Keisuke Nishida

Spectropolarimetric Observation of an Emerging Flux Region: Triggering Mechanisms of Ellerman Bombs

A high spatial resolution observation of an emerging flux region (EFR) was made using a vector magnetograph and a Hα Lyot filtergraph with the Domeless Solar Telescope at Hida Observatory on 2006 October 22. In Hα wing images, we could see many Ellerman bombs (EBs) in the EFR. Observations in two modes, slit scan and slit fixed, were performed with the vector magnetograph, along with the Hα filtergraph. Using the Hα wing images, we detected 12 EBs during the slit scan observation period and 9 EBs during the slit fixed observation period. With the slit scan observation, we found that all the EBs were distributed in the area where the spatial gradient of vertical field intensity was large, which indicates the possibility of rapid topological change in the magnetic field in the area of EBs. With the slit fixed observation, we found that EBs were distributed in the areas of undulatory magnetic fields, in both the vertical and horizontal components. This paper is the first to report the undulatory pattern in the horizontal components of the magnetic field, which is also evidence for emerging magnetic flux triggered by the Parker instability. These results allow us to confirm the association between EBs and emerging flux tubes. Three triggering mechanisms for EBs are discussed with respect to emerging flux tubes: 9 out of 21 EBs occurred at the footpoints of emerging flux tubes, 8 occurred at the top of emerging flux tubes, and 4 occurred in the unipolar region. Each case can be explained by magnetic reconnection in the low chromosphere.

Stability of the Liquid State of Imidazolium-Based Ionic Liquids under High Pressure at Room Temperature

To understand the stability of the liquid phase of ionic liquids under high pressure, we investigated the phase behavior of a series of 1-alkyl-3-methylimidazolium tetrafluoroborate ([Cnmim][BF4]) homologues with different alkyl chain lengths for 2 ≤ n ≤ 8 up to ∼7 GPa at room temperature. The ionic liquids exhibited complicated phase behavior, which was likely due to the conformational flexibility in the alkyl chain. The present results reveal that [Cnmim][BF4] falls into superpressed state around 2-3 GPa range upon compression with an implication of multiple phase or structural transitions to ∼7 GPa. Remarkably, a characteristic nanostructural organization in ionic liquids largely diminishes at the superpressed state. The behaviors of imidazolium-based ionic liquids can be classified into, at least, three patterns: (1) pressure-induced crystallization, (2) superpressurization upon compression, and (3) decompression-induced crystallization from the superpressurized glass. Interestingly, the high-pressure phase behavior was relevant to the glass transition behavior at low temperatures and ambient pressure. As n increases, the glass transition pressure (pg) decreases (from 2.8 GPa to ∼2 GPa), and the glass transition temperature increases. The results indicate that the p-T range of the liquid phase is regulated by the alkyl chain length of [Cnmim][BF4] homologues.

Density measurement of liquid FeS at high pressures using synchrotron X-ray absorption

Abstract The density of liquid iron sulfide (FeS) was measured up to 3.8 GPa and 1800 K using an X-ray absorption method. The compression curve of liquid FeS was fitted using the Vinet equation of state. The isothermal bulk modulus and its temperature and pressure derivatives were determined using a nonlinear least-squares fit. The parameter sets determined were: K0T = 2.5 ± 0.3 GPa at T = 1500 K, (dK0/dT)P = -0.0036 ± 0.0003 GPa/K, and (dK0/dP)T = 24 ± 2. These results suggest that liquid FeS is more compressible than Fe-rich liquid Fe-S.

Numerical Examination of Plasmoid-Induced Reconnection Model for Solar Flares: The Relation between Plasmoid Velocity and Reconnection Rate

The plasmoid-induced reconnection model explaining solar flares based on bursty reconnection produced by an ejecting plasmoid suggests a possible relation between the ejection velocity of a plasmoid and the rate of magnetic reconnection. In this study, we focus on the quantitative description of this relation. We performed magnetohydrodynamic simulations of solar flares by changing the values of resistivity and the plasmoid velocity. The plasmoid velocity has been changed by applying an additional force to the plasmoid to see how the plasmoid velocity affects the reconnection rate. An important result is that the reconnection rate has a positive correlation with the plasmoid velocity, which is consistent with the plasmoid-induced reconnection model for solar flares. We also discuss an observational result supporting this positive correlation.

