TasksCurrent Sheets (CS) naturally form in turbulent space plasma, as well as at the boundaries separating regions with different magnetoplasma characteristics, e.g. at the boundaries of the planetary magnetospheres, coronal structures, fronts of fast plasma flows, etc. CS determine the stability of plasma structures, the permeability of their boundaries, as well as the processes of magnetic energy release including magnetic reconnection. Of particular interest are the problems of CSs stability and energy release in collisionless space and astrophysical plasmas. CSs in space plasma are regions of effective release of magnetic energy, powerful plasma heating and acceleration of charged particles to high energies. Examples of such processes are gamma-ray bursts, the acceleration of plasma jets from active galactic nuclei, solar and stellar flares, coronal mass ejections, and magnetic substorms — phenomena related to the so-called space weather in the Solar system. One of the main unresolved issues in modern space physics is the problem of maintaining stability and accumulation of magnetic energy in CSs, and its further spontaneous release, leading to magnetic reconnection. Another important issue is energy dissipation and transfer between the kinetic (electron and ion) and macroscopic (MHD) scales. The reproduction of collisionless CSs typical for space plasma is almost impossible under laboratory conditions. So far, simple small-scale analogues with high densities and low temperatures have been investigated in laboratories, and in order to extrapolate these results to space and astrophysical scales, it is necessary to use numerical simulations. Modern multi-spacecraft missions, CLUSTER and MMS, make it possible to study “in situ” the structure and evolution of CSs in the Earth’s magnetospheres while other single spacecraft missions allow investigation of CSs in other planetary magnetospheres and in the solar wind at different regions of heliosphere. The international team of this project has a great world-level experience in creating kinetic models of CS and in the analysis of spacecraft observations of current structures in the planetary magnetospheres and solar wind. Combining the experience in theoretical and experimental studies, we plan to consider a number of unresolved issues related to the mechanisms of formation of very thin (with a thickness comparable to electronic kinetic scales) current structures and multiscale current configurations. We also plan to investigate the problem of stability and energy release in such current structures. To accomplish these tasks, we will use a large volume of observations of the CS in the terrestrial magnetotail by the multispacecraft CLUSTER and MMS missions, which allow to study 3D current structures on the ion (CLUSTER) and electronic (MMS) kinetic scales. Using both spacecraft observations and kinetic models, we plan, for the first time, to clarify the question of the universality of the kinetic mechanisms responsible for the formation of multiscale current structures, despite the differences in local plasma conditions (plasma characteristics, ion composition, characteristic temporal and spatial scales of plasma processes). For this purpose, spacecraft observations in the magnetospheres of other planets (Mars and Mercury) and in the solar wind will be used. These observations provide many scenarios for the formation of the structure and evolution of the CSs to test theories and models of energy release in them.
The project is expected to receive the following main results:
1) To determine the effects plasma ion composition and anisotropy of plasma populations on the structure and stability of the CSs.
- To determine the dependence of the parameter describing the CS embedding on the relative concentrations of light and heavy ions in planetary magnetotails (Earth, Mars, Mercury) and in the solar wind.
- To determine the dependencies of the CS embedding on the anisotropy of the velocity distribution functions of various plasma components (Vper/Vpar, where Vper is the particle velocity in the CS perpendicular to the magnetic field, and Vpar is the particle velocity at the edges of the CS parallel to the magnetic field).
- To clarify the stability of multiscale CS with respect to the development of the tearing instability. For this sake we will use the large statistics of CS observations in the Earth and Martian magnetotails to determine the positions of the embedded CSs in the parametric space relative to the "tearing-instability regions" (Zelenyi et al., 2008), in dependence on ion composition and the anisotropy of the plasma populations.
-To verify the proposed mechanisms of the formation of embedded CSs by comparing the obtained experimental dependencies with the results of self-consistent kinetic modeling of CSs for the observed plasma conditions.
2) To find the effect of an external electric field associated with the arrival of fast plasma flows on the structure and stability of the CS.
- Experimentally, on a large statistics of CLUSTER and MMS observations in the Earth magnetotail, to test the hypothesis that the application of an external electric field associated with the arrival of a fast plasma flow can cause local thinning and destabilization of the CS.
- In the case of multiple frequent CS crossings, immediately before, during and immediately after the passage of the fast plasma flow, to investigate the position and evolution of the CS parameters in the parametric space relative to the "tearing-instability regions" statistically in order to obtain quantitative dependencies of the probability of the CS transfer to the metastable state on characteristics of the fast flow (amplitude of the electric field associated with the flow, the duration of its impact on the CS).
- To make a conclusion about the influence of the electric field of fast flows on the structure and stability of the CS based on the results of observations and their comparison with the results of self-consistent PIC simulations.
3) To obtain quantitative characteristics of the CS structure (thickness, embedding parameter) and energy release processes in the tail of the Mercury magnetosphere. To draw conclusions about the similarity of the structural features of the CSs in the tail of Mercury with the CSs in the magnetotails of other planets (the Earth and Mars).
- To build a self-consistent CS model in the Hermean magnetotail, and obtain quantitative characteristics of the CS structure using the model. Compare the model profiles of the spatial distribution of the magnetic field and electric current density in the sheet with the observations of MESSENGER in the tail of Mercury.
- To compare the quantitative characteristics of the CS structure in the Hermean tail normalized to local plasma conditions with the corresponding CS characteristics observed in the tails of other planetary magnetospheres.
- To calculate energy spectra and determine the spectral indices of charged particles accelerated in the CS of Hermean magnetotail during substorms. These results will be obtained by simulating the acceleration of charged particles by the method of test particles in the prescribed fields, and by self-consistent PIC simulations. The results of both methods will also be compared with the available observational data. Special attention will be paid to clarifying the role of turbulence in the acceleration of charged particles.
4) Using statistical analysis, to obtain quantitative characteristics of the structural features (thickness, embedding) of the CS observed in the solar wind at various heliocentric distances (near the orbits of Mercury, Earth and Mars), and determine the possible magnetic reconnection regimes in them.
- To determine the structural characteristics of the CSs in the solar wind (thickness, embedding parameter, plasma characteristics, spectra of magnetic fluctuations), normalize them to local plasma conditions and draw conclusions about the similarity of the structural features of the CSs observed in the planetary magnetospheres and the solar wind;
- Based on the obtained characteristics of the CSs at different heliocentric distances, to make conclusions about possible magnetic reconnection regimes in them.
- To build analytical models of energy release and acceleration of plasma particles in the CSs in the solar wind. Obtaining the energy spectra of accelerated particles (electrons and protons), to determine the spectral indices and compare the results with the corresponding results obtained by the test-particle and PIC simulations.