部分开放多极磁场中的日冕磁绳灾变

采用球坐标下二维三分量理想MHD模型,研究部分开放多极背景磁场中日冕磁绳的灾变现象.背景磁场由含3个闭合双极场的冕流和带赤道中性电流片的开放场构成,磁绳位于中心双极场的下方,其特性由环向磁通和轴向磁通表征.对给定的环向磁通,存在轴向磁通的一个临界值;对给定的轴向磁通,也存在环向磁通的一个临界值.在该临界值以下,磁绳附着于太阳表面,系统处于平衡状态;该临界值一旦被超越,磁绳将脱离太阳表面向上喷发,说明部分开放多极背景磁场中的日冕磁绳系统存在灾变现象.本文算例表明,灾变点对应的磁能阈值超过对应部分开放场(中心双极场开放,两侧的双极场仍维持闭合)能量约15%,其超过部分可为日冕物质抛射一类太阳爆发提供能源.

Coronal Flux Rope Catastrophe in Partly-Open Multipolar Magnetic Field

Using a 2.5-dimantional ideal MHD model in spherical coordinates, the catastrophe of a coronal magnetic flux rope embedded in a partly-open multipolar background magnetic field has been studied. The background field consists of a coronal streamer containing three closed bipolar fields and an open magnetic field with an equatorial current sheet. The magnetic flux rope lies below the central bipolar field, characterized by its annular and axial magnetic fluxes. There exists a critical axial flux for a given annular flux, or a critical annular flux for a given axial flux such that below each of these critical fluxes, the magnetic flux rope remains to be attached to the solar surface and the whole system stays in equilibrium. On the other hand, if the critical fluxes are exceeded, the flux rope leaves the solar surface and erupts upward, implying an existence of catastrophe for a coronal flux rope embedded in a partly-open multipolar background magnetic field. For the given example, the corresponding threshold of magnetic energy at the catastrophic point is found to be about 15% larger than the energy of the corresponding partly-open magnetic field in which the central bipolar field is fully open whereas those at the flank remain closed. The excessive energy may serve as an energy source for solar explosions such as coronal mass ejections.