arXiv subjects

Y. Katsukawa

Publications and source records attributed to Y. Katsukawa.

At least 37 records · Page 2Linked to original sources

arXiv2016

Spectropolarimetric capabilities of Ca II 8542 A line

The next generation of space and ground-based solar missions aim to study the magnetic properties of the solar chromosphere using the infrared Ca II lines and the He I 10830 Å line. The former seem to be the best candidates to study the stratification of magnetic fields in the solar chromosphere and their relation to the other thermodynamical properties underlying the chromospheric plasma. The purpose of this work is to provide a detailed analysis of the diagnostic capabilities of the Ca II 8542 Å line, anticipating forthcoming observational facilities. We study the sensitivity of the Ca II 8542 Å line to perturbations applied to the physical parameters of reference semi-empirical 1D model atmospheres using response functions and we make use of 3D MHD simulations to examine the expected polarization signals for moderate magnetic field strengths. Our results indicate that the Ca II 8542 Å line is mostly sensitive to the layers enclosed between $\log$ $τ=[0,-5.5]$, under the physical conditions that are present in our model atmospheres. In addition, the simulated magnetic flux tube generates strong longitudinal signals in its centre and moderate transversal signals, due to the vertical expansion of magnetic field lines, in its edge. Thus, observing the Ca II 8542 Å line we will be able to infer the 3D geometry of moderate magnetic field regions.

astro-ph.SR↗

arXiv2012

Power spectra of velocities and magnetic fields on the solar surface and their dependence on the unsigned magnetic flux density

We have performed power spectral analysis of surface temperatures, velocities, and magnetic fields, using spectro-polarimetric data taken with the Hinode Solar Optical Telescope. When we make power spectra in a field-of-view covering the super-granular scale, kinetic and thermal power spectra have a prominent peak at the granular scale while the magnetic power spectra have a broadly distributed power over various spatial scales with weak peaks at both the granular and supergranular scales. To study the power spectra separately in internetwork and network regions, power spectra are derived in small sub-regions extracted from the field-of-view. We examine slopes of the power spectra using power-law indices, and compare them with the unsigned magnetic flux density averaged in the sub-regions. The thermal and kinetic spectra are steeper than the magnetic ones at the sub-granular scale in the internetwork regions, and the power-law indices differ by about 2. The power-law indices of the magnetic power spectra are close to or smaller than -1 at that scale, which suggests the total magnetic energy mainly comes from either the granular scale magnetic structures or both the granular scale and smaller ones contributing evenly. The slopes of the thermal and kinetic power spectra become less steep with increasing unsigned flux density in the network regions. The power-law indices of all the thermal, kinetic, and magnetic power spectra become similar when the unsigned flux density is larger than 200 Mx cm^-2.

astro-ph.SR↗

arXiv2012

Requirements for the Analysis of Quiet Sun Internetwork Magnetic Elements with EST and ATST

The quiet Sun internetwork is permeated by weak and highly inclined magnetic fields whose physical properties, dynamics, and magnetic interactions are not fully understood. High spatial resolution magnetograms show them as discrete magnetic elements that appear/emerge and disappear/cancel continuously over the quiet Sun surface. The 4-m European Solar Telescope (EST) and the Advanced Technology Solar Telescope (ATST) will obtain two-dimensional, high cadence, high precision polarimetric measurements at the diffraction limit (30 km). Here, we compile the basic requirements for the observation of internetwork fields with EST and ATST (field of view, cadence, instrument configuration, etc). More specifically, we concentrate on the field-of-view requirements. To set them we analyze the proper motion of internetwork magnetic elements across the solar surface. We use 13 hours of magnetograms taken with the Hinode satellite to identify and track thousands of internetwork magnetic element in an isolated supergranular cell. We calculate the velocity components of each element and the mean distance they travel. The results show that, on average, magnetic elements in the interior of supergranular cells move toward the network. The radial velocity is observed to depend on the distance to the center of the supergranule. Internetwork magnetic elements travel 400 on average. These results suggest that ATST and EST should cover, at least, one supergranular cell to obtain a complete picture of the quiet Sun internetwork.

