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Author name code: isaak
ADS astronomy entries on 2022-09-14
author:"Isaak, George R."
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Title: Anomalous variations in low-degree helioseismic mode
frequencies
Authors: Howe, R.; Chaplin, W. J.; Elsworth, Y.; Hill, F.; Komm,
R. W.; Isaak, G. R.; New, R.
2006MNRAS.369..933H Altcode: 2006MNRAS.tmp..504H
We compare changes in the frequencies of solar acoustic modes with
degree between 0 and 2, as derived from Global Oscillation Network Group
(GONG), Birmingham Solar Oscillations Network (BiSON) and Michelson
Doppler Imager (MDI) spectra obtained between 1995 and 2003. We find
that, after the solar-activity dependence has been removed from the
frequencies, there remain variations that appear to be significant,
and are often well correlated between the different data sets. We
consider possible explanations for these fluctuations, and conclude
that they are likely to be related to the stochastic excitation of the
modes. The existence of such fluctuations has possible relevance to
the analysis of other low-degree acoustic mode spectra such as those
from solar-type stars.
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Title: Solar FLAG hare and hounds: on the extraction of rotational
p-mode splittings from seismic, Sun-as-a-star data
Authors: Chaplin, W. J.; Appourchaux, T.; Baudin, F.; Boumier, P.;
Elsworth, Y.; Fletcher, S. T.; Fossat, E.; García, R. A.; Isaak,
G. R.; Jiménez, A.; Jiménez-Reyes, S. J.; Lazrek, M.; Leibacher,
J. W.; Lochard, J.; New, R.; Pallé, P.; Régulo, C.; Salabert, D.;
Seghouani, N.; Toutain, T.; Wachter, R.
2006MNRAS.369..985C Altcode: 2006MNRAS.tmp..515C; 2006astro.ph..6748C
We report on results from the first solar Fitting at Low-Angular
degree Group (solar FLAG) hare-and-hounds exercise. The group
is concerned with the development of methods for extracting the
parameters of low-l solar p-mode data (`peak bagging'), collected by
Sun-as-a-star observations. Accurate and precise estimation of the
fundamental parameters of the p modes is a vital pre-requisite of all
subsequent studies. Nine members of the FLAG (the `hounds') fitted
an artificial 3456-d data set. The data set was made by the `hare'
(WJC) to simulate full-disc Doppler velocity observations of the
Sun. The rotational frequency splittings of the l = 1, 2 and 3 modes
were the first parameter estimates chosen for scrutiny. Significant
differences were uncovered at l = 2 and 3 between the fitted splittings
of the hounds. Evidence is presented that suggests this unwanted bias
had its origins in several effects. The most important came from the
different way in which the hounds modelled the visibility ratio of
the different rotationally split components. Our results suggest that
accurate modelling of the ratios is vital to avoid the introduction of
significant bias in the estimated splittings. This is of importance
not only for studies of the Sun, but also of the solar analogues
that will be targets for asteroseismic campaigns. <P />Solar FLAG
URL: http://bison.ph.bham.ac.uk/~wjc/Research/FLAG.html <P />E-mail:
wjc@bison.ph.bham.ac.uk ‡ <P />George Isaak passed away in 2005 June
5, prior to the completion of this work. He is greatly missed by us all.
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Title: On model predictions of the power spectral density of radial
solar p modes
Authors: Chaplin, W. J.; Houdek, G.; Elsworth, Y.; Gough, D. O.;
Isaak, G. R.; New, R.
2005MNRAS.360..859C Altcode: 2005MNRAS.tmp..510C
We investigate the frequency dependence of the power spectral density
of low-degree solar p modes by comparing measurements with the results
of a stochastic-excitation model. In the past it was common practice
to use the total power in such investigations. Using the maximum of
the power spectral density instead provides a direct comparison with
the measured mode heights in the observed power spectrum. This method
permits a more careful calibration of the adjustable parameters in the
excitation model, a model which we present here, for the first time,
in a format that precisely and unambiguously relates the amplitudes
of the modes of oscillation to the Reynolds stress in the equilibrium
model. We find that errors in the theory of the linear mode damping
rates, particularly at low frequency, have a dramatic impact on
the predictions of the mode heights in the spectral density, whereas
parameter changes in the stochastic excitation model, within a plausible
domain of parameter space, have a comparatively small effect.
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Title: Noise characteristics of full-disc helioseismic observations
made by resonant scattering spectrometers
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Miller, B. A.;
New, R.; Pintér, B.
2005MNRAS.359..607C Altcode: 2005MNRAS.tmp..294C
Resonant scattering spectrometers (RSSs) have been used to make
high-precision full-disc helioseismic observations since the 1970s. They
are capable of very high-precision determinations of line-shift, meaning
that they are used to obtain precise velocity measurements, and, for
suitably configured RSSs, the disc-averaged longitudinal magnetic field
(SMMF). In order to exploit fully the very extensive high-precision
data sets, it is essential to understand the noise characteristics of
the instruments. This paper re-examines the consequences for velocity
and SMMF determinations of there being noise on the scattered light
signals measured by an RSS. It presents a theoretical description,
and analysis of simulations, which match well the noise variations
found recently in both BiSON velocity and SMMF observations. It also
outlines a method for using the differences between the power spectra
of redundant channels to analyse instrumental noise characteristics.
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Title: The Search for Correlation between BiSON SMMF Data and
CME Events
Authors: Chaplin, William J.; Dumbill, Andrew M.; Elsworth, Yvonne
P.; Isaak, George R.; McLeod, Clive P.; Miller, Brek A.; New, Roger;
Pintér, Balázs
2005HiA....13..141C Altcode:
The Birmingham Solar Oscillation Network (BiSON) has acquired high
precision solar mean magnetic field (SMMF) data on a 40-second
cadence for a decade. We present first attempts to compare such data
from recent years with the occurence of CME's as recorded by LASCO
using correlation techniques applied to measurements from different
BiSON instruments to maximise the sensitivity to CME related SMMF
responses. SMMF measurements were recorded at the time of occurence
of several hundreds CME's.
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Title: Phase Variation as a New Tool in the Investigation of the
Excitation and Damping of Solar p-modes
Authors: Simoniello, R.; Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.;
New, R.
2004ApJ...616..594S Altcode:
We report on a novel way of using the phase evolution in solar
oscillation data as a tool in the study of p-mode excitation and
damping. A mathematical formalism is presented, backed by extensive
simulations, that enables the determination of the line width of
modes close to the peak of the solar oscillation spectrum. The method
potentially avoids the usual problem of strong correlation between the
width and the height of the modes. We also identify further possible
uses of the technique.
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Title: Flag Hare-And Exercise: on the Extraction of Sectoral Mode
Splittings from Full-Disc Sun-As Data
Authors: Chaplin, W. J.; Appourchaux, T.; Baudin, F.; Boumier, P.;
Elsworth, Y.; Fletcher, S. T.; Fossat, E.; García, R. A.; Isaak,
G. R.; Jiménez, A.; Jiménez-Reyes, S. J.; Lazrek, M.; Lochard, J.;
New, R.; Pallé, P.; Régulo, C.; Salabert, D.; Toutain, T.
2004ESASP.559..356C Altcode: 2004soho...14..356C
No abstract at ADS
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Title: Impact of Observational Duty Cycle on P-Mode Eigenfrequencies
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Miller, B. A.;
New, R.; Salabert, D.
2004ESASP.559..364C Altcode: 2004soho...14..364C
No abstract at ADS
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Title: Twenty-Eight Years of BISON Data
Authors: Miller, B. A.; Hale, S. J.; Elsworth, Y.; Chaplin, W. J.;
Isaak, G. R.; New, R.
2004ESASP.559..571M Altcode: 2004soho...14..571M
No abstract at ADS
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Title: The Phase of Global Solar Oscillations
Authors: Simoniello, R.; Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.;
Miller, B. A.; New, R.
2004ESASP.559..251S Altcode: 2004soho...14..251S
No abstract at ADS
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Title: Novel Techniques for the Identification of Noise Contributions
to Full-Disc Helioseismic Power Spectra
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Miller, B. A.;
New, R.; Pintér, B.
2004ESASP.559..360C Altcode: 2004soho...14..360C
No abstract at ADS
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Title: On comparing estimates of low-l solar p-mode frequencies from
Sun-as-a-star and resolved observations
Authors: Chaplin, W. J.; Appourchaux, T.; Elsworth, Y.; Isaak, G. R.;
Miller, B. A.; New, R.
2004A&A...424..713C Altcode:
Low-angular-degree (low-l) solar p modes provide a sensitive probe
of the radiative interior and core of the Sun. Estimates of their
centroid frequencies can be used to constrain the spherically symmetric
structure of these deep-lying layers. The required data can be extracted
from two types of observation: one where the modes are detected in
integrated sunlight, i.e., a Sun-as-a-star view; and a second where
the visible disc is imaged onto many pixels, and the collected images
then decomposed into their constituent spherical harmonics. While the
imaging strategy provides access to all of the individual components
of a multiplet, the Sun-as-a-star technique is sensitive to only
about two thirds of these (average over l=0 to 3) with those modes
that are detected having different levels of visibility. Because
the various components can have contrasting spatial structure over
the solar surface, they can respond very differently to changes in
activity along the solar cycle. Since the Sun-as-a-star and resolved
analyses take as input a different “subset” of modes, the extracted
frequency estimates are expected to differ depending upon the phase
of the cycle. Differences also arise from the types of models used to
fit the modes. Here, we present expressions that allow the sizes of
these differences to be predicted.
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Title: Impact of observational duty cycle on the measurement of
low-ℓ solar p-mode frequencies
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Miller, B. A.;
New, R.; Salabert, D.
2004A&A...424..301C Altcode:
We investigate the impact of a multi-site, ground-based observational
window function on the extraction of estimates of the frequencies
of low-angular-degree (low-ℓ) solar p modes from decade-long
datasets. To effect this study we have made use of some ≈10 yr of
full-disc, “Sun-as-a-star” Doppler velocity data collected by the
Birmingham-Solar Oscillations Network (BiSON). A coherent combination
of observations made by all six BiSON sites provided the principal
time series of data. This set was then modulated by a whole series
of different window functions and the resulting sets analyzed. The
windows were made from different combinations and numbers of BiSON
stations. We find that for the majority of low-ℓ modes the bias in
the frequencies given by the effects of the window function is not
significant. However, for modes above ≈3300 μ Hz, and some ℓ=2
modes near ≈2500 μ Hz, the bias is important when the frequencies
are extracted from long datasets. <P />Appendix A is only available
in electronic form at http://www.edpsciences.org
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Title: The solar cycle as seen by low-l p-mode frequencies: comparison
with global and decomposed activity proxies
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Miller, B. A.;
New, R.
2004MNRAS.352.1102C Altcode: 2004MNRAS.tmp..158C
We present a detailed study of variations observed in low-angular
degree solar p-mode frequencies during solar cycles 22 and 23,
and their relation to global and spatially decomposed proxies of
the surface activity. To do so, we have analysed 11yr of unresolved
(Sun-as-a-star) Doppler velocity observations of the solar disc made by
the Birmingham Solar-Oscillations Network (BiSON). The sensitivity of
these observations to different azimuthal orders, m, is such that for
the degree range studied (0 <=l<= 2) extracted frequencies can be
regarded as providing a measure of the response of the sectoral modes
(with |m| =l). <P />After allowing for the dependence of the frequency
shifts on mode frequency and inertia, we find the average l= 0 shift is
significantly weaker than that at l= 2; the magnitude of the average l=
1 shift lies in between the two. The comparative sizes of the shifts
are observed to match those of the corresponding spherical harmonic
(Legendre) components of both the Kitt Peak magnetogram (KPMI) and
HeI equivalent width activity indices, and reflect, therefore, the
sensitivity of the observed mode components to the distribution of
activity over the solar surface (i.e. a `spatial' contribution to the
shifts). When the falling and rising parts of the cycles are analysed
independently, we uncover a significant difference in behaviour at l=
1. Possible reasons for this are discussed.
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Title: Extended search for correlation between solar mean magnetic
field BiSON data and coronal mass ejections
Authors: Chaplin, William J.; Dumbill, Andrew M.; Elsworth, Yvonne
P.; Isaak, George R.; McLeod, Clive P.; Miller, Brek A.; New, Roger;
Pintér, Balázs
2004SoPh..220..307C Altcode:
The Birmingham Solar-Oscillations Network (BiSON) has acquired
high-precision solar mean magnetic field (SMMF) data on a 40-s
cadence for a decade. We present attempts to compare such data
from recent years with the occurrence of coronal mass ejections
(CMEs) as recorded by LASCO, using correlation techniques applied
to measurements from different BiSON instruments to maximise the
sensitivity to CME-related SMMF responses. SMMF measurements were
recorded at the time of occurrence of several hundred CMEs. No CME
event shows a convincing response in our SMMF data at short periods
setting a threshold amplitude of 12 mG. By averaging data sets we are
able to set lower thresholds, which depend somewhat on the distribution
of response strengths. A brief summary of the very first results of
this study is also given in Chaplin et al.
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Title: Solar p-mode frequencies at ℓ=2: What do analyses of
unresolved observations actually measure?
Authors: Chaplin, W. J.; Appourchaux, T.; Elsworth, Y.; Isaak, G. R.;
Miller, B. A.; New, R.; Toutain, T.
2004A&A...416..341C Altcode:
We have studied in detail the extraction of estimates of ℓ=2 p-mode
frequencies from unresolved observations of the visible disc of the
Sun. Examples of data of this type include ground-based observations
made by the Birmingham Solar-Oscillations Network (BiSON), and
space-borne observations made by the GOLF and VIRGO/SPM instruments
on board the ESA/NASA SOHO satellite. The fitting of the modes is
complicated in practice by the asymmetric arrangement in frequency
of the three components (m=-2, 0 and 2) that are prominent in such
data. In order to investigate the effect of this we used a series of
10-yr artificial datasets into which varying degrees of asymmetry were
introduced. The sets were designed to mimic the characteristics of the
BiSON and GOLF data, and were analyzed both with and without the BiSON
window function from the period 1992 through 2001. Since reliable
estimates of the asymmetry have only recently been extracted from
unresolved observations (Chaplin et al. \cite{Chaplin03}a) it has for a
long time been standard practice to fit the ℓ=2 modes to a model that
assumes a symmetrically arranged multiplet. We have tested the impact
of this on the accuracy of the extracted frequencies. Furthermore,
we demonstrate that asymmetric models can be successfully applied,
provided the data are of sufficient length and quality. We also discuss
the implications of our simulations for analyses of real solar data.
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Title: Studies of the solar mean magnetic field with the Birmingham
Solar-Oscillations Network (BiSON)
Authors: Chaplin, W. J.; Dumbill, A. M.; Elsworth, Y.; Isaak, G. R.;
McLeod, C. P.; Miller, B. A.; New, R.; Pintér, B.
2003MNRAS.343..813C Altcode:
The first analysis of 10 yr of solar mean magnetic field data
from the Birmingham Solar-Oscillations Network (BiSON) instrument
at Sutherland, SA is presented. Long-period (>1 d) variations
correlate well with those observed by the Stanford group, but our
daily mean values have a statistical precision over 10 times better
than previous work. Differences between the absolute values determined
from Sutherland and Stanford are discussed, as are systematic annual
variations of the order of 15 per cent. At high frequencies (>0.5
mHz), the mean noise spectral density for our decade of observations
is 0.53 G<SUP>2</SUP> Hz<SUP>-1</SUP>, setting a lower limit to the
noise level than that obtained from brief observations by GOLF on
board the ESA/NASA satellite SOHO in 1996. The high-frequency mean
noise spectral density is different in different years. The lowest
annual value, obtained for 1997, is 0.22 G<SUP>2</SUP> Hz<SUP>-1</SUP>,
whilst the highest, for 1995, is 1.46 G<SUP>2</SUP> Hz<SUP>-1</SUP>. It
is not yet clear whether these variations are related to solar activity.
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Title: Observation of, and temporal variations in, solar p-mode
multiplet frequency asymmetries at l= 2
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Miller, B. A.;
New, R.; Thiery, S.; Boumier, P.; Gabriel, A. H.
