Slow magnetoacoustic waves in coronal loops: EIT vs TRACE

On May 13, 1998 the EIT (Extreme-Ultraviolet Imaging Telescope) and TRACE (Transition Region And Coronal Explorer) instruments produced simultaneous high cadence image sequences of the same active region (AR 8218). TRACE achieved a 25 sec cadence in the FeIX/X (171 Angstrom) bandpass while EIT achieved a 15 sec cadence (operating in 'shutterless mode', SOHO JOP 80) in the FeXII (195 Angstrom) bandpass. These high cadence observations in two complementary wavelengths have revealed the existence of weak transient disturbances in an extended coronal loop system, These propagating disturbances (PDs) seem to be a common phenomenon in this part of the active region, The disturbances originate from small scale brightenings at the footpoints of the loops and propagate along the loops. The apparent propagation speeds roughly vary between 65 and 150 km s(-1) which is close to the expected sound speed of the coronal loops, The measured propagation speeds seem to suggest that the transients are sound (or slow) wave disturbances.