Abstract
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.
Highlights
IntroductionSolar eruptions and flares are two main manifestations of magnetic energy release in the corona of the Sun. Hudson (2000) conjectured that a new phenomenon termed “implosion”
Solar eruptions and flares are two main manifestations of magnetic energy release in the corona of the Sun. Hudson (2000) conjectured that a new phenomenon termed “implosion”would accompany these energy release processes, based on the assumption of the dominance of Lorentz force in the coronal dynamics, and the equivalence of magnetic energy and magnetic pressure
As these peripheral loop contractions were always observed face-on and accompanied by eruptions from central magnetic structures, the possibility could not be ruled out that apparent contraction is a projection effect due to the inclination of the loop plane pushed by the erupting structure, rather than a real contraction
Summary
Solar eruptions and flares are two main manifestations of magnetic energy release in the corona of the Sun. Hudson (2000) conjectured that a new phenomenon termed “implosion”. Remarkable coronal loop contractions in extreme ultraviolet at the periphery of active regions, with speeds of tens to hundreds of km s−1, were reported in a few events ranging from Geostationary Operational Environmental Satellite (GOES) class B to X (Liu & Wang 2009, 2010; Gosain 2012; Liu et al 2012; Sun et al 2012; Simões et al 2013; Yan et al 2013; Kushwaha et al 2015; Wang et al 2016) As these peripheral loop contractions were always observed face-on and accompanied by eruptions from central magnetic structures (like a filament or an arcade eruption), the possibility could not be ruled out that apparent contraction is a projection effect due to the inclination of the loop plane pushed by the erupting structure, rather than a real contraction
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