The structure and dynamics of a bright point as seen with Hinode, SoHO and TRACE

Abstract

Context: Solar coronal bright points have been studied for more than three decades, but some fundamental questions about their formation and evolution still remain unanswered.
Aims: Our aim is to determine the plasma properties of a coronal bright point and compare its magnetic topology extrapolated from magnetogram data with its appearance in X-ray images.
Methods: We analyse spectroscopic data obtained with EIS/Hinode, Ca ii H and G-band images from SOT/Hinode, UV images from TRACE, X-ray images from XRT/Hinode and high-resolution/high-cadence magnetogram data from MDI/SoHO.
Results: The BP comprises several coronal loops as seen in the X-ray images, while the chromospheric structure consists of tens of small bright points as seen in Ca ii H. An excellent correlation exists between the Ca ii H bright points and increases in the magnetic field strength, implying that the Ca ii H passband is a good indicator for the concentration of magnetic flux. Furthermore, some of the Ca ii H bright points are the locations of the loop foot-points as determined from a comparison between the extrapolated magnetic field configuration and the X-ray images. Doppler velocities between 6 and 15 km s^-1 are derived from the Fe xii and Fe xiii lines for the bright point region, while for Fe xiv and Si vii they are in the range from -15 to +15 km s^-1. The coronal electron density is 3.7 × 10⁹ cm^-3. An excellent correlation is found between the positive magnetic flux and the X-ray light-curves.
Conclusions: The remarkable agreement between the extrapolated magnetic field configuration and some of the loops composing the bright point as seen in the X-ray images suggests that a large fraction of the magnetic field in the bright point is close to potential. However, some loops in the X-ray images do not have a counterpart in the extrapolated magnetic field configuration implying a non-potential component. The close correlation between the positive magnetic flux and the X-ray emission suggests that energy released by magnetic reconnection is stimulated by flux emergence or cancellation.