Fine-scale activity driven by magnetic reconnection within coronal microjets

¹ School of Earth and Space Sciences, Peking University, Beijing 100871, China
² IRAP, Université Toulouse III – Paul Sabatier, CNRS, CNES, Toulouse, France

^⋆⋆ Corresponding authors: arouillard@irap.omp.eu, jshept@pku.edu.cn

Received: 30 November 2024
Accepted: 29 March 2025

Abstract

Context. Parker Solar Probe and Solar Orbiter have revealed the ubiquitous presence of magnetic switchbacks and jets in solar wind forming close to the Sun. While many studies suggest a causal link via solar magnetic reconnection, the specific mechanisms remain unclear. Some numerical simulations propose that small flux ropes generated within reconnecting current sheets could escape with the expanding solar wind, causing the measured velocity spikes. Others suggest that multiple wave modes excited during magnetic reconnection grow and steepen into switchbacks.

Aims. Our aim is to study the substructure and fine-scale activity of a microjet reported in our previous study. The microjet is believed to have occurred within a coronal source region and its switchback and velocity spike bursts were measured by Solar Orbiter.

Methods. We exploited both surface magnetograms and coronal imagery at different wavelengths to derive some basic properties of these small microjets and their substructure, including their kinematics, temperature distribution, and energy. We then used a 2.5D magnetohydrodynamic model of the solar corona to study the physical mechanisms that could produce these microjets.

Results. The main jet exhibits small-scale activity in ultraviolet imaging with clear intermittent releases of bright structures propagating the stalk of its pseudo-streamer-like structure. The wave energy flux density associated with the formation of the microjet and its subcomponents is comparable to that of switchbacks derived from in situ measurements. Our numerical simulations of the emergence of a magnetic loop reconnecting with the ambient unipolar magnetic field via a tearing-mode instability reproduces the kinematic properties, periodicity, and temperature distribution of the microjet substructures.

Conclusions. We conclude that the small-scale dynamics of the microjet results from magnetic reconnection. In 2.5D magnetohydrodynamic simulations, the released magnetic islands undergo secondary magnetic reconnection, which generates bursts of small jets and large-amplitude magnetic fluctuations released in the solar wind. We speculate that these waves evolve into bursts of switchbacks and velocity spikes in the upper corona via a secondary evolution process, whereby the velocity shear induced by the jet-forcing folds in large-amplitude Alfvénic fluctuations.