On the injection of helicity by shearing motion of fluxes in relation to Flares and CMEs

Astronomy and Astrophysics – Astrophysics – Solar and Stellar Astrophysics

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Submitted to The Astrophysical Journal

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An investigation of helicity injection by photospheric shear motions is presented for two active regions, NOAA 11158 and 11166, using LOS magnetic field observations obtained from the Helioseismic and magnetic Imager on-board Solar Dynamics Observatory. We derived the horizontal velocities in the active regions from Differential Affine Velocity Estimator(DAVE) technique. During the six day evolution period of the ARs, we found persistent strong shear motions at the maximum velocity in the range of 0.5-0.7 \kmps along the magnetic polarity inversion line and outward flows from the peripheral regions of the sunspots. The helicities injected in NOAA 11158 and 11166 during the six days' period were estimated as $13.30\times10^{42}$Mx$^2$ and $9.5\times10^{42}$Mx$^2$, respectively. Temporal profiles of helicity injection showed impulsive variations at the onset times of flares/CMEs due to the negative helicity injection in the dominant region of positive helicity density. The spatial examination of helicity density maps showed that these variations resulted mainly due to the negative helicity injection in the regions of opposite helicity that were co-spatial with flaring sites. These co-spatial and co-temporal variations of helicity injection with flares are interpreted to be due to the relaxation from the state of high shear by the observed motions to a lower or shear-free state releasing energy in the form of eruptive events. This agrees with the simulations by \citet{kusano2004} for the triggering mechanism of flares. However, for the flares of smaller magnitude no clear evidence of such changes was available. Our study suggests that the existence of opposite helicity fluxes can trigger eruptive events, viz., flares and CMEs, and promises to be useful in forecasting the transient activity of ARs.

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