PRELIMINARY RESULTS ON THE SOLAR
PHOTOSPHERIC DYNAMICS OBSERVED
WITH VAMOS
M. Oliviero, P.F. Moretti, G. Severino, Th. Straus, M. Magrı̀ and A.
Tripicchio
INAF - Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131
Napoli, Italy
Abstract. The intensity and velocity fluctuations, observed simultaneously, are a
powerful diagnostic tool of the dynamics of the solar atmosphere. The phase relation between the fluctuations can improve our knowledge of the solar background,
its relation with the acoustic sources, and its interaction with the solar acoustic
oscillations. Furthermore, the opposite asymmetries observed along the p-mode line
profiles in the intensity and velocity power spectra contain information about the
source of the solar acoustic oscillations. For these reasons, it is relevant to study the
height dependence of the asymmetries and phases in the solar atmosphere. In this
paper, we present the results from the analysis of observations performed by the
VAMOS instrument in the potassium 769.9 nm line and Na I D lines, and compare
the measured phases with those obtained at different layers in the solar atmosphere
by different instruments, spanning from the base of the photosphere to the low
chromosphere.
Keywords: solar photosphere, solar oscillations
1. Introduction
Observing the intensity (I) fluctuations of the solar photosphere, simultaneously with the velocity (V) fluctuations, greatly improves our
diagnostic potential of the photospheric dynamics. In fact, the combination of the I and V data allows to investigate the I-V cross-spectrum.
This cross-spectrum, which consists of two real spectra, i.e. the I-V
phase difference and coherence spectra, has been demonstrated to be
an irreplaceable observational constraint in addition to the I and V
power spectra (Deubner et al., 1996; Straus et al., 1999; Jiménez et al.,
1999; Severino et al., 2001b). The I-V cross spectrum is turning out
to be a powerful tool - a gold mine - in particular to study the solar
“background” signal of the global oscillations, as pointed out by Severino et al. (1998). The I-V phase background has a low frequency part
which was discovered by Deubner et al. (1990) and called the plateauinterridge regime. The overall background, observed with GONG and
MDI/SOHO in the Ni I 676.8 nm line, has a step-like behaviour with
c 2002 Kluwer Academic Publishers. Printed in the Netherlands.
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M. OLIVIERO ET AL.
negative phase values at low frequencies (Straus et al., 1999), and
positive values at high frequency (Oliviero et al., 1999). Further, the
coherence has two local maxima located in the centers of negative and
positive phase regimes. Crossing a single p-mode line profile, the I-V
phase peak has a typical shark-fin shape, and the coherence peak is
surrounded by two side dips, the high frequency dip deeper than the
low frequency one (Oliviero et al., 1999).
These characteristic p-mode line profiles of the cross-spectrum and
the asymmetries of the p-mode line profiles in the I and V power
spectra are consistent with a model of the solar background signal
including a coherent1 component, composed of both a background correlated and a background uncorrelated with the modes (Severino et al.,
2001b), in addition to the incoherent component or noise, described
by the well-known Harvey’s model (Harvey, 1985). In particular, the
line asymmetries are produced by the coherent correlated background.
A background component correlated with the mode was invoked to
reverse the asymmetries in the I and V power spectra (Roxburg and
Vorontsov, 1997; Nigam et al., 1998), but only by the help of the I-V
cross-spectrum was it possible to connect this proposed background
to the really observed solar background (Skartlien and Rast, 2000;
Severino et al., 2001b). The interpretation of these components in terms
of precise physical phenomena, and specifically their relationship with
the acoustic source is currently matter of debate.
We expect that the character of the solar atmospheric dynamics
changes significantly with height, as the comprehensive work of Deubner and coworkers demonstrated for the high values of the spherical
harmonic degree ` (Hill et al., 1991). It is of great interest to extend
the study of the I-V cross spectrum as a function of the photospheric
level to medium and low `-values. This can be accomplished by using
different spectral lines as diagnostics. In this paper we present the analysis of the first results obtained with the new version of the VAMOS
instrument, which operates in the K I 769.9 nm line and explores the
range 5 < ` < 300 (Severino et al., 2001a), and discuss the VAMOS
spectra in comparison with other observations.
