Hisaki Satellite Research Articles

Simulation of Dawn‐To‐Dusk Electric Field in the Jovian Inner Magnetosphere via Region 2‐Like Field‐Aligned Current

AbstractThe 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.

Jupiter’s X-ray aurora during UV dawn storms and injections as observed byXMM–Newton, Hubble, andHisaki

ABSTRACTWe present results from a multiwavelength observation of Jupiter’s northern aurorae, carried out simultaneously by XMM–Newton, the Hubble Space Telescope (HST), and the Hisaki satellite in 2019 September. HST images captured dawn storms and injection events in the far-ultraviolet aurora several times during the observation period. Magnetic reconnection occurring in the middle magnetosphere caused by internal drivers is thought to start the production of those features. The field lines then dipolarize, which injects hot magnetospheric plasma from the reconnection site to enter the inner magnetosphere. Hisaki observed an impulsive brightening in the dawnside Io plasma torus (IPT) during the final appearance of the dawn storms and injection events, which is evidence that a large-scale plasma injection penetrated the central IPT between 6 and 9RJ (Jupiter radii). The extreme ultraviolet aurora brightened and XMM–Newton detected an increase in the hard X-ray aurora count rate, suggesting an increase in electron precipitation. The dawn storms and injections did not change the brightness of the soft X-ray aurora and they did not ‘switch-on’ its commonly observed quasi-periodic pulsations. Spectral analysis of the X-ray aurora suggests that the precipitating ions responsible for the soft X-ray aurora were iogenic and that a power-law continuum was needed to fit the hard X-ray part of the spectra. The spectra coincident with the dawn storms and injections required two power-law continua to get good fits.

Spatially Asymmetric Increase in Hot Electron Fraction in the Io Plasma Torus During Volcanically Active Period Revealed by Observations by Hisaki/EXCEED From November 2014 to May 2015

AbstractThe satellite Io, which has volcanoes and is located at 5.9 RJ from the center of Jupiter, is a powerful plasma source in the magnetosphere. The heavy ions originating from Io form a torus‐like structure and emit radiation. The pickup energy and hot electrons are believed to power the Io plasma torus. Voyager data showed that a trace amount of hot electrons (at several hundreds of eV) exist in the torus. The origin of hot electrons, that is, plasma heating and/or transport mechanisms, have been mentioned in previous research. However, the contribution of each mechanism toward supplying hot electrons remains poorly understood. To address this issue, we explored the time variation and spatial structure of hot electrons by spectroscopic observations using the Hisaki satellite. In this study, the radial distributions of plasma densities and temperatures were derived from the emission line intensities in the extreme ultraviolet range of day of year (DOY) 331 in 2014 to DOY 134 in 2015, which includes the Io's volcanically active period. We found that hot electrons inside the torus began to increase particularly on the duskside ~40 days after the onset of volcanic activity. This result suggests that the mass increase in the torus with volcanic activity enhanced the plasma transport from the outside within a specific region or via a local heating process.

Transient Change of Io's Neutral Oxygen Cloud and Plasma Torus Observed by Hisaki

AbstractIo's atmospheric oxygen atoms are heated by atmospheric sputtering and escape from Io's gravity, forming a neutral oxygen cloud around Io's orbit. This neutral cloud is important as a source of the Io oxygen plasma torus. Previous studies derived the distribution and density of the equilibrium neutral oxygen cloud. However, little is known about the evolution of the neutral cloud. In this study, we analyzed Hisaki satellite observations of the spatial distribution of OI 130.4 nm emissions around Io's orbit during transient strong density enhancement in the torus in 2015 (called high density period). Comparing time variations of OI and OII 83.4 nm emissions, we estimated that the lifetimes of O+ in this period were about 21 days in the high density period and 41 days in the normal density period. Hisaki observations are consistent with a decrease in the lifetime of O+ when the density in the torus increases. The radial distribution showed the neutral oxygen cloud spread outward up to 8.6 Jupiter radii during the high density period. We also show that during the high density period, the neutral oxygen number density at Io's orbit (where north‐south thickness is assumed to be 1.2 Jupiter radii) increased to cm−3, more than three times the value during the normal density period ( cm−3). The azimuthal distribution showed a dense region around Io and a longitudinally uniform, diffuse region distributed along Io's orbit that enlarges during the high density period.

