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 A were acquired at a cadence of 16 s. This spectral region was chosen because it contains strong emission lines of C II, formed in the upper chromosphere/lower transition 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 (3-5 km s-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 3-7 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 40-60 s. The C II Doppler shift precedes the O VI Doppler shift by 3-10 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 30-50 s. Phase difference 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 photosphere up to the base of the corona.