Evidence for wave harmonics in cool loops

Abstract

Aims.Using temporal series image data from the Coronal Diagnostic Spectrometer (CDS) on SOHO, we study oscillations found in intensity (radiance) measurements from the transition region O v 629 line with the aim of finding evidence of wave harmonics in cool loops. Methods.Using standard Fourier techniques, together with a pre-whitening method, we measure all statistically significant frequencies present in oscillations found in flaring active region loops at the temperature of O v. By measuring the distances travelled by three Propagating Disturbances (PDs), estimates of their propagation speeds are calculated. Results.Evidence is found for harmonics in the measured oscillations suggesting the presence of standing waves in the loops. We find that PDs in cool O v active region loops have sub-sonic velocities of between 12–24 km s-1, suggesting they are moved either by slow mode (acoustic or slow magnetoacoustic) wave propagation or by bulk flows along the loops triggered by the flaring that also produces the PDs. Conclusions.We find evidence that standing waves are present in flaring cool transition region loops. This evidence comes in the form of oscillations showing harmonic frequencies that match those expected for standing fast kink waves.