Azimuthal Variation in the Io Plasma Torus Observed by the Hisaki Satellite From 2013 to 2016

AbstractIn the Jovian magnetosphere, sulfur and oxygen ions supplied by the satellite Io are distributed in the so‐called Io plasma torus. The plasma torus is located in the inner area of the magnetosphere and the plasma in the torus corotates with the planet. The density and the temperature of the plasma in the torus have significant azimuthal variations. In this study, data from three‐year observations obtained by the Hisaki satellite, from December 2013 to August 2016, were used to investigate statistically the azimuthal variations and to find out whether the variations were influenced by the increase in neutral particles from Io. The azimuthal variation was obtained from a time series of sulfur ion line ratios, which were sensitive to the electron temperature and the sulfur ion mixing ratio S3+/S+. The major characteristics of the azimuthal variation in the plasma parameters were consistent with the dual hot electron model, proposed to explain previous observations. On the other hand, the Hisaki data showed that the peak System III longitude in the S3+/S+ ratio was located not only around 0°–90°, as in previous observations, but also around 180°–270°. The rotation period, the System IV periodicity, was sometimes close to the Jovian rotation period. Persistent input of energy to electrons in a limited longitude range of the torus is associated with the shortening of the System IV period.

Development of ground pipeline system for high-level scientific data products of the Hisaki satellite mission and its application to planetary space weather

The Hisaki satellite is the first-ever space telescope mission dedicated to planetary sciences. Atmospheres and magnetospheres of our solar system planets are continuously monitored by the extreme ultraviolet (EUV) spectrometer onboard Hisaki. This paper describes a data pipeline system developed for processing high-level scientific and ancillary data products from the Hisaki mission. The telemetry data downlinked from the satellite are stored in a ground telemetry database, processed in the pipeline to imaging spectral data with a 1-min temporal resolution and ancillary data products, and then archived in a public database. The imaging spectra can be further reduced to higher-level data products for practical scientific use. For example, light curves of the power emitted from Jupiter’s aurora and plasma torus with a temporal resolution of 10-min can be reduced from the imaging spectral data; the reduced light curves reveal the transport processes of energy and mass in Jupiter’s magnetosphere and associated interplanetary solar wind conditions. Continuous monitoring with Hisaki will contribute considerably to our understanding of space weather relating to planets in our solar system.

Corotation of Bright Features in the Io Plasma Torus

AbstractElectron energy distribution in the Io plasma torus (IPT) is non‐Maxwellian. The “hot” components induce extreme ultraviolet radiation, although their energy source remains unknown. One potential mechanism that may preserve the energy of hot electrons is inwardly directed plasma motion in the Jovian magnetosphere. Therefore, understanding the high‐energy component of the electron energy distribution is important. The extreme ultraviolet spectrometer onboard the HISAKI satellite has started the observation of the IPT. We show that bright transient features in one ansa of the IPT correlate with those in the other ansa after 5 hr. Because it takes 5 hr (one half of the rotation cycle of Jupiter) for a batch of plasma to move from one ansa to the other, the correlation indicates that the transient features are identical and that they survive for greater than 5 hr. Since the time scale of the radiative cooling process is ~3 hr, this fact suggests that injected hot electrons survive against cooling via Coulomb collision with ambient electrons for greater than 2 hr. Assuming the relationship with the cooling time, we can deduce the hot electron temperature from the brightening duration. Here we report the occasional hot electron injections, presumed to exceed 150 up to 650 eV, into the IPT (approximately 15% out of all events). For the most of events, the temperature of injected electron is lower than 150 eV.

