Electromagnetic ion cyclotron (EMIC) waves are believed to cause the loss of relativistic electrons from the outer radiation belt into the atmosphere due to pitch angle scattering. However, it is still unclear whether all EMIC waves can scatter relativistic electrons or which conditions are favorable for pitch angle scattering by EMIC waves. In this study, we performed a two-year data analysis of EMIC waves and relativistic electron precipitation (REP) caused by EMIC waves, from 1 November 2016 to 31 October 2018. EMIC waves were observed using a ground-based magnetometer installed at Athabasca (ATH), Canada. REP events were identified from very low-frequency radio waves propagated from the transmitters at the NDK and NLK stations (North Dakota and Seattle, USA, respectively) to the receiver installed at ATH. The magnetic local time dependence of EMIC waves showed higher occurrence rates in the dawn sector. In contrast, EMIC waves accompanied by REP were localized in the dusk sector and were likely to occur during geomagnetic substorms. We found that EMIC waves accompanied by REP were associated with the main phase of geomagnetic storms and occurred inside the plasmapause. These results suggest that the EMIC waves that cause REP occur in the overlap region between the ring current and dense cold plasma during the main phase of geomagnetic storms. This is consistent with previous studies describing that the electron resonant energy with EMIC waves is lower in regions with high plasma density.

In this study, by simulating the wave-particle interactions, we show that sub-relativistic/relativistic electron microbursts form the high-energy tail of pulsating aurora (PsA). Whistler-mode chorus waves that propagate along the magnetic field lines at high latitudes cause precipitation bursts of electrons with a wide energy range from a few keV (PsA) to several MeV (relativistic microbursts). The rising tone elements of chorus waves cause individual microbursts of sub-relativistic/relativistic electrons and the internal modulation of PsA with a frequency of a few Hz. The chorus bursts for a few seconds cause the microburst trains of sub-relativistic/relativistic electrons and the main pulsations of PsA. Our simulation studies demonstrate that both PsA and relativistic electron microbursts originate simultaneously from pitch angle scattering by chorus wave-particle interactions.

A specialized ground-based system has been developed for simultaneous observations of pulsating aurora (PsA) and related magnetospheric phenomena with the Arase satellite. The instrument suite is composed of 1) six 100-Hz sampling high-speed all-sky imagers (ASIs), 2) two 10-Hz sampling monochromatic ASIs observing 427.8 and 844.6 nm auroral emissions, 3) Watec Monochromatic Imagers, 4) a 20-Hz sampling magnetometer and 5) a 5-wavelength photometer. The 100-Hz ASIs were deployed in four stations in Scandinavia and two stations in Alaska, which have been used for capturing the main pulsations and quasi 3 Hz internal modulations of PsA at the same time. The 10-Hz sampling monochromatic ASIs have been operative in Tromsø, Norway with the 20-Hz magnetometer and the 5-wavelength photometer. Combination of these multiple instruments with the European Incoherent SCATter (EISCAT) radar enables us to reveal the energetics/electrodynamics behind PsA and further to detect the low-altitude ionization due to energetic electron precipitation during PsA. In particular, we intend to derive the characteristic energy of precipitating electrons during PsA by comparing the 427.8 and 844.6 nm emissions from the two monochromatic ASIs. Since the launch of the Arase satellite, the data from these instruments have been examined in comparison with the wave and particle data from the satellite in the magnetosphere. In the future, the system will be utilized not only for studies of PsA but also for other categories of aurora in close collaboration with the planned EISCAT_3D project.

Electrostatic electron cyclotron harmonic (ECH) waves are generally excited in the magnetic equator region, in the midnight and the morning sectors during geomagnetically active conditions, and cause the pitch angle scattering by cyclotron resonance. The scattered electrons precipitate into the Earth’s atmosphere and cause auroral emission. However, there is no observational evidence that ECH waves actually scatter electrons into the loss cone in the magnetosphere. In this study, from simultaneous wave and particle observation data obtained by the Arase satellite equipped with a high-pitch angular resolution electron analyzer, we present evidence that the ECH wave intensity near the magnetic equator is correlated with an electron flux inside the loss cone with energy of about 5 keV. The simulation suggests that this electron flux contributes to auroral emission at 557.7 nm with intensity of about 200 R.

