The evolution and space weather effects of solar coronal holes

Abstract

In recent years the role of space weather forecasting has grown tremendously as our society increasingly relies on satellite dependent technologies. As solar activity is the foremost important aspect of space weather, the forecasting of are and CME related transient geomagnetic storms has become a primary initiative. Minor magnetic storms caused by coronal holes (CHs) have also proven to be of high importance due to their long lasting and recurrent geomagnetic effects. In order to forecast CH related geomagnetic storms, the author developed an automated CH detection package (Coronal Hole Automated Recognition and Monitoring; CHARM) to replace the user-dependent CH boundary intensity thresholding methods used in previous studies. CHARM uses a local intensity thresholding method to identify low intensity regions in extreme ultraviolet or X-ray images. CHs are known to be regions of “open” magnetic field and predominant polarity, which allowed the differentiation of CHs from other low intensity regions using magnetograms. An additional algorithm (Coronal Hole Evolution; CHEVOL) was developed and used in conjunction with CHARM to study individual CHs by tracking their boundary evolution. It is widely accepted that the short term changes in CH boundaries are due to the interchange reconnection between the CH open field lines and small loops. In order to test the interchange reconnection model, the magnetic reconnection rate and the diffusion coefficient at CH boundaries were determined using observed CH boundary displacement velocities. The results were found to be in agreement with those determined by the theory. Our results also indicate that the short-term CH boundary evolution is caused by random granular motions bringing open and small closed field lines close, and thus accommodating the magnetic reconnection and displacement of field lines. The Minor Storm (MIST) algorithm was developed by the author to build on the CHARM package, providing a fast and consistent way to link CHs to high-speed solar wind (HSSW) periods detected at Earth. This allowed us to carry out a long-term analysis (2000-2009) to study the relationship between CHs, the corresponding HSSW properties, and the geomagnetic indices. The study found a strong correlation between the velocity and proton plasma temperature of the HSSW stream. This indicates that the heating and acceleration of the solar wind plasma in CHs is closely related, and perhaps are caused by the same mechanism. Many authors accept this mechanism to be Alfven wave heating. The research presented in this thesis includes the small scale analysis of individual CHs on time scales of days, which is complemented with large scale analysis of CH groups on time scales of years. This allowed us to further our understanding of CH evolution as a whole.