Observation of Magnetic reconnection at a 3D null point associated with a solar eruption

TL;DR

This study provides direct observational evidence of magnetic reconnection at a 3D null point during a solar eruption, revealing a two-step process involving the null point and flux rope interaction.

Contribution

First observational demonstration of magnetic reconnection at a null point during a solar eruption, distinguishing it from current sheet reconnection and linking it to flare features.

Findings

Reconnection at the null point caused the secondary flare peak.

The null point reconnection produced a high-lying cusp structure.

The eruption involved a two-step process with null point interaction.

Abstract

Magnetic null has long been recognized as a special structure serving as a preferential site for magnetic reconnection (MR). However, the direct observational study of MR at null-points is largely lacking. Here, we show the observations of MR around a magnetic null associated with an eruption that resulted in an M1.7 flare and a coronal mass ejection. The GOES X- ray profile of the flare exhibited two peaks at 02:23 UT and 02:40 UT on 2012 November 8, respectively. Based on the imaging observations, we find that the first and also primary X- ray peak was originated from MR in the current sheet underneath the erupting magnetic flux rope (MFR). On the other hand, the second and also weaker X-ray peak was caused by MR around a null-point located above the pre-eruption MFR. The interaction of the null-point and the erupting MFR can be described as a two-step process. During the first step, the erupting and fast expanding MFR passed through the null-point, resulting in a significant displacement of the magnetic field surrounding the null. During the second step, the displaced magnetic field started to move back, resulting in a converging inflow and subsequently the MR around the null. The null-point reconnection is a different process from the current sheet reconnection in this flare; the latter is the cause of the main peak of the flare, while the former is the cause of the secondary peak of the flare and the conspicuous high-lying cusp structure.