P. Vemareddy, A. Ambastha, R. A. Maurya, J. Chae
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.
View original:
http://arxiv.org/abs/1202.5195
No comments:
Post a Comment