Radiance and Doppler shift distributions across the network of the quiet Sun

¹ Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany e-mail: tianhui924@gmail.com
² Department of Geophysics, Peking University, Beijing, PR China e-mail: chuanyitu@pku.edu.cn
³ School of Space Science and Physics, Shandong Univ. at Weihai, Weihai, Shandong, PR China e-mail: xld@ustc.edu.cn

Received: 7 April 2008
Accepted: 27 June 2008

Abstract

Aims. The radiance and Doppler-shift distributions across the solar network provide observational constraints of two-dimensional modeling of transition-region emission and flows in coronal funnels. These distributions have not, however, been studied in detail and we attempt an investigation for a quiet Sun region.

Methods. Two different methods, dispersion plots and average-profile studies, were applied to investigate these distributions for three EUV lines. In the dispersion plots, we divided the entire quiet Sun region scanned by SUMER into a bright and a dark part according to an image of Fe xii taken by EIT during the scanning; we plotted intensities and Doppler shifts in each bin as determined according to a filtered intensity of Si ii. We also studied the difference in height variations of the magnetic field as extrapolated from the MDI magnetogram, in and outside network, in the two parts. For the average-profile study, we selected 74 individual cases and derived the average profiles of intensities and Doppler shifts across the network. Cases with large values of blue shift of Ne viii were further studied.

Results. The dispersion plots reveal that the intensities of Si ii and C iv increase from network boundary to network center in both the bright and dark parts. However, the intensity of Ne viii shows different trends, namely increasing in the bright part and decreasing in the dark part. In both parts, the Doppler shift of C iv increases steadily from internetwork to network center. The height variations in the magnetic field imply a more homogeneous magnetic structure at greater heights and clearly reflect the different magnetic structures in the two regions. The average-profile study reveals that the intensities of the three lines all decline from the network center to internetwork region. The binned intensities of Si ii and Ne viii have a good correlation. We also find that the large blue shift of Ne viii does not coincide with large red shift of C iv.

Conclusions. Our results suggest that the network structure is still prominent at the layer where Ne viii is formed in the quiet Sun, and that the magnetic structures expand more strongly in the dark part than in the bright part of this quiet Sun region. Our results might also hint for a scenario of magnetic reconnection between open funnels and side loops.