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The ionospheres of Jupiter's icy moons have been observed by in situ plasma measurements and radio science. However, their spatial structures have not yet been fully characterized. To address this, we developed a new ray tracing method for modeling the radio occultation of the ionospheres using Jovian auroral radio sources. Applying our method to Jovian auroral radio observations with the Galileo spacecraft, we derived the electron density of the ionosphere of Ganymede and Callisto. For Ganymede's ionosphere, we found that the maximum electron density on the surface was 76.5–288.5 cm−3in the open magnetic field line regions and 5.0–20.5 cm−3in the closed magnetic field line region during the Galileo Ganymede 01 flyby. The difference in the electron density distribution was correlated with the accessibility of Jovian magnetospheric plasma to the atmosphere and surface of the moons. These results indicated that electron‐impact ionization of the Ganymede exosphere and sputtering of the surface water ice were effective for the producing Ganymede's ionosphere. For Callisto's ionosphere, we found that the densities were approximately 350 and 12.5 cm−3on the night side hemisphere during Callisto 09 and 30 flybys, respectively. These results combined with previous observations indicated that atmospheric production through sublimation controlled the ionospheric density of Callisto. This method is also applicable to upcoming Jovian radio observation data from the Jupiter Icy Moon Explorer, JUICE. We developed a ray tracing method for Jupiter's icy moon ionospheric occultation of Jovian auroral radio sourcesThe ionospheric density of Ganymede is associated with electron‐impact ionization and atmospheric production via ion sputteringThe Ionospheric density of Callisto is associated with atmospheric production by solar illumination on its surface We developed a ray tracing method for Jupiter's icy moon ionospheric occultation of Jovian auroral radio sources The ionospheric density of Ganymede is associated with electron‐impact ionization and atmospheric production via ion sputtering The Ionospheric density of Callisto is associated with atmospheric production by solar illumination on its surface

We present the simultaneous and conjugated auroral emission and particle data obtained by a low‐altitude polar‐orbiting micro‐satellite, Reimei, for elucidating their latitudinal distributions and variations in the nightside auroral oval. Here are reported a few notable examples of the Reimei observations with high time and spatial resolutions, namely ∼120 msec. and ∼1.2 km × 1.2 km for multispectral auroral images and 40 msec. for energy‐pitch angle distributions of electrons and ions with energies of 10 eV–12 keV, respectively. The auroral images show various fine‐scale auroral activities characterized by the following types of auroral forms and variations: faint bands, streaming multiple arcs, shearing arcs, and vortices/curls, which are typical of the latitudinal properties of auroras. The particle analyzer simultaneously observed various properties of electron energy‐pitch angle and latitudinal distributions, and their temporal variations, each of which corresponds to a type of the auroral activities. Their features are summarized below. Reimei repetitively observed inverted‐V signatures of low‐energy (<1 keV) field‐aligned electrons in addition to the higher‐energy (several keV) diffuse electrons in low‐latitude auroral oval. In more active regions at higher latitudes, the dominant energy flux responsible for the multiple‐arc emissions was carried by the well‐known inverted‐V electron precipitation. The rapidly rotating vortices or so‐called curls of fine‐scale discrete auroras near the poleward boundary of the auroral oval were closely associated with the significant energy fluxes of spiky field‐aligned electron bursts with energy‐time dispersions produced by dispersive Alfvén waves. Plasma observations with auroral imaging in nightside auroral oval present clear characteristics of several types of particle accelerationsThe image‐particle data reveal correlations and latitudinal properties/dependence of plasma dynamics with the forms and motions of aurorasEnergy‐pitch angle distributions are crucial for elucidating the particle acceleration processes recognized in energy‐time spectrograms Plasma observations with auroral imaging in nightside auroral oval present clear characteristics of several types of particle accelerations The image‐particle data reveal correlations and latitudinal properties/dependence of plasma dynamics with the forms and motions of auroras Energy‐pitch angle distributions are crucial for elucidating the particle acceleration processes recognized in energy‐time spectrograms

