@DavidHammen's answer to If a solar flare happened during a total lunar eclipse, would the Earth block the flare from hitting the moon? And for how long? describes the regular lashing of the Moon by the Earth's magnetotail. Go read it first then come back; I'll wait.
The Falcon and the Snowman The Moon and the Magnetotail says:
Anyone can tell when the moon is inside the magnetotail. Just look: “If the moon is full, it is inside the magnetotail,” says Stubbs. “The moon enters the magnetotail three days before it is full and takes about six days to cross and exit on the other side.”
It is during those six days that strange things can happen.
The most dramatic effects are intermittent and sporadic:
[...] Much of this is pure speculation, Stubbs cautions. No one can say for sure what happens on the moon when the magnetotail hits, because no one has been there at the crucial time. “Apollo astronauts never landed on a full moon and they never experienced the magnetotail.”
The best direct evidence comes from NASA’s Lunar Prospector spacecraft, which orbited the moon in 1998-99 and monitored many magnetotail crossings. During some crossings, the spacecraft sensed big changes in the lunar nightside voltage, jumping “typically from -200 V to -1000 V,” says Jasper Halekas of UC Berkeley who has been studying the decade-old data.
Question: How did the Lunar Prospector spacecraft measure the voltage of the Moon?