Chihiro Tao

Magnetic field variations in the Jovian magnetotail induced by solar wind dynamic pressure enhancements

[1] In order to understand the response of the Jovian magnetosphere to solar wind dynamic pressure enhancements, we investigate magnetic field variations observed by the Galileo spacecraft. The lack of solar wind monitoring just upstream of the Jovian magnetosphere is overcome by simulating a one-dimensional magnetohydrodynamic (MHD) propagation of the solar wind from the Earth. We identify the events with an increase of the solar wind dynamic pressure >0.25 nPa at the Jovian orbit. Characteristic magnetic field variations are found in the Jovian magnetosphere for all of the nine events. The rectangular waveform due to the Jovian rotation disappears for eight of the nine events. Magnetic field disturbances in the frequency range from 0.3 to 10 mHz are enhanced simultaneously. The maximum amplitude of the disturbances is in proportional to the maximum amplitude of the solar wind dynamic pressure. We suggest that the current sheet is greatly deformed and reconnection bursts are induced under the compressed magnetosphere.

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.

Rotational modulation and local time dependence of Saturn's infrared H3+ auroral intensity

[1]xa0Planetary auroral emissions reveal the configuration of magnetospheric field-aligned current systems. In this study, Cassini Visual and Infrared Mapping Spectrometer (VIMS) observations of Saturns pre-equinox infrared H3+ aurorae were analysed to show (a) rotational modulation of the auroral intensity in both hemispheres and (b) a significant local time dependence of the emitted intensity. The emission intensity is modulated by the ‘planetary period’ rotation of auroral current systems in each hemisphere. The northern auroral intensity also displays a lesser anti-phase dependence on the southern rotating current system, indicating that part of the southern current system closes in the northern hemisphere. The southern hemisphere aurorae were most intense in the post-dawn sector, in agreement with some past measurements of auroral field-aligned currents, UV aurora and SKR emitted power. A corresponding investigation of the northern hemisphere auroral intensity reveals a broader dawn-noon enhancement, possibly due to the interaction of the southern rotating current system with that of the north. The auroral intensity was reduced around dusk and post-midnight in both hemispheres. These observations can be explained by the interaction of a rotating field-aligned current system in each hemisphere with one fixed in local time, which is related to the solar wind interaction with magnetospheric field lines.

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.

Neutral wind control of the Jovian magnetosphere‐ionosphere current system

[1]xa0In order to clarify the role of neutral dynamics in the Jovian magnetosphere-ionosphere-thermosphere coupling system, we have developed a new numerical model that includes the effect of neutral dynamics on the coupling current. The model calculates axisymmetric thermospheric dynamics and ion composition by considering fundamental physical and chemical processes. The ionospheric Pedersen current is obtained from the thermospheric and ionospheric parameters. The model simultaneously solves the torque equations of the magnetospheric plasma due to radial currents flowing at the magnetospheric equator, which enables us to update the electric field projected onto the ionosphere and the field-aligned currents (FACs) depending upon the thermospheric dynamics. The self-consistently calculated temperature and ion velocity are consistent with observations. The estimated neutral wind field captures the zonally averaged characteristics in previous three-dimensional models. The energy extracted from the planetary rotation is mainly used for magnetospheric plasma acceleration below 73.5° latitude while consumed in the upper atmosphere, mainly by Joule heating at above 73.5° latitude. The neutral wind dynamics contributes to a reduction in the electric field of 22% compared with the case of neutral rigid corotation. About 90% of this reduction is attributable to neutral winds below the 550-km altitude in the auroral region. The calculated radial current in the equatorial magnetosphere is smaller than observations. This indicates that the enhancement of the background conductance and/or the additional radial current at the outer boundary would be expected to reproduce the observed current.

Long‐term modulations of Saturn's auroral radio emissions by the solar wind and seasonal variations controlled by the solar ultraviolet flux

[1]xa0Saturns auroral activities have been suggested to be controlled by the seasonal variations of the polar ionospheric conductivities and atmospheric conditions associated with the solar extreme ultraviolet (EUV) flux. However, they have not yet been explained self-consistently by only the seasonal solar EUV effects. This study investigates the long-term variations of Saturnian Kilometric Radiation (SKR) as a proxy of the auroral activities, which were observed by Cassinis Radio and Plasma Wave Science experiment mostly during the southern summer (DOY (day of year) 001 2004 to DOY 193 2010). We deduced the height distribution of the SKR source region in the Northern (winter) and Southern (summer) Hemispheres from the remote sensing of SKR spectra. The peak spectral density of the southern (summer) SKR was found to be up to 100 times greater than that of the northern (winter) SKR, and the altitude of the peak flux was similar (∼ 0.8 Rs) in the Northern and Southern Hemispheres. The spectral densities in both hemispheres became comparable with each other around equinox in August 2009. These results suggest a stronger SKR source region during the summer than the winter related to the seasonal EUV effect, which is opposite to the trend observed in the Earths kilometric radiation. A long-term correlation analysis was performed for the SKR, solar EUV flux, and solar wind parameters extrapolated from Earths orbit by an magnetohydrodynamical simulation focusing on variations on timescales longer than several weeks. We confirmed clear positive correlations between the solar wind dynamic pressure and peak flux density in both the Southern and Northern Hemispheres during the declining phase of the solar cycle. We conclude that the solar wind variations on the timescale of the solar cycle control the SKR source region. In addition, it was also confirmed that the south-to-north ratios of SKR power flux and source altitudes are positively correlated with the solar EUV flux. This result strongly supports a seasonal EUV effect on the SKR source region. The variations of SKR activity over both seasonal and solar cycles are discussed in comparison to the terrestrial case.