2003MNRAS.343..343C Altcode:
The predominant contribution to the frequency splitting of low-l solar
p modes arises from the rotation of the solar interior and this lifts
the frequency degeneracy in l to give a symmetric pattern where the
observed (synodic) separation between adjacent m (i.e. for |Δm|= 1)
is ~400 nHz. Magnetic fields can also contribute to the splitting, but
they do so in such a way as to introduce asymmetries in the arrangement
of the components within each multiplet. In disc-integrated data this
effect may become apparent when l>= 2. Here, we attempt to extract
estimates of the frequency asymmetries at l= 2 from the analysis
of disc-integrated data collected by the ground-based Birmingham
Solar Oscillations Network (BiSON) and the GOLF instrument on board
the ESA/NASA SOHO satellite. Our analyses demonstrate that we have
evidence for there being non-zero asymmetries present (significance
~3-4σ) during an epoch coincident with high levels of surface
activity close to the maximum of solar cycle 23. The asymmetries are
indistinguishable from zero at minimum levels of activity near the cycle
22/23 boundary. <P />We also compare the observed asymmetries with those
calculated from a model that is based upon the recent predictions of
Moreno-Insertis & Solanki. While the level of agreement between
the two is found to be reasonable, the observations suggest (though
with poor constraints placed upon this) that the influence on the
mode frequencies of high-latitude activity may not be as strong as in
the model.
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Title: A Comparison of Low-Degree Solar p-Mode Parameters from BiSON
and GONG: Underlying Values and Temporal Variations
Authors: Howe, R.; Chaplin, W. J.; Elsworth, Y. P.; Hill, F.; Komm,
R.; Isaak, G. R.; New, R.
2003ApJ...588.1204H Altcode:
Approximately 5 years of the l=0 time series from the GONG project
have been analyzed using the algorithm developed for the BiSON
zero-dimensional data. The data cover the period 1995-2000. The results
are compared with those from a parallel analysis of contemporaneous
BiSON data and also with the results of the traditional GONG analysis
of the low-degree time series. The spectra analyzed were prepared
using the multitaper spectral analysis technique used in the recent
reanalysis of the GONG data. We consider both solar cycle trends
and temporally averaged values for mode frequencies, line widths,
amplitudes, and asymmetry parameters.
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Title: Degree dependence of mode parameters with solar activity in
BiSON data
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; New, R.
2003ESASP.517..119C Altcode: 2003soho...12..119C
The line-width, velocity power and rate of supply of energy to the
low-degree p modes of the Sun are investigated over falling phase of
cycle 22 and the rising phase of cycle 23 by fitting in the Fourier
transform domain. We see that for the first time for low-degree modes
we are able to parameterise the solar activity dependence as a function
of the angular degree of the mode.
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Title: Probing the solar core with BiSON: the challenge at low l
and low frequency
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Miller, B. A.;
New, R.; Pintér, B.
2003ESASP.517..183C Altcode: 2003soho...12..183C
In this contribution we touch upon a few issues of relevance to the
current status of low-angular-degree (low-l) p-mode Helioseismology. In
particular: the precision in frequency, both historic and current,
achievable at low l; the quest to extend the low-frequency detection
threshold nearer to the p-mode fundamental; and the level of agreement
between frequencies extracted from different data using various
analysis techniques.
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Title: Comparing results from the GONG l = 0 and BiSON time series
Authors: Howe, R.; Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Komm,
R. W.; New, R.
2003ESASP.517..303H Altcode: 2003soho...12..303H
Approximately 5 years of the l = 0 time series from the GONG project
have been analysed using the algorithm developed for the BiSON
0-dimensional data. The data cover the period 1995-2000. The results
are compared with those from a parallel analysis of contemporaneous
BiSON data, and also with the results of the traditional GONG analysis
of the low-degree time series. The spectra analysed were prepared
using the multitaper spectral analysis technique used in the recent
re-analysis of the GONG data. We consider both solar-cycle trends
and temporally averaged values for mode frequencies, linewidths,
amplitudes and asymmetry parameters.
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Title: High-frequency interference peaks in BiSON data
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Marchenkov,
K. I.; Miller, B. A.; New, R.
2003ESASP.517..247C Altcode: 2003soho...12..247C
We have analyzed 9 yr of non-imaged Doppler velocity observations of
the visible disc of the Sun in an effort to search for pseudo-mode-like
structure in the data above the acoustic cut-off frequency of the solar
atmosphere (≍5400 μHz). These data were collected by the ground-based
Birmingham Solar-Oscillations Network (BiSON) over the period 1992
January through 2000 December. Our analysis uncovers the presence of a
pseudo-mode-like structure above the acoustic cut-off frequency that
persists up to ≍8500 μHz, with a spacing between adjacent peaks
(or troughs) of ~68 μHz. The signature - which disappears at higher
frequencies - has a slightly different repeat period (i.e., frequency
separation between successive peaks or troughs) to that found by Garcia
et al. (1998) in full-disc GOLF data.
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Title: The Search for Correlation Between BISON Smmf Data and Cme's
Authors: Chaplin, William J.; Dumbill, Andy M.; Elsworth, Yvonne;
Isaak, George R.; McLeod, Clive P.; Miller, Brek A.; New, Roger;
Pinter, Balazs
2003IAUJD...3E..30C Altcode:
The Birmingham Solar Oscillation Network (BiSON) has acquired high
precision solar mean magnetic field (SMMF) data on a 40-second
cadence for a decade. We present first attempts to compare such data
from recent years with the occurence of CME's as recorded by LASCO
using correlation techniques applied to measurements from different
BiSON instruments to maximise the sensitivity to CME related SMMF
responses. SMMF measurements were recorded at the time of occurence
of several hundreds CME's.
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Title: On the measurement bias of low-l solar p-mode excitation
parameters: The impact of a ground-based window function
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Miller, B. A.;
New, R.; Pintér, B.; Thiery, S.
2003A&A...398..305C Altcode:
We present a study of the impact of a ground-based, multi-station window
function on estimates of the power and damping of low-l solar p modes
extracted from fits to resonant structure in the frequency domain. The
window functions come from six-site observations made by the Birmingham
Solar-Oscillations Network (BiSON) over the 10-yr period beginning 1991
January. Two strategies were adopted. In the first, we used an 800-d
time series of continuous observations made by the GOLF instrument
on board the ESA/NASA SOHO satellite. These data were modulated by a
variety of BiSON window functions, with fractional duty cycles ranging
from ~ 0.4 to ~ 0.8, and the resulting series analyzed. In the second
we generated artificial 10-yr time series and studied the effect on
these of the complete BiSON window.
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Title: Does the Energy Supplied to Low-l Solar p-Modes Vary over
the Activity Cycle?
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Miller, B. A.;
New, R.; Toutain, T.
2003ApJ...582L.115C Altcode:
We report on the average behavior of the excitation and damping of
low angular degree (low l) solar p-mode oscillations over the decade
from 1991 to 2000 using both long and short time duration Fourier
transforms. The data in question were collected by the ground-based
Birmingham Solar Oscillations Network. Throughout most of the
period under study, the energy supply rate to the modes remains
roughly constant-implying a near-constant level of forcing-while the
damping and velocity power show a fairly smooth increase and decrease,
respectively, in response to increasing levels of solar activity (in
line with previous findings). However, here we uncover evidence of
there being a sharp increase in the mode velocity power over a brief
period of approximately 100 days centered on 1998 late March. The
magnitude and sign of this are contrary to the expectation based on
the long-timescale, solar-cycle trend; such unusual behavior is absent
in the damping. This implies that the forcing of, or rate of energy
supplied to, the modes increased in magnitude over this period.
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Title: Peak finding at low signal-to-noise ratio: low-ℓ solar
acoustic eigenmodes at n≤9 from the analysis of BiSON data
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Marchenkov,
K. I.; Miller, B. A.; New, R.; Pinter, B.; Appourchaux, T.
2002MNRAS.336..979C Altcode:
We make use of 9 yr of full-disc helioseismic data - as collected by
the ground-based Birmingham Solar-Oscillations Network (BiSON) - to
search for low-frequency, low-angular-degree (low-l) acoustic modes. A
range of tests are applied to the power spectrum of the observations
that search for prominent mode-like structure: strong spikes, structure
spanning several bins signifying the presence of width (from damping),
and the occurrence of prominent multiplet structure at l>= 1 arising
principally from the solar rotation and made from several spikes
separated suitably in frequency. For each test we present analytical
expressions that allow the probability that the uncovered structure is
part of the broad-band noise background to be assessed. These make use
of the cumulative binomial (Bernoulli) distribution and serve to provide
an objective measure of the significance of the detections. This work
has to date uncovered nine significant detections of non-broad-band
origin that we have identified as low-l modes with radial overtone
numbers n<= 9.
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Title: An oblique magnetic rotator in the Sun?
Authors: Isaak, G. R.; Isaak, K. G.
2002AN....323..436I Altcode:
The current paradigm for the origin of sunspots and a global solar
magnetic field is a dynamo, located near the bottom of the convection
zone (CZ). We propose an alternative model. By interpolating in
evolution between early-type stars and evolved white dwarfs and neutron
stars, we suggest that the Sun (and indeed probably most stars) has
a fossil magnetic field in the form of an oblique magnetic rotator
(OMR) in the radiative zone (RZ). This rotator is electromagnetically
screened by the skin-effect, produced by the differentially rotating
CZ. The OMR provides a natural explanation for many solar phenomena,
including the rigid rotation of the interior of the Sun, the long-term
phase stability of the solar cycle, as well as diametrically opposing
active longitudes. Nutation of the OMR can be invoked to explain the
solar cycle and some of its features, including the sunspot butterfly
diagram, Hale's law for the polarity of sunspots and their reversal
every 11 years. An extension of the OMR model provides a natural
explanation of the spatial and temporal variations of sunspots based
on such a primordial magnetic field. Here, sunspots are bundles of
magnetic flux that have broken off the extremities of the OMR. These
rise by magnetic buoyancy and, on reaching the RZ-CZ boundary, suffer
a deflection in a direction dependent upon the algebraic difference
between the tangential zone velocities. This reverses during the
nutation cycle and thus the leading and trailing sunspots reverse in
magnetic polarity every 11 years. The flux fibrils, tubes, or possibly
ropes, convey magnetic flux, energy, angular momentum, and, perhaps
most importantly, frozen-in matter to the solar surface, each with a
fingerprint characteristic of the radiative zone. More tentatively,
the flux tubes may originate from even deeper within the Sun - the
nuclear active zone (NZ). The resulting isotopic enhancements would
have implications on abundance in the interstellar medium, and so on
nucleosynthesis and its cosmological inferences.
---------------------------------------------------------
Title: Observation of, and variations in, multiplet frequency
asymmetries at low l
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Marchenkov,
K. I.; Miller, B. A.; New, R.; Pinter, B.; Thiery, S.
2002ESASP.508...71C Altcode: 2002soho...11...71C
The predominant contribution to the frequency splitting of low-l solar
p modes arises from the rotation of the solar interior and this lifts
the frequency degeneracy in l to give a symmetric pattern where the
observed (synodic) separation between adjacent m (i.e., for |Δm| = 1)
is ~400nHz. Magnetic fields can also contribute to the splitting, but
they do so in such a way as to introduce asymmetries in the arrangement
of the components within each multiplet. In full-disc data this effect
may become apparent when l >= 2. Here, we extract estimates of the
frequency asymmetry for l = 2, investigate variations over the activity
cycle and compare the results with the near-surface predictions of
Dziembowski et al. (2000) which are based upon observations from
higher l.
---------------------------------------------------------
Title: Does the energy supply rate to the p modes vary over the
solar cycle?
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; New, R.
2002ESASP.508...33C Altcode: 2002soho...11...33C
The rate of supply of energy to the low-degree p modes of the Sun
is investigated using both long and short time-duration Fourier
transforms. We see that, although on average the energy supply rate to
the modes is independent of solar activity, there is a period of high
solar activity where the energy-supply rate is increased. Furthermore
we see that the correlations between the mode line width and the power
in the modes are different on the falling phase of cycle 22 and the
rising phase of cycle 23 with a strong correlation between the energy
supply rate and the mode power on the rising phase of cycle 23.
---------------------------------------------------------
Title: On the measurement precision of solar p-mode eigenfrequencies
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Miller, B. A.;
New, R.
2002MNRAS.330..731C Altcode:
We make use of 3456d of observations of the low-l p-mode oscillations
of the Sun in order to study the evolution over time of the measurement
precision of the radial eigenfrequencies. These data were collected by
the ground-based Birmingham Solar-Oscillations Network (BiSON) between
1991 January and 2000 June. When the power spectrum of the complete
time series is fitted, the analysis yields frequency uncertainties
that are close to those expected from the returned coherence times of
the modes. The slightly elevated levels compared with the prediction
appear to be consistent with a degradation of the signal-to-noise
ratio in the spectrum that is the result of the influence of the
window function of the observations (duty cycle 71 per cent). The
fractional frequency precision reaches levels of a several parts in
10<SUP>6</SUP> for many of the modes. The corresponding errors reported
from observations made by the GOLF instrument on board the ESA/NASA
SOHO satellite, when extrapolated to the length of the BiSON data set,
are shown to be (on average) about ~25 per cent smaller than their BiSON
counterparts owing to the uninterrupted nature of the data from which
they were derived. <P />An analysis of the BiSON data in contiguous
segments of different lengths, T, demonstrates that the frequency
uncertainties scale as T<SUP>-1/2</SUP>. This is to be expected in the
regime where the coherence (life) times of the modes, τ<SUB>nl</SUB>,
are smaller than the observing time T (the `oversampled' regime). We
show that mode detections are only now beginning to encroach on the
`undersampled' regime (where T<τ<SUB>nl</SUB>).
---------------------------------------------------------
Title: Variation of acoustic mode centroid frequencies over the
solar cycle
Authors: Chaplin, W. J.; Appourchaux, T.; Elsworth, Y.; Isaak, G. R.;
New, R.
2002AdSpR..29.1881C Altcode:
Together with a brief historical overview, we use high-quality
helioseismic data collected by three different observational
programmes during the declining phase of activity cycle 22, and a
substantial portion of the rising phase of the current cycle (23),
to study the phenomenological nature of the cycle-induced (centroid)
eigenfrequencies. Our analyses (for 1600 ≤ ν ≤ 4000 μHz) make
use of observations made by the ground-based GONG over the angular
degree range 4 ≤ l ≤ 150; the ground-based BiSON over 0 ≤ l ≤
2; and the VIRGO/LOI instrument on board the ESA/NASA SOHO satellite
over 0 ≤ l ≤ 8. We show that GONG shifts averaged over different
ranges in l, together with the BiSON and LOI data averaged over
their full quoted ranges, all scale at a given frequency with the
normalized mode inertia ratio Q<SUB>nl</SUB> (Christensen-Dalsgaard
& Berthomieu 1991). This is to be expected if the time-dependent
perturbation affecting the modes is confined in the surface layers;
the excellent agreement also reflects favourably on the external
consistency of the different observations. We have also analyzed
the frequency dependence of the shifts by fitting a power-law of the
form δν <SUB>nl</SUB> ∝ (ν <SUB>nl</SUB>/ E<SUB>nl</SUB> to the
data (where the E<SUB>nl</SUB> are the mode inertias, and α is the
power-law index to be extracted). Previous studies have suggested that
a relation with α = 0 provides an adequate description of the shifts
up to ν ≈ 3500 μHz. However, here we show that while nevertheless
describing the shifts well up to ∼ 2500 μHz, the linear scaling
breaks down conspicuously at higher frequencies. Above this threshold,
the shifts follow a power-law dependence with α ∼ 2.
---------------------------------------------------------
Title: Rigid rotation of the solar core? On the reliable extraction
of low-l rotational p-mode splittings from full-disc observations
of the Sun
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Marchenkov,
K. I.; Miller, B. A.; New, R.
2001MNRAS.327.1127C Altcode:
We present low-l rotational p-mode splittings from the analysis
of 8yr of observations made by the Birmingham Solar-Oscillations
Network (BiSON) of the full solar disc. These data are presented
in the light of a thorough investigation of the fitting techniques
used to extract them. Particular attention is paid to both the origin
and magnitude of bias present in these estimates. An extensive Monte
Carlo strategy has been adopted to facilitate this study - in all,
several thousand complete, artificial proxies of the 96-month data
set have been generated to test the analysis of real `full-disc'
data. These simulations allow for an assessment of any complications
in the analysis which might arise from variations in the properties
of the p modes over the 11-yr solar activity cycle. The use of such
an extended data set affords greater precision in the splittings,
and by implication the rotation rate inferred from these data, and
reduces bias inherent in the analysis, thereby giving a more accurate
determination of the rotation. The grand, weighted sidereal average
of the BiSON set is 434+/-2nHz, a value consistent with that expected
were the deep radiative interior (r/R<0.5) to rotate at the same
frequency, and in the same `rigid' manner, as the more precisely and
accurately studied outer part of the radiative zone.
