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Spectroscopic measurements of AR NOAA 10871, obtained with the ExtremeUltraviolet Normal Incidence Spectrograph (EUNIS) sounding rocket instrument on2006 April 12, reveal velocity oscillations in the He II 303.8 Angstromemission line formed at T ~ 50000K. The oscillations appear to arise in abright active-region loop arcade about 25" wide which crosses the EUNIS slit.The period of these transition region oscillations is 26 +/- 4 s, coupled witha velocity amplitude of +/- 10 km/s, detected over 4 complete cycles. Similaroscillations are observed in lines formed at temperatures up to T ~ 4K,but we find no evidence for the coupling of these velocity oscillations withcorresponding phenomena in the corona. We interpret the detected oscillationsas originating from an almost purely adiabatic plasma, and infer that they aregenerated by the resonant transmission of MHD waves through the loweractive-region atmospheres. Through use of seismological techniques, weestablish that the observed velocity oscillations display wave properties mostcharacteristic of fast-body global sausage modes.

Transition-Region Velocity Oscillations Observed by EUNIS-06