2. The Instrument and the Observations
The VAMOS (Velocity And Magnetic Observations of the Sun) is a solar imager, based on the technology of the magneto-optical filter (MOF,
1
We define as coherent a dynamic process whose I and V fluctuations have a
fixed temporal phase difference. Furthermore, we call correlated two processes with
a fixed phase difference between their velocity or intensity fluctuations.
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PRELIMINARY VAMOS RESULTS ON THE PHOTOSPHERIC DYNAMICS
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Figure 1. The VAMOS on the 40 cm stellar telescope in the East tower of the
Capodimonte observatory.
Table I. Main characteristics of the VAMOS instrument
Filter type:
Blue operating wavelength:
Red operating wavelength:
Field-of-view:
Spatial resolution:
Best time resolution:
Dopplergram sensitivity:
Magnetic field range:
Sensor:
Frame grabber:
Max image dynamic range:
MOF with potassium vapour cells
(769.89 − 0.01) nm
(769.89 + 0.01) nm
full-disk
approx. 12 arcsec
1 image/160 ms
approx. 10 m/s/pixel
approx. 20-2500 G
699x288 pixels CCD 8-bit video camera
768x512 pixels (linear or logarithmic)
16 bits/pixel (through image summation)
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M. OLIVIERO ET AL.
e.g. Cacciani and Fofi, 1978), which is able to obtain high cadence
observations of the Sun’s intensity (I) and velocity (V) fluctuations,
and longitudinal magnetic field (B) component at the photospheric
level, where the K I 769.9 nm line is formed. The instrument acquires
intensity images in the red and blue wings of the potassium line, alternatively. These images are combined in real time to produce the
intensity, velocity and magnetic field images, as described in Severino
et al. (2001a). The old version of this instrument (Cacciani et al., 1997),
with two sodium vapour cells, observed in the Na I D lines (Oliviero
et al., 1998). The new instrument, with two potassium vapour cells,
became operational in May 1999, and it has been used for campaign
observations since then. Currently, the VAMOS is located in the East
tower of the Capodimonte observatory main building and is piggybacked on a 40 cm stellar telescope (Figure 1). The instrument main
characteristics are listed in Table I. Moreover, a complete and updated
information on the VAMOS can be found on the web.
The data set we used for this paper consists of 5 consecutive days
of observation, from 19 to 23 June 2000. The 5 daily runs are 432 min,
758 min, 764 min, 734 min, and 684 min long, respectively (corresponding to 39 µHz, 22 µHz, 22 µHz, 23 µHz, and 24 µHz wide frequency bins
in the Fourier domain). For each day, we acquired velocity and intensity
images both at one minute cadence. Due to the adopted acquisition
procedure, the intensity time series have a time lag of 21 sec with respect
to the velocity time series.
We also used a set of data from the old version of the VAMOS,
consisting in 256 velocity and intensity images acquired in the Na I D
lines once per minute on February 20, 1997, and with a similar time lag
between the two time series. Due to the lower frequency resolution of
the sodium data (65 µHz) in respect to the potassium data, the sodium
has been used only for determining the phase difference on the mode
and in the background, and not for analysis of the mode line profiles.
3. Data Analysis
We used the calibration and spherical harmonic analysis procedures
developed in IDL for the VAMOS project by Oliviero et al. (1998). In
this package, the sum of all theoretical contributions to the Earth-Sun
line-of-sight velocity is used to calibrate the dopplergrams, also taking
into account the center-to-limb dependence of the solar line profiles.
