Height distribution of the power of 3-min oscillations over sunspots

¹ Institute of Solar-Terrestrial Physics, Lermontov st., 126a, PO Box 291, 664033 Irkutsk, Russia
e-mail: kobanov@iszf.irk.ru
² Physics Department, University of Warwick, Coventry CV4 7AL, UK

Received: 23 October 2009
Accepted: 28 September 2010

Abstract

Context. The height structure of 3-min oscillations over sunspots is studied in the context of the recently discovered effect of height inversion: over the umbra, the spatial location of the maximum of chromospheric 3-min oscillation power corresponds to the relative decrease in the power of photospheric oscillations.

Aims. We investigate whether the height inversion of the power of 3-min oscillations is a common feature of the spatial structure of the oscillations for the majority of sunspots.

Methods. Spectrogram sequences of Hα 6563 Å and Fe i 6569 Å over sunspots, acquired with very high cadency (about 2 s or better) are obtained. The distribution of the oscillation power of the line-of-sight velocity signal is studied by using methods of wavelet frequency filtration and Fourier analysis.

Results. The effect of the height inversion is found in 9 of 11 analyzed active regions. The interpretation of this effect is possibly connected to both the decrease in the level of photosphere in sunspot umbrae and the magnetic field topology.