Enhancement of the Jovian Magnetospheric Plasma Circulation Caused by the Change in Plasma Supply From the Satellite Io

AbstractThe innermost Galilean satellite, Io, supplies a large amount of volcanic gasses to the Jovian magnetosphere. The fast rotation of Jupiter and the outward transport of ionized gasses are responsible for forming a huge and rotationally dominant magnetosphere. The plasma supply from the satellite has a key role in the characterization of the Jovian magnetosphere. In fact, significant variations of the plasma population in the inner magnetosphere caused by the volcanic eruptions in Io were found in early 2015, using a continuous data set of the Io plasma torus obtained from an extreme ultraviolet spectroscope onboard the Hisaki satellite. The time evolution of the Io plasma torus radial distribution showed that the outward transport of plasma through 8 RJ from Jupiter was enhanced for approximately 2 months (from the end of January to the beginning of April 2015). Intense short‐lived auroral brightenings––which represent transient energy releases in the outer part of the magnetosphere—occurred frequently during this period. The short‐lived auroral brightenings accompanied well‐defined sporadic enhancements of the ion brightness in the plasma torus, indicating a rapid inward transport of energy from the outer part of the magnetosphere and the resultant enhancement of hot electron population in the inner magnetosphere. This evidently shows that the change in a plasma source in the inner magnetosphere affects a large‐scale radial circulation of mass and energy in a rotationally dominant magnetosphere.

The Oxygen Neutral Cloud Surrounding Jupiter’s Volcanic Moon

Japan’s Hisaki satellite takes measurements of faint oxygen emissions from Io.

Spatial Distribution of Io's Neutral Oxygen Cloud Observed by Hisaki

AbstractWe report on the spatial distribution of a neutral oxygen cloud surrounding Jupiter's moon Io and along Io's orbit observed by the Hisaki satellite. Atomic oxygen and sulfur in Io's atmosphere escape from the exosphere mainly through atmospheric sputtering. Some of the neutral atoms escape from Io's gravitational sphere and form neutral clouds around Jupiter. The extreme ultraviolet spectrograph called EXCEED (Extreme Ultraviolet Spectroscope for Exospheric Dynamics) installed on the Japan Aerospace Exploration Agency's Hisaki satellite observed the Io plasma torus continuously in 2014–2015, and we derived the spatial distribution of atomic oxygen emissions at 130.4 nm. The results show that Io's oxygen cloud is composed of two regions, namely, a dense region near Io and a diffuse region with a longitudinally homogeneous distribution along Io's orbit. The dense region mainly extends on the leading side of Io and inside of Io's orbit. The emissions spread out to 7.6 Jupiter radii (RJ). Based on Hisaki observations, we estimated the radial distribution of the atomic oxygen number density and oxygen ion source rate. The peak atomic oxygen number density is 80 cm−3, which is spread 1.2 RJ in the north‐south direction. We found more oxygen atoms inside Io's orbit than a previous study. We estimated the total oxygen ion source rate to be 410 kg/s, which is consistent with the value derived from a previous study that used a physical chemistry model based on Hisaki observations of ultraviolet emission ions in the Io plasma torus.

Response of Jupiter's Aurora to Plasma Mass Loading Rate Monitored by the Hisaki Satellite During Volcanic Eruptions at Io