Several observations using the Hubble Space Telescope reported that the brightness morphology of the oxygen OI]135.6 nm emissions on Europa's atmosphere has a north‐south asymmetry which changes with the position of Europa with respect to the Jovian magnetospheric plasma sheet. Similar north‐south asymmetry of Io's auroral limb glow has been explained by higher electron flux into the atmosphere on the hemisphere that faces the plasma sheet center. This explanation, however, has not yet been evaluated for the case of Europa quantitatively. In this study, we used a test particle simulation for the Jovian magnetospheric electrons to estimate the brightness of the 135.6 nm aurora in Europa's atmosphere and evaluate the cause of the north‐south asymmetry with the previously suggested idea, in which the strong deceleration of the magnetospheric flux tube results in the inhomogeneous electron flux into Europa's atmosphere (the “slow‐down effect”). Our simulation successfully recreates the systematically changing north‐south asymmetry of Europa's oxygen aurora brightness using the “slow‐down effect.” With deceleration into 10% of the background plasma flow, the maximum north‐to‐south brightness ratio is estimated at 2.17 and 2.56 on the trailing (plasma‐upstream) and leading (downstream) side, respectively. However, the previously observed brightness ratio is larger on the trailing side (up to ∼5). The results indicate that additional model scenarios are required to fully explain the north‐south asymmetry of Europa's oxygen aurora morphology. Europa is one of Jupiter's icy moons and possesses a tenuous oxygen atmosphere. A ultraviolet oxygen aurora at 135.6 nm is generated due to the collision between atmospheric oxygen molecules and magnetospheric electrons in Europa's atmosphere, and the aurora has a time‐variable north‐south asymmetric structure. We simulated electron motion near Europa and calculated the aurora brightness to investigate how the spatial distribution of Europa's oxygen aurora becomes north‐south asymmetric. Previous studies qualitatively explain that the strong deceleration of the magnetospheric plasma flux tube results in the unequal electron flux into the atmosphere to generate the north‐south asymmetric aurora structure, but this explanation has never been quantitatively evaluated for the case of Europa. Based on this “slow‐down effect” scenario, we successfully reproduced the time‐variation of the north‐south asymmetry of Europa's oxygen aurora. However, our model estimates a larger north‐to‐south brightness ratio on the leading side, whereas previous auroral observations show that the trailing hemisphere has more pronounced north‐south asymmetry. This result indicates that the “slow‐down effect” cannot fully explain the structure of Europa's oxygen aurora and that additional model scenarios are required to understand the auroral north‐south asymmetry. Morphology of Europa’s oxygen 135.6 nm aurora was reproduced by test particle method to investigate the north‐south aurora asymmetryWe confirmed that a flux tube decelerated to 10% of the background plasma flow creates the periodic changes of the north‐south asymmetryBut the slow‐down scenario does not explain the larger north‐south asymmetry observed on the trailing hemisphere Morphology of Europa’s oxygen 135.6 nm aurora was reproduced by test particle method to investigate the north‐south aurora asymmetry We confirmed that a flux tube decelerated to 10% of the background plasma flow creates the periodic changes of the north‐south asymmetry But the slow‐down scenario does not explain the larger north‐south asymmetry observed on the trailing hemisphere

To understand the mechanism of the increased frequency of intervals of pulsations of diminishing periods (IPDPs), we analyzed IPDP‐type electromagnetic ion cyclotron (EMIC) waves that occurred on 19 April 2017, using ground and satellite observations. Observations by low‐altitude satellites and ground‐based magnetometers indicate that the increased IPDP frequency is caused by an inward (i.e., Earthward) shift of the EMIC wave source region. The EMIC wave source region moves inward along the mid‐latitude trough, which we used as a proxy for the plasmapause location. A statistical analysis shows that increases in the IPDP frequency showed a positive correlation with polar cap potentials. These results suggest an enhanced convection electric field causes an inward shift of the source region. The inward shift of the source region allows EMIC waves to scatter relativistic electrons over a wide range of radial distances during the IPDP event. This mechanism suggests that IPDP‐type EMIC waves are more likely to scatter relativistic electrons than other EMIC waves. We also show that the decreased phase‐space density of relativistic electrons in the outer radiation belt is consistent with the extent of the source region and the resonant energy of EMIC waves, implying a possible contribution of EMIC waves to outer radiation belt loss during the main phase of geomagnetic storms. An inward shift of the electromagnetic ion cyclotron (EMIC) wave source region causes the increased intervals of pulsations of diminishing period frequencyThe inward shift allows EMIC waves to scatter relativistic electrons over a wide radial distanceThe decrease in electron phase‐space density is consistent with the source region's extent and the EMIC waves’ resonant energy An inward shift of the electromagnetic ion cyclotron (EMIC) wave source region causes the increased intervals of pulsations of diminishing period frequency The inward shift allows EMIC waves to scatter relativistic electrons over a wide radial distance The decrease in electron phase‐space density is consistent with the source region's extent and the EMIC waves’ resonant energy

