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The Van Allen Probes mission contributed to the discovery of the relativistic (~500 keV-2 MeV) and ultra-relativistic (~>2 MeV) electron three-belt structure in Earth's radiation belts. This structure results from the partial depletion of the preexisting outer belt and the replenishment of a new outer belt. Ultra-low frequency and very-low frequency waves are believed to play important roles in these processes, and substorm injections are usually not responsible for the formation of the external outer belt. In this study, based on observations from the Arase and NOAA-18 (National Oceanic and Atmospheric Administration-18) satellites, we report an electron three-belt event in the energy range of ~100-200 keV (i.e., sub-relativistic electrons). According to the evolution of the phase space density and the dawn-dusk asymmetric flux enhancement during this event, we conclude that the depletion of the upper part of the original outer belt was due to outward radial diffusion accompanied by magnetopause shadowing effect and convection, and the formation of the external outer belt was due to increased electron flux from convection as well as substorm injections. This discovery and its preliminary explanation may help understand the electron three-belt structure in radiation belts more comprehensively. [ABSTRACT FROM AUTHOR]

High-power transmission experiments in the very low frequency (VLF) mode have been conducted by the US Air Force Research Laboratory's Demonstration and Science Experiments (DSX) satellite in the radiation belts using a novel transmitter that automatically tunes to find the resonance frequency of the transmitter circuit including the antenna. The resulting voltage-frequency curves are used to derive antenna impedance at the resonance. The analysis shows that the antenna reactance is far less than that of a dipole antenna in free space. The derived radiation resistance is up to several tens of kilo Ohms. Most interestingly, it is found that the radiation resistance is inversely proportional to the square of transmission wave frequency. The transmitted power can be up to 80 W for the DSX transmitter with an 82-m long tip-to-tip antenna, showing that the high-power VLF transmission is feasible. Whistler wave transmission inside the higher-density plasmasphere is more efficient. Data analysis indicates that the antenna impedance does not vary systematically with the antenna orientation angle relative to the ambient magnetic field. The previous dominant theoretical studies yield not only incorrect values of the impedance but a completely different frequency dependence than that derived from DSX experiments. Instead, the recent theories correctly capture both the antenna impedance magnitude and the frequency dependence. [ABSTRACT FROM AUTHOR]

Ultra-low-frequency (ULF) waves are known to radially diffuse hundreds-keV to few-MeV electrons in the magnetosphere, as the range of drift frequencies of such electrons overlaps with the frequencies of the waves, leading to resonant interactions. The theoretical framework for this process is described by analytic expressions of the resonant interactions between electrons and toroidal and poloidal ULF wave modes in a background magnetic field. However, most expressions estimate the radial diffusion rates based on estimates of the power of ULF waves that are obtained either from spacecraft close to the equatorial plane or from the ground. In this study, using multiyear measurements from the THEMIS and Arase missions, we present a statistical analysis of the distribution of ULF wave power in magnetic latitude and local time and show that the wave power of the radial and azimuthal components of the magnetic field increases away from the magnetic equator. Our result could have significant implications for the radial diffusion rates as currently estimated. [ABSTRACT FROM AUTHOR]

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+ 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 lefthanded 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+ 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. [ABSTRACT FROM AUTHOR]

Stable auroral red (SAR) arcs are optical events with dominant 630.0-nm emission caused by low-energy electron heat flux into the topside ionosphere from the inner magnetosphere. SAR arcs are observed at subauroral latitudes and often occur during the recovery phase of magnetic storms and substorms. Past studies concluded that these low-energy electrons were generated in the spatial overlap region between the outer plasmasphere and ring-current ions and suggested that Coulomb collisions between plasmaspheric electrons and ring-current ions are more feasible for the SAR-arc generation mechanism rather than Landau damping by electromagnetic ion cyclotron waves or kinetic Alfvén waves. This work studies three separate SAR-arc events with conjunctions, using all-sky imagers and inner magnetospheric satellites (Arase and Radiation Belt Storm Probes [RBSP]) during non-storm-time substorms on December 19, 2012 (event 1), January 17, 2015 (event 2), and November 4, 2019 (event 3). We evaluated for the first time the heat flux via Coulomb collision using full-energy-range ion data obtained by the satellites. The electron heat fluxes due to Coulomb collisions reached ~109 eV/cm²/s for events 1 and 2, indicating that Coulomb collisions could have caused the SAR arcs. RBSP-A also observed local enhancements of 7-20-mHz electromagnetic wave power above the SAR arc in event 2. The heat flux for the freshly detached SAR arc in event 3 reached ~108 eV/cm²/s, which is insufficient to have caused the SAR arc. In event 3, local flux enhancement of electrons (<200 eV) and various electromagnetic waves were observed, these are likely to have caused the freshly detached SAR arc. [ABSTRACT FROM AUTHOR]

