explanation blue bibcodes open ADS page with paths to full text
Author name code: felipe
ADS astronomy entries on 2022-09-14
author:Felipe, Tobias
---------------------------------------------------------
Title: Limitations of the Ca II 8542 Å Line for the Determination
of Magnetic Field Oscillations
Authors: Felipe, Tobias; Socas Navarro, Hector; Sangeetha, C. R.;
Milic, Ivan
2021ApJ...918...47F Altcode: 2021arXiv210702160F
Chromospheric umbral oscillations produce periodic brightenings
in the core of some spectral lines, known as umbral flashes. They
are also accompanied by fluctuations in velocity, temperature, and,
according to several recent works, magnetic field. In this study, we
aim to ascertain the accuracy of the magnetic field determined from
inversions of the Ca II 8542 Å line. We have developed numerical
simulations of wave propagation in a sunspot umbra. Synthetic Stokes
profiles emerging from the simulated atmosphere were computed and
then inverted using the NICOLE code. The atmospheres inferred from
the inversions have been compared with the original parameters from
the simulations. Our results show that the inferred chromospheric
fluctuations in velocity and temperature match the known oscillations
from the numerical simulation. In contrast, the vertical magnetic field
obtained from the inversions exhibits an oscillatory pattern with a ~300
G peak-to-peak amplitude, which is absent in the simulation. We have
assessed the error in the inferred parameters by performing numerous
inversions with slightly different configurations of the same Stokes
profiles. We find that when the atmosphere is approximately at rest,
the inversion tends to favor solutions that underestimate the vertical
magnetic field strength. On the contrary, during umbral flashes,
the values inferred from most of the inversions are concentrated at
stronger fields than those from the simulation. Our analysis provides
a quantification of the errors associated with the inversions of the
Ca II 8542 Å line and suggests caution with the interpretation of
the inferred magnetic field fluctuations.
---------------------------------------------------------
Title: Multiple Stokes I inversions for inferring magnetic fields
in the spectral range around Cr I 5782 Å
Authors: Kuckein, C.; Balthasar, H.; Quintero Noda, C.; Diercke, A.;
Trelles Arjona, J. C.; Ruiz Cobo, B.; Felipe, T.; Denker, C.; Verma,
M.; Kontogiannis, I.; Sobotka, M.
2021A&A...653A.165K Altcode: 2021arXiv210711116K
<BR /> Aims: In this work, we explore the spectral window containing
Fraunhofer lines formed in the solar photosphere, around the
magnetically sensitive Cr I lines at 5780.9, 5781.1, 5781.7, 5783.0,
and 5783.8 Å, with Landé g-factors between 1.6 and 2.5. The goal is
to simultaneously analyze 15 spectral lines, comprising Cr I, Cu I,
Fe I, Mn I, and Si I lines, without the use of polarimetry, to infer
the thermodynamic and magnetic properties in strongly magnetized
plasmas using an inversion code. <BR /> Methods: Our study is based
on a new setup at the Vacuum Tower Telescope (VTT, Tenerife), which
includes fast spectroscopic scans in the wavelength range around
the Cr I 5781.75 Å line. The oscillator strengths log(gf) of all
spectral lines, as well as their response functions to temperature,
magnetic field, and Doppler velocity, were determined using the Stokes
Inversion based on Response functions (SIR) code. Snapshot 385 of the
enhanced network simulation from the Bifrost code serves to synthesize
all the lines, which are, in turn, inverted simultaneously with SIR to
establish the best inversion strategy. We applied this strategy to VTT
observations of a sunspot belonging to NOAA 12723 on 2018 September
30 and compared the results to full-disk vector field data obtained
with the Helioseismic and Magnetic Imager (HMI). <BR /> Results: The
15 simultaneously inverted intensity profiles (Stokes I) delivered
accurate temperatures and Doppler velocities when compared with the
simulations. The derived magnetic fields and inclinations achieve
the best level of accuracy when the fields are oriented along the
line-of-sight (LOS) and less accurate when the fields are transverse to
the LOS. In general, the results appear similar to what is reported in
the HMI vector-field data, although some discrepancies exist. <BR />
Conclusions: The analyzed spectral range has the potential to deliver
thermal, dynamic, and magnetic information for strongly magnetized
features on the Sun, such as pores and sunspots, even without the use
of polarimetry. The highest sensitivity of the lines is found in the
lower photosphere, on average, around log τ = −1. The multiple-line
inversions provide smooth results across the whole field of view
(FOV). The presented spectral range and inversion strategy will be
used for future VTT observing campaigns.
---------------------------------------------------------
Title: Performance of solar far-side active region neural detection
Authors: Broock, E. G.; Felipe, T.; Asensio Ramos, A.
2021A&A...652A.132B Altcode: 2021arXiv210609365B
Context. Far-side helioseismology is a technique used to infer the
presence of active regions in the far hemisphere of the Sun based on
the interpretation of oscillations measured in the near hemisphere. A
neural network has recently been developed to improve the sensitivity
of the seismic maps to the presence of far-side active regions. <BR
/> Aims: Our aim is to evaluate the performance of the new neural
network approach and to thoroughly compare it with the standard
method commonly applied to predict far-side active regions from seismic
measurements. <BR /> Methods: We have computed the predictions of active
regions using the neural network and the standard approach from five
years of far-side seismic maps as a function of the selected threshold
in the signatures of the detections. The results have been compared
with direct extreme ultraviolet observations of the far hemisphere
acquired with the Solar Terrestrial Relations Observatory. <BR />
Results: We have confirmed the improved sensitivity of the neural
network to the presence of far-side active regions. Approximately 96%
of the active regions identified by the standard method with a strength
above the threshold commonly employed by previous analyses are related
to locations with enhanced extreme ultraviolet emission. For this
threshold, the false positive ratio is 3.75%. For an equivalent
false positive ratio, the neural network produces 47% more true
detections. Weaker active regions can be detected by relaxing the
threshold in their seismic signature. For almost the entire range
of thresholds, the performance of the neural network is superior
to that of the standard approach, delivering a higher number of
confirmed detections and a lower rate of false positives. <BR />
Conclusions: The neural network is a promising approach for improving
the interpretation of the seismic maps provided by local helioseismic
techniques. Additionally, refined predictions of magnetic activity in
the non-visible solar hemisphere can play a significant role in space
weather forecasting.
---------------------------------------------------------
Title: Signatures of sunspot oscillations and the case for
chromospheric resonances
Authors: Felipe, Tobías
2021NatAs...5....2F Altcode: 2020arXiv200710471F; 2020NatAs.tmp..148F
Sunspots host a large variety of oscillatory phenomena, whose
properties depend on the nature of the wave modes and the magnetic and
thermodynamic structure of the spot. Umbral chromospheric oscillations
exhibit significant differences compared to their photospheric
counterparts. They show an enhanced power and a shorter dominant period,
from waves with an amplitude of a few hundred meters per second in the
five-minute band at the photosphere, to amplitudes of several kilometers
per second in the three-minute band at the chromosphere. Various models
have been proposed to explain this behaviour, including the presence
of a chromospheric resonance cavity between the photosphere and the
transition region. Jess et al. (2020, Nature Astronomy, 4, 220) claimed
the detection of observational evidence supporting this model, obtained
from the comparison of spectropolarimetric observations and numerical
simulations. Here, it is shown that the observational insight reported
by Jess et al. is not a common property of sunspots. More importantly,
numerical modelling also shows that it is not an unequivocal signature
of an acoustic resonator.
---------------------------------------------------------
Title: Downflowing umbral flashes as evidence of standing waves in
sunspot umbrae
Authors: Felipe, T.; Henriques, V. M. J.; de la Cruz Rodríguez, J.;
Socas-Navarro, H.
