Recent progresses in theoretical studies and satellite observations for collisionless magnetic reconnection

Abstract

As an essential mechanism in large scale fast magnetic energy releases and field reconfigurations processes in space, astrophysical, and laboratory plasmas, magnetic reconnection, particularly collisionless magnetic reconnection, has been studied for more than 65 years. Many progresses have been achieved in recent years and basic features of the process have been well understood, largely due to more and more satellite observation data available in the last decade. However, a few outstanding issues are still remained unresolved. We in the paper review the development of collisionless magnetic reconnection studies and major achievements in recent years, and also briefly discuss the open questions remained to be answered in studies of collisionless magnetic reconnection. Magnetic reconnection is thought an essential mechanism in many plasma physics processes in space, laboratory, and astrophysical objects, related to configurations relaxation and fast release of magnetic field and energy, observed in laboratory experiments and satellites observations and studied in numerical simulations and theoretical analysis [1]. The concept of magnetic reconnection was first proposed as a process of oppositely directed magnetic field lines merging and annihilated at a magnetic null, in an effort to understand the solar flare phenomenon It has then become a fundamental model in magnetic reconnection studies. Nevertheless, the Sweet-Parker reconnection rate is on the order of square root of the electrical resistivity. Due to the low collisionality in space and solar plasmas, the rate is too slow to be counted for fast events such as solar flares.Another model was then proposed by Petschek, with a fast driven and an X-point geometry, to solve the problem [5]. It was claimed in the Petschek model that a fast reconnection rate could be almost on the order of logarithm of resistivity, much faster than the Sweet-Parker rate. It was however found latterly that in high resolution simulations, the X-point geometry could not be realized in high Lundquist number regime of S≥10 4 [6]. And in the low Lundquist number regime (S<10 3 ), Petschek reconnection rate and Sweet-Parker reconnection rate would be more or less on the same order. On the other hand, collisionless mechanisms such as the finite Larmor radius (FLR) effect rather than collisional

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A brief of recent research progress on ionospheric disturbances

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