A statistical study of wave propagation in coronal holes

Abstract

Aims.To find evidence for propagating magnetoacoustic waves in equatorial and polar coronal hole locations. Methods: Using temporal series data from the Coronal Diagnostic Spectrometer (CDS) on SOHO, we study oscillations found in radiant flux and velocity measurements from transition region (O V 629) and coronal lines (Mg X 624, Si XII 520). We use Fourier techniques to measure phase delays between flux ("intensity") oscillations and between velocity oscillations of different transition region-corona and corona-corona line pairs. We also measure the phase delays between flux and velocity oscillations (I-V) in the three spectral lines investigated. Results: We find outwardly propagating slow magnetoacoustic waves in both of the coronal hole regions studied. The propagation speeds are found to be lower than those found in off-limb locations. We find evidence for a resonant cavity or "Doppler" effect, whereby the measured phases are present at fixed integer intervals of f/4 (90° of phase) and 3f/8 (135° of phase) instead of the expected interval of f or 360°. We find, in addition, from the I-V phases, evidence for standing waves at coronal temperatures in the lines of Mg X 624 and Si XII 520. Correlations are found between the locations where the phases are measured and localised brightenings in both equatorial and polar coronal holes. This suggests that the slow magnetoacoustic waves are originating preferentially from bright areas within the coronal holes which we take to be the locations of concentrated magnetic field (loops, bright points). Finally, we find evidence that in these bright regions along the slit, the measured phases tend to occur at a spectrum of frequencies, perhaps suggesting the presence of discrete propagating wave packets. Conclusions: .We conclude that propagating slow magnetoacoustic waves are present in equatorial and polar coronal hole locations and that they occur preferentially in bright regions that are associated with magnetic field concentrations in the form of loops or bright points. In addition, we conclude that some resonant cavity effect is affecting the propagating waves, perhaps resulting in the standing waves that are found at coronal temperatures.