Multi-wavelength study of Network Bright Points near the limb*

Abstract

We analyze the morphology of network bright points (NBPs), their relation to fine dark mottles and their temporal variations, using observations of a quiet region near the polar solar limb, obtained during the solar minimum. For our analysis we used an image-processing method for the selection and identification of NBPs. Further we constructed 'maximum power frequency' maps computing the power spectrum for each pixel of a field and selecting the frequency of maximum spectral power as the most representative oscillating frequency for this pixel. The morphological analysis of our data indicates that the enhancement of NBPs is part of a complicated process that is responsible for the formation of mottles. The analysis of the lifetimes of NBPs indicates that although the general patterns of NBPs remain constant for time intervals larger than an hour, the lifetime of individual bright points is of the order of 16 min. Furthermore, our results indicate an association of the lifetime of NBPs with that of mottles. From the analysis of temporal variations we confirm that the power of NBPs is smaller than the corresponding power for intra-network points at both the 3-min and the 5-min oscillating modes and in both the Mg b1−0.4 A and the continuum. Our analysis revealed enhanced power in the 7-min range, which is theoretically considered a candidate frequency for transverse waves related to NBPs at the base of the chromosphere (Kalkofen, 1997). However, we consider more likely that this period is related to the evolution of individual NBPs.