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* News 04/04/20 * The archive is using a new backend database. This has thrown up a few SQL errors in the last few days. If you have any issues please email adavey@nso.edu with either the number of eprint you are trying to edit or a link to your preprint.

Intrusion of Magnetic Peninsula toward the Neighboring Opposite-polarity Region That Triggers the Largest Solar Flare in Solar Cycle 24  

Yumi Bamba   Submitted: 2020-05-05 00:17

The largest X9.3 solar flare in solar cycle 24 and the preceding X2.2 flare occurred on September 6, 2017, in the solar active region NOAA 12673. This study aims to understand the onset mechanism of these flares via analysis of multiple observational datasets from the Hinode and Solar Dynamics Observatory and results from a non-linear force-free field extrapolation. The most noticeable feature is the intrusion of a major negative-polarity region, appearing similar to a peninsula, oriented northwest into a neighboring opposite-polarity region. We also observe proxies of magnetic reconnection caused by related to the intrusion of the negative peninsula: rapid changes of the magnetic field around the intruding negative peninsula; precursor brightening at the tip of the negative peninsula, including a cusp-shaped brightening that shows a transient but significant downflow (~100 km s-1) at a leg of the cusp; a dark tube-like structure that appears to be a magnetic flux rope that erupted with the X9.3 flare; and coronal brightening along the dark tube-like structure that appears to represent the electric current generated under the flux rope. Based on these observational features, we propose that (1) the intrusion of the negative peninsula was critical in promoting the push-mode magnetic reconnection that forms and grows a twisted magnetic flux rope that erupted with the X2.2 flare, (2) the continuing intrusion progressing even beyond the X2.2 flare is further promoted to disrupt the equilibrium that leads the reinforcement of the magnetic flux rope that erupted with the X9.3 flare.

Authors: Yumi Bamba, Satoshi Inoue, Shinsuke Imada
Projects: Hinode/EIS,Hinode/SOT,Hinode/XRT,SDO-AIA,SDO-HMI

Publication Status: published (ApJ)
Last Modified: 2020-05-06 13:30
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The Role of a Tiny Brightening in a Huge Geo-effective Solar Eruption Leading to the St Patrick's Day Storm  

Yumi Bamba   Submitted: 2019-02-13 19:09

The largest magnetic storm in solar cycle 24 was caused by a fast coronal mass ejection (CME) that was related to a small C9.1 flare that occurred on 15 March 2015 in solar active region (AR) NOAA 12297. The purpose of this study is to understand the onset mechanism of the geo-effective huge solar eruption. We focused on the C2.4 flare that occurred prior to the C9.1 flare of the filament eruption. The magnetic field structure in the AR was complicated: there were several filaments including the one that erupted and caused the CME. We hence carefully investigated the photospheric magnetic field, brightenings observed in the solar atmosphere, and the three-dimensional coronal magnetic field extrapolated from nonlinear force-free field modeling, using data from Hinode and Solar Dynamics Observatory. We found three intriguing points : (1) There was a compact but noticeably highly twisted magnetic field structure that is represented by a small filament in the C2.4 flaring region, where a tiny precursor brightening was observed before the C2.4 flare. (2) The C2.4 flaring region is located in the vicinity of a foot point of the closed field that prohibits the filament from erupting. (3) The filament shows a sudden eruption after the C2.4 flare and accompanying small filament eruption. From our analysis, we suggest that a small magnetic disturbance that was represented by the tiny precursor brightening at the time of the C2.4 flare is related to the trigger of the huge filament eruption.

Authors: Yumi Bamba, Satoshi Inoue, and Keiji Hayashi
Projects: Hinode/SOT,SDO-AIA,SDO-HMI

Publication Status: accepted by the Astrophysical Journal
Last Modified: 2019-02-14 13:18
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Evaluation of Applicability of a Flare Trigger Model based on Comparison of Geometric Structures  

Yumi Bamba   Submitted: 2018-02-01 18:54

The triggering mechanism(s) and critical condition(s) of solar flares are still not completely clarified, although various studies have attempted to elucidate them. We have also proposed a theoretical flare-trigger model based on MHD simulations Kusano et al. 2012, in which two types of small-scale bipole field, the so-called Opposite Polarity (OP) and Reversed Shear (RS) types of field, can trigger flares. In this study, we evaluated the applicability of our flare-trigger model to observation of 32 flares that were observed by the Solar Dynamics Observatory (SDO), by focusing on geometrical structures. We classified the events into six types, including the OP and RS types, based on photospheric magnetic field configuration, presence of precursor brightenings, and shape of the initial flare ribbons. As a result, we found that approximately 30% of the flares were consistent with our flare-trigger model, and the number of RS type triggered flares is larger than that of the OP type. We found none of the sampled events contradicts our flare model, although we cannot clearly determine the trigger mechanism of 70% of the flares in this study. We carefully investigated the applicability of our flare-trigger model and the possibility that other models can explain the other 70% of the events. Consequently, we concluded that our flare-trigger model has certainly proposed important conditions for flare-triggering.

