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Unambiguous Evidence of Coronal Implosions During Solar Eruptions and Flares  

Juntao Wang   Submitted: 2018-04-09 05:21

In the implosion conjecture, coronal loops contract as the result of magnetic energy release in solar eruptions and flares. However, after almost two decades, observations of this phenomenon are still rare, and most of previous reports are plagued by projection effects so that loop contraction could be either true implosion or just a change in loop inclination. In this paper, to demonstrate the reality of loop contractions in the global coronal dynamics, we present four events with the continuously contracting loops in an almost edge-on geometry from the perspective of SDO/AIA, which are free from the ambiguity caused by the projection effects, also supplemented by contemporary observations from STEREO for examination. In the wider context of observations, simulations and theories, we argue that the implosion conjecture is valid in interpreting these events. Furthermore, distinct properties of the events allow us to identify two physical categories of implosion. One type demonstrates a rapid contraction at the beginning of the flare impulsive phase, as magnetic free energy is removed rapidly by a filament eruption. The other type, which has no visible eruption, shows a continuous loop shrinkage during the entire flare impulsive phase which we suggest manifests the ongoing conversion of magnetic free energy in a coronal volume. Corresponding scenarios are described, which can provide reasonable explanations for the observations. We also point out that implosions may be suppressed in cases when a heavily-mass-loaded filament is involved, possibly serving as an alternative account for their observational rarity.

Authors: Juntao Wang, P. J. A. Simões, and L. Fletcher
Projects: None

Publication Status: Accepted for publication in ApJ
Last Modified: 2018-04-10 16:45
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Observations of Reconnection Flows in a Flare on the Solar Disk  

Juntao Wang   Submitted: 2017-08-30 04:26

Magnetic reconnection is a well-accepted part of the theory of solar eruptive events, though the evidence is still circumstantial. Intrinsic to the 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. We report on a unique set of SDO/AIA imaging and Hinode/EIS spectroscopic observations of the disk flare SOL2016-03-23T03:54 in which all four flows are present simultaneously. This includes spectroscopic evidence for a plasma upflow in association with large-scale expanding closed inflow 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. Observations of coronal reconnection flows are still rare, and most events are observed at the solar limb, obscured by complex foregrounds, making their relationship to the flare ribbons, cusp field and arcades formed in the lower atmosphere difficult to interpret. The disk location and favorable perspective of this event have removed these ambiguities giving a clear picture of the reconnection dynamics.

Authors: Juntao Wang, P. J. A. Simões, N. L. S. Jeffrey, L. Fletcher, P. J. Wright, and I. G. Hannah
Projects: None

Publication Status: Accepted for publication in ApJL
Last Modified: 2017-08-30 12:29
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Arcade Implosion Caused by a Filament Eruption in a Flare  

Juntao Wang   Submitted: 2016-10-20 02:51

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. We report here on the analysis of a well-observed implosion in the form of an arcade contraction associated with a filament eruption, during the C3.5 flare SOL2013-06-19T07:29. A sequence of events including magnetic flux-rope instability and distortion, followed by filament eruption and arcade implosion, lead us to conclude that the implosion arises from the transfer of magnetic energy from beneath the arcade as part of the global magnetic instability, rather than due to local magnetic energy dissipation in the flare. The observed net contraction of the imploding loops, which is found also in nonlinear force-free field extrapolations, reflects a permanent reduction of magnetic energy underneath the arcade. This event shows that, in addition to resulting in expansion or eruption of overlying field, flux-rope instability can also simultaneously implode unopened field due to magnetic energy transfer. It demonstrates the "partial opening of the field" scenario, which is one of the ways in 3D to produce a magnetic eruption without violating the Aly-Sturrock hypothesis. In the framework of this observation we also propose a unification of three main concepts for active region magnetic evolution, namely the metastable eruption model, the implosion conjecture, and the standard "CSHKP" flare model.

Authors: Juntao Wang, P. J. A. Simões, L. Fletcher, J. K. Thalmann, H. S. Hudson, and I. G. Hannah
Projects: None

Publication Status: Accepted for publication in ApJ.
Last Modified: 2016-10-20 13:42
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Unambiguous Evidence of Coronal Implosions During Solar Eruptions and Flares
Observations of Reconnection Flows in a Flare on the Solar Disk
Arcade Implosion Caused by a Filament Eruption in a Flare

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