行星际日冕物质抛射引起福布斯下降的一维随机微分模拟

福布斯下降(Forbush decrease,FD)是银河宇宙线(galactic cosmic rays,GCRs)受短期剧烈太阳活动调制的重要现象之一.本文设GCRs进入由行星际日冕物质抛射(interplanetary coronal mass ejection,ICME)及其前沿激波共同形成的扰动区时,其径向扩散系数κ_(rr)受抑制变为μ(r)·κ_(rr)(0μ(r)1),且抑制强度与粒子位置处的太阳风等离子体速度正相关.对任意时刻的扰动区,抑制系数μ(r)在激波处最小为μ(r_(sh)),并按指数规律增大,在ICME尾部归一.CME爆发时,μ(r_(sh))取全局最小值μm.在扰动区向日球层外传播的过程中,μ(r_(sh))逐步恢复为1.在此基础上,根据GOES和ACE卫星观测确定模型参数,用一维随机微分方程描述GCRs在日球层内的传播,并采用倒向随机方法模拟了一个由独立Halo ICME调制GCRs引起的2005年5月15日FD事件.计算所得地面中子通量的主相、恢复相及其在CME到达地球前的增加过程,均与Oulu中子探测器观测结果一致.

Interplanetary coronal mass ejection induced forbush decrease event:a simulation study with one-dimensional stochastic differential method

Forbush decrease (FD) event is one of the most important short-term modulations of galactic cosmic rays (GCRs) caused by intense solar activities such as interplanetary coronal mass ejection (ICME). The modulation mechanisms of GCRs by the disturbed interplanetary magnetic fields (IMF) of ICME and the accompanying forward interplanetary shock (IP) are not clear yet. In this work, we present a one-dimensional dynamic model of the GCR barrier driven by ICME. In our model, the time dependent radial diffusion coefficient κ_rr of GCRs is depressed to be μ(r)·κ_rr (0 < μ(r) ≤ 1) as they run into the disturbed IMF. The scale factor μ(r) is inversely proportional to the local solar wind speed away from the Sun. Within the disturbed area at any time, μ(r) increases exponentially from the local minimum μ(r_sh)) at the IP front to 1 at the end of the ICME tail. In addition, μ(r_sh)) switches gradually from its global minimum μ_m at the bursting of the CME to 1 as the shock moving toward the outer boundary of the heliosphere. The geometrical and dynamic parameters of the ICME and IP are derived from the observations of GOES and ACE satellites. Based on the stochastic transport theory, the one-dimensional backward stochastic differential equation (SDE) method is adopted to simulate the transport of GCRs modulated by single halo ICME. The evolution of the neutron flux at the ground is calculated according to the recently reported proton-neutron yield function. As an example, the FD event on 15 May 2005, caused by the CME event bursting on 13 May 2005, is studied and simulated. The results show that the calculated neutron flux evolution, including not only the main and recovery phases, but also the pre-enhancement before the arriving of the CME at the Earth, is consistent with the observation of Oulu neutron monitor. According to the trajectories of GCRs, it can be found that, the per-enhancement of the neutron flux is a result of the scattering by the forward IP passing 1 AU. Before the IP reaches the switch cutoff R_c, GCRs are evidently confined in the sheath between the IP and CME. After that, the GCRs will stay for longer time in the magnetic cloud of the ICME as a result of the damping of IP strength. The parameterzed one-dimensional GCRs modulation model and the SDE method, as have been confirmed by the neutron monitor observation on the Earth, can be used further to calculate and predict the GCRs fluxes of other places, such as the Mars, in the heliosphere.