This package was applied separately to the intensity and velocity images time series. We divided each intensity image by its average, then
we applied a two-point difference filter to the normalized image time
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PRELIMINARY VAMOS RESULTS ON THE PHOTOSPHERIC DYNAMICS
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series to remove the center-to-limb variation of solar intensity, and,
finally, we calculated the spherical harmonic coefficients. Starting from
the velocity images, we computed the calibrated dopplergrams, which
was then decomposed into spherical harmonics, and we applied the twopoint difference filter to the resulting velocity coefficient time series. In
this way, we obtained the intensity and velocity coefficient time series
as a function of the degree ` and the azimuthal order m. The present
analysis is limited to ` ≤ 200, because at higher ` the signal to noise
ratio for the intensity is low, and also because the spatial resolution cannot be better than the 1200 corresponding to the instrument diffraction
limit. Moreover, since the full solar disk images have been apodized,
we have a resulting `-leakage ∆` ' 4.
The total coefficient time series of the intensity and velocity fluctuations were constructed by merging the 5 data sets, and assuming
zero for the values of the coefficients in absence of observations. The
full data set is 6196 min long, with a duty cycle of about 54%, and the
frequency bins in the spectra are about 2.7 µHz wide. We computed the
intensity and velocity power spectra (PI,V ) and, from the complex crossspectrum (TI TV∗ ), the spectrum (ΦI−V ) of the phase difference between
the intensity and velocity fluctuations, and the coherence spectrum
(COH), as follows
`
X
PI,V (`, ν) =
|TI,V (`, m, ν)|2
(1)
TI (`, m, ν)TV∗ (`, m, ν)
(2)
m=−`
ΦI−V = arg
`
X
m=−`
P
`
m=−` TI (`, m, ν)TV∗ (`, m, ν)
i .
COH = hP
`
∗ (`, m, ν)
T
(`,
m,
ν)T
m=−`
I
V
(3)
Here ν is the frequency, ∗ denotes the complex conjugate quantity, and
TI (`, m, ν) and TV (`, m, ν) are the Fourier transforms of the intensity
and velocity coefficients. Coherence is a measure of the statistical stability of the phase difference, i.e. the phase scatter with m is lower
when coherence is higher. Note that the Fourier transforms have been
corrected for both the frequency shift produced by the solar rotation, as
described in Oliviero et al. (1999), and the phase lag, linearly depending
on the frequency, due to the 21 sec time lag between intensity and
velocity images.
Before presenting our results, it is worthwhile to look at the mdependence of the I-V phase differences, shown in Figure 2 for ` = 150,
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M. OLIVIERO ET AL.
Figure 2. I-V phase variation as function of m at ` = 150, after averaging the
cross-spectrum over frequency. The solid line is a linear fit to the data.
after averaging the cross-spectrum over frequency. We note a significant, approximately linear, phase variation with m, common to both
the p-modes and the background, and with essentially the same slope
for all `-values. A systematic rotation between the I and V images could
produce this effect. A possible source for such a rotation might be a
spatial shift, which simulates a rotation in the central part of the solar
disk. We estimated that a systematic spatial shift of the order of 2 pixels
(∼ 800 at disk center) would be sufficient to produce the observed phase
variation. As a consequence, we removed a linear fit of the m-variation
from the phase of the cross-spectrum, before computing the phase
difference and the coherence values shown in Figures 3, 4, 5 and 6.
The m-variation of the phase differences, present in the VAMOS data,
suggested us to look also at GONG data, where we found an analogous
variation, but with smaller slope (∼ 25◦ for GONG and ∼ 100◦ for
VAMOS in the range ∆m = 150). We conclude that it is worthwhile to
review the GONG analysis of the cross-spectrum performed by Oliviero
et al. (1999).
