115 research outputs found
Spiral-shaped wavefronts in a sunspot umbra
Solar active regions show a wide variety of oscillatory phenomena. The
presence of the magnetic field leads to the appearance of several wave modes,
whose behavior is determined by the sunspot thermal and magnetic structure. We
aim to study the relation between the umbral and penumbral waves observed at
the high photosphere and the magnetic field topology of the sunspot.
Observations of the sunspot in active region NOAA 12662 obtained with the
GREGOR telescope (Observatorio del Teide, Spain) were acquired on 2017 June 17.
The data set includes a temporal series in the Fe I 5435 \AA\ line obtained
with the imaging spectrograph GREGOR Fabry-P\'erot Interferometer (GFPI) and a
spectropolarimetric raster map acquired with the GREGOR Infrared Spectrograph
(GRIS) in the 10830 \AA\ spectral region. The Doppler velocity deduced from the
restored Fe I 5435 \AA\ line has been determined, and the magnetic field vector
of the sunspot has been inferred from spectropolarimetric inversions of the Ca
I 10839 \AA\ and the Si I 10827 \AA\ lines. A two-armed spiral wavefront has
been identified in the evolution of the two-dimensional velocity maps from the
Fe I 5435 \AA\ line. The wavefronts initially move counterclockwise in the
interior of the umbra, and develop into radially outward propagating running
penumbral waves when they reach the umbra-penumbra boundary. The horizontal
propagation of the wavefronts approximately follows the direction of the
magnetic field, which shows changes in the magnetic twist with height and
horizontal position. The spiral wavefronts are interpreted as the visual
pattern of slow magnetoacoustic waves which propagate upward along magnetic
field lines. Their apparent horizontal propagation is due to their sequential
arrival to different horizontal positions at the formation height of the Fe I
5435 \AA\ line, as given by the inclination and orientation of the magnetic
field.Comment: Accepted for publication in A&
Height variation of the cutoff frequency in a sunspot umbra
In the solar atmosphere, the acoustic cutoff frequency is a local quantity
which depends on the atmospheric height. It separates the low-frequency
evanescent waves from the high-frequency propagating waves. We measure the
cutoff frequency of slow magnetoacoustic waves at various heights of a sunspot
umbra and compare the results with the estimations from several analytical
formulae. We analyzed the oscillations in the umbra of a sunspot belonging to
active region NOAA 12662 observed in the 10830 \AA\ spectral region with the
GREGOR Infrared Spectrograph and in the Fe I 5435 \AA\ line with the GREGOR
Fabry-P\'erot Interferometer. Both instrumets are attached to the GREGOR
telescope at the Observatorio del Teide, Tenerife, Spain. We have computed the
phase and amplification spectra between the velocity measured from different
pairs of lines that sample various heights of the solar atmosphere. The cutoff
frequency and its height variation have been estimated from the inspection of
the spectra. At the deep umbral photosphere the cutoff frequency is around 5
mHz and it increases to 6 mHz at higher photospheric layers. At the
chromosphere the cutoff is mHz. The comparison of the
observationally determined cutoff with the theoretically predicted values
reveals an agreement in the general trend and a reasonable match at the
chromosphere, but also significant quantitative differences at the photosphere.
Our analyses show strong evidence of the variation of the cutoff frequency with
height in a sunspot umbra, which is not fully accounted for by current
analytical estimations. This result has implications for our understanding of
wave propagation, the seismology of active regions, and the evaluation of
heating mechanisms based on compressible waves.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
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