Correlation between coronal hole and quiet Sun intensities: Evidence for continuous reconnection

¹ Centre for Astrophysics, University of Central Lancashire, Preston, UK e-mail: kpr@iiap.res.in
² DAMTP, University of Cambridge, Cambridge, UK

Received: 14 October 2004
Accepted: 5 November 2004

Abstract

Intensities of 12 strong EUV emission lines in a coronal hole and the neighbouring quiet Sun region have been obtained from SOHO/CDS observations made in August and September 1996. The field of view is 1 arcmin wide by 4 arcmin, with a series of observations made at different spatial locations on the boundary of the north polar hole and its large equatorial extension, the “Elephant's Trunk”. The formation temperatures of the observed lines vary from 0.083 MK to 1.10 MK and hence they represent increasing heights in the solar atmosphere from the upper chromosphere and transition region to the low corona. Line intensities in the vicinity of the coronal hole boundary were examined. For lines formed in the transition region, a correlation was found between intensities in the coronal hole and those in the adjacent quiet Sun region. However, for lines formed higher up in the low corona, the correlation gradually disappears. Moreover, it is found that in the lower transition region the coronal hole emission is brighter than that in the quiet Sun. We conclude that the correlation observed in the transition region implies that flux transport is not impeded by the presence of the coronal hole boundary, suggesting that reconnection is taking place which changes the status of the footpoints from closed to open and vice versa, maintaining the location of the boundary against the movement of the flux. The magnetic reconnection events provide energy in the corona which conducts down the field lines until it is absorbed in the transition region. The brightness of the coronal hole in the lower transition region lines can be explained if the energy conducted down the open field lines is able to penetrate to lower temperature levels than it does on the closed field of the quiet Sun region. This is possible because, for any given temperature in this range, the density in the coronal hole is lower than it is in the quiet Sun.