Observations and mode identification of sausage waves in a magnetic pore

Aims. We aim to determine the phase speed of an oscillation in a magnetic pore using only intensity images at one height. The observations were obtained using the CRisp Imaging SpectroPolarimeter at the Swedisch 1-m Solar Telescope and show variations in both cross-sectional area and intensity in a magnetic pore. Methods. We have designed and tested an observational method to extract the wave parameters that are important for seismology. We modelled the magnetic pore as a straight cylinder with a uniform plasma both inside and outside the flux tube and identify different wave modes. Using analytic expressions, we are able to distinguish between fast and slow modes and obtain the phase speed of the oscillations. Results. We found that the observed oscillations are slow modes with a phase speed around 5 km s-1. We also have strong evidence that the oscillations are standing harmonics