Direct Observation of Two-step Magnetic Reconnection in a Solar Flare

Abstract

We report observations of an eruptive X2.8 flare on 2013 May 13, which shows two distinct episodes of energy release in the impulsive phase. The first episode is characterized by the eruption of a magnetic flux rope, similar to the energyrelease process in most standard eruptive flares. While the second episode, which is stronger than the first normal one and shows enhanced high-energy X-ray and even γ-ray emissions, is closely associated with magnetic reconnection of a largescale loop in the aftermath of the eruption. The reconnection inflow of the loop leg is observed in the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) 304Å passband and accelerates towards the reconnection region to a speed as high as ∼130 km/s. Simultaneously the corresponding outflow jets are observed in the AIA hot passbands with a speed of ∼740 km/s and mean temperature of ∼14 MK. RHESSI observations show a strong burst of hard X-ray (HXR) and γ-ray emissions with hard electron spectra of δ ≈ 3, exhibiting a soft-hard-harder behavior. A distinct altitude decrease of the HXR loop-top source coincides with the inward swing of the loop leg observed in the AIA 304Å passband, which is suggested to be related to the coronal implosion. This fast inflow of magnetic flux contained in the loop leg greatly enhances the reconnection rate and results in very efficient particle acceleration in the secondstep reconnection, which also helps to achieve a second higher temperature peak up to T ≈ 30 MK.

Solar Flare–CME Coupling throughout Two Acceleration Phases of a Fast CME

“…Considering the role of magnetic reconnection in the CME acceleration by feeding magnetic flux, this would generally correspond to the time that the reconnection outflow jets need to reach the lower part of the erupting flux rope on Alfvénic timescales. According to the observation, the reconnection site at the time of the second step of reconnection is located low in the corona, about 20 Mm above the solar limb (see Figure 4, where the loop-leg inflow is swept in and outflow plasmas originate; also details in Gou et al 2017), and the height of the flux rope's lower boundary is measured as ∼0.6R e by the GCS model (Figure 3(a)). If we assume that the distance between these two is 0.5R e , and the Alfvén speed is of the order of 1000 kms −1 (as inferred from the speeds of SADLs in Figure 5), the time delay is about 6 minutes.…”

“…The eruption produces a fast halo CME with a velocity of ∼1850 kms −1 (according to the SOHO/LASCO CME catalog 10 ), and an intense longduration X2.8 flare that starts at 15:48 UT and peaks at 16:05 UT. This flare is associated with strong particle acceleration as observed in emissions of high-energy HXRs and γ-rays (see details in Gou et al 2017). Here we concentrate on the dynamic evolution of the CME in the solar corona, especially on the impulsive acceleration process and its relation to the flare energy release and high-energy particle acceleration.…”

“…Gou et al (2019) presented detailed observations of the buildup of the magnetic flux rope and large-scale CME from the coalescence of multiple small-scale plasmoids during the early stage of the flare. Gou et al (2017) focused on two distinct episodes of flare energy release associated with two-step reconnection. The first episode is characterized by the "standard" flux-rope eruption, and the second one is initiated by the reconnection of a loop leg behind the eruption, which leads to even stronger particle acceleration observed in emissions of high-energy HXRs and γ-rays.…”

Statistical Properties of Ribbon Evolution and Reconnection Electric Fields in Eruptive and Confined Flares

“…, 2013 ; Gou et al. , 2017 ). The signatures of magnetic reconnection, such as plasma inflow to the current sheet, reconnection outflows, associated energy release in form of plasma heating, and particle acceleration, are best observed on the solar limb.…”

Particle Acceleration at the Pileup Collision of the Twin Shock

“…Depending on whether there is reconnection between the magnetic field turbulence that occurred on the posterior CME-driven shock and that produced by the preceding CME, the pre-accelerated ions inside the preceding CME's driver can be processed by the posterior CME, leading to an enhancement of ions that are compositionally interplanetary CME-like . In addition, Gou et al (2017) analyzed a model on the impulsive flare on 2013 May 13. They deduced that two-step magnetic reconnections between double consecutive flare eruptions play major roles in the production of the impulsive SEP event (for convenience, here called the twin-flare scenario).…”