Coronal implosions in solar eruptions and flares

Wang, Juntao (2018) Coronal implosions in solar eruptions and flares. PhD thesis, University of Glasgow.

Abstract

Coronal implosions - the convergence motion of plasmas and entrained magnetic field in the corona due to a reduction in magnetic pressure - can help to locate and track sites of magnetic energy release or redistribution during solar flares and eruptions. Although this conjecture was proposed almost two decades ago, observa- tions of such phenomena are still rare, and even our understanding of it is far from complete. In this thesis, following an introduction to the background and techniques used, we first generalise the implosion idea based on its spirit concerning about the relationship between magnetic energy release and field shrinkage, which allows us to unite and explain three different phenomena, that is, peripheral implosions, inflows and dipolarisations, using only one single principle. Previous observations of apparent contractions in the periphery of active regions are mainly in a face-on state, which cannot exclude the possibilty of field inclining instead of a real contraction as the cause. This then motivates us to study an excellent event observed near the solar disk center, and evidence from both observations and coronal magnetic field extrapolations is found to support the implosion idea. In a unification of three main concepts for active region magnetic evolution implied by the observation, namely the metastable eruption model, the implosion conjecture, and the standard “CSHKP” flare model, the contraction of the field is explained by the removal of the erupting filament originally underneath rather than local magnetic energy dissipation in a flare invoked by previous authors.
However, the observation and extrapolation results in the work above are indirect and still not adequate, as the complex structure of the solar atmosphere, and the simplified assumption and preprocessing in the extrapolation may lead us to a wrong conclusion. Thus in the following four carefully seleted events with the continuously contracting loops in an almost edge-on geometry are for the first time investigated, demonstrating the reality of contraction of field lines in the global coronal dynamics unambiguously. Meanwhile, two categories of implosions, flare- and eruption-driven, are identified, which could be interpreted well in the framework of the implosion conjecture, disproving other authors’ proposal. We also revisit one of the original assumptions of the implosion conjecture which may fail when a heavily-mass-loaded filament is involved, and in this case implosions can be suppressed, possibly served as an alternative explanation for their observational rarity.
In the end, we move on to one of the generalised implosion types, i.e., the inflow, and also study other reconnection flows associated with it. Intrinsic to the well- accepted reconnection picture of a solar eruptive event, particularly in the standard model for two-ribbon flares (“CSHKP” model), are an advective flow of magnetized plasma into the reconnection region, expansion of field above the reconnection region as a flux rope erupts, retraction of heated post-reconnection loops, and downflows of cooling plasma along those loops. However, the evidence of these flows is still circumstantial and rare. We report in this work on a unique set of SDO/AIA imaging and Hinode/EIS spectroscopic observations of a flare in which all four flows are present simultaneously. This also includes spectroscopic evidence for a plasma upflow at the edge of the active region claimed by previous authors, which we suggest decomposing into two components, one associated with open field at quasi- separatrix layers, the other with large-scale expanding closed arcade field. The reconnection inflows are symmetric, and consistent with fast reconnection, and the post-reconnection loops show a clear cooling and deceleration as they retract. Unlike previous events observed at the solar limb which are obscured by complex foregrounds and thus makes the relationship between the plasma flows, the flare ribbons, cusp field and arcades formed in the lower atmosphere difficult to interpret, the disk location and favorable perspective of this event studied here have removed these ambiguities giving a clear picture of the reconnection dynamics.
We end with a brief chapter summarizing the thesis and suggesting some future work.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Sun, atmosphere, corona, flares, coronal mass ejections, implosions, magnetic field.
• Subjects: Q Science > Q Science (General); Q Science > QB Astronomy
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Supervisor's Name: Fletcher, Professor Lyndsay
• Date of Award: 2018
• Depositing User: Mr Juntao Wang
• Unique ID: glathesis:2018-31010
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 14 Nov 2018 13:59
• Last Modified: 16 Jan 2019 14:05
• URI: https://theses.gla.ac.uk/id/eprint/31010

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