Magnetic Reconnection Triggered by the Parker Instability in the Galaxy: Two-dimensional Numerical Magnetohydrodynamic Simulations and Application to the Origin of X-Ray Gas in the Galactic Halo

摘要

We propose the Galactic flare model for the origin of the X-ray gas in the Galactic halo. For this purpose, we examine the magnetic reconnection triggered by Parker instability (magnetic buoyancy instability), by performing the two-dimensional resistive numerical magnetohydrodynamic simulations. As a result of numerical simulations, the system evolves through the following phases. Parker instability occurs in the Galactic disk. In the nonlinear phase of Parker instability, the magnetic loop inflates from the Galactic disk into the Galactic halo and collides with the antiparallel magnetic field, so that the current sheets are created in the Galactic halo. The tearing instability occurs and creates the plasmoids (magnetic islands). Just after the plasmoid ejection, further current sheet thinning occurs in the sheet, and the anomalous resistivity sets in. Petschek reconnection starts and heats the gas quickly in the Galactic halo. It also creates the slow and fast shock regions in the Galactic halo. The magnetic field (B ~ 3 μG), for example, can heat the gas (n ~ 10-3 cm-3) to a temperature of ~106 K via the reconnection in the Galactic halo. The gas is accelerated to Alfvén velocity (~300 km s-1). Such high-velocity jets are the evidence of the Galactic flare model we present in this paper, if the Doppler shift of the bipolar jet is detected in the Galactic halo.