Transmission and conversion of magnetoacoustic waves on the magnetic canopy in a quiet Sun region

Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, National Observatory of Athens, 15236 Penteli, Greece
e-mail: jkonto@noa.gr; georgia@noa.gr; kostas@noa.gr

Received: 14 April 2014
Accepted: 30 May 2014

Abstract

Context. We present evidence for the conversion and transmission of wave modes on the magnetic flux tubes that constitute mottles and form the magnetic canopy in a quiet Sun region.

Aims. Our aim is to highlight the details and the key parameters of the mechanism that produces power halos and magnetic shadows around the magnetic network observed in Hα.

Methods. We use our previous calculations of the magnetic field vector and the height of the magnetic canopy, and based on simple assumptions, we determine the turning height, i.e., the height at which the fast magnetoacoustic waves reflect at the chromosphere. We compare the variation of 3, 5, and 7 min power in the magnetic shadow and the power halo with the results of a two-dimensional model on mode conversion and transmission. The key parameter of the model is the attack angle, which is related to the inclination of the magnetic field vector at the canopy height. Our analysis takes also into account that 1) there are projection effects on the propagation of waves; 2) the magnetic canopy and the turning height are curved layers; 3) waves with periods longer than 3 min only reach the chromosphere in the presence of inclined magnetic fields (ramp effect); 4) mottles in Hα are canopy structures; and 5) the wings of Hα contain mixed signal from low- and high-β plasma.

Results. The dependence of the measured power on the attack angle follows the anticipated by the two-dimensional model very well. Long-period slow waves are channeled to the upper chromospheric layers following the magnetic field lines of mottles, while short-period fast waves penetrate the magnetic canopy and are reflected back higher, at the turning height.

Conclusions. Although both magnetoacoustic modes contribute to velocity signals, making the interpretation of observations a challenging task, we conclude that conversion and transmission of the acoustic waves into fast and slow magnetoacoustic waves are responsible for forming power halos and magnetic shadows in the quiet Sun region.