Tomoki Kimura

Cassini observations of ion and electron beams at Saturn and their relationship to infrared auroral arcs

We present Cassini Visual and Infrared Mapping Spectrometer observations of infrared auroral emissions from the noon sector of Saturns ionosphere revealing multiple intense auroral arcs separated by dark regions poleward of the main oval. The arcs are interpreted as the ionospheric signatures of bursts of reconnection occurring at the dayside magnetopause. The auroral arcs were associated with upward field-aligned currents, the magnetic signatures of which were detected by Cassini at high planetary latitudes. Magnetic field and particle observations in the adjacent downward current regions showed upward bursts of 100–360 keV light ions in addition to energetic (hundreds of keV) electrons, which may have been scattered from upward accelerated beams carrying the downward currents. Broadband, upward propagating whistler waves were detected simultaneously with the ion beams. The acceleration of the light ions from low altitudes is attributed to wave-particle interactions in the downward current regions. Energetic (600 keV) oxygen ions were also detected, suggesting the presence of ambient oxygen at altitudes within the acceleration region. These simultaneous in situ and remote observations reveal the highly energetic magnetospheric dynamics driving some of Saturns unusual auroral features. This is the first in situ identification of transient reconnection events at regions magnetically conjugate to Saturns magnetopause.

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.

Asymmetric distribution of reconnection jet fronts in the Jovian nightside magnetosphere

Magnetic reconnection plays important roles in mass transport and energy conversion in planetary magnetospheres. It is considered that transient reconnection causes localized auroral arcs or spots in the Jovian magnetosphere, by analogy to the case in the Earths magnetosphere. However, the local structures of transient reconnection events (i.e., magnetospheric plasma parameters) and their spatial distribution have not been extensively investigated for the Jovian magnetosphere. Here we examine plasma velocity and density during strong north-south magnetic field events in the Jovian nightside magnetosphere, which may be associated with tail reconnection. We find prominent reconnection jet fronts predominantly on the dawnside of the nightside magnetosphere, which would be a signature unique to rotation-dominant planetary magnetospheres. The observed plasma structures are consistent with significant field-aligned currents which would generate localized aurora.

Weakening of Jupiter's main auroral emission during January 2014

In January 2014 Jupiters FUV main auroral oval decreased its emitted power by 70% and shifted equatorward by ∼1°. Intense, low-latitude features were also detected. The decrease in emitted power is attributed to a decrease in auroral current density rather than electron energy. This could be caused by a decrease in the source electron density, an order of magnitude increase in the source electron thermal energy, or a combination of these. Both can be explained either by expansion of the magnetosphere or by an increase in the inward transport of hot plasma through the middle magnetosphere and its interchange with cold flux tubes moving outward. In the latter case the hot plasma could have increased the electron temperature in the source region and produced the intense, low-latitude features, while the increased cold plasma transport rate produced the shift of the main oval.

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.

Magnetic reconnection in the Jovian tail: X-line evolution and consequent plasma sheet structures

Magnetic reconnection in planetary magnetospheres plays important roles in energy and mass transfer in the steady state, and also possibly in transient large-scale disturbances. In this paper we report observations of a reconnection event in the Jovian magnetotail by the Galileo spacecraft on 17 June 1997. In addition to the tailward retreat of a main X-line, signatures of recurrent X-line formations are found by close examination of energetic particle anisotropies. Furthermore, detailed analyses of multi-instrumental data for this period provide various spatiotemporal features in the plasma sheet. A significant density decrease was detected in the central plasma sheet, indicative of the transition to lobe (open field line) reconnection from plasma sheet (closed field line) reconnection. When Galileo vertically swept through the plasma sheet, a velocity layer structure was observed. We also analyze a strong southward magnetic field which is similar to dipolarization fronts observed in the terrestrial magnetotail: the ion flow (∼450 km s−1) was observed behind the magnetic front, whose thickness of 10000–20000 km was of the order of ion inertial length. The electron anisotropy in this period suggests an anomalously high-speed electron jet, implying ion-electron decoupling behind the magnetic front. Particle energization was also seen associated with these structures. These observations suggest that X-line evolution and consequent plasma sheet structures are similar to those in the terrestrial magnetosphere, whereas their generality in the Jovian magnetosphere and influence on the magnetospheric/ionospheric dynamics including transient auroral events need to be further investigated with more events.

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.

Auroral evidence of radial transport at Jupiter during January 2014

We present Jovian auroral observations from the 2014 January Hubble Space Telescope (HST) campaign and investigate the auroral signatures of radial transport in the magnetosphere alongside contemporaneous radio and Hisaki EUV data. HST FUV auroral observations on day 11 show, for the first time, a significantly superrotating polar spot poleward of the main emission on the dawnside. The spot transitions from the polar to main emission region in the presence of a locally broad, bright dawnside main emission feature and two large equatorward emission features. Such a configuration of the main emission region is also unreported to date. We interpret the signatures as part of a sequence of inward radial transport processes. Hot plasma inflows from tail reconnection are thought to flow planetward and could generate the superrotating spot. The main emission feature could be the result of flow shears from prior hot inflows. Equatorward emissions are observed. These are evidence of hot plasma injections in the inner magnetosphere. The images are thought to be part of a prolonged period of reconnection. Radio emissions measured by Wind suggest that hectometric (HOM) and non-Io decametric (DAM) signatures are associated with the sequence of auroral signatures, which implies a global magnetospheric disturbance. The reconnection and injection interval can continue for several hours.