Effects of a velocity shear on double current sheet systems: Explosive reconnection and particle acceleration

Abstract

The effect of a parallel velocity shear on the explosive phase of a double current sheet system is investigated within the 2D resistive magnetohydrodynamic framework. We further explore the effect of this shear on acceleration of test particles. The general evolution pattern of the double current sheets is similar for all sub-Alfvénic shears with respect to the initial transient phase, the onset of the plasmoid instability, and the final relaxation phase. We find that the theoretical scaling of the reconnection rate with shear holds if the rate is measured when the islands have a similar size. The larger island widths for lower shears greatly enhance the reconnection rate during the explosive phase. We have further examined the modification of the energy spectrum of the accelerated particles in the presence of a shear. Our results also show that the flow only modifies the high energy tail of the particle spectrum and has negligible effect on the power-law index. Individual particle trajectories help to explore the various mechanisms associated with the acceleration. Based on the location of the particles, the acceleration mechanisms are found to vary. We highlight the importance of the convective electric field in the inflow as well as the outflow region inside large magnetic islands in the acceleration of particles. The interaction and reflection of the particles with the reconnection exhausts inside the large scale primary magnetic islands is found to have a significant effect on the energization of the particles.

Thermally enhanced tearing in solar current sheets: Explosive reconnection with plasmoid-trapped condensations

“…We compute the growth rates for the linear and nonlinear phases of the evolution and estimate the scaling relations with the resistivities for different nonlinear phases. We find that the growth rate obtained from our model is faster by an order of magnitude, and weaker with resistivity in comparison with earlier works by Otto & Birk (1992), Zhang & Ma (2011), Akramov &Baty (2017), andPaul &Vaidya (2021), who assume the medium is adiabatic. We find the occurrence of the explosive nature of the evolution within the resistivity domain of η = 5 × 10 −3 to 10 −4 in our work.…”

“…To estimate the linear growth rate, γ lin , we calculate the growth rate in the linear regime by taking the mean value of the slope, which gives γ lin = 3.76 × 10 −1 t −1 A . This value is larger by an order of magnitude compared to the studies of the double current sheet problem (Otto & Birk 1992;Zhang & Ma 2011;Akramov & Baty 2017;Paul & Vaidya 2021), where the radiative cooling effect (or other non-adiabatic effects, e.g., thermal conduction) is not incorporated. This implies that the higher linear growth rate can be ascribed to the non-adiabatic effects of the radiative cooling and background heating.…”

“…On the other hand, double current layer models are also seen to give rise to resistive instabilities known as double tearing modes (DTMs). Double current layer models with two widely seperated current layers can develop a single standard tearing mode on each layer that will influence each other later in the nonlinear evolution stage (Keppens et al 2013;Paul & Vaidya 2021). In contrast, DTMs are tearing modes that are intrinsically coupled and that co-develop on nearby resonant surfaces.…”

“…We also quantify the number of generated plasmoids as influenced by different physical parameters, and its scaling relation with the Lundquist number. In various earlier works, Zhang & Ma (2011); Keppens et al (2013); Akramov & Baty (2017); Paul & Vaidya (2021) have studied the tearing mode effect in explosive reconnection events for adiabatic conditions. By including the radiative energy loss in an optically thin medium and background heating to investigate the effect of thermal instability in a current sheet model, we determine whether explosive reconnection may be triggered in a greater number of Lundquist regimes than in pure resistive MHD alone.…”

A Volumetric Study of Flux Transfer Events at the Dayside Magnetopause

“…Magnetic reconnection is a fundamental process occurring in space plasma wherein a rearrangement of topology of the oppositely oriented magnetic field lines leads to an overall magnetic relaxation of the system. Numerous observations, simulations, and laboratory experiments have routinely attested to the affiliation of the process with energy conversion and particle acceleration (Yamada et al 1997;de Gouveia Dal Pino & Kowal 2015;Li et al 2017;Dahlin 2020;Ergun et al 2020;Paul & Vaidya 2021). A seminal paper by Dungey (1961) presented the idea that the process of reconnection is a key component of the solar wind-magnetosphere interaction.…”