92 research outputs found
Signatures of the impact of flare ejected plasma on the photosphere of a sunspot light-bridge
We investigate the properties of a sunspot light-bridge, focusing on the
changes produced by the impact of a plasma blob ejected from a C-class flare.
We observed a sunspot in active region NOAA 12544 using spectropolarimetric
raster maps of the four Fe I lines around 15655 \AA\ with the GREGOR Infrared
Spectrograph (GRIS), narrow-band intensity images sampling the Fe I 6173 \AA\
line with the GREGOR Fabry-P\'erot Interferometer (GFPI), and intensity broad
band images in G-band and Ca II H band with the High-resolution Fast Imager
(HiFI). All these instruments are located at the GREGOR telescope at the
Observatorio del Teide, Tenerife, Spain. The data cover the time before,
during, and after the flare event. The analysis is complemented with
Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI)
data from the Solar Dynamics Observatory (SDO). The physical parameters of the
atmosphere at differents heights were inferred using spectral-line inversion
techniques. We identify photospheric and chromospheric brightenings, heating
events, and changes in the Stokes profiles associated to the flare eruption and
the subsequent arrival of the plasma blob to the light bridge, after traveling
along an active region loop. The measurements suggest that these phenomena are
the result of reconnection events driven by the interaction of the plasma blob
with the magnetic field topology of the light bridge.Comment: Accepted for publication in A&
Properties of the inner penumbral boundary and temporal evolution of a decaying sunspot
It was empirically determined that the umbra-penumbra boundaries of stable
sunspots are characterized by a constant value of the vertical magnetic field.
We analyzed the evolution of the photospheric magnetic field properties of a
decaying sunspot belonging to NOAA 11277 between August 28 - September 3, 2011.
The observations were acquired with the spectropolarimeter on-board of the
Hinode satellite. We aim to proof the validity of the constant vertical
magnetic-field boundary between the umbra and penumbra in decaying sunspots. A
spectral-line inversion technique was used to infer the magnetic field vector
from the full-Stokes profiles. In total, eight maps were inverted and the
variation of the magnetic properties in time were quantified using linear or
quadratic fits. We found a linear decay of the umbral vertical magnetic field,
magnetic flux, and area. The penumbra showed a linear increase of the vertical
magnetic field and a sharp decay of the magnetic flux. In addition, the
penumbral area quadratically decayed. The vertical component of the magnetic
field is weaker on the umbra-penumbra boundary of the studied decaying sunspot
compared to stable sunspots. Its value seem to be steadily decreasing during
the decay phase. Moreover, at any time of the shown sunspot decay, the inner
penumbra boundary does not match with a constant value of the vertical magnetic
field, contrary to what was seen in stable sunspots. During the decaying phase
of the studied sunspot, the umbra does not have a sufficiently strong vertical
component of the magnetic field and is thus unstable and prone to be
disintegrated by convection or magnetic diffusion. No constant value of the
vertical magnetic field was found for the inner penumbral boundary.Comment: Accepted for publication in Astronomy & Astrophysics, 6 pages, 7
figure
Ca II 8542 \AA\ brightenings induced by a solar microflare
We study small-scale brightenings in Ca II 8542 \AA\ line-core images to
determine their nature and effect on localized heating and mass transfer in
active regions. High-resolution 2D spectroscopic observations of an active
region in the Ca II 8542 \AA\ line were acquired with the GFPI attached to the
1.5-meter GREGOR telescope. Inversions of the spectra were carried out using
NICOLE. We identified three brightenings of sizes up to 2"x2". We found
evidence that the brightenings belonged to the footpoints of a microflare (MF).
The properties of the observed brightenings disqualified the scenarios of
Ellerman bombs or IRIS bombs. However, this MF shared some common properties
with flaring active-region fibrils or flaring arch filaments (FAFs): (1) FAFs
and MFs are both apparent in chromospheric and coronal layers according to the
AIA channels, and (2) both show flaring arches with lifetimes of about 3.0-3.5
min and lengths of about 20". The inversions revealed heating by 600 K at the
footpoint location in the ambient chromosphere during the impulsive phase.