astro-ph.SR↗

arXiv2010

A new type of small-scale downflow patches in sunspot penumbrae

A sunspot penumbra is observationally examined to reveal properties of small-scale flow structures and how they are related to the filamentary magnetic structures and the Evershed flow. We also study how the photospheric dynamics is related to chromospheric activities. The study is based on data analysis of spectro-polarimetric observations of photospheric Fe I lines with the Solar Optical Telescope aboard Hinode in a sunspot penumbra at different heliocentric angles. Vector magnetic fields and velocities are derived using the spectro-polarimetric data and a Stokes inversion technique. An observation with a Ca II H filtergram co-spatial and co-temporal with the spectro-polarimetric one is also used to study possible chromospheric responses. We find small patches with downflows at photospheric layers. The downflow patches have a size of 0.5" or smaller and have a geometrical configuration different from that of the Evershed flow. The downflow velocity is about 1 km/s at lower photspheric layers, and is almost zero in the upper layers. Some of the downflow patches are associated with brightenings seen in Ca II H images. The downflows are possible observational signatures of downward flows driven by magnetic reconnection in the interlaced magnetic field configuration, where upward flows make brightenings in the chromosphere. Another possibility is that they are concentrated downward flows of overturning magnetoconvection.

astro-ph.SR↗

arXiv2009

Supersonic Evershed flow outside Sunspots

We report on the discovery of mostly horizontal field channels just outside sunspot penumbrae (in the so-called `moat' region) that are seen to sustain supersonic flows (line-of-sight component of 6 km s{-1}). The spectral signature of these supersonic flows corresponds to circular polarization profiles with an additional, satellite, third lobe of the same sign as the parent sunspot' Stokes V blue lobe, for both downflows and upflows. This is consistent with an outward directed flow that we interpret as the continuation of the magnetized Evershed flow outside sunspots at supersonic speeds. In Stokes Q and U, a clear signature of a transverse field connecting the two flow streams is observed. Such an easily detectable spectral signature should allow for a clear identification of these horizontal field channels in other spectropolarimetric sunspot data. For the spot analyzed in this paper, a total of 5 channels with this spectral signature have been unambiguously found.

astro-ph.SR↗

arXiv2008

Giant Chromospheric Anemone Jet Observed with Hinode and Comparison with Magnetohydrodynamic Simulations: Evidence of Propagating Alfven Waves and Magnetic Reconnection

Hinode discovered a beautiful giant jet with both cool and hot components at the solar limb on 2007 February 9. Simultaneous observations by the Hinode SOT, XRT, and TRACE 195 satellites revealed that hot (5x10^6 K) and cool (10^4 K) jets were located side by side and that the hot jet preceded the associated cool jet (1-2 minutes). A current-sheet-like structure was seen in optical (Ca IIH), EUV (195A), and soft X-ray emissions, suggesting that magnetic reconnection is occurring in the transition region or upper chromosphere. Alfven waves were also observed with Hinode SOT. These propagated along the jet at velocities of 200 km/s with amplitudes (transverse velocity) of 5-15 km/s and a period of 200 s. We performed two-dimensional MHD simulation of the jets on the basis of the emerging flux-reconnection model, by extending Yokoyama and Shibata's model. We extended the model with a more realistic initial condition (10^6 K corona) and compared our model with multiwavelength observations. The improvement of the coronal temperature and density in the simulation model allowed for the first time the reproduction of the structure and evolution of both the cool and hot jets quantitatively, supporting the magnetic reconnection model. The generation and the propagation of Alfven waves are also reproduced self-consistently in the simulation model.

