An EUV Bright Point as seen by SUMER, CDS, MDI and EIT on-board SoHO

Abstract

This paper presents the formation, evolution and decay of a coronal bright point via a spectroscopic analysis of its transition region counterpart and the evolution of the underlying magnetic bipole during 3 days of almost continuous observations. The data were obtained with various instruments on-board SoHO, including the SUMER spectrograph in the transition region line S vi 933.40 Å, CDS in the He i 584.33, O v 629.73 and Mg ix 368.06 Å lines, plus MDI and EIT. The existence of the coronal feature is strongly correlated with the evolution of the underlying bipolar region. The lifetime of the bright point from the moment when it was first visible in the EIT images until its complete disappearance was ~ hrs. Furthermore, the bright point only became visible at coronal temperatures when the two converging opposite magnetic polarities were ~7000 km apart. As far as the temporal coverage of the data permits, we found that the bright point disappeared at coronal temperatures after a full cancellation of one of the magnetic polarities. The spectroscopic analysis reveals the presence of small-scale (~´´) transient brightenings within the bright point with a periodicity of ~6 min. The Doppler shift in the bright point was found to be in the range of -10 to 10 although it is dominated by a red-shifted emission which is associated with regions characterized by stronger “quiet” Sun photospheric magnetic flux. Small-scale brightenings within the bright point show velocity variations in the range 3–6 . In general the bright point has a radiance ~4 times higher than that of the network. No relation was found between the bright point and the UV explosive event phenomena.