清水 敏文

Abstract Magnetic helicity is a physical parameter used to quantify the complexity of magnetic fields, providing an indication of the energy state in the coronal magnetic structure. We investigate the temporal evolution of magnetic helicity and its relationship to the occurrence of a variety of flares in the solar active region NOAA 12297, which was well observed using the Solar Dynamics Observatory/Helioseismic and Magnetic Imager in 2015 March. The active region produced many M-class flares and an X-class flare in two distinctive areas, both of which had a similar magnetic evolution, i.e., the opposite polarity of an emerging flux developed beside a preexisting sunspot, but exhibited flares with different magnitudes and frequencies. We derived the spatiotemporal evolution of the magnetic helicity injections and evaluated how spinning and braiding helicity injections evolved with time in the two areas. In one area, we observed a remarkable evolution, in which a negative spinning helicity injection in the preexisting sunspot increased in a positive helicity system, followed by the occurrence of the X-class flare. The negative helicity injection was clearly caused by the flux emergence that developed along the outer edge of the preexisting sunspot. The other area showed positive braiding helicity injections, with spinning helicity injections fluctuating concurrently with flux emergence, changing their signs several times, i.e., variable energy, and helicity input. The observed temporal behaviors of the helicity injections may explain different types of flare occurrences in the regions.

We developed a scan mirror mechanism (SMM) that enable a slit-based spectrometer or spectropolarimeter to precisely and quickly map an astronomical object. The SMM, designed to be installed in the optical path preceding the entrance slit, tilts a folding mirror and then moves the reflected image laterally on the slit plane, thereby feeding a different one-dimensional image to be dispersed by the spectroscopic equipment. In general, the SMM is required to scan quickly and broadly while precisely placing the slit position across the field-of-view (FOV). These performances are in high demand for near-future observations, such as studies on the magnetohydrodynamics of the photosphere and the chromosphere. Our SMM implements a closed-loop control system by installing electromagnetic actuators and gap-based capacitance sensors. Our optical test measurements confirmed that the SMM fulfills the following performance criteria: i) supreme scan-step uniformity (linearity of 0.08%) across the wide scan range (± 1005 ″), ii) high stability (3 σ= 0.1 ″), where the angles are expressed in mechanical angle, and iii) fast stepping speed (26 ms). The excellent capability of the SMM will be demonstrated soon in actual use by installing the mechanism for a near-infrared spectropolarimeter onboard the balloon-borne solar observatory for the third launch, Sunrise III.

Nanoflares and the shock formation of magnetohydrodynamic waves in the solar chromosphere have been considered as key physical mechanisms of the heating of the chromosphere and corona. To investigate candidates of their signature in the mm-wavelength, a tiny active region located on the solar disk was observed with the Atacama Large millimeter and sub-millimeter Array (ALMA) at 3 mm, coordinated with observatories on orbit including Hinode SOT spectro-polarimeter in the Cycle 4 solar campaign (19 March 2017). ALMA’s spatial resolution was moderate, far from the best performance, but it provided stable conditions that are suitable to investigate temporal variations in the mm-wavelength. We determined that the noise level is less than 20 K (σ) over 1 hour in the 20-s cadence time series of synthesized ALMA images. The time variations with amplitudes above the noise level were observed throughout the field of view, but variations exceeding 200 K, corresponding to energy input to the chromosphere on the order of 10^20-22 erg, were localized in two locations. One location was on the polarity inversion line, where tiny concentrated magnetic patches exist in weak field and a tiny magnetic flux may be emergent. The other location was at the outer edge of a bipolar magnetic region, which was under development with a successive series of magnetic flux emergence. This observation suggests that nanoflare-class energy inputs in the chromosphere can occur associated with emerging flux activities.