The presence of the dawn‐to‐dusk electric field of about 4 mV/m in the Jovian inner magnetosphere and its response to the enhancement of the solar wind dynamic pressure are still a mystery of the rotation‐dominated Jovian magnetosphere. Previous studies have suggested that magnetosphere‐ionosphere (M‐I) coupling via Region 2‐like (R2‐like) field‐aligned current (FAC) could be the origin of the Jovian dawn‐to‐dusk electric field. This study investigates whether the dawn‐to‐dusk electric field is formed from this scenario by using a Jovian ionosphere model and a two‐dimensional ionospheric potential solver. Our results show that the dawn‐dusk asymmetry in the ionospheric potential form even at middle latitudes and that the dawn‐to‐dusk electric field is induced in the inner magnetosphere if the electric potential is mapped to the magnetospheric equatorial plane. Around the Io orbit, the calculated electric field strength for the ionosphere without meteoroid influx is too large, 200 mV/m at dawn and 88 mV/m at dusk. One of the solutions is to consider long‐lived meteoric ions in the Jovian ionosphere, which reduce the electric field strength to 15 mV/m at dawn and 12 mV/m at dusk. The model also shows that the electric field strength increases with the intensity of R2‐like FAC, consistent with its response to the solar wind dynamic pressure observed by the Hisaki satellite. The dawn‐to‐dusk electric field in the Jovian inner magnetosphere via Region 2‐like field‐aligned current was simulatedEnhancement of ionospheric conductance by meteoric ions weakens the dawn‐to‐dusk electric fieldThe simulated dawn‐to‐dusk electric field for the case including meteoroid influx can explain Hisaki observations The dawn‐to‐dusk electric field in the Jovian inner magnetosphere via Region 2‐like field‐aligned current was simulated Enhancement of ionospheric conductance by meteoric ions weakens the dawn‐to‐dusk electric field The simulated dawn‐to‐dusk electric field for the case including meteoroid influx can explain Hisaki observations

We report the first comprehensive observations of Jovian synchrotron radiation (JSR) and H3+emission from the Jovian thermosphere to investigate the generation process of short‐term (days to weeks) variations in the Jovian radiation belt. The observations were made by the Giant Metrewave Radio Telescope and NASA Infrared Telescope Facility during November 2011. The total flux density of JSR increased by approximately 5% between 6–9 November and 12–17 November, associated with the increased solar UV/EUV flux. From 7 to 14 November, a possible rise in the infrared H3+emission was observed in the middle‐latitude region, corresponding to a temperature variation of approximately 10 K. These results are consistent with the scenario that the solar UV/EUV heating causes variations in the thermospheric temperature and JSR. Radio images along the equatorial region showed that the JSR intensity decreased inside 1.5 Jovian radii (RJ) and the peak position shifted outward. This implies that energetic electrons are attenuated by some internal loss process, despite the simultaneous increase in radial diffusion. A physical model for the radiation belt shows that such an internal loss process can explain the observed variation of brightness distribution. Typical loss time scale is longer than strong diffusion limit, which suggests the existence of some pitch angle diffusion process such as wave‐particle interaction. Thus, variations of the total JSR flux density and thermospheric temperature seem consistent with the scenario, and the brightness distribution of JSR can be explained by the increase in radial diffusion accompanied by internal loss processes. First comprehensive study to reveal short‐term changes in Jovian radiation beltIntensity of Jovian synchrotron radiation and H3+varied with solar UV/EUV fluxIncrease in radial diffusion and internal loss process can explain radio image

