The variation of coronal holes with solar cycle

Abstract

Coronal holes are commonly known to be the source of the open magnetic flux and the high-speed solar wind. In EUV and X-ray emission lines, coronal holes are seen as the regions with reduced emission due to lower temperatures and electron density than the surrounding quiet sun. Observing the evolution of coronal holes over the course of a solar cycle, can lead to a greater understanding of the evolution of the solar dynamo and can advance our knowledge in forecasting space weather. Synoptic data from the Normal Incidence Spectrometer (NIS), which is part of the Coronal Diagnostic Spectrometer (CDS) onboard the Solar and Heliospheric Observatory (SoHO), have been used in this work to observe the evolution of coronal holes during cycle 23, from mid-1996 through to mid-2007 (solar minimum to solar minimum).
Towards the end of 1998, communication with SoHO was recovered after being temporarily lost for several months. On recovery of SoHO, it was noticed that the spectral lines had acquired wings, making the Gaussian profiles broader. This broadening now extends profiles beyond the boundaries of some spectral windows; this prevents a direct measurement of the background of the spectral line. Here, I present a new method of obtaining an estimate of the background for both the NIS bands, as well as alternative values with which to describe the wings of the broadened Gaussian spectral profiles. The new method for the background level shows a considerable improvement over the background level determined by a fitting routine; I demonstrate that the resultant intensity could be over-estimated by up to 30%.
Previous studies which investigated the coronal hole area variation were either limited to or concentrated on the poiar coronal holes, or else the models experienced large uncertainties when determining the area of coronal holes close to the solar limb. However, the isotropic nature of the radiation emitted from the low corona does allow for coronal hole areas to be determined at any location on the solar disc, right up to the limb.
Imaging the coronal lines Mg ix and Mg x show many dark low-emission regions which belong to filaments and their channels as well as coronal holes. It is not until the temperature has been derived from the ratio of Mg x/Mg ix, that we are able to see coronal holes as the only regions cooler than the surrounding quiet sun. Observing the typical coronal hole temperature through the solar cycle shows that the temperature of the holes varies in phase with the solar activity cycle; with values of 1.17 ± 0.06 MK and 1.09 ± 0.02 MK at solar maximum and near solar minimum, respectively.
Using this low-temperature property of coronal holes, an automated method has been developed to distinguish the coronal hole areas using the large Mg x/Mg ix temperature ratio datasets from May 1998 onwards. Prior to this, the Mg x line was not included in the synoptic study; therefore the coronal hole regions were identified manually from Mg ix intensity images. The results show a negative correlation between the synoptic coronal hole area and solar activity; areas of -12% of the total solar surface area are found around solar minimum, where the coronal holes are predominantly found at the polar regions, and
this falls to r.6% around solar maximum.
The motion of the coronal holes over the solar cycle has also been observed using maps created from the Mg x/Mg ix temperature ratio. It is shown that, in the northern hemisphere, the old-cycle coronal hole leaves the pole in May 1999 and the new opposite polarity polar coronal hole begins to re-establish at the pole from September 2001. The southern hemisphere polar hole vacates the pole in May 2000 and begins to reform in January 2004. This shows that the northern hemisphere polarity reversed first, followed by the southern hemisphere approximately two years later. An interesting behaviour was noticed at the poles when, shortly after a polar coronal hole established, the coronal hole was displaced from the pole. This occurred three times in both hemispheres until the polar coronal holes finally settled in June 2005 (North) and November 2006 (South). These displacements could be due to interactions between holes of opposite and of like polarity at mid-latitudes.
Combining the synoptic coronal hole areas with the daily disk-averaged magnetic field data, obtained by Kitt-Peak and SOLIS facilities, the variation of the open magnetic flux over cycle 23 is also presented. Shortly after the old-cycle northern polar coronal hole vacates the polar region, the open flux is shown to steadily increase up until '.2003, approximately two years after solar maximum. . The open flux begins to return to solar minimum levels in January 2004, when both polar coronal holes are attempting to encompass the pole. These events signify that there is a strong connection between the evolution of open flux and the processes involved in reversing the polarity of the Sun.