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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−3 in the open magnetic field line regions and 5.0–20.5 cm−3 in 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−3 on 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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.Key Points: 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 [ABSTRACT FROM AUTHOR]