---------------------------------------------------------
Title: Changes in convective properties over the solar cycle: effect
on p-mode damping rates
Authors: Houdek, G.; Chaplin, W. J.; Appourchaux, T.;
Christensen-Dalsgaard, J.; Däppen, W.; Elsworth, Y.; Gough, D. O.;
Isaak, G. R.; New, R.; Rabello-Soares, M. C.
2001MNRAS.327..483H Altcode:
Measurements of both solar irradiance and p-mode oscillation
frequencies indicate that the structure of the Sun changes with
the solar cycle. Balmforth, Gough & Merryfield investigated the
effect of symmetrical thermal disturbances on the solar structure
and the resulting pulsation frequency changes. They concluded that
thermal perturbations alone cannot account for the variations in both
irradiance and p-mode frequencies, and that the presence of a magnetic
field affecting acoustical propagation is the most likely explanation
of the frequency change, in the manner suggested earlier by Gough &
Thompson and by Goldreich et al. Numerical simulations of Boussinesq
convection in a magnetic field have shown that at high Rayleigh number
the magnetic field can modify the preferred horizontal length scale
of the convective flow. Here, we investigate the effect of changing
the horizontal length scale of convective eddies on the linewidths
of the acoustic resonant mode peaks observed in helioseismic power
spectra. The turbulent fluxes in these model computations are obtained
from a time-dependent, non-local generalization of the mixing-length
formalism. The modelled variations are compared with p-mode linewidth
changes revealed by the analysis of helioseismic data collected by
the Birmingham Solar-Oscillations Network (BiSON); these low-degree
(low-l) observations cover the complete falling phase of solar activity
cycle 22. The results are also discussed in the light of observations
of solar-cycle variations of the horizontal size of granules and with
results from 2D simulations by Steffen of convective granules.
---------------------------------------------------------
Title: The phenomenology of solar-cycle-induced acoustic
eigenfrequency variations: a comparative and complementary analysis
of GONG, BiSON and VIRGO/LOI data
Authors: Chaplin, W. J.; Appourchaux, T.; Elsworth, Y.; Isaak, G. R.;
New, R.
2001MNRAS.324..910C Altcode:
We use high-quality helioseismic data collected by three different
observational programmes during the declining phase of activity
cycle 22,Q7 and a substantial portion of the rising phase of the
current cycle (23), to study the phenomenological nature of the
cycle-induced (centroid) eigenfrequency variations. We have analysed
the frequency dependence of the shifts by fitting a power law of the
form δν<SUB>nl</SUB>~(ν<SUB>nl</SUB>)<SUP>α</SUP>/E<SUB>nl</SUB>
to the data (where the E<SUB>nl</SUB> are the mode inertias, and
α is the power-law index to be extracted). Previous studies have
suggested that a relation with α=0 provides an adequate description
of the shifts up to ν~3500μHz. However, here we show that while
nevertheless describing the shifts well up to ~2500μHz, the linear
scaling breaks down conspicuously at higher frequencies. Above this
threshold, the shifts follow a power-law dependence with α~2. Our
analyses (for 1600<=ν<=4000μHz) make use of observations made
by the ground-based GONG over the angular degree range 4<=l<=150
the ground-based BiSON over 0<=l<=2 and the VIRGO/LOI instrument
on board the ESA/NASA SOHO satellite over 0<=l<=8. We show that
GONG shifts averaged over different ranges in l, together with the BiSON
and LOI data averaged over their full quoted ranges, all scale at fixed
frequency with the normalized mode inertia ratio Q<SUB>nl</SUB>Q1. This
is to be expected if the solar-cycle perturbation affecting the modes
is confined in the surface layers; the excellent agreement also reflects
favourably on the external consistency of the different observations.
---------------------------------------------------------
Title: Changes to low-ll solar p-mode frequencies over the solar
cycle: correlations on different time-scales
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Marchenkov,
K. I.; Miller, B. A.; New, R.
2001MNRAS.322...22C Altcode:
We have studied variations in the frequencies of low-l solar p modes
through the analysis of nine years of helioseismic data collected
by the Birmingham Solar Oscillations Network (BiSON)†. This is the
first time that such a long data set has been explored with the extra
accuracy afforded by fitting the modes to asymmetric profiles. The epoch
covered (1991-99) spans the declining activity phase of solar cycle 22,
and a substantial portion of the initial activity increase during cycle
23. The complete time series has been split into contiguous segments of
length 27, 54, 108 and 216d in order to facilitate the study of changes
occurring on different time-scales. Further, we have characterized the
observed shifts as a function of six well-known indicators of solar
activity. These indices reflect changes taking place in the photosphere,
chromosphere and corona, but only over the visible hemisphere of the
Sun. Since the low-l eigenfrequencies respond to global variations
in activity, we discuss the implications of this mismatch for the
analyses performed. We demonstrate that, as expected, the low-l modes
adjust to changes in the activity measures on time-scales as short as
a few months. Our analysis indicates that all six proxies correlate
equally well (at the level of precision of the data) with the measured
shifts. Further, the sensitivity of the shifts to changes in five of
the activity indices is the same, to within ~15per cent (1σ) or so,
on the falling and rising phases considered. There is, however, a
slight suggestion that the sensitivity to changes in the disc-averaged
line-of-sight magnetic field component (as determined from daily Kitt
Peak magnetograms) may be higher on the rising phase of the cycle.
---------------------------------------------------------
Title: Non linear inversion for the sound speed in the solar interior
using BiSON and SOI/MDI p-mode frequencies
Authors: Marchenkov, K. I.; Roxburgh, I. W.; Vorontsov, S. V.; Chaplin,
W. J.; Elsworth, Y.; Isaak, G. R.; New, R.
2001ESASP.464..531M Altcode: 2001soho...10..531M
The results of the global sound-speed inversion obtained with solar
p-mode frequencies provided by the recent high-quality observational
data (BiSON, SOI/MDI) are presented and discussed. The iterative
nonlinear inversion technique used here is a generalization (for the
case of exact solution of the adiabatic oscillation equations) of the
Born quasiasymptotic approximate inversion developed by Marchenkov et
al. (2000).
---------------------------------------------------------
Title: BiSON
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Marchenkov, K.;
Miller, B. A.; New, R.; Pinter, B.
2001ESASP.464..313C Altcode: 2001soho...10..313C
The current status of the Birmingham Solar Oscillation Network (BiSON)
is discussed.
---------------------------------------------------------
Title: Helioseismic signatures of localized structural discontinuities
in BiSON and GONG data
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; New, R.
2001ESASP.464...79C Altcode: 2001soho...10...79C
We use high-quality helioseismic data collected by BiSON and GONG in
an attempt to search for variations over time of the influence on the
acoustic mode frequencies of localized structural perturbations located
beneath the solar surface. Our analysis of the BiSON data is aimed at
a study of the effects of the He II ionization zone. We use GONG data
(4 <= l <= 140) to extract estimates of the solar-cycle-induced
eigenfrequency shifts over the range 1600 <= ν <= 4000
μHz. A careful analysis of these data may have uncovered tentative
evidence for an oscillatory behaviour that would be the signature of
a time-dependent solar cycle perturbation located approximately ~20
Mm beneath the photosphere.
---------------------------------------------------------
Title: A long-term look at acoustic modes with a nine-year BiSON
spectrum
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Marchenkov, K.;
Miller, B. A.; New, R.; Pinter, B.
2001ESASP.464...51C Altcode: 2001soho...10...51C
We present data taken from BiSON to illuminate solar-activity related
changes in the oscillation parameters. We also use the full dataset
to hunt for low order p-modes.
---------------------------------------------------------
Title: Stellar Seismology, Stellar Ages, and the Cosmological Constant
Authors: Isaak, G. R.; Isaak, K. G.
2001ASPC..245..199I Altcode: 2001aats.conf..199I; 2001astro.ph..9444I
Solar seismology has allowed precision measurements of both the static
and dynamic structure of our local star, the Sun. In the near future,
seismology of solar-like stars of different ages and masses, necessarily
restricted by angular resolution to low l-modes, will allow studies of
the internal structure of stars at various stages of evolution. Such
studies will test not only the theory of stellar evolution, but also
allow the determination of ages of stars from the helium content in
their cores. Such observations can be made photometrically from space,
but also spectroscopically from the ground. We outline ground-based
schemes. By correlating the external properties of nearby stars with
their internal properties, it will be possible to extend local studies
to distant open and globular clusters, and thereby to obtain an age
of the Universe, based on many stars. The combination of the age,
the density parameter Omega and Hubble's constant will allow strong
limits to be placed on the cosmological constant.
---------------------------------------------------------
Title: Characteristics of full-disc BiSON power spectra above the
acoustic cut-off frequency of the solar atmosphere
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Marchenkov,
K. I.; Miller, B. A.; New, R.
2001ESASP.464..191C Altcode: 2001soho...10..191C
We have analyzed 60 months of non-imaged Doppler velocity
observations of the visible disc of the Sun in an effort to search
for pseudo-mode-like structure in the data above the acoustic cut-off
frequency of the solar atmosphere (≍5400 μHz). These data were
collected by the ground-based Birmingham Solar-Oscillations Network
(BiSON). Our analysis uncovers marginally significant evidence for
the presence of a pseudo-mode-like structure in the range 5900 <=
ν <= 6600 μHz, with a spacing between adjacent peaks (or troughs)
of ~70 μHz. The signature - which, owing to the low signal to noise,
disappears at higher frequencies - has a similar repeat period and phase
to that found by Garcia et al. (1998) in full-disc GOLF data. However,
the zero-to-peak power is a factor of ~10 weaker in strength.
---------------------------------------------------------
Title: Probing the solar cycle: a comparative and complementary
analysis of GONG BiSON and VIRGO/LOI eigenfrequency shifts
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; New, R.;
Appourchaux, T.
2001ESASP.464...83C Altcode: 2001soho...10...83C
We use high-quality helioseismic data collected by GONG (over 4 <=
l <= 140), BiSON (0 <= l <= 2) and VIRGO/LOI (0 <= l <=
8) to study the phenomenological nature of the cycle-induced (centroid)
eigenfrequency variations. We have analyzed the frequency dependence
of the shifts by fitting a power-law of the form δν<SUB>nl</SUB> ~
(ν<SUB>nl</SUB>)<SUP>α</SUP>/E<SUB>nl</SUB> to the data (where the
E<SUB>nl</SUB> are the mode inertias, and α is the power-law index to
be extracted). Previous studies have suggested that a relation with
α = 0 provides an adequate description of the shifts up to ν ≍
3500 μHz. However, here we show that while nevertheless describing
the shifts well up to ~2500 μHz, the linear scaling breaks down
conspicuously at higher frequencies. Above this threshold, the shifts
follow a power-law dependence with α ~ 2. We also ahow that GONG
shifts, averaged over different ranges in l together with the BiSON
and LOI data averaged over their full quoted ranges, all scale at
fixed frequency with the normalized mode inertia ratio Q<SUB>nl</SUB>
(Christensen-Dalsgaard & Berthomieu 1991). This is to be expected
if the solar-cycle perturbation affecting the modes is confined in
the surface layers; the excellent agreement also reflects favourably
on the external consistency of the different observations.
---------------------------------------------------------
Title: g-mode detection: Where do we stand?
Authors: Appourchaux, T.; Andersen, B.; Berthomieu, G.; Chaplin, W.;
Elsworth, Y.; Finsterle, W.; Frölich, C.; Gough, D. O.; Hoeksema,
T.; Isaak, G.; Kosovichev, A.; Provost, J.; Scherrer, P.; Sekii, T.;
Toutain, T.
2001ESASP.464..467A Altcode: 2001soho...10..467A
We review the recent developments in determining the upper limits to
g-mode amplitudes obtained by SOHO instruments, GONG and BiSON. We
address how this limit can be improved by way of new helioseismic
instruments and/or new collaborations, hopefully providing in the not
too distant future unambiguous g-mode detection.
---------------------------------------------------------
Title: Observational Upper Limits to Low-Degree Solar g-Modes
Authors: Appourchaux, T.; Fröhlich, C.; Andersen, B.; Berthomieu, G.;
Chaplin, W. J.; Elsworth, Y.; Finsterle, W.; Gough, D. O.; Hoeksema,
J. T.; Isaak, G. R.; Kosovichev, A. G.; Provost, J.; Scherrer, P. H.;
Sekii, T.; Toutain, T.
2000ApJ...538..401A Altcode:
Observations made by the Michelson Doppler Imager (MDI) and Variability
of solar IRradiance and Gravity Oscillations (VIRGO) on the Solar and
Heliospheric Observatory (SOHO) and by the ground-based Birmingham
Solar Oscillations Network (BiSON) and Global Oscillations Network
Group (GONG) have been used in a concerted effort to search for solar
gravity oscillations. All spectra are dominated by solar noise in the
frequency region from 100 to 1000 μHz, where g-modes are expected to be
found. Several methods have been used in an effort to extract any g-mode
signal present. These include (1) the correlation of data-both full-disk
and imaged (with different spatial-mask properties)-collected over
different time intervals from the same instrument, (2) the correlation
of near-contemporaneous data from different instruments, and (3) the
extraction-through the application of complex filtering techniques-of
the coherent part of data collected at different heights in the solar
atmosphere. The detection limit is set by the loss of coherence
caused by the temporal evolution and the motion (e.g., rotation)
of superficial structures. Although we cannot identify any g-mode
signature, we have nevertheless set a firm upper limit to the amplitudes
of the modes: at 200 μHz, they are below 10 mm s<SUP>-1</SUP> in
velocity, and below 0.5 parts per million in intensity. The velocity
limit corresponds very approximately to a peak-to-peak vertical
displacement of δR/R<SUB>solar</SUB>=2.3×10<SUP>-8</SUP> at the
solar surface. These levels which are much lower than prior claims,
are consistent with theoretical predictions.
---------------------------------------------------------
Title: Source of excitation of low-l solar p modes: characteristics
and solar-cycle variations
Authors: Chaplin, W. J.; Appourchaux, T.; Elsworth, Y.; Isaak, G. R.;
Miller, B. A.; New, R.
2000MNRAS.314...75C Altcode:
We investigate various properties of the excitation source that
is responsible for driving the acoustic p-mode oscillations of the
Sun. Current prejudice places this in the superadiabatic layer of
the convection zone. We consider in detail how the precise nature of
the resonant mode spectrum is modified: (i) as a result of the impact
of different source-multipole mixtures; and (ii) as a function of the
radial extent of the source. To do this, we model the observed resonant
spectra with the solutions to a simple, one-dimensional wave equation
which is intended to describe the essential elements of the solar
resonant acoustic cavity. Further, we also fit these models to the
low-l peaks in a high-resolution power spectrum generated from data
collected by the Birmingham Solar-Oscillations Network (BiSON). We
also use the extensive BiSON data set to search for variations in the
source characteristics over the solar cycle.
---------------------------------------------------------
Title: Variations in the excitation and damping of low-l solar p
modes over the solar activity cycle<SUP>*</SUP>
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Miller, B. A.;
New, R.
2000MNRAS.313...32C Altcode:
We have searched helioseismic data collected by the Birmingham Solar
Oscillations Network (BiSON) for solar-cycle changes to those low-l
p-mode parameters that relate to the excitation and damping of the
resonances. These data - collected between 1991 and 1997 - cover the
complete declining phase of solar activity cycle 22 (up to and including
the cycle 22/23 boundary). Over the range 2600<=ν<=3600μHz,
we uncover a mean 24+/-3per cent increase in the frequency-domain
linewidths; a mean decrease of 46+/-5per cent decrease in the mode
heights, and a mean decrease of 22+/-3per cent in the modal velocity
powers. The rate at which energy is supplied to the modes remains
constant, at the level of precision of the observations (measured
change 0+/-4per cent). We use expressions derived from the equation of
a damped harmonic oscillator to illustrate the diagnostic properties
of the observables: these indicate that both the signs and relative
sizes of the extracted variations can arise from changes solely to
the net damping; the net forcing of the modes need not change. The
results possibly hint at the changes being maximal at frequencies near
~3100μHz. They might therefore suggest an origin for the observed
variations that is peaked in the superadiabatic layer of the convection
zone, which couples most strongly to the eigenfunctions of modes at
the centre of the p-mode spectrum.
