Full kinetic simulations of plasma flow interactions with meso- and microscale magnetic dipoles

Abstract

We examined the plasma flow response to meso-and microscale magnetic dipoles by performing three-dimensional full particle-in-cell simulations. We particularly focused on the formation of a magnetosphere and its dependence on the intensity of the magnetic moment. The size of a magnetic dipole immersed in a plasma flow can be characterized by a distance L from the dipole center to the position where the pressure of the local magnetic field becomes equal to the dynamic pressure of the plasma flow under the magnetohydrodynamics (MHD) approximation. In this study, we are interested in a magnetic dipole whose L is smaller than the Larmor radius of ions r iL calculated with the unperturbed dipole field at the distance L from the center. In the simulation results, we confirmed the clear formation of a magnetosphere consisting of a magnetopause and a tail region in the density profile, although the spatial scale is much smaller than the MHD scale. One of the important findings in this study is that the spatial profiles of the plasma density as well as the current flows are remarkably affected by the finite Larmor radius effect of the plasma flow, which is different from the Earth's magnetosphere. The magnetopause found in the upstream region is located at a position much closer to the dipole center than L. In the equatorial plane, we also found an asymmetric density profile with respect to the plasma flow direction, which is caused by plasma gyration in the dipole field region. The ion current layers are created in the inner region of the dipole field, and the electron current also flows in the region beyond the ion current layer because ions with a large inertia can closely approach the dipole center. Unlike the ring current structure of the Earth's magnetosphere, the current layers in the microscale dipole fields are not circularly closed around the dipole center. Since the major current is caused by the particle gyrations, the current is independently determined to be in the direction of the electron and ion gyrations, which are the same in both the upstream and downstream regions. The present analysis on the formation of a magnetosphere in the regime of a microscale magnetic dipole is significant for understanding the solar wind response to the crustal magnetic anomalies on the Moon surface, such as were recently observed by spacecraft. V C 2014 AIP Publishing LLC. [http://dx.

The Formation of a Magnetosphere with Implicit Particle-in-Cell Simulations

“…From the equatorial plane, we can clearly see that the maximum electron density located inside the dipole, is close to the back dipole center. This spiral distribution mainly attribute to the gyration of electron [1]. The stretching of the tail is visible from the elongated blue region along x direction, which is consistent with the development stage in Figure 2.…”

“…Several simulations of magnetosphere using explicit PIC codes are reported in Refs. [3,8,4,1]. We developed a new relativistic particle mover with adaptive sub-cycling to resolve in time the fast gyration of particles located in strong dipolar magnetic field.…”

Formation of collisionless shocks in magnetized plasma interaction with kinetic-scale obstacles

“…In particular, this work showed not only that non-Maxwellian particle distributions are generated from the interaction with the mini magnetosphere, but also that the plasma deflection occurs due to microscopic collective electric fields associated with charge separation between electrons and ions, which can only be appropriately captured using PIC simulations. Ashida et al 28 studied the interaction between an unmagnetized plasma flow and magnetic obstacles with sub-Larmor radius magnetic obstacles, showing that mini magnetospheres can be formed even for obstacles sizes smaller than the ion gyroradius. Other recent works 14,29 with full PIC simulations show that enhanced proton flux around lunar magnetic anomalies can be responsible for the appearance of dark lanes on lunar swirls, regions on the lunar surface commonly found around mini magnetospheres that receive enhanced ageing from direct interaction with the solar wind.…”

Solar wind plasma interaction with Gerasimovich lunar magnetic anomaly

“…Apart from observations, several attempts have been made to model the solar wind plasma interaction with lunar magnetic anomalies theoretically, numerically, and experimentally using single dipole [ Siscoe and Goldstein , ; Harnett and Winglee , , ; Poppe et al , ; Kallio et al , ; Wang et al , , ; Shaikhislamov et al , ; Jarvinen et al , ; Deca et al , ; Ashida et al , ] and multidipole [ Harnett and Winglee , ] approximations. Generally, these models, consistent with observations, have shown that strong magnetic anomalies may shield the lunar surface from solar wind and have a direct impact on surface weathering and swirl albedo markings [ Blewett et al , ; Poppe et al , ].…”