Temporal variability in the Doppler-shift of solar transition region lines

Abstract

High cadence datasets taken in C III 977 Å, O VI 1032 Å and Ne VIII 720 Å were analysed in an effort to establish the extent of the variability in the Doppler-shift of typical mid-transition region lines. The shortest time-scale variability seems to occur in the network boundary regions where the line-shift can vary by 7-8 km s^-1 in less than 1 min. The internetwork region also shows variability although this tends to be longer lived, ~ 2-3 min. The average line-shift in C III is a red-shift which ranges from ~ 2 km s^-1 to ~ 20 km s^-1 with an average value for all regions selected being around 10 km s^-1 in very good agreement with that derived by others. The red-shift values indicate a clear difference between network and internetwork regions, with the largest red-shift being present at the network boundary. For O VI, this gives an average red-shift ranging from 5 to 10 km s^-1. For Ne VIII, there is a 13 km s^-1 difference between internetwork and bright network plasma with the bright network being more red-shifted. This could imply that the bright network regions are dominated by spicule down-flow.\ In the second part we present results from 2-dimensional (2D) dissipative magnetohydrodynamic (MHD) simulations of the response of the solar transition region to micro-scale energy depositions. A variety of temperatures at which the energy deposition takes place as well as the amount of energy deposited are examined. This work is a continuation of previous related simulations where small-scale energy depositions were modelled in 1D hydrodynamics. The observable consequences of such transient events are then computed for three transition region lines, namely C IV 1548 Å, O VI 1032 Å, and Ne VIII 770 Å, under the consideration of non-equilibrium ionization.