On the statistical detection of propagating waves in polar coronal holes

TL;DR

This study analyzes spectral data from a solar coronal hole to detect and characterize propagating waves, revealing slow magnetoacoustic waves with different behaviors in network and internetwork regions.

Contribution

It introduces a statistical method to detect propagating waves in solar coronal data and distinguishes wave propagation characteristics in different regions.

Findings

Detected ~25 min period compressional waves.

Measured subsonic propagation speeds indicating slow magnetoacoustic waves.

Identified differences in wave propagation between network and internetwork regions.

Abstract

Waves are important for the heating of the solar corona and the acceleration of the solar wind. We have examined a long spectral time series of a northern coronal hole observed on the 20th October 1996, with the SUMER spectrometer onboard SoHO. The observations were obtained in a transition region N IV 765 A line and in a low coronal Ne VIII 770 A line. Our observations indicate the presence of compressional waves with periods of ~25 min. Using Fourier techniques, we measured the phase delays between intensity as well as velocity oscillations in the two chosen lines. From this we are able to measure the travel time of the propagating oscillations and, hence, the propagation speeds of the waves producing the oscillations. We found that there is a difference in the nature of the propagation in bright ('network') and dark ('internetwork') regions with the latter sometimes showing evidence for downwardly propagating waves that is not seen in the former. As, in all cases, the measured propagation speeds are subsonic, we concluded that the detected waves are slow magnetoacoustic in nature.