Abstract Details
| Name: rohit Affiliation: ncra-tifr Conference ID: ASI2017_649 Title : On the energisation of particles by fast magnetic reconnection. Authors and Co-Authors : 1) Rohit Sharma, NCRA-TIFR, Pune 2) Dhrubaditya Mitra, NORDITA, Stockholm, Sweden. 3) Divya Oberoi, NCRA-TIFR, Pune Abstract Type : Poster Abstract Category : Sun and the Solar System Abstract : Magnetic reconnection occurs in a wide variety of astrophysical systems. It is often invoked to explain the production of a accelerated population of charged particles. It is vital for the solar corona in context with nano-flare theory, to explain the coronal heating problem. The Sweet-Parker theory predicts the magnetic reconnection rates to be $\gamma \propto 1/\sqrt{S}$, where $S = L V_{A}/\eta$ is Lundquist number where L is the thickness of current sheet, $V_{\rm A}$ is the Alfven velocity and \$\eta$ is the magnetic diffusivity. Within a reconnection model, if the reconnection rate goes to zero as $\S \to \infty$ then the reconnection process is defined to be {\it slow}; otherwise it is called {\it fast}. Clearly, slow reconnection is not of relevance in most astrophysical problems. From direct numerical simulations (DNS) of two-dimensional magnetohydrodynamics (MHD), Lourerio et al. (2009) have shown that at large enough turbulent intensities reconnection can be fast -- the reconnection rate becomes independent of the Lundquist number. We investigate the same question using three-dimensional DNS. We find that the reconnection rate obeys Sweet-Parker scaling for low turbulent intensity runs. Runs with large turbulent intensity develops numerous small scale magnetic features, which increases the reconnection rates making reconnection fast, v.i.z., for large enough turbulent intensity reconnection rate departs significantly from Sweet-Parker behaviour, becomes almost a constant as a function of the Lundquist number. The extent of this departure is more in 3D as compared to 2D runs. Another consequence of the magnetic reconnection is that the magnetic energy is dissipated to energise the particles in the process of reconnection. Various solar observations suggests the presence of non-thermal population of particles. For example, the coherent emission mechanisms requires a supra-thermal population of particles in velocity space in order to produce the radio bursts. We further study the energisation of test-particles in the same setup. We find that the speed of the energised particles obeys a Maxwellian distribution, whose variance also obeys Sweet-Parker scaling for small turbulent intensity but depends weakly on the Lundquist number for large turbulent intensity. Furthermore, the variance increases with the strength of the reconnecting magnetic field. |