Dust-driven Dynamos in Accretion Disks

Magnetically driven astrophysical jets are related to accretion and involve toroidal magnetic field pressure inflating poloidal magnetic field flux surfaces. Examination of particle motion in combined gravitational and magnetic fields shows that these astrophysical jet toroidal and poloidal magnetic fields can be powered by the gravitational energy liberated by accreting dust grains that have become positively charged by emitting photo-electrons. Because a dust grain experiences magnetic forces after becoming charged, but not before, charging can cause irreversible trapping of the grain, so dust accretion is a consequence of charging. Furthermore, charging causes canonical angular momentum to replace mechanical angular momentum as the relevant constant of the motion. The resulting effective potential has three distinct classes of accreting particles distinguished by canonical angular momentum, namely (1) “cyclotron-orbit,” (2) “Speiser-orbit,” and (3) “zero canonical angular momentum” particles. Electrons and ions are of class 1, but depending on mass and initial orbit inclination, dust grains can be of any class. Light-weight dust grains develop class 1 orbits such that the grains are confined to nested poloidal flux surfaces, whereas grains with a critical weight such that they experience comparable gravitational and magnetic forces can develop class 2 or class 3 orbits, respectively producing poloidal and toroidal field dynamos.

磁驱动的天体物理喷流与吸积有关，并涉及环形磁场压力使极向磁场通量面膨胀。对在引力场和磁场共同作用下的粒子运动的研究表明，这些天体物理喷流的环形和极向磁场可以由吸积因发射光电子而带正电的尘埃颗粒所释放的引力能提供动力。因为尘埃颗粒在带电之后（而非之前）会受到磁力作用，带电会导致颗粒不可逆地被捕获，所以尘埃吸积是带电的结果。此外，带电导致正则角动量取代机械角动量成为相关的运动常量。由此产生的有效势根据正则角动量区分出三类不同的吸积粒子，即（1）“回旋轨道”，（2）“斯皮泽轨道”和（3）“零正则角动量”粒子。电子和离子属于第1类，但根据质量和初始轨道倾角，尘埃颗粒可以属于任何一类。轻质尘埃颗粒形成第1类轨道，使得颗粒被限制在嵌套的极向通量面内，而具有临界质量使得其受到相当的引力和磁力的颗粒可以分别形成第2类或第3类轨道，从而分别产生极向场和环形场发电机效应。