Takeshi Sakanoi

Traveling ionospheric disturbances detected in the FRONT Campaign

The F-region Radio and Optical measurement of Nighttime TID (FRONT) campaign was conducted to clarify the non-classical features of traveling ionospheric disturbances (TIDs) at mid-latitudes in May, 1998 and August, 1999. A cluster of all-sky CCD cameras and a GPS receiver network observed a wide area of the ionosphere over Japan to detect the spatial structure and temporal evolution of TIDs. The propagation direction of the nighttime TID detected during the FRONT campaign periods is restricted to the southwest. The time evolution of their amplitude indicates that the TID structure is intensified as it travels from high-latitudes to low-latitudes. The significant coincidence between the structures of 630 nm band airglow and total electron content indicates that the perturbations take place in the bottomside of the ionospheric F region. Coherent echoes from the field-aligned irregularities were observed by the MU radar in the nights when the TID activity was high.

Traveling ionospheric disturbances observed in the OI 630‐nm nightglow images over Japan by using a Multipoint Imager Network during the FRONT Campaign

Pilot observations using a network of five all-sky imagers (ASIs) were conducted during the new moon period of May 19–22, 1998 as part of the F-region Radio and Optical measurement of Nighttime TID (FRONT) campaign. The network observation enabled us to track propagation of medium-scale traveling ionospheric disturbances (TIDs) in the OI 630-nm nightglow over a distance of more than 2500 km. The TIDs were observed every night during the campaign period, but occurrence was limited from evening to midnight. They have horizontal wavelengths of 200–600 km, travel a horizontal distance of more than 1000 km, and last for more than three hours. In every case, the TIDs moved southwestward with a velocity of 83–137 m/s. Using dual-site TID images, the altitude of the TID structures in the 630-nm nightglow was calculated to be ∼260 km, which corresponds to the bottom side of the mid-latitude ionospheric F layer.

Relationship between field‐aligned currents and inverted‐V parallel potential drops observed at midaltitudes

The inverted-V field-aligned acceleration region existing in the altitude range of several thousand kilometers plays an essential role for the magnetosphere-ionosphere coupling system. The adiabatic plasma theory predicts a linear relationship between field-aligned current density (J∥) and parallel potential drop (Φ∥), that is, J∥ = KΦ∥, where K is the field-aligned conductance. We examined this relationship using the charged particle and magnetic field data obtained from the Akebono (Exos D) satellite. The potential drop above the satellite was derived from the peak energy of downward electrons, while the potential drop below the satellite was derived from two different methods: the peak energy of upward ions and the energy-dependent widening of electron loss cone. On the other hand, field-aligned current densities in the inverted-V region were estimated from the Akebono magnetometer data. Using these potential drops and field-aligned current densities, we estimated the linear field-aligned conductance KJΦ. Further, we obtained the corrected field-aligned conductance KJΦC by applying the full Knights formula to the current-voltage relationship. We also independently estimated the field-aligned conductance KTN from the number density and the thermal temperature of magnetospheric source electrons which were obtained by fitting accelerated Maxwellian functions for precipitating electrons. The results are summarized as follows: (1) The latitudinal dependence of parallel potential drops is characterized by a narrow V-shaped structure with a width of 0.4°−1.0°. (2) Although the inverted-V potential region exactly corresponds to the upward field-aligned current region, the latitudinal dependence of upward current intensity is an inverted-U shape rather than an inverted-V shape. Thus it is suggested that the field-aligned conductance KJΦC changes with a V-shaped latitudinal dependence. In many cases, KJΦC values at the edge of the inverted-V region are about 5–10 times larger than those at the center. (3) By comparing KJΦC with KTN, KJΦC is found to be about 2–20 times larger than KTN. These results suggest that low-energy electrons such as trapped electrons, secondary and back-scattered electrons, and ionospheric electrons significantly contribute to upward field-aligned currents in the inverted-V region. It is therefore inferred that non adiabatic pitch angle scattering processes play an important role in the inverted-V region.

