Emission heights of coronal bright points on Fe XII radiance map

TL;DR

This study investigates the emission heights of coronal bright points (BPs) using magnetic field extrapolation and radiance maps, revealing most BPs are located around 5 Mm height and on top of magnetic loops.

Contribution

It provides a detailed analysis of the emission heights of BPs and their relation to magnetic field structures, using a novel correlation method with height-dependent magnetic field extrapolations.

Findings

Most BPs are located near neutral lines at about 5 Mm height.

Correlations between Fe XII intensity and magnetic field components peak below 20 Mm.

Most BPs are situated on the tops of magnetic loops.

Abstract

We study the emission heights of the coronal bright points (BPs) above the photosphere in the bipolar magnetic loops that are apparently associated with them. As BPs are seen in projection against the disk their true emission heights are unknown. The correlation of the BP locations on the Fe XII radiance map from EIT with the magnetic field features (in particular neutral lines) was investigated in detail. The coronal magnetic field was determined by an extrapolation of the photospheric field to different altitudes above the disk. It was found that most BPs sit on or near a photospheric neutral line, but that the emission occurs at a height of about 5 Mm. Some BPs, while being seen in projection, still seem to coincide with neutral lines, although their emission takes place at heights of more than 10 Mm. Such coincidences almost disappear for emissions above 20 Mm. We also projected the upper segments of the 3-D magnetic field lines above different heights, respectively, on to the x-y plane. The shape of each BP was compared with the respective field-line segment nearby. This comparison suggests that most coronal BPs are actually located on the top of their associated magnetic loops. Finally, we calculated for each selected BP region the correlation coefficient between the Fe XII intensity enhancement and the horizontal component of the extrapolated magnetic field vector at the same x-y position in planes of different heights, respectively. We found that for almost all the BP regions we studied the correlation coefficient, with increasing height, increases to a maximal value and then decreases again. The height corresponding to this maximum was defined as the correlation height, which for most bright points was found to range below 20 Mm.