Statistics – Applications
Scientific paper
Mar 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt.........2t&link_type=abstract
PhD Thesis
Statistics
Applications
3
Mhd, Numerical Simulations, X-Ray, Galaxy, Magnetic Field
Scientific paper
In this thesis, we perform two-dimensional (2D) resistive magnetohydrodynamic (MHD) numerical simulations of the magnetic reconnection in interstellar medium. Part I is introduction. The motivation of the study is to investigate the origin of hot gas in interstellar medium. A scenario for generating X-ray gas in Galaxy is proposed, and examined by performing 2D MHD simulations with simple assumptions (Part II). The magnetic reconnection triggered by a supernova (Part III) and Parker instability (Part IV) are studied in detail, by performing 2D MHD simulations. Furthermore, the magnetic reconnection is also studied by performing three-dimensional (3D) MHD numerical simulation in (Part V). % Finally, we discuss and summarize the thesis (Parts VI and VII). Part I First, we review observation of Galactic Ridge X-ray Emission (GRXE) and its problems. Second, we describe observation of interstellar magnetic field briefly. Third, we review magnetic reconnection, theoretical models, numerical simulations, observations and experiments, and tearing instability. Forth, Parker instability (undular mode of magnetobuoyancy instability) is mentioned. Finally, we show the purpose of this thesis. Part II We present a scenario for the origin of the hot plasma in Galaxy as a model of strong X-ray emission [sim 3-10 keV; LX(2-10 keV) sim 1038 erg s-1], called GRXE, which has been observed near to the galactic plane. GRXE is thermal emission from a hot component (sim 7 keV) and a cool component (sim 0.8 keV). Observations suggest that the hot component is diffuse, and that it is not escaping away freely. Both what heats the hot component and what confines it in Galactic ridge still remain puzzling, while the cool component is believed to be created by supernovae. We propose a new scenario: the hot component is heated by magnetic reconnection, and confined by a helical magnetic field produced by magnetic reconnection. We solved 2D MHD equations numerically to study how magnetic reconnection, triggered by a supernova explosion, creates hot plasmas and magnetic islands (helical tubes), and how the magnetic islands confine the hot plasmas in Galaxy. The supernova shock is one of the possible mechanisms to trigger reconnection in Galaxy. We conclude that magnetic reconnection is able to heat the GRXE plasma if the magnetic field is localized in an intense flux tube with Blocal sim 30 muG. Part III This is the main part of the thesis. We examine the magnetic reconnection triggered by a supernova shock (or a point explosion) in interstellar medium, by performing 2D MHD numerical simulations with high spatial resolution. The magnetic reconnection starts long after the supernova shock (fast-mode MHD shock wave) passes a current sheet. The current sheet evolves as follows: (i) The tearing-mode instability is excited by the supernova shock. The current sheet becomes thin in the nonlinear phase of tearing instability. (ii) The current-sheet thinning is saturated when the current-sheet thickness becomes comparable to that of Sweet-Parker current sheet. After that, Sweet-Parker type reconnection starts, and the current-sheet length increases. (iii) The secondary tearing-mode instability occurs in the thin Sweet-Parker current sheet. (iv) As a result, further current-sheet thinning occurs, because gas density decreases in the current sheet. The anomalous resistivity sets in, and Petschek type reconnection starts. The interstellar gas is accelerated and heated. The magnetic energy is released quickly while magnetic islands are moving in the current sheet during Petschek type reconnection. (v) Magnetic reconnection stops because the gas pressure increases in the current sheet near left and right boundaries. The released magnetic energy is determined by the interstellar magnetic field strength, not by the energy of initial supernova nor distance between the supernova and the current sheet. We suggest that magnetic reconnection is a possible mechanism to generate X-ray gas in Galaxy. Part IV We examine the magnetic reconnection triggered by Parker instability and the resulting magnetic structure in Galaxy, by performing 2D MHD numerical simulations with simple assumptions. We assume, as the initial condition, that there exists horizontal magnetic flux sheet unstable to Parker instability in the disk, and anti-parallel field in the halo, because there is another magnetic field (which is not parallel) generated by Colioris force or shear motion. The magnetic field in the disk inflates toward Galactic halo, collide with magnetic field in the halo, and trigger the magnetic reconnection. The magnetic reconnection accelerates the interstellar gas, and releases the magnetic energy to heat the gas in Galactic halo. The released magnetic energy is determined by the interstellar magnetic field strength. We suggest that the magnetic reconnection is a possible mechanism to generate X-ray plasma in Galactic halo. Part V We examine the magnetic reconnection triggered by a supernova shock (or a pont explosion) in interstellar medium, by performing 3D MHD numerical simulations. We extend the 2D numerical simulations, performed in part III, to 3D. All assumption is same with those in 2D model except for the grid size and simulation region size. The magnetic reconnection occurs in the current sheet, which is similar to 2D model with the large glid size. We study the 3D effects on the magnetic reconnection. Part VI We discuss how can we apply the results to magnetic reconnections in the actual interstellar medium. We estimate the actual time scale and length scale of the magnetic reconnection and tearing instability. Magnetic reconnection will occur by the current-sheet thinning by the secondary tearing instability in nonlinear phase. Otherwise, the magnetic reconnection will occur through third, forth, and fifth tearing instability. We discuss the implications for the observations and possibility of oblique shock in reconnection jets and particle acceleration. Applications to other astrophysical objects such as Galactic center and interstellar jet and star formation, and other galaxies and cluster of galaxies are also discussed. Part VII We conclude and summarize this thesis. We end off this thesis by the description of future works.
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