T. Maeshiro

Measurement of Magnetic Helicity Injection and Free Energy Loading into the Solar Corona

We develop a new methodology that can determine magnetic helicity flux as well as Poynting flux across the photosphere based on magnetograph observation. By applying this method, we study the injection mechanism of magnetic helicity and magnetic free energy into the solar corona. In order to derive the helicity and energy fluxes, first the velocity tangential to the solar surface is constructed by applying a correlation tracking technique on the magnetic observation, and second, the velocity component normal to 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 and energy fluxes separately for the shear motion effect and for the flux emergence effect. Based on this new method, NOAA Active Region 8100 was analyzed from 1997 November 1 to 5 using data observed by the Solar and Heliospheric Observatory Michelson Doppler Interferometer and the vector magnetograph at the National Astronomical Observatory of Japan (NAOJ) in Tokyo. The results indicate that the photospheric shear motion and the flux emergence process have equally contributed to the helicity injection and have supplied magnetic helicity of opposite signs into this active region.

The Trigger Mechanism of Solar Flares in a Coronal Arcade with Reversed Magnetic Shear

We have investigated the possibility that magnetic reconnection between oppositely sheared magnetic loops works as a trigger mechanism of solar flares, based on three-dimensional numerical simulations. The simulations were carried out by applying a slow footpoint motion, which reverses a preloaded magnetic shear, in the vicinity of the magnetic neutral line. The simulation results clearly indicated that the reversal of magnetic shear can cause a large-scale eruption of the magnetic arcade through a series of two different kinds of magnetic reconnections. The first reconnection is initiated by the resistive-tearing mode instability growing on the magnetic shear inversion layer and annihilates the sheared magnetic fluxes, which are oppositely directed along the magnetic neutral line. As a result of this, the magnetic arcade collapses into the reconnection point, and a new current sheet is generated above and below the shear inversion layer. The generation of new current sheets is followed by another magnetic reconnection, which drives the eruption of the sheared magnetic arcade. Mutual excitation of the two reconnections may explain the explosive property of the flare onset.

Annihilation of magnetic helicity: A new model for solar flare onset

Abstract Although magnetic reconnection is believed to be a key process for solar flares, the onset mechanism of flares remains as a long-standing problem. In this paper, we propose a new model for solar flares, in which magnetic reconnection converts oppositely-sheared field into the shear-free field. This process can be understood as the annihilation of magnetic helicities with different signs, and it is consistent with the recent results of the helicity observations. Numerical simulations were carried, which show that, if the helicity is sharply reversed within a magnetic arcade, reconnection quickly grows in the helicity inversion layer, driving explosive dynamics. The explosive process appears as a result of nonlinear instability, which grows faster than exponentially with time. Based on the results, we predict that the coexistence of the positive and negative helicity is crucial for the onset of flares.

Magnetic helicity injection and sigmoidal coronal loops

We studied the relationship between magnetic helicity injection and the formation of sigmoidal loops. We analyzed seven active regions: three regions showed coronal loops similar to the potential field, and four regions showed the sigmoidal loops. The magnetic helicity injection rate was evaluated using the method proposed by Kusano et al. In order to compare the helicity of regions of various sizes, we defined the normalized helicity injection rate as the magnetic helicity injection rate divided by the magnetic flux squared. We found that the sigmoidal regions and nonsigmoidal regions have comparable normalized helicity injection rates. Next, we calculated the magnetic helicity content of the sigmoidal loops by using the magnetic flux tube model (Longcope & Welsch) and compared it with the magnetic helicity injected from around the footpoints of three sigmoidal loops. For two sigmoidal loops, it is found that these values are comparable. Another loop showed significant disagreement between helicity injection rate and its magnetic helicity content. Excluding this region on the basis of its complexity (perhaps multiple loops forming a sigmoidal loop), we can conclude that geometric twist of the sigmoidal loops is consistent with the magnetic helicity injected from around the footpoints of the sigmoidal loops.

A STATISTICAL STUDY OF THE CORRELATION BETWEEN MAGNETIC HELICITY INJECTION AND SOFT X-RAY ACTIVITY IN SOLAR ACTIVE REGIONS

The correlation between magnetic helicity injection across the photosphere and soft X-ray activity in the solar corona is statistically investigated for seven active regions appearing in the years 1997-2000. The magnetic helicity flux into the solar corona is analyzed by the induction equation method, using magnetograms observed by the Michelson Doppler Imager on SOHO and by the vector magnetograph at the National Astronomical Observatory of Japan. Soft X-ray activity is evaluated from the data observed by the Yohkoh soft X-ray telescope (SXT). Soft X-ray activity of active regions in nonflare phases is found to correlate better with unsigned magnetic helicity flux than with the simple integration of the magnetic helicity flow. In addition, several magnetic variables, e.g., magnetic flux and electric current flux, are investigated, and it is confirmed that any fluxes given by the area integration of magnetic variables are well correlated with soft X-ray activity. However, for the magnetic helicity flow, not only the whole area flux but also the local intensity correlates well with the soft X-ray intensity. The relation between the spatial structure of the magnetic shear and soft X-ray activity is also investigated, and it is revealed that structural complexity in the magnetic shear tends to increase the efficiency of energy liberation in the solar corona. These results indicate that the magnetic helicity injected from the photosphere is relevant to the heating process in the solar corona, although several magnetic variables, not only magnetic helicity, could be related to that.

Relation between magnetic helicity injection and flare activities in active region NOAA 8100

Abstract The analysis of the magnetic helicity injection into active region NOAA 8100 is shown. This region is known as a source of many coronal activities such as flares. It is found that most of the flare events occurred about half a day after the helicity injection rate changed its sign. And the positions of Hα emission in flares well correspond to the helicity inversion lines in space. These results suggest that the sign-reversal of the helicity injection rate is a key signature of flare activities.

Measurement of magnetic helicity flux into the solar corona

Abstract 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.

Simulation study of the Solar flare onset mechanism and the self-organization in the Solar coronal plasma

Three-dimensional magnetobydrodynamics of the Solar coronal plasma is investigated by numerical simulation, aiming to understand the mechanisms of the Solar flare onset. It is demonstrated by the simulations that the resistive tearing mode instability growing on the magnetic shear inversion layer can drive the large-scale eruption through the mutual excitation of double reconnections. It is also revealed that the instability is able to cause the magnetohydrodynamic energy relaxation, in which the typical sigmoidal structure is self-organized prior to the onset of eruption. The simulation insults predict that both the formation of sigmoids and the onset of flares should occur around the electric current sheet where the magnetic shear is steeply reversed. It is consistent with the reversed-shear flare model and tlie vector inagnetograph observations.

Helicity injection into the solar corona

Abstract We developed a new methodology to measure magnetic helicity as well as magnetic energy injection into the solar corona from magnetograph observations, and we studied the relationship between helicity injection and X-ray activity in the solar corona. In order to calculate the gauge-invariant helicity flux and the Poynting flux across the photosphere, first the velocity utangenital to the photospheric surface is constructed by applying the correlation tracking technique on SOHO/MDI observations, and second the normal velocity is calculated by solving the induction equation as an inverse-problem using data from SOHO/MDI and the Solar Flare Telescope at NAOJ in Tokyo. The helicity injection as well as the free energy build-up is analyzed for active region NOAA 8100 from November 1 to 4, 1997. The results indicate that the emerging flux and the shear flow inject magnetic helicity of opposite sign (positive and negative, respectively) into the active region prior to a series of flares. Furthermore it is found that during the helicity injection process magnetic free energy in the amount of 5×1032 erg was supplied, which is much more than the X-ray flux emitted by the flares.