Astron. Astrophys. 327, 365-376 (1997)

Randomly sampling the chromospheric peak power distribution

J.G. Doyle ¹, G.H.J. van den Oord ² and E. O'Shea <sup>1

1</sup> Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland (email: jgd@star.arm.ac.uk; eos@star.arm.ac.uk)
² Sterrekundig Instituut, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands (G.H.J.vandenOord@fys.ruu.nl)

Received 1 April 1997 / Accepted 16 June 1997

Abstract

We have analyzed the UV continuum light curves of 2535 pixels obtained during fifteen separate observations of active regions with the Ultraviolet Spectrometer and Polarimeter (UVSP) on board the solar maximum mission in 1989. Specifically we have looked for periodicities. In the power spectra of 738 light curves (i.e., 29%), evidence was found for periodicities at multiple frequencies. For each power spectrum, containing significant power, we determined the frequency at which the maximum power is found. The distribution , which describes the number of pixels having maximum power at frequency , is strongly concentrated in the 2-5 mHz band with a distinct maximum at 3-3.5 mHz. No pixels had their maximum power above 10 mHz and only a few had their maximum power in the 5-10 mHz range. The oscillations in the 2-5 mHz range are probably related to evanescent acoustic waves driven by the photospheric five minute oscillations. In the 2-5 mHz band, the distribution resembles very much the power spectra as have been observed near the temperature minimum. This suggests that the photospheric power distribution can be interpreted as a probability distribution for finding a given frequency in the UV continuum light curves. In the 2-5 mHz band the spread of the maximum powers and the spread of the count rates, at any frequency, is much larger than above 5 mHz where both are relatively constant. No clear correlation is found between the maximum power and the count rate in a pixel. Furthermore, no evidence is found for emission from the shocks which have recently been invoked to explain the formation of Ca II grains. Whether this is due to temperature effects is unclear. Our general procedure adopted to estimate the confidence level in the power spectrum of a light curve containing photon noise can be equally applied to SUMER and CDS data from SOHO.

Key words: methods: data analysis Sun: chromosphere Sun: oscillations Sun: UV radiation

Send offprint requests to: J.G. Doyle

Contents

• 1. Introduction
• 2. Observational data
• 3. The fourier transform
• 4. Results
• 5. Discussion and conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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