Acoustic power absorption and enhancement generated by slow and fast MHD waves

Evidence of solar cycle velocity/intensity amplitude changes consistent with the mode conversion theory

¹ PMOD/WRC Physikalisch-Meteorologisches Observatorium Davos-World Radiation Center, 7260 Davos Dorf, Switzerland e-mail: [rosaria.simoniello;wolfgang.finsterle]@pmodwrc.ch
² Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot; CEA, IRFU, SAp, centre de Saclay, 91191 Gif-sur-Yvette, France
³ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot; 5 place Jules Janssen, 92190 Meudon, France e-mail: rgarcia@cea.fr
⁴ IAC, Instituto de Astrofisíca de Canarias, 38205 La Laguna, Tenerife, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain e-mail: [salabert;ajm]@iac.es
⁶ School of Physics and Astronomy, University of Birmingham, Edgbaston, B15 2TT Birmingham, UK e-mail: ype@bison.ph.bham.ac.uk
⁷ Max-Planck-Institute for Solar System Research, Max-Planck-Strasse 2, 37197 Katlenburg-Lindau, Germany e-mail: schunker@mps.mpg.de

Received: 7 August 2009
Accepted: 1 March 2010

Abstract

We used long duration, high quality, unresolved (Sun-as-a star) observations collected by the ground based network BiSON and by the instruments GOLF and VIRGO on board the ESA/NASA SOHO satellite to search for solar-cycle-related changes in mode characteristics in velocity and continuum intensity for the frequency range between 2.5 mHz 6.8 mHz. Over the ascending phase of solar cycle 23 we found a suppression in the p-mode amplitudes both in the velocity and intensity data between 2.5 mHz 4.5 mHz with a maximum suppression for frequencies in the range between 2.5 mHz 3.5 mHz. The size of the amplitude suppression is 13 ± 2 per cent for the velocity and 9 ± 2 per cent for the intensity observations. Over the range of 4.5 mHz 5.5 mHz the findings hint within the errors to a null change both in the velocity and intensity amplitudes. At still higher frequencies, in the so called High-frequency Interference Peaks (HIPs) between 5.8 mHz 6.8  mHz, we found an enhancement in the velocity amplitudes with the maximum 36 ± 7 per cent occurring for 6.3 mHz 6.8 mHz. However, in intensity observations we found a rather smaller enhancement of about 5 ± 2 per cent in the same interval. There is evidence that the frequency dependence of solar-cycle velocity amplitude changes is consistent with the theory behind the mode conversion of acoustic waves in a non-vertical magnetic field, but there are some problems with the intensity data, which may be due to the height in the solar atmosphere at which the VIRGO data are taken.