Magnetic flux supplement to coronal bright points
Figure 5. The formation process of a MBF and the associated BP (No. 1 in Table 1), which is formed by bipolar emergence. The top row gives the BP region as seen in AIA 193 Å at the indicated time. The middle row shows the corresponding HMI magnetograms. The bottom row shows the normalised AIA 193 Å lightcurve (left), and the positive (middle) and negative (right) magnetic fluxes obtained in the blue and yellow boxes marked in the top two rows. The contours of positive (red) and negative (blue) magnetic flux density are over-plotted in the top row. The yellow and blue circles over-plotted on the magnetograms denote the locations of the negative and positive polarities of the MBF. The dotted lines over-plotted in the bottom row denote the time of the images shown on the top and middle rows.
Coronal bright points (BPs) are associated with magnetic bipolar features (MBFs) and magnetic can- cellation. Here, we investigate how BP-associated MBFs form and how the consequent magnetic cancellation occurs. We analyse longitudinal magnetograms from the Helioseismic and Magnetic Imager to investigate the photospheric magnetic flux evolution of 70 BPs. From images taken in the 193 Å passband of the Atmospheric Imaging Assembly (AIA) we dermine that the BPs´ lifetimes vary from 2.7 to 58.8 hours. The formation of the BP MBFs is found to involve three processes, namely emergence, conver- gence and local coalescence of the magnetic fluxes. The formation of a MBF can involve more than one of these processes. Out of the 70 cases, flux emergence is the main process of a MBF buildup of 52 BPs, mainly convergence is seen in 28, and 14 cases are associated with local coalescence. For MBFs formed by bipolar emergence, the time dierence between the flux emergence and the BP appearance in the AIA 193 Å passband varies from 0.1 to 3.2 hours with an average of 1.3 hours. While magnetic cancellation is found in all 70 BPs, it can occur in three dierent ways: (I) between a MBF and small weak magnetic features (in 33 BPs); (II) within a MBF with the two polarities moving towards each other from a large distance (34 BPs); (III) within a MBF whose two main polarities emerge in the same place simultaneously (3 BPs). While a MBF builds up the skeleton of a BP, we find that the magnetic activities responsible for the BP heating may involve small weak fields.
Last Revised: 2016 January 15th