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PRELIMINARY VAMOS RESULTS ON THE PHOTOSPHERIC DYNAMICS
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4. Results
In Figure 3, the `-ν diagrams of the intensity and velocity power, the
phase difference and coherence spectra are shown. The p-mode ridges
are evident in the diagrams of PV and PI , in particular the velocity
power shows that they extend to `-values greater than 200, while the
intensity power is more noisy and contaminated by a vertical spurious
band at high-`. To comment on the results, we show, as an example,
the computed spectra at ` = 75 (Figure 4) and the phase difference and
coherence as a function of ` at constant frequency (Figure 5). On the pmodes, in correspondence of the velocity power maxima, the phases are
positive and close to about 160◦ (see Figure 4 in the frequency range
from 2 to 5 mHz). At low frequencies, below 2 mHz, where the plateau2
discovered by Deubner et al. (1990) is located, the phase difference
ΦI−V has a regime of negative values around −160◦ (Figure 4). In this
regime, the phases decrease with increasing `, passing from values close
to −140◦ , at ` = 30, to values close to −180◦ , at ` = 170 (Figure 5).
However, caution must be used in considering this `-variation because
the corresponding coherence, i.e. a measure of the phase scatter in
respect to the azimuthal order m, decreases with increasing ` (Figure 5).
Finally, the coherence is relatively high on the p-modes ranging from
0.4 to 0.9 (Figure 4), while is lower than 0.4 in the negative phase
regime (Figures 4 and 5). Note that from the analysis of sodium data
we found values of phase difference close to the potassium results, i.e.
160◦ on the p-mode and −150◦ in the background.
Following Duvall et al. (1993), we have also computed, for all four
spectra, the mean p-mode line profiles for radial order n=4 and degree
`=130−170 (Figure 6), shifting the modes in respect to the `=150
mode, so that they overlap, and averaging the spectra over the degree.
Before shifting, we corrected the power spectra for the effects of the twopoint difference filter and the image integration times of 30.72 sec and
6.24 sec for the velocity and intensity time series, respectively. These
corrections were performed according to Komm et al. (1998), who applied a similar detrending to compare GONG and SOHO/MDI power
spectra. The velocity and intensity power show the p-mode line asymmetries, but less clearly as in Duvall et al. (1993) and Oliviero et al.
(2001). Because our data have a lower signal to noise ratio with respect
to the data quoted above, we would expect an asymmetry partly hidden
by noise (Rast and Bogdan, 1998). However, part of the difference may
2
Note that the negative phase regime at low frequencies extends to higher
frequencies intruding in the interridge region. We hereafter refer to this plateauinterridge regime simply as “the background”. The high frequency background of
positive phase values will be not discussed in this paper.
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M. OLIVIERO ET AL.
Figure 3. `-ν diagrams of the velocity and intensity power, phase difference and coherence spectra, from the June 19-23 full data set. The power spectra are in arbitrary
units, color and gray bars refer to phase differences and coherence, respectively.
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PRELIMINARY VAMOS RESULTS ON THE PHOTOSPHERIC DYNAMICS
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Figure 4. PV , PI , ΦI−V and COH spectra for ` = 75, from the June 19-23 full data
set. Note that, to better illustrate the phase difference transition between the p-mode
and the low frequency regime, we used the range [0◦ , 360◦ ] for the ΦI−V plot. In this
way, below 2 mHz the value of −160◦ of the negative phase regime becomes 200◦ .
be due also to the different spectral lines used in the observations and
hence to a possible dependence of the line asymmetries on the height in
the solar atmosphere. The phase difference and coherence profiles differ
clearly from the GONG measurements (Oliviero et al., 1999; Oliviero
et al., 2001). In particular, the phase difference has a reversed “sharkfin” shape as a consequence of the different phase in the background.
Furthermore, the two asymmetric dips in the wings of the coherence
profile, observed in GONG data, are now missing.
In the [0◦ , 360◦ ] range used for ΦI−V in Figures 4 and 6, the phase
difference in between the ridges is about 190◦ , close to the phase value
in the background at frequencies below 2 mHz, while on the modes
the maximum of the velocity power spectrum corresponds to a phase
value of about 160◦ . As expected, the coherence spectrum peaks in
correspondence of the p-mode resonance frequency.
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M. OLIVIERO ET AL.
Figure 5. ΦI−V and COH variations as a function of ` at constant frequency
ν = 1.5 mHz, from the June 19-23 full data set. The solid line in the left panel
roughly indicates the phase variation.
