Magnetoacoustic wave propagation in off-limb polar regions

Abstract

Aims.To find evidence of magnetoacoustic waves in off-limb polar regions. 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 and coronal spectral lines. 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). Results.The phase delays measured between different line pairs, when plotted over a -180$\degr$ to +180$\degr$ range, line up along diagonal lines corresponding to measurable and fixed time delays. The slopes of these diagonal lines suggest the outward propagation of waves. Using the measured time delays, we estimate propagation speeds for the different line pairs that indicate that the waves producing the observed phase delays are magnetoacoustic waves propagating at speeds close to the sound speed. In addition, we find that the phases occur at fixed integer frequencies of f/4 (90$\degr$) and 3f/16 (67.5$\degr$), instead of the expected interval of f (360$\degr$), indicating that a "Doppler effect" is acting on the waves. Conclusions.From I-V measurements, we find evidence for fast magnetoacoustic waves to be predominantly present at coronal temperatures, while at transition region temperatures slow magnetoacoustic waves are more common. We find strong evidence for outwardly propagating slow magnetoacostic waves in off-limb polar regions and an indication that these propagating waves are influenced by some form of resonant cavity through which they pass.