Instabilities In Current Sheets

Current sheets are basic plasma structures in nature,which have been observed in the magnetotail,magnetopause,bow shock and Heliosphere.Current sheets are strongly inhomogeneous narrow transition layers between two regions of oppositely directed magnetic fields,Important transport process are known to happen in this narrow layer,most notably is the process of magnetic reconnection,during which magnetic energy is converted to plasma kinetic and thermal energy.Magnetic reconnection is believed to play a central role in many violent plasma phenomena such as solar flares, coronal mass ejections,magnetospheric substorms,and sawtooth crashes in tokamaks. The collisionless instabilities in current sheet are crucial for determining the onset conditions and time scales of magnetic reconnection.Tearing mode in itself is a process of magnetic reconnection.Various drift instabilities do not reconnect the magnetic,however, can influence the magnetic reconnection onset through anomalous resistivity or through their effects on tearing mode.In this paper,by using a formally exact method to solve the linear Vlasov-Maxwell system,tearing mode and drift kink mode are investigated.The orbit integrals are treated numerically using the exact unperturbed particle orbits,and the resulting eigenvalue problem of the integro-differential equations is solved using the spectral method.It is found that tearing mode and drift kink mode in the current sheet with a superthermal velocity distribution(kappa-distribution) have smaller growth rate than their counterparts in the current sheet with an Maxwellian distribution.The effects of distribution function anisotropy on tearing mode are investigated. For Harris sheets,the electron temperature anisotropy has important influence on tear-ing mode:electron temperature anisotropy is strongly destabilizing when Te⊥＞Te‖and strongly stabilizing when Te⊥＜Te‖.The ion temerature anisotropy has the same destabilizing/stabilizing effect as electrons on tearing mode.However,compared with electron temperature anisotropy,the ion temerature anisotropy has much less influence on tearing mode.We proposed a new current sheet model,namely Kappa-Maxwell sheet,which has a Maxwellian velocity distribution in the direction parallel to the magnetic field and a Kappa velocity distribution in the direction perpendicular to the magnetic field. This kind of current sheet mode can well represent the velocity distribution observed in the numerical simulation of lower-hybrid drift instability.We found that the tearing mode in Kappa-Maxwell sheet has much larger growth rate than its counterpart in isotropic Harris sheet.The explanation is that the perpendicular effective temperature is greater than parallel temperature in Kappa-Maxwell sheets and this kind of temperature anisotropy is known to be able to enhance the tearing mode growth rate.In chapter 5,in the framework of local theory,we investigated the effect of background population in current sheet on the lower-hybrid drift instability.The results indicate that the background population has a stabilizing effect on lower-hybrid drift instability and reduces the phase velocity of the mode.