Auroral kilometric radiation (AKR) is known to be transient emissions generated by rapidly accelerated electrons together with sudden auroral activation in the polar magnetosphere. In contrast, the characteristics and relationship with the auroral acceleration of rather continuous AKR emissions are not well understood. We examine the emission using long‐term data and report that the continuous AKR emission frequency changes with universal time (UT) as the Earth rotates, indicating that the Earth is a spin‐modulated variable radio source. The observed UT variation of AKR frequency means that the acceleration altitude changes periodically with planetary rotation. The observations indicate that the diurnal wobble of the tilted geomagnetic field in the solar wind flow alters the magnetosphere‐ionosphere (M‐I) coupling state in the polar magnetosphere, giving rise to periodic variation of auroral particle acceleration altitude. These observations of planetary radio wave properties provide insight into the physics of planetary particle acceleration. We found universal time control of AKR frequency.Earth is a spin‐modulated variable source.Acceleration altitude changes periodically.

Ionospheric Pedersen and Hall conductances play significant roles in electromagnetic coupling between the planetary ionosphere and magnetosphere. Several observations and models have suggested the existence of meteoric ions with interplanetary origins in the lower part of Jupiter’s ionosphere; however, no models have considered the contributions of meteoric ions to ionospheric conductance. This study is designed to evaluate the contribution of meteoric ions to ionospheric conductance by developing an ionospheric model combining a meteoroid ablation model and a photochemical model. We find that the largest contribution to Pedersen and Hall conductivities occurs in the meteoric ion layer at altitudes of 350–600 km due to the large concentration of meteoric ions resulting from their long lifetimes of more than 100 Jovian days. Pedersen and Hall conductances are enhanced by factors of 3 and 10, respectively, in the middle‐ and low‐latitude and auroral regions when meteoric ions are included. The distribution of Pedersen and Hall conductances becomes axisymmetric in the middle‐ and low‐latitude regions. Enhanced axisymmetric ionospheric conductance should impact magnetospheric plasma convection. The contribution of meteoric ions to the ionospheric conductance is expected to be important only on Jupiter in our solar system because of Jupiter’s intense magnetic and gravitational fields. Meteoric ions dominate the Jovian lower ionosphere due to their long lifetimesPedersen and Hall conductances are enhanced, and they are independent of local time due to the large concentrations of meteoric ionsThe contribution of meteoric ions to conductance results from their large densities and altitudinal coincidence with the conductive layer Meteoric ions dominate the Jovian lower ionosphere due to their long lifetimes Pedersen and Hall conductances are enhanced, and they are independent of local time due to the large concentrations of meteoric ions The contribution of meteoric ions to conductance results from their large densities and altitudinal coincidence with the conductive layer

We investigate the time and location where ULF waves and whistler‐mode chorus contributed to the net flux enhancement of relativistic electrons during the magnetic storm of May 2017. During the early recovery phase, both ULF and chorus waves contribute to the enhancement of relativistic electron fluxes, but ULF waves play roles of the inward diffusion. During the late recovery phase, both Van Allen Probe‐B and Arase show that whistler‐mode chorus contributes to the flux enhancement confined in the L‐value. The CRCM coupled with BATS‐R‐US simulation qualitatively reproduces the global evolution of ULF waves. Although the electron flux is underestimated by the simulation, this study reveals a large anisotropy of hot electrons in the region where whistler‐mode chorus waves were actually observed by satellites. In addition, the estimated magnetic field curvature on the dayside is small during the recovery phase. Furthermore, we investigate the control of wave evolution. Both observations and the simulation suggest that the observed ULF waves in the frequency range of ∼2–5 mHz are excited by the enhancement of the solar wind dynamic pressure. Observations also indicate that whistler‐mode chorus on the nightside is predominantly excited by hot electrons with temperature anisotropy, whereas the dayside chorus is enhanced by the change of the magnetic field line configuration. The estimated spatial distributions of electron anisotropy and magnetic field curvature provide an explanation for the presence of enhanced whistler‐mode chorus in the dusk sector, which is far from the usual location of wave generation. ULF waves play roles of radially inward diffusion during the early recovery phaseThe whistler‐mode chorus during the late recovery phase contributes to the flux enhancement confined in the L‐valueOn the dayside, the magnetic field curvature controls the occurrence of whistler‐mode chorus ULF waves play roles of radially inward diffusion during the early recovery phase The whistler‐mode chorus during the late recovery phase contributes to the flux enhancement confined in the L‐value On the dayside, the magnetic field curvature controls the occurrence of whistler‐mode chorus

