Contracting and Erupting Components of Sigmoidal Active Regions

TL;DR

This study analyzes five solar eruptions from sigmoidal active regions using high-resolution EUV imaging, revealing that loop contraction occurs before eruptions and correlates with flare energy release, providing insights into eruption dynamics.

Contribution

It provides detailed observations of contracting and erupting components in solar eruptions, highlighting the timing and energy relationships, which were not fully characterized before.

Findings

Loop contraction always precedes eruption initiation.

Contraction correlates with flare energy release and peak X-ray flux.

Contracting loops do not return to their original positions, indicating a new equilibrium.

Abstract

It is recently noted that solar eruptions can be associated with the contraction of coronal loops that are not involved in magnetic reconnection processes. In this paper, we investigate five coronal eruptions originating from four sigmoidal active regions, using high-cadence, high-resolution narrowband EUV images obtained by the Solar Dynamic Observatory (SDO}). The magnitudes of the flares associated with the eruptions range from the GOES-class B to X. Owing to the high-sensitivity and broad temperature coverage of the Atmospheric Imaging Assembly (AIA) onboard SDO, we are able to identify both the contracting and erupting components of the eruptions: the former is observed in cold AIA channels as the contracting coronal loops overlying the elbows of the sigmoid, and the latter is preferentially observed in warm/hot AIA channels as an expanding bubble originating from the center of the sigmoid. The initiation of eruption always precedes the contraction, and in the energetically mild events (B and C flares), it also precedes the increase in GOES soft X-ray fluxes. In the more energetic events, the eruption is simultaneous with the impulsive phase of the nonthermal hard X-ray emission. These observations confirm the loop contraction as an integrated process in eruptions with partially opened arcades. The consequence of contraction is a new equilibrium with reduced magnetic energy, as the contracting loops never regain their original positions. The contracting process is a direct consequence of flare energy release, as evidenced by the strong correlation of the maximal contracting speed, and strong anti-correlation of the time delay of contraction relative to expansion, with the peak soft X-ray flux. This is also implied by the relationship between contraction and expansion, i.e., their timing and speed.