explanation blue bibcodes open ADS page with paths to full text
Author name code: settele
ADS astronomy entries on 2022-09-14
author:"Settele, Axel"
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Title: Acceleration Effects in MDI Magnetogram Data
Authors: Norton, A.; Settele, A.
2003SoPh..214..227N Altcode:
Acceleration effects are found in the Michelson Doppler Imager
(MDI) magnetogram data because changes in the line profiles during
the 30-s measurement are introduced by underlying p-mode velocity
variations. This imparts an oscillatory component to the magnetic flux
signal. Simulated profiles using Maltby M and Harvard Smithsonian
Reference Atmospheres (HSRA) are shifted in accordance with a given
velocity amplitude and period and the MDI algorithm for data measurement
is applied. The simulated oscillatory component to the magnetic flux
density always has a phase difference with respect to the underlying
velocity of −90°. The maximum introduced RMS amplitude is a function
of velocity amplitude and field strength, but realistic errors are
on the order of 5/2000 G, or 0.25% of field strength. Comparison
of simulations with observations shows RMS amplitudes of MDI flux
density are much greater than predicted by this effect. A 2-component
HSRA model, tested to determine if stronger fields with smaller fill
factors could fit the data, still can not reproduce the observations. It
is concluded that oscillatory amplitudes of magnetic flux density
measured with MDI are not due to acceleration effects, although the
effect could contribute up to 25% of the measured amplitude. Attempts
to remove acceleration effects from the magnetic flux signal are not
successful. Also, we confirm that velocities measured in linearly
polarized light in the vicinity of a strong magnetic field contain
larger errors than velocities measured in circularly polarized light
(Yang and Norton, 2001).
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Title: Temporal and spatial variations of the magnetic field vector
in sunspots
Authors: Settele, A.; Sigwarth, M.; Muglach, K.
2002A&A...392.1095S Altcode:
In order to look for magnetic field vector oscillations in sunspots we
used data measured with the Advanced Stokes Polarimeter at the Dunn
Solar Telescope. We analyzed two time series of 65 and 110 min that
were taken by scanning repeatedly a 6" 24 x 75" field of view, while
obtaining the full Stokes vector in the lines FeI 630.15 nm and 630.25
nm. An inversion was carried out and a Fast Fourier Transform (FFT)
analysis was used to find oscillatory phenomena. We discuss possible
unwanted effects that lead to apparent magnetic field oscillations and
find an average amplitude of (B, psi ) 5.8 G/0.23<SUP>o</SUP> rms by
excluding these effects, which also means that only 6% and 22% of the
two sunspot umbrae respectively remained for analysis. If we smooth
the power spectra over 2x2 pixels, all significant power disappears.
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Title: Systematic errors in measuring solar magnetic fields with a
FPI spectrometer and MDI
Authors: Settele, A.; Carroll, T. A.; Nickelt, I.; Norton, A. A.
2002A&A...386.1123S Altcode:
We will show the potential errors of observations using instruments,
where a line spectrum is obtained not instantly but by scanning through
the spectral line in time. We give two examples in simulations of
a sunspot observation with the measuring procedure of a typical
Fabry-Perot interferometer (FPI) spectrometer and the Michelson
Doppler Imager (MDI) on board of the Solar and Heliospheric Observatory
(SOHO). The effect described is inherent for observations with a low
intensity input signal, where the accumulation time for one image and
therefore the whole process of measuring a spectrum is relatively high
compared to the timescale of the underlying physical processes. We show
that our simulated data produce an error in magnetic field strength and
- in a time series - result in an apparent oscillation of the magnetic
field strength, while the input contained only velocity oscillations. As
the FPI spectrometer is being used more and more widely, one has to
be aware of the disadvantages and potential errors when interpreting
its data. We encourage every observer to do the simple simulation as
described for every specific setup used.
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Title: Waves in Sunspots: Resonant Transmission and the Adiabatic
Coefficient
Authors: Settele, A.; Staude, J.; Zhugzhda, Y. D.
