Magnetic reconnection is a process that the magnetic fields in plasmas can rapidly rearrange themselves, meanwhile releases energy and accelerated particles. Numerous explosive energy events in the solar-terrestrial space are caused by the reconnection, including the well-known aurorae, but the onset mechanism of reconnection itself has been controversial for decades.

The corporation group consisted by LU Quanming, WANG Rongsheng from University of Science and Technology of China and foreign colleagues use spacecraft observations and particle-in-cell (PIC) simulations to show that magnetotail reconnection starts from electron reconnection in the presence of a strong external driver. This study is published in Nature Communications.

Since the triggering time for the magnetic reconnection in the Earth's magnetotail is transient (only a few seconds to tens of seconds), it's hard for satellites to directly detect the particle dynamics. So far, theoretical and simulation efforts have led to two distinct evolutionary mechanisms.

In mechanism one, magnetotail reconnection starts from small-scale reconnection caused by electron kinetics (electron reconnection) a strong external driver; and in mechanism two, ion kinetics initiates magnetotail reconnection directly on a larger scale. 

The research team made detection of electron reconnection in the magnetotail by high-resolution measurements through the Magnetospheric Multiscale (MMS) spacecraft, the electron reconnection does occur within a strongly externally driven environment, which provides experimental evidence for the mechanism one.

Illustration of Earth's magnetic field with the two MMS areas of study outlined. (Image by NASA)

To better understand the observational results, researchers use PIC simulations to examine the possible mechanism. Starting from a mild, small-scale electron reconnection, it then develops into bursty ion reconnection, and finally the magnetotail reconnection.

This study provides both in situ detection of electron kinetics with a strong external driver in the magnetotail reconnection and simulation results, which helps solve the long-standing conundrum of magnetotail reconnection onset.

(Written by ZHANG Liying, edited by HU Dongyin, USTC News Center)