2021A&A...645L..12F Altcode: 2021arXiv210104188F
Context. Umbral flashes are sudden brightenings commonly visible
in the core of some chromospheric lines. Theoretical and numerical
modeling suggests that they are produced by the propagation of shock
waves. According to these models and early observations, umbral flashes
are associated with upflows. However, recent studies have reported
umbral flashes in downflowing atmospheres. <BR /> Aims: We aim to
understand the origin of downflowing umbral flashes. We explore how
the existence of standing waves in the umbral chromosphere impacts the
generation of flashed profiles. <BR /> Methods: We performed numerical
simulations of wave propagation in a sunspot umbra with the code
MANCHA. The Stokes profiles of the Ca II 8542 Å line were synthesized
with the NICOLE code. <BR /> Results: For freely propagating waves,
the chromospheric temperature enhancements of the oscillations are
in phase with velocity upflows. In this case, the intensity core of
the Ca II 8542 Å atmosphere is heated during the upflowing stage of
the oscillation. However, a different scenario with a resonant cavity
produced by the sharp temperature gradient of the transition region
leads to chromospheric standing oscillations. In this situation,
temperature fluctuations are shifted backward and temperature
enhancements partially coincide with the downflowing stage of the
oscillation. In umbral flash events produced by standing oscillations,
the reversal of the emission feature is produced when the oscillation
is downflowing. The chromospheric temperature keeps increasing while
the atmosphere is changing from a downflow to an upflow. During the
appearance of flashed Ca II 8542 Å cores, the atmosphere is upflowing
most of the time, and only 38% of the flashed profiles are associated
with downflows. <BR /> Conclusions: We find a scenario that remarkably
explains the recent empirical findings of downflowing umbral flashes
as a natural consequence of the presence of standing oscillations
above sunspot umbrae.
---------------------------------------------------------
Title: Chromospheric Resonances above Sunspots and Potential
Seismological Applications
Authors: Felipe, Tobias; Kuckein, Christoph; González Manrique,
Sergio Javier; Milic, Ivan; Sangeetha, C. R.
2020ApJ...900L..29F Altcode: 2020arXiv200810623F
Oscillations in sunspot umbrae exhibit remarkable differences
between the photosphere and chromosphere. We evaluate two competing
scenarios proposed for explaining those observations: a chromospheric
resonant cavity and waves traveling from the photosphere to upper
atmospheric layers. We have employed numerical simulations to
analyze the oscillations in both models. They have been compared with
observations in the low (Na I D<SUB>2</SUB>) and high (He I 10830 Å)
chromosphere. The nodes of the resonant cavity can be detected as
phase jumps or power dips, although the identification of the latter
is not sufficient to claim the existence of resonances. In contrast,
phase differences between velocity and temperature fluctuations reveal
standing waves and unequivocally prove the presence of an acoustic
resonator above umbrae. Our findings offer a new seismic method to probe
active region chromospheres through the detection of resonant nodes.
---------------------------------------------------------
Title: Numerical determination of the cutoff frequency in solar models
Authors: Felipe, T.; Sangeetha, C. R.
2020A&A...640A...4F Altcode: 2020arXiv200600526F
Context. In stratified atmospheres, acoustic waves can only propagate
if their frequency is higher than the cutoff value. The determination of
the cutoff frequency is fundamental for several topics in solar physics,
such as evaluating the contribution of the acoustic waves to the
chromospheric heating or the application of seismic techniques. However,
different theories provide different cutoff values. <BR /> Aims:
We developed an alternative method to derive the cutoff frequency
in several standard solar models, including various quiet-Sun and
umbral atmospheres. The effects of magnetic field and radiative losses
on the cutoff are examined. <BR /> Methods: We performed numerical
simulations of wave propagation in the solar atmosphere using the
code MANCHA. The cutoff frequency is determined from the inspection
of phase-difference spectra computed between the velocity signal
at two atmospheric heights. The process is performed by choosing
pairs of heights across all the layers between the photosphere and
the chromosphere to derive the vertical stratification of the cutoff
in the solar models. Result. The cutoff frequency predicted by the
theoretical calculations departs significantly from the measurements
obtained from the numerical simulations. In quiet-Sun atmospheres,
the cutoff shows a strong dependence on the magnetic field for
adiabatic wave propagation. When radiative losses are taken into
account, the cutoff frequency is greatly reduced and the variation
of the cutoff with the strength of the magnetic field is lower. The
effect of the radiative losses in the cutoff is necessary to understand
recent quiet-Sun and sunspot observations. In the presence of inclined
magnetic fields, our numerical calculations confirm that the cutoff
frequency is reduced as a result of the reduced gravity experienced by
waves that propagate along field lines. An additional reduction is also
found in regions with significant changes in the temperature, which is
due to the lower temperature gradient along the path of field-guided
waves. <BR /> Conclusions: Our results show solid evidence that the
cutoff frequency in the solar atmosphere is stratified. The cutoff
values are not correctly captured by theoretical estimates. In addition,
most of the widely used analytical cutoff formulae neglect the effect
of magnetic fields and radiative losses, whose role is critical for
determining the evanescent or propagating nature of the waves.
---------------------------------------------------------
Title: Chromospheric resonant cavities in umbrae: unequivocal
detection and seismic applications
Authors: Felipe, T.; Kuckein, C.; González Manrique, S. J.; Milic,
I.; Sangeetha, C. R.
2020sea..confE.196F Altcode:
Umbral chromospheric oscillations exhibit significant differences
compared to their photospheric counterparts. We evaluate two competing
scenarios proposed for explaining those observations: a chromospheric
resonant cavity and waves traveling from the photosphere to upper
atmospheric layers. The oscillatory signatures of both models have been
determined from numerical simulations, and they have been compared to
observations. We find that a high-frequency peak in the He I 10830 Å
power spectra cannot discriminate between both theories, contrary to the
claims of Jess et al. (2019). In contrast, phase differences between
velocity and temperature fluctuations reveal a standing pattern and
unequivocally prove the presence of an acoustic cavity above umbrae. Our
findings offer a new seismic method to probe sunspot chromospheres
through the identification of resonant nodes in phase spectra.
---------------------------------------------------------
Title: The magnetic structure and dynamics of a decaying active region
Authors: Kontogiannis, Ioannis; Kuckein, Christoph; González
Manrique, Sergio Javier; Felipe, Tobias; Verma, Meetu; Balthasar,
Horst; Denker, Carsten
2020IAUS..354...53K Altcode:
We study the evolution of the decaying active region NOAA 12708, from
the photosphere up to the corona using high resolution, multi-wavelength
GREGOR observations taken on May 9, 2018. We utilize spectropolarimetric
scans of the 10830 Å spectral range by the GREGOR Infrared Spectrograph
(GRIS), spectral imaging time-series in the Na ID<SUP>2</SUP> spectral
line by the GREGOR Fabry-Pérot Interferometer (GFPI) and context
imaging in the Ca IIH and blue continuum by the High-resolution Fast
Imager (HiFI). Context imaging in the UV/EUV from the Atmospheric
Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO)
complements our dataset. The region under study contains one pore with a
light-bridge, a few micro-pores and extended clusters of magnetic bright
points. We study the magnetic structure from the photosphere up to the
upper chromosphere through the spectropolarimetric observations in He
II and Si I and through the magnetograms provided by the Helioseismic
and Magnetic Imager (HMI). The high-resolution photospheric images
reveal the complex interaction between granular-scale convective
motions and a range of scales of magnetic field concentrations in
unprecedented detail. The pore itself shows a strong interaction with
the convective motions, which eventually leads to its decay, while,
under the influence of the photospheric flow field, micro-pores
appear and disappear. Compressible waves are generated, which are
guided towards the upper atmosphere along the magnetic field lines of
the various magnetic structures within the field-of-view. Modelling
of the He i absorption profiles reveals high velocity components,
mostly associated with magnetic bright points at the periphery
of the active region, many of which correspond to asymmetric Si I
Stokes-V profiles revealing a coupling between upper photospheric
and upper chromospheric dynamics. Time-series of Na ID<SUP>2</SUP>
spectral images reveal episodic high velocity components at the same
locations. State-of-the-art multi-wavelength GREGOR observations allow
us to track and understand the mechanisms at work during the decay
phase of the active region.
---------------------------------------------------------
Title: Inversions of synthetic umbral flashes: a selection of
wavelength sampling
Authors: Felipe, T.; Esteban Pozuelo, S.
