Observations of the upper solar chromosphere with SUMER

The structure and dynamics of the solar chromosphere are still matters of debate. The chromospheric network reflecting the supergranulation of the outer convection zone of the Sun is a prominent feature of the lower solar atmosphere that extends into the transition zone between chromosphere and corona. In particular, the physics of the so-called "nonmagnetic" chromosphere in internetwork regions as well as the physics of the magnetic network are not yet fully understood. Here we present observations of the H I Lyman continuum obtained in areas of the undisturbed Sun by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument on the Solar and Heliospheric Observatory (SOHO). The observing sequences are unique in the sense that they cover the spectral range from 67 nm to 93 nm with the highest cadence the SUMER spectrometer can achieve operating near the limit of its mechanism performance, telemetry allocation, and memory capabilities. In this wavelength range not only the Lyman continuum but also many extreme-ultraviolet emission lines (N II, N III, S IV , O II, O III, O IV, O V, Ne VIII, and Mg IX) are prominent, allowing the investigation of radiation formed at temperatures representative of regions from the chromosphere to the corona. Brightenings have been identified that are presumed to be related to the well-known 3 min oscillations as seen, for instance, in Ca II H 2v and K 2v observations. The relative temporal variations of the continuum radiance near 77 nm were typically 20% to 40%, whereas simultaneously recorded transition-region lines varied by about 40% of their lowest values in phase with the continuum. In the corona, the Ne VIII and Mg IX lines with formation temperatures of 620 000 K and 950 000 K, respectively, experienced relative changes of 10% and displayed no phase relationship with the transition-region lines or the continuum. Radiance variations in the spatial regime across the solar disk show a higher correlation between the chromosphere and the corona than between the transition region and the corona. The observations will be discussed with a view towards providing constraints for modelling chromospheric structure and dynamics. chromosphere - Sun: transition region - Sun: corona - Sun: UV radiation