Rapid intensity and velocity variations in solar transition region lines

We have obtained short exposure (3 s) time series of strong upper chromospheric and transition region emission lines from the quiet Sun with the SUMER instrument onboard SOHO during two 1 hour periods in 1996. With a Nyqvist frequency of 167 mHz and relatively high count rates the dataset is uniquely suited for searching for high frequency variations in intensity and Doppler velocity. From Monte-Carlo experiments taking into account the photon-counting statistics we estimate our detection limit to correspond to a wave-packet of four periods coherent over 3′′ with a Doppler-shift amplitude of 2.5km s−1 in the darkest internetwork areas observed in C iii. In the network the detection limit is estimated to be 1.5km s−1. Above 50 mHz we detect wave-packet amplitudes above 3km s−1 less than 0.5% of the time. Between 20 and 50 mHz we detect some wave-packets with a typical duration of four periods and amplitudes up to 8km s−1. At any given internetwork location these wave-packets are present 1% of the time. In the 10–20 mHz range we see amplitudes above 3km s−1 12% of the time. At lower frequencies our dataset is consistent with other SUMER datasets reported in the literature. The chromospheric 3–7 mHz signal is discernible in the line emission. In the internetwork this is the dominant oscillation frequency but higher frequencies (7–10 mHz) are often present and appear coherent in Doppler velocity over large spatial regions (≈ 40”). Wavelet analysis implies that these oscillations have typical durations of 1000s. The network emission also shows a 5 mHz signal but is dominated by low frequency variations (of < 4 mHz) in both intensity and velocity. The oscillations show less power in intensity than in velocity. We find that while both red and blue shifted emission is observed, the transition region lines are on average red shifted between 5–10km s−1 in the network. A net red shift is also found in the internetwork emission but it is smaller (< 4km s−1). The line widths do not differ much between the internetwork and network, the non-thermal line widths increase with increasing temperature of line formation from 30km s−1 for the C ii 1334 Å line to 45km s−1 for the O vi 1032 Å line. By constructing scatterplots of velocity versus intensity we find that in the network a mean redshift is correlated with a high mean intensity. In the Send offprint requests to: V. Hansteen internetwork regions we do not find any correlation between the intensity and the Doppler velocity.