B. A. Miller

Solar p-mode frequencies over three solar cycles

We analyze thirty years of solar oscillations data collected by the Birmingham Solar Oscillations Network (BiSON). Estimates of the mean frequency shifts of low-degree p-modes have been extracted over a period spanning solar cycles 21-23. Two methods of analysis are used to extract the frequency shifts: one method uses results on fitted frequencies of individual modes, which are then averaged to give mean frequency shifts; the other method uses cross-correlations of power frequency spectra made from subsets of the data shifted in time. The frequency shifts are correlated against six proxies of solar activity, which are sensitive to magnetic and irradiance variability at a range of locations from the photosphere to the corona. We find proxies that have good sensitivity to the effects of weak-component magnetic flux—which is more widely distributed in latitude than the strong flux in the active regions—are those that follow the frequency shifts most consistently over the three cycles. This list includes the Mg II H and K core-to-wing data, the 10.7 cm radio flux, and the He I equivalent width data. While the two methods of analysis give consistent results, use of the cross-correlation function to measure mean frequency shifts returns less precise values in cases in which the duty cycle is greater than 30%. Estimation of uncertainties from the cross-correlation method also requires that proper allowance be made for strong correlations in the data.

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.

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.

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.

Impact of the Solar Activity Cycle on Frequency Separation Ratios in Helioseismology

Roxburgh & Vorontsov have recently proposed the use of ratios of small to large frequency separations of low angular degree p-modes as a means of eliminating from asteroseismic data the unwanted influence of the structure of the near-surface layers of stars. Here we have studied the impact of the solar activity cycle on the magnitude of these so-called frequency separation ratios using data collected by Sun-as-a-star observations. The ratios are observed to change with the shifting level of global solar activity. The effect, which we detect in BiSON Doppler velocity data at a marginal level of significance, is shown to be a consequence of the influence of acoustic asphericity from the surface activity on the azimuthally dependent Sun-as-a-star frequencies. The results suggest that any analysis that makes use of ratios formed from long helioseismic data sets may therefore show effects of bias. While the effect is less significant in shorter data sets, of length similar to what will soon be available from asteroseismic campaigns, an approximate doubling of the effects from the solar asphericity may be sufficient to cause complications for stellar analyses.

The Search for Correlation between BiSON SMMF Data and CME Events

We present first attempts to compare the Birmingham Solar-Oscillations Network (BiSON) high precision solar mean magnetic field (SMMF) data of four years with the occurrence of CMEs (coronal mass ejections) as recorded by LASCO on board SOHO. The BiSON magnetic measurement technique is given in Chaplin et al. (2003). Particularly interesting results of recent SMMF high-cadence observations have come from studies of correlation between the SMMF determined by MDI and the occurrence of CMEs (Boberg and Lundstedt 2000 and Boberg et al 2002). Two frequency ranges, centered on 13 and 90 minutes, have been identified as possibly correlating with CME occurrence. We have used BiSON SMMF data from two sites to investigate CME related SMMF signals to try to confirm the MDI results. To search methodically through our data set we have developed two correlation techniques suited to short (up to 32 minutes) and long (up to 3 hours) period wavelets, respectively. For short periods we analyzed SMMF data in the vicinity of CMEs, and for long periods we compared SMMF results for days with and without recorded CMEs. In neither period range have we yet clearly identified correlations between SMMF power excesses and CME onsets. For the details of the techniques and the results see Chaplin et al. (2004).