---------------------------------------------------------
Title: Skew-symmetric solar P modes in low-l BiSON ^* data
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Miller, B. A.;
New, R.
1999MNRAS.308..424C Altcode:
The p-mode oscillations of the Sun are manifestations of resonantly
trapped acoustic waves propagating within its interior. The effective
size of the resulting resonant cavity changes with the properties of
the modes - the interaction of this phenomenon with a highly localized
excitation source in the upper layers of the convection zone gives
rise to skew-symmetric resonant profiles whose degree of asymmetry
changes with frequency. Here, we have fitted low-angular-degree (low-l)
resonant p-mode peaks - in a power spectrum generated from 32 months
of BiSON Doppler velocity observations of the visible solar disc -
to a skew-symmetric formalism to account for this effect. We present
the fitted frequencies, fine-structure spacings [d_0(n) and d_1(n)]
and mode-skewness estimates; and discuss the quantitative impact of
fitting a skew, rather than symmetric, limit model. We also consider
the reliability of the extracted parameters through the application
of a useful statistical test, and extensive Monte Carlo fits to
artificial data.
---------------------------------------------------------
Title: Rotation of the solar core from BiSON and LOWL frequency
observations
Authors: Chaplin, W. J.; Christensen-Dalsgaard, J.; Elsworth, Y.;
Howe, R.; Isaak, G. R.; Larsen, R. M.; New, R.; Schou, J.; Thompson,
M. J.; Tomczyk, S.
1999MNRAS.308..405C Altcode:
Determination of the rotation of the solar core requires very accurate
data on splittings for the low-degree modes which penetrate to the core,
as well as for modes of higher degree to suppress the contributions
from the rest of the Sun to the splittings of the low-degree modes. Here
we combine low-degree data based on 32 months of observations with the
BiSON network and data from the LOWL instrument. The data are analysed
with a technique that specifically aims at obtaining an inference of
rotation that is localized to the core. Our analysis provides what we
believe is the most stringent constraint to date on the rotation of
the deep solar interior.
---------------------------------------------------------
Title: An analysis of solar p-mode frequencies extracted from BiSON
data: 1991-1996
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Lines, R.;
McLeod, C. P.; Miller, B. A.; New, R.
1998MNRAS.300.1077C Altcode:
We present a comprehensive frequency analysis of Doppler velocity
observations of the visible solar disc made by the Birmingham Solar
Oscillations Network (BiSON) from 1990-1996, i.e. covering the
falling phase of activity cycle 22, up to and including the cycle
22/23 boundary. We have fitted low-degree (low-l) solar p modes in
a variety of power spectra of differing lengths generated from these
data. The analysis of the extracted frequencies reveals the expected
clear solar-cycle dependence; in addition, there is now sufficient
accuracy in the data to show that the low-l modal eigenfrequencies are
less affected by the solar cycle than their higher l counterparts. The
observed low-degree frequency shifts up to ~3900muHz are consistent
- at the level of precision of the data - with an inverse mode-mass
scaling. At frequencies above this, the blending of modes adjacent in
frequency space makes it increasingly difficult to extract reliable
frequency estimates. However, our data show indications of a turnover
and possibly also an eventual sign change in the solar cycle shifts
at frequencies above ~4000muHz, as seen in higher l data. We have
parametrized the observed shifts as a function of the 10.7-cm radio
flux, and produced an activity-corrected, average frequency table
which incorporates eigenfrequencies from 18 4-month and 9 8-month
spectra. We also present the fitted frequencies from a 32-month
power spectrum, generated from data collected at or close to solar
minimum. In addition, we also searched for frequency asymmetries in
the l=2 mode multiplets. These could result from strong near-surface
magnetic activity, or a buried magnetic field. Our fits merely place
an upper limit to any mean asymmetry - over the range 10<=n<=21 -
of between ~-80 and ~170nHz (3sigma).
---------------------------------------------------------
Title: Solar p-mode excitation: further insight from recent low-l
BiSON helioseismological data
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Lines, R.;
McLeod, C. P.; Miller, B. A.; New, R.
1998MNRAS.298L...7C Altcode:
We present measurements of low-degree solar p-mode velocity powers,
energies and energy supply rates between ~1600muHz and ~4700muHz,
as derived from the analysis of several years of high-quality
helioseismological data collected by the Birmingham Solar-Oscillations
Network (BiSON). For the radial (l=0) modes, we find that the total
energy E per mode (kinetic plus potential) peaks at ~3200muHz,
reaching ~10^28erg. The rate at which energy is supplied to the
modes E=2piDeltanu.E, where Deltanu is the FWHM modal line width, is
observed to plateau between ~3000 and ~4000muHz, reaching a maximum of
~10^23ergs^-1. At frequencies below the plateau, the supply rate follows
a simple power law with a dependence of E~nu^7.0+/-0.4 and at very-high
frequencies the data follow E~nu^-4.5+/-1.1. These assume modal inertias
normalized at T=T_eff with the mode-mass calculation normalized at
the optical depth of the BiSON observations, the fitted power laws in
E are approximately ~nu^6.5+/-1.1 and ~nu^-6.2+/-1.1 respectively.
---------------------------------------------------------
Title: Solar p-mode Excitation: Further Insights from Recent Low-l
BiSON Helioseismological Data
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Lines, R.;
McLeod, C. P.; Miller, B. A.; New, R.
1998ESASP.418..879C Altcode: 1998soho....6..879C
We present measurements of low-degree solar p-mode velocity powers,
energies and energy supply rates between ~1600 μ Hz and ~4700 μ
Hz, as derived from the analysis of several years of high-quality
helioseismological data collected by the Birmingham Solar-Oscillations
Network (BiSON). For the radial (ell = 0) modes, we find that the
total energy E per mode (kinetic plus potential) peaks at ~3200 μ Hz,
reaching ~10<SUP>28</SUP> ergs. The rate at which energy is supplied
to the modes dot{E} = 2pi Δ ν cdot E, where Δ ν is the fwhm modal
line width, is observed to plateau between ~3000 μ Hz and ~4000 μ
Hz, reaching a maximum of ~10<SUP>23</SUP> ergs s<SUP>-1</SUP>. At
frequencies below the plateau, the supply rate follows a simple
power law with a dependence of dot{E} propto nu<SUP>7.0 plus or minus
0.4</SUP>; and at very-high frequencies the data follow dot{E} propto
nu<SUP>-4.5 plus or minus 1.1</SUP>.
---------------------------------------------------------
Title: Low Degree p-mode solar cycle trends from BISON data
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; McLeod, C. P.;
Miller, B. A.; New, R.
1998IAUS..185..171C Altcode:
The resonant p-mode oscillations of the Sun are manifestations of
trapped, standing sound waves in the solar interior. The frequencies
of the discrete spectrum of sustained modes are consequently modified
by the mechanical properties of the layers through which the waves
traverse. Quasi-periodic changes to the solar structure -- on an 11
(22)-year timescale -- result in certain measurable mode properties
being affected. Here, we have used high-quality Doppler velocity data,
collected in integrated sunlight by the Birmingham Solar-Oscillations
Network (BiSON), in order to assess the resulting impact on the
low-degree modes of oscillation over the falling phase of solar cycle
22, and the beginning of cycle 23.
---------------------------------------------------------
Title: Further inside into p-mode excitation and damping from recent
BiSON data
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; McLeod, C. P.;
Miller, B. A.; New, R.
1998IAUS..185..221C Altcode:
The resonant p-mode oscillations of the Sun are believed to be excited
stochastically in its outer layers. Here, we seek to gain additional
insight into the p-mode excitation and damping problem by following
the power evolution, with time, of individual low-degree (low-ell)
multiplets. Sine-wave fits have been performed -- at the appropriate
frequencies -- on short stretches of high-quality Doppler velocity data,
collected in integrated sunlight by the Birmingham Solar-Oscillations
Network (BiSON). The results of these analyses will be presented
and discussed.
---------------------------------------------------------
Title: Observational Upper Limits for Low-Degree Solar g-modes
Authors: Fröhlich, C.; Finsterle, W.; Andersen, B.; Appourchaux, T.;
Chaplin, W. J.; Elsworth, Y.; D. O. Gough; Hoeksema, J. T.; Isaak,
G. R.; Kosovichev, A. G.; Provost, J.; Scherrer, P. H.; Sekii, T.;
Toutain, T.
1998ESASP.418...67F Altcode: 1998soho....6...67F
No abstract at ADS
---------------------------------------------------------
Title: Touching on the effects of an imperfect Window Function
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; McLeod, C. P.;
Miller, B. A.; New, R.
1998IAUS..185...47C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A comparison of low-degree solar p-mode frquencies from BISON
and LOI
Authors: Appourchaux, T.; Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.;
McLeod, C. P.; Miller, B. A.; New, R.
1998IAUS..185...45A Altcode:
Here, we compare the low-degree solar p-mode frequencies returned from
the analysis of two, contemporaneous, independent helioseismological
data sets collected during 1996. The first comprises Doppler velocity
observations of the 770-nm line of potassium, made in integrated
sunlight by the six-station, terrestrial Birmingham Solar-Oscillations
Network (BiSON). The second consists of irradiance distribution
measurements of the solar disc, made at 500rm nm, by the Luminosity
Oscillations Imager (LOI), which is part of the VIRGO experiment on
the ESA/NASA SOHO satellite.
---------------------------------------------------------
Title: Comparative Studies of Low-Order and Low-Degree Solar p Modes
Authors: Appourchaux, T.; Andersen, B.; Chaplin, W.; Elsworth, Y.;
Finsterle, W.; Frohlich, C.; Gough, D.; Hoeksema, J. T.; Isaak, G.;
Kosovichev, A.; Provost, J.; Scherrer, P.; Sekii, T.; Toutain, T.
1998ESASP.418...95A Altcode: 1998soho....6...95A
The amplitudes of solar p-modes decrease steeply with decreasing
radial order below about 17. The background solar signal (solar noise)
in general increases steadily with decreasing frequency. For the
irradiance and radiance measurements with VIRGO or SOI/MDI on SOHO this
combination makes it difficult to detect low degree modes below about
1.8 mHz. The solar noise as observed in velocity with SOI/MDI or the
ground based BISON network is significantly lower in this region than
in intensity measurements. This allows low degree modes to be observed
close to 1 mHz. We present results of detection and charaterization
of the lowest order observable p-modes both in velocity and intensity
measurements. Where applicable the properties of the modes observed
with the two methods are compared.
---------------------------------------------------------
Title: The Extraction of Rotational Splittings from Monte Carlo
Simulations of Unresolved Low-l Data
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Miller, B. A.;
New, R.
1998ESASP.418..135C Altcode: 1998soho....6..135C
The rotation of the Sun lifts the frequency degeneracy in ell of the
acoustic eigenmode spectrum. The extraction of reliable estimates
of these splittings is a particularly challenging task. Here, we
address the problem in the context of the analysis of full-disc
helioseismological data. In essence, a set of “zero-dimensional”
data are collected. Consequently: (i) a power spectrum of the data will
consist of many closely spaced resonant peaks, with the most prominent
aspect being an alternating quadrupole-monopole and octupole-dipole
mode-pair structure; (ii) the data cannot be spatially filtered to
extract “single-mode” power spectra -- however, this does mean that
they are unaffected by spatial sideband leakage (which presents its
own set of problems for resolved data); and (iii) the composition of
the power spectrum may be further complicated by the introduction
of temporal sidebands and other window-function-aliased power as a
result of an imperfect observational duty cycle. At low frequencies,
the modes are very weak -- however, their long coherence times give
sharp, well defined resonant peaks in the frequency domain, enabling
the m-dependent structure of the mode to be well resolved (given a
sufficiently long integration time). At higher frequencies, despite
increased signal-to-noise, the increasing mode line widths lead to a
substantial blending of adjacent features in the frequency domain -- as
the frequency-domain line widths increase still further, so the problems
associated with extracting reliable estimates of the mode splittings
become ever more severe. Here, we have used artificial p-mode data --
generated by a model which is based upon a randomly forced, damped
harmonic oscillator -- to assess the veracity of maximum-likelihood
fitting approaches for different parts of the low-ell spectrum. Among
other points of interest, the results of these simulations reveal a
clear tendency for the formal uncertainty of the fitted splitting to
be anti-correlated with the magnitude of the splitting for modes at
higher n.
---------------------------------------------------------
Title: A search l=2 asymmetries in BISON data
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; McLeod, C. P.;
Miller, B. A.; New, R.
1998IAUS..185..169C Altcode:
The rotation of the Sun lifts the frequency degeneracy in the angular
degree ell of the resonant p-mode oscillations of the solar cavity,
giving rise to a multiplet structure, each resulting component being
identified by an azimuthal order m. A buried magnetic field will
perturb these frequencies still further via the Lorentz force. Here,
we have used low-degree solar p-mode data, collected by the Birmingham
Solar-Oscillations Network (BiSON) in integrated sunlight, in an effort
to search for such an effect in modes of degree ell=2. The Lorentz
perturbation will shift the outer, sectoral (m=pm 2) components in
the same sense, while the m=0 component remains unaffected, giving the
integrated-sunlight-observed ell=2 multiplet an asymmetric frequency
structure. The degree of asymmetry is expected to change with the solar
cycle. Results obtained by attempting to fit explicitly the asymmetric
structure, and by overlaying many independent mode realizations,
will be presented and discussed.
---------------------------------------------------------
Title: Solar internal sound speed as inferred from combined BiSON
and LOWL oscillation frequencies
Authors: Basu, Sarbani; Christensen-Dalsgaard, J.; Chaplin, W. J.;
Elsworth, Y.; Isaak, G. R.; New, R.; Schou, J.; Thompson, M. J.;
Tomczyk, S.
1997MNRAS.292..243B Altcode: 1997astro.ph..2105B
Observations of the Sun with the LOWL instrument provide a homogeneous
set of solar p-mode frequencies from low to intermediate degree that
allow one to determine the structure of much of the solar interior
avoiding systematic errors that are introduced when different data sets
are combined, i.e., principally the effects of solar cycle changes on
the frequencies. Unfortunately, the LOWL data set contains very few of
the lowest-degree modes, which are essential for determining reliably
the structure of the solar core - in addition, these lowest-degree data
have fairly large associated uncertainties. However, observations made
by the Birmingham Solar-Oscillations Network (BiSON) in integrated
sunlight provide high-accuracy measurements of a large number of
low-degree modes. In this paper we demonstrate that the low-degree mode
set of the LOWL data can be successfully combined with the more accurate
BiSON data, provided the observations are contemporaneous for those
frequencies where the solar cycle induced effects are important. We
show that this leads to a factor of 2 decrease in the error on the
inferred sound speed in the solar core. We find that the solar sound
speed is higher than in solar models for r<0.2Rsolar. The density
of the solar core is, however, lower than that in solar models.
---------------------------------------------------------
Title: Techniques used in the analysis of data collected by the
Birmingham Solar-Oscillations Network (BiSON). II. Frequency domain
analysis & data merging
Authors: Chaplin, W. J.; Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod,
C. P.; Miller, B. A.; New, R.
1997A&AS..125..195C Altcode:
The Birmingham Solar-Oscillations Network (BiSON) consists of 6
solar observatories dedicated to the collection of full-disc solar
Doppler velocity data -- these data are sensitive to the lowest spatial
degrees of oscillation (0 <= l <= 4). In Paper I of this series,
we described the calibration of raw data from the BiSON instruments
to produce solar velocity residuals representing the oscillations
of the Sun's surface. In this paper, we discuss the combination of
velocity residuals into time series -- including a thorough discussion
of the treatment of data overlaps between sites -- and the analysis
of the power spectra computed from such time series to derive the
characteristics of the acoustic modes.
---------------------------------------------------------
Title: Solar p-mode linewidths from recent BiSON helioseismological
data
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; McLeod, C. P.;
Miller, B. A.; New, R.
1997MNRAS.288..623C Altcode:
We present low-degree solar p-mode linewidths from the analysis of
several high-resolution frequency spectra generated from high-quality
Doppler velocity data collected - in integrated sunlight - by the
Birmingham Solar-Oscillations Network (BiSON) between 1991 July and
1995 July. We have used linewidths, Deltanu, measured in a 32-month
frequency spectrum, to test the power-law dependence of the widths
at low frequencies. Over the range 1473<=nu<=1822Hz, we find
Deltanu~nu^7.0+/-1.5. If an effort is made to correct for finite
observing time systematics, the fitted index increases to 7.3
(and even higher, to 7.8, if a naive, single-bin-width correction
is applied). This is somewhat steeper than reported in previous
observations of intermediate-degree modes, and is more in line
with theoretical calculations which predict Deltanu~nu^8 over this
frequency range. If one assumes that the steep dependence reported
here persists down to lower frequencies, low-degree-mode coherence
(e-folding) times at 800Hz of >14yr, and at 300 muHz (i.e. near
the frequency expected for the fundamental) of >13000yr are implied.
