Limb Sounding: A new remote sensing technique for lunar dusty exosphere monitoring

Introduction

The lunar dust environment in the exosphere, coupled with the other weather environments — plasma, electromagnetic field, and the surface regolith — is a highly dynamic system in nature.  Particularly, since the discovery of the horizon glow in the 60s and 70s, scientists (and astronauts) have discussed the source of the emission. A potential, and widely accepted, hypothesis is the levitation of lunar dust in the near-terminator exosphere.  However, it is still controversial due to the lack of experimental evidence. Recent measurements, particularly a dedicated in situ dust instrument on the LADEE spacecraft, did not find such dust components (Horanyi et al., 2015), raising questions about the levitated dust hypothesis.

 

Here, we propose a new, complementary technique of measurements using recently established energetic neutral atom imaging.  The technique, named "Limb Sounding", can offer the exospheric density profile and levitated dust composition in the terminator region in a remote sensing manner (Futaana et al., 2024).  In this presentation, we outline the technique, introduce its theory, and discuss its feasibility. We will also depict a simple system implementation as a pathfinder mission.

 

Outline of limb sounding

Figure 1 shows two main processes occurring in the dusty exosphere. The charge exchange with exospheric atoms neutralizes the solar wind (panel 1) and collision with a dust grain (panel 2). Depending on the size of the grain, the impinging solar wind proton is neutralized and decelerated. In both cases, the neutralized solar wind (also called energetic neutral atoms, ENAs) can reach the spacecraft that will be measured during the ingress to or egress from the lunar shadow. This geometry is required since the dusty exosphere is between the Sun and the spacecraft, and thus, the solar wind protons pass through the exosphere. Since a part of the solar wind will lose its charge states and be converted to energetic neutral atoms (ENAs) by several mechanisms in the dusty exosphere, the ENAs contain information about the dusty exosphere.

 

ENAs produced by exospheric gas: ENAs produced by the lunar exospheric neutrals were studied by Futaana et al. (2008).  They conducted a forward model using an ideal exosphere and claimed that the ENA flux is high enough to be measured by an established ENA instrument. Theoretically, the flux is proportional to the exospheric density and the path length of the solar wind in the exosphere.  Thus, the measured ENA flux contains the information of the exospheric density and spatial distribution (scale height). By time series measurements using the moving spacecraft, various paths of the measurements within an ingress (and egress) path can be measured.

 

ENAs produced by dust grains: Collision with dust grains is the other source of ENA production. Collier et al. (2009) established a theory of the dust-originated ENA production and demonstrated that the flux is measurable if the solar wind speed is high enough.  The detection also depends on the dust grain size distribution.  Here, we extended their theory to consider the energy loss and angular spread.  This extension allows us to simulate the expected energy and directional distribution and their time series. This will further enable us to assess the feasibility of retrieving the dust grain size distribution and its height profile.

 

These two processes can be distinguished by measuring the energy of the hydrogen: The charge exchange does not alter the energy, but the dust grain penetration will decelerate the solar wind proton during the neutralization process.

 

Summary

In this presentation, we will outline the limb sounding technique, highlighting its theoretical feasibility. Particularly, their angular and energy distributions will be discussed. We further describe an example system to demonstrate the technique using a CubeSat as a pathfinder. With a relatively simple measurement system, the limb sounding technique can provide an opportunity to investigate the dynamic lunar dusty exosphere. Once it is validated, the measurement principle will potentially provide a long-term monitoring system of the lunar dusty exosphere, possibly contaminated by human activities in the coming decades.

Figure 1: Illustration of the Limb Sounding technique. (Top) A production mechanism of energetic neutral atoms (ENAs) by exospheric gas via the charge exchange mechanism. (Bottom) Another production mechanism of ENAs, i.e., the solar wind penetrating through dust grains in the exosphere.