Figure 6. Mean p-mode line profiles for radial order n = 4 and degree ` = 130 − 170,
from the June 19-23 full data set. The dashed lines in the velocity and intensity
power are a linear extension of the background from the low-frequency side of the
mode.
5. Discussion
It is worthwhile to compare our I-V phase differences on the p-modes in
the neighbourhood of ν = 3.3 mHz and at low-`, i.e. ` < 200, with the
phase differences at the same frequency as function of height at high-`,
i.e. ` >
∼ 200, based on the work of Deubner and other authors, as re-
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PRELIMINARY VAMOS RESULTS ON THE PHOTOSPHERIC DYNAMICS
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Figure 7. I-V phase variation for p-modes, as function of height in the solar atmosphere. The shadowed band represents schematically the phase variation and its
scatter as inferred from the measurements, in different spectral lines, presented by
Hill et al. (1991), Figure 18, and Masiello et al. (1998), Figure 4. The symbols with
error bars represent, in order of increasing height, the measurements on the p-modes
from GONG (Oliviero et al., 1999), VAMOS with potassium cells (this work), and
VAMOS with sodium cells (Oliviero et al., 1998). For all phases we assumed an error
of ±20◦ , estimated from the data scatter. We assumed an error of ±100 Km to take
into account the uncertainty of the line formation height.
ported in Hill et al. (1991), and Masiello et al. (1998). This comparison
is given in Figure 7 by using the range [0◦ , 360◦ ] for the phases, and by
representing schematically the scatter of the high-` values with the 40◦
wide band. The phase in the Ni I line was computed from the analysis of
36 days of GONG observations (Oliviero et al., 1999), while the phase
in the Na I D lines was computed from only 256 min long intensity
and velocity time series acquired with the old version of the VAMOS
instrument (Oliviero et al., 1998). The high-` phase differences in the
deep photosphere are lower than 90◦ , that is the I-V phase of an adiabatic evanescent wave in an isothermal atmosphere; they jump to values
much higher than the adiabatic value in the medium photosphere, and,
finally, tend smoothly back to the adiabatic value in the high photosphere and low chromosphere. On the other hand, our low-` phase
differences on the p-modes (actually, the three values corresponding to
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M. OLIVIERO ET AL.
Ni I (GONG), K I and Na I (VAMOS) in order of increasing height) have
values somewhat greater for both K I and Na I. Moreover, the VAMOS
I-V phase differences do not match the about 90◦ observed by Deubner
et al. (1996) in the same potassium line. A candidate for explaining
the higher phases observed by VAMOS is the intensity-velocity crosstalk which can affect at different degrees the MOF systems (Moretti
and Severino, 2002). In fact, differently from high-` data, the VAMOS
measures do not refer to the line center, but are based on two points
in the line profile, i.e. the red and blue operating wavelengths listed in
Table I. The actual position of the solar line in respect to these wavelengths depends on the instrument’s offset velocity which varies with
the position on the disk and with the observation time. In this way, the
instrument observes, generally, two different intensity levels in the line
profile. As a test of the effect due to the offset velocity in the VAMOS
data, we divided a daily run into two 4h time series, corresponding to
the morning and afternoon, respectively, and, hence, to different values
of the Earth-Sun relative velocity. After having performed the usual
space-time analysis for both series separately, we found that the morning I-V phases differ by ∼ 20◦ with respect to the afternoon phases, in
particular in the morning they are smaller on the modes and greater in
the low frequency background. Moreover, another source of cross-talk
can be a spurious transmission between the two operating wavelengths
of the VAMOS. This might occurr when the effective temperature of
the potassium inside the cell is not sufficiently high (Cacciani et al.,
1994). In our case, a potassium temperature decrease might be due to
a less efficient heating of the potassium in the cell stems caused by the
consumption of the potassium reservoir. We can estimate by excess the
effect of this spurious transmission on our I-V phase difference following
Moretti and Severino (2002, see their Equation 1 for the intensity power
affected by velocity cross-talk). If we assume that the departure of the
VAMOS gain, i.e. the root of the ratio between intensity and velocity
power, from the GONG gain (Oliviero et al., 1999) is completely due
to cross-talk, we found an upper limit for the phase variation of about
60◦ . On the other hand, we note that with a correction much smaller
than this one our K I phase difference would be consistent with the
high-` phases (Figure 7). We expect that cross-talk affects the Na I
phase difference to a lower extension in respect to the potassium data,
because in this case both the observed gain is closer to the GONG gain
and the measured phases agree rather well with those observed by the
MOF in Kanzelhöhe. How much of the discrepance between low- and
high-` phase differences can be explained by the details of the I-V phase
determination with a spectrometer like VAMOS, is a question which
deserves to be settled quantitatively, also in view of the large use of
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PRELIMINARY VAMOS RESULTS ON THE PHOTOSPHERIC DYNAMICS
13
the MOF-based instruments (e.g. Kanzelhöhe, LOW-`, Mount Wilson,
Università di Roma La Sapienza). In particular, for a precise correction
of the cross-talk effect it is necessary to know the instrument passband.
