Chromospheric and Transition Region Internetwork Oscillations: A Signature of Upward‐propagating Waves

Abstract

We analyze spectral time series obtained on 1997 April 25 with the SUMER instrument on SOHO. Line and continuum data near 1037 were acquired at a cadence of 16 s. This spectral region was A? chosen because it contains strong emission lines of C II, formed in the upper chromosphere/lower tran- sition region ; O VI, formed in the upper transition region ; and neighboring continuum emission formed in the middle chromosphere. The time series reveal oscillatory behavior. Subsonic (3È5kms~1 amplitude) Doppler velocity oscillations in the C II and O VI lines, with periods between 120 and 200 s, are prominent. They are seen as large-scale coherent oscillations, typically of 3È7 Mm length scale, occasionally approaching 15 Mm, visible most clearly in internetwork regions. The Doppler velocity oscillations are related to oscillations seen in the continuum intensity, which precede upward velocity in C II by 40È60 s. The C II Doppler shift precedes the O VI Doppler shift by 3È10 s. Oscillations are also present in the line intensities, but the intensity amplitudes associated with the oscillations are small. The continuum intensity precedes the C II intensity by 30È50 s. Phase di†erence analysis shows that there is a preponderance of upward-propagating waves in the upper chromosphere that drive an oscillation in the transition region plasma, thus extending the evidence for upward-propagating waves from the photo- sphere up to the base of the corona.