Kentaro Yaji

Characteristics of hard X-ray double sources in impulsive solar flares

Abstract Imaging observations of solar flare hard X-ray sources with the Hard X-ray Telescope aboard the Yohkoh satellite have revealed that hard X-ray emissions (>30 keV) originate most frequently from double sources. The double sources are located on both sides of the magnetic neutral line, suggesting that the bulk of hard X-rays is emitted from footpoints of flaring magnetic loops. We also found that hard X-rays from the double sources are emitted simultaneously within a fraction of second and that the weaker source tends to be located in the stronger magnetic field region, showing a softer spectrum. Physical implications on the observed characteristics of the hard X-ray double sources are discussed.

OUTFLOW STRUCTURE OF THE QUIET SUN CORONA PROBED BY SPACECRAFT RADIO SCINTILLATIONS IN STRONG SCATTERING

Radio scintillation observations have been unable to probe flow speeds in the low corona where the scattering of radio waves is exceedingly strong. Here we estimate outflow speeds continuously from the vicinity of the Sun to the outer corona (heliocentric distances of 1.5-20.5 solar radii) by applying the strong scattering theory to radio scintillations for the first time, using the Akatsuki spacecraft as the radio source. Small, nonzero outflow speeds were observed over a wide latitudinal range in the quiet-Sun low corona, suggesting that the supply of plasma from closed loops to the solar wind occurs over an extended area. The existence of power-law density fluctuations down to the scale of 100 m was suggested, which is indicative of well-developed turbulence which can play a key role in heating the corona. At higher altitudes, a rapid acceleration typical of radial open fields is observed, and the temperatures derived from the speed profile show a distinct maximum in the outer corona. This study opened up a possibility of observing detailed flow structures near the Sun from a vast amount of existing interplanetary scintillation data.

A Superhot Flare Observed by Yohkoh

We report the first solar flare for which direct X-ray imaging (with focusing optics) has shown the location of superhot (T ~ 30 MK) plasma. This flare consists of two separate loop structures. The smaller structure dominates the soft X-ray emission, but the longer structure develops with persistently higher temperatures, which overlaps with low-energy hard X-ray sources with very soft spectra. Using a two-temperature model, we describe how the soft X-ray images lead to the inference of superhot temperatures in the latter structure, as we combine three broadband measurements at slightly different effective wavelengths.

Magnetic field configuration in impulsive solar flares revealed with Yohkoh and Nobeyama Radioheliograph

Abstract The results of an analysis of 25 solar impulsive flares, observed simultaneously with microwave and X-ray imaging instruments, are presented. From the relative locations of microwave, hard X-ray, and soft X-ray sources, we have found that, for fourteen out of the 25 cases, at least two loops are involved. Typical sizes of the two loops are different from each other; one is typically ≤ 20″ and the other 30″–80″. In seven out of remaining thirteen cases, circular polarization images in microwaves show the bipolar structure at the location of a HXR source, indicating the existence of a small loop. These observations suggest that in the majority of impulsive flare events a system of two or more loops (or bipoles) interacts with each other, releasing magnetic energy and producing energetic electrons. Detailed analysis of two out of the 25 events supports this result.

A Semi-analytical Model for Wind-fed Black Hole High-mass X-Ray Binaries: State Transition Triggered by Magnetic Fields from the Companion Star

We propose a mechanism of state transition in wind-fed black hole binaries (high-mass X-ray binaries) such as Cyg X-1 and LMC X-1. Modeling a line-driven stellar wind from the companion by two-dimensional hydrodynamical calculations, we investigate the processes of wind capture by and accretion onto the black hole. We assume that the wind acceleration is terminated at the He II ionization front because ions responsible for line-driven acceleration are ionized within the front, i.e. He III region. It is found that the mass accretion rate inferred from the luminosity is remarkably smaller than the capture rate. Considering the difference, we construct a model for the state transition based on the accretion flow being controlled by magneto-rotational instability. The outer flow is torus like, and plays an important role to trigger the transition. The model can explain why state transition does occur in Cyg X-1, while not in LMC X-1. Cyg X-1 exhibits a relatively low luminosity, and then the He II ionization front is located and can move between the companion and black hole, depending on its ionizing photon flux. On the other hand, LMC X-1 exhibits too high luminosity for the front to move considerably; the front is too close to the companion atmosphere. The model also predicts that each state of high-soft or low-hard would last fairly long because the luminosity depends weakly on the wind velocity. In the context of the model, the state transition is triggered by a fluctuation of the magnetic field when its amplitude becomes comparable to the field strength in the torus-like outer flow.

Loop-Loop Interaction in Impulsive Solar Flares Inferred from Microwave and X-Ray Images

We have analyzed fourteen solar impulsive flares simultaneously observed with microwave and X-ray imaging instruments (the Nobeyama Radioheliograph and the Yohkoh X-ray telescopes). All of the events analyzed are short-lived, consisting of a single or multiple spikes with each spike duration of about 10 s and showing quite hard, hard X-ray spectra with relatively small photon counts in the HXT L-band. Hence, we believe that these events reveal most clearly the behavior of energetic electrons without severely contamination by thermal emission from super-hot (~ 30MK) flare plasma.

Electron Acceleration Site of the 1992 Sep. 6 Flare

In this report, we present results of our analyses of M2.4 -class impulsive flare at 0514 UT on Sep. 6, 1992, based upon microwave, X-ray and optical observations.