Modeling the Lunar Wake Response to a CME Using a Hybrid PIC Model

In the solar wind, a low-density wake region forms downstream of the nightside lunar surface. In this study, we use a series of 3D hybrid particle-in-cell simulations to model the response of the lunar wake to a passing coronal mass ejection (CME). Average plasma parameters are derived from the Wind spacecraft located at 1 au during three distinct phases of a passing halo (Earth-directed) CME on 2015 June 22. Each set of plasma parameters, representing the shock/plasma sheath, a magnetic cloud, and plasma conditions we call the mid-CME phase, are used as the time-static upstream boundary conditions for three separate simulations. These simulation results are then compared with results that use nominal solar wind conditions. Results show a shortened plasma void compared to nominal conditions and a distinctive rarefaction cone originating from the terminator during the CME’s plasma sheath phase, while a highly elongated plasma void reforms during the magnetic cloud and mid-CME phases. Developments of electric and magnetic field intensification are also observed during the plasma sheath phase along the central wake, while electrostatic turbulence dominates along the plasma void boundaries and 2–3 lunar radii R M downstream in the central wake during the magnetic cloud and mid-CME phases. The simulations demonstrate that the lunar wake responds in a dynamic way with the changes in the upstream solar wind during a CME.

Electrostatic Solitary Structures in Space Plasmas: Soliton Perspective

Occurrence of electrostatic solitary waves (ESWs) is ubiquitous in space plasmas, e.g., solar wind, Lunar wake and the planetary magnetospheres. Several theoretical models have been proposed to interpret the observed characteristics of the ESWs. These models can broadly be put into two main categories, namely, Bernstein–Green–Kruskal (BGK) modes/phase space holes models, and ion- and electron- acoustic solitons models. There has been a tendency in the space community to favor the models based on BGK modes/phase space holes. Only recently, the potential of soliton models to explain the characteristics of ESWs is being realized. The idea of this review is to present current understanding of the ion- and electron-acoustic solitons and double layers models in multi-component space plasmas. In these models, all the plasma species are considered fluids except the energetic electron component, which is governed by either a kappa distribution or a Maxwellian distribution. Further, these models consider the nonlinear electrostatic waves propagating parallel to the ambient magnetic field. The relationship between the space observations of ESWs and theoretical models is highlighted. Some specific applications of ion- and electron-acoustic solitons/double layers will be discussed by comparing the theoretical predictions with the observations of ESWs in space plasmas. It is shown that the ion- and electron-acoustic solitons/double layers models provide a plausible interpretation for the ESWs observed in space plasmas.

Do Impulsive Solar-Energetic-Electron (SEE) Events Drive High-Voltage Charging Events on the Nightside of the Moon?

When the Earth’s moon is in the supersonic solar wind, the darkside of the Moon and the lunar plasma wake can be very dangerous charging environments. In the absence of photoelectron emission (dark) and in the absence of cool plasma (wake), the emission or collection of charge to reduce electrical potentials is difficult. Unique extreme charging events may occur during impulsive solar-energetic-electron (SEE) events when the lunar wake is dominated by relativistic electrons, with the potential to charge and differentially charge objects on and above the lunar surface to very-high negative electrical potentials. In this report the geometry of the magnetic connections from the Sun to the lunar nightside are explored; these magnetic connections are the pathways for SEEs from the Sun. Rudimentary charging calculations for objects in the relativistic-electron environment of the lunar wake are performed. To enable these charging calculations, secondary-electron yields for impacts by relativistic electrons are derived. Needed lunar electrical-grounding precautions for SEE events are discussed. Calls are made 1) for future dynamic simulations of the plasma wake in the presence of time-varying SEE-event relativistic electrons and time-varying solar-wind magnetic-field orientations and 2) for future charging calculations in the relativistic-electron wake environment and on the darkside lunar surface.

A Double-Disturbed Lunar Plasma Wake

<p>Under nominal solar wind conditions, a low density wake region forms downstream of the nightside lunar surface.  However, the lunar plasma environment undergoes a transformation as the Moon passes through the Earth’s magnetotail, with the warm plasma typically not having a strong flow, and thus the wake structure disappears.  However, while in the tail, there can be a sudden intense change due to solar-driven events such as coronal mass ejections.  With a new planned human presence on the Moon, it is important to understand the near-surface plasma environment’s response to these extreme conditions.  We investigate the response of the lunar wake to a passing coronal mass ejection on 2012 March 8 while crossing the Earth’s magnetotail using both a large-scale MHD model of the Earth’s global magnetosphere and smaller-scale 3-D hybrid-PIC simulations.</p><p>The CME plasma shock was detected by the Wind spacecraft around 10:30 UT and in the Earth’s magnetotail around 11:20 UT by the ARTEMIS spacecraft in lunar orbit.  Wind observations are used as time-dependent up-stream conditions for a 24-hour global magnetosphere MHD simulation run through NASA’s Community Coordinated Modeling Center using the OpenGGCM model.  Extracted plasma parameters from the ARTEMIS spacecraft following the plasma shock are used as upstream static boundary conditions for hybrid-PIC simulations using the AMITIS code.</p><p>Results for the hybrid-PIC lunar wake simulations performed during a momentary jump in magnetotail plasma velocity and density show a short misaligned plasma void relative to nominal SW conditions.  MHD results indicate that changes near the Moon appear as a result of a warped magnetopause boundary moving inward after 11:00 UT, causing the Moon to enter the magnetosheath.  These results also show a number of plasmoids developing and propagating down the tail, including one seen at 11:20 UT that corresponds temporarily with plasmoid-like features in the ARTEMIS magnetic field profiles.</p>