2019A&A...632A..75F Altcode: 2019arXiv191013980F
Context. Imaging spectrographs are popular instruments used to obtain
solar data. They record quasi-monochromatic images at selected
wavelength positions. By scanning the spectral range of the line,
it is possible to obtain bidimensional maps of the field-of-view
with a moderate spectral resolution. <BR /> Aims: In this work, we
evaluate the quality of spectropolarimetric inversions obtained from
various wavelength samplings during umbral flashes. <BR /> Methods:
We computed numerical simulations of nonlinear wave propagation in
a sunspot and constructed synthetic Stokes profiles in the Ca II
8542 Å line during an umbral flash using the NLTE code NICOLE. The
spectral resolution of the Stokes profiles was downgraded to various
cases with differences in the wavelength coverage. A large set of
wavelength samplings was analyzed and the performance of the inversions
was evaluated by comparing the inferred chromospheric temperature,
velocity, and magnetic field with the actual values at the chromosphere
of the numerical simulation. <BR /> Results: The errors in the inverted
results depend to a large extent on the location of the wavelength
points across the profile of the line. The inferred magnetic field
improves with the increase of the spectral resolution. In the case
of velocity and temperature, low spectral resolution data produce a
match of the inverted atmospheres with the actual values comparable
to wavelength samplings with finer resolution, while providing a
higher temporal cadence in the data acquisition. <BR /> Conclusions:
We validated the NLTE inversions of spectropolarimetric data from the Ca
II 8542 Å during umbral flashes, during which the atmosphere undergoes
sudden dramatic changes due to the propagation of a shock wave. Our
results favor the use of fine spectral resolution for analyses that
focus on the inference of the magnetic field, whereas the estimation
of temperature and velocity fluctuations can be performed with lower
spectral resolution.
---------------------------------------------------------
Title: Improved detection of far-side solar active regions using
deep learning
Authors: Felipe, T.; Asensio Ramos, A.
2019A&A...632A..82F Altcode: 2019arXiv191101099F
Context. The analysis of waves on the visible side of the Sun
allows the detection of active regions on the far side through local
helioseismology techniques. Knowing the magnetism in the whole Sun,
including the non-visible hemisphere, is fundamental for several space
weather forecasting applications. <BR /> Aims: Seismic identification of
far-side active regions is challenged by the reduced signal-to-noise
ratio, and only large and strong active regions can be reliable
detected. Here we develop a new method to improve the identification
of active region signatures in far-side seismic maps. <BR /> Methods:
We constructed a deep neural network that associates the far-side
seismic maps obtained from helioseismic holography with the probability
that active regions lie on the far side. The network was trained with
pairs of helioseismic phase-shift maps and Helioseismic and Magnetic
Imager (HMI) magnetograms acquired half a solar rotation later, which
were used as a proxy for the presence of active regions on the far
side. The method was validated using a set of artificial data, and
it was also applied to actual solar observations during the period of
minimum activity of solar cycle 24. <BR /> Results: Our approach shows
a higher sensitivity to the presence of far-side active regions than
standard methods that have been applied up to date. The neural network
can significantly increase the number of detected far-side active
regions, and will potentially improve the application of far-side
seismology to space weather forecasting.
---------------------------------------------------------
Title: Origin of the chromospheric three-minute oscillations in
sunspot umbrae
Authors: Felipe, T.
2019A&A...627A.169F Altcode: 2019arXiv190609797F
Context. Sunspot umbrae show a change in the dominant period of
their oscillations from five minutes (3.3 mHz) in the photosphere
to three minutes (5.5 mHz) in the chromosphere. <BR /> Aims: In this
paper, we explore the two most popular models proposed to explain the
three-minute oscillations: the chromospheric acoustic resonator and the
propagation of waves with frequency above the cutoff value directly from
lower layers. <BR /> Methods: We employ numerical simulations of wave
propagation from the solar interior to the corona. Waves are driven
by a piston at the bottom boundary. We have performed a parametric
study of the measured chromospheric power spectra in a large number
of numerical simulations with differences in the driving method, the
height of the transition region (or absence of transition region),
the strength of the vertical magnetic field, and the value of the
radiative cooling time. <BR /> Results: We find that both mechanisms
require the presence of waves with periods in the three-minute band
at the photosphere. These waves propagate upward and their amplitude
increases due to the drop of the density. Their amplification is
stronger than that of evanescent low-frequency waves. This effect is
enough to explain the dominant period observed in chromospheric spectral
lines. However, waves are partially trapped between the photosphere and
the transition region, forming an acoustic resonator. This chromospheric
resonant cavity strongly enhances the power in the three-minute
band. <BR /> Conclusions: The chromospheric acoustic resonator model
and the propagation of waves in the three-minute band directly from
the photosphere can explain the observed chromospheric three-minute
oscillations. They are both important in different scenarios. Resonances
are produced by waves trapped between the temperature minimum and the
transition region. Strong magnetic fields and radiative losses remove
energy from the waves inside the cavity, resulting in resonances with
weaker amplitude.
---------------------------------------------------------
Title: Spiral-shaped wavefronts in a sunspot umbra
Authors: Felipe, T.; Kuckein, C.; Khomenko, E.; Thaler, I.
2019A&A...621A..43F Altcode: 2018arXiv181011257F
Context. 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. <BR /> Aims: 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. <BR />
Methods: Observations of the sunspot in active region NOAA 12662
obtained with the GREGOR telescope (Observatorio del Teide, Tenerife,
Spain) were acquired on 2017 June 17. The data set includes a temporal
series in the Fe I 5435 Å line obtained with the imaging spectrograph
GREGOR Fabry-Pérot Interferometer (GFPI) and a spectropolarimetric
raster map acquired with the GREGOR Infrared Spectrograph (GRIS)
in the 10 830 Å spectral region. The Doppler velocity deduced from
the restored Fe I 5435 Å line has been determined, and the magnetic
field vector of the sunspot has been inferred from spectropolarimetric
inversions of the Ca I 10 839 Å and the Si I 10 827 Å lines. <BR
/> Results: A two-armed spiral wavefront has been identified in the
evolution of the two-dimensional velocity maps from the Fe I 5435 Å
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. <BR /> Conclusions:
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 Å line, as given by the inclination and orientation of the
magnetic field. <P />The movie associated to Fig. 2 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201834367/olm">https://www.aanda.org</A>
---------------------------------------------------------
Title: Height variation of the cutoff frequency in a sunspot umbra
Authors: Felipe, T.; Kuckein, C.; Thaler, I.
2018A&A...617A..39F Altcode: 2018arXiv180605856F
Context. In the solar atmosphere, the acoustic cutoff frequency is
a local quantity that depends on atmospheric height. It separates
low-frequency evanescent waves from high-frequency propagating
waves. <BR /> Aims: 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. <BR
/> Methods: We analyzed the oscillations in the umbra of a sunspot
belonging to active region NOAA 12662 observed in the 10 830 Å spectral
region with the GREGOR Infrared Spectrograph and in the Fe I 5435 Å
line with the GREGOR Fabry-Pérot Interferometer. Both instruments
are attached to the GREGOR telescope at the Observatorio del Teide,
Tenerife, Spain. We computed the phase and amplification spectra between
the velocity measured from various pairs of lines that sample various
heights of the solar atmosphere. The cutoff frequency and its height
variation were estimated from the inspection of the spectra. <BR />
Results: 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 3.1 mHz. A 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. <BR /> Conclusions: 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.
---------------------------------------------------------
Title: Inversions of synthetic umbral flashes: Effects of scanning
time on the inferred atmospheres
Authors: Felipe, T.; Socas-Navarro, H.; Przybylski, D.