2001SoPh..202..281S Altcode:
We investigate linear acoustic-gravity waves in three different
semi-empirical model atmospheres of large sunspot umbrae. The sunspot
filter theory is applied, that is, the resonant transmission of
vertically propagating waves is modelled. The results are compared
with observed linear sunspot oscillations. For three umbral models we
present the transmission coefficients and the energy density of the
oscillations with the maxima of transmission. The height dependence of
the adiabatic coefficient (the ratio of specific heats) γ strongly
influences the calculated resonance frequencies. The variable γ can
explain the observed closely spaced resonance period peaks. The first
resonance in the 3 min range is interpreted as a resonance of the upper
chromosphere only, while the higher order peaks are resonances of the
whole chromosphere.
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Title: Magneto-Atmosphaerische Wellen in der Umbra von Sonnenflecken;
Magnetoatmospheric waves in the umbra of sunspots
Authors: Settele, A.
2001PhDT.........3S Altcode:
No abstract at ADS
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Title: Problems in the interpretation of sunspot oscillation
measurements
Authors: Settele, A.; Staude, J.; Zhugzhda, Y. D.
2001ESASP.464..247S Altcode: 2001soho...10..247S
Sunspot oscillations are observed in spectral lines formed at different
depths in the sunspot atmosphere. The interpretation of measurements in
chromospheric lines is problematic as the lines are formed in NLTE. So
the measured power spectra do not originate from one specific depth in
the atmosphere but are smoothed out over a larger region. This will also
influence the phase shifts of the waves measured in different spectral
lines and might cause some misinterpretation. These complications
are illustrated by our model calculations: Modelling the transmission
of magneto-atmospheric waves through the sunspot atmosphere by means
of the sunspot filter approach, we show the "real" power spectra at
different atmospheric depths. These results are compared with Ca II-H/K
power spectra gained from the NLTE calculations of the Ca II-H/K lines
in an atmosphere perturbed by our waves. All results are preliminary.
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Title: Magneto-Atmosphärische Wellen in der Umbra von Sonnenflecken
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Title: Magneto-Atmosphärische Wellen in der Umbra von Sonnenflecken
Authors: Settele, Axel
2001PhDT.......431S Altcode:
No abstract at ADS
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Title: Resonant Transmission of Magneto-atmospheric Waves in Sunspot
Umbrae
Authors: Staude, J.; Rendtel, J.; Settele, A.
2001IAUS..203..320S Altcode:
Oscillations of velocity and intensity, of the magnetic field in
the photosphere as well, have been observed at all height levels of
the atmosphere above sunspots from the photosphere up to the lower
corona. We discuss the nonstationary behavior of intensity and velocity
oscillations; they have been observed by SOHO in transition region lines
formed in the EUV plume of a sunspot during a continuous time series
of 4.3 hours. Applying a wavelet analysis, we find periods around 3 mHz
with different periods in the observed EUV lines. Moreover, we discuss
the observed oscillations within the frame of model calculations
for the resonant transmission of magneto-atmospheric waves through
realistic models of the sunspot subphotosphere and atmosphere.
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Title: The Influence of the Adiabatic Coefficient and the Magnetic
Field on Sunspot Oscillations
Authors: Settele, A.; Zhugzhda, Y. D.; Staude, J.
1999ESASP.448..367S Altcode: 1999ESPM....9..367S; 1999mfsp.conf..367S
No abstract at ADS
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Title: A new method to calculate the resonant response of a sunspot
model atmosphere to magneto-atmospheric waves
Authors: Settele, A.; Zhugzhda, Y. D.; Staude, J.
1999AN....320..147S Altcode:
In order to understand the observed oscillations in sunspots we
present a new method for calculating the resonant response of a
realistic semi-empirical model of the sunspot umbral atmosphere and
subphotosphere to magneto-atmospheric waves in a vertical magnetic
field. The depth dependence of both the adiabatic coefficient and the
turbulent pressure is taken into account. This requires an extension
of the wave equations by Ferraro & Plumpton (1958). We compare
the coefficients of wave transmission, reflection, and conversion
between fast mode and slow mode waves for different assumptions,
compare the results with those from earlier modelling efforts, and
point out possible sources of mistakes. The depth dependence of the
adiabatic coefficient strongly influences the resulting spectrum of
resonance frequencies. The condition of a conservation of wave flux
is violated if the depth dependence of the turbulent pressure is not
properly considered.