Authors: Yumi Bamba and Kanya Kusano
Projects: SDO-AIA,SDO-HMI

Publication Status: Accepted by the Astrophysical Journal
Last Modified: 2018-02-05 22:08
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Triggering Process of the X1.0 Three-ribbon Flare in the Great Active Region NOAA 12192  

Yumi Bamba   Submitted: 2017-04-03 20:04

The solar magnetic field in a flare-producing active region (AR) is much more complicated than theoretical models, which assume a very simple magnetic field structure. The X1.0 flare, which occurred in AR 12192 on 2014 October 25, showed a complicated three-ribbon structure. To clarify the trigger process of the flare and to evaluate the applicability of a simple theoretical model, we analyzed the data from Hinode/Solar Optical Telescope and the Solar Dynamics Observatory/Helioseismic and Magnetic Imager, Atmospheric Imaging Assembly. We investigated the spatio-temporal correlation between the magnetic field structures, especially the non-potentiality of the horizontal field, and the bright structures in the solar atmosphere. As a result, we determined that the western side of the positive polarity, which is intruding on a negative polarity region, is the location where the flare was triggered. This is due to the fact that the sign of the magnetic shear in that region was opposite that of the major shear of the AR, and the significant brightenings were observed over the polarity inversion line (PIL) in that region before flare onset. These features are consistent with the recently proposed flare-trigger model that suggests that small reversed shear (RS) magnetic disturbances can trigger solar flares. Moreover, we found that the RS field was located slightly off the flaring PIL, contrary to the theoretical prediction. We discuss the possibility of an extension of the RS model based on an extra numerical simulation. Our result suggests that the RS field has a certain flexibility for displacement from a highly sheared PIL, and that the RS field triggers more flares than we expected.

Authors: Y. Bamba, S. Inoue, K. Kusano, and D. Shiota
Projects: Hinode/SOT,SDO-AIA,SDO-HMI

Publication Status: Published
Last Modified: 2017-04-04 08:09
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Comparison between Hinode/SOT and SDO/HMI, AIA Data for the Study of the Solar Flare Trigger Process  

Yumi Bamba   Submitted: 2014-07-07 11:06

Understanding the mechanism that produces solar flares is important not only from the scientific point of view but also for improving space weather predictability. There are numerous observational and computational studies, which attempted to reveal the onset mechanism of solar flares. However, the underlying mechanism of flare onset remains elusive. 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) 1600Å 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.

Authors: Yumi Bamba, Kanya Kusano, Shinsuke Imada, Yusuke Iida
Projects: Hinode/SOT,SDO-AIA,SDO-HMI

Publication Status: Accepted to PASJ, Hinode special issue
Last Modified: 2014-07-09 13:41
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Study on Triggering Process of Solar Flares Based on Hinode/SOT Observations  

Yumi Bamba   Submitted: 2013-09-21 06:19

We investigated four major solar flare events that occurred in active regions NOAA 10930 (December 13 and 14, 2006) and NOAA 11158 (February 13 and 15, 2011) by using data observed by the Solar Optical Telescope (SOT) onboard the Hinode satellite. To reveal the trigger mechanism of solar flares, we analyzed the spatio-temporal correlation between the detailed magnetic field structure and the emission image of the Ca H line at the central part of flaring regions for several hours prior to the onset of flares. We observed in all the flare events that the magnetic shear angle in the flaring regions exceeded 70 degrees, as well as that characteristic magnetic disturbances developed at the centers of flaring regions in the pre-flare phase. These magnetic disturbances can be classified into two groups depending on the structure of their magnetic polarity inversion lines; the so-called ''Opposite-Polarity'' and ''Reversed-Shear'' magnetic field recently proposed by our group, although the magnetic disturbance in one event of the four samples is too subtle to clearly recognize the detailed structure. The result suggests that some major solar flares are triggered by rather small magnetic disturbances. We also show that the critical size of the flare-trigger field varies among flare events and briefly discuss how the flare-trigger process depends on the evolution of active regions.

Authors: Y. Bamba, K. Kusano, T. T. Yamamoto, and T. J. Okamoto
Projects: Hinode/SOT

Publication Status: in press.
Last Modified: 2013-09-25 17:53
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Study on Triggering Process of Solar Flares Based on Hinode/SOT Observations  

Yumi Bamba   Submitted: 2013-09-21 06:19

We investigated four major solar flare events that occurred in active regions NOAA 10930 (December 13 and 14, 2006) and NOAA 11158 (February 13 and 15, 2011) by using data observed by the Solar Optical Telescope (SOT) onboard the Hinode satellite. To reveal the trigger mechanism of solar flares, we analyzed the spatio-temporal correlation between the detailed magnetic field structure and the emission image of the Ca H line at the central part of flaring regions for several hours prior to the onset of flares. We observed in all the flare events that the magnetic shear angle in the flaring regions exceeded 70 degrees, as well as that characteristic magnetic disturbances developed at the centers of flaring regions in the pre-flare phase. These magnetic disturbances can be classified into two groups depending on the structure of their magnetic polarity inversion lines; the so-called ''Opposite-Polarity'' and ''Reversed-Shear'' magnetic field recently proposed by our group, although the magnetic disturbance in one event of the four samples is too subtle to clearly recognize the detailed structure. The result suggests that some major solar flares are triggered by rather small magnetic disturbances. We also show that the critical size of the flare-trigger field varies among flare events and briefly discuss how the flare-trigger process depends on the evolution of active regions.

Authors: Y. Bamba, K. Kusano, T. T. Yamamoto, and T. J. Okamoto
Projects: Hinode/SOT

Publication Status: in press.
Last Modified: 2013-09-30 10:01
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Abstracts by Author
Intrusion of Magnetic Peninsula toward the Neighboring Opposite-polarity Region That Triggers the Largest Solar Flare in Solar Cycle 24
The Role of a Tiny Brightening in a Huge Geo-effective Solar Eruption Leading to the St Patrick?s Day Storm
Evaluation of Applicability of a Flare Trigger Model based on Comparison of Geometric Structures
Triggering Process of the X1.0 Three-ribbon Flare in the Great Active Region NOAA 12192
Comparison between Hinode/SOT and SDO/HMI, AIA Data for the Study of the Solar Flare Trigger Process
Study on Triggering Process of Solar Flares Based on Hinode/SOT Observations
Study on Triggering Process of Solar Flares Based on Hinode/SOT Observations

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