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Simulation Study of Hemispheric Phase-Asymmetry in the Solar Cycle  

Kanya Kusano   Submitted: 2016-12-12 23:37

Observations of the sun suggest that solar activities systematically create north-south hemispheric asymmetries. For instance, the hemisphere in which the sunspot activity is more active tends to switch after the early half of each solar cycle. Svalgaard & Kamide (2013) recently pointed out that the time gaps of polar field reversal between the north and south hemispheres are simply consequences of the asymmetry of sunspot activity. However, the mechanism underlying the asymmetric feature in solar cycle activities is not yet well understood. In this paper, in order to explain the cause of the asymmetry from the theoretical point of view, we investigate the relationship between the dipole- and quadrupole-type components of the magnetic field in the solar cycle using the mean-field theory based on the flux transport dynamo model. As a result, we found that there are two different attractors of the solar cycle, in which either the north or the south polar field is first reversed, and that the flux transport dynamo model well explains the phase-asymmetry of sunspot activity and the polar field reversal without any ad hoc source of asymmetry.

Authors: D. Syukuya, K. Kusano
Projects: None

Publication Status: Accepted for publication in ApJ
Last Modified: 2016-12-21 12:21
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Magnetic Field Structures Triggering Solar Flares and Coronal Mass Ejections  

Kanya Kusano   Submitted: 2012-10-02 22:14

Solar flares and coronal mass ejections (CMEs), 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 December 13, 2006 and the M-class flare on February 13, 2011, 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 time scale of changes in the small magnetic structures.

Authors: K. Kusano, Y. Bamba, T. T. Yamamoto, Y. Iida, S. Toriumi, A. Asai
Projects: Hinode/SOT

Publication Status: ApJ, in press
Last Modified: 2012-10-03 11:25
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Study of Magnetic Helicity in the Solar Corona  

Kanya Kusano   Submitted: 2004-05-11 20:03

Several new methodologies to detect the magnetic helicity injection activity across the photosphere into the solar corona have been developed recently. We briefly review the basic principle of the typical two methods, so-called the local correlation tracking (LCT) method and the induction equation (IE) method. The reliability of them is examined using a three-dimensional model field. The benchmark examination indicates that, if the vector magnetic field data are available, the IE method can greatly improve the accuracy of the helicity flux measurement. The physical implication of the recent results of the magnetic helicity measurement is also discussed. In particular, we emphasize the possibility that not only the intensity but also the complexity of the magnetic helicity injection from the photosphere could activate the energy liberation process in the solar corona.

Authors: K.Kusano, T.Maeshiro, T.Yokoyama, and T.Sakurai
Projects: None

Publication Status: Solar-B Science Meeting proceedings (2004)
Last Modified: 2004-05-11 20:03
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ANNIHILATION OF MAGNETIC HELICITY: A NEW MODEL OF SOLAR FLARE ONSET MECHANISM  

Kanya Kusano   Submitted: 2003-05-13 00:10

Although magnetic reconnection is believed as a key process of solar flares, the onset mechanism of flares remains as a long-standing problem. In this paper, we propose a new model that solar flares can be triggered by magnetic reconnection to convert the oppositely-sheared field into the shear-free field. This process can be understood as the annihilation between magnetic helicities of the different signs, and is consistent with the recent results of the helicity observations. Numerical simulation was carried out to examine the model, and it indicates that, if the helicity is sharply reversed within a magnetic arcade, reconnection quickly grows at the helicity inversion layer, driving the explosive dynamics. The explosive process appears as a result of the nonlinear instability, which can grow faster than exponential function of time. These results well support the annihilation-of-helicity model as the flare onset mechanism. Based on the results, we predict that the coexistence of the positive and negative helicity is crucial for the onset of flares.

Authors: K.Kusano, T.Yokoyama, T.Maeshiro, T.Sakurai
Projects: None

Publication Status: COSPAR, Adv. Space Research, in press
Last Modified: 2003-05-13 00:10
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MEASUREMENT OF MAGNETIC HELICITY FLUX INTO THE SOLAR CORONA  

Kanya Kusano   Submitted: 2003-05-13 00:01

We developed a new methodology which can determine magnetic helicity flux across the photosphere based on the magnetograph observation. In order to derive the helicity flux, first the velocity tangential to the solar surface is constructed by applying a correlation tracking technique on the magnetic observation, and secondly the velocity component across the photosphere is derived from the condition that the magnetic evolution must be consistent with the induction equation. Through this procedure, we can determine the helicity flow across the photosphere as a function of time and space. Based on this new method, we analyzed magnetic helicity of the active regions NOAA 9026 and 9077, using the data taken by SOHO/MDI and the vector magnetograph at NAOJ/Tokyo. As a result, it was revealed that positive and negative helicity is supplied in each region simultaneously. In particular, temporary activation of the helicity injection of the both signs was observed prior to X-class flare events in the GOES classification. The investigation on the helicity distribution suggests some relationship between flare onset and the increase of complexity in the helicity structure.

Authors: K.Kusano, T.Maeshiro, T.Yokoyama, T.Sakurai
Projects: None

Publication Status: COSPAR, Adv. Space Research, in press
Last Modified: 2003-05-13 00:01
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Abstracts by Author
Simulation Study of Hemispheric Phase-Asymmetry in the Solar Cycle
Magnetic Field Structures Triggering Solar Flares and Coronal Mass Ejections
Study of Magnetic Helicity in the Solar Corona
ANNIHILATION OF MAGNETIC HELICITY: A NEW MODEL OF SOLAR FLARE ONSET MECHANISM
MEASUREMENT OF MAGNETIC HELICITY FLUX INTO THE SOLAR CORONA

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