astro-ph↗

arXiv2008

Magnetic Landscape of Sun's Polar Region

We present the magnetic landscape of the polar region of the Sun that is unprecedented in terms of high spatial resolution, large field of view, and polarimetric precision. These observations were carried out with the Solar Optical Telescope aboard \emph{Hinode}. Using a Milne-Eddington inversion, we found many vertically-oriented magnetic flux tubes with field strength as strong as 1 kG that are scattered in latitude between 70-90 degree. They all have the same polarity, consistent with the global polarity of the polar region. The field vectors were observed to diverge from the center of the flux elements, consistent with a view of magnetic fields that expand and fan out with height. The polar region is also covered with ubiquitous horizontal fields. The polar regions are the source of the fast solar wind channelled along unipolar coronal magnetic fields whose photospheric source is evidently rooted in the strong field, vertical patches of flux. We conjecture that vertical flux tubes with large expansion around the photosphere-corona boundary serve as efficient chimneys for Alfven waves that accelerate the solar wind.

astro-ph↗

arXiv2008

The properties of penumbral microjets inclination

We investigate the dependence of penumbral microjets inclination on the position within penumbra. The high cadence observations taken on 10 November 2006 with the Hinode satellite through the \ion{Ca}{ii} H and G--band filters were analysed to determine the inclination of penumbral microjets. The results were then compared with the inclination of the magnetic field determined through the inversion of the spectropolarimetric observations of the same region. The penumbral microjet inclination is increasing towards the outer edge of the penumbra. The results suggest that the penumbral microjet follows the opening magnetic field lines of a vertical flux tube that creates the sunspot.

astro-ph↗

arXiv2008

Disintegration of Magnetic Flux in Decaying Sunspots as Observed with the Hinode SOT

Continuous observations of sunspot penumbrae with the Solar Optical Telescope aboard \textit{Hinode} clearly show that the outer boundary of the penumbra fluctuates around its averaged position. The penumbral outer boundary moves inward when granules appear in the outer penumbra. We discover that such granules appear one after another while moving magnetic features (MMFs) are separating from the penumbral ``spines'' (penumbral features that have stronger and more vertical fields than those of their surroundings). These granules that appear in the outer penumbra often merge with bright features inside the penumbra that move with the spines as they elongate toward the moat region. This suggests that convective motions around the penumbral outer boundary are related to the disintegration of magnetic flux in the sunspot. We also find that dark penumbral filaments frequently elongate into the moat region in the vicinity of MMFs that detach from penumbral spines. Such elongating dark penumbral filaments correspond to nearly horizontal fields extending from the penumbra. Pairs of MMFs with positive and negative polarities are sometimes observed along the elongating dark penumbral filaments. This strongly supports the notion that such elongating dark penumbral filaments have magnetic fields with a ``sea serpent''-like structure. Evershed flows, which are associated with the penumbral horizontal fields, may be related to the detachment of the MMFs from the penumbral spines, as well as to the formation of the MMFs along the dark penumbral filaments that elongate into the moat region.

astro-ph↗

arXiv2008

The intensity contrast of solar granulation: comparing Hinode SP results with MHD simulations

The contrast of granulation is an important quantity characterizing solar surface convection. We compare the intensity contrast at 630 nm, observed using the Spectro-Polarimeter (SP) aboard the Hinode satellite, with the 3D radiative MHD simulations of V{ö}gler & Sch{ü}ssler (2007). A synthetic image from the simulation is degraded using a theoretical point-spread function of the optical system, and by considering other important effects. The telescope aperture and the obscuration by the secondary mirror and its attachment spider, reduce the simulated contrast from 14.4 % to 8.5 %. A slight effective defocus of the instrument brings the simulated contrast down to 7.5 %, close to the observed value of 7.0 %. A proper consideration of the effects of the optical system and a slight defocus, lead to sufficient degradation of the synthetic image from the MHD simulation, such that the contrast reaches almost the observed value. The remaining small discrepancy can be ascribed to straylight and slight imperfections of the instrument, which are difficult to model. Hence, Hinode SP data are consistent with a granulation contrast which is predicted by 3D radiation MHD simulations.

