Search

The electromagnetic interaction between Europa and the plasma sheet in the Jovian magnetosphere generates Alfvén waves, ultimately generating auroral footprints in Jupiter's atmosphere. The position of Europa's auroral footprint is a proxy for travel time of the Alfvén waves. We measured Europa's footprint position using the far‐ultraviolet images of Jupiter obtained by the Hubble Space Telescope in two observing campaigns in 2014 and 2022. The measured footprint position indicates a longer Alfvén travel time in the 2022 campaign. We retrieved the plasma sheet parameters at Europa's orbit from the footprint position by tracing the Alfvén waves launched at Europa and found an increase of both mass density and temperature in the plasma sheet in 2022. The Poynting flux generated at Europa is calculated with the retrieved plasma sheet parameters, which suggests the total energy transfer from Europa to its auroral footprint is similar to the case of Io. Plain Language Summary: Europa is an obstacle to the plasma corotating with Jupiter's magnetosphere. Through the interaction between Europa and the magnetospheric plasma flow, Alfvén waves are launched at Europa. The Alfvén waves propagate along the field line and ultimately generate auroral emissions at locations distant from the instantaneous magnetic footprint of Europa. The position of Europa's auroral footprint is a proxy for the travel time of the Alfvén waves. We measured the position of Europa's auroral footprint using the far‐ultraviolet images of Jupiter obtained by the Hubble Space Telescope in two observing campaigns in 2014 and 2022. We found large deviations of the footprint position between the two observing campaigns. By tracing the Alfvén waves launched at Europa, we retrieved plasma mass density and temperature at Europa's orbit from the measured footprint position. It is revealed that time variation in the plasma mass density and temperature caused the deviations in the footprint position. We also calculated the Poynting flux generated at Europa using the retrieved plasma parameters and found that the total energy transfer from Europa to its auroral footprint is similar to the case of Io. Key Points: We measured the equatorial lead angle of Europa's auroral footprint in Jupiter's atmosphere with the HST data taken in 2014 and 2022Plasma conditions at Europa's orbit are retrieved from the measured lead angle by tracing the Europa‐originated Alfvén wavesChanges in the plasma conditions at Europa's orbit can account for the variation of the footprint lead angle [ABSTRACT FROM AUTHOR]

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]

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]

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]

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]

Abstract: Temporal variation of Jupiter's northern aurora during enhanced Io volcanic activity was detected using the EXCEED spectrometer on board the Hisaki Earth‐orbiting planetary space telescope. It was found that in association with reported Io volcanic events in early 2015, auroral power and estimated field‐aligned currents were enhanced during day of year 40–120. Furthermore, the far ultraviolet color ratio decreased during the event, indicating a decrease of auroral electron mean energy and total acceleration by <30%. During the episode of enhanced Io volcanic activity, Jupiter's magnetosphere contains more source current via increased suprathermal plasma density by up to 42%; therefore, it would have required correspondingly less electron acceleration to maintain the enhanced field‐aligned current and corotation enforcement current. Sporadic large enhancements in auroral emission detected more frequently during the active period could have been contributed by nonadiabatic magnetospheric energization. [ABSTRACT FROM AUTHOR]

Abstract: We developed a new numerical model of the Jovian magnetosphere–ionosphere coupling current system in order to investigate the effects of diurnal variation of ionospheric conductance. The conductance is determined by ion chemical processes that include the generation of hydrogen and hydrocarbon ions by solar EUV radiation and auroral electrons precipitation. The model solves the torque equations for magnetospheric plasma accelerated by the radial currents flowing along the magnetospheric equator. The conductance and magnetospheric plasma then change the field-aligned currents (FACs) and the intensity of the electric field projected onto the ionosphere. Because of the positive feedback of the ionospheric conductance on the FAC, the FAC is the maximum on the dayside and minimum just before sunrise. The power transferred from the planetary rotation is mainly consumed in the upper atmosphere on the dayside, while it is used for magnetospheric plasma acceleration in other local time (LT) sectors. Further, our simulations show that the magnetospheric plasma density and mass flux affect the temporal variation in the peak FAC density. The enhancement of the solar EUV flux by a factor of 2.4 increases the FAC density by 30%. The maximum density of the FAC is determined not only by the relationship between the precipitating electron flux and ionospheric conductance, but also by the system inertia, i.e., the inertia of the magnetospheric plasma. A theoretical analysis and numerical simulations reveal that the FAC density is in proportion to the planetary angular velocity on the dayside and to the square of the planetary angular velocity on the nightside. When the radial current at the outer boundary is fixed at values above 30MA, as assumed in previous model studies, the peak FAC density determined at latitude 73°–74° is larger than the diurnal variable component. This result suggests large effects of this assumed radial current at the outer boundary on the system. [Copyright &y& Elsevier]

We observed Jupiter's infrared aurora using the Subaru Infrared Camera and Spectrometer (IRCS) with adaptive optics, in order to compare horizontal and vertical emission profiles among H 3 + fundamental, H 3 + overtone, and H 2 emissions. The fundamental and overtone lines show similar horizontal distributions around the main auroral oval for both hemispheres. There is no correlation between the fundamental lines and temperature; however, a correlation seems to exist for the overtone lines. Although both the fundamental and overtone lines are related to electron precipitation, there are differences in the precipitating electron energies that influence the emissions, because of their different emission altitudes. In addition, the morphological difference between the H 2 and H 3 + emission structures is significant. That is, the intensity of the H 3 + emission is enhanced around the main oval, but that of the H 2 emission is not. The peak altitudes of the H 3 + fundamental, H 3 + overtone, and H 2 are 650, 870, and 830km for the north and 700, 910, and 950km for the south, with the uncertainties of approximately 100km. This finding, that the fundamental emission peak altitude is found to be lower than those of the overtone and H 2 emissions, is consistent with the theoretical models and derived temperatures for the emission peak altitude. We also find similar peak altitudes for the H 2 and H 3 + overtone emissions. This is in accordance with previous observations, which have revealed that the altitude of H 2 is higher than the model-based expectation. Collisional de-excitation of H 2 can dominate at lower altitudes, which might explain the higher emission peak altitude compared to that of the model. [ABSTRACT FROM AUTHOR]