The Role of fast magnetosonic waves in the release and conversion via reconnection of energy stored by a current sheet |
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Dana Longcope Submitted: 2012-07-25 08:27
Using a simple two-dimensional, zero-beta model, we explore the manner by which reconnection at a current sheet releases and dissipates free magnetic energy. We find that only a small fraction (3%-11% depending on current sheet size) of the energy is stored close enough to the current sheet to be dissipated abruptly by the reconnection process. The remaining energy, stored in the larger-scale field, is converted to kinetic energy in a fast magnetosonic disturbance propagating away from the reconnection site, carrying the initial current and generating reconnection-associated flows (inflow and outflow). Some of this reflects from the lower boundary (the photosphere) and refracts back to the X-point reconnection site. Most of this inward wave energy is reflected back again, and continues to bounce between X-point and photosphere until it is gradually dissipated, over many transits. This phase of the energy dissipation process is thus global and lasts far longer than the initial purely local phase. In the process a significant fraction of the energy (25%-60%) remains as undissipated fast magnetosonic waves propagating away from the reconnection site, primarily upward. This flare-generated wave is initiated by unbalanced Lorentz forces in the reconnection-disrupted current sheet, rather than by dissipation-generated pressure, as some previous models have assumed. Depending on the orientation of the initial current sheet the wave front is either a rarefaction, with backward directed flow, or a compression, with forward directed flow.
Authors: Dana Longcope, Lucas Tarr
Projects: None
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Publication Status: ApJ accepted
Last Modified: 2012-07-25 14:19
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A Model for the Origin of High Density in Loop-top X-ray Sources |
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Dana Longcope Submitted: 2011-07-14 10:58
Super-hot looptop sources, detected in some large solar flares, are compactsources of HXR emission with spectra matching thermal electron populationsexceeding 30 megakelvins. High observed emission measure, as well as inferenceof electron thermalization within the small source region, both provideevidence of high densities at the looptop; typically more than an order ofmagnitude above ambient. Where some investigators have suggested such densityenhancement results from a rapid enhancement in the magnetic field strength, wepropose an alternative model, based on Petschek reconnection, whereby looptopplasma is heated and compressed by slow magnetosonic shocks generatedself-consistently through flux retraction following reconnection. Under steadyconditions such shocks can enhance density by no more than a factor of four.These steady shock relations (Rankine-Hugoniot relations) turn out to beinapplicable to Petschek's model owing to transient effects of thermalconduction. The actual density enhancement can in fact exceed a factor of tenover the entire reconnection outflow. An ensemble of flux tubes retractingfollowing reconnection at an ensemble of distinct sites will have a collectiveemission measure proportional to the rate of flux tube production. This rate,distinct from the local reconnection rate within a single tube, can be measuredseparately through flare ribbon motion. Typical flux transfer rates and loopparameters yield emission measures comparable to those observed in super-hotsources.
Authors: D.W. Longcope, S.E. Guidoni
Projects: None
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Publication Status: ApJ (accepted)
Last Modified: 2011-07-14 15:24
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A Quantitative Model of Energy Release and Heating by Time-dependent, Localized Reconnection in a Flare with a Thermal Loop-top X-ray Source |
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Dana Longcope Submitted: 2011-06-22 09:57
We present a quantitative model of the magnetic energy stored and thenreleased through magnetic reconnection for a flare on 26 Feb 2004. This flare,well observed by RHESSI and TRACE, shows evidence of non-thermal electrons onlyfor a brief, early phase. Throughout the main period of energy release there isa super-hot (T>30 MK) plasma emitting thermal bremsstrahlung atop the flareloops. Our model describes the heating and compression of such a source bylocalized, transient magnetic reconnection. It is a three-dimensionalgeneralization of the Petschek model whereby Alfvén-speed retraction followingreconnection drives supersonic inflows parallel to the field lines, which formshocks heating, compressing, and confining a loop-top plasma plug. Theconfining inflows provide longer life than a freely-expanding orconductively-cooling plasma of similar size and temperature. Superposition ofsuccessive transient episodes of localized reconnection across a current sheetproduces an apparently persistent, localized source of high-temperatureemission. The temperature of the source decreases smoothly on a time scaleconsistent with observations, far longer than the cooling time of a singleplug. Built from a disordered collection of small plugs, the source need nothave the coherent jet-like structure predicted by steady-state reconnectionmodels. This new model predicts temperatures and emission measure consistentwith the observations of 26 Feb 2004. Furthermore, the total energy released bythe flare is found to be roughly consistent with that predicted by the model.Only a small fraction of the energy released appears in the super-hot source atany one time, but roughly a quarter of the flare energy is thermalized by thereconnection shocks over the course of the flare. All energy is presumed toultimately appear in the lower-temperature T<20 MK, post-flare loops.
Authors: D.W. Longcope, A.C. Des Jardins, T. Carranza-Fulmer, J. Qiu
Projects: RHESSI
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Publication Status: Solar Physics, vol. 267, pp.107-139 (2010)
Last Modified: 2011-06-22 12:25
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Helicity Transport and Generation in the Solar Convection Zone |
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Dana Longcope Submitted: 2003-04-24 10:50
Magnetic helicity provides a theoretical tool for characterizing the solar dynamo and the evolution of the coronal field. The magnetic helicity may be inferred from several types of observation including vector magnetograms of the photospheric magnetic fields. The helicity of an active region reflects, to some degree, the twist in the magnetic field below it. Photospheric observations reveal a tendency for left-handed chirality in the Northern hemisphere, although one-quarter to one-third of the active regions twist in the opposite sense. This means that coronal magnetic field has negative helicity in the North. Sub-photospheric fields will have left-handed twist in the North, although the net helicity also depends on the
writhe of the flux tube axes. We show that buffeting by turbulence, the so-called Sigma-effect, can explain the handedness and level of intrinsic variation of observed twist. This mechanism does not generate helicity, rather it produces twist and writhe of opposite signs. In this scenario, helicity of one sign propagates into the corona, while opposing helicity propagates downward in the form of torsional Alfvén waves.
Authors: Longcope, D.W. and Pevtsov, A.A.
Projects:
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Publication Status: COSPAR, Adv. in Space Research, in press
Last Modified: 2003-04-28 11:25
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