---------------------------------------------------------
Title: The observation and simulation of stochastically excited
solar p modes
Authors: Chaplin, W. J.; Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod,
C. P.; Miller, B. A.; New, R.
1997MNRAS.287...51C Altcode:
We use low-degree p-mode data, collected in integrated sunlight by
the Birmingham Solar-Oscillations Network (BiSON), to derive the
distribution of the observed strengths of the solar oscillations. We
demonstrate that certain features of the observations cannot be
explained by a model of the oscillations that is based upon a
stochastically forced, damped harmonic oscillator. The solution of
the equation of motion of the oscillator, as derived by the use of the
Laplace transform, is presented, and its application to the simulation
of real p-mode data, in the form of a coded algorithm, is discussed. A
variety of tests are applied - both in the time and in the frequency
domains - to artificially generated time series. These confirm that
the model serves both as a useful diagnostic tool and for providing
additional insight into the p-mode excitation and damping problem.
---------------------------------------------------------
Title: The Solar Core: New Low-l p-Mode Fine-Spacing Results from
BiSON
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; McLeod, C. P.;
Miller, B. A.; New, R.
1997ApJ...480L..75C Altcode: 1997astro.ph..2141C
The fine-structure spacing d<SUB>l</SUB>(n) = ν<SUB>l,n</SUB>
- ν<SUB>l+2,n-1</SUB> for low-degree solar p modes of angular
degree l and radial order n is sensitive to conditions in the deep
radiative interior of the Sun. Here we present fine-structure spacings
derived from the analysis of nearly 5 years of helioseismological data
collected between 1991 July and 1996 February by the Birmingham Solar
Oscillations Network (BiSON). These data cover 9 <= n <= 28 for
d<SUB>0</SUB>(n), and 11 <= n <= 27 for d<SUB>1</SUB>(n). The
measured spacings are much more precise and cover a greater range
than earlier measurements from BiSON data (Elsworth et al. 1990a). The
predicted fine-structure spacings for a “standard” solar model are
clearly excluded by the BiSON data (at ~10 σ) models that include
helium and heavy-element settling provide a much better match to the
observed spacings (see also Elsworth et al. 1995). Since the inclusion
of core settling in solar models will tend to increase slightly
the predicted neutrino flux, the BiSON fine-structure data appear
to reinforce previous conclusions, i.e., an astrophysical solution
to the solar neutrino problem seems unlikely. <P />Birmingham Solar
Oscillations Network; http://bison.ph.bham.ac.uk.
---------------------------------------------------------
Title: On the spectroscopic detection from the ground of Earth-like
planets circling solar-like stars
Authors: Isaak, G. R.
1997IAUJD..13E...6I Altcode:
Planets with masses and orbits similar to those of the Earth, revolving
around a solar-like star, give rise to recoil effects with amplitudes
of the order of ~0.1 m s^{-1}, and periodicities of the order of
a year. Spectroscopic methods, developed by the author in the late
1950's -- extended by him, together with collaborators, since the
early 1970's -- led to the discovery of global solar oscillations in
1979, and can be extended to tackle the above problem. The methods,
and their potential, will be presented and discussed.
---------------------------------------------------------
Title: Solar core rotation from low-degree BiSON p-mode splittings:
1981-95
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; McLeod, C. P.;
Miller, B. A.; New, R.
1996MNRAS.283L..31C Altcode:
In this paper, we present an analysis of the rotational splittings of
low-degree solar p modes, as measured in full-disc helioseismological
data collected by the Birmingham Solar-Oscillations Network (BiSON)
between 1981 and 1995. These data provide coverage over solar
activity cycles 21 and 22. Rotationally split multiplets were fitted
in two 4-month and seven 8-month frequency spectra with a technique
that minimizes a maximum-likelihood function consistent with chi^2
2-d.o.f. statistics. The fitted modes used in the analysis map the
range 1<=l<=3 and 2.0<=nu<=3.1mHz. The lower limit to the
analysed frequency range was determined by the poorer quality of the
pre-1990 data; the upper, very conservative, cut-off was imposed in
order to avoid using data where the linewidths of the modes begin to
increase substantially above 1muHz. Global splitting averages (for
1<=l<=3) were computed over the quoted mode-frequency range by:
(i) weighting each datum according to the scatter observed in the
sectoral splittings as a function of l and (ii) weighting each datum
according to its formal uncertainty, as computed by the mode-fitting
procedure. The l-weighted averages show a slight decline over the period
1981-95: however, the gradient is only significant at the ~2sigma
level (-3.8+/-1.8nHz yr^-1). If one were to interpret any trends in
the low-degree splittings as being indicative of a modification of
the rotational behaviour of the solar core (for r/R_solar<=0.25),
we would estimate that a ~60 per cent change in the core rotation rate
is excluded at the 3sigma level by the l-weighted data. The `formally'
weighted averages imply a very significant change with epoch. However,
an analysis of the data appears to show that the formal uncertainties
may somewhat underestimate the actual errors on the measured splittings,
leading to an overestimation of the significance of any inferred change
with epoch.
---------------------------------------------------------
Title: Low-frequency, low-degree solar p-mode measurements from
recent BiSON* data
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Lines, R.;
McLeod, C. P.; Miller, B. A.; New, R.
1996MNRAS.282L..15C Altcode:
We present high-quality measurements of the frequencies of some
acoustic modes of oscillation of the Sun, as determined from recent
full-disc BiSON data. These cover the range 0<=<~<=3 and
7<=<e1>n<=</e1>14. Several of the mode frequencies
have been measured to accuracies as high as 5 parts in 10^6, as
determined by the formal mode-fitting uncertainties. When compared
with the model frequencies of the AARHUS group (model OPAL1 from
Basu et al.) in which the Livermore equation of state, the Livermore
opacities and both helium and heavy element diffusion and settling
have been incorporated, the frequency differences (in the sense BiSON
minus OPAL1) are typically of the order of only ~-0.1 muHz over this
frequency region.
---------------------------------------------------------
Title: BiSON Performance
Authors: Chaplin, William J.; Elsworth, Yvonne; Howe, Rachel; Isaak,
George R.; McLeod, Clive P.; Miller, Brek A.; van der Raay, H. B.;
Wheeler, Sarah J.; New, Roger
1996SoPh..168....1C Altcode:
Since 1981 we have been operating the Birmingham Solar-Oscillations
Network (BiSON), a global network of resonant-scattering spectrometers,
observing the low-l solar p modes. Here we discuss historical
developments, culminating in the establishment of a 6-station network
in 1992 September, and the subsequent performance of that network. The
data record of each station from 1992 to 1994 has been analysed in terms
of weather and equipment breakdowns. Our early experience suggests that
the best long-term coverage possible with a 6-station network is limited
in practice to about 80%, which falls short of previous predictions.
---------------------------------------------------------
Title: Low-degree, L=4 modes in full-disc BiSON* helioseismological
data
Authors: Chaplin, W. J.; Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod,
C. P.; Miller, B. A.; New, R.
1996MNRAS.280.1162C Altcode:
We demonstrate that some L=4 acoustic eigenmodes of the Sun are clearly
distinguishable in long power spectra generated from Doppler velocity
residuals collected in unimaged sunlight by the six-station Birmingham
Solar Oscillations Network (BiSON). Modes covering the range n=17
to 21 have been measured in 16-month-long power spectra. They have
typical amplitudes -when measured in full-disc Doppler velocity data
- of less than ~1 cm s^-1 per root bin, and their frequencies have
been determined to fractional accuracies of the order of 3 parts in
10^5. The sensitivity of the whole-disc observations to these modes -
as implied by the data - appears to be approximately a factor of 2
greater than the theoretical predictions of Christensen-Dalsgaard.
---------------------------------------------------------
Title: Solar core rotation: low-degree solar p-mode rotational
splitting results from BiSON*
Authors: Chaplin, W. J.; Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod,
C. P.; Miller, B. A.; New, R.
1996MNRAS.280..849C Altcode:
In a recent paper (Elsworth et al.), we presented rotational splitting
measurements of low-degree, low-frequency solar p modes, which, for
the first time, show visually well-separated components at frequencies
as low as ~1.5 mHz. At the present time, these data appear to rule out
a rapidly rotating solar core. Here, we expand our analysis of these
data to a full discussion of the statistical properties of the measured
splittings. We have considered two fully independent Fourier spectra,
generated from Doppler velocity residuals collected by the six-station
Birmingham Solar-Oscillations Network (BiSON) between 1992 January 1 and
1994 August 23 (Fourier spectra 16A and 16C from Elsworth et al.). We
present formal uncertainties from maximum-likelihood fits to the mode
multiplets, analyse and compare the statistical properties of the sets,
and search for possible trends in the splitting measures with n.
---------------------------------------------------------
Title: Recent results from the Birmingham Solar-Oscillations Network
(BiSON).
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; McLeod, C. P.;
Miller, B. A.; New, R.
1996BAAS...28..936C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Recent Results from the Birmingham Solar-Oscillations Network
(BiSON)
Authors: Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; McLeod, C. P.;
Miller, B. A.; New, R.
1996AAS...188.6904C Altcode: 1996BAAS...28R.936C
The Birmingham Solar-Oscillations Network (BiSON) consists of 6
semi and fully automatic observatories, dedicated to the collection
of low-degree solar oscillations data in integrated sunlight. The
network was established in 1981 with two permanent stations; the
addition of several more sites culminated with the addition of a
sixth in 1992. Here, we give an update on the current status of the
network. In addition, we discuss recent important scientific results
from analysed data, including: low-degree rotational splitting results
covering the period 1981 to 1995; and the measurement of low-frequency
modes, down to ~ 1.2mHz, in a power spectrum generated from 32 months
of recent BiSON data.
---------------------------------------------------------
Title: Observing the sun with the Birmingham Solar-Oscillations
Network (BISON)
Authors: Chaplin, W. C.; Elsworth, Y.; Isaak, G. R.; Lines, R.;
McLeod, C. P.; Miller, B. A.; New, R.; van der Raay, H. B.
1996Obs...116...32C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Techniques used in the analysis of solar oscillations data from
the BiSON (University of Birmingham) network. I. Daily calibration.
Authors: Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod, C. P.; Miller,
B. A.; New, R.; Wheeler, S. J.
1995A&AS..113..379E Altcode:
In this, the first of a pair of linked papers, we discuss the methods
used to convert raw data from the BiSON network of resonant-scattering
spectrometers to velocity residuals, from which the low-degree,
five-minute solar oscillations are studied. We describe our standard
calibration and some alternative methods. The rejection of bad data,
and the corrections applied for unwanted low-frequency effects, are
also discussed.
---------------------------------------------------------
Title: Slow rotation of the Sun's interior
Authors: Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod, C. P.; Miller,
B. A.; New, R.; Wheeler, S. J.; Gough, D. O.
1995Natur.376..669E Altcode:
THE rotation of the Sun is not that of a rigid body; at its surface,
the gas near the poles has a lower angular velocity than that near
the equator<SUP>1</SUP>. This latitudinal variation persists to
the base of the convection zone, below which the angular velocity
becomes approximately uniform<SUP>2,3</SUP>. Any variations
of angular velocity at much greater depths are, however, poorly
constrained<SUP>4-10</SUP>. Observations of solar oscillation modes
have been used to probe density variations in the Sun; rotational
splitting of degenerate modes, although difficult to resolve, provides
important constraints on the dynamical structure<SUP>11</SUP>. Here we
report observations of rotationally split modes made over a three-year
period with the Birmingham Solar Oscillations Network. Our results
indicate that there is a substantial region inside the Sun that is
rotating more slowly than the surface. This situation seems likely
to be transient—the minimum-energy state would have all the deeper
regions rotating with the same angular velocity—and is at variance
with our current ideas about the rotational evolution of main-sequence
stars<SUP>12</SUP>. We have no solution to the dynamical problem
this poses.
---------------------------------------------------------
Title: The Recent Performance of the 6-STATION BISON Network
Authors: Chaplin, W. J.; Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod,
C. P.; Miller, B. A.; van der Raay, H. B.
1995ESASP.376b.391C Altcode: 1995help.confP.391C; 1995soho....2..391C
No abstract at ADS
---------------------------------------------------------
Title: Automated Photometry of Bright Stars Using the BISON Network
Authors: Monks, P. D.; Isaak, G. R.; Lines, R.; Litherland, J. W.;
McLeod, C. P.
1995ESASP.376b.539M Altcode: 1995soho....2..539M; 1995help.confP.539M
No abstract at ADS
---------------------------------------------------------
Title: Some Notes on the Calibration of Doppler Velocity Residuals
Collected by a Resonant Scattering Spectrometer
Authors: Chaplin, W. J.; Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod,
C. P.; Miller, B. A.; New, R.
1995ESASP.376b.163C Altcode: 1995soho....2..163C; 1995help.confP.163C
Discusses three alternative calibration techniques for Doppler velocity
data collected by a resonant-scattering spectrometer viewing the
unimaged solar disc, that allow long-term, low-frequency information
to be preserved. In the first, the effects of curvature in the solar
reference line are removed by normalizing the observed ratio, R(v) -
the difference between the resonantly-scattered intensities in the blue
and red wings of the source line, normalized by the scattered sum -
by a function that reflects the variation of the total intensity over
the instrument bandwidth. In the second method, a linearized function is
formed from a power series of R(v). A straight forward example of this
technique appears to be subject to systematic, low-frequency errors
of the order of ≡1 to 2 m s<SUP>-1</SUP>. And in the third method,
Doppler velocity data collected over a full observing season could,
in principle, form an internal reference - a look-up curve.
---------------------------------------------------------
Title: The Measurement of L = 4 Modes in Full-Disc BISON Data
Authors: Chaplin, W. J.; Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod,
C. P.; Miller, B. A.
1995ESASP.376b.387C Altcode: 1995help.confP.387C; 1995soho....2..387C
No abstract at ADS
---------------------------------------------------------
Title: An Investigation of the Correlation Between Unusually Large
Amplitude P-Modes and Solar Activity
Authors: Chaplin, W. J.; Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod,
C. P.; Miller, B. A.; New, R.
1995ESASP.376b.335C Altcode: 1995help.confP.335C; 1995soho....2..335C
No abstract at ADS
---------------------------------------------------------
Title: A search for small-amplitude, acoustic, p-mode oscillations
on alpha CMi
Authors: Bedford, D. K.; Chaplin, W. J.; Coates, D. W.; Davies, A. R.;
Innis, J. L.; Isaak, G. R.; Speake, C. C.
1995MNRAS.273..367B Altcode:
We have searched for small-amplitude, acoustic, p-mode oscillations
on the F5 IV-V subgiant alpha CMi (HR 2943) in high-quality
Doppler velocity measurements made in 1991, 1992 and 1993 with a
magneto-optical-filter spectrometer. An internal precision of 0.13
m^-2 s^-2 muHz^-1 (1 sigma) has been achieved in one of the 1993
data sets. Conclusive evidence for the presence of a p-mode spectrum
may be established by extracting the mean first-order-half spacing
1/2Deltav. Fourier analysis of the 1991 and 1992 data appears
to indicate the presence of a strong concentration of periodic
power. However, the analysis of substantially higher quality 1993 data
fails to reveal a strong peak; we are therefore forced to conclude
tentatively that we have, at present, failed to find conclusive evidence
for an acoustic oscillation spectrum on alpha CMi.
---------------------------------------------------------
Title: Fine Spacing of L = 0, L = 2 Acoustic Eigenmodes and the
Solar Neutrino Problem: Particle Physics and Cosmological Implications
Authors: Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod, C. P.; Miller,
B. A.; Wheeler, S. J.; New, R.
1995ASPC...76...51E Altcode: 1995gong.conf...51E
No abstract at ADS
---------------------------------------------------------
Title: Observations of Global Solar Oscillations in Moonlight
Authors: Fussell, J. A.; Brazier, R. I.; Davies, A. R.; Isaak, G. R.;
McCleod, C. P.; Morgan-Vandome, S. C.; Speake, C. C.