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 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+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. Electromagnetic ion cyclotron (EMIC) waves in the magnetosphere have four major occurrence regions excited by possibly different generation processesFor He‐EMIC waves in the afternoon sector, injected particles and off‐equatorial sources are the major driver at L∼ 6 and L> 8, respectivelyH‐EMIC waves are generated by enhancing dynamic pressure in the noon sector at L< 6, or suprathermal protons in the dawn sector at L> 8 Electromagnetic ion cyclotron (EMIC) waves in the magnetosphere have four major occurrence regions excited by possibly different generation processes For He‐EMIC waves in the afternoon sector, injected particles and off‐equatorial sources are the major driver at L∼ 6 and L> 8, respectively H‐EMIC waves are generated by enhancing dynamic pressure in the noon sector at L< 6, or suprathermal protons in the dawn sector at L> 8

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 ∼109eV/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 ∼108eV/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. Stable auroral red (SAR) arcs are aurora with an optical red emission from oxygen atoms at latitudes slightly lower than the auroral oval and often occur during storm‐time substorms. The oxygen excitation is caused by low‐energy electrons transferred from the inner magnetosphere to the ionosphere. Past studies concluded that these low‐energy electrons were generated in the spatial overlap region between the plasmaspheric cold electrons and high‐energy ring‐current ions and suggested that Coulomb collisions are more plausible as the generation mechanism of SAR arcs than are electromagnetic waves. In this study, we report three SAR‐arc events observed by all‐sky imagers and inner magnetospheric satellites (Arase and Radiation Belt Storm Probes). We evaluated for the first time the three possible activation mechanisms using the data for full‐energy‐range plasma and electromagnetic wave data obtained by the satellites. As predicted previously, the calculated heat flux could have caused the SAR arcs in two of the events, but there were mixed cases in which electromagnetic waves were observed. When Arase flew above a freshly generated SAR arc, the flux of low‐energy electrons (<200 eV) was enhanced and various electromagnetic waves were observed, which may be responsible for its generation. Heat fluxes estimated from Radiation Belt Storm Probes (RBSP) reached the order of 109eV/cm2/s, indicating that Coulomb collisions could be the cause of stable auroral red (SAR) arcsThe RBSP‐A satellite observed local enhancements of 7–20‐mHz electromagnetic wave power above a SAR arcArase observed increases of electron fluxes (<200 eV) and various electromagnetic waves above a freshly detached SAR arc Heat fluxes estimated from Radiation Belt Storm Probes (RBSP) reached the order of 109eV/cm2/s, indicating that Coulomb collisions could be the cause of stable auroral red (SAR) arcs The RBSP‐A satellite observed local enhancements of 7–20‐mHz electromagnetic wave power above a SAR arc Arase observed increases of electron fluxes (<200 eV) and various electromagnetic waves above a freshly detached SAR arc

Quasi‐periodic variations of a few to several days are observed in the energetic plasma and magnetic dipolarization in Jupiter's magnetosphere. Variation in the plasma mass flux related to Io's volcanic activity is proposed as a candidate for the variety of the period. Using a long‐term monitoring of Jupiter's northern aurora by the Earth‐orbiting planetary space telescope Hisaki, we analyzed the quasi‐periodic variation seen in the auroral power integrated over the northern pole for 2014–2016, which included monitoring Io's volcanically active period in 2015 and the solar wind near Jupiter during Juno's approach phase in 2016. Quasi‐periodic variation with periods of 0.8–8 days was detected. The difference between the periodicities during volcanically active and quiet periods is not significant. Our data set suggests that the difference of period between volcanically active and quiet conditions is below 1.25 days. This is consistent with the expected difference estimated from a proposed relationship based on a theoretical model applied to the plasma variation of this volcanic event. The periodicity does not show a clear correlation with the auroral power, central meridional longitude, nor Io phase angle. The periodic variation is continuously observed in addition to the auroral modulation due to solar wind variation. Furthermore, Hisaki auroral data sometimes shows particularly intense auroral bursts of emissions lasting <10 h. We find that these bursts coincide with peaks of the periodic variations. Moreover, the occurrence of these bursts increases during the volcanically active period. This auroral observation links parts of previous observations to give a global view of Jupiter's magnetospheric dynamics. Quasi‐periodic variations of a few to several days seen in Jupiter's polar‐integrated northern aurora observed by HisakiAuroral bursts <10 h sometimes seen at peak of periodic variation, whose occurrence increases with Io's volcanic activityThis periodic variation additionally seen in aurora intensity enhancements associated with solar wind variations Quasi‐periodic variations of a few to several days seen in Jupiter's polar‐integrated northern aurora observed by Hisaki Auroral bursts <10 h sometimes seen at peak of periodic variation, whose occurrence increases with Io's volcanic activity This periodic variation additionally seen in aurora intensity enhancements associated with solar wind variations

