Yasumasa Kasaba

Repeated injections of energy in the first 600?ms of the giant flare of SGR?1806 - 20

The massive flare of 27 December 2004 from the soft γ-ray repeater SGR 1806–20, a possible magnetar, saturated almost all γ-ray detectors, meaning that the profile of the pulse was poorly characterized. An accurate profile is essential to determine physically what was happening at the source. Here we report the unsaturated γ-ray profile for the first 600 ms of the flare, with a time resolution of 5.48 ms. The peak of the profile (of the order of 107 photons cm-2 s-1) was reached ∼50 ms after the onset of the flare, and was then followed by a gradual decrease with superposed oscillatory modulations possibly representing repeated energy injections with ∼60-ms intervals. The implied total energy is comparable to the stored magnetic energy in a magnetar (∼ 1047 erg) based on the dipole magnetic field intensity (∼ 1015 G), suggesting either that the energy release mechanism was extremely efficient or that the interior magnetic field is much stronger than the external dipole field.On December 27, 2004, plasma particle detectors on the GEOTAIL spacecraft detected an extremely strong signal of hard X-ray photons from the giant flare of SGR1806-20, a magnetar candidate. While practically all gamma-ray detectors on any satellites were saturated during the first ~500 ms interval after the onset, one of the particle detectors on GEOTAIL was not saturated and provided unique measurements of the hard X-ray intensity and the profile for the first 600 ms interval with 5.48 ms time resolution. After ~50 ms from the initial rapid onset, the peak photon flux (integrated above ~50 keV) reached the order of 10^7 photons sec^{-1} cm^{-2}. Assuming a blackbody spectrum with kT=175 keV, we estimate the peak energy flux to be 21 erg sec^{-1} cm^{-2} and the fluence (for 0-600 ms) to be 2.4 erg cm^{-2}. The implied energy release comparable to the magnetic energy stored in a magnetar (~10^{47} erg) suggests an extremely efficient energy release mechanism.

Statistical studies of plasma waves and backstreaming electrons in the terrestrial electron foreshock observed by Geotail

We present statistical studies of the direction finding of 2f p radiation and the spatial distribution of plasma waves and energetic particles in the terrestrial electron foreshock observed by Geotail. First, we investigate the geometry of the electron foreshock which is assumed to be the 2f p radio source. The 2f p radio source is likely to be in the leading region of the electron foreshock where the most intense Langmuir waves are observed. The Langmuir wave activities and the population of energetic electrons gradually decrease in the region beyond 10 R E from the contact point. The decreasing rate of Langmuir wave activity is very small, about 10 -0.008 /R E . We also find that in the region around the contact point of the tangential interplanetary magnetic field (IMF) lines and the bow shock, the observed amplitude of the 2f p radiation seems to become weak. We think that it is due to the weak activities of Langmuir waves in the region close to the contact point and/or the directivity of 2f p radiation along the tangential IMF line. Next, we investigate the influence of the solar wind conditions on the activities in the electron foreshock. We confirm a positive correlation of the 2f p radio activity with the solar wind kinetic energy flux and a decrease of 2f p radio activity with decreasing IMF cone angle resulting in IMF lines tangent to the far flank of the bow shock. The 2f p radio activity is more affected by both parameters than the amplitude of Langmuir waves is affected. This suggests that the 2f p radio emissivity is very sensitive to the energy of Langmuir waves as expected from the generation process of 2f p radiation. Such high sensitivity also supports the concentration of the radio emissivity in the leading region of the electron foreshock and the limitation of the radio source extension along the magnetic field line. We also reinvestigate the comparison between the terrestrial and Venusian foreshocks. The differences between them suggest the nonsimilarity of shocks with different sizes.

