Go Murakami

Transient internally driven aurora at Jupiter discovered by Hisaki and the Hubble Space Telescope

Jupiters auroral emissions reveal energy transport and dissipation through the planets giant magnetosphere. While the main auroral emission is internally driven by planetary rotation in the steady state, transient brightenings are generally thought to be triggered by compression by the external solar wind. Here we present evidence provided by the new Hisaki spacecraft and the Hubble Space Telescope that shows that such brightening of Jupiters aurora can in fact be internally driven. The brightening has an excess power up to similar to 550 GW. Intense emission appears from the polar cap region down to latitudes around Ios footprint aurora, suggesting a rapid energy input into the polar region by the internal plasma circulation process.

Telescope of extreme ultraviolet (TEX) onboard SELENE: science from the Moon

The Upper Atmosphere and Plasma Imager (UPI) is to be launched in 2007 and sent to the Moon. From the lunar orbit, two telescopes are to be directed towards the Earth. The Moon has no atmosphere, which results in there being no active emission near the spacecraft; consequently, we will have a high-quality image of the near-Earth environment. As the Moon orbits the Earth once a month, the Earth will also be observed from many different directions. This is called a “science from the Moon”. The two telescopes are mounted on a two-axis gimbal system, the Telescope of Extreme ultraviolet (TEX) and Telescope of Visible light (TVIS). TEX detects the O II (83.4 nm) and He II (30.4 nm) emissions scattered by ionized oxygen and helium, respectively. The targets of extreme-ultraviolet (EUV) imaging are the polar ionosphere, the polar wind, and the plasmasphere and inner magnetosphere. The maximum spatial and time resolutions are 0.09 Re and 1 min, respectively.

Variation of Jupiter's aurora observed by Hisaki/EXCEED: 2. Estimations of auroral parameters and magnetospheric dynamics

Jupiters auroral parameters are estimated from observations by a spectrometer EXCEED (Extreme Ultraviolet Spectroscope for Exospheric Dynamics) onboard JAXAs Earth-orbiting planetary space telescope Hisaki. EXCEED provides continuous auroral spectra covering the wavelength range over 80–148 nm from the whole northern polar region. The auroral electron energy is estimated using a hydrocarbon color ratio adopted for the wavelength range of EXCEED, and the emission power in the long wavelength range 138.5–144.8 nm is used as an indicator of total emitted power before hydrocarbon absorption and auroral electron energy flux. The quasi-continuous observations by Hisaki provide the auroral electron parameters and their relation under different auroral activity levels. Short- (within one planetary rotation) enhancements of auroral power accompany increases of the electron number flux rather than the electron energy variations. The relationships between the auroral electron energy (~70–400 keV) and flux (1026–1027 /s, 0.08–0.9 μA/m2) estimated from the observations over a 40-day interval are in agreement with field-aligned acceleration theory when incorporating probable magnetospheric parameters. Applying the electron acceleration theory to each observation point, we explore the magnetospheric source plasma variation during these power-enhanced events. Possible scenarios to explain the derived variations are (i) an adiabatic variation of the magnetospheric plasma under a magnetospheric compression and/or plasma injection, and (ii) a change of the dominant auroral component from the main emission (main aurora) to the emission at the open-closed boundary.

Evidence for global electron transportation into the jovian inner magnetosphere

Hot electron plasma moves in from Io Scientists have known that solar radiation ionizes the gases from Ios volcanoes to create a torus of plasma around Jupiter, but how that plasma moves is unclear. To investigate this, Yoshioka et al. monitored the temperature of the hot electron plasma as a function of distance from the planet with the Hisaki Earth-orbiting satellite. The fraction of hot electrons decreases only gradually with distance from Jupiter, which implies a rapid resupply of these electrons from outside Ios orbit. Science, this issue p. 1581 Near-Earth satellite measurements in the extreme ultraviolet examine a charged torus produced by volcanoes on Jupiter’s moon Io. Jupiter’s magnetosphere is a strong particle accelerator that contains ultrarelativistic electrons in its inner part. They are thought to be accelerated by whistler-mode waves excited by anisotropic hot electrons (>10 kiloelectron volts) injected from the outer magnetosphere. However, electron transportation in the inner magnetosphere is not well understood. By analyzing the extreme ultraviolet line emission from the inner magnetosphere, we show evidence for global inward transport of flux tubes containing hot plasma. High-spectral-resolution scanning observations of the Io plasma torus in the inner magnetosphere enable us to generate radial profiles of the hot electron fraction. It gradually decreases with decreasing radial distance, despite the short collisional time scale that should thermalize them rapidly. This indicates a fast and continuous resupply of hot electrons responsible for exciting the whistler-mode waves.

