Oscillations of the sun's chromosphere. II - H-alpha line centre and wing filtergram time sequences

Details Oscillations of the sun's chromosphere. II - H-alpha line centre and wing filtergram time sequences Oscillations of the sun's chromosphere. II - H-alpha line centre and wing filtergram time sequences

Mar 1985

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985a%26a...144..443k&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 144, no. 2, March 1985, p. 443-451.

Astronomy and Astrophysics

Astrophysics

33

Chromosphere, Filtergrams, H Alpha Line, Solar Atmosphere, Solar Oscillations, Brightness Distribution, Fourier Analysis, Phase Shift, Power Spectra, Spectrum Analysis, Time Series Analysis

Scientific paper

In order to investigate the dynamics of the solar chromosphere we perform a Fourier analysis of time sequences (total duration 128 min) of Hα photographic filtergrams taken simultaneously at disc centre in line centre and ±0.5 Å from the line centre. The results are: (i) At low frequencies (periods >450 s) the brightness fluctuations are caused by the temporal evolution of Hα structures at the boundaries of the chromospheric network. We observe that much of the coarse chromospheric structure survives the 128 min time span. We derive lifetimes of 2-8 min for the small-scale structure and 5-20 min for the larger structures. (ii) The modal structure of the 5 min oscillation is clearly visible in the power spectra of the three filtergram. The fundamental (f) mode can be followed to high horizontal wavenumbers kh ≍3.7 Mm-1 and follows the expected relationship ω2f = gkh, where g is the surface gravity. (iii) A chromospheric resonant mode cannot be found in the k - ω plane. (iv) The higher resonant p modes reach into the acoustic wave domain (periods T ≍ 150 s) and thus require the transition zone as the upper reflecting layer. (v) We find no evidence for internal gravity waves. (vi) From a coherence and phase analysis we conclude that the brightness fluctuations of the chromospheric structure seen in -0.5 Å lead those in +0.5 Å by 2 min at kh =2 Mm -1 and by 4 min at kh =0.5 Mm-1. (vii) From the same coherence and phase analysis we can identify acoustic waves in the solar atmosphere with periods as short as 80 s. They possess as much power as the 5 min oscillations and are seen better outside the chromospheric network than within the network. (viii) The phase difference between intensity in Hα line centre and velocity, constructed from the two Hα wing filtergrams, decreases from about 90° at low frequencies and high wavenumbers to 0° at high frequencies and low wavenumbers. Tentatively we interpret this as a change from mainly standing waves for the low order p modes to mainly upward propagating waves in the acoustic wave domain. Acoustic waves are thus candidates for the heating process of the inter-network regions.

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