Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： 10.3847/1538-4357/836/1/40

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：EDP Sciences  

The solar dynamo problem is the question of how the cyclic variation in the solar magnetic field is maintained. One of the important processes is the transport of magnetic flux by surface convection. To reveal this process, the dependence of the squared displacement of magnetic flux concentrations on the elapsed time is investigated in this paper via a feature-recognition technique and a continual five-day magnetogram. This represents the longest time scale over which a satellite observation has ever been performed for this problem. The dependence is found to follow a power law and differ significantly from that of diffusion transport. Furthermore, there is a change in the behavior at a spatial scale of 103.8 km. A super-diffusion behavior with an index of 1.4 is found at smaller scales, while changing to a sub-diffusion behavior with an index of 0.6 on larger ones. We interpret this difference in the transport regime as coming from the network-flow pattern.

DOI： 10.1051/swsc/2016022

Scopus

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

In this second paper in the series, we investigate occurrence frequencies of apparent unipolar processes, cancellation, and emergence of patch structures in quiet regions. Apparent unipolar events are considerably more frequent than cancellation and emergence, per our definition, which is consistent with Lamb et al. Furthermore, we investigate the frequency distributions of changes in flux during apparent unipolar processes and find that they concentrate around the detection limit of the analysis. Combining these findings with the results of our previous paper, Iida et al., which found that merging and splitting are more dominant than emergence and cancellation, these results support the understanding that apparent unipolar processes are actually interactions with and among patches below the detection limit and that there still are numerous flux interactions between the flux range in this analysis and below the detection limit. We also investigate occurrence frequency distributions of flux decrease during cancellation. We found a relatively strong dependence, 2.48 +/- 0.26 as a power-law index. This strong dependence on flux is consistent with the model, which was suggested in the previous paper.

DOI： 10.1088/0004-637X/814/2/134

Web of Science

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Publishing type：Research paper (scientific journal)  

DOI： 10.1088/0004-637X/799/2/139

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OXFORD UNIV PRESS  

To elucidate the flare trigger mechanism, we have analyzed several flare
events which were observed by Hinode/Solar Optical Telescope (SOT), in our
previous study. Because of the limitation of SOT field of view, however, only
four events in the Hinode data sets have been utilizable. Therefore, increasing
the number of events is required for evaluating the flare trigger models. We
investigated the applicability of data obtained by the Solar Dynamics
Observatory (SDO) to increase the data sample for a statistical analysis of the
flare trigger process. SDO regularly observes the full disk of the sun and all
flares although its spatial resolution is lower than that of Hinode. We
investigated the M6.6 flare which occurred on 13 February 2011 and compared the
analyzed data of SDO with the results of our previous study using Hinode/SOT
data. Filter and vector magnetograms obtained by the Helioseismic and Magnetic
Imager (HMI) and filtergrams from the Atmospheric Imaging Assembly (AIA) 1600A
were employed. From the comparison of small-scale magnetic configurations and
chromospheric emission prior to the flare onset, we confirmed that the trigger
region is detectable with the SDO data. We also measured the magnetic shear
angles of the active region and the azimuth and strength of the flare-trigger
field. The results were consistent with our previous study. We concluded that
statistical studies of the flare trigger process are feasible with SDO as well
as Hinode data. We also investigated the temporal evolution of the magnetic
field before the flare onset with SDO.

DOI： 10.1093/pasj/psu091

Web of Science

arXiv

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Other Link： http://arxiv.org/pdf/1407.1887v1

Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We present a comparison of the Solar Dynamics Observatory (SDO) analysis of NOAA Active Region (AR) 11158 and numerical simulations of flux-tube emergence, aiming to investigate the formation process of this flare-productive AR. First, we use SDO/Helioseismic and Magnetic Imager (HMI) magnetograms to investigate the photospheric evolution and Atmospheric Imaging Assembly (AIA) data to analyze the relevant coronal structures. Key features of this quadrupolar region are a long sheared polarity inversion line (PIL) in the central delta-sunspots and a coronal arcade above the PIL. We find that these features are responsible for the production of intense flares, including an X2.2-class event. Based on the observations, we then propose two possible models for the creation of AR 11158 and conduct flux-emergence simulations of the two cases to reproduce this AR. Case 1 is the emergence of a single flux tube, which is split into two in the convection zone and emerges at two locations, while Case 2 is the emergence of two isolated but neighboring tubes. We find that, in Case 1, a sheared PIL and a coronal arcade are created in the middle of the region, which agrees with the AR 11158 observation. However, Case 2 never builds a clear PIL, which deviates from the observation. Therefore, we conclude that the flare-productive AR 11158 is, between the two cases, more likely to be created from a single split emerging flux than from two independent flux bundles.