1995ASPC...76..452F Altcode: 1995gong.conf..452F
No abstract at ADS
---------------------------------------------------------
Title: Rotational Splitting of Low-Degree Solar P Modes
Authors: Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod, C. P.; Miller,
B. A.; Wheeler, S. J.; New, R.; Gough, D. O.
1995ASPC...76...43E Altcode: 1995gong.conf...43E
No abstract at ADS
---------------------------------------------------------
Title: Performance of the BISON Network 1981-Present
Authors: Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod, C. P.; Miller,
B. A.; van der Raay, H. B.; Wheeler, S. J.; New, R.
1995ASPC...76..392E Altcode: 1995gong.conf..392E
No abstract at ADS
---------------------------------------------------------
Title: Statistical Distribution of Solar p-Mode Amplitudes
Authors: Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod, C. P.; Miller,
B. A.; Wheeler, S. J.; New, R.
1995ASPC...76..318E Altcode: 1995gong.conf..318E
No abstract at ADS
---------------------------------------------------------
Title: High-precision, longitudinal, disc-averaged magnetic field
measurements of α Canis Minoris and β Leporis.
Authors: Bedford, D. K.; Chaplin, W. J.; Davies, A. R.; Innis, J. L.;
Isaak, G. R.; Speake, C. C.
1995A&A...293..377B Altcode:
High-precision longitudinal, disc-averaged magnetic field measurements
have been made on two late-type stars, α CMi (HR 2943) and β Lep (HR
1829). A magneto-optical filter has been used to measure the degree
of circular polarization in the wings of a line of neutral potassium
(7699 A) formed in the atmosphere of the target star. From 10 nights
of data taken on α CMi on the 1.9-m reflector at the South African
Astronomical Observatory (SAAO) in 1993 January, an upper limit to
any net longitudinal field component present - assuming the field to
be constant over the period of observation - of -0.40+/-0.26G was
established. Similarly, upper limits for β Lep of -4.3+/-6.4G and
-0.7+/-1.6G were established, from 4 and 10 nights of data taken
in 1992 and 1993 January respectively. No significant, periodic,
day-to-day variations were seen in the three time series of data.
---------------------------------------------------------
Title: High-precision velocity observations of Procyon A -
II. Measurement of the orbital motion of Procyon A during 1986-90.
Authors: Innis, J. L.; Isaak, G. R.; Speake, C. C.; Chaplin, W. J.;
Brazier, R. I.; Jones, A. R.
1994MNRAS.271..573I Altcode:
Radial velocity observations of high internal precision have been
obtained, with the Birmingham stellar spectrometer, of the F5 IV-V star
Procyon A. The velocities have been derived using a `null technique' by
determining the time when the topocentric velocity of Procyon A passes
through zero (which occurs in January each year). At this time, the
barycentric velocity of the star is equal to the barycentric velocity
of the instrument, which can be calculated to a high accuracy. Seven
such velocities have been determined from five observing runs during
the years 1986 to 1990 inclusive. The internal precision and external
errors (e.g. our zero-point) cannot be uniquely determined from so
few points but the former appears to be in the range 10 to 40 m s-
. We have compared our data with the orbit of Procyon recently derived
by Irwin et al. The mean annual acceleration of Procyon A over the two
years 1988 to 1990, according to the orbit of Irwin et al., is 133 m
s- yr . Our measured value is 122 m s1 yr , with an error of the order
oft 10 m s- , in reasonable agreement with the value obtained by Irwin
et al. A detailed comparison will be possible when several more years
of data have been obtained. Key words: techniques: radial velocities -
binaries: spectroscopic - stars: individual: Procyon.
---------------------------------------------------------
Title: Corrigendum - Low-L P-Mode Solar Eigenfrequency Measurements
from the Birmingham Network
Authors: Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod, C. P.; New, R.
1994MNRAS.270..720E Altcode:
Key words: Sun: oscillations.
---------------------------------------------------------
Title: Solar p-Mode Frequencies and Their Dependence on Solar
Activity: Recent Results from the BISON Network
Authors: Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod, C. P.; Miller,
B. A.; New, R.; Speake, C. C.; Wheeler, S. J.
1994ApJ...434..801E Altcode:
We present here high-accuracy determinations of the frequencies of low-l
solar p-modes and their solar-cycle dependence. The data were obtained
using the Birmingham network of solar spectrometers (BISON). The
precision of the measurements is discussed. Our previously published
results of a significant frequency shift between solar minimum and
solar maximum, apparently independent of l and similar to that found by
other workers for intermediate-l modes, is confirmed and extended. This
suggests that at most only a small fraction of the variation is due to
the solar core. Sets of frequencies at high and low solar activity, and
an average corrected for solar-activity effects, are presented. There
is now evidence that the solar-activity dependence of the frequencies
varies across the 5 minute spectrum.
---------------------------------------------------------
Title: Solar Seismology - the Velocity Continuum Spectrum
Authors: Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod, C. P.; Miller,
B. A.; New, R.; Speake, C. C.; Wheeler, S. J.
1994MNRAS.269..529E Altcode:
The discrete spectrum of global solar modes was discovered in 1979
by Claverie et al. Here we report the first determination of the
continuum spectrum in velocity, where atmospheric and instrumental
effects have been significantly reduced. Crosscorrelations of velocity
measurements at four observing stations of a global network have
made this possible. The solar noise power is a factor of 5 lower than
expectations. Key words: methods: data analysis - Sun: oscillations.
---------------------------------------------------------
Title: The longitudinal magnetic field of Procyon.
Authors: Bedford, D. K.; Chaplin, W. J.; Innis, J. L.; Isaak, G. R.;
McLeod, C. P.; Speake, C. C.
1994MNRAS.269..639B Altcode:
An upper limit of 4.3 G (3 u) on the longitudinal magnetic field, and
its variation, of the bright star Procyon is placed by the extended
measurement of the circular polarization in the blue and red wings
of the 770-nm absorption line of neutral potassium. Key words: line:
profiles - polarization - stars: individual: Procyon - stars: magnetic
fields.
---------------------------------------------------------
Title: An Epoxy Coated Flux Collector for Stellar Seismology
Authors: Bedford, D. K.; Chaping, W. J.; Isaak, G. R.; Litherland,
J. W.
1994MNRAS.269..679B Altcode:
A programme directed towards the discovery in solar-like stars of
small-amplitude, pressure-driven oscillations analogous to those
observed of the Sun, and to search for planetary companions, has been
pursued for several years, principally at the SAAO 1.9-m telescope. Here
we describe a 0.9-m, spun epoxy flux collector of - 1-arcmin quality,
a prototype towards larger, inexpensive flux collectors dedicated to a
stellar seismology programme. This prototype has been used to gather
preliminary stellar data. Key words: techniques: miscellaneous -
telescopes - stars: oscillations.
---------------------------------------------------------
Title: A high-precision determination of the radial velocity of the
G5 II giant beta Lep.
Authors: Bedford, D. K.; Chaplin, W. J.; Davies, A. R.; Innis, J. L.;
Isaak, G. R.; Speake, C. C.
1994MNRAS.269..435B Altcode:
We apply a null technique to measure to a high accuracy the
barycentric radial velocity of the G5 II giant P Lep (HR 1829). We use a
magneto-optical filter as a spectroscopic bridge to determine the time
at which the geocentric velocity of the star passes through zero. From
data taken on the 1.9-m reflector at the South African Astronomical
Observatory (SAAO) in 1992 and 1993 January, the barycentric radial
velocity of P Lep is calculated to be -13.6+0.2 and -13.576+0.040 km
s ' respectively. Key words: techniques: radial velocities - stars:
individual: P Lep - stars: oscillations.
---------------------------------------------------------
Title: Solar Structure from Global Studies of the Five-Minute
Oscillation
Authors: Claverie, A.; Isaak, G. R.; McLeod, C. P.; van der Raay,
H. B.; Roca Cortes, T.
1994snft.book..407C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The variation in the strength of low-l solar p-modes - 1981-92
Authors: Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod, C. P.; Miller,
B. A.; Speake, C. C.; Wheeler, S. J.; New, R.
1993MNRAS.265..888E Altcode:
The acoustic eigenmodes of the sun are a well-recognized probe of
the structure of the solar interior. We report here on a measurement
of the strength of the low-l p-mode oscillations throughout a solar
cycle. There is an increase of 35 +/-5 percent in the strength of
the modes from solar maximum to solar minimum, consistent for all
the l-values measured. The observations were made with the Birmingham
Solar Oscillations Network.
---------------------------------------------------------
Title: Metal epoxy-coated mirrors for stellar seismology
Authors: Bedford, David K.; Chaplin, W. J.; Isaak, G. R.
1993SPIE.1931...96B Altcode:
A 0.9 m diameter f/1.8 paraboloidal mirror of moderate quality
(approximately 2 arcmin) has been produced by spinning an epoxy- coated
spherical substrate. This mirror has been used to build a prototype
flux collector for stellar seismology. The objective is to produce 2
m mirrors for this application.
---------------------------------------------------------
Title: Avalanche Photodiodes in Stellar Spectroscopy
Authors: Davies, A. R.; Isaak, G. R.; Speake, C. C.
1993ASPC...42..493D Altcode: 1993gong.conf..493D
No abstract at ADS
---------------------------------------------------------
Title: High Precision Velocity Measurements of the Star Procyon -
a Possible Stellar Signal
Authors: Bedford, D. K.; Chaplin, W. J.; Davies, A. R.; Innis, J. L.;
Isaak, G. R.; Speake, C. C.
1993ASPC...42..383B Altcode: 1993gong.conf..383B
No abstract at ADS
---------------------------------------------------------
Title: High-Frequency Solar Velocity Noise
Authors: Elsworth, Y.; Howe, R.; Isaak, G.; McLeod, C.; Miller, B.;
Speake, C.; Wheeler, S.; New, R.
1993ASPC...42..107E Altcode: 1993gong.conf..107E
No abstract at ADS
---------------------------------------------------------
Title: Low-l p-mode solar eigenfrequency measurements from the
Birmingham Network
Authors: Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod, C. P.; New, R.
1991MNRAS.251P...7E Altcode:
Observations of the acoustic oscillations of the sun taken over a
period of eight years are presented and discussed in terms of their
implications regarding the nature of the deep solar interior. Stable
atomic standards are used to integrate light from the whole solar disc,
and the data taken over the course of the measurements are averaged
to examine the mean frequencies for all the modes. The quality of the
data allows comparisons with solar theories as well as the analysis
of long-term solar cycle trends.
---------------------------------------------------------
Title: High-precision velocity observations of Procyon A - I. Search
for p-mode oscillations from 1988 and 1990 observations.
Authors: Innis, J. L.; Isaak, G. R.; Speake, C. C.; Brazier, R. I.;
Williams, H. K.
1991MNRAS.249..643I Altcode:
An analysis for the presence of small, low-degree p-mode
oscillations of high-precision velocity observations of Procyon is
presented. Examination of solar data taken during the Procyon observing
runs at higher counting rates yields correspondingly lower noise levels,
indicating that the spectrometer performance is, as yet, essentially
limited by photon statistics. The power spectra are also studied for
possible periodicities due to the presence of equifrequency spaced
modes of comparable power to the background noise. It is observed that
while some periodicities were seen they were not reproducible in all
three data sets, and therefore are probably not of stellar origin.
---------------------------------------------------------
Title: Oscillation observations.
Authors: Hill, Frank; Deubner, Franz-Ludwig; Isaak, George
1991sia..book..329H Altcode:
This chapter reviews recent observations of solar oscillations. The
oscillations discussed are global and local 5-min p-modes, the 160-min
oscillation and oscillations in the solar atmosphere. Experimental
and data reduction methods are described. Summaries of recent results
are provided, including measurements of frequencies, amplitudes, line
widths and splittings. Other topics include active-region tomography,
solar cycle changes, the chromospheric cavity and diameter measurements.
---------------------------------------------------------
Title: A preliminary investigation of the suitability of the
Mount Stromlo coude spectrograph for very precise radial-velocity
measurements
Authors: Innis, J. L.; Isaak, G. R.; Isaak, K.
1990Obs...110..188I Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Evidence from solar seismology against non-standard solar-core
models
Authors: Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod, C. P.; New, R.
1990Natur.347..536E Altcode:
GLOBAL oscillations of the Sun<SUP>1</SUP> have been used to test
solar models<SUP>2</SUP>, but modelling the oscillation frequencies to
their measured accuracies of a few microhertz has proved difficult,
mostly owing to ignorance of the structure of the Sun's outer
layers<SUP>3</SUP>. The frequency separation between closely spaced
modes in the acoustic spectrum is expected to depend more on core
properties<SUP>4</SUP>, however, and thus to provide constraints on
models of the solar core. Our observations combine data from a global
network of observing stations, which reduces the masking effect of
daily sidebands in the spectral analysis. Here we present precision
measurements of fine structure and its variation with frequency. Our
results agree with standard solar models<SUP>5-7</SUP>, and seem
to remove the need for significant mixing<SUP>8,9</SUP> or weakly
interacting massive particles (WIMPS)<SUP>10,11</SUP> in the core, both
of which have been advanced to explain the low measured flux of solar
neutrinos<SUP>12,13</SUP>. This suggests that the solar neutrino problem
must be resolved within neutrino physics, not solar physics; neutrino
oscillations and a finite neutrino mass form a possible explanation.
---------------------------------------------------------
Title: Solar and Stellar Seismology
Authors: Isaak, G. R.
1990Obs...110...80I Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Variation of low-order acoustic solar oscillations over the
solar cycle
Authors: Elsworth, Y.; Howe, R.; Isaak, G. R.; McLeod, C. P.; New, R.
1990Natur.345..322E Altcode:
GLOBAL acoustic oscillation modes of the Sun were discovered eleven
years ago<SUP>1</SUP>. The possibility of temporal variations in the
oscillation frequencies was suggested by fluctuations in the flux of
solar neutrinos<SUP>2</SUP>, and would also be implied by changes in the
size of the solar cavity or in the speed of sound within the Sun. Our
group has studied solar oscillations for many years<SUP>1,3</SUP>;
over the past decade, a network of three stations deployed at sites
that can permit 24-h data collection has provided p-mode (acoustic)
spectra of very high quality. Here we present evidence that the
frequencies of the lowest-order (l<=2) modes have varied over
the period of observation (1977-88) in a manner that is correlated
with solar activity (as measured by sunspot number). The frequency
variation has a peak-to-peak amplitude of 0.46+/-0.06μHz, and could
reflect variations in the solar dimensions or in the sound speed in
the Sun, which might in turn be due to changes in solar temperature
and/or magnetic field.
---------------------------------------------------------
Title: Linewidths of low-degree acoustic modes of the sun
Authors: Elsworth, Y.; Isaak, G. R.; Jefferies, S. M.; McLeod, C. P.;
New, R.
1990MNRAS.242..135E Altcode:
Estimates of the spectral linewidths of low degree (l = 0 and l = 1),
5-min p-modes obtained from Doppler shift observations in 1984 (63 d)
and 1986 (63 d) are reported. The observed linewidths increase from
0.5 micro-Hz at 2000 micro-Hz to 3 micro-Hz at 3800 micro-Hz for l =
0. Comparison with other data suggests that for a given frequency the
linewidth increases with increasing values of l. On the assumption
that the linewidth is substantially due to damping processes, the
linewidths are consistent with e-folding times between 3.7 and 0.6 d.
---------------------------------------------------------
Title: Camera obscura and sunspots
Authors: Isaak, G. R.
1989Obs...109..152I Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observations of solar p-modes with L of not greater than 5
Authors: Palle, P. L.; Perez Hernandez, F.; Roca Cortes, T.; Isaak,
G. R.
1989A&A...216..253P Altcode:
Results are presented for the sun's radial velocity in integrated
sunlight. A resonant scattering spectrometer has been used to determine
the frequencies and amplitudes of p-modes with spherical harmonics
of degree (l) of less than 3, and a second spectrophotometer, which
observes part of the sun by means of a spatial filter, has been used
to measure the frequencies and amplitudes of p-modes with l of not
greater than 5. The frequencies at both ends of the p-mode 5-minute
oscillations with l of between 3 and 5, inclusively, are obtained with
greater precision than previous determinations.
---------------------------------------------------------
Title: Solar Seismology
Authors: Isaak, G. R.
1989QJRAS..30...93I Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar oscillations as seen in the Na I and K I absorption
lines.