Connecting the footpoints, a dark filamentary structure appeared in the Ca II
line-core images. Before the start of the MF, the spectra of this structure
already indicated average blueshifts, meaning upward motions of the plasma
along the LOS. During the impulsive phase, these velocities increased up to
-2.2 km/s. Downflows dominated at the footpoints. However, in the upper
photosphere, slight upflows occurred during the impulsive phase. Hence,
bidirectional flows are present in the footpoints of the MF. Conclusions: We
detected Ca II brightenings that coincided with the footpoint location of an
MF. The MF event led to a rise of plasma in the upper photosphere, both before
and during the impulsive phase. Excess mass, previously raised to at most
chromospheric layers, slowly drained downward along arches toward the
footpoints of the MF.Comment: Accepted for publication in Astronomy & Astrophysics, 13 pages, 6
figures, 1 online movi
Flare-induced changes of the photospheric magnetic field in a -spot deduced from ground-based observations
Aims: Changes of the magnetic field and the line-of-sight velocities in the
photosphere are being reported for an M-class flare that originated at a
-spot belonging to active region NOAA 11865.
Methods: High-resolution ground-based near-infrared spectropolarimetric
observations were acquired simultaneously in two photospheric spectral lines,
Fe I 10783 \AA\ and Si I 10786 \AA, with the Tenerife Infrared Polarimeter at
the Vacuum Tower Telescope (VTT) in Tenerife on 2013 October 15. The
observations covered several stages of the M-class flare. Inversions of the
full-Stokes vector of both lines were carried out and the results were put into
context using (extreme)-ultraviolet filtergrams from the Solar Dynamics
Observatory (SDO).
Results: The active region showed high flaring activity during the whole
observing period. After the M-class flare, the longitudinal magnetic field did
not show significant changes along the polarity inversion line (PIL). However,
an enhancement of the transverse magnetic field of approximately 550 G was
found that bridges the PIL and connects umbrae of opposite polarities in the
-spot. At the same time, a newly formed system of loops appeared
co-spatially in the corona as seen in 171 \AA\ filtergrams of the Atmospheric
Imaging Assembly (AIA) on board SDO. However, we cannot exclude that the
magnetic connection between the umbrae already existed in the upper atmosphere
before the M-class flare and became visible only later when it was filled with
hot plasma. The photospheric Doppler velocities show a persistent upflow
pattern along the PIL without significant changes due to the flare.
Conclusions: The increase of the transverse component of the magnetic field
after the flare together with the newly formed loop system in the corona
support recent predictions of flare models and flare observations.Comment: 8 pages, 9 figures. Accepted for publication in Astronomy &
Astrophysic
Temporal evolution of arch filaments as seen in He I 10830 \r{A}
We study the evolution of an arch filament system (AFS) and of its individual
arch filaments to learn about the processes occurring in them. We observed the
AFS at the GREGOR solar telescope on Tenerife at high cadence with the very
fast spectroscopic mode of the GREGOR Infrared Spectrograph (GRIS) in the He I
10830 \AA\ spectral range. The He I triplet profiles were fitted with analytic
functions to infer line-of-sight (LOS) velocities to follow plasma motions
within the AFS. We tracked the temporal evolution of an individual arch
filament over its entire lifetime, as seen in the He I 10830 \AA\ triplet. The
arch filament expanded in height and extended in length from 13" to 21". The
lifetime of this arch filament is about 30 min. About 11 min after the arch
filament is seen in He I, the loop top starts to rise with an average Doppler
velocity of 6 km/s. Only two minutes later, plasma drains down with supersonic
velocities towards the footpoints reaching a peak velocity of up to 40 km/s in
the chromosphere. The temporal evolution of He I 10830 \AA\ profiles near the
leading pore showed almost ubiquitous dual red components of the He I triplet,
indicating strong downflows, along with material nearly at rest within the same
resolution element during the whole observing time. We followed the arch
filament as it carried plasma during its rise from the photosphere to the
corona. The material then drained toward the photosphere, reaching supersonic
velocities, along the legs of the arch filament. Our observational results
support theoretical AFS models and aids in improving future models.Comment: Accepted for publication in Astronomy & Astrophysics, 12 pages, 15
figures, 1 online movi
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