Author name code: leifsen
ADS astronomy entries on 2022-09-14
author:"Leifsen, Torben"
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Title: High-resolution observations of the solar photosphere,
chromosphere, and transition region. A database of coordinated IRIS
and SST observations
Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Carlsson,
M.; de la Cruz Rodríguez, J.; Bose, S.; Chintzoglou, G.; Drews, A.;
Froment, C.; Gošić, M.; Graham, D. R.; Hansteen, V. H.; Henriques,
V. M. J.; Jafarzadeh, S.; Joshi, J.; Kleint, L.; Kohutova, P.;
Leifsen, T.; Martínez-Sykora, J.; Nóbrega-Siverio, D.; Ortiz, A.;
Pereira, T. M. D.; Popovas, A.; Quintero Noda, C.; Sainz Dalda, A.;
Scharmer, G. B.; Schmit, D.; Scullion, E.; Skogsrud, H.; Szydlarski,
M.; Timmons, R.; Vissers, G. J. M.; Woods, M. M.; Zacharias, P.
Bibcode: 2020A&A...641A.146R
Altcode: 2020arXiv200514175R
NASA's Interface Region Imaging Spectrograph (IRIS) provides
high-resolution observations of the solar atmosphere through ultraviolet
spectroscopy and imaging. Since the launch of IRIS in June 2013, we
have conducted systematic observation campaigns in coordination with
the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides
complementary high-resolution observations of the photosphere and
chromosphere. The SST observations include spectropolarimetric imaging
in photospheric Fe I lines and spectrally resolved imaging in the
chromospheric Ca II 8542 Å, Hα, and Ca II K lines. We present
a database of co-aligned IRIS and SST datasets that is open for
analysis to the scientific community. The database covers a variety
of targets including active regions, sunspots, plages, the quiet Sun,
and coronal holes.
Title: Variation of Low Degree P-Mode Amplitudes
Authors: Andersen, B.; Leifsen, T.
Bibcode: 2004ESASP.559..297A
Altcode: 2004soho...14..297A
No abstract at ADS
Title: Amplitude modulation of low degree p-modes - comparison of
BISON and VIRGO
Authors: Andersen, Bo; Leifsen, Torben; Chaplin, William J.; Elsworth,
Yvonne
Bibcode: 2003ESASP.517..151A
Altcode: 2003soho...12..151A
Using both VIRGO and MDI data we have previously studied the amplitude
variation of the l=0 p-modes for radial orders 12 to 32. In this study
we extend the investigation backward in time to 1992 by including
data from the BISON network. For the large amplitude modes there is a
strong correlation between the space based radiance measurements from
VIRGO and the ground based Doppler shift measurements from BISON. The
extreme rotational modulation of l=0, n=22 is confirmed to be a
phenomenon confined to the period of minimum solar activity. Also with
neighbouring l=1, n=21 a clear modulation is seen at slightly lower
frequency. Some persistent frequencies occur in other l=0,1 modes,
but not to the same level in time and amplitude.
Title: Temporal behaviour of radial p-modes
Authors: Leifsen, T.; Andersen, B. N.; Toutain, T.
Bibcode: 2001ESASP.464...63L
Altcode: 2001soho...10...63L
The amplitude modulation of solar radial p-modes has been studied in
irradiance and radiance data from VIRGO and velocity data from MDI
onboard SOHO. The amplitudes vary substantially with time. For two
modes, l=0, n=21 and 22 we find a strong rotational modulation at
the end of the old solar cycle (number 22). The modulation vanishes
during solar minimum and only weakly reappears during the rise of the
new cycle. There is a decrease in the integrated mode amplitudes with
increased activity for the modes that show rotational modulation. At
low solar activity there is little correlation between modes of
varying order, but the correlation seems to increase with increasing
solar activity.
Title: Observations of sunspot transition region oscillations
Authors: Brynildsen, N.; Maltby, P.; Leifsen, T.; Kjeldseth-Moe, O.;
Wilhelm, K.