Authors: Isaak, G. R.; McLeod, C. P.; Palle, P. L.; van der Raay,
H. B.; Roca Cortes, T.
1989A&A...208..297I Altcode:
Data on the solar p modes observed simultaneously in the solar NaI and
KI absorption lines were obtained in 1985 by operating two independent
resonant scattering spectrometers at the same site (Observatorio del
Teide, Tenerife). Since the abundances of the ground state atoms of
these two elements are not the same, different depths of the solar
photosphere are sampled. A comparison of the data obtained is given.
---------------------------------------------------------
Title: The current status of the Birmingham solar seismology network.
Authors: Aindow, A.; Elsworth, Y. P.; Isaak, G. R.; McLeod, C. P.;
New, R.; Vanderraay, H. B.
1988ESASP.286..157A Altcode: 1988ssls.rept..157A
Some aspects of the performance of a network of solar seismology
stations are presented. Duty cycles for 1986 and 1987 are
given and examples of window functions and p-mode spectra
are shown. Sideband powers down to 2% of peak power have
been achieved for spans of 16 days and 3% for 64 days. Noise
levels of 7 (ms<SUP>-1</SUP>)<SUP>2</SUP>Hz<SUP>-1</SUP> and 13
(ms<SUP>-1</SUP>)<SUP>2</SUP>Hz<SUP>-1</SUP> have been achieved in
regions just above and below the p-mode spectrum. A new station in
Birmingham is discussed. Its importance as a testing and training
station is outlined and plans to deploy replicas around the globe
are described.
---------------------------------------------------------
Title: A detector of small gradients of transparency of the
terrestrial atmosphere.
Authors: McLeod, D. B.; Isaak, G. R.
1988ESASP.286..223M Altcode: 1988ssls.rept..223M
In the study of low l p-modes of the Sun in integrated sunlight with
ground based spectrometers even small (10<SUP>-3</SUP>) gradients
in transparency across the solar disk give rise to displacements of
centroids of spectral lines at the m/s level. The authors describe
a prototype instrument which measures this atmospheric gradient
continuously and thereby makes it possible to subtract these artefacts
from the measurements.
---------------------------------------------------------
Title: High precision velocity observations of Arcturus using the
7699 Å line of potassium.
Authors: Innis, J. L.; Isaak, G. R.; Brazier, R. I.; Belmonte, Juan
A.; Palle, Pere L.; Roca Cortés, Teodoro; Jones, A. R.
1988ESASP.286..569I Altcode: 1988ssls.rept..569I
The K giant Arcturus (α Boo) was observed with the Birmingham
double magneto-optical filter spectrometer using the GHRIL facility
at the Nasmyth focus of the 4.2 m William Herschel Telescope
of the Observatorio del Roque de los Muchachos in 1988 April -
May. Approximately 80 hours of data were obtained over a 2 week
interval. The authors' preliminary analysis of the data shows the
presence of the large (≡200 m s<SUP>-1</SUP>) amplitude velocity
variation reported earlier by other observers. However, this more
extensive data set strongly suggests that this variation is not
singly periodic, as was previously indicated. The authors present
some speculative comments as to the nature of this variation, and the
implications for stellar seismology.
---------------------------------------------------------
Title: Linewidth of low degree acoustic modes of the Sun.
Authors: Elsworth, Y. P.; Isaak, G. R.; Jefferies, S. M.; McLeod,
C. P.; New, R.; Palle, Pere L.; Regulo, C.; Roca Cortés, Teodoro
1988ESASP.286...27E Altcode: 1988ssls.rept...27E
Estimates of the spectral linewidths of low degree (l = 0 and l = 1),
"5 minute" p-modes obtained from Doppler shift observations in 1984,
1986 and 1987 are reported. The observed linewidths increase from 0.5
μHz at 2000 μHz to 3.8 μHz at 4300 μHz for l = 0. Comparison with
other data suggest that for a given frequency the linewidth increases
with increasing l value. On the assumption that the linewidth is
substantially due to damping processes the linewidths are consistent
with e-folding times between 3.7 and 0.5 days.
---------------------------------------------------------
Title: High precision velocity measurements of Procyon using the
7966 Å line of potassium.
Authors: Innis, J. L.; Isaak, G. R.
1988ESASP.286..583I Altcode: 1988ssls.rept..583I
The Star Procyon (α CMi) was observed for 2 weeks using the Brimingham
double magneto-optical filter spectrometer on the 1.9 m reflector of
the South African Astronomical Observatory in 1988 January. Analysis of
these velocity data shows that the noise level (per frequency channel
of ≡1 μHz) in the power spectrum of the combined nightly runs is
approximately 12 m<SUP>2</SUP>s<SUP>-2</SUP>. These data were searched
for the presence of small scale oscillations, but none were obviously
present. On one night the surface integrated magnetic field of Procyon
was measured using a rotating quarter wave plate to select alternatively
opposite senses of circularly polarised light. Preliminary analysis
indicates that |B| is 1.3±4.0 Gauss (standard error), consistent with
a non detection at the few Gauss level.
---------------------------------------------------------
Title: Effect of atmospheric extinction on solar radial velocity
measurements.
Authors: Belmonte, Juan A.; Elsworth, Y. P.; Isaak, G. R.; New, R.;
Palle, Pere L.; Roca Cortés, Teodoro
1988ESASP.286..177B Altcode: 1988ssls.rept..177B
Differential extinction across the Earth's atmosphere affects
astronomical photometry in a well known way. Under the same
circumstances when a rotating extended object is observed
spectrometrically, a residual radial velocity is obtained which varies
during the day. In the case of integral sunlight observations, this
effect has been calculated along the day in all possible observing
situations during the year. Applications to real observations are
shown and discussed leading to some conclusions specially relevant to
ground-based networks.
---------------------------------------------------------
Title: Stellar seismology at Birmingham.
Authors: Isaak, G. R.; Innis, J. L.
1988MNSSA..47...34I Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Spectroscopic Search for Oscillations in the 769.9-NM
Potassium Line of Procyon
Authors: Isaak, G. R.; Jones, A. R.
1988IAUS..123..255I Altcode:
A double magneto optical filter attached to the Cassegrain focus of
the 1.9 m reflector of the South African Astronomical Observatory was
used to observe Procyon in the 769.9 nm line of potassium during all
clear hours of six nights. Velocity calibration was provided by the
rotational and orbital velocity changes of the observer as well as by
a continuous magnetic modulation.
---------------------------------------------------------
Title: Solar Cycle Dependence of Solar P-Modes
Authors: Isaak, G. R.; Jefferies, S. M.; McLeod, C. P.; New, R.;
van der Raay, H. B.; Palle, P. L.; Regulo, C.; Roca-Cortes, T.
1988IAUS..123..201I Altcode:
Data obtained at Izaña (Tenerife) and Haleakala (Maui), using optical
resonant scattering with a potassium vapour cell over the years 1980 -
84, are used to determine the frequencies of the low l p modes. Possible
variation in these frequencies with the solar cycle are investigated.
---------------------------------------------------------
Title: Variations in the Mean Line-Of Velocity of the Sun - 1976-1985
Authors: Jimenez, A.; Palle, P. L.; Regulo, C.; Roca-Cortes, T.;
Elsworth, Y. P.; Isaak, G. R.; Jefferies, S. M.; McLeod, C. P.; New,
R.; van der Raay, H. B.
1988IAUS..123..215J Altcode:
Measurements of the line of sight velocity of the sun with respect to
earth have been obtained at Izaña (Tenerife) during the years 1976 to
1985. The mean values found for each year show a trend of ≡30 m/s from
minimum to maximum. Their mean value is of 583.1±0.2 m/s which is 92%
of the gravitational redshift predicted by theory and their variation
seems to be related to the solar cycle with the clear exception of 1985.
---------------------------------------------------------
Title: Experience in Operating a Limited Global Network of Stations
Measuring Full-Disc Oscillations of the Sun
Authors: Elsworth, Y. P.; Isaak, G. R.; Jefferies, S. M.; McLeod,
C. P.; New, R.; van der Raay, H. B.; Palle, P. L.; Regulo, C.;
Roca-Cortes, T.
1988IAUS..123..535E Altcode:
Details are given about the operation of a two station network and of
a new semi-automatic station which has recently been added. Comparison
is made with predicted duty cycles. A possible way of quantifying the
sky quality is also given.
---------------------------------------------------------
Title: Comparison of Solar Oscillation Data Obtained from a Study
of the NA and K Fraunhoffer Absorption Lines
Authors: Isaak, G. R.; McLeod, C. P.; van der Raay, H. B.; Palle,
P. L.; Roca-Cortes, T.
1988IAUS..123...53I Altcode:
Two independent resonant scattering spectrometers, one using a sodium
and the other a potassium vapour cell, were operated simultaneously at
the same site. Due to the differing abundances of ground state atoms of
these two elements different depths of the photosphere are sampled. An
inter-comparison of solar p modes obtained with these spectrometers
is given.
---------------------------------------------------------
Title: The Solar Oscillations Spectrum and the Solar Cycle
Authors: Jimenez, A.; Palle, P. L.; Perez, J. C.; Regulo, C.;
Roca-Cortes, T.; Isaak, G. R.; McLeod, C. P.; van der Raay, H. B.
1988IAUS..123..205J Altcode:
During the summer seasons of the years 1977 to 1985 daily velocity
measurements of solar global oscillations have been obtained using a
resonant scattering spectrometer. After calculating the power spectra
of the daily residuals, the mean for each season is found. Several
discrete frequency intervals are defined in the spectrum. The mean
power in these intervals and the cut-off frequency of the p-mode
spectrum, determined for each year, are correlated with the solar
activity cycle. Furthermore, several series of 13 contiguous days for
each year are analyzed.
---------------------------------------------------------
Title: The 13-DAY Period Oscillation and the Solar Cycle
Authors: Jimenez, A.; Palle, P. L.; Regulo, C.; Roca-Cortes, T.;
Isaak, G. R.; McLeod, C. P.; van der Raay, H. B.
1988IAUS..123..211J Altcode:
From the analysis of radial velocity measurements of the Sun, obtained
at Izaña from 1976 to 1985, stable periods longer than 1 day, have
been found in the observed signal. The appearance of an oscillation
with a 13 day period has been confirmed. The comparison, for 1981 -
84, of the observations with a calibrated numerical model of the
passage of inhomogeneities (spots and plages) on the solar surface,
shows that the signal is not only due to this effect. It is believed
that a velocity field, probably related to the surface inhomogeneities,
contributes to the observed signal.
---------------------------------------------------------
Title: Continuous Magnetic Calibration of Velocity Sensitivity of
Oscillation Spectrometers
Authors: Isaak, G. R.; Jones, A. R.
1988IAUS..123..467I Altcode:
Modulation of the magnetic field in optical resonance spectrometers
in order to calibrate the slope of the observable intensity ratios in
terms of differential velocities is described.
---------------------------------------------------------
Title: Search for solar g modes.
Authors: Palle, P. L.; Roca Cortes, T.; Isaak, G. R.; van der Raay,
H. B.
1987PAICz..66..177P Altcode: 1987eram....1..177P
Using a resonant scattering spectrophotometer the authors measured
the radial velocity of the Sun at the K I 769.9 nm line with very
high resolution and temporal stability. The observations carried out
at Izaña (Tenerife) continuously for the last three years, have been
used to search for solar g modes. Individual peaks, well above noise
level, are identified and their frequencies are likely to correspond
with g modes.
---------------------------------------------------------
Title: The Limb Shift Effect and its Variation with the Solar Cycle
Authors: Anguera, M.; Pallé, P. L.; Régulo, C.; Roca Cortés, T.;
Isaak, G. R.; McLeod, C. P.; van der Raay, H. B.
1987rfsm.conf...24A Altcode:
The radial velocity limb shift effect has been measured for the K
I 7699 Å line using a resonant scattering spectrophotometer in the
summer of 1982. On the other hand, using integral sunlight, the line of
sight velocity has been measured during the years 1976 to 1986 and the
gravitational redshift determined. This value shows a variation over
those years and, when compared with the phase of the solar activity
cycle, the most probable interpretation is a change of the limb shift
effect with the cycle.
---------------------------------------------------------
Title: The global oscillation spectrum of the sun. I - Analysis of
daily power spectra of velocity measurements
Authors: Palle, P. L.; Perez, J. C.; Regulo, C.; Roca Cortes, T.;
Isaak, G. R.
1986A&A...169..313P Altcode:
Daily observations of solar global oscillations using a resonant
scattering spectrometer have been obtained by observing integral
sunlight during the summer seasons of the years 1977 to 1984. The power
spectra of the daily residuals are calculated and the mean for each
observing season is found. Several discrete frequency intervals are
defined in the spectrum which yield information on the characteristics
of the p-mode and noise levels. The power in these intervals and the
cut-off frequency of the p-mode spectrum determined for each year,
show no correlation with the solar activity cycle.
---------------------------------------------------------
Title: Linewidth and rotational splitting of low degree modes in
the 5 minute region.
Authors: Isaak, G. R.
1986ASIC..169..223I Altcode: 1986ssds.proc..223I
The linewidth of low degree modes in the 5 minute region of the Sun's
acoustic spectrum increases dramatically with frequency over the range
of 2 mHz to some 5 mHz, in approximate agreement with theory. Below some
2.5 mHz the linewidth is narrow enough to allow rotational splitting
of l = 1 modes to be measured. These measurements seem to confirm
earlier results.
---------------------------------------------------------
Title: The radial velocity of the sun as a star and the solar cycle
Authors: Jiménez, A.; Pallé, P. L.; Régulo, C.; Roca Cortés, T.;
Isaak, G. R.; McLeod, C. P.; van der Raay, H. B.
1986AdSpR...6h..89J Altcode: 1986AdSpR...6...89J
Radial velocity measurements of the sun as a star using a resonant
scattering spectrometer have been obtained at Izan~a (Tenerife) during
long observing seasons from 1976 to 1985. Its analysis shows that except
for the global oscillations with periods shorter than one day there are
stable signals with longer periods: at ~13 days with mean amplitude of
~ 3 m/s (it changes with the solar cycle) and another one of ~ 15 m/s
amplitude which shows a temporal variation of various years related
to the solar cycle. The first signal has a partial explanation as an
effect produced by the passage of active regions through the visible
surface of the sun but it does not completely explain the observed
signal. The second one, related to the limb shift, is probably due to
changes in the convection zone in connection with the solar cycle.
---------------------------------------------------------
Title: Global Oscillations of the Sun
Authors: Isaak, G. R.
1985PhyBl..41..176I Altcode: 1985PhB....41..176I
No abstract at ADS
---------------------------------------------------------
Title: Solar G modes
Authors: Isaak, G. R.; van der Raay, H. B.; Palle, P. L.; Roca Cortes,
T.; Delache, P.
1984MmSAI..55...91I Altcode:
A statistical analysis of the frequency spectrum of 85 days of
data obtained from integral solar velocity signals, has shown that
significant signals exist in the frequency range 25 - 150 μHz. The
signal structure is shown to have characteristics of constant period
spacings indicative of the existence of g modes in the solar oscillation
spectrum. Utilising the asymptotic Tassoul relation a total of 58 lines
are identified covering the l<SUB>1</SUB>, l<SUB>2</SUB>, l<SUB>3</SUB>
and l<SUB>4</SUB> modes.
---------------------------------------------------------
Title: Further experimental evidence of the 13 day solar periodicity
Authors: Isaak, G. R.; van der Raay, H. B.; Palle, P. L.; Roca
Cortes, T.
1984MmSAI..55..353I Altcode:
Extensive solar data obtained over the period May to August 1982 are
analyzed to show the continued existence, over a six-year period,
of a 13-day velocity signal of 6 m/s amplitude. Mean daily velocity
signals are shown with a superimposed best-fit sine wave, and the power
spectrum obtained from that fit is presented. Data comparing measured
and calculated relative phases for the 13.035 day period are given. The
constancy of the signal's amplitude and its long coherence time may be
partly explained by velocity signals induced by active region rotation,
but some fundamental cause, possibly related to the rapid rotation of
the solar core, is strongly suggested.
---------------------------------------------------------
Title: Observations of global solar oscillations: past, present,
future (invited paper)
Authors: Isaak, G. R.
1984MmSAI..55...45I Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Continuous observation of solar oscillations from two suitably
spaced ground stations
Authors: Claverie, A.; Isaak, G. R.; McLeod, C. P.; van der Rasy,
H. B.; Palle, P. L.; Roca Cortes, T.
1984MmSAI..55...63C Altcode:
By operating two observing stations, one at Izana on Tenerife and
the other at Haleakala on Maui, up to 22 hours of continuous solar
surface velocity data per day are obtained. Correlation of two station
data provides confirmation of the solar origin of the signals and the
improved window function and long time span of data collection yield
clear power spectra of the 5 minute oscillations. Detailed analysis
of a prominent l<SUB>1</SUB> mode indicates that coherence exists over
the 88 days of observation.
