Solar photospheric magnetic reconnection

Abstract

We investigate photospheric magnetic reconnection due to an encounter of oppositely directed vertical magnetic flux sheets, performing 2.5-dimensional magnetohydrodynamic (MHD) numerical simulations. We construct the initial flux sheets adopting the thin flux tube approximation. Since actual solar resistivity possesses a maximum at the temperature-minimum region, we adopt a resistivity model in which the resistivity is described as a function of height with a maximum (where the magnetic Reynolds number = 1000) at a middle height of our simulation box. Owing to the resistivity, the Sweet-Parker type reconnection occurs at the middle. The inflow speed (υ _i ∼ 160 m/s) is nearly equal to the speeds implied by observations of canceling magnetic features on the photosphere. Thus photospheric reconnection seems to be a cancellation mechanism. It is shown that upward propagating MHD slow mode waves are generated by an upward reconnection jet. Moreover, when we incline the initial field lines 30° from the vertical direction in the other flux sheet, Alfvén waves are also generated as a result of the reconnection. The energy flux carried by the slow modes and Alfvén waves are 10¹⁰ and 10⁸ erg/cm²/s, respectively, and the durations are 40 s. Since in models of solar spicules upward propagating slow waves or Alfvén waves are usually assumed as the initial perturbations, we compare the energy of both waves. It is found that the wave energies due to the reconnection are comparable to those assumed in spicule models. Thus the photospheric magnetic reconnection might be one of the causes of solar spicules.