Jeffersson A. Agudelo Rueda

The solar wind shows a non-adiabatic temperature profile with distance from the Sun which suggests the presence of local heating and particle-acceleration mechanisms. Turbulence and magnetic reconnection are candidate mechanisms to account for this heating. In this work, we study the formation of electric current structures and the spontaneous onset of 3D magnetic reconnection in particle-in-cell (PIC) simulations of collisionless anisotropic Alfvénic turbulence. Figure 1 shows the 3D rendering of the magnitude of the current density at a representative turbulent time in our simulation. Figure 2 shows a sketch of the setup of anisotropic counter–propagating Alfvén Waves in our elongated simulation box. In Figure 1 we identify the formation of current filaments and extended current-sheets which are mainly elongated along the background magnetic field. To study the distribution of energy among the spatial scales due to the turbulent cascade we calculate the reduced 2D power spectral density of the magnetic fluctuations (PD2B) as a function of the wavevector components parallel and perpendicular to the background magnetic field. Figure 3 visualises the broad range of scales in which the magnetic energy is distributed. It also shows an anisotropy consistent with the elongated shapes of the structures in figure 1. The energy distribution reaches smaller length scales in the perpendicular direction than in the perpendicular direction.

3D magnetic reconnection is fundamentally different from 2D. Therefore, we propose a set of indicators to find reconnection sites in 3D. Our indicators are: C1) High Current-density structures; C2) Non-zero parallel electric fields; C3) Heated particles; C4) Fast ions and electrons and C5) Strong gradients in at least one component of the magnetic field and magnetic null regions. Figure 4 visualises our set of indicators in a subset of our simulation box. This figure shows some regions that satisfy C1, C3, C4, and C5 as well as the presence of a highly twisted magnetic flux rope and a magnetic flux tubes. The white circle marks the reconnection region where 4 of the 5 indicators are present. Since the electric field is highly sensitive to the particle noise due to the finite number of particles, the application of the indicator C2 in 3D PIC simulations is rather limited. Using our indicators, we identify spontaneous formation of reconnection events. In particular we study one in which a highly twisted magnetic flux rope breaks and reconnects with the surrounding magnetic tubes. Afterwards, we study the bulk ion velocity and the magnetic field measured along a 1D trajectory passing through and near the reconnection region. Figure 5 shows the cartesian components and magnitude of the ion velocity and magnetic field. We observe the presence of slow-mode-polarised fluctuations, rotations in the magnetic field and changes in the sign of the correlation between magnetic field and ion velocity consistent with reconnection exhausts observed in the solar wind.

To summarise, we establish a set of indicators for magnetic reconnection in 3D PIC simulations. We observe the spontaneous onset of 3D magnetic reconnection in a PIC simulation of decaying Alfvénic turbulence. The profiles of ion speed and magnetic field are consistent with reconnection events in the solar wind. Our results can be compared with measurements made by PSP and Solar Orbiter.