We present statistical analyses of whistler-mode waves observed by Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS). Although some observations showed rising tone elements of the lunar whistler-mode waves similar to the terrestrial chorus emissions, it remains unknown whether a banded structure typically seen in chorus is common to the lunar waves. In this study, we automatically detected whistler-mode waves from 9 years of ARTEMIS data and classified them into four types of spectral shapes: lower band only, upper band only, banded, and no-gap. We first show that magnetic connection to the lunar surface is a dominant factor in the wave generation; the occurrence rate of whistler mode waves is more than 10 times larger on magnetic field lines connected to the Moon than on unconnected field lines. Then we compared the field line connected events according to the position of the Moon and the condition of the field-line foot point (day/night and existence of lunar magnetic anomalies). The results show that (i) almost no banded event is observed in any circumstances, suggesting that generation mechanisms for the two band structure on the terrestrial chorus are largely ineffective around the Moon, and (ii) the wave occurrence rate depends on the foot point conditions, presumably affected by electrostatic/magnetic reflections deforming the velocity distribution of the resonant electrons. Thus, our results provide implications for the two band structure formation and new insights to fundamental processes of the Moon-plasma interaction.

The temporal variation of the energetic electron flux distribution caused by whistler mode chorus waves through the cyclotron resonant interaction provides crucial information on how electrons are accelerated in the Earth’s inner magnetosphere. This study employing a data-driven test-particle simulation demonstrates that the rapid deformation of energetic electron distribution observed by the Arase satellite is not simply explained by a quasi-linear diffusion mechanism, but is essentially caused by nonlinear scattering: the phase trapping and the phase dislocation. In response to upper-band whistler chorus bursts, multiple nonlinear interactions finally achieve an efficient flux enhancement of electrons on a time scale of the chorus burst. A quasi-linear diffusion model tends to underestimate the flux enhancement of energetic electrons as compared with a model based on the realistic dynamic frequency spectrum of whistler waves. It is concluded that the nonlinear phase trapping plays an important role in the rapid flux enhancement of energetic electrons observed by Arase.

Inner magnetospheric electrons are precipitated into the ionosphere via pitch-angle (PA) scattering by lower band chorus (LBC), upper band chorus (UBC), and electrostatic electron cyclotron harmonic (ECH) waves at different magnetic latitudes. The PA scattering efficiency of low-energy electrons (0.1–10 keV) is yet to be investigated via in situ observations because of difficulties in flux measurements inside loss cones at the magnetosphere. In this study, we demonstrate that LBC, UBC, and ECH waves contribute to PA scattering of electrons at different energies using the Arase (ERG) satellite observation data and successively detected the loss cone filling, i.e., strong diffusion. For LBC waves, strong diffusion occurred at energies above ~1 keV, whereas it occurred below ~1 keV for UBC and ECH waves. The occurrence rate of the strong diffusion by high-amplitude LBC (>50 pT), UBC (>20 pT), and ECH (>10 mV/m) waves, respectively, reached ~70%, 20%, and 40% higher than that without simultaneous wave activity. The energy range where the occurrence rate was high agreed with the range where the PA diffusion rate of each wave exceeded the strong diffusion level based on the quasilinear theory.

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.

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.

We report on discrete rising-tone elements of whistler-mode waves observed by Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) in the vicinity of the Moon. The two-probe ARTEMIS observations suggest that a free energy source for the wave generation is provided by electron anisotropy resulting from lunar surface absorption and magnetic reflection. High time resolution dynamic spectra reveal that the waves consist of multiple rising tone elements, exhibiting striking similarities to the well-known whistler-mode chorus in planetary magnetospheres. The observed frequency sweep rates are generally consistent with those predicted by the nonlinear growth theory of chorus emissions by Omura et al. (2008). These results imply that whistler-mode waves can grow nonlinearly into chorus-like emissions even around airless bodies without magnetospheres and that a well-defined dipole field is not a prerequisite for the chorus generation.