To this aim, we are planning to set up a diode laser system to measure
the VAMOS transmission before the observations.
On the other hand, in the low frequency background (at ν ∼ 1.5 mHz)
our low-` phase differences seem to decrease with height, from ∼ 300◦ in
the deep photosphere to ∼ 200◦ in the low chromosphere. For the high-`
phase differences, however, it is not easy to extract the corresponding
variation with height from the published data, as accurately as for the
p-modes. Due to the low power of the background, a real distinction
of the plateau regime in the `-ν diagram is necessary for this purpose.
With more detailed review of those data which permit the calculation
of `-ν phase diagram, it should be possible to derive a similar figure as
Figure 7 for the low-frequency background.
6. Perspectives
The final goal of the space-time analysis of the I and V atmospheric
fluctuations is to improve our physical knowledge of solar atmospheric
dynamics. In this context, the results on the I-V phase differences and
p-mode line profile asymmetries add important information that is not
fully exploited at this moment. The super-adiabatic phase values of the
I-V phase differences in the medium photosphere are usually interpreted
as an effect of the radiative damping of the acoustic-gravity waves. On
the other hand, deeper in the photosphere, where radiative damping is
even more effective, the measure of I-V phases, which are significantly
lower than their adiabatic value, requires a different mechanism to
be explained. This may be related to the interaction with convective
motions, as suggested by Houdek et al. (1995), or, may reflect the
highly variable structure of the photospheric basic layers, where, in
particular, going inwards the acoustic cutoff frequency drops abruptly
(e.g. Mihalas and Mihalas, 1984, Figure 54.2) and, hence, a wave of
fixed frequency should get an increasingly progressive character.
A better comprehension of wave propagation and damping in the
deep photosphere, from both the observational and theoretical points
of view, is really fundamental. This is true not only because of the
intrinsic interest of the problem, but also because this atmospheric
level is the site where most of the solar acoustic power is generated
and, moreover, because it behaves as the upper turning point for global
acoustic modes. In particular, the treatment of high-` p-modes, whose
diagnostic potential is still far from to beeing exhausted (e.g. Gough
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M. OLIVIERO ET AL.
et al., 1996), would benefit from a progress in the physical modeling
of the deep photosphere and top convection zone, which is abreast
of the great development of numerical simulation (e.g. Nordlund and
Stein, 2001; Stein and Nordlund, 2001; Georgobiani et al., 2000; Steffen,
1991). Furthermore, we note that to increase the diagnostic power of
I-V phase differences it would require also a quantitative analysis of
the filter effect done by the radiative transfer in the different spectral
lines used for probing the solar atmosphere. This analysis would be
useful both for determining the line formation levels, and for clarifying
to what extension the observed intensity fluctuations are a proxy of
the temperature and the other associated thermodynamic fluctuations
(Severino et al., 1998).
