National Astronomical Observatoy of Japan

There are many evidences of magnetic reconnection in jets; i.e., change of magnetic field topology at the footpoint active regions, and so on. Shibata et al. found two types of the interaction (reconnection) between emerging flux and coronal field; the "anemone-jet" type and the "two-sided-loops/jets" type. The former occurs when emerging flux appears in coronal holes. In this case, a jet is ejected in a vertical direction. On the other hand, the latter occurs when emerging flux appears in quiet regions, and two loop brightenings (or jets) occur in the horizontal direction at both sides of the emerging flux.

Movie 1: X-ray jet observed by the soft X-ray telescope aboard Yohkoh (Shibata et al. 1994). This type is called "anemone-jet" which occurs when emerging flux (rising bipolar magnetic flux) appears in coronal holes, where coronal magnetic field fields are open and nearly vertical or oblique. The left panel shows positive images, and the right panel shows negative images. 100" corresponds to 72000 km on the solar surface.

Movie 2: X-ray loop brightening (or horizontal jets) observed by the soft X-ray telescope aboard Yohkoh (Shibata et al. 1994, 1998). This type is called "two-sided loop (brightenings)" which occur when emerging flux appear in quiet regions, where coronal magnetic field fields are closed and roughly horizontal.

Yokoyama and Shibata (1995,1996) performed 2D (and 2.5D) MHD numerical simulations of magnetic reconnection occurring in the current sheet between emerging flux and overlying pre-existing coronal magnetic fields, by extending the simulation study by Shibata et al. (1992) and applied the results to X-ray jets ("anemone-jet" and "two-sided-loop") discovered by Yohkoh. They succeeded to reproduce many observed features of X-ray jets, such as two types of jets (Movie 1 and 2), whip-like motion of jets, a gap between the footpoint of a jet and the brightest part of a footpoint flare, converging shape of jets, and so on (Shibata et al. 1994, 1996, Shimojo et al. 1996).

It should be noted that the emergence of magnetic flux (rising motion of magnetic loop) itself is self-consistently simulated in these numerical simulations. The basic physics of the emerging flux (Shibata et al. 1989) is the same as that of the nonlinear evolution of the Parker instability in galactic disks (Matsumoto et al. 1988).

The initial condition of these numerical simulations is as follows: The gas layer consists of three layers (isothermal hot corona with temperature T = 25 T_0), isothermal cool chromosphere/photosphere (T = T_0), and convection zone in the deepest layer) in magneto-hydrostatic equilibrium under uniform gravity. The density decreases by more than five decades from the bottom of the gas layer to the corona. Initially, horizontal magnetic flux sheet is embedded in the convection zone just below the photosphere (z < 0), which is unstable to the Parker instability (undular mode of magnetic buoyancy instability). The plasma beta (= gas pressure/magnetic pressure) in the initial flux sheet is 4. On the other hand, nearly uniform horizontal or oblique magnetic filed is assumed in the corona, where the plasma beta is 0.2. The unit of the length and time are H and H/Cs, where H (= 200 km) and Cs (= 10 km/s) are the pressure scale height and sound speed in the cool layer (chromospher/photosphere).

We assume anomalous resistivity model;

eta = eta_0 (v_d - v_c)^2 (for v_d > v_c),

eta = 0 (for v_d < v_c),

where eta is the magnetic diffusivity, v_d = j/rho is the effective electron - ion drift velocity of the electric current (j), rho is the mass density, v_c is the critical velocity (e.g., ion thermal velocity) above which the anomalous resistivity sets in. The effective magnetic Reynolds number (= L V_A/eta) is 300 - 3000, where L is the length of the current sheet, and V_A is the Alfven speed just above the current sheet. This type of resistivity is essential to model fast reconnection (Petschek type reconnection) as shown by Yokoyama and Shibata (1994). They showed that if we assume uniform resistivity, the reconnection becomes Sweet-Parker type even if magnetic reconnection is driven by external force (magnetic buoyancy force in our case).

(Yokoyama and Shibata 1994)

(Yokoyama and Shibata 1995, 1996)

Movie 3: 2D MHD numerical simulations of magnetic reconnection between emerging flux and overlying coronal field (Yokoyama and Shibata 1995, 1996). The case of horizontal coronal magnetic field, as a model of two-sided loop brightenings type. These figures show the density distribution, velocity vectors, and magnetic field lines.

Movie 4: The same as in Movie 3 but for
temperature (Yokoyama and Shibata 1995, 1996).

Movie 5: 2D MHD numerical simulations of magnetic reconnection occurring
between emerging flux and overlying coronal field
(Yokoyama and Shibata 1995, 1996). The case of
oblique coronal magnetic field, as a model of anemone-jet type.
These figures show the
density distribution, velocity vectors, and magnetic field lines.

Movie 6: The same as in Movie 5 but for
temperature (Yokoyama and Shibata 1995, 1996).

Shibata,K., Tajima,T., Steinolfson,R. and Matsumoto,R., (1989)

Shibata, K., Nozawa, S., and Matsumoto, R., (1992)

Shibata, K., Nitta, N., Strong, K. T., et al. (1994)

Shibata, K., Yokoyama, T., Shimojo, M., (1996)

Shibata, K., Nitta, N., Shimojo, M., and Yashiro, S. (1998) in preparation.

Shimojo, M., Hashimoto, S., Shibata, K., et al. (1996)

Yokoyama, T., & Shibata, K. (1994),

Yokoyama, T., & Shibata, K. (1995),

Yokoyama, T., & Shibata, K. (1996),

produced by K. Shibata

in 1998.