astro-ph↗

arXiv2008

On-orbit Performance of the Solar Optical Telescope aboard Hinode

On-orbit performance of the Solar Optical Telescope (SOT) aboard Hinode is described with some attentions on its unpredicted aspects. In general, SOT revealed an excellent performance and has been providing outstanding data. Some unexpected features exist, however, in behaviors of the focus position, throughput and structural stability. Most of them are recovered by the daily operation i.e., frequent focus adjustment, careful heater setting and corrections in data analysis. The tunable filter contains air bubbles which degrade the data quality significantly. Schemes for tuning the filter without disturbing the bubbles have been developed and tested, and some useful procedures to obtain Dopplergram and magnetogram are now available. October and March when the orbit of satellite becomes nearly perpendicular to the direction towards the sun provide a favorable condition for continuous runs of the narrow-band filter imager.

astro-ph↗

arXiv2008

Frequent Occurrence of High-speed Local Mass Downflows on the Solar Surface

We report on new spectro-polarimetric measurements with simultaneous filter imaging observation, revealing the frequent appearance of polarization signals indicating high-speed, probably supersonic, downflows that are associated with at least three different configurations of magnetic fields in the solar photosphere. The observations were carried out with the Solar Optical Telescope onboard the {\em Hinode} satellite. High speed downflows are excited when a moving magnetic feature is newly formed near the penumbral boundary of sunspots. Also, a new type of downflows is identified at the edge of sunspot umbra that lack accompanying penumbral structures. These may be triggered by the interaction of magnetic fields sweeped by convection with well-concentrated magnetic flux. Another class of high speed downflows are observed in quiet sun and sunspot moat regions. These are closely related to the formation of small concentrated magnetic flux patches. High speed downflows of all types are transient time-dependent mass motions. These findings suggest that the excitation of supersonic mass flows are one of the key observational features of the dynamical evolution occurring in magnetic-field fine structures on the solar surface.

astro-ph↗

arXiv2008

Relationships between magnetic foot points and G-band bright structures

Magnetic elements are thought to be described by flux tube models, and are well reproduced by MHD simulations. However, these simulations are only partially constrained by observations. We observationally investigate the relationship between G-band bright points and magnetic structures to clarify conditions, which make magnetic structures bright in G-band. The G-band filtergrams together with magnetograms and dopplergrams were taken for a plage region covered by abnormal granules as well as ubiquitous G-band bright points, using the Swedish 1-m Solar Telescope (SST) under very good seeing conditions. High magnetic flux density regions are not necessarily associated with G-band bright points. We refer to the observed extended areas with high magnetic flux density as magnetic islands to separate them from magnetic elements. We discover that G-band bright points tend to be located near the boundary of such magnetic islands. The concentration of G-band bright points decreases with inward distance from the boundary of the magnetic islands. Moreover, G-band bright points are preferentially located where magnetic flux density is higher, given the same distance from the boundary. There are some bright points located far inside the magnetic islands. Such bright points have higher minimum magnetic flux density at the larger inward distance from the boundary. Convective velocity is apparently reduced for such high magnetic flux density regions regardless of whether they are populated by G-band bright points or not. The magnetic islands are surrounded by downflows.These results suggest that high magnetic flux density, as well as efficient heat transport from the sides or beneath, are required to make magnetic elements bright in G-band.

astro-ph↗

arXiv2008

Transient horizontal magnetic fields in solar plage regions

We report the discovery of isolated, small-scale emerging magnetic fields in a plage region with the Solar Optical Telescope aboard Hinode. Spectro-polarimetric observations were carried out with a cadence of 34 seconds for the plage region located near disc center. The vector magnetic fields are inferred by Milne-Eddington inversion. The observations reveal widespread occurrence of transient, spatially isolated horizontal magnetic fields. The lateral extent of the horizontal magnetic fields is comparable to the size of photospheric granules. These horizontal magnetic fields seem to be tossed about by upflows and downflows of the granular convection. We also report an event that appears to be driven by the magnetic buoyancy instability. We refer to buoyancy-driven emergence as type1 and convection-driven emergence as type2. Although both events have magnetic field strengths of about 600 G, the filling factor of type1 is a factor of two larger than that of type2. Our finding suggests that the granular convection in the plage regions is characterized by a high rate of occurrence of granular-sized transient horizontal fields.

