Birmingham Solar Oscillations Network Data Research Articles

Atmospheric Extinction Coefficients in the Ic Band for Several Major International Observatories: Results from the BiSON Telescopes, 1984–2016

Abstract Over 30 years of solar data have been acquired by the Birmingham Solar Oscillations Network (BiSON), an international network of telescopes used to study oscillations of the Sun. Five of the six BiSON telescopes are located at major observatories. The observational sites are, in order of increasing longitude: Mount Wilson (Hale) Observatory (MWO), California, USA; Las Campanas Observatory, Chile; Observatorio del Teide, Izaña, Tenerife, Canary Islands; the South African Astronomical Observatory, Sutherland, South Africa; Carnarvon, Western Australia; and the Paul Wild Observatory, Narrabri, New South Wales, Australia. The BiSON data may be used to measure atmospheric extinction coefficients in the band (approximately 700–900 nm), and presented here are the derived atmospheric extinction coefficients from each site over the years 1984–2016.

BiSON data preparation: a correction for differential extinction and the weighted averaging of contemporaneous data

The Birmingham Solar Oscillations Network (BiSON) has provided high-quality high-cadence observations from as far back in time as 1978. These data must be calibrated from the raw observations into radial velocity and the quality of the calibration has a large impact on the signal-to-noise ratio of the final time series. The aim of this work is to maximize the potential science that can be performed with the BiSON data set by optimizing the calibration procedure. To achieve better levels of signal-to-noise ratio, we perform two key steps in the calibration process: we attempt a correction for terrestrial atmospheric differential extinction; and the resulting improvement in the calibration allows us to perform weighted averaging of contemporaneous data from different BiSON stations. The improvements listed produce significant improvement in the signal-to-noise ratio of the BiSON frequency–power spectrum across all frequency ranges. The reduction of noise in the power spectrum will allow future work to provide greater constraint on changes in the oscillation spectrum with solar activity. In addition, the analysis of the low-frequency region suggests that we have achieved a noise level that may allow us to improve estimates of the upper limit of g-mode amplitudes.

Low-frequency, low-degree solar p-mode properties from 22 years of Birmingham Solar Oscillations Network data

The solar low-degree low-frequency modes of oscillation are of particular interest as their frequencies can be measured with very high precision and hence provide good constraints on seismic models. Here we detect and characterize these valuable measures of the solar interior from a 22 yr Birmingham Solar Oscillations Network data set. We report mode frequencies, line widths, heights, amplitudes, and rotational splitting, all with robust uncertainties. The new values of frequency, rotational splitting, amplitude, and line width we provide will help place new constraints on hydrostatic and rotational structure, plus diagnostics of near-surface convection. Further to this, by assuming simple power laws, we extrapolate mode properties to lower frequencies. We demonstrate that the low-l low-frequency p modes have a low signal-to-noise ratio and that this cannot be overcome simply by continued observation. It will be necessary to observe the Sun in novel ways to ‘beat’ the intrinsic granulation noise.

Quasi-biennial variations in helioseismic frequencies: can the source of the variation be localized?

We investigate the spherical harmonic degree (l) dependence of the ‘seismic’ quasi-biennial oscillation (QBO) observed in low-degree solar p-mode frequencies, using Sun-as-a-star Birmingham Solar Oscillations Network data. The amplitude of the seismic QBO is modulated by the 11-yr solar cycle, with the amplitude of the signal being largest at solar maximum. The amplitude of the signal is noticeably larger for the l= 2 and 3 modes than for the l= 0 and 1 modes. The seismic QBO shows some frequency dependence but this dependence is not as strong as observed in the 11-yr solar cycle. These results are consistent with the seismic QBO having its origins in shallow layers of the interior (one possibility being the bottom of the shear layer extending 5 per cent below the solar surface). Under this scenario the magnetic flux responsible for the seismic QBO is brought to the surface (where its influence on the p modes is stronger) by buoyant flux from the 11-yr cycle, the strong component of which is observed at predominantly low latitudes. As the l= 2 and 3 modes are much more sensitive to equatorial latitudes than the l= 0 and 1 modes the influence of the 11-yr cycle on the seismic QBO is more visible in l= 2 and 3 mode frequencies. Our results imply that close to solar maximum the main influence of the seismic QBO occurs at low latitudes (<45°), which is where the strong component of the 11-yr solar cycle resides. To isolate the latitudinal dependence of the seismic QBO from the 11-yr solar cycle we must consider epochs when the 11-yr solar cycle is weak. However, away from solar maximum, the amplitude of the seismic QBO is weak making the latitudinal dependence hard to constrain.