2018A&A...614A..73F Altcode: 2018arXiv180205028F
Context. The use of instruments that record narrowband images at
selected wavelengths is a common approach in solar observations. They
allow scanning of a spectral line by sampling the Stokes profiles with
two-dimensional images at each line position, but require a compromise
between spectral resolution and temporal cadence. The interpretation
and inversion of spectropolarimetric data generally neglect changes in
the solar atmosphere during the scanning of line profiles. <BR /> Aims:
We evaluate the impact of the time-dependent acquisition of various
wavelengths on the inversion of spectropolarimetric profiles from
chromospheric lines during umbral flashes. <BR /> Methods: Numerical
simulations of nonlinear wave propagation in a sunspot model were
performed with the code MANCHA. Synthetic Stokes parameters in the Ca
II 8542 Å line in NLTE were computed for an umbral flash event using
the code NICOLE. Artificial profiles with the same wavelength coverage
and temporal cadence from reported observations were constructed and
inverted. The inferred atmospheric stratifications were compared with
the original simulated models. <BR /> Results: The inferred atmospheres
provide a reasonable characterization of the thermodynamic properties
of the atmosphere during most of the phases of the umbral flash. The
Stokes profiles present apparent wavelength shifts and other spurious
deformations at the early stages of the flash, when the shock wave
reaches the formation height of the Ca II 8542 Å line. These features
are misinterpreted by the inversion code, which can return unrealistic
atmospheric models from a good fit of the Stokes profiles. The
misguided results include flashed atmospheres with strong downflows,
even though the simulation exhibits upflows during the umbral flash,
and large variations in the magnetic field strength. <BR /> Conclusions:
Our analyses validate the inversion of Stokes profiles acquired by
sequentially scanning certain selected wavelengths of a line profile,
even in the case of rapidly changing chromospheric events such as
umbral flashes. However, the inversion results are unreliable during
a short period at the development phase of the flash.
---------------------------------------------------------
Title: Helioseismic Constraints on the Subsurface Flows of the
Averaged Supergranule
Authors: Braun, Douglas C.; Duvall, Thomas L., Jr.; Felipe, Tobias;
DeGrave, Kyle
2018tess.conf11506B Altcode:
We report progress on constraining the subsurface flow properties
of supergranulation from helioseismic holography applied to HMI/SDO
observations of over 63,000 individual supergranules. First, using
surface-focused measurements, we confirm the advantages of broader
phase-speed filters in reducing diffraction effects, noted by Duvall
and collaborators in prior time-distance analyses. Second, we expand
the type of measurements to include deep-focusing geometries. Third,
we compare all measurements with predictions made using numerical
wave-propagation simulations performed with the 3D MANCHA code using
a number of prescribed flow patterns. These model flows include those
inferred from prior time-distance analyses as well as a model based
on results of recent fully-convective MURaM-based computations. While
the model predictions are in general qualitative agreement with the
ensemble averaged measurements, no single model is fully consistent
with the complete set of measurements. This suggests there is room
for improvement in constraining the subsurface flows. <P />This work
is supported by NASA grant 80NSSC18K0068 awarded to NWRA, as well as
by the NASA High-End Computing program at Ames Research Center.
---------------------------------------------------------
Title: Signatures of the impact of flare-ejected plasma on the
photosphere of a sunspot light bridge
Authors: Felipe, T.; Collados, M.; Khomenko, E.; Rajaguru, S. P.;
Franz, M.; Kuckein, C.; Asensio Ramos, A.
2017A&A...608A..97F Altcode: 2017arXiv170806133F
<BR /> Aims: 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. <BR /> Methods: We observed a sunspot in active
region NOAA 12544 using spectropolarimetric raster maps of the four
Fe I lines around 15 655 Å with the GREGOR Infrared Spectrograph,
narrow-band intensity images sampling the Fe I 6173 Å line with
the GREGOR Fabry-Pérot Interferometer, and intensity broad-band
images in G-band and Ca II H-band with the High-resolution Fast
Imager. 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 and Helioseismic and Magnetic Imager
data from the Solar Dynamics Observatory. The physical parameters of
the atmosphere at differents heights were inferred using spectral-line
inversion techniques. <BR /> Results: We identify photospheric and
chromospheric brightenings, heating events, and changes in the Stokes
profiles associated with the flare eruption and the subsequent arrival
of the plasma blob to the light bridge, after traveling along an
active region loop. <BR /> Conclusions: 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. <P />Movies attached to Figs. 1 and 3 are available at <A
href="http://www.aanda.org/10.1051/0004-6361/201731374/olm">http://www.aanda.org</A>
---------------------------------------------------------
Title: Detection of emission in the Si I 1082.7 nm line core in
sunspot umbrae
Authors: Orozco Suárez, D.; Quintero Noda, C.; Ruiz Cobo, B.;
Collados Vera, M.; Felipe, T.
2017A&A...607A.102O Altcode: 2017arXiv170906773O
Context. Determining empirical atmospheric models for the solar
chromosphere is difficult since it requires the observation and
analysis of spectral lines that are affected by non-local thermodynamic
equilibrium (NLTE) effects. This task is especially difficult in sunspot
umbrae because of lower continuum intensity values in these regions
with respect to the surrounding brighter granulation. Umbral data is
therefore more strongly affected by the noise and by the so-called
scattered light, among other effects. <BR /> Aims: The purpose of this
study is to analyze spectropolarimetric sunspot umbra observations
taken in the near-infrared Si I 1082.7 nm line taking NLTE effects into
account. Interestingly, we detected emission features at the line core
of the Si I 1082.7 nm line in the sunspot umbra. Here we analyze the
data in detail and offer a possible explanation for the Si I 1082.7 nm
line emission. <BR /> Methods: Full Stokes measurements of a sunspot
near disk center in the near-infrared spectral range were obtained
with the GRIS instrument installed at the German GREGOR telescope. A
point spread function (PSF) including the effects of the telescope,
the Earth's atmospheric seeing, and the scattered light was constructed
using prior Mercury observations with GRIS and the information provided
by the adaptive optics system of the GREGOR telescope during the
observations. The data were then deconvolved from the PSF using a
principal component analysis deconvolution method and were analyzed
via the NICOLE inversion code, which accounts for NLTE effects in the
Si I 1082.7 nm line. The information of the vector magnetic field was
included in the inversion process. <BR /> Results: The Si I 1082.7 nm
line seems to be in emission in the umbra of the observed sunspot after
the effects of scattered light (stray light coming from wide angles)
are removed. We show how the spectral line shape of umbral profiles
changes dramatically with the amount of scattered light. Indeed, the
continuum levels range, on average, from 44% of the quiet Sun continuum
intensity to about 20%. Although very low, the inferred levels are in
line with current model predictions and empirical umbral models. The Si
I 1082.7 nm line is in emission after adding more that 30% of scattered
light so that it is very sensitive to a proper determination of the
PSF. Additionally, we have thoroughly investigated whether the emission
is a byproduct of the particular deconvolution technique but have not
found any evidence to the contrary. Only the circular polarization
signals seem to be more sensitive to the deconvolution strategy
because of the larger amount of noise in the umbra. Interestingly,
current umbral empirical models are not able to reproduce the emission
in the deconvolved umbral Stokes profiles. The results of the NLTE
inversions suggests that to obtain the emission in the Si I 1082.7 nm
line, the temperature stratification should first have a hump located
at about log τ = -2 and start rising at lower heights when moving into
the transition region. <BR /> Conclusions: This is, to our knowledge,
the first time the Si I 1082.7 nm line is seen in emission in sunspot
umbrae. The results show that the temperature stratification of current
umbral models may be more complex than expected with the transition
region located at lower heights above sunspot umbrae. Our finding might
provide insights into understanding why the sunspot umbra emission in
the millimeter spectral range is less than that predicted by current
empirical umbral models.
---------------------------------------------------------
Title: Helioseismic holography of simulated sunspots: dependence of
the travel time on magnetic field strength and Wilson depression
Authors: Felipe, T.; Braun, D. C.; Birch, A. C.
2017A&A...604A.126F Altcode: 2017arXiv170509135F
Improving methods for determining the subsurface structure of sunspots
from their seismic signature requires a better understanding of the
interaction of waves with magnetic field concentrations. We aim to
quantify the impact of changes in the internal structure of sunspots
on local helioseismic signals. We have numerically simulated the
propagation of a stochastic wave field through sunspot models with
different properties, accounting for changes in the Wilson depression
between 250 and 550 km and in the photospheric umbral magnetic field
between 1500 and 3500 G. The results show that travel-time shifts at
frequencies above approximately 3.50 mHz (depending on the phase-speed
filter) are insensitive to the magnetic field strength. The travel
time of these waves is determined exclusively by the Wilson depression
and sound-speed perturbation. The travel time of waves with lower
frequencies is affected by the direct effect of the magnetic field,
although photospheric field strengths below 1500 G do not leave a
significant trace on the travel-time measurements. These results could
potentially be used to develop simplified travel-time inversion methods.
