Spectroscopic measurements of AR NOAA 10871 , obtained with the Extreme Ultraviolet Normal Incidence Spectrograph ( EUNIS ) sounding rocket instrument on 2006 April 12 , reveal velocity oscillations in the He II 303.8Å emission line formed at T~ { } \approx~ { } 5 ~ { } \times~ { } 10 ^ { 4 } K. The oscillations appear to arise in a bright active-region loop arcade about 25 ^ { \prime \prime } wide which crosses the EUNIS slit .
The period of these transition region oscillations is 26 \pm 4 s , coupled with a velocity amplitude of \pm~ { } 10 km/s , detected over 4 complete cycles .
Similar oscillations are observed in lines formed at temperatures up to T \approx 4 \times 10 ^ { 5 } K , but we find no evidence for the coupling of these velocity oscillations with corresponding phenomena in the corona .
We interpret the detected oscillations as originating from an almost purely adiabatic plasma , and infer that they are generated by the resonant transmission of MHD waves through the lower active-region atmospheres .
Through use of seismological techniques , we establish that the observed velocity oscillations display wave properties most characteristic of fast-body global sausage modes .