Unambiguous Evidence of Coronal Implosions during Solar Eruptions and Flares

Abstract

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 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 shows the ongoing conversion of magnetic free energy in a coronal volume. Corresponding scenarios are described that 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.

Evolution of Photospheric Vector Magnetic Field Associated with Moving Flare Ribbons as Seen by GST

“…Furthermore, the inward collapse of coronal field might also be accompanied by an upward turning of fields in the peripheral regions . These are well in line with observations of flareinduced contraction of coronal loops (e.g., Liu et al 2012b;Gosain 2012;Simões et al 2013;Wang et al 2018b), and with photospheric observations that flaring sites usually exhibit an enhancement of horizontal magnetic field B h and penumbral structure at the center, surrounded by regions of weakened B h and penumbrae; also, the resulting Lorentz-force change seems to be able to drive the observed surface flows and sunspot motions (see references above). It should be noted that although the overall magnetic field in three dimension (3D) must become more potential after the release of magnetic energy, the near-surface field could become more stressed after flares/CMEs (e.g., Jing et al 2008;Liu et al 2012a).…”

Observations of a prominence eruption and loop contraction

“…8 and 9). The upward motion of loops has been observed in other eruptive events and interpreted as the upward progression of the magnetic reconnection site into the corona due to the effect of the erupting magnetic structure, frequently with a filament present within, on the observed loops (Liu & Wang 2009;Gosain 2012;Simões et al 2013;Wang et al 2018). HXR coronal sources together with conjugate foot-point sources are a natural consequence of the coronal magnetic reconnection.…”

“…The observed speeds varies from 1.5 to 39 km s −1 . Wang et al (2018) considered four events with loop contraction. In the first event, the contraction occurred during the impulsive phase.…”

Flare-productive active regions

“…As a more recent study, Wang et al. (2018c) analyzed four flare events using SDO/AIA and STEREO and demonstrated the existence of real contractions of loops. They identified two categories of implosion, which are (1) a rapid contraction at the beginning of the flare impulsive phase, as magnetic free energy is removed rapidly by a filament eruption; and (2) a continuous loop shrinkage during the entire flare impulsive phase that corresponds to ongoing conversion of magnetic free energy in a coronal volume.

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