305 research outputs found
Detection of the Horizontal Divergent Flow prior to the Solar Flux Emergence
It is widely accepted that solar active regions including sunspots are formed
by the emerging magnetic flux from the deep convection zone. In previous
numerical simulations, we found that the horizontal divergent flow (HDF) occurs
before the flux emergence at the photospheric height. This Paper reports the
HDF detection prior to the flux emergence of NOAA AR 11081, which is located
away from the disk center. We use SDO/HMI data to study the temporal changes of
the Doppler and magnetic patterns from those of the reference quiet Sun. As a
result, the HDF appearance is found to come before the flux emergence by about
100 minutes. Also, the horizontal speed of the HDF during this time gap is
estimated to be 0.6 to 1.5 km s^-1, up to 2.3 km s^-1. The HDF is caused by the
plasma escaping horizontally from the rising magnetic flux. And the interval
between the HDF and the flux emergence may reflect the latency during which the
magnetic flux beneath the solar surface is waiting for the instability onset to
the further emergence. Moreover, SMART Halpha images show that the
chromospheric plages appear about 14 min later, located co-spatial with the
photospheric pores. This indicates that the plages are caused by plasma flowing
down along the magnetic fields that connect the pores at their footpoints. One
importance of observing the HDF may be the possibility to predict the sunspot
appearances that occur in several hours.Comment: 32 pages, 8 figures, 3 tables, accepted for publication in Ap
Evolution of Eruptive Flares II. The Occurrence of the Locally Enhanced Resistivity in the Preflare Phase
Twist, Writhe & Helicity in the inner penumbra of a sunspot
The aim of this work is the determination of the twist, writhe, and self
magnetic helicity of penumbral filaments located in an inner Sunspot penumbra.
To this extent, we inverted data taken with the spectropolarimeter (SP) aboard
Hinode with the SIR (Stokes Inversion based on Response function) code. For the
construction of a 3D geometrical model we applied a genetic algorithm
minimizing the divergence of the magnetic field vector and the net
magnetohydrodynamic force, consequently a force-free solution would be reached
if possible. We estimated two proxies to the magnetic helicity frequently used
in literature: the force-free parameter and the current helicity term. We show
that both proxies are only qualitative indicators of the local twist as the
magnetic field in the area under study significantly departures from a
force-free configuration. The local twist shows significant values only at the
borders of bright penumbral filaments with opposite signs on each side. These
locations are precisely correlated to large electric currents. The average
twist (and writhe) of penumbral structures is very small. The spines (dark
filaments in the background) show a nearly zero writhe. The writhe per unit
length of the intraspines diminishes with increasing length of the tube axes.
Thus, the axes of tubes related to intraspines are less wrung when the tubes
are more horizontal. As the writhe of the spines is very small, we can conclude
that the writhe reaches only significant values when the tube includes the
border of a intraspine.Comment: 7 pages, 4 figures; Astrophysical Journal, in pres
Supersonic Downflows at the Umbra-Penumbra Boundary of Sunspots
High resolution spectropolarimetric observations of 3 sunspots taken with
Hinode demonstrate the existence of supersonic downflows at or close to the
umbra-penumbra boundary which have not been reported before. These downflows
are confined to large patches, usually encompassing bright penumbral filaments,
and have lifetimes of more than 14 hr. The presence of strong downflows in the
center-side penumbra near the umbra rules out an association with the Evershed
flow. Chromospheric filtergrams acquired close to the time of the
spectropolarimetric measurements show large, strong, and long-lived
brightenings in the neighborhood of the downflows. The photospheric intensity
also exhibit persistent brightenings comparable to the quiet Sun.
Interestingly, the orientation of the penumbral filaments at the site of the
downflows is similar to that resulting from the reconnection process described
by Ryutova et al. The existence of such downflows in the inner penumbra
represents a challenge for numerical models of sunspots because they have to
explain them in terms of physical processes likely affecting the chromosphere.Comment: Accepted for publication in Ap
3D MHD Flux Emergence Experiments: Idealized models and coronal interactions
This paper reviews some of the many 3D numerical experiments of the emergence
of magnetic fields from the solar interior and the subsequent interaction with
the pre-existing coronal magnetic field. The models described here are
idealized, in the sense that the internal energy equation only involves the
adiabatic, Ohmic and viscous shock heating terms. However, provided the main
aim is to investigate the dynamical evolution, this is adequate. Many
interesting observational phenomena are explained by these models in a
self-consistent manner.Comment: Review article, accepted for publication in Solar Physic
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