Comparison of quiet-Sun radiances measured by CDS and SUMER on SOHO

Space Science Reviews - Since the beginning of the SOHO (Solar and Heliospheric Observatory) mission an intercalibration programme was carried out which included simultaneous observations of the...     

Why does the perpendicular electric field increase at the edge of auroral arcs?

Radar, rocket and satellite measurements often indicate that there is a strong increase and subsequent decrease in the perpendicular electric field when traversing one edge of an auroral arc. The analysis of rocket measurements, presented here, shows that above an auroral arc there is a small gradient in the electric field due to polarization effects in the ionosphere, but that the strong increase at the edge of the arc can only be explained if the field-aligned currents associated with the arc are taken into account.     

Ion Temperatures as Observed in a Solar Coronal Hole

From the widths of the extreme ultraviolet (EUV) lines measured by the SUMER instrument on SOHO, it was found previously (Tu et al. 1998) that the average temperature of Ne⁺⁷, at heights (relative to h₀) above the southern solar limb from 17″ to 64″, ranges between 1.3 and 5 × 10⁶ K and of Ne⁺⁶ between 1 and 4 × 10⁶ K. For mass-per-charge numbers greater than 4 the temperatures of the ions increase slightly with increasing mass-per-charge, while the thermal speed reveals no clear trend. We present a new data set with exposure times much longer than the ones in the previous study. The results obtained from line width analysis of Fe XII (1242 Å), Mg X (1249 Å), Mg VIII (772 Å) Ne VIII (770 Å), and Si VIII (1445 Å) support our previous study. In this case, the trend of increasing temperature begins at a mass-per-charge number of 3. A qualitative explanation based on ion-cyclotron-resonance heating within linear kinetic theory is suggested.     

Search for Signatures of a Coronal Hole in Transition Region Lines Near Disk Center

The analysis of data taken by SUMER near disk center, where a small coronal hole is observed in EIT images, is performed. From the measurements of Doppler non-thermal velocities and intensities, we search for the diagnostics and the signature of small scale structures in the coronal hole using transition region lines. Transition region lines in the range of 7 × 10⁴ K to 2.5 × 10⁵ K have a non-thermal velocity excess of 4.0 to 5.5 km s^-1 relative to the contiguous quiet Sun. While the average intensity is lower in the coronal hole than in the quiet area, this result shows an increase of turbulence at the base of the high speed solar wind. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Morphology of the Solar Upper Atmosphere During the Sunspot Minimum

The solar upper atmosphere (SUA) is defined as the volume above the photosphere occupied by plasmas with electron temperatures, T _e, above 2×10⁴ K. Until the Skylab era, only little was known about the morphology of the SUA, while the quality of the spectroscopic observations was continually improving. A spherically symmetric atmosphere was assumed at that time, in which the temperature increased with height. With advances in the observational techniques, it became apparent that the morphology of the SUA was very complex even during the minimum of the magnetic activity cycle. In particular, spectroscopic measurements with high spectral and spatial resolution, which were made in the light of ultraviolet emission lines representing a variety of temperatures, led to the conclusion that most of the radiation from the solar transition region could not be explained by assuming a continuous chromosphere-corona interface, but rather by a region of unresolved fine structures. Recent observational results obtained by modern instruments, such as the Extreme-ultraviolet Imaging Telescope (EIT), the Large Angle Spectroscopic Coronagraph (LASCO), and the Solar Ultraviolet Measurements of (SUMER) spectrograph on the Solar and Heliospheric Observatory (SOHO), as well as the Transition Region and Coronal Explorer (TRACE), and their interpretations will be presented in this review of our understanding of the morphology of the SUA.