---------------------------------------------------------
Title: Solar oscillations and the effect of a comet impact
Authors: Isaak, G. R.; van der Raay, H. B.; Palle, P. L.; Roca
Cortes, T.
1984MmSAI..55..263I Altcode:
If such external influences as the impact of a comet on the solar
surface could destroy or mask the coherence of observed solar
oscillations, the scope and effectiveness of extended study in this
field would be considerably limited. It is therefore important to
investigate any possible effects due to the recently reported comet
impacts on the sun. Three such events were recorded by Sheely et
al. (1982), and the last of these, on 8.0 UT July 20, 1981, occurred
while full disk optical resonant scattering determinations of the
solar velocity spectrum were being made. It is found that its effect
on the 5-min oscillations is not noticeable,and that it is therefore
meaningful to study solar oscillation data in order to resolve the
question as to the internal rotation of the sun.
---------------------------------------------------------
Title: Laboratory Work on Noise and Stability of Detectors and
Calibration Lamps
Authors: Isaak, G. R.
1984srps.conf..133I Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A New Method for Determining the Helium Abundance in the
Solar Atmosphere
Authors: Isaak, G. R.
1983SoPh...82..205I Altcode: 1983IAUCo..66..205I
Recent observations of a cut-off frequency in the acoustic modes of the
Sun (Claverie et al., 1981b) should help determine the mean molecular
weight and, thereby, the helium abundance in the visible layers of
the solar atmosphere. A first preliminary result of Y = 0.42 ± 0.04
is obtained for an assumed minimum photospheric temperature of 4400
K ± 200 K.
---------------------------------------------------------
Title: Rotational splitting of solar five-minute oscillations of
low degree
Authors: Claverie, A.; Isaak, G. R.; McLeod, C. P.; van der Raay,
H. B.; Roca Cortes, T.
1983SoPh...82..233C Altcode: 1983IAUCo..66..233C
An analysis of 28 contiguous days of whole disk observations of the
solar surface by means of optical resonant scattering in the K 769.9
nm line, taken at the Teide Observatory at Izana during July-August
1980, have thus far yielded two significant facts. Firstly when the
results of an iterative sine-wave fitting procedure are considered
in the period range 2-3 h, although the expected daily harmonics
corresponding to 1/8, 1/10, 1/11, and 1/12 of a day are clearly seen
the l/9th contribution is significantly absent. It is suggested that
this results from an interference between a signal of 160 min (1/9th
of a day) with the daily harmonic. It is further pointed out that the
observatories at which the 160 min oscillation has been seen, Crimea,
Pic du Midi, and Stanford are all separated by integral numbers of
160 min, and thus the phase of the 160 min oscillation relative to the
daily observation window is constant. However, the Teide Observatory
is situated at a half integral number of 160 min periods relative to
the others. Thus when constructive interference exist at the first
three sites destructive interference will exist at the latter. It is
thus concluded that the non-existence of a peak corresponding to the
1/9th harmonic of a day in the sine-wave fit data is strong indirect
evidence for the existence of the 160 min signal.
---------------------------------------------------------
Title: Is there an oblique magnetic rotator inside the Sun?
Authors: Isaak, G. R.
1983SoPh...82..235I Altcode: 1983IAUCo..66..235I
The size of the rotational splitting recently observed (Claverie et
al., 1981) is correlated with the 12.2<SUP>d</SUP> variation in the
measurements of solar oblateness observed by Dicke (1976) and implies
a convection zone of depth of 0.1 R<SUB>⊙</SUB>. The near equality
of amplitudes of global velocity oscillations (Claverie et al., 1981)
of the various m components of the l = 1 and l = 2 modes as seen from
the Earth viewing the Sun nearly along the equator is unexpected for
pure rotational splitting. It is suggested that a magnetic perturbation
is present and an oblique asymmetric magnetic rotator with magnetic
fields of a few million gauss is responsible. A more detailed account
was submitted to Nature.
---------------------------------------------------------
Title: Solar core rotation
Authors: Claverie, A.; Isaak, G. R.; McLeod, C. P.; van der Raay,
H. B.; Palle, P. L.; Roca Cortes, T.
1982Natur.299..704C Altcode:
Data from solar line of sight velocity measurements are presented
to demonstrate the existence of a synodic signal with a 6.5 m/sec
amplitude. Recordings of alternate measurements of the resonantly
scattered light intensity were made at 1 sec intervals and a mean
ratio for the laboratory and solar lines was calculated over a 42 sec
interval, yielding 900 points each day of observation. An oscillation
which was in phase at both observation points was found. A peak
was found at 0.88 micro-Hz, with a 13.15 day period. The 6.6 m/sec
velocity amplitude of the peak was calculated to indicate a mean
surface displacement of 1,000,000 m. The magnitude of the Doppler
shift is taken as evidence that a core moving at a greater speed than
the surface of the sun is responsible for the observed signal. It is
further suggested that the core is moving twice as fast as the surface.
---------------------------------------------------------
Title: Abundances of Elements of Cosmological Interest: Discussion
Authors: Isaak, G. R.
1982RSPTA.307...35I Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Is the Sun an oblique magnetic rotator?
Authors: Isaak, G. R.
1982Natur.296..130I Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A two-dimensional solar spectrometer.
Authors: Brookes, J. R.; Isaak, G. R.; van der Raay, H. B.
1981SoPh...74..503B Altcode:
A precise two-dimensional positioning device has been developed for
use in conjunction with a resonant scattering spectrometer to study
the spatial distribution of solar velocity fields. The principle of
operation and constructional details are discussed and the experimental
performance is evaluated. As an illustration of its use preliminary
data obtained from a meridional scan are presented.
---------------------------------------------------------
Title: Short-Period Intensity Fluctuations of Integral Sunlight
Authors: Claverie, A.; Isaak, G. R.; McLeod, C. P.
1981SoPh...74...73C Altcode:
An attempt has been made to detect short-period solar luminosity
fluctuations in the vicinity of 5 min, analogous to the observed
velocity oscillation. Using silicon photodiodes to monitor integral
sunlight, an upper limit for the amplitude of the intensity fluctuations
of 3 x 10<SUP>-5</SUP> rms was found.
---------------------------------------------------------
Title: Solar oscillations: Past, present, and future
Authors: Isaak, G. R.
1981SoPh...74...43I Altcode:
Observation of global oscillations of the Sun constitutes a primitive
seismology of the solar interior. The frequencies, if correctly
identified with definite normal modes of vibration, provide a measure
of the average velocity of sound in the interior and thereby of
its composition and temperature. Fine structure in the frequencies
of nonradial modes may provide information on their character
(multiplicity) and on the rotation of the solar interior. Study of
the amplitudes and phase fluctuations of the vibrations may clarify
the excitation and damping of the vibrations.
---------------------------------------------------------
Title: Structure of the 5-minute solar oscillations 1976 1980
Authors: Claverie, A.; Isaak, G. R.; McLeod, C. P.; van der Raay,
H. B.; Roca Cortes, T.
1981SoPh...74...51C Altcode:
The discrete structure in the 5 min velocity oscillations of the solar
surface has been confirmed by a re-analysis of data obtained between
1976 and 1979, and in addition a preliminary analysis of 1980 data
show excellent consistency of the determined frequencies over the
five year period. It is further shown that atmospheric transparency,
as measured by the power in the solar intensity fluctuations, shows no
correlation with the measured amplitude of the velocity fluctuations,
over 2 orders of magnitude.
---------------------------------------------------------
Title: A two dimensional solar spectrometer
Authors: Brookes, J. R.; Isaak, G. R.; van der Raay, H. B.
1981JPhE...14.1288B Altcode:
A precise two-dimensional positioning device has been developed for
use in conjunction with a resonant scattering spectrometer to study
the spatial distribution of solar oscillations. The principle of
operation and constructional details are discussed and the experimental
performance is evaluated by analyzing the results obtained from
sequentially scanning four quadrants of the solar disc.
---------------------------------------------------------
Title: Rapid rotation of the solar interior
Authors: Claverie, A.; Isaak, G. R.; McLeod, C. P.; van der Raay,
H. B.; Roca Cortes, T.
1981Natur.293..443C Altcode:
Evidence for the rapid internal rotation of the sun obtained from
Doppler shift measurements of the Fraunhofer absorption lines is
reported. An improved version of the Doppler shift optical resonance
spectrometer was used to record the resonantly scattered intensities
corresponding to the two wings of the 769.9-nm Fraunhofer absorption
lines of neutral potassium on 28 continuous days in July and August
1980. A total of 33 lines was identified which were found to be split
into components corresponding to the l = 1, l = 2 and l = 0 modes of
the spherical harmonic. Superposed frequency spectra for each mode
exhibit a width indicative of high-Q oscillations, with a mean value
of 0.75 microHz. Comparison of this value with that anticipated for
the case of uniform rotation (0.4 microHz) reveals that the core of
the sun rotates two to nine times as rapidly as the surface.
---------------------------------------------------------
Title: Global Oscillations of the Sun
Authors: Isaak, G. R.
1981IrAJ...15..119I Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Structure in the 5 minute oscillations of integral sunlight
Authors: Claverie, A.; Isaak, G. R.; McLeod, C. P.; van der Raay,
H. B.; Roca Cortes, T.
1980A&A....91L...9C Altcode:
The discovery of structure in the 5 minute oscillations of the
sun, integrated over the full disk, was reported by Claverie et
al. (1979). This paper presents additional information on the discrete
structure in the 5-minute velocity oscillation observed in integral
sunlight. Measurements taken at Pic-du-Midi and Izana (on Tenerife) from
1976 to 1979 show good consistency, and some 25 discrete frequencies
(established to an accuracy of about 1:1000) are tabulated.
---------------------------------------------------------
Title: Solar oscillations, stellar oscillations and cosmology
Authors: Isaak, G. R.
1980Natur.283..644I Altcode:
Two implications of the recent observations<SUP>1</SUP> of low angular,
high radial overtones of the whole Sun are reported here. The first
implication is that other main sequence stars are likely to be
oscillating in similar modes and that precision spectroscopy as well
as photometry from space is capable of detecting these oscillations,
thereby extending seismology to stars. The second implication is a solar
helium abundance Y≲0.17 with implications for cosmological models.
---------------------------------------------------------
Title: The Latest Results of the Velocity Spectroscopy of the Sun
Authors: Claverie, A.; Isaak, G. R.; McLeod, C. P.; van der Raay,
H. B.; Roca-Cortes, T.
1980LNP...125..181C Altcode: 1980nnsp.work..181C
No abstract at ADS
---------------------------------------------------------
Title: Solar structure from global studies of the 5-minute oscillation
Authors: Claverie, A.; Isaak, G. R.; McLeod, C. P.; van der Raay,
H. B.; Roca Cortes, T.
1979Natur.282..591C Altcode:
An overall view of the solar surface is presented with emphasis
on the study of low 1 value oscillations. Line of sight velocity
measurements of the whole solar disk were made using optical resonance
spectroscopy. Constancy of the peak of spectral emission is demonstrated
in two ways; first, the observed peaks in the power spectra are numbered
sequentially and plotted against the observed frequency of the order
of the peaks. Secondly, the mean power spectra data are screened by
a high pass filter by subtracting a moving mean over three points and
then subjecting the resulting points to an autocorrelation.
---------------------------------------------------------
Title: A resonant-scattering solar spectrometer
Authors: Brookes, J. R.; Isaak, G. R.; van der Raay, H. B.
1978MNRAS.185....1B Altcode:
Summary. The resonant-scattering spectrometer, developed in Birmingham
and used to study the radial velocity of the solar surface, is
described as it was during 1976. Possible instrumental sources of
error are analysed and evaluated. It is shown that angular changes in
the light path through the electro-optic modulator can give rise to
an oscillatory error of up to 1 rn/s, and that no other instrumental
effects give rise to errors of this order.
---------------------------------------------------------
Title: The observation of a rotating body using high-resolution
spectroscopy
Authors: Brookes, J. R.; Isaak, G. R.; van der Raay, H. B.
1978MNRAS.185...19B Altcode:
Summary - If a high-resolution specotroscopic instrument is used to
observe a rotating body in the light from a narrow spectral line,
significant factors affect the apparent field of view when both
the line width and the instrumental resolution, expressed as their
equivalent Doppler velocities, are less than, or comparable to, the
surface velocity of the observed body. The particular case of the
Sun observed with the Birmingham resonant scattering spectrometer
is considered, and the effect on the apparent velocity oscillations
produced by supergranulation is discussed.
---------------------------------------------------------
Title: The search for solar oscillations, 1974 to 1976.
Authors: Brookes, J. R.; Isaak, G. R.; McLeod, C. P.; van der Raay,
H. B.; Roca Cortes, T.
1978MNRAS.184..759B Altcode:
Using a resonant scattering spectrometer, observations of integral solar
light have been recorded and analyzed to determine the existence of
solar oscillations. Although the data from individual days do confirm
the existence of long-period oscillations, these do not seem to be
consistent in either period, amplitude or phase. Possible interpretation
and further experimental verifications are considered.
---------------------------------------------------------
Title: Does the Sun ring?
Authors: Isaak, G. R.
1978PhB....29..127I Altcode:
An investigation of solar oscillations is proposed, whereby small
velocity perturbations of the surface layers are studied in order
to measure sound waves (or phonons) in the solar atmosphere. Such
observations would also provide information on the solar
interior. Excitation mechanisms include shocks from the core due to
uneven or explosive nuclear burning, and flares from the surface
with energies on the order of 10 to the 23rd J for a photospheric
velocity amplitude of 1 m/sec. Fraunhofer absorption lines are used
as indicators of Doppler shifts to evaluate velocity noise. Attention
is given to detailed observations of solar cells, where oscillation
is more pronounced than on the sun as a whole. It is pointed out that
oscillations occurring on the sun probably occur on most, if not all,
other stars.
---------------------------------------------------------
Title: Solar Oscillations
Authors: Brookes, J. R.; Isaak, G. R.; McLeod, C. P.; van der Raay,
H. R.; Roca Cortes, T.
1978pfsl.conf..115B Altcode: 1978ESPM....2..115B
No abstract at ADS
---------------------------------------------------------
Title: Oscillations of the Sun
Authors: Isaak, G. R.
1976Obs....96..221I Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Talk on Solar Oscillations
Authors: Isaak, G. R.
1976Obs....96..132I Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Observation of free oscillations of the Sun
Authors: Brookes, J. R.; Isaak, G. R.; van der Raay, H. B.
1976Natur.259...92B Altcode:
The Fraunhofer absorption lines for potassium and sodium on the Sun
are compared with the corresponding lines in the laboratory using a
resonant optical scattering method. The observed shifts between the Sun
and laboratory lines may be interpreted in terms of the gravitational
redshift (GRS), motion of the laboratory relative to the Sun and
oscillatory terms which may be related to oscillations of the Sun.
---------------------------------------------------------
Title: Improved Limit on the Absence of Dispersion of the Velocity
of Light
Authors: Isaak, G. R.
1969Natur.223..161I Altcode:
LABORATORY measurements have duly verified that the velocity of light
is independent of the velocity of the source to an accuracy of 1.3 in
10<SUP>4</SUP> for γ-rays from π<SUP>0</SUP> meson decay at 6 GeV
(ref. 1) and is also isotropic to better than 6 in 10<SUP>8</SUP>
(ref. 2), 2 in 10<SUP>8</SUP> (ref. 3) and 2 in 10<SUP>10</SUP>
(ref. 4) in a frame of reference attached to the Earth. The best
evidence that the velocity of light is independent of frequency
probably depends on astronomical observations of Cepheids in the nearest
galaxies, and makes it possible to set a limit of 1 in 10<SUP>8</SUP>
on dispersion. Somewhat poorer limits over a much wider range of
frequencies, from the visible to the radio-frequency region, have been
provided by observations of flare stars<SUP>5</SUP>.
---------------------------------------------------------
Title: An Atomic Beam Spectrophotometer
Authors: Isaak, G. R.
1961Natur.189..373I Altcode:
No abstract at ADS
---------------------------------------------------------
Title: An Atomic Beam Spectrophotometer
Authors: Isaak, G. R.
1961Natur.189.4373I Altcode:
No abstract at ADS
