Abstract
Abstract 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.
Highlights
As one can imagine, the Moon is an obstacle in the solar wind plasma flow
A clear and distinct void with compressed magnetic and electric fields forms downstream from the lunar nightside. This downstream void is much shorter than that modeled by Holmström et al (2012) with nominal solar wind conditions and a 45° Parker spiral interplanetary magnetic field (IMF), with wake refilling aided by the higher plasma temperatures enhancing the plasma expansion process (Fatemi 2014)
Examining different phases of a coronal mass ejection (CME) using hybrid simulations highlights changes the lunar wake can undergo over the course of hours
Summary
The Moon is an obstacle in the solar wind plasma flow. Observations by the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun (THEMIS-ARTEMIS, an extension of the Time History of Events and Macroscale Interactions during Substorms mission) spacecraft indicate that associated plasma disturbances create strong electric fields just downstream of the terminator and the formation of an extended wake structure that is gradually refilled by electrons and ions downstream (Zhang et al 2014; Xu et al 2019). There are several modeling studies focusing on the lunar wake structure under nominal solar wind conditions using MHD descriptions (Spreiter et al 1970; Cui & Lei 2008; Xie et al 2013; Michel 2014) as well as particle (Farrell et al 1998; Birch & Chapman 2001; Nakagawa 2013) and hybrid PIC models (Wang et al 2011; Fatemi et al 2012, 2013; Holmström et al 2012; Vernisse et al 2013; Poppe et al 2014). Several studies have already addressed the near-surface lunar wake environment and effects of CMEs and solar energetic particle (SEP) events at the lunar surface. We will conclude with a discussion and conclusions of our results
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