Bibcode: 2000SoPh..191..129B
Altcode:
Oscillations with a period of 3 minutes are observed in the transition
region of six sunspots with the Solar and Heliospheric Observatory
- SOHO joint observing programme for velocity fields in sunspot
regions. Observations of the transition region lines O v λ629
and N v λλ1238, 1242 with the SUMER instrument show significant
differences in the amplitude of the 3-minute oscillations from one
sunspot to another, both in intensity and line-of-sight velocity. In
four sunspots the central part of the umbra is observed. Two of these
sunspots show coincidence between the maxima in peak line intensity
and velocity directed towards the observer, as is expected for an
upward-propagating acoustic wave. The two other sunspots show large
oscillation amplitudes and a difference of 25° between maxima in
intensity and blue shift. The possible effect of partial wave reflection
on the observed phase relation is discussed. For one sunspot only a part
of the umbra, close to the penumbra, was observed and the observations
show a difference of 50° between maxima in intensity and blueshift. For
the smallest sunspot the observations are found to be contaminated by
contributions from an area without oscillations. Observed oscillations
in line width are small, but probably significant in two sunspots. The
observations of NOAA 8378 allow us to compare simultaneous recordings
of the oscillations in the chromospheric Si ii λ1260 line with the
oscillations in the transition region lines. We question the suggestion
by Fludra (1999) that the sunspot transition region oscillations are
a typical feature of the sunspot plumes.
Title: Sunspot Transition Region Oscillations in NOAA 8156
Authors: Brynildsen, N.; Leifsen, T.; Kjeldseth-Moe, O.; Maltby, P.;
Wilhelm, K.
Bibcode: 1999ApJ...511L.121B
Altcode: 1998astro.ph.12012B
Based on observations obtained with the Solar and Heliospheric
Observatory joint observing program for velocity fields in sunspot
regions, we have detected 3 minute transition region umbral oscillations
in NOAA 8156. Simultaneous recordings of O V λ629 and N V λ1238,
λ1242 with the SUMER instrument give the spatial distribution of power
in the 3 minute oscillations, both in intensity and in line-of-sight
velocity. Comparing loci with the same phase, we find that the entire
umbral transition region oscillates. The observed maxima in peak line
intensity are nearly in phase with the maxima in velocity directed
toward the observer. We discuss the suggestion that the waves are
upward-propagating acoustic waves.
Title: Sources of Amplitude Modulation of Solar P-Modes
Authors: Leifsen, Torben; Andersen, Bo; Appourchaux, Thierry
Bibcode: 1998ESASP.418..939L
Altcode: 1998soho....6..939L
We study the amplitude variation with time of the solar radial
p-modes. Continuous datasets from more than two years of observations
with the SOHO/VIRGO SPM and LOI instruments were used. A Hilbert
transform method was used to study the time variation of the amplitudes
of the l = 0-3 modes with radial order 12-32. The observed amplitudes of
the modes vary substantially with time on a large range of timescales
up to more than a solar rotation. As expected the power spectra of
the amplitude variation show little or no consistent periodicities for
most of the modes. However, for some of the modes, specifically for it
l = 0, n = 21 and 22, a strong modulation is observed with the solar
sidereal rotation frequency. This is a very surprising result as the
{l} = 0 modes should be insensitive to the solar rotation. In contrast
the SPM blue channel irradiance observations show a modulation with
the solar synodic rotation frequency. This is as one would expect as
active regions rotate over the solar disc and thereby modulate the
irradiance signal. A comparison with MDI velocity data show that the
amplitude modulation of the modes closely resembles the modulation as
observed with VIRGO/SPM. This shows that the observed modulation is
not an effect of the irradiance variation due to active regions, but
rather a modulation of the mode itself by rotation. This conclusion is
strengthened by the fact that the symmetric {l} = 0 modes are modulated
by the sidereal and not the synodic rotation frequency. The two years
of observations cover times of both low and higher solar activity as
the activity of the new solar cycle started in the summer of 1997. In
the solar irradiance one can see the onset of the new solar cycle
as a frequency shift to lower frequencies as the activity appear at
higher solar latitudes. Surprisingly a similar shift is also seen in
the modulation of the two modes, indicating that the modulation of
the modes is closely connected to the latitudes of solar activity.
