Mullard Space Science Laboratory

L. M. Green
Aspects of the relationship between active regions and coronal mass ejections
2002 (supervisor: J. L. Culhane)

This thesis seeks to understand further the role of solar flares and coronal mass ejections in the evolution of the solar corona, and the relationship between the two phenomena. The thesis starts with a description of the solar atmosphere and the physics governing the magnetic fields in this region. Active regions are then discussed as they are the location of the flares studied in this thesis, and are related to the coronal mass ejections selected. The magnetic flux which emerges into the active regions is likely to be twisted and distorted. Such structure in the field can be described by the parameter magnetic helicity, which is also introduced. A discussion on coronal mass ejections and their relationship to flares and helicity concludes the introduction section.

Various instruments have been used in order to obtain a complete analysis of the chosen events. These instruments include both space--borne and ground based instruments. There is a section to explain the workings of each instrument which include the Yohkoh Soft X–ray Telescope and Hard X–ray Telescope, SoHO Extreme–ultraviolet Imaging Telescope, Large Angle and Spectroscopic Coronagraph and Michelson Doppler Imager, the GOES X–ray flux monitor and two ground based Hα telescopes in Austria and Japan. The various data analysis techniques are also briefly described.

The relationship between coronal mass ejections and flares has been debated since the 1970s. This thesis investigates the effect of the ejections on the long–term flare activity in certain active regions near solar minimum. It is found that the ejections significantly alter the magnetic environment in the flaring active regions to produce a situation where less energetic flares occur. It is also found that at the start of a period of high CME activity in one particular active region, an imbalance between the positive and negative line of sight magnetic flux forms. This may also be related to a change of the magnetic environment resulting from a process contributing to CME onset, or even possibly as a direct consequence of the CME itself. The magnetic field component along the observers line of sight is measured and so will be sensitive to changes in orientation of the field.

Coronal mass ejections are thought to be an important process in the solar corona as they are the means by which plasma, and more importantly, magnetic field are removed from one solar cycle to the next. Twisting and writhing of bundles of magnetic field lines results in a quantity known as magnetic helicity which is a well preserved quantity. Coronal mass ejections also therefore offer a natural method to remove magnetic helicity and prevent an endless accumulation in the corona. The source of helicity for a rotating active region which produces many coronal mass ejections has been studied in this thesis. It has been found that the action of differential rotation on the footpoints of the coronal flux tubes cannot provide enough helicity to provide a source for the observed number of ejections. Instead the source must be provided by the emergence of twisted and distorted flux from below the photosphere.

The results in Chapter 3 are commensurate with previous work which suggests that high intensity flares are likely to be accompanied by a coronal mass ejection. Chapter 5 in this thesis details the study of a highly energetic flare that was expected to be accompanied by a coronal mass ejection but was, in fact, confined to the lower corona. It is found that the flare is likely to be the result of an interaction between emerging flux and pre–existing flux low in the corona. Reconnection occurs and a fast expansion is observed in one of the loops. The work suggests that a fast injection of twist into the expanding loop may have occurred, and that flares are a method by which helicity is transferred in coronal structures.


Mullard Space Science Laboratory - Holmbury St Mary - Dorking - Surrey - RH5 6NTTelephone: +44 (0) 1483 204100 - Copyright © 1999-2005 UCL

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