Multi-component structure of solar and stellar transition regions

Emission lines from the solar transition region between the chromosphere and the corona often show a two Gaussian component profile with a core and a broad second component contributing up to 25% to the total intensity. For the first time a systematic study of the broadening and Doppler shift of the second weaker components is performed using SUMER on SOHO to explore the spatial structures of the second components. It is found that the two component structure is basically restricted to the bright chromospheric network. The narrow core component shows the familiar transition region redshifts, with hardly any blueshifts in the network. The broad second components are blueshifted compared to the core, but are still predominantly redshifted. However, quite large areas in the network (up to 20′′× 20′′) show concentrations of blueshifts in the second component. In the inter-network the line profile has a single Gaussian shape and shows small redand also some blueshifts. It is suggested that the two components in the network correspond with two spatially unresolved physical regimes in quiet Sun network: small scale loops and larger scale coronal loop structures anchored in the network. The footpoint regions of the latter are of a funnel-type and form a “canopy” above internetwork regions of the chromosphere. Shocks propagating upward from the non-magnetic chromosphere interact with this canopy, which leads to the transition region inter-network emission. A further analysis, especially of emission lines originating from higher temperatures, is required to confirm this scenario. The distribution and correlations of the line intensities, shifts and widths show that these physical regimes are heated by different mechanisms. This sheds new light on the interpretation of stellar observations in terms of coronal heating. A comparison to existing studies of stellar transition regions shows the need for more thorough theoretical investigations on the formation of stellar transition region lines.