Acoustic waves in the solar atmosphere at high spatial resolution

¹ Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany e-mail: nbello@kis.uni-freiburg.de
² Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany e-mail: kneer@astro.physik.uni-goettingen.de; olok@astro.physik.uni-goettingen.de
³ Central Astronomical Observatory of the Russian Academy of Sciences, Pulkovskoye chaussee 65/1, 196140 St. Petersburg, Russia

Received: 4 April 2009
Accepted: 10 August 2009

Abstract

Aims. The energy supply for the radiative losses of the quiet solar chromosphere is studied. On the basis of high spatial resolution data, we investigate the amount of energy flux carried by acoustic waves in the solar photosphere.

Methods. Time sequences from quiet Sun disc centre were obtained with the “Göttingen” Fabry-Perot spectrometer at the Vacuum Tower Telescope, Observatorio del Teide/Tenerife, in the non-magnetic Fe i 5576 Å line. The data were reconstructed with speckle methods. The velocity and intensity fluctuations at line minimum were subjected to Fourier and wavelet analyses. The energy fluxes at frequencies higher than the acoustic cutoff frequency (period U ≈ 190 s) were corrected for the transmission of the solar atmosphere, which reduces the signal from short-period waves.

Results. Both Fourier and wavelet analysis give an amount of energy flux of ~3000 W m^-2 at a height  km. Approximately 2/3 of it is carried by waves in the 5-10 mHz range, and 1/3 in the 10-20 mHz band. Extrapolation of the flux spectra gives an energy flux of 230-400 W m^-2 at frequencies ν > 20 mHz. We find that the waves occur predominantly above inter-granular areas.

Conclusions. We conclude that the acoustic flux in waves with periods shorter than the acoustic cutoff period can contribute to the basal heating of the solar chromosphere, in addition to the atmospheric gravity waves found recently.