---------------------------------------------------------
Title: Dependence of sunspot photospheric waves on the depth of the
source of solar p-modes
Authors: Felipe, T.; Khomenko, E.
2017A&A...599L...2F Altcode: 2017arXiv170200997F
Photospheric waves in sunspots moving radially outward at speeds faster
than the characteristic wave velocities have been recently detected. It
has been suggested that they are the visual pattern of p-modes excited
around 5 Mm beneath the sunspot's surface. Using numerical simulations,
we performed a parametric study of the waves observed at the photosphere
and higher layers that were produced by sources located at different
depths beneath the sunspot's surface. The observational measurements
are consistent with waves driven between approximately 1 Mm and 5 Mm
below the sunspot's surface.
---------------------------------------------------------
Title: Synthetic polarimetric spectra from stellar prominences
Authors: Felipe, T.; Martínez González, M. J.; Asensio Ramos, A.
2017MNRAS.465.1654F Altcode: 2016arXiv161009282F
Stellar prominences detected in rapidly rotating stars serve as probes
of the magnetism in the corona of cool stars. We have synthesized the
temporal evolution of the Stokes profiles generated in the He I 10
830 and 5876 Å triplets during the rotation of a prominence around
a star. The synthesis was performed with the HAZEL code using a cloud
model in which the prominence is characterized by a slab located at a
fixed latitude and height. It accounts for the scattering polarization
and Zeeman and Hanle effects. Several cases with different prominence
magnetic field strengths and orientations have been analysed. The
results show an emission feature that drifts across the profile while
the prominence is out of the stellar disc. When the prominence eclipses
the star, the intensity profile shows an absorption. The scattering
induced by the prominence generates linear polarization signals
in Stokes Q and U profiles, which are modified by the Hanle effect
when a magnetic field is present. Due to the Zeeman effect, Stokes V
profiles show a signal with very low amplitude when the magnetic field
along the line of sight is different from zero. The estimated linear
polarization signals could potentially be detected with the future
spectropolarimeter Mid-resolution InfRAreD Astronomical Spectrograph,
to be attached to Gran Telescopio Canarias telescope.
---------------------------------------------------------
Title: Forward modeling for local solar seismology
Authors: Felipe, Tobías
2017psio.confE...6F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Three-dimensional structure of a sunspot light bridge
Authors: Felipe, T.; Collados, M.; Khomenko, E.; Kuckein, C.; Asensio
Ramos, A.; Balthasar, H.; Berkefeld, T.; Denker, C.; Feller, A.;
Franz, M.; Hofmann, A.; Joshi, J.; Kiess, C.; Lagg, A.; Nicklas, H.;
Orozco Suárez, D.; Pastor Yabar, A.; Rezaei, R.; Schlichenmaier,
R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka, M.; Solanki,
S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Volkmer, R.;
von der Lühe, O.; Waldmann, T.
2016A&A...596A..59F Altcode: 2016arXiv161104803F
Context. Active regions are the most prominent manifestations of solar
magnetic fields; their generation and dissipation are fundamental
problems in solar physics. Light bridges are commonly present during
sunspot decay, but a comprehensive picture of their role in the
removal of the photospheric magnetic field is still lacking. <BR />
Aims: We study the three-dimensional configuration of a sunspot,
and in particular, its light bridge, during one of the last stages of
its decay. <BR /> Methods: We present the magnetic and thermodynamical
stratification inferred from full Stokes inversions of the photospheric
Si I 10 827 Å and Ca I 10 839 Å lines obtained with the GREGOR
Infrared Spectrograph of the GREGOR telescope at the Observatorio del
Teide, Tenerife, Spain. The analysis is complemented by a study of
continuum images covering the disk passage of the active region, which
are provided by the Helioseismic and Magnetic Imager on board the Solar
Dynamics Observatory. <BR /> Results: The sunspot shows a light bridge
with penumbral continuum intensity that separates the central umbra from
a smaller umbra. We find that in this region the magnetic field lines
form a canopy with lower magnetic field strength in the inner part. The
photospheric light bridge is dominated by gas pressure (high-β),
as opposed to the surrounding umbra, where the magnetic pressure
is higher. A convective flow is observed in the light bridge. This
flow is able to bend the magnetic field lines and to produce field
reversals. The field lines merge above the light bridge and become
as vertical and strong as in the surrounding umbra. We conclude that
this occurs because two highly magnetized regions approach each other
during the sunspot evolution. <P />Movies associated to Figs. 2 and 13
are available at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Tracing p-mode Waves from the Photosphere to the Corona in
Active Regions
Authors: Zhao, Junwei; Felipe, Tobías; Chen, Ruizhu; Khomenko, Elena
2016ApJ...830L..17Z Altcode:
Atmosphere above sunspots is abundant with different types
of waves. Among these waves are running penumbral waves in the
chromosphere, quasi-periodic oscillations in the lower coronal loops,
and recently reported running waves in sunspots’ photosphere, all of
which were interpreted as magnetoacoustic waves by some authors. Are
these waves in different atmospheric layers related to each other,
what is the nature of these waves, and where are the ultimate sources
of these waves? Applying a time-distance helioseismic analysis over a
suite of multi-wavelength observations above a sunspot, we demonstrate
that the helioseismic p-mode waves are able to channel up from the
photosphere through the chromosphere and transition region into the
corona, and that the magnetoacoustic waves observed in different
atmospheric layers are a same wave originating from the photosphere
but exhibiting differently under different physical conditions. We
also show waves of different frequencies travel along different
paths, which can be used to derive the physical properties of the
atmosphere above sunspots. Our numerical simulation of traveling
of waves from a subphotospheric source qualitatively resembles the
observed properties of the waves and offers an interpretation of the
shapes of the wavefronts above the photosphere.
---------------------------------------------------------
Title: Helioseismic Holography of Simulated Sunspots: Magnetic and
Thermal Contributions to Travel Times
Authors: Felipe, T.; Braun, D. C.; Crouch, A. D.; Birch, A. C.
2016ApJ...829...67F Altcode: 2016arXiv160804893F
Wave propagation through sunspots involves conversion between waves of
acoustic and magnetic character. In addition, the thermal structure
of sunspots is very different than that of the quiet Sun. As a
consequence, the interpretation of local helioseismic measurements of
sunspots has long been a challenge. With the aim of understanding these
measurements, we carry out numerical simulations of wave propagation
through sunspots. Helioseismic holography measurements made from
the resulting simulated wavefields show qualitative agreement with
observations of real sunspots. We use additional numerical experiments
to determine, separately, the influence of the thermal structure of the
sunspot and the direct effect of the sunspot magnetic field. We use the
ray approximation to show that the travel-time shifts in the thermal
(non-magnetic) sunspot model are primarily produced by changes in the
wave path due to the Wilson depression rather than variations in the
wave speed. This shows that inversions for the subsurface structure
of sunspots must account for local changes in the density. In some
ranges of horizontal phase speed and frequency there is agreement
(within the noise level in the simulations) between the travel times
measured in the full magnetic sunspot model and the thermal model. If
this conclusion proves to be robust for a wide range of models, it
would suggest a path toward inversions for sunspot structure.
---------------------------------------------------------
Title: Magnetic and Thermal Contributions to Helioseismic Travel
times in Simulated Sunspots
Authors: Braun, Douglas; Felipe, Tobias; Birch, Aaron; Crouch,
Ashley D.
