Hiroki Kurokawa

Yohkoh observations of the creation of high-temperature plasma in the flare of 16 December 1991

Yohkoh observations of an impulsive solar flare which occurred on 16 December, 1991 are presented. This flare was a GOES M2.7 class event with a simple morphology indicative of a single flaring loop. X-ray images were taken with the Hard X-ray Telescope (HXT) and soft X-ray spectra were obtained with the Bragg Crystal Spectrometer (BCS) on board the satellite. The spectrometer observations were made at high sensivity from the earliest stages of the flare, are continued throughout the rise and decay phases, and indicate extremely strong blueshifts, which account for the majority of emission in Caxix during the initial phase of the flare. The data are compared with observations from other space and ground-based instruments. A balance calculation is performed which indicates that the energy contained in non-thermal electrons is sufficient to explain the high temperature plasma which fills the loop. The cooling of this plasma by thermal conduction is independently verified in a manner which indicates that the loop filling factor is close to 100%. The production of ‘superhot’ plasma in impulsive events is shown to differ in detail from the morphology and mechanisms appropriate for more gradual events.

X-RAY BRIGHT POINT FLARES DUE TO MAGNETIC RECONNECTION

Ground-based optical observations coordinated with Yohkoh/SXT X-ray observations of an old, disintegrating bipolar active region AR NOAA 7493 (May 1, 1993) provided a multiwavelength data base to study a flaring ‘active region’ X-ray bright point (XBP) of about 16 hr lifetime, and the activity related to it in different layers of the solar atmosphere. The XBP appeared to be related to a new minor bipole of about 1020 Mx. Superposed on a global evolution of soft X-ray brightness, the XBP displayed changes of brightness, lasting for 1–10 min. During the brightenings the XBP apparently had a spatial structure, which was (tiny) loop-like rather than point-like. The X-ray brightenings were correlated with chromospheric activity: (i) brightenings of underlying chromospheric faculae, and (ii) appearance of strong turbulent velocities in the arch filament system. We propose that the XBP brightenings were due to reconnection of the magnetic field lines (sketched in 3D) between the new bipole and a pre-existing plage field induced by the motion of one of the new pores (v = 0.2 km s−1) towards the plage, and that the XBP itself was a reconnected hot loop between them.

High-resolution spectral observation during the impulsive phase of a flare

High-resolution observations of the flare on October 21, 1989 were made with the Domeless Solar Telescope of the Hida Observatory. The following new results have been obtained: (a) during the impulsive phase of the flare, the spectral line asymmetry has spatial fine structures of 1″–2″; (b) for several points in the flare region the line profile alternatively changes between blue asymmetry and red asymmetry within a few seconds. A possible explanation has been suggested.

Chromospheric ejections and their signatures in X-ray observed by Yohkoh

Abstract Surges, or condensations of chromospheric material, are commonly observed in active regions when new emerging magnetic flux (EMF) occurs. In order to study EMF-related phenomena in the corona, observing campaigns were organized with the Yohkoh X-ray satellite and ground based observatories. EMFs could be detected in magnetograms made in Potsdam and Hawaii, surge and arch filament system (AFS) events (on Oct 7 1991, May 1 1993, respectively) with the Multichannel Subtractive Double Pass instruments operating at Meudon and on the Canary Islands and with the Hida telescope at Kyoto University. Bright X-ray loops coincident with chromospheric surge activity were deteted in the high resolution Yohkoh SXT images. We also found coincident brightness variations between an X-ray bright point (XBP) and the underlying chromospheric plage related to EMF. We interpret the appearance of AFS as condensation of material among magnetic field lines, the X-ray bright points as the result of a magnetic reconnection process, and surges as ejection of cold plasma bubbles squeezed between field lines as a result of reconnection.