Slow magnetoacoustic waves in coronal loops: EIT and TRACE

On May 13, 1998 the EIT (Extreme ultraviolet Imaging Telescope) on board of SoHO (Solar and Heliospheric Observatory) and TRACE (Transition Region And Coronal Explorer) instruments produced simultaneous high cadence image seq uences of the same active region (AR 8218). TRACE achieved a 25 s cadence i n the Fe IX (171 Angstrom) bandpass while EIT achieved a 15 s cadence (oper ating in "shutterless mode", SoHO JOP 80) in the Fe XII (195 Angstrom) band pass. These high cadence observations in two complementary wavelengths have revealed the existence of weak transient disturbances in an extended coron al loop system. These propagating disturbances (PDs) seem to be a common ph enomenon in this part of the active region. The disturbances originate from small scale brightenings at the footpoints of the loops and propagate alon g the loops. The projected propagation speeds roughly vary between 65 and 1 50 km s(-1) for both instruments which is close to and below the expected s ound speed in the coronal loops. The measured slow magnetoacoustic propagat ion speeds seem to suggest that the transients are sound (or slow) wave dis turbances. This work differs from previous studies in the sense that it is based on a multi-wavelength observation of an entire loop bundle at high ca dence by two EUV imagers. The observation of sound waves along the same pat h shows that they propagate along the same loop, suggesting that loops cont ain sharp temperature gradients and consist of either concentric shells or thin loop threads, at different temperatures.