Abstract
Abstract The process of magnetic reconnection when studied in nature or when modeled in 3D simulations differs in one key way from the standard 2D paradigmatic cartoon: it is accompanied by many fluctuations in the electromagnetic fields and plasma properties. We developed a diagnostics to study the spectrum of fluctuations in the various regions around a reconnection site. We define the regions in terms of the local value of the flux function that determines the distance from the reconnection site, with positive values in the outflow and negative values in the inflow. We find that fluctuations belong to two very different regimes depending on the local plasma beta (defined as the ratio of plasma and magnetic pressures). The first regime develops in the reconnection outflows where beta is high and it is characterized by a strong link between plasma and electromagnetic fluctuations, leading to momentum and energy exchanges via anomalous viscosity and resistivity. But there is a second, low-beta regime: it develops in the inflow and in the region around the separatrix surfaces, including the reconnection electron diffusion region itself. It is remarkable that this low-beta plasma, where the magnetic pressure dominates, remains laminar even though the electromagnetic fields are turbulent.
Highlights
The process of magnetic reconnection when studied in Nature or when modeled in 3D simulations differs in one key way from the standard 2D paradigmatic cartoon: it is accompanied by much fluctuations in the electromagnetic fields and plasma properties
One of the longest-known instabilities connected with reconnection is the lower hybrid drift instability (LHDI), long suspected to play a role in promoting reconnection [13] and observed in space [14] and in laboratory[15]
In a previous study [49] based on a similar 3D reconnection simulation, we investigated all these fluctuations collectively determining the spectrum to have a power law distribution compatible with a turbulent cascade and with the same index observed in space [11]
Summary
Our investigation uses the 3D fully kinetic particle in cell code iPic3D [44]. The present simulation is similar to the one reported in [45], it uses a standard Harris equilibrium [46]: B(y) = B0 tanh(y/L)x + Bgz (1). A guide field Bg/B0 = 0.1 and a background plasma of nb/n0 = 0.1 is imposed, where B0 is the asymptotic in plane field and n0 is the peak Harris density This choice of initial equilibrium corresponds to plasma beta (ratio of plasma pressure and magnetic pressure) peaking in the center of the sheet: β. Where Cs = (m/2πkTs)3/2 is the normalization coefficient of the Maxwellian distribution and ve0 = −2ck(Te + Ti)/eB0L is the drift speed of the electrons required to support the Harris current when the simulation frame corresponds to the ion frame In this frame the background plasma and the Harris ions are not drifting and the overall system has no velocity shear (since the electron mass is much smaller and the center of mass speed is essentially the speed of the ions that is initially zero). We measure in each of these regions how the fluctuations for a given quantity are statistically distributed and report the histogram of the fluctuations with respect to the mean in each region using a color scale
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