DOI： 10.1007/s11207-014-0502-1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We report a detailed event analysis on the M6.6-class flare in the active
region (AR) NOAA 11158 on 2011 February 13. AR 11158, which consisted of two
major emerging bipoles, showed prominent activities including one X- and
several M-class flares. In order to investigate the magnetic structures related
to the M6.6 event, particularly the formation process of a flare-triggering
magnetic region, we analyzed multiple spacecraft observations and numerical
results of a flare simulation. We observed that, in the center of this
quadrupolar AR, a highly sheared polarity inversion line (PIL) was formed
through proper motions of the major magnetic elements, which built a sheared
coronal arcade lying over the PIL. The observations lend support to the
interpretation that the target flare was triggered by a localized magnetic
region that had an intrusive structure, namely a positive polarity penetrating
into a negative counterpart. The geometrical relationship between the sheared
coronal arcade and the triggering region was consistent with the theoretical
flare model based on the previous numerical study. We found that the formation
of the trigger region was due to a continuous accumulation of the small-scale
magnetic patches. A few hours before the flare occurrence, the series of
emerged/advected patches reconnected with a preexisting fields. Finally, the
abrupt flare eruption of the M6.6 event started around 17:30 UT. Our analysis
suggests that, in a triggering process of a flare activity, all magnetic
systems of multiple scales, not only the entire AR evolution but also the fine
magnetic elements, are altogether involved.

DOI： 10.1088/0004-637X/773/2/128

Web of Science

arXiv

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Other Link： http://arxiv.org/pdf/1306.2451v1

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Solar flares and coronal mass ejections, the most catastrophic eruptions in our solar system, have been known to affect terrestrial environments and infrastructure. However, because their triggering mechanism is still not sufficiently understood, our capacity to predict the occurrence of solar eruptions and to forecast space weather is substantially hindered. Even though various models have been proposed to determine the onset of solar eruptions, the types of magnetic structures capable of triggering these eruptions are still unclear. In this study, we solved this problem by systematically surveying the nonlinear dynamics caused by a wide variety of magnetic structures in terms of three-dimensional magnetohydrodynamic simulations. As a result, we determined that two different types of small magnetic structures favor the onset of solar eruptions. These structures, which should appear near the magnetic polarity inversion line (PIL), include magnetic fluxes reversed to the potential component or the nonpotential component of major field on the PIL. In addition, we analyzed two large flares, the X-class flare on 2006 December 13 and the M-class flare on 2011 February 13, using imaging data provided by the Hinode satellite, and we demonstrated that they conform to the simulation predictions. These results suggest that forecasting of solar eruptions is possible with sophisticated observation of a solar magnetic field, although the lead time must be limited by the timescale of changes in the small magnetic structures.

DOI： 10.1088/0004-637X/760/1/31

Web of Science

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Language：English   Publishing type：Doctoral thesis  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Frequencies of magnetic patch processes on the supergranule boundary, namely, flux emergence, splitting, merging, and cancellation, are investigated through automatic detection. We use a set of line-of-sight magnetograms taken by the Solar Optical Telescope (SOT) on board the Hinode satellite. We found 1636 positive patches and 1637 negative patches in the data set, whose time duration is 3.5 hr and field of view is 112 '' x 112 ''. The total numbers of magnetic processes are as follows: 493 positive and 482 negative splittings, 536 positive and 535 negative mergings, 86 cancellations, and 3 emergences. The total numbers of emergence and cancellation are significantly smaller than those of splitting and merging. Further, the frequency dependence of the merging and splitting processes on the flux content are investigated. Merging has a weak dependence on the flux content with a power-law index of only 0.28. The timescale for splitting is found to be independent of the parent flux content before splitting, which corresponds to similar to 33 minutes. It is also found that patches split into any flux contents with the same probability. This splitting has a power-law distribution of the flux content with an index of -2 as a time-independent solution. These results support that the frequency distribution of the flux content in the analyzed flux range is rapidly maintained by merging and splitting, namely, surface processes. We suggest a model for frequency distributions of cancellation and emergence based on this idea.