A stable auroral red (SAR) arc is an aurora with a dominant 630 nm emission at subauroral latitudes. SAR arcs have been considered to occur due to the spatial overlap between the plasmasphere and the ring‐current ions. In the overlap region, plasmaspheric electrons are heated by ring‐current ions or plasma waves, and their energy is then transferred down to the ionosphere where it causes oxygen red emission. However, there have been no study conducted so far that quantitatively examined plasma and electromagnetic fields in the magnetosphere associated with SAR arc. In this paper, we report the first quantitative evaluation of conjugate measurements of a SAR arc observed at 2204 UT on 28 March 2017 and investigate its source region using an all‐sky imager at Nyrölä (magnetic latitude: 59.4°N), Finland, and the Arase satellite. The Arase observation shows that the SAR arc appeared in the overlap region between a plasmaspheric plume and the ring‐current ions and that electromagnetic ion cyclotron waves and kinetic Alfven waves were not observed above the SAR arc. The SAR arc was located at the ionospheric trough minimum identified from a total electron content map obtained by the GNSS receiver network. The Swarm satellite flying in the ionosphere also passed the SAR arc at ~2320 UT and observed a decrease in electron density and an increase in electron temperature during the SAR‐arc crossing. These observations suggest that the heating of plasmaspheric electrons via Coulomb collision with ring‐current ions is the most plausible mechanism for the SAR‐arc generation. A stable auroral red (SAR) arc is an aurora with an optical red emission at latitudes slightly lower than the auroral zone. SAR arcs have been considered to occur due to the spatial overlap between the low‐energy plasmaspheric electrons and the high‐energy ring‐current ions. In the overlap region, plasmaspheric electrons are heated by ring‐current ions or plasma waves, and their energy is then transferred down to the upper atmosphere to cause the red emission. However, there have been no study conducted so far that quantitatively examined plasma and electromagnetic fields in the magnetospheric source region of SAR arcs. In this paper, we report the first quantitative evaluation of a SAR arc using an all‐sky imager at Nyrölä, Finland, and the Arase satellite. The Arase observation shows that the SAR arc appeared in the overlap region between a plasmaspheric plume and the ring‐current ions in the inner magnetosphere. The electromagnetic waves associated with the SAR arc were not observed. These observations suggest that the heating of plasmaspheric electrons by ring‐current ions is the most plausible mechanism for the SAR‐arc generation. This result provides direct evidence of the previous theoretical expectation on the generation mechanism of red aurora at lower latitudes. The conjugate measurements of a detached SAR arc on 28 March 2017 are analyzed using observations from the Arase satelliteSwarm and GNSS‐TEC data show that the electron density decreased and the electron temperature increased in the ionosphere above the SAR arcThe observed plasmas and electromagnetic fields suggest that Coulomb collision is the most plausible mechanism for the SAR‐arc generation

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. Three different types of relativistic electron precipitation (REP) events are observed at International Space Station (ISS)Electromagnetic ion cyclotron waves were observed during a REP event at conjugate locations near the magnetic equatorWhistler mode waves were observed during other REP events at conjugate locations near the magnetic equator Several different kinds of plasma waves were identified in the magnetosphere as the possible cause of relativistic electron precipitation (REP) events at International Space Station (ISS).