清水 敏文

By examining the magnetic structure at sites in the bright coronal interiors of active regions that are not flaring but exhibit persistent strong coronal heating, we establish some new characteristics of the magnetic origins of this heating. We have examined the magnetic structure of these sites in five active regions, each of which was well observed by both the Yohkoh SXT and the Marshall Space Flight Center Vector Magnetograph and showed strong shear in its magnetic field along part of at least one neutral line (polarity inversion). Thus, we can assess whether this form of nonpotential field structure in active regions is a characteristic of the enhanced coronal heating and vice versa. From 27 orbits' worth of Yohkoh SXT images of the five active regions, we have obtained a sample of 94 persistently bright coronal features (bright in all images from a given orbit), 40 long (≳ 20,000 km) neutral-line segments having strong magnetic shear throughout (shear angle greater than 45°), and 39 long neutral-line segments having weak magnetic shear throughout (shear angle less than 45°). From this sample, we find that (1) all of our persistently bright coronal features are rooted in magnetic fields that are stronger than 150 G, (2) nearly all (95%) of these enhanced coronal features are rooted near neutral lines (closer than 10,000 km), (3) a great majority (80%) of the bright features are rooted near strong-shear portions of neutral lines, (4) a great majority (85%) of long strong-shear segments of neutral lines have persistently bright coronal features rooted near them, (5) a large minority (40%) of long weak-shear segments of neutral lines have persistently bright coronal features rooted near them, and (6) the brightness of a persistently bright coronal feature often changes greatly over a few hours. From these results, we conclude that most persistent enhanced heating of coronal loops in active regions (1) requires the presence of a polarity inversion in the magnetic field near at least one of the loop footpoints, (2) is greatly aided by the presence of strong shear in the core magnetic field along that neutral line, and (3) is controlled by some variable process that acts in this magnetic environment. We infer that this variable process is low-lying reconnection accompanying flux cancellation. © 1997. The American Astronomical Society. All rights reserved.

A XUV Doppler telescope was developed using multilayer optics. A high-spectral resolution is achieved by adopting the multilayer materials. Whole-sun images were obtained in an XUV emission line, Fe XIV 211Å to carry out the velocity-field measurement with detection limit of 100 km/s.

We present the development status of the normal incidence XUV multilayer mirrors for XUV Doppler Telescope, which observes coronal velocity fields of the whole Sun. The telescope has two narrow band-pass multilayer mirrors tuned to slightly longer and shorter wavelengths around the Fe XIV line at 211.3 Å. From the intensity difference of the images taken with these two bands, we can obtain Dopplergram of 1.8 MK plasma of the whole Sun. It is required that the multilayer has high wavelength-resolution (λ/Δλ ∼ 30 per mirror), anti-reflection coating for intense He II 304 Å emission line and high d-spacing uniformity of ∼ 1%.

© 1996 SPIE. All rights reserved. We present an overview of an ongoing Japanese sounding rocket project with the Solar XUV Doppler telescope. The telescope employs a pair of normal incidence multilayer mirrors and a back-thinned CCD, and is designed to observe coronal velocity field of the whole sun by measuring line-of-sight Doppler shifts of the Fe XIV 211 A line. The velocity detection limit is estimated to be better than 100 km/s. The telescope will be launched by the Institute of Space and Astronautical Science (ISAS) in 1998, when the solar activity is going to be increasing towards the cycle 23 activity maximum. Together with the overview of the telescope, the current status of the development of each telescope components including multilayer mirrors, telescope structure, image stabilization mechanism, and focal plane assembly, are reviewed. The observation sequence during the flight is also briefly described.

We present recent observations of quiet regions near the center of the solar disk using the Advanced Stokes Polarimeter. These observations reveal a component of the solar magnetic field heretofore unobserved: isolated, small-scale (typically 1″-2″ or smaller), predominantly horizontal magnetic flux structures in the solar photosphere. These features occur in isolation of the well-known, nearly vertical flux concentrations usually seen in the photospheric "network." Hence we ascribe this horizontal flux to the photospheric "internetwork." They reveal themselves by the distinct signature of the Stokes Q and U polarization profiles, which are symmetric about the line center. The polarization signals are weak, with peak amplitudes typically ∼0.1%-0.2% of the continuum intensity in the resolved spectral profiles, but they are well above the noise level of these observations (≈0.05%). Such magnetic fields are weak (significantly less than 1000 G) and largely horizontal owing to the absence, or near absence, of accompanying Stokes V polarization when observed at the center of the solar disk. These horizontal field elements are often associated with blueshifted Stokes line profiles, and they often occur between regions of opposite polarity (but weak) Stokes V profiles. The horizontal elements are short-lived, typically lasting ∼5 minutes. Our observations suggest that we are viewing the emergence of small, concentrated loops of flux, carried upward either by granular convection or magnetic buoyancy. Even though these entities show weak field strengths, they also seem to be fairly common, implying that they could carry the order of 1024 Mx of magnetic flux to the surface on a daily basis. However, further observational study is needed to identify the specific nature of this phenomenon. © 1996. The American Astronomical Society. All rights reserved.

