The dynamic topology of the solar corona : mapping the Sun’s three dimensional magnetic skeleton

Sun--Corona--Magnetic fields--Mathematical models;Solar magnetic fields

Williams, Benjamin Matthew ; Parnell, Clare Elizabeth ; Parnell, Clare Elizabeth

University:University of St Andrews

Department:Mathematics & Statistics (School of)

关键词: Sun--Corona--Magnetic fields--Mathematical models;    Solar magnetic fields;   

Others  :  https://research-repository.st-andrews.ac.uk/bitstream/handle/10023/14637/BenjaminWilliamsPhDThesis.pdf?sequence=3&isAllowed=y

来源: DR-NTU

【 摘 要 】

Observations of the surface of the Sun reveal multi-scaled, mixed magnetic featuresthat carpet the entire solar surface. Not surprisingly, the global magnetic fieldsextrapolated from these observations are highly complex. This thesis explores thetopology of the Sun’s global coronal magnetic fields. The magnetic skeleton of amagnetic field provides us with a way of examining the magnetic field andquantifying its complexity.Using specialised codes to find the magnetic skeletons which were written during thecourse of this work, we first examine potential field extrapolations of the global solarcoronal magnetic field determined from observed synoptic magnetograms from theHeliospheric Magnetic Imager on the Solar Dynamics Observatory. The resolution ofthe PFSS models is found to be very important for discovering the true nature of theglobal magnetic skeleton. By increasing the maximum number of harmonics used inthe potential field extrapolations and, therefore, the grid resolution, 60 times morenull points may be found in the coronal magnetic field. These high resolution fieldsalso have a large global separator network which connects the coronal magnetic fieldover large distances and involves between 40 % and 60 % of all the null points in thesolar atmosphere. This global separator network exists at both solar minimum andsolar maximum and has separators that reach high into the solar atmosphere(> 1R☉) even though they connect null points close to the solar surface.These potential field extrapolations are then compared with magnetohydrostatic(MHS) extrapolations of the coronal magnetic field which also provide us withinformation about the plasma in the corona. With a small component of electriccurrent density in the direction perpendicular to the radial direction, these MHS fieldsare found to have a plasma beta and pressure typical of the corona. As this smallcomponent of electric current density grows, the heliospheric current sheet is warpedsignificantly and the magnetic field, plasma beta and pressure become unphysical.Torsional spine reconnection is also studied local to a single null point. First using adynamical relaxation of a spiral null point under non-resistive magnetohydrodynamics(MHD) to a MHS equilibrium is form in which a current layer has built up aroundthe spine lines. Then the reconnection under resistive MHD in this current sheet isstudied. The current about the spine lines is dissipated and the magnetic energy ismainly converted into heat directly as the field lines untwist about the spine line.