As far as the solar p-mode line profiles are concerned, their asymmetries have been used to investigate the nature of the acoustic sources
and, in particular, the source depth (Nigam et al., 1998; Kumar and
Basu, 1999) and type (Rast and Bogdan, 1998). The reversal of the
asymmetry between intensity and velocity power was attributed to
the interaction of the modes with a component of the solar background which is correlated with the mode (Roxburg and Vorontsov,
1997; Nigam et al., 1998). Moreover, in their fit of the power and
I-V phase spectra obtained by Oliviero et al. (1999), Skartlien and
Rast (2000) identified the acoustic source and the mode-correlated
background with convective downdraft events occurring in the darkest
intergranular lanes and comparable to the seismic events observed by
Goode and co-workers (Strous et al., 2000, and references therein).
Recently, Severino et al. (2001b) showed that both the asymmetry and
its reversal, as well as the phase difference and coherence (the latter
of which has never been modeled before) can be explained by using
a coherent background only partly correlated to the p-modes, without
hypothesis on the acoustic source depth. Furthermore, they indicated
that caution should be exercised in the interpretation of models of
helioseimic spectra which do not use all four spectra (intensity and
velocity power, and I-V phase difference and coherence) as constraints.
Knowing the height dependence of the p-mode line asymmetries, I-V
phase differences and coherence in the solar atmosphere is potentially
helpfull to clarify the nature of the solar helioseimic background and its
relation with acoustic sources and the p-modes. In our opinion, at the
moment, such a kind of information from ground based MOF instruments has not reached yet the highest possible level of confidence in
order to be used with profit to complement e.g. GONG or MDI/SOHO
data. Therefore, we plan in the near future to make more effort to
optimize the VAMOS data acquisition as well as the analysis of the
combined I and V fluctuations in the potassium resonance line.
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PRELIMINARY VAMOS RESULTS ON THE PHOTOSPHERIC DYNAMICS
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Acknowledgements
We thank E. Cascone for assistance in implementing the VAMOS on the
40 cm stellar telescope. We acknowledge the support of the Ministero
dell’Università e della Ricerca Scientifica e Tecnologica (MURST).
References
Cacciani, A. and Fofi, M.: 1978, Solar Phys. 59, 179.
Cacciani, A., Marmolino, C., Moretti, P. F., Oliviero, M., Severino, G., and Smaldone, L. A.: 1997, in G. Cauzzi and C. Marmolino (eds.), The Inconstant Sun,
Mem. SaIt. 68 N.2, 467.
Cacciani, A., Rosati, P., Ricci, D., Egidi, A., Apporchaux, T., Marquedant, R.J.,
Smith, E.J.: 1994, Theoretical and Experimental Study of the Magneto Optical
Filter, JPL internal report #D11900.
Deubner, F.-L., Waldschik, Th., and Steffens S.: 1996, Astron. Astrophys. 307, 936.
Deubner, F.-L., Fleck, B., Marmolino, C., and Severino, G.: 1990, Astron. Astrophys.
236, 509.
Duvall, T.L., Jr., Jefferies, S.M., Harvey, J.W., Osaki, Y., and Pomerantz, M.A.:
1993, Astrophys. J. 410, 829.
Georgobiani, D., Kosovichev, A.G., Nigam, R., Nordlund, Å., Stein, R.F.: 2000,
Astrophys. J. 530, L139.
Gough, D.O., Leibacher, J.W., Scherrer, P.H., Toomre, J.: 1996, Science 272, 1281.
Harvey,J.W.: 1985, in E.J. Rolfe and B. Battrick (eds.), Future Missions in Solar
Heliospheric and Space Plasma Physics, ESA SP-235, 199.
Hill, F., Deubner, F.-L., Isaak, G.: 1991, in A. N. Cox, W. C. Livingston, and M. S.
Matthews (eds.), Solar interior and atmosphere, the University of Arizona Press,
Tucson, p. 329.