AbstractThe production and transport of plasma mass are essential processes in the dynamics of planetary magnetospheres. At Jupiter, it is hypothesized that Io's volcanic plasma carried out of the plasma torus is transported radially outward in the rotating magnetosphere and is recurrently ejected as plasmoid via tail reconnection. The plasmoid ejection is likely associated with particle energization, radial plasma flow, and transient auroral emissions. However, it has not been demonstrated that plasmoid ejection is sensitive to mass loading because of the lack of simultaneous observations of both processes. We report the response of plasmoid ejection to mass loading during large volcanic eruptions at Io in 2015. Response of the transient aurora to the mass loading rate was investigated based on a combination of Hisaki satellite monitoring and a newly developed analytic model. We found that the transient aurora frequently recurred at a 2–6 day period in response to a mass loading increase from 0.3 to 0.5 t/s. In general, the recurrence of the transient aurora was not significantly correlated with the solar wind, although there was an exceptional event with a maximum emission power of ~10 TW after the solar wind shock arrival. The recurrence of plasmoid ejection requires the precondition that an amount comparable to the total mass of magnetosphere, ~1.5 Mt, is accumulated in the magnetosphere. A plasmoid mass of more than 0.1 Mt is necessary in case that the plasmoid ejection is the only process for mass release.

The time variation of atomic oxygen emission around Io during a volcanic event observed with Hisaki/EXCEED

Io has an atmosphere produced by volcanism and sublimation of frosts deposited around active volcanoes. However, the time variation of atomic oxygen escaping Io's atmosphere is not well known. In this paper, we show a significant increase in atomic oxygen around Io during a volcanic event. Brightening of Io's extended sodium nebula was observed in the spring of 2015. We used the Hisaki satellite to investigate the time variation of atomic oxygen emission around Io during the same period. This investigation reveals that the duration of atomic oxygen brightness increases from a volcanically quiet level to a maximum level during the same approximate time period of 30 days as the observed sodium brightness. On the other hand, the recovery of the atomic oxygen brightness from the maximum to the quiet level (60 days) was longer than that of the sodium nebula decreasing (40 days). Additionally, a dawn-dusk asymmetry of the atomic oxygen emission is observed.

Three-year of observations of Jupiter’s aurora and Io plasma torus variabilities by earth orbiting extreme-ultraviolet spectroscope HISAKI

Extreme Ultraviolet spectrograph, EXCEED, on-board the HISAKI satellite is designed for observing tenuous gas and plasma around planets in the solar system. It enables us to obtain continuous and long-term data set and find time variability in the planetary magnetosphere and ionosphere with time scales of several hours to months. Here, we introduce new findings of Jupiter’s UV aurora and plasma emissions from the Io plasma torus obtained from the HISAKI observation.

Transient brightening of Jupiter's aurora observed by the Hisaki satellite and Hubble Space Telescope during approach phase of the Juno spacecraft

AbstractIn early 2014, continuous monitoring with the Hisaki satellite discovered transient auroral emission at Jupiter during a period when the solar wind was relatively quiet for a few days. Simultaneous imaging made by the Hubble Space Telescope (HST) suggested that the transient aurora is associated with a global magnetospheric disturbance that spans from the inner to outer magnetosphere. However, the temporal and spatial evolutions of the magnetospheric disturbance were not resolved because of the lack of continuous monitoring of the transient aurora simultaneously with the imaging. Here we report the coordinated observation of the aurora and plasma torus made by Hisaki and HST during the approach phase of the Juno spacecraft in mid‐2016. On day 142, Hisaki detected a transient aurora with a maximum total H2 emission power of ~8.5 TW. The simultaneous HST imaging was indicative of a large “dawn storm,” which is associated with tail reconnection, at the onset of the transient aurora. The outer emission, which is associated with hot plasma injection in the inner magnetosphere, followed the dawn storm within less than two Jupiter rotations. The monitoring of the torus with Hisaki indicated that the hot plasma population increased in the torus during the transient aurora. These results imply that the magnetospheric disturbance is initiated via the tail reconnection and rapidly expands toward the inner magnetosphere, followed by the hot plasma injection reaching the plasma torus. This corresponds to the radially inward transport of the plasma and/or energy from the outer to the inner magnetosphere.

