High frequency and high wavenumber solar oscillations: detection of chromospheric modes
Abstract
We determine the frequencies of solar oscillations covering a wide range of degree (100 < l < 4000) and frequency (1.5 < \nu < 10 mHz) using the ring diagram technique applied to power spectra obtained from MDI (Michelson Doppler Imager) data. The chromospheric g-mode ridge is clearly visible in these power spectra at frequencies of 1.5--3.5 mHz. This ridge is located close to the expected position for chromospheric g-modes and has the correct shape in the wavenumber-frequency (or l-\nu) diagram. The chromospheric p-modes appear as ridges connecting the normal p-mode ridges quite similar to the expected avoided crossings in theoretical chromospheric models. Although we cannot detect all the expected avoided crossings in the power spectra, the behavior of frequency separation between adjacent modes of the same degree (\nu_{n+1,l}-\nu_{n,l}) indicate that such features may be present between all p-mode ridges at frequencies of 5--6 mHz. The f-mode ridge extends up to l of about 3000, where the line width becomes very large, implying a damping time which is comparable to the time period. The frequencies of high degree f-modes are significantly different from those given by the simple dispersion relation \omega^2=gk.Keywords
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