11:30 AM - 11:45 AM
[PEM15-10] 2-D PIC simulations of surface charging processes on the nightside of Phobos
Keywords:Phobos, Surface charging, particle-in-cell simulation
The solar wind, the stream of electrically charged particles that the Sun blows out into space, generates a complex plasma environment around an airless small-body and its surface charging. The nightside charging process is not well understood because of the wake formation and other kinetic plasma processes. It is considered that the wake formation process is basically explained by ambipolar diffusion. Previous studies investigated the nightside potential of the Martian moon Phobos using an analytical model based on a self-similar solution [Farrell et al., 2018]. They suggested that the potential of electric charges reaches about -200 V or more on the nightside of Phobos, which is potentially large enough to affect sensitive equipment. However, their analytic model cannot apply for the deep nightside region because of the lack of reproducing kinetic effects, such as ion acceleration and gyro motion. For this limitation, midnight surface potential has not been clarified.
To estimate the midnight surface charging, we utilize self-consistent 2-D particle-in-cell(PIC) simulations that can solve the plasma environment, including kinetic effects. For efficient calculations, we assume scaled-down parameters that are not the real-size radius of Phobos. First, we compare PIC results to two analytic models, the ambipolar diffusion model and the current balance model. A comparison between the self-similar model and PIC results indicates a large gap within several Debye lengths (λD) from the nightside surface. Although the current balance model shows almost the same tendency as PIC results, there are physical limitations that arisen from the simple assumption. This indicates that the current balance model can apply to the surface only with solar zenith angles of < 98°.
Next, in order to evaluate the effect of scaled-down assumption, we calculate the surface potential for four cases of different body sizes (r = 50, 100, 150, and 200 λD). Based on the previous implication about the ion deflection into the wake region [Nakagawa, 2013], we performed a scaling analysis and revealed the general surface charging process at the nightside of the body. Charging processes are explained by three steps. The first step is the polar sheath formation. In the polar region, ions and electrons continuously impact and the surface gets charged by maintaining the current balance. This process generates an ion sheath near the surface. In the sheath region, stronger electric fields are generated under the assumption in a larger radius case. The second step is ion acceleration by this strong electric field. This step explains the process of ion penetration into the deep wake region. The final step is electron access into the deep nightside region. At the nightside surface near the void, more electrons can hit the surface together with deflected ions. This process leads to strong negative charging at the nightside in the larger radius case.
Our PIC simulations contribute to an understanding of the plasma environment on the nightside of Phobos. These implications can be useful for the Japanese future mission, Mars Moon eXploration (MMX).
To estimate the midnight surface charging, we utilize self-consistent 2-D particle-in-cell(PIC) simulations that can solve the plasma environment, including kinetic effects. For efficient calculations, we assume scaled-down parameters that are not the real-size radius of Phobos. First, we compare PIC results to two analytic models, the ambipolar diffusion model and the current balance model. A comparison between the self-similar model and PIC results indicates a large gap within several Debye lengths (λD) from the nightside surface. Although the current balance model shows almost the same tendency as PIC results, there are physical limitations that arisen from the simple assumption. This indicates that the current balance model can apply to the surface only with solar zenith angles of < 98°.
Next, in order to evaluate the effect of scaled-down assumption, we calculate the surface potential for four cases of different body sizes (r = 50, 100, 150, and 200 λD). Based on the previous implication about the ion deflection into the wake region [Nakagawa, 2013], we performed a scaling analysis and revealed the general surface charging process at the nightside of the body. Charging processes are explained by three steps. The first step is the polar sheath formation. In the polar region, ions and electrons continuously impact and the surface gets charged by maintaining the current balance. This process generates an ion sheath near the surface. In the sheath region, stronger electric fields are generated under the assumption in a larger radius case. The second step is ion acceleration by this strong electric field. This step explains the process of ion penetration into the deep wake region. The final step is electron access into the deep nightside region. At the nightside surface near the void, more electrons can hit the surface together with deflected ions. This process leads to strong negative charging at the nightside in the larger radius case.
Our PIC simulations contribute to an understanding of the plasma environment on the nightside of Phobos. These implications can be useful for the Japanese future mission, Mars Moon eXploration (MMX).