Global Observations of Chromospheric Oscillations

Details Global Observations of Chromospheric Oscillations Global Observations of Chromospheric Oscillations

May 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...180.1705h&link_type=abstract

American Astronomical Society, 180th AAS Meeting, #17.05; Bulletin of the American Astronomical Society, Vol. 24, p.753

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Nearly 30 years ago, observations of Doppler velocities from small areas of the solar disk showed both ~ 300-s period oscillations of the photosphere and ~ 200-s period oscillations of the chromosphere. Theoretical models of the solar atmosphere suggest that the 200-s period oscillations may be modes resulting from trapping of acoustic waves in a chromospheric acoustic cavity, or they may be excitation of the acoustic cutoff frequency, or a resonance in the atmospheric frequency transmission. Since the advent of helioseismology there have been many searches for the 200-s oscillations in global observations but so far without clear confirmation of the early observations. We analyzed full-disk observations made at South Pole in 1988 using the CaII K-line and Tucson observations using the Hα line. Global l-nu spectra of these observations (l <=150, nu <=16 mHz) show the well-known p-modes and their high-frequency extensions. The background power shows nearly l-independent features that we associate with non-periodic fluctuations having characteristic decay times of 225 and 90 sec, and with periodic fluctuations having peak power at about 180-s period. We identify the 180-s feature with chromospheric oscillations previously detected in small area observations. In the Hα line observations, it is the strongest feature of the oscillation power spectrum. It is not clear from the observations if the 180-s feature is a signature of trapped chromospheric modes or is a manifestation of one of the other physical processes which have been proposed.

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