Fast Magnetic Reconnection in the Solar Chromosphere Mediated by the Plasmoid Instability

Abstract

Magnetic reconnection in the partially ionized solar chromosphere is studied in 2.5-dimensional magnetohydrodynamic simulations including radiative cooling and ambipolar diffusion. A Harris current sheet with and without a guide field is considered. Characteristic values of the parameters in the middle chromosphere imply a high magnetic Reynolds number of ∼ 10 6 -10 7 in the present simulations. Fast magnetic reconnection then develops as a consequence of the plasmoid instability without the need to invoke anomalous resistivity enhancements. Multiple levels of the instability are followed as it cascades to smaller scales, which approach the ion inertial length. The reconnection rate, normalized to the asymptotic values of magnetic field and Alfvén velocity in the inflow region, reaches values in the range ∼ 0.01-0.03 throughout the cascading plasmoid formation and for zero as well as for strong guide field. The out-flow velocity reaches ≈ 40 km s −1 . Slow-mode shocks extend from the X-points, heating the plasmoids up to ∼ 8 × 10 4 K. In the case of zero guide field, the inclusion of ambipolar diffusion and radiative cooling both cause a rapid thinning of the current sheet (down to ∼ 30 m) and early formation of secondary islands. Both of these processes have very little effect on the plasmoid instability for a strong guide field. The reconnection rates, temperature enhancements, and upward out-flow velocities from the vertical current sheet correspond well to their characteristic values in chromospheric jets.

Review on Current Sheets in CME Development: Theories and Observations

“…In this sense, some parameters, like η e and d determined by either high temperatures observed by UVCS or high densities observed by coronagraphs, should be considered effective and average. Different from our previous concepts that the cascading process transfers the energy associated with large scale structures to that associated with small scales until the kinetic process dominates, recent numerical experiments for magnetic reconnection revealed that an inverse process, namely merging or coalescence of magnetic islands, takes place simultaneously (e.g., see Bárta et al 2011aBárta et al , 2011bShen et al 2011Shen et al , 2013Mei et al 2012;Ni et al 2012aNi et al , 2012bNi et al , 2015. It is probably this inverse process that causes the energy spectra, which is usually a power law, to possess an index steeper than that of the Kolmogorov value of 5/3.…”

Heating Mechanisms in the Low Solar Atmosphere Through Magnetic Reconnection in Current Sheets

“…The numerical results by Leake et al (2012Leake et al ( , 2013 demonstrate that the recombination is the main effect to cause fast magnetic reconnection in the up chromosphere. Our latest paper (Ni et al 2015) verifies that the plasmoid instability is very important to lead to fast magnetic reconnection at around the middle of the chromosphere, where the neutral-ion collisional mean free path is smaller than the critical width of the current sheet for secondary islands appearing. The speeds of reconnection out flows are in the range of observed speeds of jets in solar chromosphere.…”

Statistical Study of Network Jets Observed in the Solar Transition Region: a Comparison Between Coronal Holes and Quiet-Sun Regions

“…If the generation mechanism is magnetic reconnection in the chromosphere (e.g. Shibata et al, 2007;Yurchyshyn, Abramenko, and Goode, 2013;Deng et al, 2015;Ni et al, 2015), it would indicate that there is no significant difference of magnetic structures in the chromospheric layers of CH and QS. In CH, it is probably small chromospheric loops that reconnect with open flux in the network.…”