Initiation of hot coronal loop oscillations: Spectral features

Abstract

We explore the excitation of hot loop oscillations observed with the SUMER spectrograph on SOHO by analysing and spectral line profiles in the initial phase of the events. We investigate all 54 Doppler shift oscillations in 27 flare-like events, whose physical parameters have been measured so far. In nearly 50% of the cases, the spectral evolution reveals the presence of two spectral components, one of them almost undisturbed, the other highly shifted. We find that the shifted component reaches maximum Doppler shift (on the order of 100-300 km s-1) and peak intensity almost simultaneously. The velocity amplitude of the shifted component has no correlation with the oscillation amplitudes. These features imply that in these events the initial shifts are not caused by the locally oscillating plasma (or waves), but most likely by a pulse of hot plasma travelling along the loop through the slit position. This interpretation is also supported by several examples showing that standing slow mode waves are set up immediately after the initial line shift pulse (standing slow mode waves are inferred from the 1/4-period phase relationship between the velocity and intensity oscillations). We re-measure the physical parameters of the 54 Doppler oscillations by fitting the time profiles excluding the first peak, and find that the periods are almost unchanged, damping times are shorter by 5%, and amplitudes are smaller by 37% than measured when the first peak is included. We also measure the velocity of the net (background) flow during the oscillations, which is found to be nearly zero. Our result of initial hot flows supports the model of single footpoint (asymmetric) excitation, but contradicts chromospheric evaporation as the trigger.