What is the average electrical charge on the lunar surface?

Question

Known facts:

The lunar surface is covered by regolith.
The Solar wind imparts a charge to the lunar surface

Wikipedia says

On the daylit side of the Moon, solar ultraviolet and X-ray radiation is energetic enough to knock electrons out of atoms and molecules in the lunar soil

The same article further states

On the night side, the dust is negatively charged by electrons in the solar wind.

Given both sides of Luna acquire an electrical charge,

Was the magnitude of the acquired charge on either side ever quantified?
What is the average charge magnitude on either side of the lunar soil?

I think that your question implies that there's a lit side and an unlit side of the Moon that don't change over time, when actually all sides of the Moon do receive periods of sunlight as the Moon circles the Earth. — Don Branson, Sep 02 '13 at 13:29

score 10 · Accepted Answer · edited Sep 02 '13 at 22:28

Yes, the ARTEMIS satellite monitors the lunar surface (a link to mission data).

The lunar surface gets ionized by the plasma, cosmic rays, and coronal mass ejections from the Sun.

Solar System Exploration Division, NASA Goddard Space Flight Center and Space Sciences Laboratory, University of California paper on Lunar surface charging: Magnitude and implications as a function of space and time (PDF) suggests that:

Theoretically, the Moon should charge to small positive values of ~+5-10V on the sunlit hemisphere (where photoemission dominates), and to larger negative values of ~-100V on the shad-owed hemisphere (where photoemission is absent)

Another paper on Lunar surface charging: A global perspective using Lunar Prospector data by that same group of researches states:

There are significant uncertainties in lunar surface charging processes, and very little is known about either spatial or temporal values.

What is the average electrical charge on the lunar surface?

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