We present the results of a search for radio emission from active-region transient brightenings identified in Yohkoh soft X-ray telescope observations of active region AR 7260. We present detailed observations of four events in which 17 GHz radio emission is clearly detected in observations by the Nobeyama radioheliograph. The time profiles of the 17 GHz data are very similar to those of the soft X-ray fluxes, and the 17 GHz flux is very close to that expected from plasma with the temperature and emission measure derived for the soft X-ray-emitting material from filter ratios. No impulsive nonthermal radio emission was detected from any of the four events, although each was at least GOES class B1 in soft X-rays. Weak hard X-rays may have been detected by GRO/BATSE from the strongest of the events, but not from two others. These negative results leave open the possibility that there is a difference between active region transient brightenings and solar flares, in that the former do not convert a significant amount of the released energy into accelerated electrons. However, confirmation of this hypothesis will require a larger sample of events.

Spectroscopic observations of coronal emission lines were carried out at the Norikura Solar Observatory in cooperation with the soft X-ray telescope on board the Yohkoh satellite to study the plasma distributions at different temperatures. Intensity and velocity distributions in Fe λ5303 (green), Fe λ6374 (red), and Ca xv λ5694 (yellow) lines are compared with the soft X-ray images. It is found that the soft X-ray images closely resemble those of the yellow line that represents a rather high temperature component of the corona. On the other hand the low-temperature component seen in the green and the red lines shows quite a different distribution from that of the high-temperature component the low-temperature component consists of many thin loops or streaks, while the high-temperature component is more diffuse. We find that the active elements of the cool component, i.e., complex loop systems, rapid changes of small structures, and localized large plasma motions, all tend to be cospatial with the hot component.

Solar active-region temperatures have been determined from the full-Sun spectra of helium-like sulphur (S xv) observed by the Bragg Crystal Spectrometer on board the Yohkoh satellite. The average temperature deduced from S xv is demonstrated to vary with the solar activity level: A temperature of 2.5 × 106 K is derived from the spectra taken during low solar activity, similar to the general corona, while 4 × 106 K is obtained during a higher activity phase. For the latter, the high-temperature tail of the differential emission measure of active regions is found most likely due to the superposition of numerous flare-like events (micro/nano-flares). © 1995 Kluwer Academic Publishers.

Frequent transient brightenings have been discovered in solar active regions using the Yohkoh Soft X-ray Telescope. We examine the possibility that these transient brightenings have much contribution to heating of the active-region corona. Imaging observations provide the following physical conditions: temperature, 4-8 MK; emission measure, 10(44.5)-10(47.5) cm(-3); electron density, 2 x 10(9)-2 x 10(10) cm(-3); gas pressure, 5-20 dyne cm(-2); loop length, 5 x 10(3)-4 x 10(4) km; loop width, 2 x 10(3)-7 x 10(3) km; and duration, 2-7 min. The energy involved in the observed transient brightenings is estimated to range from 10(25) to, 10(29) erg. The frequency distribution as a function of the energy can be represented by a single power-law with an index of 1.5-1.6 in the energy range greater than 10(27) erg, although the distribution deviates from a power-law in the energy range less than 10(27) erg due to the instrument dead time and/or obscuration by bright coronal features. The single power-law of the frequency distribution thus appears to extend from the solar flare range down to an energy of 10(27) erg. The total energy supplied by transient brightenings and flares, assuming that the power-law continues to lower energy, is estimated to be at most a factor of 5 smaller than the heating rate required for the active-region corona. We need weaker events with an occurrence rate much higher than the extrapolated power-law to explain the heating of the active-region corona with transient brightenings.

We have found from the observations of the Yohkoh Soft X-ray Telescope (SXT) that the corona above active regions expands occasionally, and almost continually in the cases of "active" active regions. This is contrary to the commonly accepted idea of magnetohydrostatic equilibrium of these regions. The key to this discovery has been a movie representation of the Yohkoh-SXT data, which, for the first time, provides adequate sampling and continuity for this purpose. The movies show ubiquitous expansions above the active regions, with velocities in the range of a few to a few tens km s-1 as measured when they are on the limb. The expansion appears to preserve the overall structure of the active-region corona. We suggest that the expansion may have a physical relationship with the transient loop brightenings found within the active regions. This finding of almost continual expansion of the active-region corona may affect some of the basic ideas concerning active regions, as well as those of the mass-loss from the Sun and Sun-like stars.