Transition-Region Velocity Oscillations Observed by EUNIS-06

Abstract

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 303.8 Angstrom emission line formed at T ~ 50000K. The oscillations appear to arise in a bright active-region loop arcade about 25" wide which crosses the EUNIS slit. The period of these transition region oscillations is 26 +/- 4 s, coupled with a velocity amplitude of +/- 10 km/s, detected over 4 complete cycles. Similar oscillations are observed in lines formed at temperatures up to T ~ 400000K, 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.

7 pages, 5 figures, accepted into ApJ

References in corpus (4)

• MHD Sausage Mode Oscillations in Coronal Loops
• The Magnetic Structure of Coronal Loops Observed by TRACE
• High-Frequency Oscillations in a Solar Active Region observed with the Rapid Dual Imager
• Twisting Flux Tubes as a cause of Micro-Flaring Activity

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