Asymmetries of Solar p ‐Mode Line Profiles (original) (raw)
Related papers
Line Asymmetry of Solar p ‐Modes: Reversal of Asymmetry in Intensity Power Spectra
The Astrophysical Journal, 1999
The sense of line asymmetry of solar p-modes in the intensity power spectra is observed to be opposite of that seen in the velocity power spectra. Theoretical calculations provide a good understanding and fit to the observed velocity power spectra whereas the reverse sense of asymmetry in the intensity power spectrum has been poorly understood. We show that when turbulent eddies arrive at the top of the convection zone they give rise to an observable intensity fluctuation which is correlated with the oscillation they generate, thereby affecting the shape of the line in the p-mode power spectra and reversing the sense of asymmetry (this point was recognized by Nigam et al. and Roxburgh & Vorontsov). The addition of the correlated noise displaces the frequencies of peaks in the power spectrum. Depending on the amplitude of the noise source the shift in the position of the peak can be substantially larger than the frequency shift in the velocity power spectra. In neither case are the peak frequencies precisely equal to the eigenfrequencies of p-modes. We suggest two observations which can provide a test of the model discussed here.
Monthly Notices of the Royal Astronomical Society, 1999
We use the solutions to a simple, one-dimensional wave equation ± which is intended to describe the essential elements of the solar resonant acoustic cavity ± as formalistic models to which to fit low-`modes in observational helioseismic power spectra. We have analysed data collected in velocity, by the ground-based Birmingham Solar-Oscillations Network (BiSON), and in intensity, by the full-disc VIRGO Sun photometers (SPM) on board the ESA/NASA SOHO satellite.
On the Asymmetry of Solar Acoustic Line Profiles
The Astrophysical Journal, 1998
We study a simpliÐed model of solar acoustic oscillations and show how asymmetries in spectral lines depend both on the acoustic source depth, as previously recognized, and on the acoustic source type. We provide a uniÐed description of modal line asymmetries and high-frequency pseudomode locations, suggesting an inversion on power spectra minima to determine source properties and a correction to Lorentzian line shapes based upon the relative locations of spectral peaks and valleys. We also consider nonadiabatic e †ects due to Newtonian cooling and demonstrate that these do not lead to notable di †erences between velocity and intensity power spectral line shapes. We argue more generally that it is unlikely that any nonadiabatic e †ect can be responsible for the observed di †erences. Finally, we discuss the importance of both multiplicative and additive background power to the spectra and show how additive noise can reduce the apparent line asymmetry of a mode. We note that information on solar convective motions can be potentially extracted from three components of the acoustic power spectra : the additive background yielding information on the spectrum of nonoscillatory motions at the height of observation, the multiplicative background reÑecting the source spectrum, and the power minima providing the source depth and physical nature. For stochastically excited linear waves only the Ðrst of these contributes signiÐcantly to spectral di †erences between observed variables.
The Astrophysical Journal, 1998
An accurate determination of the frequencies of low-degree solar p-modes is an important task of helioseismology. Using 679 days of solar oscillation data observed in Doppler velocity and continuum intensity from two Solar and Heliospheric Observatory instruments (the Michelson Doppler Imager and the SunPhotoMeter), we show that fitting the spectra with Lorentzian profiles leads to systematic differences between intensity and velocity frequencies as large as 0.1 mHz for angular degrees , 1, and 2 because of the opposite asymmetry l ϭ 0 between intensity and velocity. We use a physics-based asymmetrical line shape to fit p-mode lines, and we demonstrate that their asymmetry is statistically significant and that frequency differences are considerably reduced. These measurements provide more accurate estimates of the solar eigenfrequencies. We discuss inferences of the structure of the solar core.
Secular variations in the spectrum of solar p-modes
Solar Physics, 1994
The solar p-mode spectrum of very low I is measured with high accuracy for a long enough period of time so as to allow the search for solar cycle variations, in this paper solar cycle variations of the frequency and energy of the modes are confirmed. Moreover, a slight variation,within errors, of its rotational splitting with the solar cycle, is suggested.
Source of excitation of low- solar p modes: characteristics and solar-cycle variations
Monthly Notices of the Royal Astronomical Society, 2000
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-`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.
Structure of the Solar Core: Effect of Asymmetry of Peak Profiles
The Astrophysical Journal, 2000
Recent studies have established that peaks in solar oscillation power spectra are not Lorentzian in shape, but have a distinct asymmetry. Fitting a symmetric Lorentzian profile to the peaks therefore produces a shift in frequency of the modes. Accurate determination of low-frequency modes is essential to infer the structure of the solar core by inversion of the mode frequencies. In this paper we investigate how the changes in frequencies of low-degree modes obtained by fitting symmetric and asymmetric peak profiles change the inferred properties of the solar core. We use data obtained by the Global Oscillations at Low Frequencies (GOLF) project on board the SoHO spacecraft. Two different solar models and inversion procedures are used to invert the data to determine the sound speed in the solar core. We find that for a given set of modes no significant difference in the inferred sound-speed results from taking asymmetry into account when fitting the low-degree modes.
Asymmetry reversal in solar acoustic modes
The power spectra of solar acoustic modes are asymmetric, with velocity having more power on the low frequency side of the peak and intensity having more power on the high frequency side. This effect exists in both observations and simulations, and it is believed to be caused by the correlated background noise. We study the temperature near the solar surface by means of a 3D hydrodynamic simulation of convection with a detailed treatment of radiation. The temperature spectrum at optical depth τ cont = 1 has opposite asymmetry to the velocity spectrum, whereas the temperature measured at a fixed geometrical depth, corresponding to < τ cont >= 1, has the same asymmetry as velocity. We believe that the asymmetry reversal in temperature at τ cont = 1 (and therefore in intensity) occurs partly because of the radiative transfer effects. High temperature sensitivity of the opacity suppresses temperature fluctuations on opposite sides of the mode peaks differently, thus causing the asymmetry reversal.
Inferences on the solar envelope with high-degree modes
Astronomy and Astrophysics, 2002
We investigate the structure of the Sun by helioseismic inversion of a set of p-mode frequencies which includes new precise observations of modes with high degree (l < 1000) obtained from the MDI instrument on the SOHO satellite (Rhodes et al. 1998). Such data have the potential to improve the resolution of the solar structure in the near-surface region, to provide detailed tests of the equation of state and constrain the envelope helium abundance. In order to suppress the uncertainties in the treatment of the surface layers in helioseismic inversion procedures, we introduce here the use of a new surface term, developed on the basis of higher-order asymptotic theory of acoustic modes and suitable for the handling of high-degree mode frequencies.
Variation of the low-degree solar acoustic mode parameters over the solar cycle
2002
VIRGO/SPM is a helioseismic sunphotometer on board SOHO that observes the diskintegrated sunlight irradiance at three different colors (red, green, and blue). The data obtained for SPM since the beginning of the SOHO mission, April 1996, to March 2001 have been used to study the differences of the p-mode parameters during the solar activity cycle. These time series have been divided in sub-series of 100 days, transformed to power spectra and averaged in sets of three to yield a total number of six averaged power spectra (around one per year). A new way of analyzing the power spectrum has been applied to the six power spectra of each color; it consists of fitting the whole p-mode spectrum at once with a unique background. The results for the frequencies, line widths, power, mode energy, energy rate fed in the mode and splittings along the activity cycle are found, compared and discussed.