The geocoronal responses to the geomagnetic disturbances

AbstractAtomic hydrogen atoms in the terrestrial exosphere resonantly scatter solar Lyman alpha (121.6 nm) radiation, observed as the hydrogen geocorona. Measurements of scattered solar photons allow us to probe time‐varying distributions of exospheric hydrogen atoms. The Hisaki satellite with the extreme ultraviolet spectrometer (EXtreme ultraviolet spectrosCope for ExosphEric Dynamics: EXCEED) was launched in September 2013. EXCEED acquires spectral images (52–148 nm) of the atmospheres/magnetospheres of planets from Earth orbit. Due to its low orbital altitude (~1000 km), the images taken by the instrument also contain the geocoronal emissions. In this context, EXCEED has provided quasi‐continuous remote sensing observations of the geocorona with high temporal resolution (~1 min) since 2013. These observations provide a unique database to determine the long‐term behavior of the exospheric density structure. In this paper, we report exospheric structural responses observed by EXCEED to geomagnetic disturbances. Several geomagnetic storms with decreases of Dst index occurred in February 2014 and the Lyman alpha column brightness on the night side of the Earth increased abruptly and temporarily by approximately 10%. Hisaki reveal that the time lag between the peaks of the magnetic activity and the changes in the Lyman alpha column brightness is found to be about 2 to 6 h during storms. In order to interpret the observational results, we evaluate quantitatively the factors causing the increase. On the basis of these results, a coupling effect via charge exchange between the exosphere and plasmasphere causes variations of the exospheric density structure.

Response of Jupiter's inner magnetosphere to the solar wind derived from extreme ultraviolet monitoring of the Io plasma torus

AbstractBecause Jupiter's magnetosphere is huge and is rotationally dominated, solar wind influence on its inner part has been thought to be negligible. Meanwhile, dawn‐dusk asymmetric features of this region have been reported. Presence of dawn‐to‐dusk electric field is one of the leading explanations of the asymmetry; however, the physical process of generating such an intense electric field still remains unclear. Here we present long and continuous monitoring of the extreme ultraviolet emissions from the Io plasma torus in Jupiter's inner magnetosphere made by the Hisaki satellite between December 2013 and March 2014. We found five occasions where the dusk/dawn brightness ratio was enhanced above 2.5 in response to rapid increase of the solar wind dynamic pressure. The enhancement is achieved as the dusk region brightens and the dawn region dims. The observation indicates that dawn‐to‐dusk electric field in the inner magnetosphere is enhanced under compressed conditions.

Characteristics of solar wind control on Jovian UV auroral activity deciphered by long‐term Hisaki EXCEED observations: Evidence of preconditioning of the magnetosphere?

AbstractWhile the Jovian magnetosphere is known to have the internal source for its activity, it is reported to be under the influence of the solar wind as well. Here we report the statistical relationship between the total power of the Jovian ultraviolet aurora and the solar wind properties found from long‐term monitoring by the spectrometer EXCEED (Extreme Ultraviolet Spectroscope for Exospheric Dynamics) on board the Hisaki satellite. Superposed epoch analysis indicates that auroral total power increases when an enhanced solar wind dynamic pressure hits the magnetosphere. Furthermore, the auroral total power shows a positive correlation with the duration of a quiescent interval of the solar wind that is present before a rise in the dynamic pressure, more than with the amplitude of dynamic pressure increase. These statistical characteristics define the next step to unveil the physical mechanism of the solar wind control on the Jovian magnetospheric dynamics.

Jupiter's X‐ray and EUV auroras monitored by Chandra, XMM‐Newton, and Hisaki satellite

