Variations of the Low l Solar Acoustic Spectrum Correlated with the Activity Cycle
Solar cycle variation of the frequencies and of the power of solar acoustic oscillations are investigated. Integrated sunlight data from 1977 to 1988 obtained at the Observatorio del Teide (Izaíia, Tenerife), using a resonant scattering spectrophotometer, is analyzed in 60 day time strings and their power spectra are calculated from 2 to 3.8 mHz. To study the frequency variation, each power spectrum is cross-correlated with the one corresponding to the 1981 series and the shifts of the centroids of the cross-correlation peaks are calculated. The results show a clear variation in frequency of the cross-correlation peaks of −0.37 ± 0.04 µHz peak to peak as solar activity cycle goes from maximum to minimum. Moreover, this effect is found to depend on the 1 value of the modes, being absent for l = 0 and of 0.42 ± 0.06 µHz for l = 1. These results can be interpreted as an amplitude modulation between modes of the same multiplet, probably as a consequence of the action of strong magnetic fields. As low l modes penetrate deeply into the Sun’s interior, these observations suggest changes in its structure correlated with the solar activity cycle. When the power of the modes is calculated, using the same series as before, and its change along the solar cycle is studied, a variation of ∼ 40% is found, the power being higher when solar activity is at its minimum. If this effect is independent of the l value of the p-modes, the results can be interpreted in terms of a change in the efficiency of the excitation mechanism of such modes. Indeed, if turbulent convection is such a mechanism, a change in the characteristic size of the granulation would account for the observed effect. Alternatively, another explanation could be a selective change in the efficiency of the excitation and/or damping mechanisms of the l ≤ 3 modes in front of other l value modes.
KeywordsPower Spectrum Solar Activity Solar Cycle Excitation Mechanism Solar Activity Cycle
Unable to display preview. Download preview PDF.
- Elsworth Y., Isaak G.R., Jefferies S.M., McLeod C.P., New R., Pallé P.L., Rgulo C., Roca Cortés T.,(1989), ’Linewidth of low degree acoustic modes of the Sun’. M.N.R.A.S., (in press).Google Scholar
- Jiménez J.A., Pallé P.L., Pérez J.C., Régulo C., Roca Cortés T., Isaak G.R., McLeod C.P., van der Raay H.B.,(1988) in J. Christensen-Dalsgaard and S. Frandsen (eds.) éAdvances in Helio and Asteroseismologyé, Reidel, 205.Google Scholar
- Libbrecht K., (1988). ‘The excitation and damping of solar p-modes’.in E.J. Rolfe (ed.) ’Seismology of the sun and sun-like stars’, ESA SP-286, 3Google Scholar
- Muller R. and Roudier T., (1984). ‘Variability of the structure of the granulation over the solar activity cycle’ in ‘The Hydromagnetics of the Sun’, ESA SP-220, 51.Google Scholar
- Pallé P.L., Régulo C. and Roca Cortés T.,(1989), ‘Solar cycle induced variations of the low 1 solar acoustic spectrum’, Ast. & Ap. (in press).Google Scholar
- Pallé P.L., Pérez Hernández F., Roca Cortés T., Isaak G.R., (1989), ‘Observation of solar p-modes with i < 5’. Astron. & Astrophys., (in press)Google Scholar