Abstract:
We examined the contribution of
fi
eld strength gradients for the degree of net vertical current
(
NVC
)
neutralization in
active regions
(
ARs
)
. We used photospheric vector magnetic
fi
eld observations of AR 11158 obtained by
Helioseismic and Magnetic Imager
on board
SDO
and
Hinode
. The vertical component of the electric current is
decomposed into twist and shear terms. The NVC exhibits systematic evolution owing to the presence of the sheared
polarity inversion line between rotating and shearing magnetic regions. We found that the sign of shear current
distribution is opposite in dominant pixels
(
60%
–
65%
)
to that of twist current distribution, and its time pro
fi
le bears
no systematic trend. This result indicates that the gradient of magnetic
fi
eld strength contributes to an opposite signed,
though smaller in magnitude, current to that contributed by the magnetic
fi
eld direction in the vertical component of
the current. Consequently, the net value of the shear current is negative in both polarity regions, which when added to
the net twist current reduces the direct current value in the north
(
B
z
>
0
)
polarity, resulting in a higher degree of NVC
neutralization. We conjecture that the observed opposite signs of shear and twist currents are an indication, according
to Parker, that the direct volume currents of
fl
ux tubes are canceled by their return currents, which are contributed by
fi
eld strength gradients. Furthermore, with the increase of spatial resolution, we found higher values of twist, shear
current distributions. However, the resolution effect is more useful in resolving the
fi
eld strength gradients, and
therefore suggests more contribution from shear current for the degree of NVC neutralization.
