Small-scale dynamics of photospheric magnetic activities and their chromospheric responses

The evolution of photospheric magnetic fields is considered as the fundamental source of forming atmospheric structures and triggering most solar activities, including flares and mass ejections on various scales (CMEs, jets, etc.). With the implementation of high-resolution observational instruments, small-scale details of magnetic features are recognized that can provide important information regarding the evolution in active regions and the connection between photospheric magnetic reconnection and jet-like ejections in the quiet Sun. This research takes advantage of the exceptionally high-resolution measurements of vector magnetic field and imaging observations by the Goode Solar Telescope, and UV/EUV imaging observations from space-based instruments. The studied topics include structural evolution of penumbra and shear flows in response to a flare eruption, flux emergence in the formation of an active region, and small-scale magnetic reconnections in the photosphere in a coronal boundary. The main findings in this dissertation work are listed as follows: Using high-resolution imaging observation in the TiO band by GST and photospheric vector magnetic field observation by the Helioseismic and Magnetic Imager, strong shear flow expansion in a precursor kernel near the polarity inversion line (PIL) is found to be closely associated with the M6.5 flare in AR 12371, which indicates a linkage between small-scale magnetic changes and the flare eruption. The flow field of the penumbra in the flaring core region is calculated using the differential affine velocity estimator. An enhancement of penumbral flows and an extension of penumbral fibrils after the first peak of the flare hard X-ray (HXR) emission are observed to be accompanied by a rapid increase of the horizontal field. These results provide evidence of the back reaction theory of coronal restructuring on the photosphere as a result of flare energy release. Taking advantage of high-resolution magnetic field measurements, TiO continuum, and H-alpha observations by GST, small-scale magnetic flux emergences in the AR 12665 are observed in two types of topology: magnetic flux sheet emergence associated with the newly forming granules, and the traditional magnetic flux loop emergence. Both types of flux emergence are associated with the darkening of granular boundaries, while only flux sheets elongate granules along the direction of emerging magnetic fields and expand laterally. H-alpha observations reveal transient brightenings in the events of magnetic loop emergence, which are most probably the signatures of Ellerman bombs. Statistical study of small-scale magnetic reconnections around the coronal hole boundary is achieved by tracking magnetic features with the Southwest Automatic Magnetic Identification Suite (SWAMIS). The tracking results show that the magnetic energy release rate by small-scale cancellation inside the coronal hole is 3 times higher than that outside the coronal hole. Spicules are likely associated with these kinds of reconnections, while only a small portion of the release magnetic energy is directly deposited to hot plasma

Large Misalignment between Stellar Bar and Dust Pattern in NGC 3488 Revealed by Spitzer and SDSS

A large position angle misalignment between the stellar bar and the distribution of dust in the late-type barred spiral NGC 3488 was discovered, using mid-infrared images from the Spitzer Space Telescope and optical images from the Sloan Digital Sky Survey (SDSS). The angle between the bar and dust patterns was measured to be 25+-2deg, larger than most of the misalignments found previously in barred systems based on Ha or HI/CO observations. The stellar bar is bright at optical and 3.6um, while the dust pattern is more prominent in the 8um band but also shows up in the SDSS u and g-band images, suggesting a rich interstellar medium environment harboring ongoing star formation. This angular misalignment is unlikely to have been caused by spontaneous bar formation. We suggest that the stellar bar and the dust pattern may have different formation histories, and that the large misalignment was triggered by a tidal interaction with a small companion. A statistical analysis of a large sample of nearby galaxies with archival Spitzer data indicates that bar structure such as that seen in NGC 3488 is quite rare in the local Universe.Comment: 18 pages, 4 figures, 1 table, accepted for publication in New Astronom

Flare differentially rotates sunspot on Sun's surface

Sunspots are concentrations of magnetic field visible on the solar surface (photosphere). It was considered implausible that solar flares, as resulted from magnetic reconnection in the tenuous corona, would cause a direct perturbation of the dense photosphere involving bulk motion. Here we report the sudden flare-induced rotation of a sunspot using the unprecedented spatiotemporal resolution of the 1.6 m New Solar Telescope, supplemented by magnetic data from the Solar Dynamics Observatory. It is clearly observed that the rotation is non-uniform over the sunspot: as the flare ribbon sweeps across, its different portions accelerate (up to ∼50° h−1) at different times corresponding to peaks of flare hard X-ray emission. The rotation may be driven by the surface Lorentz-force change due to the back reaction of coronal magnetic restructuring and is accompanied by a downward Poynting flux. These results have direct consequences for our understanding of energy and momentum transportation in the flare-related phenomena

Study on Temperature Force Control Mechanism of CRTSⅡ Slab Track: Control Conditions of Temperature Cracking

Diseases such as track slab arching and joint concrete crushing of China Railway Track System (CRTS)II slab track were caused by huge temperature force, which seriously threatens driving safety of trains. In this study, a longitudinal weak connection scheme of CRTSII slab track was proposed to adjust the temperature force in track slab and reduce diseases of longitudinal continuous track slab. This paper focuses on the cracking characteristics of the longitudinal heterogeneous concrete composite structure. The equation which was originally developed to calculate crack width and structure stress under temperature loads, was put forward to consider deformation difference of different elastic modulus. The influence law of various parameters was analyzed. The reinforcement stress and crack width of CRTSII slab track after longitudinal connection weakening were calculated, and the reasonable limit value of tensile force of connection reinforcement and the minimum value of bond resistance of reinforcement in joint position were obtained. The result shows that, in order to reduce the bond resistance between the joint material and the reinforcement, the elastic modulus of the elastic material should be less than 5000 MPa; in order to ensure that the reinforcement does not produce large stress, the elastic modulus of the joint should be greater than 1000 MPa