of PsA have been identified in the past by sounding rocket and satellite observations, but the spatial distributions of precipitating electron energies of PsA have never been estimated. In this study, using the data from ground-based all-sky cameras at two wavelengths of 427.8 nm and 844.6 nm, we estimated the temporal and spatial variations in the precipitating electron energy of PsA. The results showed that the spatial distribution of precipitating electron energies was not uniform in the PsA patch, suggesting that the coherent spatial scale of the wave-particle interactions is smaller than each PsA patch size.

The RBSP and the Arase satellites have different inclinations and sometimes they fly both near the equator and off the equator on the same magnetic field line, simultaneously. Such conjunction events give us opportunities to compare the electron density at different latitudes. In this study, we analyzed the plasma waves observed by Arase and RBSP-A or B during the three conjunction events during and after the 7 Sep 2017 storm event. The electron number density at the satellite positions were estimated from frequencies of the UHR emissions obtained by the HFA/PWE onboard the Arase and the Waves instrument onboard RBSP, respectively. During the three conjunction events, the satellites passed through the plume, inner trough (the narrow region with low electron density between main body of the plasmasphere and the plume), plasmatrough with variable electron density, and partially-refilled plasmasphere. The power-law index m for the inner trough and plume was inferred to be 6~8 and ~0, respectively. This is interpreted to mean that the trough was close to collisionless and the plume was near diffusive equilibrium. In the plasmatrough with the varying density, both the high-density and low-density regions had m~0. The low-density portion of this region may have a different origin from the inner trough, because of the different m-indices. For the partially-refilled plasmasphere in the storm recovery phase, the power-law index m showed negative values, meaning that the density in the equatorial plane was higher than at higher latitudes.

During the second half of 2019, a series of recurring, moderate geomagnetic storms (Dst ≈ - 70 nT) emerged after a sequence of high-speed solar wind streams (Vsw ≥ 600 km/s) impacted the magnetosphere. During one of these storms, intense substorm activity was also recorded (SML ≈ - 2000 nT on August 31 and September 1), as well as a longer-lasting solar wind pressure pulse. We investigate this series of events, using particle measurements from three missions that recorded significant enhancements of relativistic electron fluxes: the Van Allen Probes, Arase and Galileo 207 & 215 satellites. We use both the flux intensity and the phase space density (PSD) of electrons, along with interplanetary parameters and information on ultra-low frequency (ULF) and chorus wave activity for a detailed analysis of this event. Our study demonstrates the importance of substorm injections, even during moderate or weak geomagnetic storms. The presence of seed electrons at L* = 4-5, in addition to intense ULF and chorus wave activity, seems to result in very efficient electron acceleration to relativistic and ultra-relativistic energies. This work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870437 for the SafeSpace project.

The butterfly distribution is one of the characteristic features of the electron pitch angle distribution in electron radiation belts, and it has the maximum electron flux intensity at the pitch angle between the loss cone and 90 degrees. In this Letter, we report the results of test-particle simulations to reveal that the lower-band whistler chorus can be an important cause of the butterfly distribution of relativistic electrons. Our simulations show that the combination of two nonlinear scattering processes, that are phase-trapping and phase-bunching, contributes to the wave-driven butterfly distribution. The butterfly pitch angle distribution of relativistic electrons is proposed to be a signature of the nonlinear scattering processes due to intense whistler chorus waves in the heart of the outer radiation belt.

Whistler mode chorus waves scatter magnetospheric electrons and cause precipitation into the Earth’s atmosphere. Previous measurements showed that nightside chorus waves are indeed responsible for diffuse/pulsating aurora. Although chorus waves and electron precipitation have also been detected on the dayside, their link has not been illustrated (or demonstrated) in detail compared to the nightside observations. Conventional low-altitude satellite observations do not well resolve the energy range of 10–100 keV, hampering verification on resonance condition with chorus waves. In this paper we report observations of energetic electrons with energies of 30–100 keV that were made by the electron sensor installed on the NASA’s sounding rocket RockSat-XN. It was launched from the Andøya Space Center on the dayside (MLT ~ 11 h) at the L-value of ~ 7 on 13 January 2019. Transient electron precipitation was observed at ~ 50 keV with the duration of <100 s. A ground station at Kola peninsula in Russia near the rocket’s footprint observed intermittent emissions of whistler-mode waves simultaneously with the rocket observations. The energy of precipitating electrons is consistent with those derived from the quasi-linear theory of pitch angle scattering by chorus waves through cyclotron resonance, assuming a typical dayside magnetospheric electron density. Precise interaction region is discussed based on the obtained energy spectrum below 100 keV.