astro-ph↗

arXiv2008

Moat flow in the vicinity of sunspots for various penumbral configurations

High-resolution time series of sunspots have been obtained with the Swedish 1m Solar Telescope between 2003 and 2006 at different locations on the solar disc. Proper motions in seven different active regions have been studied. The analysis has been done by applying local correlation tracking to every series of sunspots, each of them more than 40 minutes long. The sunspots' shapes include a different variety of penumbral configurations. We report on a systematic behaviour of the large-scale outflows surrounding the sunspots, commonly known as moat flows, that are essentially present only when preceded by a penumbra not tangential but perpendicular to the sunspot border. We present one case for which this rule appears not to be confirmed. We speculate that the magnetic neutral line, which is located in the vicinity of the anomalous region, might be responsible for blocking the outflow. These new results confirm the systematic and strong relation between the moat flows and the existence of penumbrae. A comparative statistical study between moats and standard granulation is also performed.

astro-ph↗

arXiv2008

Coronal transverse magnetohydrodynamic waves in a solar prominence

Solar prominences are cool 10$^4$ Kelvin plasma clouds supported in the surrounding 10$^6$ Kelvin coronal plasma by as-yet undetermined mechanisms. Observations from \emph{Hinode} show fine-scale threadlike structures oscillating in the plane of the sky with periods of several minutes. We suggest these transverse magnetohydrodynamic waves may represent Alfvén waves propagating on coronal magnetic field lines and these may play a role in heating the corona.

astro-ph↗

arXiv2007

Flare Ribbons Observed with G-band and FeI 6302A Filters of the Solar Optical Telescope on Board Hinode

The Solar Optical Telescope (SOT) on board Hinode satellite observed an X3.4 class flare on 2006 December 13. Typical two-ribbon structure was observed, not only in the chromospheric CaII H line but also in G-band and FeI 6302A line. The high-resolution, seeing-free images achieved by SOT revealed, for the first time, the sub-arcsec fine structures of the "white light" flare. The G-band flare ribbons on sunspot umbrae showed a sharp leading edge followed by a diffuse inside, as well as previously known core-halo structure. The underlying structures such as umbral dots, penumbral filaments and granules were visible in the flare ribbons. Assuming that the sharp leading edge was directly heated by particle beam and the diffuse parts were heated by radiative back-warming, we estimate the depth of the diffuse flare emission using the intensity profile of the flare ribbon. We found that the depth of the diffuse emission is about 100 km or less from the height of the source of radiative back-warming. The flare ribbons were also visible in the Stokes-V images of FeI 6302A, as a transient polarity reversal. This is probably related to "magnetic transient" reported in the literature. The intensity increase in Stokes-I images indicates that the FeI 6302A line was significantly deformed by the flare, which may cause such a magnetic transient.

astro-ph↗

arXiv2007

A Tale Of Two Spicules: The Impact of Spicules on the Magnetic Chromosphere

We use high-resolution observations of the Sun in Ca II H 3968 A from the Solar Optical Telescope on Hinode to show that there are at least two types of spicules that dominate the structure of the magnetic solar chromosphere. Both types are tied to the relentless magnetoconvective driving in the photosphere, but have very different dynamic properties. ``Type-I'' spicules are driven by shock waves that form when global oscillations and convective flows leak into the upper atmosphere along magnetic field lines on 3-7 minute timescales. ``Type-II'' spicules are much more dynamic: they form rapidly (in ~10s), are very thin (<200km wide), have lifetimes of 10-150s (at any one height) and seem to be rapidly heated to (at least) transition region temperatures, sending material through the chromosphere at speeds of order 50-150 km/s. The properties of Type II spicules suggest a formation process that is a consequence of magnetic reconnection, typically in the vicinity of magnetic flux concentrations in plage and network. Both types of spicules are observed to carry Alfven waves with significant amplitudes of order 20 km/s.

astro-ph↗