1,552 research outputs found
Structure and Stability of Magnetic Fields in Solar Active Region12192 Based on Nonlinear Force-Free Field Modeling
We analyze a three-dimensional (3D) magnetic structure and its stability in
large solar active region(AR) 12192, using the 3D coronal magnetic field
constructed under a nonlinear force-free field (NLFFF) approximation. In
particular, we focus on the magnetic structure that produced an X3.1-class
flare which is one of the X-class flares observed in AR 12192. According to our
analysis, the AR contains multiple-flux-tube system, {\it e.g.}, a large flux
tube, both of whose footpoints are anchored to the large bipole field, under
which other tubes exist close to a polarity inversion line (PIL). These various
flux tubes of different sizes and shapes coexist there. In particular, the
later are embedded along the PIL, which produces a favorable shape for the
tether-cutting reconnection and is related to the X-class solar flare. We
further found that most of magnetic twists are not released even after the
flare, which is consistent with the fact that no observational evidence for
major eruptions was found. On the other hand, the upper part of the flux tube
is beyond a critical decay index, essential for the excitation of torus
instability before the flare, even though no coronal mass ejections (CMEs) were
observed. We discuss the stability of the complicated flux tube system and
suggest the reason for the existence of the stable flux tube. In addition, we
further point out a possibility for tracing the shape of flare ribbons, on the
basis of a detailed structural analysis of the NLFFF before a flare.Comment: 24 pages, 9 figures, accepted for publication in The Astrophysical
Journa
Improvement of solar cycle prediction: Plateau of solar axial dipole moment
Aims. We report the small temporal variation of the axial dipole moment near
the solar minimum and its application to the solar cycle prediction by the
surface flux transport (SFT) model. Methods. We measure the axial dipole moment
using the photospheric synoptic magnetogram observed by the Wilcox Solar
Observatory (WSO), the ESA/NASA Solar and Heliospheric Observatory Michelson
Doppler Imager (MDI), and the NASA Solar Dynamics Observatory Helioseismic and
Magnetic Imager (HMI). We also use the surface flux transport model for the
interpretation and prediction of the observed axial dipole moment. Results. We
find that the observed axial dipole moment becomes approximately constant
during the period of several years before each cycle minimum, which we call the
axial dipole moment plateau. The cross-equatorial magnetic flux transport is
found to be small during the period, although the significant number of
sunspots are still emerging. The results indicates that the newly emerged
magnetic flux does not contributes to the build up of the axial dipole moment
near the end of each cycle. This is confirmed by showing that the time
variation of the observed axial dipole moment agrees well with that predicted
by the SFT model without introducing new emergence of magnetic flux. These
results allows us to predict the axial dipole moment in Cycle 24/25 minimum
using the SFT model without introducing new flux emergence. The predicted axial
dipole moment of Cycle 24/25 minimum is 60--80 percent of Cycle 23/24 minimum,
which suggests the amplitude of Cycle 25 even weaker than the current Cycle 24.
Conclusions. The plateau of the solar axial dipole moment is an important
feature for the longer prediction of the solar cycle based on the SFT model.Comment: 5 pages, 3 figures, accepted for publication in A&A Lette
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