Title: Time Dependence of Solar Noise Observed with VIRGO
Authors: Andersen, Bo; Leifsen, Torben; Appourchaux, Thierry; Frohlich,
Claus; Jiménez, Antonio; Wehrli, Christoph
Bibcode: 1998ESASP.418...83A
Altcode: 1998soho....6...83A
The effect of non- and quasiperiodic solar surface structures dominate
the power spectra of solar irradiance and radiance over a broad range
of time scales. Only in the p-mode region above about 2 mHz and in the
rotationally dominated region below about 3 μ Hz there seems to be
narrow stationary peaks. The solar background signal, or solar noise has
clear large scale quasi-stationary structures that seems to be closely
correlated to the combination of timescales and contrasts of the solar
surface sources. The dominant sources are the solar granulation and
supergranulation. The solar noise determines the detection limit in
the search for g-modes. Thus an increased understanding of it may be
helpfull in this search. In addition this understanding will be usefull
to determine the properities of stellar small scale surface structures
from the data from future asteroseismology space missions. In this
study we use the VIRGO data to study the time variation of the solar
noise with timescales from about one day to about one year.
Title: Statistical Properties of the Amplitude Modulation of Solar
p-Modes
Authors: Andersen, Bo; Aarset, Magne; Leifsen, Torben; Appourchaux,
Thierry; Frohlich, Claus; Hoeksema, J. T.; Jiménez, Antonio; Toutain,
Thierry
Bibcode: 1998ESASP.418..897A
Altcode: 1998soho....6..897A
The low degree solar p-modes show amplitude modulation at all observable
timescales. For some modes a large fraction of this modulation seems
to be correlated to the solar rotation. For other modes there seems
to be little or no deterministic component in the modulation. Only
intermittent correlation between the modulation of different modes
have been observed. This is to be expected if the excitation of modes
is completely stochastic. None of the observational methods observe the
modes directly. In different ways they all observe the solar atmospheres
response to the modes. This implies that the modes may be modulated by
this response. By studying the statistical properties of the different
observed modes we attempt to discriminate between variations in the
modes themselves and the atmospheric response. In this work we study the
statistical properties of the mode amplitude variations for radial order
p-modes observed with the VIRGO and SOI/MDI instruments on SOHO. The
time scales studied span the region from 0.2 μ Hz to 15 muHz. Here,
we are modelling the amplitude modulation, utilising the concepts of
state space models, as a stochastic process and study the properties
of this model as function of radial order and line width of the modes.
Title: Amplitude modulation of radial p-modes from Virgo
Authors: Leifsen, T.; Andersen, B. N.; Appourchaux, T.; Frohlich,
C.; Jimenez, A.; Toutain, T.; Wehrli, C.
Bibcode: 1998IAUS..185..113L
Altcode:
We present results from wavelet analysis of more than one year of
data from the VIRGO Sun Photometers (SPM) and the VIRGO Luminosity
Oscillation Imager (LOI) onboard the SOHO spacecraft. The temporal
behaviour of p-modes with l=0--2 is presented. The analysis of
the l=0 modes show a modulation of the mode amplitudes with the
solar rotation. This result is quite surprising as the l=0 modes
should not be sensitive to the solar rotation. Possible effects of
varying solar activity on the modulation of the mode amplitudes is
investigated. The effect of the modulation on the fitting of mode lines
and determination of the mode frequencies is also studied. Wavelet
analysis has a fundamental limitation in the ability to achieve
simultaneous high frequency and time resolution. In order to be able
to study the rotationally split components of the l=1 modes with good
time resolution, we apply a spatial filtering technique on the LOI
data to separate the different components.
