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

¹ Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland, UK e-mail: dps@arm.ac.uk
² School of Mathematics and Statistics, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9SS, UK
³ Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany

Received: 4 February 2008
Accepted: 6 September 2008

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.