Short-period waves in the solar atmosphere

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  • Astron. Nachr./AN 324, No. 4, 356 (2003) / DOI 10.1002/asna.200310126

    Short-period waves in the solar atmosphere

    M. WUNNENBERG, A. ANDJIC and F. KNEERUniversitats-Sternwarte Gottingen, Geismarlandstr. 11, D-37083 Gottingen, Germany

    Received 30 October 2002; accepted 7 January 2003; published online 20 May 2003

    1. Introduction

    The heating of the lower and middle chromosphere may becaused by the following process: Turbulent flow fields inthe convection zone generate upward propagating acousticwaves, which form shocks at about 500 km and dissipate theirenergy. As these waves have short periods (< 100 s) and oc-cur on small scales, we need high spatial and temporal reso-lution for detecting them.

    2. Observations and data processing

    The data for this contribution were taken in August 2000 atsolar disk center with the Gottingen two-dimensional spec-trometer, which is based on two Fabry-Perot interferometers.By scanning through the Fe I 5434 A line a time series of41.25 min with a cadence of 25 s was taken. The line centerof this non-magnetic line is formed at about 600 km.Broadband and narrow-band images were taken simultane-ously in order to allow later reconstruction as described inKrieg et al. (1999). Bisectors were calculated at every pointin the field of view (2713) and then intensity and veloc-ity maps at different bisector heights could be determined.Images of the time series, which belong to the same atmo-spheric height, were correlated and destretched to give 3D-data-boxes (x, y, t).

    Fig. 1. Time slices of broadband images (left) and line center veloc-ities (right).

    Correspondence to: wunnenbe@uni-sw.gwdg.de

    Fig. 1 shows time slices from these boxes, one for the in-tensities of broadband images, where the evolution of gran-ules can be seen and another for the velocities near line cen-ter, obtained from the narrow-band images.For each point (xi, yi) of the 3D-boxes (i.e. one row in thetime slice) a wavelet analysis with Morlet wavelets was per-formed with a code developed by Torrence & Compo (1998).The wavelet power spectra for periods between one and twominutes were calculated.

    3. Results and outlook

    In Fig. 2 temporal velocity fluctuations, measured near linecenter, at one point in the field of view are presented togetherwith the wavelet power spectrum.

    0 10 20 30 40-1.0-0.5

    0.0

    0.5

    1.0

    velo

    city

    [km/

    s]

    0 10 20 30 40time [min]

    1.01.21.41.61.8

    perio

    d [m

    in]

    Fig. 2. Velocity fluctuations (upper panel) and appertaining waveletpower spectrum in the 12 min range (lower panel).

    Thus, we are able to show the existence of short-periodwaves in the solar atmosphere.More and detailed information can be found in Wunnenberget al. (2002).With the new GREGOR telescope better spatial and tempo-ral resolution will be obtained and then we would be able todetect short-period waves with periods at about 20 s.

    References

    Krieg, J., Wunnenberg, M., Kneer, F., Koschinsky, M., Ritter, C.:1999, A&A 343, 983

    Torrence, C., Compo, G.P.: 1998, Bull. Amer. Meteor. Soc. 79, 61(wavelet software is available at URL:http://paos.colorado.edu/research/wavelets)

    Wunnenberg, M., Kneer, F., Hirzberger, J.: 2002, A&A 395, L51

    c2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0004-6337/03/0406-0356 $ 17.50+.50/0