Astron. Astrophys. 324, 704-712 (1997)

Dynamics of the solar photosphere

I. Two-dimensional spectroscopy of mesoscale phenomena

Th. Strausand D. Bonaccini <sup>1, 2

1</sup> Osservatorio Astronomico di Capodimonte, Via Moiariello 16, I-80131 Napoli, Italy
² European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching, Germany

Received 28 May 1996 / Accepted 10 October 1996

Abstract

We address the "mesogranulation" phenomenon by analyzing a spectral time series, taken at disk center with a two-dimensional spectroscopy device and covering a period of 4 hours. This tunable device was composed by a Fabry-Perot interferometer mounted in tandem with an Universal Birefringent Filter (UBF). We calculate spatial power spectra, spatio-temporal power, phase difference and coherence spectra at different low photospheric levels, in order to investigate the nature of the mesoscale phenomena.

At the lowest levels, mesostructures appear as a part of an extended distribution of granular sizes without further distinction from granulation. Here, the plasma flows are driven by convection. On the other hand, a different mesoscale phenomenon emerges at levels as high as approximately 200-300 km above , at medium spatial () and medium temporal () frequencies. This phenomenon is distinct from convection by its non-convective phase difference values (, ) and by its different propagation character (almost horizontal propagation).

By these properties, the mesoscale phenomena in the higher photosphere can be identified as internal gravity waves in the solar atmosphere.

Key words: Sun: atmosphere, granulation, photosphere, oscillations convection hydrodynamics

Send offprint requests to: Th. Straus

Contents

• 1. Introduction
• 2. Observation and data reduction
• 3. Results and discussion
  • 3.1. Power spectra
  • 3.2. Phase difference spectra
  • 3.3. Fourier filtered time series
• 4. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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