A new efficient method for determining weighted power spectra: detection of low-frequency solar p-modes by analysis of BiSON data

We present a new and highly efficient algorithm for computing a power spectrum made from evenly spaced data which combines the noise-reducing advantages of the weighted fit with the computational advantages of the Fast Fourier Transform (FFT). We apply this method to a 10-year data set of the solar p-mode oscillations obtained by the Birmingham Solar Oscillations Network (BiSON) and thereby uncover three new low-frequency modes. These are the l=2, n=5 and n=7 modes and the l=3, n=7 mode. In the case of the l=2, n=5 modes, this is believed to be the first such identification of this mode in the literature. The statistical weights needed for the method are derived from a combination of the real data and a sophisticated simulation of the instrument performance. Variations in the weights are due mainly to the differences in the noise characteristics of the various BiSON instruments, the change in those characteristics over time and the changing line-of-sight velocity between the stations and the Sun. It should be noted that a weighted data set will have a more time-dependent signal than an unweighted set and that, consequently, its frequency spectrum will be more susceptible to aliasing.

IS THE CURRENT LACK OF SOLAR ACTIVITY ONLY SKIN DEEP?

The Sun is a variable star whose magnetic activity and total irradiance vary on a timescale of approximately 11 years. The current activity minimum has attracted considerable interest because of its unusual duration and depth. This raises the question: what might be happening beneath the surface where the magnetic activity ultimately originates? The surface activity can be linked to the conditions in the solar interior by the observation and analysis of the frequencies of the Sun's natural seismic modes of oscillation-the p modes. These seismic frequencies respond to changes in activity and are probes of conditions within the Sun. The Birmingham Solar-Oscillations Network (BiSON) has made measurements of p-mode frequencies over the last three solar activity cycles, and so is in a unique position to explore the current unusual and extended solar minimum. We show that the BiSON data reveal significant variations of the p-mode frequencies during the current minimum. This is in marked contrast to the surface activity observations, which show little variation over the same period. The level of the minimum is significantly deeper in the p-mode frequencies than in the surface observations. We observe a quasi-biennial signal in the p-mode frequencies, which has not previously been observed at mid-and low-activity levels. The stark differences in the behavior of the frequencies and the surface activity measures point to activity-related processes occurring in the solar interior, which are yet to reach the surface, where they may be attenuated.

BiSON Data Show Change in Solar Structure with Magnetic Activity

We find evidence for an activity-dependent change in solar structure at the second helium ionization zone (~0.98 R☉). We use low-degree p-mode frequencies obtained from 13 years of observations by the Birmingham Solar Oscillations Network (BiSON) to track changes in the amplitude of the rapid-variation signature imposed by the bump in the adiabatic exponent, Γ1, at the He II ionization zone. This is the first indication of structural change detected using only low-degree data and confirms the recent results of high-degree and local helioseismic studies.

Extended search for correlation between solar mean magnetic field BiSON data and coronal mass ejections

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.

Solar p-mode frequencies at $\mathsf{\ell=2}$: What do analyses of unresolved observations actually measure?

We have studied in detail the extraction of estimates of 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 (, 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]a) it has for a long time been standard practice to fit the 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.

Peak finding at low signal-to-noise ratio: low- solar acoustic eigenmodes at n&lt;= 9 from the analysis of BiSON data

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 greater than or equal to 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 less than or equal to 9.

On the measurement precision of solar p-mode eigenfrequencies

We make use of 3456 d 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(6) 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 similar to25 per cent smaller than their BiSON counterparts owing to the uninterrupted nature of the data from which they were derived. An analysis of the BiSON data in contiguous segments of different lengths, T, demonstrates that the frequency uncertainties scale as T-1/2. This is to be expected in the regime where the coherence (life) times of the modes, tau(nl), 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(nl)).

Source of excitation of low- solar p modes: characteristics and solar-cycle variations

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.

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 which allows 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-two 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 < 0.2Rsun. The density of the solar core is, however, lower than that in solar models.

The Solar Core: New Low-[CLC][ITAL]l p[/ITAL][/CLC]-Mode Fine-Spacing Results from B[CLC]i[/CLC]SON

The fine-structure spacing dl(n) = νl,n - νl+2,n-1 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 d0(n), and 11 ≤ n ≤ 27 for d1(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.