Oscillations of the Sun's chromosphere. VI. K grains, resonances, and gravity waves

Details Oscillations of the Sun's chromosphere. VI. K grains, resonances, and gravity waves Oscillations of the Sun's chromosphere. VI. K grains, resonances, and gravity waves

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...274..584k&link_type=abstract

Astronomy and Astrophysics, Vol. 274, p. 584 (1993)

Astronomy and Astrophysics

Astrophysics

46

Sun: Chromosphere, Sun: Oscillations

Scientific paper

We present observations of simultaneous filtergram time sequences in Mg b2, Ca K and Hα obtained from quiet Sun disc centre with the Vacuum Tower Telescope at Observatorio del Teide, Tenerife. Fourier analyses are performed to obtain power, coherence and phase spectra in the k-ω plane. There, the dominant features are the wellknown ridges of the 5 min resonant modes. Yet in the chromosphere the ridges extend to high wavenumbers (wavelengths ≍ 1.3 Mm) and to high frequencies (periods ≍ 105 s). Neither the famous chromospheric "3 min" oscillations nor an oscillation at the acoustic cutoff frequency (period 210 s) appear exceptionally pronounced. The signature of gravity waves is indicated from phase relations.
We distinguish between the behaviour in the interior of the chromospheric network and on the boundary. The network boundary behaves less oscillatory than the interior. In snapshots of chromospheric intensities the K grains (Beckers 1964), or, synonymously the bright cell points, appear in the cell interior. They represent the phases of high temperature of a wave field with partly resonant and coherent properties. (We take intensity fluctuations as proxies for temperature fluctuations.) The waves are only partly upward propagating p-modes with a multitude of eigenvalues in frequency and wavenumber, like the subphotospheric p-modes. We suggest that an excitation mechanism acts within the chromosphere itself to drive the waves. This could explain the phase relations between intensity and velocity oscillations.

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