2016SPD....47.0701B Altcode:
The interpretation of local helioseismic measurements of sunspots
has long been a challenge, since waves propagating through sunspots
are potentially affected by both mode conversion and changes in the
thermal structure of the spots. We carry out numerical simulations of
wave propagation through a variety of models which alternately isolate
either the thermal or magnetic structure of the sunspot or include
both of these. We find that helioseismic holography measurements made
from the resulting simulated wavefields show qualitative agreement
with observations of real sunspots. Using insight from ray theory,
we find that travel-time shifts in the thermal (non-magnetic) sunspot
model are primarily produced by changes in the wave path due to the
Wilson depression rather than variations in the wave speed. This shows
that inversions for the subsurface structure of sunspots must account
for local changes in the density. In some ranges of horizontal phase
speed and frequency there is agreement (within the noise level of the
measurements) between the travel times measured in the full magnetic
sunspot model and the thermal model. If this conclusion proves to
be robust for a wide range of models, it suggests a path towards
inversions for sunspot structure. This research has been funded
by the Spanish MINECO through grant AYA2014-55078-P, by the NASA
Heliophysics Division through NNX14AD42G and NNH12CF23C, and the NSF
Solar Terrestrial program through AGS-1127327.
---------------------------------------------------------
Title: Tracing Helioseismic Waves from the Photosphere to the Corona
Authors: Zhao, Junwei; Felipe, Tobias; Chen, Ruizhu; Khomenko, Elena
2016SPD....4730307Z Altcode:
Can p-mode waves in sunspots propagate to the chromosphere and the
corona? And what are their counterparts in different atmospheric
heights? In order to study the connection between the photospheric
p-mode waves and the waves observed above the photosphere, we use a
helioseismic analysis technique, namely time-distance helioseismology,
and analyze multi-height observations from different instruments. We
find clear evidences that some p-mode waves in the photosphere, running
penumbral waves in the chromosphere, and the periodic disturbances in
the coronal fan structures are actually same magnetoacoustic waves that
exhibit differently at the different atmospheric heights. The 6-mHz
waves, with inclined wavefronts, propagate slantingly upward along
magnetic field lines. The 3-mHz waves, forming backward-'C'-shape
wavefronts, propagate mostly horizontally. Through numerical
simulations, we demonstrate that these p-mode waves that can travel
upward to the corona, possibly originate from sources located a few
megameters beneath sunspots' surface.
---------------------------------------------------------
Title: Synthetic Observations of Wave Propagation in a Sunspot Umbra
Authors: Felipe, T.; Socas-Navarro, H.; Khomenko, E.
2014ApJ...795....9F Altcode: 2014arXiv1408.6565F
Spectropolarimetric temporal series from Fe I λ6301.5 Å and Ca II
infrared triplet lines are obtained by applying the Stokes synthesis
code NICOLE to a numerical simulation of wave propagation in a sunspot
umbra from MANCHA code. The analysis of the phase difference between
Doppler velocity and intensity core oscillations of the Fe I λ6301.5
Å line reveals that variations in the intensity are produced by
opacity fluctuations rather than intrinsic temperature oscillations,
except for frequencies between 5 and 6.5 mHz. On the other hand, the
photospheric magnetic field retrieved from the weak field approximation
provides the intrinsic magnetic field oscillations associated to wave
propagation. Our results suggest that this is due to the low magnetic
field gradient of our sunspot model. The Stokes parameters of the
chromospheric Ca II infrared triplet lines show striking variations as
shock waves travel through the formation height of the lines, including
emission self-reversals in the line core and highly abnormal Stokes V
profiles. Magnetic field oscillations inferred from the Ca II infrared
lines using the weak field approximation appear to be related with
the magnetic field strength variation between the photosphere and
the chromosphere.
---------------------------------------------------------
Title: Influence of Magnetic and Thermal Effects on Helioseismic
Travel-time Shifts in Sunspot Models
Authors: Felipe, Tobias; Braun, Douglas; Crouch, Ashley D.; Birch,
Aaron
2014AAS...22420206F Altcode:
Sunspots are one of the most prominent manifestations of solar
magnetic activity and have been studied using local helioseismology for
decades. Recent modeling and observational studies indicate that the
interpretation of travel-time shifts is still subject to uncertainties
regarding the physical causes of the wave perturbations. Numerical wave
propagation has proved useful in addressing this problem. In this work,
we have analyzed travel-time shifts obtained from three dimensional
numerical simulations of wave propagation in a magnetohydrostatic
sunspot-like atmosphere. In particular, we isolate the individual
effects of the magnetic field and thermal perturbations on the
measurements by means of simulations where only one kind of perturbation
(magnetic or thermal) is included. The resulting travel-time shift maps,
obtained by applying helioseismic holography to the photospheric Doppler
signals in the simulated domain, will be compared and discussed. We
plan to make the artificial data available to the community for the
development and validation of other helioseismic methods. This work
is supported by the NASA SDO Science Center program (through contract
NNH09CE41C) and by the NASA Living With a Star Program (through grant
NNX14AD42G).
---------------------------------------------------------
Title: Evaluation of the Capability of Local Helioseismology to
Discern between Monolithic and Spaghetti Sunspot Models
Authors: Felipe, T.; Crouch, A. D.; Birch, A. C.
2014ApJ...788..136F Altcode: 2014arXiv1405.0036F
The helioseismic properties of the wave scattering generated
by monolithic and spaghetti sunspots are analyzed by means of
numerical simulations. In these computations, an incident f- or p
<SUB>1</SUB>-mode travels through the sunspot model, which produces
absorption and phase shift of the waves. The scattering is studied by
inspecting the wavefield, computing travel-time shifts, and performing
Fourier-Hankel analysis. The comparison between the results obtained
for both sunspot models reveals that the differences in the absorption
coefficient can be detected above noise level. The spaghetti model
produces a steep increase of the phase shift with the degree of the
mode at short wavelengths, while mode mixing is more efficient for the
monolithic model. These results provide a clue for what to look for
in solar observations to discern the constitution of sunspots between
the proposed monolithic and spaghetti models.
---------------------------------------------------------
Title: Numerical Simulations of Multiple Scattering of the f-mode
by Flux Tubes
Authors: Felipe, T.; Crouch, A.; Birch, A.
2013ApJ...775...74F Altcode: 2013arXiv1308.1139F
We use numerical simulations to study the absorption and phase shift
of surface-gravity waves caused by groups of magnetic flux tubes. The
dependence of the scattering coefficients on the distance between
the tubes and their positions is analyzed for several cases with two
or three flux tubes embedded in a quiet Sun atmosphere. The results
are compared with those obtained neglecting completely or partially
multiple scattering effects. We show that multiple scattering has a
significant impact on the absorption measurements and tends to reduce
the phase shift. We also consider more general cases of ensembles of
randomly distributed flux tubes, and we have evaluated the effects on
the scattering measurements of changing the number of tubes included
in the bundle and the average distance between flux tubes. We find
that for the longest wavelength incoming waves, multiple scattering
enhances the absorption, and its efficiency increases with the number
of flux tubes and the reduction of the distance between them.
---------------------------------------------------------
Title: Three-dimensional Numerical Simulations of Fast-to-Alfvén
Conversion in Sunspots
Authors: Felipe, T.
2012ApJ...758...96F Altcode: 2012arXiv1208.5726F
The conversion of fast waves to the Alfvén mode in a realistic
sunspot atmosphere is studied through three-dimensional numerical
simulations. An upward propagating fast acoustic wave is excited
in the high-β region of the model. The new wave modes generated
at the conversion layer are analyzed from the projections of the
velocity and magnetic field in their characteristic directions,
and the computation of their wave energy and fluxes. The analysis
reveals that the maximum efficiency of the conversion to the slow
mode is obtained for inclinations of 25° and low azimuths, while
the Alfvén wave conversions peak at high inclinations and azimuths
between 50° and 120°. Downward propagating Alfvén waves appear
at the regions of the sunspot where the orientation of the magnetic
field is in the direction opposite to the wave propagation, since at
these locations the Alfvén wave couples better with the downgoing
fast magnetic wave which is reflected due to the gradients of the
Alfvén speed. The simulations show that the Alfvén energy at the
chromosphere is comparable to the acoustic energy of the slow mode,
being even higher at high inclined magnetic fields.