AbstractJupiter's X‐ray auroral emission in the polar cap region results from particles which have undergone strong field‐aligned acceleration into the ionosphere. The origin of precipitating ions and electrons and the time variability in the X‐ray emission are essential to uncover the driving mechanism for the high‐energy acceleration. The magnetospheric location of the source field line where the X‐ray is generated is likely affected by the solar wind variability. However, these essential characteristics are still unknown because the long‐term monitoring of the X‐rays and contemporaneous solar wind variability has not been carried out. In April 2014, the first long‐term multiwavelength monitoring of Jupiter's X‐ray and EUV auroral emissions was made by the Chandra X‐ray Observatory, XMM‐Newton, and Hisaki satellite. We find that the X‐ray count rates are positively correlated with the solar wind velocity and insignificantly with the dynamic pressure. Based on the magnetic field mapping model, a half of the X‐ray auroral region was found to be open to the interplanetary space. The other half of the X‐ray auroral source region is magnetically connected with the prenoon to postdusk sector in the outermost region of the magnetosphere, where the Kelvin‐Helmholtz (KH) instability, magnetopause reconnection, and quasiperiodic particle injection potentially take place. We speculate that the high‐energy auroral acceleration is associated with the KH instability and/or magnetopause reconnection. This association is expected to also occur in many other space plasma environments such as Saturn and other magnetized rotators.

Local electron heating in the Io plasma torus associated with Io from HISAKI satellite observation

AbstractIo‐correlated brightness change in the Io plasma torus (IPT) was discovered by the Voyager spacecraft, showing evidence of local electron heating around Io. However, its detailed properties and the cause of electron heating are still open issues. The extreme ultraviolet spectrograph on board the HISAKI satellite continuously observed the IPT from the end of December 2013 to the middle of January 2014. The variation in the IPT brightness showed that clear periodicity associated with Io's orbital period (42 h) and that the bright region was located downstream of Io. The amplitude of the periodic variation was larger at short wavelengths than at long wavelengths. From spectral analyses, we found that Io‐correlated brightening is caused by the increase in the hot electron population in the region downstream of Io. We also found that the brightness depends on the system III longitude and found primary and secondary peaks in the longitude ranges of 100–130° and 250–340°, respectively. Io's orbit crosses the center of the IPT around these longitudes. This longitude dependence suggests that the electron heating process is related to the plasma density around Io. The total radiated power from the IPT in January 2014 was estimated to be 1.4 TW in the wavelength range from 60 to 145 nm. The Io‐correlated component produced 10% of this total radiated power. The interaction between Io and the IPT continuously produces a large amount of energy around Io, and 140 GW of that energy is immediately converted to hot electron production in the IPT.

Field-of-View Guiding Camera on the HISAKI (SPRINT-A) Satellite

HISAKI (SPRINT-A) satellite is an earth-orbiting Extreme UltraViolet (EUV) spectroscopic mission and launched on 14 Sep. 2013 by the launch vehicle Epsilon-1. Extreme ultraviolet spectroscope (EXCEED) onboard the satellite will investigate plasma dynamics in Jupiter’s inner magnetosphere and atmospheric escape from Venus and Mars. EUV spectroscopy is useful to measure electron density and temperature and ion composition in plasma environment. EXCEED also has an advantage to measure spatial distribution of plasmas around the planets. To measure radial plasma distribution in the Jovian inner magnetosphere and plasma emissions from ionosphere, exosphere and tail separately (for Venus and Mars), the pointing accuracy of the spectroscope should be smaller than spatial structures of interest (20 arc-seconds). For satellites in the low earth orbit (LEO), the pointing displacement is generally caused by change of alignment between the satellite bus module and the telescope due to the changing thermal inputs from the Sun and Earth. The HISAKI satellite is designed to compensate the displacement by tracking the target with using a Field-Of-View (FOV) guiding camera. Initial checkout of the attitude control for the EXCEED observation shows that pointing accuracy kept within 2 arc-seconds in a case of “track mode” which is used for Jupiter observation. For observations of Mercury, Venus, Mars, and Saturn, the entire disk will be guided inside slit to observe plasma around the planets. Since the FOV camera does not capture the disk in this case, the satellite uses a star tracker (STT) to hold the attitude (“hold mode”). Pointing accuracy during this mode has been 20–25 arc-seconds. It has been confirmed that the attitude control works well as designed.