One‐ and two‐dimensional simulations of electron beam instability: Generation of electrostatic and electromagnetic 2f p waves

We have performed computer simulations of the self-consistent nonlinear evolution of electrostatic and electromagnetic 2f p waves excited by electron beams with electromagnetic particle code. In both one- and two-dimensional periodic systems an electrostatic 2f p wave is generated at twice the wave number of forward propagating Langmuir waves by wave-wave coupling. This wave grows with the forward propagating Langmuir wave in the nonlinear stage of the simulations. The electrostatic 2f p wave in the simulations is saturated at about -20 ∼ -30 dB of that of the Langmuir waves. It is larger than the value expected from observations in the terrestrial electron foreshock. The electromagnetic 2f p wave is only excited in two-dimensional systems. The magnitude of the electromagnetic 2f p wave is correlated with the backward propagating Langmuir wave, not with the electrostatic 2f p wave. This result suggests that the electromagnetic 2f p wave is excited by the wave-wave coupling of forward and backward propagating Langmuir waves. The typical power density estimated from a reasonable amplitude of Langmuir wave is of the same order or much weaker than the value typically observed around the electron foreshock.

Plasma waves in the upstream and bow shock regions observed by GEOTAIL

Abstract Upstream waves in the foreshock of the Earths bow shock are the manifestation of microscopic plasma dynamics caused by the solar wind interaction with the bow shock. Though the past wave observation has revealed many interesting features of the foreshock plasma waves, the lack of waveform observation could not provide sufficient information to understand the micro-physics of wave-particle interactions beyond certain points. The present paper describes the preliminary results of the GEOTAIL plasma wave observation in the upstream as well as in the bow shock regions. The waveform capture (WFC) receiver has revealed interesting waveforms in these regions which are to provide a clue of understanding the microphysics involved in the wave generation in the upstream and bow shocks. Based on the observed information, we classify the electron and ion foreshock regions into more detailed structures. We also have carried out some simple computer simulations to understand some of the observed wave phenomena.

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.

The BepiColombo/MMO model payload and operation plan

Abstract The Institute of Space and Astronautical Science (ISAS) of Japan plans to contribute the Mercury Magnetospheric Orbiter (MMO) to the BepiColombo program, the ESA Cornerstone mission to the planet Mercury. The principal objective of the MMO is to study the magnetic field and magnetosphere of Mercury. The ISAS Mercury exploration working group has performed the definition study of the MMO mission in cooperation with the ESA/ESTEC BepiColombo project team. This paper briefly reviews the scientific objectives, and describes the model payload and its operation plan.

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.

The angular distribution of auroral kilometric radiation observed by the GEOTAIL spacecraft

We examine the angular distribution of auroral kilometric radiation (AKR) based on a 38-month data set of GEOTAIL plasma wave observations. We show that the AKR illumination pattern becomes narrower with increasing frequency above 300 kHz. The difference is more evident in the duskside zone and when geomagnetic conditions become more disturbed. We also show that AKR is more active on the winter hemisphere especially for the higher frequency range.

Initial observations of auroras by the multi-spectral auroral camera on board the Reimei satellite

The small Japanese scientific satellite Reimei was launched successfully from Baikonur Space Center into a sun-synchronous (noon-midnight meridian) polar orbit at an altitude of ∼630 km on August 23, 2005. Auroral fine structure observations have been performed by the multi-spectral auroral camera (MAC) and electron/ion spectrum analyzers (ESA/ISA) on board Reimei. The MAC is a three channel camera system used to simultaneously observe the temporal dynamics of the auroral fine structure at the wavelengths of 428 nm (N2+ 1NG), 558 nm (O), and 670 nm (N2 1PG). Since its successful launch, Reimei has been operated continuously and produced lots of unique data on the auroral fine structures with the MAC and ESA/ISA. The initial observation data from MAC has shown the temporal dynamics of auroral fine structure, such as the vortex street, appearing in the poleward edge of the auroral oval. The significant differences in the auroral shapes between the MAC channels suggest the existence of different emission processes. Further, the height profiles of the aurora and airglow are clearly seen in the image data in the limb direction. It is expected that the various mechanisms and dynamics of auroral fine structures can be investigated by analyzing the Reimei observation data.