308 research outputs found
Initial Observations of Sunspot Oscillations Excited by Solar Flare
Observations of a large solar flare of December 13, 2006, using Solar Optical
Telescope (SOT) on Hinode spacecraft revealed high-frequency oscillations
excited by the flare in the sunspot chromosphere. These oscillations are
observed in the region of strong magnetic field of the sunspot umbra, and may
provide a new diagnostic tool for probing the structure of sunspots and
understanding physical processes in solar flares.Comment: 10 pages, 6 figures, ApJL in pres
Changes in the subsurface stratification of the Sun with the 11-year activity cycle
We report on the changes of the Sun's subsurface stratification inferred from
helioseismology data. Using SOHO/MDI (SOlar and Heliospheric
Observatory/Michelson Doppler Imager) data for the last 9 years and, more
precisely, the temporal variation of f-mode frequencies, we have computed the
variation of the radius of subsurface layers of the Sun by applying
helioseismic inversions. We have found a variability of the ``helioseismic''
radius in antiphase with the solar activity, with the strongest variations of
the stratification being just below the surface around 0.995.
Besides, the radius of the deeper layers of the Sun, between 0.975
and 0.99 changes in phase with the 11-year cycle.Comment: 14 pages, 7 figures, accepted in ApJ
Reconstruction of Solar Subsurfaces by Local Helioseismology
Local helioseismology has opened new frontiers in our quest for understanding
of the internal dynamics and dynamo on the Sun. Local helioseismology
reconstructs subsurface structures and flows by extracting coherent signals of
acoustic waves traveling through the interior and carrying information about
subsurface perturbations and flows, from stochastic oscillations observed on
the surface. The initial analysis of the subsurface flow maps reconstructed
from the 5 years of SDO/HMI data by time-distance helioseismology reveals the
great potential for studying and understanding of the dynamics of the quiet Sun
and active regions, and the evolution with the solar cycle. In particular, our
results show that the emergence and evolution of active regions are accompanied
by multi-scale flow patterns, and that the meridional flows display the
North-South asymmetry closely correlating with the magnetic activity. The
latitudinal variations of the meridional circulation speed, which are probably
related to the large-scale converging flows, are mostly confined in shallow
subsurface layers. Therefore, these variations do not necessarily affect the
magnetic flux transport. The North-South asymmetry is also pronounced in the
variations of the differential rotation ("torsional oscillations"). The
calculations of a proxy of the subsurface kinetic helicity density show that
the helicity does not vary during the solar cycle, and that supergranulation is
a likely source of the near-surface helicity.Comment: 17 pages, 10 figures, in "Cartography of the Sun and the Stars",
Editors: Rozelot, Jean-Pierre, Neiner, Corali
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
Evidence for collapsing fields in corona and photosphere during the 15 February 2011 X2.2 flare: SDO AIA and HMI Observations
We use high-resolution images of the sun obtained by the SDO/AIA instrument
to study the evolution of the coronal loops in a flaring solar active region.
During 15 February 2011 a X-2.2 class flare occurred in NOAA 11158, a
sunspot complex. We identify three distinct phases of the
coronal loop dynamics during this event: (i) {\it Slow rise phase}: slow rising
motion of the loop-tops prior to the flare in response to slow rise of the
underlying flux rope, (ii) {\it Collapse phase}: sudden contraction of the
loop-tops with lower loops collapsing earlier than the higher loops, and (iii)
{\it Oscillation phase}: the loops exhibit global kink oscillations after the
collapse phase at different periods, with period decreasing with decreasing
height of the loops. The period of these loop oscillations is used to estimate
the field strength in the coronal loops of different loop lengths in this
active region. Further, we also use SDO/HMI observations to study the
photospheric changes close to the polarity inversion line (PIL). The
longitudinal magnetograms show step-wise permanent decrease in the magnetic
flux after the flare over a coherent patch along the PIL. Further, we examine
the HMI Stokes I,Q,U,V profiles over this patch and find that the Stokes-V
signal systematically decreases while the Stokes-Q and U signal increases after
the flare. These observations suggest that close to the PIL the field
configuration became more horizontal after the flare. We also use HMI vector
magnetic field observations to quantify the changes in the field inclination
angle and found an inward collapse of the field lines towards the polarity
inversion line (PIL) by 10. These observations are consistent
with the "coronal implosion" scenario and its predictions about flare related
photospheric field changes.Comment: 27 pages, 7 figures, in press (Astrophysical Journal
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