DOI： 10.1088/0004-637X/752/2/149

Web of Science

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1088/2041-8205/751/1/L9

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：EDP SCIENCES S A  

Context. The anelastic approximation is often adopted in numerical calculations with low Mach numbers, such as those including stellar internal convection. This approximation requires so-called frequent global communication, because of an elliptic partial differential equation. Frequent global communication is, however, negative factor for the parallel computing performed with a large number of CPUs.
Aims. We test the validity of a method that artificially reduces the speed of sound for the compressible fluid equations in the context of stellar internal convection. This reduction in the speed of sound leads to longer time steps despite the low Mach number, while the numerical scheme remains fully explicit and the mathematical system is hyperbolic, thus does not require frequent global communication.
Methods. Two- and three-dimensional compressible hydrodynamic equations are solved numerically. Some statistical quantities of solutions computed with different effective Mach numbers (owing to the reduction in the speed of sound) are compared to test the validity of our approach.
Results. Numerical simulations with artificially reduced speed of sound are a valid approach as long as the effective Mach number (based on the lower speed of sound) remains less than 0.7.

DOI： 10.1051/0004-6361/201118268

Web of Science

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ASTRONOMICAL SOC PACIFIC  

The cancellation is a convergence and a disappearance of the two opposite polarities and is thought as a main process of the flux disappearance in the photosphere. We investigate the spatial structures and time evolutions of Doppler velocity field around the cancellations in the quiet Sun by using Filtergram (FG) onboard Hiltode. We found the characteristic redshifts in 7 cancellations, blue shifts in 2 cancellations, and no characteristic Doppler velocity in 3 events. It is found that the stable downflow is coincident with the flux decrease. These results suggest that the Omega-loop submergence is a dominant scenario in the cancellation of the quiet Sun.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ASTRONOMICAL SOC PACIFIC  

We investigate the frequency of magnetic activities, namely flux emergence, splitting, merging, and cancellation, through an automatic detection in order to understand the generation of the power-law distribution of magnetic flux reported by Parnell et al. (2009). Quiet Sun magnetograms observed in the Na I 5896 angstrom line by the Hinode Solar Optical Telescope are used in this study. The longitudinal fluxes of the investigated patches range from approximate to 10(17) Mx to approximate to 10(19) Mx. Emergence and cancellation are much less frequent than merging and splitting. The time scale for splitting is found to be approximate to 33 minutes and independent of the flux contained in the splitting patch. Moreover magnetic patches split into any flux contents with equal probability. It is shown that such a fragmentation process leads to a distribution with a power-law index -2. Merging has a very weak dependence on flux content, with a power-law index of only -0.33. These results suggest that (1) magnetic patches are fragmented by splitting, merging, and tiny cancellation; and (2) flux is removed from the photosphere through tiny cancellations after these fragmentations.

Web of Science

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

A cancellation is thought to be a basic process of the photospheric magnetic field and plays an important role in magnetic flux budget and in various solar activities. There are two major theoretical scenarios for this phenomena, the "U-loop emergence" and the "Omega-loop submergence" models. It is important to clarify which is the dominant process during the cancellation for the estimation of the solar magnetic flux transport through the surface. We study the vector magnetic field and velocity structures around a quiet-Sun cancellation by using the Solar Optical Telescope on board the Hinode satellite. Transverse magnetic field connecting the canceling magnetic features and strong long-lasting Doppler redshift signal are found. The transverse field is observed in the first spectropolarimetric observation after the occurrence of the cancellation while the redshift is clearly delayed to the cancellation by 20 minutes. These results indicate that the observed cancellation is an "Omega-loop submergence."

DOI： 10.1088/0004-637X/713/1/325

Web of Science

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J-GLOBAL

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J-GLOBAL

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Article

CiNii Books

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Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

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J-GLOBAL

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Article

CiNii Books

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Language：Japanese   Publisher：京都大学大学院理学研究科附属天文台  

CiNii Article

CiNii Books

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