We performed a statistical study of electromagnetic ion cyclotron (EMIC) wave distributions and their coupling with energetic protons in the inner magnetosphere using the Arase satellite data from May 2017 to December 2020. We investigated the energetic proton pitch-angle distributions and partial thermal pressures associated with EMIC waves using inter-calibrated proton data in the energy range of 30 eV/q-187 keV/q. With a cold plasma approximation, we computed the proton minimum resonance energies using the observed EMIC wave frequency and plasma density values. We found that the EMIC waves had left-handed polarization near the magnetic equator close to the threshold of proton cyclotron instability, and propagated to higher latitudes along the field line with polarization reversal. H-EMIC waves showed two peak occurrence regions in the morning and noon sectors at L=7.5-9 outside the plasmasphere. The flux enhancements associated with morning side H-EMIC waves appeared at E<1 keV/q among all pitch angles, while H-EMIC waves in the noon sector exhibited flux enhancement in field-aligned directions at E=1-100 keV/q. He-EMIC waves showed a broad occurrence region from 12 to 20 magnetic local time at L=5.5-8.5 inside the plasmasphere with strong flux enhancements at all pitch-angle ranges at E=1-100 keV/q. The proton minimum resonance energy using the obtained central frequency was consistent with the observed flux enhancements at different peak occurrence regions. We conclude that the free energy sources of EMIC waves in different geomagnetic environments drive the two different types of EMIC waves, and they interact with energetic protons at different energy ranges.

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 surveyed the subsurface structure of Chryse and Acidalia Planitiae using data from the Mars SHAllow RADar sounder (SHARAD) onboard the Mars Reconnaissance Orbiter (MRO). Several subsurface reflectors were identified in these regions, but these reflectors do not constitute apparent subsurface structures larger than 30 km. The bulk dielectric constants of the uppermost layers at two locations were estimated as 5.3 and 5.9 from the combination of high-resolution images and topographic data. These values were used to constrain the possible bulk porosity (approximately 28 % and 25 %) and the upper limit for the volume fraction of water ice (~43 % and ~50 %). The estimated porosities of the shallow subsurface layers in these locations can be explained by the emplacement of basaltic rock or till that may contain some ice-cemented regolith.

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.

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.

We surveyed the subsurface structure in eastern Coprates and Capri Chasmata in the equatorial region using high-resolution visible images, digital terrain models, and radar sounding data. We identified subsurface reflectors in four areas of the chasmata. At the stratigraphic exposure on the chasmata walls, the corresponding depth of the reflector is ~60 m. The bulk dielectric constants of the layers above the reflectors are calculated as 3.4-4.0, suggesting a rock-air mixture with ~46.1% porosity, or a rock-air-ice mixture with ~21.2% water-ice fraction. Recent climate models suggest that water-ice is unstable on the surface around the equatorial regions. However, considering the recent high obliquity that occurred ~0.4 Ma and a slow diffusivity of water-ice, the existence of subsurface water-ice deeper than a few meters cannot be ruled out. If water-ice is actually contained in the layer, our results show the maximum volume of putative water-ice in the chasmata is 16.6 km.