---------------------------------------------------------
Title: Scattering of the f-mode by Small Magnetic Flux Elements from
Observations and Numerical Simulations
Authors: Felipe, T.; Braun, D.; Crouch, A.; Birch, A.
2012ApJ...757..148F Altcode: 2012arXiv1208.4024F
The scattering of f-modes by magnetic tubes is analyzed using
three-dimensional numerical simulations. An f-mode wave packet is
propagated through a solar atmosphere embedded with three different
flux tube models that differ in radius and total magnetic flux. A
quiet-Sun simulation without a tube present is also performed as a
reference. Waves are excited inside the flux tube and propagate along
the field lines, and jacket modes are generated in the surroundings
of the flux tube, carrying 40% as much energy as the tube modes. The
resulting scattered wave is mainly an f-mode composed of a mixture of m
= 0 and m = ±1 modes. The amplitude of the scattered wave approximately
scales with the magnetic flux. A small amount of power is scattered
into the p <SUB>1</SUB>-mode. We have evaluated the absorption and
phase shift from a Fourier-Hankel decomposition of the photospheric
vertical velocities. They are compared with the results obtained
from the ensemble average of 3400 small magnetic elements observed in
high-resolution MDI Doppler datacubes. The comparison shows that the
observed dependence of the phase shift with wavenumber can be matched
reasonably well with the simulated flux tube model. The observed
variation of the phase shifts with the azimuthal order m appears to
depend on details of the ensemble averaging, including possible motions
of the magnetic elements and asymmetrically shaped elements.
---------------------------------------------------------
Title: Comparison of Numerical and Observational Scattering of the
f-mode by Small Magnetic Elements
Authors: Felipe, Tobias; Braun, D. C.; Crouch, A. D.; Birch, A. C.
2012AAS...22010906F Altcode:
The observed scattering of the f-mode by small magnetic elements
is studied through Fourier-Hankel analysis and compared with
three-dimensional numerical simulations of the scattering produced by
magnetic flux tube models. The numerical setup consists of an f-mode
wave packet which is propagated through a realistic solar atmosphere
embedded with a flux tube model. A quiet Sun simulation without a
tube present is also performed as a reference. Sausage (m=0) and kink
(m=±1) modes are excited in the flux tube and propagate along the field
lines, and jacket modes are generated in the surroundings of the flux
tube, carrying 40% as much energy as the tube modes. The resulting
scattered wave is mainly an f-mode composed of a mixture of m=0 and
m=±1 modes. We find the observed dependence of the phase shift with
wavenumber for an ensemble average of about 3400 magnetic elements
can be matched reasonably well with the simulated flux tube model. The
observed variation with azimuthal order m of the phase-shifts appears
to depend on details of the ensemble averaging, including possible
motions of the magnetic elements and asymmetrically shaped elements. <P
/>This research has been funded by NASA through projects NNH09CE43C,
NNH09CF68C, and NNH07CD25C.
---------------------------------------------------------
Title: Magneto-acoustic wave energy in sunspots: observations and
numerical simulations
Authors: Felipe, T.; Khomenko, E.; Collados, M.; Beck, C.
2011hsa6.conf..630F Altcode:
We have reproduced some sunspot wave signatures obtained
from spectropolarimetric observations through 3D MHD
numericalsimulations. The results of the simulations arecompared with
the oscillations observed simultaneously at different heights from the
SiI lambda10827Å line, HeI lambda10830Å line, the CaII H core and
the FeI blends at the wings of the CaII H line. The simulations show
a remarkable agreement with the observations, and we have used them
to quantify the energy contribution of the magneto-acoustic waves to
the chromospheric heating in sunspots. Our findings indicate that the
energy supplied by these waves is 5-10 times lower than the amount
needed to balance the chromospheric radiative losses.
---------------------------------------------------------
Title: Numerical simulations of scattering of f-modes by magnetic
flux tubes
Authors: Felipe, Tobias; Birch, Aaron C.; Crouch, Ashley D.; Braun,
Douglas C.
2011sdmi.confE..80F Altcode:
The scattering of the f-mode by a magnetic flux tube is analyzed
using three-dimensional numerical simulations. An f-mode wave packet
is propagated through a realistic solar atmosphere embedded with a
flux tube of 200 km radius and 1600 G field strength. A quiet Sun
simulation without the tube being present is also performed as a
reference. Sausage (m=0) and kink (m=± 1) modes are excited in the
magnetic tube and propagate downward along the field lines, while the
resulting scattered wave is mainly an f-mode composed of a mixture
of m=0 and m=± 1 modes. Low power is also scattered into high-order
acoustic p-modes. We have evaluated the absorption and phase shift
from a Fourier-Hankel decomposition of the vertical velocities.
---------------------------------------------------------
Title: Magnetoacoustic Wave Energy from Numerical Simulations of an
Observed Sunspot Umbra
Authors: Felipe, T.; Khomenko, E.; Collados, M.
2011ApJ...735...65F Altcode: 2011arXiv1104.4138F
We aim at reproducing the height dependence of sunspot wave signatures
obtained from spectropolarimetric observations through three-dimensional
MHD numerical simulations. A magnetostatic sunspot model based on
the properties of the observed sunspot is constructed and perturbed
at the photosphere, introducing the fluctuations measured with the Si
I λ10827 line. The results of the simulations are compared with the
oscillations observed simultaneously at different heights from the He
I λ10830 line, the Ca II H core, and the Fe I blends in the wings of
the Ca II H line. The simulations show a remarkable agreement with the
observations. They reproduce the velocity maps and power spectra at
the formation heights of the observed lines, as well as the phase and
amplification spectra between several pairs of lines. We find that the
stronger shocks at the chromosphere are accompanied with a delay between
the observed signal and the simulated one at the corresponding height,
indicating that shocks shift the formation height of the chromospheric
lines to higher layers. Since the simulated wave propagation matches
very well the properties of the observed one, we are able to use the
numerical calculations to quantify the energy contribution of the
magnetoacoustic waves to the chromospheric heating in sunspots. Our
findings indicate that the energy supplied by these waves is too low to
balance the chromospheric radiative losses. The energy contained at the
formation height of the lowermost Si I λ10827 line in the form of slow
magnetoacoustic waves is already insufficient to heat the higher layers,
and the acoustic energy which reaches the chromosphere is around 3-9
times lower than the required amount of energy. The contribution of
the magnetic energy is even lower.
---------------------------------------------------------
Title: Local Helioseismology of Small-Scale Magnetic Elements
Authors: Crouch, Ashley D.; Braun, D. C.; Felipe, T.; Birch, A. C.;
Duvall, T. L.
2011SPD....42.1604C Altcode: 2011BAAS..43S.1604C
We will discuss recent progress in the measurement and modeling of the
interaction of helioseismic waves with small-scale magnetic elements. We
will present measurements of the Hankel analysis phase shifts
and absorption coefficients associated with an average small-scale
magnetic element, measured using ensemble-averaging techniques. We
will show results from theoretical calculations and the numerical
simulations of wave interactions with thin magnetic flux tubes. We
will compare the Hankel analysis measurements with the predictions
from these theoretical models, and discuss how these results pertain
to the local helioseismology of magnetic flux concentrations. This
work is supported by NASA contract NNH09CE43C.
---------------------------------------------------------
Title: Magneto-acoustic waves in sunspots from observations and
numerical simulations
Authors: Felipe, T.; Khomenko, E.; Collados, M.; Beck, C.
2011JPhCS.271a2040F Altcode: 2010arXiv1009.5512F
We study the propagation of waves from the photosphere to the
chromosphere of sunspots. From time series of cospatial Ca II H
(including its line blends) intensity spectra and polarimetric spectra
of Si I λ 1082.7 nm and He I λ 1083.0 nm we retrieve the line-of-sight
velocity at several heights. The analysis of the phase difference and
amplification spectra shows standing waves for frequencies below 4 mHz
and propagating waves for higher frequencies, and allows us to infer
the temperature and height where the lines are formed. Using these
observational data, we have constructed a model of sunspot, and we
have introduced the velocity measured with the photospheric Si I λ
1082.7 nm line as a driver. The numerically propagated wave pattern
fits reasonably well with the observed using the lines formed at higher
layers, and the simulations reproduce many of the observed features. The
observed waves are slow MHD waves propagating longitudinally along
field lines.