Houdek, G., Balmforth, N.J., and Christensen-Dalsgaard, J.: 1995, in J.T. Hoeksema, V. Domingo, B. Fleck and B. Battrick (eds.), Fourth SOHO Workshop:
Helioseismology, ESA SP-376, 447.
Jiménez, A., Roca Cortés, T., Severino, G., and Marmolino, C.: 1999, Astrophys. J.
525, 1042.
Komm, R.W., Anderson, E., Hill, F., Howe, R., Kosovichev, A.G., Scherrer, P.H.,
Schou, J., Fodor, I., and Stark, P.: 1998, in S. Korzennik and A. Wilson (eds.),
SOHO6/GONG98 Workshop: Structure and Dynamics of the Interior of the Sun
and Sun-like Stars, ESA SP-418, 253.
Kumar, P. and Basu, S.: 1999, Astrophys. J. 519, 389.
Masiello, G., Marmolino, C., and Straus, Th.: 1998, in S. Korzennik and A. Wilson
(eds.), SOHO6/GONG98 Workshop: Structure and Dynamics of the Interior of
the Sun and Sun-like Stars, ESA SP-418, 261.
Mihalas, D. and Mihalas, B.W.: 1984, Foundations of Radiation Hydrodynamics,
Oxford University Press, New York, p. 219.
Moretti, P. F. and Severino, G.: 2002, Astron. Astrophys. 384, 638.
Nigam, R., Kosovichev, A.G., Scherrer, P.H., and Schou, J.: 1998, Astrophys. J. 495,
L115.
Nordlund, Å. and Stein, R.F.: 2001, Astrophys. J. 546, 576.
oliviero.tex; 29/07/2002; 19:05; p.15
16
M. OLIVIERO ET AL.
Oliviero, M., Severino, G., and Straus, Th.: 1998, in S. Korzennik and A. Wilson
(eds.), SOHO6/GONG98 Workshop: Structure and Dynamics of the Interior of
the Sun and Sun-like Stars, ESA SP-418, 275.
Oliviero, M., Severino, G., and Straus, Th.: 2001, in A. Wilson (ed.), SOHO
10/GONG 2000 Workshop: Helio- and Asteroseismology at the Dawn of the
Millennium, ESA SP-464, 669.
Oliviero, M., Severino, G., Straus, Th., Jefferies, S. M., Appourchaux, T.: 1999,
Astrophys. J. 516, L45.
Rast, M.P. and Bogdan, T.J.: 1998, Astrophys. J. 496, 527.
Roxburg, I.W. and Vorontsov, S.V.: 1997, MNRAS 292, L33.
Severino, G., Straus, Th., Jefferies, S.M.: 1998, in S. Korzennik and A. Wilson
(eds.), SOHO6/GONG98 Workshop: Structure and Dynamics of the Interior of
the Sun and Sun-like Stars, ESA SP-418, 53.
Severino, G., Moretti, P. F., Oliviero, M., and the VAMOS team: 2001a, in A.
Wilson (ed.), SOHO 10/GONG 2000 Workshop: Helio- and Asteroseismology at
the Dawn of the Millennium, ESA SP-464, 337.
Severino, G., Magrı̀, M., Oliviero, M., Straus, Th., and Jefferies, S.M.: 2001b,
Astrophys. J. 561, 444.
Skartlien, R. and Rast, M.: 2000, Astrophys. J. 535, 464.
Steffen, M.: 1991, in L. Crivellari, I. Hubeny and D.G. Hummer (eds.), Stellar
Atmospheres: Beyond Classical Models, NATO ASI-Series C 341, 247.
Stein, R.F. and Nordlund, Å.: 2001, Astrophys. J. 546, 585.
Straus, Th., Severino, G., Deubner, F.-L., Fleck, B., Jefferies, S.M., and Tarbell, T.:
1999, Astrophys. J. 516, 949.
Strous, L.H., Goode, P.R., and Rimmele, T.R.: 2000, Astrophys. J. 535, 1000.
oliviero.tex; 29/07/2002; 19:05; p.16