Response of Jupiter's auroras to conditions in the interplanetary medium as measured by the Hubble Space Telescope and Juno

We present the first comparison of Jupiters auroral morphology with an extended, continuous and complete set of near-Jupiter interplanetary data, revealing the response of Jupiters auroras to the interplanetary conditions. We show that for ∼1-3 days following compression region onset the planets main emission brightened. A duskside poleward region also brightened during compressions, as well as during shallow rarefaction conditions at the start of the program. The power emitted from the noon active region did not exhibit dependence on any interplanetary parameter, though the morphology typically differed between rarefactions and compressions. The auroras equatorward of the main emission brightened over ∼10 days following an interval of increased volcanic activity on Io. These results show that the dependence of Jupiters magnetosphere and auroras on the interplanetary conditions are more diverse than previously thought.

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

While 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

Jupiters 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 Jupiters 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.

Conjunction study of plasmapause location using ground-based magnetometers, IMAGE-EUV, and Kaguya-TEX data

[1] A statistical study comparing the plasmapause location determined using extreme ultraviolet (EUV) and cross-phase measurements was performed over 50 days in May-July 2000 and 1 day in May 2008. In EUV images the plasmapause location was estimated using the sharp gradient in the brightness of 30.4 nm He + emission. We have taken EUV images obtained by the IMAGE and the Kaguya satellites, which were operated in a solar maximum and minimum periods, respectively. In the ground-based cross-phase measurement, the plasmapause was defined as a steep drop of mass density in its radial profile. Mass density was inferred from the eigenfrequency of field line resonances in the ULF band (∼1―1000 mHz), which was deduced from geomagnetic field data using cross-phase analysis. The two measurements of the plasmapause have been compared in a same meridian at the same time and very good agreement was found in 18 of 19 events. Our result clearly indicates that the He + and mass density plasmapause are usually detected at the same place with the error range of ± 0.4 R E . In only one event, the He + and the mass density defined plasmapauses were not colocated. This event may be due to the difference of refilling time between He + and other dominant species.

High-resolution imaging detector using five microchannel plates and a resistive anode encoder

We have developed a high-resolution imaging detector with five microchannel plates (MCPs) in a set of V and Z stacks and a resistive anode encoder (RAE) for future space applications. In a position-sensitive system with a RAE, the spatial resolution depends on the signal-to-noise ratios at the anode terminals. Therefore, a high and stable electron gain of MCPs allows the position determination of each photoelectron event with a high spatial resolution. We investigated the effect of the potentials applied to the detector on the pulse height distribution (PHD) and the spatial resolution by means of calculations and experiments. The calculations showed that the negative interstack potential reduced the size of the electron cloud at the Z-stack input by approximately 80%. The result suggests that, under such a condition, the Z-stack MCP is operated in the completely saturated mode and exhibits a narrow PHD. On the other hand, in the measurements, applying the negative interstack potential reduced the width of the PHD by approximately 60%. As a result, the spatial resolution of 45 microm, corresponding to 480x480 pixels, was achieved. The results enable us to optimize and apply the technique to future missions.

Development of Mg/SiC multilayer mirrors

A multilayer coating mirror of Mo/Si is usually used for space science in the spectral range of extreme ultraviolet (EUV), especially for He-II (30.4 nm) radiation, because it is highly stable under vacuum and atmosphere. It has the fairly high reflectivity of 15-20%. But the space science community needs the coating of higher reflectivity at 30.4 nm radiation for the future satellite missions, especially for the small satellite (to reduce the size of optics). In this work, for developing a new multilayer mirror for He-II radiation, we report the performance of a multilayer consisting of Mg/SiC and the aging in reflectivity under atmosphere and vacuum.