Title: The Effect of Amplitude Modulation on Asymmetries of Solar
p-Modes
Authors: Andersen, Bo; Leifsen, Torben; Appourchaux, Thierry; Frohlich,
Claus; Hoeksema, J. T.; Toutain, Thierry
Bibcode: 1998ESASP.418..893A
Altcode: 1998soho....6..893A
The low degree solar p-modes show an asymmetry in their line profiles
that both depend on the observational technique and on the radial
order of the modes. Typically the modes determined from observations
with Doppler shift methods show an asymmetry towards lower frequency
while the opposite is the case for modes determined from irradiance and
radiance observations. The difference in asymmetry is probably caused by
near surface effects. If the modes are fitted with symmetric functions
this leads to a systematic shift in the frequency determinations for
the two data sets. This may lead to systematic differences in the
inversions based on the different frequencies. All the mode amplitudes
show significant time variation. The typical centroid frequency also
varies with timescales from a few days to the solar cycle. These
variations may set absolute limits to the observable accuracy of the
frequency determination. These time modulations may influence the
observed line profiles and thus the frequency determinations. Here we
present results of the effect on the mode asymmetry of the amplitude
modulation of low degree modes observed with VIRGO and SOI/MDI. The
range in timescales is from a few days to a few solar rotations.
Title: First results from VIRGO on SoHO
Authors: Frohlich, C.; Andersen, B. N.; Appourchaux, T.; Berthomieu,
G.; Crommelynck, D. A.; Domingo, V.; Fichot, A.; Finsterle, W.;
Gómez, M. F.; Gough, D.; Jiménez, A.; Leifsen, T.; Lombaerts, M.;
Pap, J. M.; Provost, J.; Roca Cortés, T.; Romero, J.; Roth, H. -J.;
Sekii, T.; Telljohann, U.; Toutain, T.; Wehrli, C.
Bibcode: 1997IAUS..181...67F
Altcode:
No abstract at ADS
Title: First Results from VIRGO, the Experiment for Helioseismology
and Solar Irradiance Monitoring on SOHO
Authors: Fröhlich, Claus; Andersen, Bo N.; Appourchaux, Thierry;
Berthomieu, Gabrielle; Crommelynck, Dominique A.; Domingo, Vicente;
Fichot, Alain; Finsterle, Wolfgang; Gómez, Maria F.; Gough, Douglas;
Jiménez, Antonio; Leifsen, Torben; Lombaerts, Marc; Pap, Judit M.;
Provost, Janine; Roca Cortés, Teodoro; Romero, José; Roth, Hansjörg;
Sekii, Takashi; Telljohann, Udo; Toutain, Thierry; Wehrli, Christoph
Bibcode: 1997SoPh..170....1F
Altcode:
First results from the VIRGO experiment (Variability of solar IRradiance
and Gravity Oscillations) on the ESA/NASA Mission SOHO (Solar and
Heliospheric Observatory) are reported. The observations started
mid-January 1996 for the radiometers and sunphotometers and near the
end of March for the luminosity oscillation imager. The performance of
all the instruments is very good, and the time series of the first 4-6
months are evaluated in terms of solar irradiance variability, solar
background noise characteristics and p-mode oscillations. The solar
irradiance is modulated by the passage of active regions across the
disk, but not all of the modulation is straightforwardly explained in
terms of sunspot flux blocking and facular enhancement. Helioseismic
inversions of the observed p-mode frequencies are more-or-less in
agreement with the latest standard solar models. The comparison of
VIRGO results with earlier ones shows evidence that magnetic activity
plays a significant role in the dynamics of the oscillations beyond
its modulation of the resonant frequencies. Moreover, by comparing
the amplitudes of different components ofp -mode multiplets, each of
which are influenced differently by spatial inhomogeneity, we have
found that activity enhances excitation.
Title: Wavelet Analysis of IPHIR Data
Authors: Leifsen, T.; Hanssen, A.; Andersen, B. N.; Toutain, T.
Bibcode: 1995ASPC...76..520L
Altcode: 1995gong.conf..520L
No abstract at ADS
Title: Solar Noise Simulations in Irradiance
Authors: Andersen, B. N.; Leifsen, T. E.; Toutain, T.
