Reconnection and turbulence in the Earth’s magnetotail

Magnetic reconnection (MR) and turbulence are fundamental plasma process responsible for explosive magnetic energy release and redistribution over scales in laboratory, space and astrophysical plasmas. Although, MR, turbulence and the associated multi-scale structures can be studied independently, a major breakthrough can be achieved through simultaneous investigations of these processes. Examples of common occurrence of these phenomena are represented by sawtooth crashes in fusion devices, solar flares and polar aurora associated explosions in the Earth`s magnetotail. The extent of space plasma systems affected by magnetic explosions is huge, however, MR is preferentially occurring at thin, highly localized current structures. The size of laboratory fusion devices does not allow us to fully reproduce the large-scale structures associated with MR.The remote solar observation techniques are biased by line-of-sight effects. After all, it is not fully known how MR works in turbulent plasmas, why it is so fast and how it is accelerating particles to high energies. It turned out that the ideal test-bed for validating the theoretical MR models is the Earth`s magnetosphere, where in-situ multi-spacecraft field and particle measurements are available. Single-spacecraft missions cannot differentiate between spatial and temporal variations, important for recognizing the true multi-scale physics behind MR. Previous multi-spacecraft studies (Cluster and Themis) concentrated on MR studies in the near-Earth space (within distances < 30 RE, where RE is the Earth`s radius), supposing laminar flow configurations. Much of our knowledge about fast MR in collisionless plasmas comes from these missions. Besides single-spacecraft and fortuitous rare alignment of two-spacecraft studies, a comprehensive multi-spacecraft survey of MR has not yet been accomplished in distances larger than 30 RE in the deep-tail. Nevertheless, it was revealed that, the tailward progression and evolution of MR, farther on 30 RE, is highly dynamic, turbulent, characterized by complex 3D current and magnetic structures, multiple X-lines and particle acceleration centers. The unprecedented two-spacecraft NASA ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of Moon`s Interaction with the Sun) mission addresses the physics of MR, turbulence and particle acceleration processes near the lunar orbit (~60 RE, we restrict ourselves to the processes in the magnetotail). This project aims to reach a deeper understanding of MR in space and astrophysical plasmas through observation and identification of favorable conditions for different theoretical scenarios of MR in the deep magnetotail at lunar orbit. The scenarios considered in this project include the two-fluid Hall model, turbulent reconnection and plasmoid instability. In order to learn how particles are accelerated, we will accomplish two-probe characterization of turbulence, electric and magnetic field structures, together with a careful inspection of particle distribution functions. The changing ARTEMIS probe separations allow us to study the spatial structure of fluctuations and the intriguing 3D topology of magnetic structures, such as MR associated plasmoids or flux ropes. These structures are interesting by their own merit, but our primary goal is to understand how they affect the reconnection rate. Though the plasma, field parameters and the typical scales and energy levels of the dynamic magnetotail are different from the conditions in the solar corona or astrophysical regimes, we believe that the analysis of turbulent emerging modes and structures will help us to understand these plasma phenomena fundamentally better.

The geomagnetic field is generated in the Earths core. It is so strong that it extends out into space over the ionosphere where it interacts with the solar wind. In the project Reconnection and turbulence in the magnetotail (Rekonnektion und Turbulenzen im irdischen Magnetschweif) the geophysical and the plasma physical aspects of solar wind magnetosphere interaction processes were studied. The results of numerous space missions testified that magnetic reconnection (MR) plays a key role in those interaction processes. MR on the dayside boundary of the magnetosphere can interconnect geomagnetic and interplanetary magnetic fields, load the newly connected magnetic structures with solar wind plasma and transport the magnetic flux and plasma to the nightside magnetospheric tail. The addition of flux, energy and plasma to the magnetotail generates thin current sheets embedded into oppositely oriented magnetic fields, where MR can occur again, preferably in a distance between 15 and 30 RE (RE Earths radius) in anti-sunward direction. The explosion drives energy, plasma and particle flows in anti-sunward and sunward direction, leading to stormy perturbations of the magnetic field on the Earth. Using numerical simulations and multi-point measurements in the magnetotail it was shown that, MR can be triggered by pressure perturbations in a distance of ~ 60 RE, even without addition of magnetic flux to the tail. It was found, that despite the large distance from Earth, MR affects the near-Earth space and perturbs the geomagnetic field on the ground. Multi-point observations in the magnetotail revealed that MR associated electron acceleration is a multi-step process and different mechanisms are responsible for particle acceleration as the distance from the MR site increases. MR heated plasma exhibits anisotropy relative to the local magnetic field direction. It was also found that the temperature anisotropy driven instabilities are strongest near the MR site. The MR associated abrupt energy conversion processes and the outflows can generate waves and turbulence. Interestingly, plasma turbulence can generate filamentary thin current sheets. It was found that in the compressed turbulent dayside region, in front of the magnetosphere, these current structures are very frequent. The thin current sheets can eventually reconnect. Multiple signatures of the occurrence of MR in such a turbulent environment were found for the first time. The findings of the project help us to better understand our space plasma environment.