Magnetic reconnection resulting from flux emergence: implications for jet formation in the lower solar atmosphere?

¹ Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland
e-mail: jyd@arm.ac.uk
² School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, PR China

Received: 17 June 2011
Accepted: 26 September 2011

Abstract

Aims. We aim at investigating the formation of jet-like features in the lower solar atmosphere, e.g. chromosphere and transition region, as a result of magnetic reconnection.

Methods. Magnetic reconnection as occurring at chromospheric and transition regions densities and triggered by magnetic flux emergence is studied using a 2.5D MHD code. The initial atmosphere is static and isothermal, with a temperature of 2 × 10⁴ K. The initial magnetic field is uniform and vertical. Two physical environments with different magnetic field strength (25 G and 50 G) are presented. In each case, two sub-cases are discussed, where the environments have different initial mass density.

Results. In the case where we have a weaker magnetic field (25 G) and higher plasma density (N_e = 2 ×    10¹¹ cm^-3), valid for the typical quiet Sun chromosphere, a plasma jet would be observed with a temperature of 2–3  ×    10⁴ K and a velocity as high as 40 kms^-1. The opposite case of a medium with a lower electron density (N_e = 2 × 10¹⁰ cm^-3), i.e. more typical for the transition region, and a stronger magnetic field of 50 G, up-flows with line-of-sight velocities as high as  ~90 kms^-1 and temperatures of 6  ×  10⁵ K, i.e. upper transition region – low coronal temperatures, are produced. Only in the latter case, the low corona Fe ix 171 Å shows a response in the jet which is comparable to the O v increase.

Conclusions. The results show that magnetic reconnection can be an efficient mechanism to drive plasma outflows in the chromosphere and transition region. The model can reproduce characteristics, such as temperature and velocity for a range of jet features like a fibril, a spicule, a hot X-ray jet or a transition region jet by changing either the magnetic field strength or the electron density, i.e. where in the atmosphere the reconnection occurs.