G. R. Isaak

Solar internal sound speed as inferred from combined BiSON and LOWL oscillation frequencies

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 < 0.2 R.. The density of the solar core is, however, lower than that in solar models.

Solar p-mode frequencies and their dependence on solar-activity - recent results from the BISON network

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.

Observational Upper Limits to Low-Degree Solar g-Modes

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-1 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☉ = 2.3 × 10-8 at the solar surface. These levels which are much lower than prior claims, are consistent with theoretical predictions.

A Comparison of Low-Degree Solar p-Mode Parameters from BiSON and GONG: Underlying Values and Temporal Variations

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.

On the measurement bias of low- solar p-mode excitation parameters: The impact of a ground-based window function

We present a study of the impact of a ground-based, multi-station window function on estimates of the power and damping of low-� 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. 4t o∼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.

Solar p-mode frequencies at l = 2: What do analyses of unresolved observations actually measure?

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. 2003a) 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.

The radial velocity of the sun as a star and the solar cycle

Abstract Radial velocity measurements of the sun as a star using a resonant scattering spectrometer have been obtained at Izana (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.

Structure of the 5-minute solar oscillations 1976–1980

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.

Does the Energy Supplied to Low-l Solar p-Modes Vary over the Activity Cycle?

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.

Solar Oscillations: Past, Present, and Future

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.After a brief historical review emphasizing global velocity spectroscopy an account is given of the present status of the observations of global oscillations in the range of periods of 3 to 160 min.Finally the future capabilities of the observational techniques and their resultant potential is discussed.