On May 31th, Research in Astronomy and Astrophysics published the latest numerical work, conducted by graduate student ZHANG Yining and Dr. YE Jing from Yunnan Observatories and their co-authors, on the turbulent properties in the large-scale solar flare current sheet (CS).
Their study quantitatively analyzes the magnetic reconnection rate, diffusivity, dissipation scale, turbulence amplitude, etc. after the appearance of turbulence caused by tearing instabilities. They find that turbulence can effectively broaden the CS width and brings an extra dissipation effect in the CS.
Solar flare is one of the most energetic events in the solar system, in which up to 1032 ergs of magnetic energy is released via magnetic reconnection. This large-scale current sheet connecting to the flare and the erupting flux rope is the main place for the magnetic reconnection process.Classical theories predict a width of the CS corresponding to the ion inertial scale of tens or hundreds of meters, while many observations found the associated width of 104-105 km. The huge discrepancy between them can be related to the turbulence occurring in the CS. Thus the accurate estimation of the energy dissipation provoked by turbulence is crucial to understand the fast energy release in the solar flare.
In doing so, researchers utilized 2D high-resolution magnetohydrodynamic (MHD) numerical simulations, based on the standard flare model in the gravitationally stratified solar atmosphere. The magnetic reconnection rate shows an apparent increase due to the appearance of tearing instabilities.
Researchers find that the appearance of the turbulence is equivalent to adding an extra dissipation term into the induction equations which could drastically enlarge the local diffusivity in the CS. According to the spectrum analysis, researchers calculate the related dissipation scale of 100-200 km, which is much bigger than the ion inertial scale. It corresponds to the width of the secondary reconnection CS between the merging plasmoids. In addition, they calculate the general width of the current sheet to be 1500-2500 km, which is consistent with observational results. Indeed, the CS width often found in observations is proven to be relevant with the Taylor scale from Biskamp’s theory.
On the other hand, researchers find that the termination shock (TS) at the flare loop top can somewhat amplify the turbulence amplitude. The amplification factor is related to the local geometry of TS, and the TS is shown to have a higher heating efficiency rather the kinetic energy transfer.
This study helps us better understand the details of the dissipation mechanism of the magnetic reconnection in the presence of turbulence in the solar flare.
Contact:
YE Jing
Yunnan Observatories, CAS
Email: yj@ynao.ac.cn
