ESTEC 7 Dec 20071

Radio Properties of the Moon:

a brief review of some important bits

Graham WoanUniversity of Glasgow

ESTEC 7 Dec 20072

lunar regolith

• Regolith electrical properties from Apollo missions:

• electrical conductivity ~10-14 /Ohm/m at d.c.

• loss tangent ~0.001 to 0.1 at ~10 MHz

• relative permittivity ~6

• Conductivity low enough to allow dipoles to be placed on the regolith without degrading beam pattern too severely (see later).

• dipole antenna pattern has largest lobe on the lunar side

• skin depth of between 1 and 100 (30 km at 1 MHz)

• possible trouble for subsurface reflections (from mascons etc)

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lunar exosphere

• Dual-frequency Luna spacecraft measurements give reasonably good evidence that an ionised layer, several kilometres thick, builds up on the illuminated side of the Moon (Vyshlov 1976)

• Max illuminated plasma frequency of ~0.5 MHz

• No layer seen during the lunar night

Luna 22

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Dust

• Photo-ionisation may levitate dust electrostatically to give a “dust fountain”:

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Lunar opacity

RAE-2 occultation of Earth

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Lunar opacity

RAE-2 occultation of Sun

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Takahashi and Woan

Radio propagation around the Moon, with subsurface penetration and a lunar density model

Lunar opacity

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Takahashi and Woan

Radio propagation at the Mapert Mountain (lunar south pole), with subsurface penetration and a lunar density model

Lunar opacity

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Surface dipole field response

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unresolved lunar radio questions

• What is the electron ‘weather’ really like on the Moon?– Diurnal variations– Solar wind induced variations– Over the terminator

• What are the electrical properties of the regolith at likely sites?

• What is the true attenuation factor of the occulting Moon at the surface?

• What is the noise floor like on the dark, far-side surface?

• Can we achieve the dynamic range needed for HI surveying out to z~100-1000?

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What are the other current/planned investigations?

• Kaguya (Japan) and Chang’e 1(China) both have exosphere monitoring experiments, and plans for future lunar low-frequency experiments (see, e.g., Maohai Huang’s talk, IAU 26). More details needed!