Bibcode: 1994SoPh..152..247A
Altcode: 1994IAUCo.143..247A; 1994svs..coll..247A
The global signature of granulation, meso- and supergranulation is
calculated using values for intensities and lifetimes from spatially
resolved observations. These simulations are compared with observations
from ACRIM, IPHIR and the SOVA-1 photometers. The results indicate that
the overall shape of the background signal in the simulations reproduce
the observations at low frequency. However when the granulation
lifetimes are about 500 seconds the simulated data do not correspond
to the observations between 1 and 2 mHz.
Title: Solar 5-minute Oscillations at 2.23 MU M
Authors: Leifsen, T.
Bibcode: 1994IAUS..154..271L
Altcode:
No abstract at ADS
Title: Observations and simulations of timeresolved CO spectra
Authors: Leifsen, T.
Bibcode: 1994chdy.conf..139L
Altcode:
No abstract at ADS
Title: Convection and Gravity Wave Interaction in the Solar Interior
Authors: Andersen, B. N.; Andreassen, O.; Wasberg, C. E.; Leifsen, T.
Bibcode: 1993ASPC...42...49A
Altcode: 1993gong.conf...49A
No abstract at ADS
Title: Solar 5-min oscillations at 2.23mm
Authors: Leifsen, T.
Bibcode: 1993ASPC...40...97L
Altcode: 1993ist..proc...97L; 1993IAUCo.137...97L
No abstract at ADS
Title: Status of the infrared solar oscillation study.
Authors: Leifsen, T.
Bibcode: 1991dsoo.conf...25L
Altcode:
Broad band intensity observations obtained with a fast multichannel
photometer at the Oslo Solar Observatory in 1987 and 1988 led to the
discovery of a region in the near infrared part of the solar spectrum
were the intensity oscillations show unusually large amplitudes. This
paper describes an ongoing project initiated in order to search for
the source of the large amplitudes and to study the possibility of
using the 2.23 μm region for intensity measurements in helio- and
asteroseismology studies. The Fourier Transform Spectrometer (FTS)
of the McMath telescope is used for the observations.
Title: New Light on Solar Infrared Intensity Oscillations
Authors: Leifsen, T.; Maltby, P.
Bibcode: 1990SoPh..125..241L
Altcode:
The detection of large-amplitude infrared solar intensity oscillations
in the 5-min region is reported. Using a broad-band multichannel
photometer, the peak-to-peak intensity variation at 2.23 μm is found
to be as high as 2.4% for a circular aperture of 1 arc min and 0.8%
in the full disk observations, i.e., remarkably higher than at the
other four observed wavelength regions.
Title: Solar infrared intensity oscillations.
Authors: Leifsen, T.; Maltby, P.
Bibcode: 1988ESASP.286..169L
Altcode: 1988ssls.rept..169L
The 5-min oscillations are found to be easily observable as intensity
variations in an infrared wavelength band centered at 2.23 μm with
bandwidth (FWHM) 65 nm. The observed peak to peak intensity variation
is 2.4% for a circular aperture of 1 arc min and 0.8% in the full disc
observations, i.e. considerably higher than in the other four observed
channels between 0.67 and 1.65 μm. In addition to the 5-min oscillation
the observed full disc power spectrum shows a strong feature centered
at 4.3 mHz. This frequency coincides with that of the fundamental
p-mode resonance of the chromosphere. Although this identification is
not proven the possibility to study the chromospheric cavity in full
disc observations is interesting.
Title: The limb effect of the K i resonance line, 769.9 n m
Authors: Andersen, B. N.; Barth, S.; Hansteen, V.; Leifsen, T.; Lilje,
P. B.; Vikanes, F.
Bibcode: 1985SoPh...99...17A
Altcode:
Low-noise observations have been obtained to search for a possible limb
effect in the K I 769.9 nm resonance line. The observations were carried
out along the north/south diameter of the solar disc. The data were
individually corrected for the effects of straylight on the velocity
measurements. A small, but significant limb effect is detected. The
total shift in the line core from center to limb corresponds to 125
m s−1 with an uncertainty of < 30 m s−1.