---------------------------------------------------------
Title: Multi-layer Study of Wave Propagation in Sunspots
Authors: Felipe, T.; Khomenko, E.; Collados, M.; Beck, C.
2010ApJ...722..131F Altcode: 2010arXiv1008.4004F
We analyze the propagation of waves in sunspots from the photosphere
to the chromosphere using time series of co-spatial Ca II H intensity
spectra (including its line blends) and polarimetric spectra of Si
I λ10,827 and the He I λ10,830 multiplet. From the Doppler shifts
of these lines we retrieve the variation of the velocity along the
line of sight at several heights. Phase spectra are used to obtain
the relation between the oscillatory signals. Our analysis reveals
standing waves at frequencies lower than 4 mHz and a continuous
propagation of waves at higher frequencies, which steepen into shocks
in the chromosphere when approaching the formation height of the Ca
II H core. The observed nonlinearities are weaker in Ca II H than in
He I lines. Our analysis suggests that the Ca II H core forms at a
lower height than the He I λ10,830 line: a time delay of about 20 s is
measured between the Doppler signal detected at both wavelengths. We fit
a model of linear slow magnetoacoustic wave propagation in a stratified
atmosphere with radiative losses according to Newton's cooling law to
the phase spectra and derive the difference in the formation height
of the spectral lines. We show that the linear model describes well
the wave propagation up to the formation height of Ca II H, where
nonlinearities start to become very important.
---------------------------------------------------------
Title: Magneto-acoustic Waves in Sunspots: First Results From a New
Three-dimensional Nonlinear Magnetohydrodynamic Code
Authors: Felipe, T.; Khomenko, E.; Collados, M.
2010ApJ...719..357F Altcode: 2010arXiv1006.2998F
Waves observed in the photosphere and chromosphere of sunspots
show complex dynamics and spatial patterns. The interpretation
of high-resolution sunspot wave observations requires modeling
of three-dimensional (3D) nonlinear wave propagation and mode
transformation in the sunspot upper layers in realistic spot model
atmospheres. Here, we present the first results of such modeling. We
have developed a 3D nonlinear numerical code specially designed to
calculate the response of magnetic structures in equilibrium to an
arbitrary perturbation. The code solves the 3D nonlinear MHD equations
for perturbations; it is stabilized by hyper-diffusivity terms and is
fully parallelized. The robustness of the code is demonstrated by a
number of standard tests. We analyze several simulations of a sunspot
perturbed by pulses of different periods at a subphotospheric level,
from short periods, introduced for academic purposes, to longer and
realistic periods of 3 and 5 minutes. We present a detailed description
of the 3D mode transformation in a non-trivial sunspot-like magnetic
field configuration, including the conversion between fast and slow
magneto-acoustic waves and the Alfvén wave, by calculation of the wave
energy fluxes. Our main findings are as follows: (1) the conversion from
acoustic to the Alfvén mode is only observed if the driving pulse is
located out of the sunspot axis, but this conversion is energetically
inefficient; (2) as a consequence of the cutoff effects and refraction
of the fast magneto-acoustic mode, the energy of the evanescent waves
with periods around 5 minutes remains almost completely below the level
β = 1; (3) waves with frequencies above the cutoff propagate field
aligned to the chromosphere and their power becomes dominating over that
of evanescent 5 minute oscillations, in agreement with observations.
---------------------------------------------------------
Title: Mode transformation and frequency change with height in 3D
numerical simulations of magneto-acoustic wave propagation in sunspots
Authors: Felipe, T.; Khomenko, E.; Collados, M.
2010arXiv1005.3684F Altcode:
Three-dimensional numerical simulations of magnetoacoustic wave
propagation are performed in a sunspot atmosphere with a computational
domain covering from the photosphere to the chromosphere. The
wave source, with properties resembling the solar spectrum, is
located at different distances from the axis of the sunspot for
each simulation. These results are compared with the theory of mode
transformation and also with observational features. Simulations show
that the dominant oscillation frequency in the chromosphere decreases
with the radial distance from the sunspot axis. The energy flux of the
different wave modes involved, including de Alfvén mode, is evaluated
and discussed.
---------------------------------------------------------
Title: Ondas MHD en la fotosfera y cromosfera de manchas solares
Authors: Felipe, Tobías
2010PhDT.......165F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observational Signatures of Numerically Simulated MHD Waves
in Small-scale Flux Sheets
Authors: Khomenko, E.; Collados, M.; Felipe, T.
2009ASPC..405..183K Altcode: 2008arXiv0801.3966K
We present some results obtained from the synthesis of Stokes profiles
in small-scale flux sheets with propagating MHD waves. To that aim,
2D flux sheets showing internal structure have been excited with 5
min period drivers, allowing non-linear waves to propagate inside the
magnetic structure. The observational signatures of these waves in
Stokes profiles of several spectral lines that are commonly used in
spectropolarimetric measurements are discussed.
---------------------------------------------------------
Title: Multi-layer Study of Wave Propagation in Sunspots
Authors: Felipe, T.; Khomenko, E.; Collados, M.; Beck, C.
2008ESPM...12.2.12F Altcode:
Observations in different spectral lines give us information about
the different layers of the solar atmosphere. Here we analyze
the propagation of waves in sunspots from the photosphere to the
chromosphere using time series of cospatial Ca II H intensity
spectra and polarimetric spectra of Si I 10827 A and He I 10830
A multiplet. From the Doppler shifts of these lines we retrieve
the temporal variations of the velocity along the line-of-sight
at several heights. Phase spectra are used to get the relation
between oscillatory signals measured at each spectral signature. Our
analysis reveals standing waves for frequencies lower than 3.5 mHz and
propagating waves for higher frequencies, which steepen into shocks
in the chromosphere. Oscillations are detectable in Ca II H wings and
they are propagated along line wing layers to the line core. Ca II H
core forms at a lower height than the He I 10830 A line. A time delay
of about 30 s is measured between the Doppler signals detected at both
wavelengths. We also find that in "cold" sunspots the Si I 10827 A forms
deeper than in the quiet sun. This type of measurements demonstrate
the importance of simultaneous co-spatial observations at different
wavelengths. Future infrastructures, such as GREGOR and EST, should
include multi-wavelength capabilities to make possible the study of
the photosphere-chromosphere connection with the highest spatial and
temporal resolution.
---------------------------------------------------------
Title: Nonlinear Numerical Simulations of Magneto-Acoustic Wave
Propagation in Small-Scale Flux Tubes
Authors: Khomenko, E.; Collados, M.; Felipe, T.
2008SoPh..251..589K Altcode: 2008SoPh..tmp...32K; 2007arXiv0710.3335K
We present results of nonlinear, two-dimensional, numerical simulations
of magneto-acoustic wave propagation in the photosphere and chromosphere
of small-scale flux tubes with internal structure. Waves with realistic
periods of three to five minutes are studied, after horizontal and
vertical oscillatory perturbations are applied to the equilibrium
model. Spurious reflections of shock waves from the upper boundary
are minimized by a special boundary condition. This has allowed us to
increase the duration of the simulations and to make it long enough to
perform a statistical analysis of oscillations. The simulations show
that deep horizontal motions of the flux tube generate a slow (magnetic)
mode and a surface mode. These modes are efficiently transformed
into a slow (acoustic) mode in the v<SUB>A</SUB><c<SUB>S</SUB>
atmosphere. The slow (acoustic) mode propagates vertically along
the field lines, forms shocks, and remains always within the flux
tube. It might effectively deposit the energy of the driver into the
chromosphere. When the driver oscillates with a high frequency, above
the cutoff, nonlinear wave propagation occurs with the same dominant
driver period at all heights. At low frequencies, below the cutoff,
the dominant period of oscillations changes with height from that
of the driver in the photosphere to its first harmonic (half period)
in the chromosphere. Depending on the period and on the type of the
driver, different shock patterns are observed.
