Solar and Magnetospheric MHD Theory Group - University of St Andrews

Emission of jets driven by reconnection of emerging magnetic fields


We have studied an active region jet originating from NOAA 8531 on May 15 1999. To study this jet, we performed 3D MHD numerical simulations of magnetic flux emergence and its subsequent reconnection with preexisting magnetic flux. Then, we compared the physical properties of the observed jet with the reconnecting outflow produced in the numerical model and we found a good qualitative and quantitative comparison. More precisely, a high-velocity upflow (~ 100 Km/sec) was observed after the emergence of new magnetic flux at the edge of the active region. The jet was recorded over a range of temperatures between 10^5 K and 1.5x10^6 K. In our numerical experiments, we find that the jet is the result of magnetic reconnection between newly emerging flux and the preexisting magnetic field of the active region. To build the active region, we use the emergence of a toroidal magnetic flux rope. When the rope intersects with the photosphere, a small active region is formed. The further emergence of the magnetic field above the photosphere leads to the development of a coronal field. Then, a second toroidal flux rope is emerging into the pre-existing, ambient field of the active region (Figure 1).
Figure 1. Distribution of Bz (colormap), total magnetic field vector (arrows) and fieldlines at the photosphere, t=15 min. The red panel indicates the region where reconnection occurs between the large-scale loops (yellow lines) of the active region and the small-scale emerging field (white lines). This is the region from where jets will be emitted due to reconnection.

When the two fields come into contact, a current sheet is formed at their interface. Reconnection sets in and high-velocity bidirectional outflows are emitted from the edges of the current sheet. The outflows are reconnection jets, which are accelerated due to the tension of the reconnected fieldlines. Shrtly after their emission, the jets are moving vertically above the reconnection interface but eventually they are directed along the ambient magnetic fieldlines (collimated flows) and, thus, they are bented and may travel all along the apex of the active region loops (Figure 2).

Figure 2. 3D topology of fieldlines around the emission of the jet, at t=20 min (left) and t=23 min (right). Yellow fieldlines represent the active region, blue the emerging field, white and green are reconnected fieldlines. Temperature (~ 1 MK) is shown by the isosurface (in red). Arrows show the direction of the magnetic field lines.

The results of our numerical experiments appear in Astronomy and Astrophysics (2009). This is a copy of the paper.