Transverse Pc1 waves propagating from magnetospheric source regions undergo mode conversion to the compressional mode in the ionosphere due to the induced Hall current. Mode converted Pc1 waves propagate across the magnetic field through the ionospheric waveguide. This process is called Pc1 wave ducting (PWD). PWDs have been observed by magnetometers on both ground and low Earth orbit satellites over a wide latitudinal and longitudinal range. In this work, we present the statistical analysis results of PWD exploiting Swarm satellites from 2015 to 2019. Spatial distributions show that the PWDs are mainly observed over the South Atlantic Anomaly longitudes, possibly due to the high Hall conductivity and F-region density, and at subauroral/auroral latitudes ( +50- 70 MLAT). The occurrence rate of PWD increases with increasing AE and |SYM-H| indices. Seasonal dependence shows that PWD exhibits a high occurrence rate during equinox and local summer while local winter hosts only a low occurrence. The asymmetry between summer and winter can be explained by the ionospheric plasma density. The high occurrence rate in equinox may result from intense geomagnetic activity during the equinox, probably due to the Russell-McPherron effect. From our statistical analysis, we conclude that the occurrence of PWD is controlled by both ionospheric plasma conditions and geomagnetic activity, and that the mode conversion and PWD occur more efficiently as plasma density increases. [ABSTRACT FROM AUTHOR]

In situ electron density profiles obtained from Arase in the night magnetic local time (MLT) sector and from RBSP-B covering all MLTs are used to study the small-scale density irregularities present in the plasmasphere and near the plasmapause. Electron density perturbations with amplitudes >10% from background density and with time-scales less than 30-min are investigated here as the small-scale density irregularities. The statistical survey of the density irregularities is carried out using nearly 2 years of density data obtained from RBSP-B and 4 months of data from Arase satellites. The results show that density irregularities are present globally at all MLT sectors and L-shells both inside and outside the plasmapause, with a higher occurrence at L > 4. The occurrence of density irregularities is found to be higher during disturbed geomagnetic and interplanetary conditions. The case studies presented here revealed: (1) The plasmaspheric density irregularities observed during both quiet and disturbed conditions are found to coexist with the hot plasma sheet population. (2) During quiet periods, the plasma waves in the whistler-mode frequency range are found to be modulated by the small-scale density irregularities, with density depletions coinciding well with the decrease in whistler intensity. Our observations suggest that different source mechanisms are responsible for the generation of density structures at different MLTs and geomagnetic conditions. [ABSTRACT FROM AUTHOR]

Magnetospheric Extremely Low-Frequency/Very Low-Frequency (ELF/VLF) waves have an important role in the acceleration and loss of energetic electrons in the magnetosphere through wave-particle interaction. It is necessary to understand the spatiotemporal development of magnetospheric ELF/VLF waves to quantitatively estimate this effect of wave-particle interaction, a global process not yet well understood. We investigated spatiotemporal development of magnetospheric ELF/VLF waves using 6 PWING ground-based stations at subauroral latitudes, Exploration of energization and Radiation in Geospace and RBSP satellites, POES/MetOp satellites, and the RAM-SCB model, focusing on the March and November 2017 storms driven by corotating interaction regions in the solar wind. Our results show that the ELF/VLF waves are enhanced over a longitudinal extent from midnight to morning and dayside associated with substorm electron injections. In the main to early storm recovery phase, we observe continuous ELF/VLF waves from ~0 to ~12 MLT in the dawn sector. This wide extent seems to be caused by frequent occurrence of substorms. The wave region expands eastward in association with the drift of source electrons injected by substorms from the nightside. We also observed dayside ELF/VLF wave enhancement, possibly driven by magnetospheric compression by solar wind, over an MLT extent of at least 5 h. Ground observations tend not to observe ELF/VLF waves in the post-midnight sector, although other methods clearly show the existence of waves. This is possibly due to Landau damping of the waves, the absence of the plasma density duct structure, and/or enhanced auroral ionization of the ionosphere in the post-midnight sector. [ABSTRACT FROM AUTHOR]

We report three different types of relativistic electron precipitation (REP) events observed at International Space Station (ISS), associated with electromagnetic ion cyclotron (EMIC) waves or whistler mode waves as observed by the Arase satellite at conjugate locations near the magnetic equator. Three different detectors installed on the ISS were complementarily used; CALET/CHD as the detector of precipitating MeV electrons, MAXI/RBM as the detector of sub-MeV electrons from horizontal and vertical directions, and SEDA-AP/SDOM to quantitatively measure the energy spectrum. The REP event on 21 August 2017 shows a quasiperiodic intensity variation at ~1 Hz which corresponds to variations of the EMIC waves at the Arase altitudes. The REP event on 24 April 2017 shows rapid and irregular intensity variation which corresponds to the amplitude variation of chorus waves, while the REP events on 26 October 2017 shows a smooth quasiperiodic time variation at ~0.2 Hz which corresponds to the amplitude variation of "electrostatic" whistler mode waves. This study clearly demonstrates that the time variation of REP events at ISS are caused by various types of plasma waves near the magnetic equator. [ABSTRACT FROM AUTHOR]