Relation between fine structure of energy spectra for pulsating aurora electrons and frequency spectra of whistler mode chorus waves

We investigate the origin of the fine structure of the energy spectrum of precipitating electrons for the pulsating aurora (PsA) observed by the low-altitude Reimei satellite. The Reimei satellite achieved simultaneous observations of the optical images and precipitating electrons of the PsA from satellite altitude (~620 km) with resolution of 40 ms. The main modulation of precipitation, with a few seconds, and the internal modulations, with a few hertz, that are embedded inside the main modulations are identified above ~3 keV. Moreover, stable precipitations at ~1 keV are found for the PsA. A “precipitation gap” is discovered between two energy bands. We identify the origin of the fine structure of the energy spectrum for the precipitating electrons using the computer simulation on the wave-particle interaction between electrons and chorus waves. The lower band chorus (LBC) bursts cause the main modulation of energetic electrons, and the generation and collapse of the LBC bursts determines on-off switching of the PsA. A train of rising tone elements embedded in the LBC bursts drives the internal modulations. A close set of upper band chorus (UBC) waves causes the stable precipitations at ~1 keV. We show that a wave power gap around the half gyrofrequency at the equatorial plane in the magnetosphere between LBC and UBC reduces the loss rate of electrons at the intermediate energy range, forming a gap of precipitating electrons in the ionosphere.

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.

Primary Black Hole Spin in OJ 287 as Determined by the General Relativity Centenary Flare

OJ 287 is a quasi-periodic quasar with roughly 12 year optical cycles. It displays prominent outbursts that are predictable in a binary black hole model. The model predicted a major optical outburst in 2015 December. We found that the outburst did occur within the expected time range, peaking on 2015 December 5 at magnitude 12.9 in the optical R-band. Based on Swift/XRT satellite measurements and optical polarization data, we find that it included a major thermal component. Its timing provides an accurate estimate for the spin of the primary black hole,

Multiscale temporal variations of pulsating auroras: On‐off pulsation and a few Hz modulation

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Low-energy ion precipitation structures associated with pulsating auroral patches

. The present outburst also confirms the established general relativistic properties of the system such as the loss of orbital energy to gravitational radiation at the 2% accuracy level, and it opens up the possibility of testing the black hole no-hair theorem with 10% accuracy during the present decade.

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

A statistical study on the cross-scale property on the temporal variations of pulsating aurora intensity was conducted on 53 events observed at the Poker Flat Research Range during the period from 1 December 2011 to 1 March 2012. The observed modulation frequency ranged from 1.5 to 3.3 Hz, and strong modulations were not seen in the frequency range higher than about 3 Hz. This suggests that the time of flight of electrons has a time-smoothing effect on the more rapid variations above 3 Hz. Furthermore, the frequency of modulation showed relatively strong correlation to auroral intensity (correlation coefficient of 0.58), and it can be explained with nonlinear wave growth theory, in which the modulation frequency increases with the wave amplitude of the whistler mode chorus. In contrast, the on-off pulsations showed no significant correlations with auroral intensity. This result probably implies that several different plasma processes with different time scales from nonlinear wave growth should be taken into account when determining the on-off periods. In particular, we suggest that long-term variations in the cold plasma density play a dominant role in controlling the conditions of wave-particle interactions that have temporal scale of the on-off pulsation periods.

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

Pulsating auroras often appear in forms of geo-stable or slowly convecting “patches.” These patches can maintain their rough shape and size over many sequences of luminosity pulsations, yet they slowly drift with ionospheric E × B convection. Because of these characteristics, there has long been a speculation that the pulsating auroral patch (PAP) is connected to flux tubes filled with enhanced cold plasma. In this study, we perform a survey on pulsating auroral events when the footprints of low-Earth-orbit satellites traversed the PAPs, with a focus on the low-energy particle signatures associated with the PAPs. As a result, we identified, in a majority (~2/3) of events, the existence of a low-energy ion precipitation structure that is collocated with the PAP, with core energies ranging from several tens of eV up to a few hundred eV. This result supports the hypothesis that a PAP connects to flux tubes filled with enhanced cold plasma. We further propose that the plasma outflows from the ionosphere are the origin of such cold plasma flux tubes. We suggest that the PAP is formed by a combination of high-energy electrons of a magnetospheric origin, the low-energy plasma structure of an ionospheric origin, and certain ELF/VLF waves that are intensified and modulated in interactions with both the hot and cold plasma populations.