THE ROLE OF A FLUX ROPE EJECTION IN A THREE-DIMENSIONAL MAGNETOHYDRODYNAMIC SIMULATION OF A SOLAR FLARE

We investigated the dynamic evolution of a three-dimensional (3D) flux rope eruption and magnetic reconnection process in a solar flare by simply extending the two-dimensional (2D) resistive magnetohydrodynamic simulation model of solar flares with low β plasma to a 3D model. We succeeded in reproducing a current sheet and bi-directional reconnection outflows just below the flux rope during the eruption in our 3D simulations. We calculated four cases of a strongly twisted flux rope and a weakly twisted flux rope in 2D and 3D simulations. The time evolution of a weakly twisted flux rope in the 3D simulation shows behaviors similar to those of the 2D simulation, while a strongly twisted flux rope in the 3D simulation clearly shows a different time evolution from the 2D simulation except for the initial phase evolution. The ejection speeds of both strongly and weakly twisted flux ropes in 3D simulations are larger than in the 2D simulations, and the reconnection rates in 3D cases are also larger than in the 2D cases. This indicates positive feedback between the ejection speed of a flux rope and the reconnection rate even in the 3D simulation, and we conclude that the plasmoid-induced reconnection model can be applied to 3D. We also found that small-scale plasmoids are formed inside a current sheet and make it turbulent. These small-scale plasmoid ejections have a role in locally increasing the reconnection rate intermittently as observed in solar flares, coupled with a global eruption of a flux rope.

Spicule Dynamics over a Plage Region

We studied spicular jets over a plage area and derived their dynamic characteristics using Hinode Solar Optical Telescope (SOT) high-resolution images. A target plage region was near to the west limb of the solar disk. This location permitted us to study the dynamics of spicular jets without any overlapping effect of spicular structures along the line of sight. In this work, to increase the ease with which we could identify spicules on the disk, we applied the image processing method ‘MadMax’ developed by Koutchmy et al. (1989). It enhances fine, slender structures (like jets), over a diffuse background. We identified 169 spicules over the target plage. This sample permited us to derive statistically reliable results regarding spicular dynamics. The properties of plage spicules can be summarized as follows: (1) In a plage area, we clearly identified spicular jet features. (2) They were shorter in length than the quiet region limb spicules, and followed a ballistic motion under constant deceleration. (3) The majority (80%) of the plage spicules showed a cycle of rise and retreat, while 10% of them faded out without a complete retreat phase. (4) The deceleration of the spicule was proportional to the velocity of ejection (i.e., the initial velocity).

Density and Elasticity Measurements for Liquid Materials

Abstract Density and elastic properties (such as bulk modulus) of liquid materials under pressure are important to understand the behavior and distribution of magma in the Earths interior. The magma behavior is also closely related to mantle differentiation in the early Earth. In this chapter, the currently proposed density, sound velocity, and elasticity measurements for noncrystalline materials at high pressure are reviewed and the advantages and potential problems of these methods are discussed.

Compressional and shear wave velocities for polycrystalline bcc-Fe up to 6.3 GPa and 800 K

Abstract The cores of the Earth and other differentiated bodies are believed to be comprised of iron and various amounts of light elements. Measuring the densities and sound velocities of iron and its alloys at high pressures and high temperatures is crucial for understanding the structure and composition of these cores. In this study, the sound velocities (vP and vS) and density measurements of body-centered cubic (bcc)-Fe were determined experimentally up to 6.3 GPa and 800 K using ultrasonic and X-ray diffraction methods. Based on the measured vP, vS, and density, we obtained the following parameters regarding the adiabatic bulk KS and shear G moduli of bcc-Fe: KS0 = 163.2(15) GPa, ∂KS/̸P = 6.75(33), ∂KS/∂T = –0.038(3) GPa/K, G0 = 81.4(6) GPa, ∂G/̸P = 1.66(14), and ∂G/∂T = –0.029(1) GPa/K. Moreover, we observed that the sound velocity–density relationship for bcc-Fe depended on temperature in the pressure and temperature ranges analyzed in this study and the effect of temperature on vS was stronger than that on vP at a constant density, e.g., 6.0% and 2.7% depression for vS and vP, respectively, from 300 to 800 K at 8000 kg/m3. Furthermore, the effects of temperature on both vP and vS at a constant density were much greater for bcc-Fe than for ε-FeSi (cubic B20 structure), according to previously obtained measurements, which may be attributable to differences in the degree of thermal pressure. These results suggest that the effects of temperature on the sound velocity–density relationship for Fe alloys strongly depend on their crystal structures and light element contents in the range of pressure and temperature studied.