Satoshi Kasahara

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.

The Energization and Radiation in Geospace (ERG) Project

The Energization and Radiation in Geospace (ERG) project for solar cycle 24 will explore how relativistic electrons in the radiation belts are generated during space storms. This geospace exploration project consists of three research teams: the ERG satellite observation team, the ground-based network observation team, and the integrated data analysis/simulation team. Satellite observation will provide in situ measurements of features such as the plasma distribution function, electric and magnetic fields, and plasma waves, whereas remote sensing by ground-based observations using, for example, HF radars, magnetometers, optical instruments, and radio wave receivers will provide the global state of the geospace. Various kinds of data will be integrated and compared with numerical simulations for quantitative understanding. Such a synergetic approach is essential for comprehensive understanding of relativistic electron generation/loss processes through crossenergy and cross-regional coupling in which different plasma populations and regions are dynamically coupled with each other. In addition, the ERG satellite will utilize a new and innovative measurement technique for wave-particle interactions that can directly measure the energy exchange process between particles and plasma waves. In this paper, we briefly review some of the profound problems regarding relativistic electron accelerations and losses that will be solved by the ERG project, and we provide an overview of the project.

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.

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.

Geospace exploration project ERG

The Exploration of energization and Radiation in Geospace (ERG) project explores the acceleration, transport, and loss of relativistic electrons in the radiation belts and the dynamics for geospace storms. This project consists of three research teams for satellite observation, ground-based network observation, and integrated data analysis/simulation. This synergetic approach is essential for obtaining a comprehensive understanding of the relativistic electron generation/loss processes of the radiation belts as well as geospace storms through cross-energy/cross-regional couplings, in which different plasma/particle populations and regions are strongly coupled with each other. This paper gives an overview of the ERG project and presents the initial results from the ERG (Arase) satellite.

Simultaneous entry of oxygen ions originating from the Sun and Earth into the inner magnetosphere during magnetic storms

became observable in the energy window of Polar were transported from the high-latitude magnetopause to the inner magnetosphere when the convection electric field was strong. When the convection electric field was weak, the ions were reflected toward the distant tail. The O 6+ ions that became observable in the energy window of Geotail were sufficiently transported from the low-latitude magnetopause to the near-Earth magnetotail regardless of the strength of the convection electric field. The observational facts may be adequately explained in terms of ion transport paths depending on the convection electric field with different entry points.

Field-aligned beams and reconnection in the jovian magnetotail

Abstract The release of plasma in the jovian magnetotail is observed in the form of plasmoids, travelling compression regions, field-aligned particle beams and flux-rope like events. We demonstrate that electrons propagate along the magnetic field lines in the plasma sheet boundary layer (PSBL), while close to the current sheet center the electron distribution is isotropic. The evidences of the counterstreaming electron beams in the PSBLs are also presented. Most of the field-aligned energetic ion beams are associated with the field-aligned electron beams and about half of them have the bipolar fluctuation of the meridional magnetic field component. Moreover they often show a normal velocity dispersion for the different species which fits well in the scenario of particle propagation from a single source. All features above are observed during jovian reconfiguration events which are typically bonded with plasma flow reversals. From all these characteristics, which are based on energetic particle and magnetic field measurements, we believe that the reconfiguration processes in the jovian magnetotail are associated with reconnection.

Pulsating aurora from electron scattering by chorus waves

Auroral substorms, dynamic phenomena that occur in the upper atmosphere at night, are caused by global reconfiguration of the magnetosphere, which releases stored solar wind energy. These storms are characterized by auroral brightening from dusk to midnight, followed by violent motions of distinct auroral arcs that suddenly break up, and the subsequent emergence of diffuse, pulsating auroral patches at dawn. Pulsating aurorae, which are quasiperiodic, blinking patches of light tens to hundreds of kilometres across, appear at altitudes of about 100 kilometres in the high-latitude regions of both hemispheres, and multiple patches often cover the entire sky. This auroral pulsation, with periods of several to tens of seconds, is generated by the intermittent precipitation of energetic electrons (several to tens of kiloelectronvolts) arriving from the magnetosphere and colliding with the atoms and molecules of the upper atmosphere. A possible cause of this precipitation is the interaction between magnetospheric electrons and electromagnetic waves called whistler-mode chorus waves. However, no direct observational evidence of this interaction has been obtained so far. Here we report that energetic electrons are scattered by chorus waves, resulting in their precipitation. Our observations were made in March 2017 with a magnetospheric spacecraft equipped with a high-angular-resolution electron sensor and electromagnetic field instruments. The measured quasiperiodic precipitating electron flux was sufficiently intense to generate a pulsating aurora, which was indeed simultaneously observed by a ground auroral imager.