The Full Moon's Passage Through
Earth's Magnetotail

The Energetics of a Full Moon

Nick Anthony Fiorenza

Moon passing through Earth's magnetotail

Illustration based on NASA's illustration

Once every lunar month, the Moon passes through Earth's magnetotail. The Earth's magnetotail (magnetic tail) is an extension of Earth's magnetic sphere, which extends outward from the core of the Earth. The magnetotail is caused by the solar wind's presure against Earth's magnetic sphere, thus always blowing away from the direction of the Sun.

The magnetotail extends well beyond the orbit of the Moon, thus the Moon enters the magnetotail during its orbit around the Earth. The Moon enters the magnetotail about three days before the Full Moon and exits about three days after. It enters just after the Gibbous Moon, in the "Pollinating Phase" of the lunar cycle, which begins a social time of magnetic attraction. It leaves the magnetotail just before the Disseminating Moon, toward the end of the "Fruit Bearing Phase."

Excerpts below are from: "The Moon and the Magnetotail" by Dr. Tony Phillips; NASA.

"During the crossing, the Moon passes through a gigantic plasma sheet of hot charged particles trapped in the tail. The lightest and most mobile of these particles, electrons, pepper the Moon's surface and give the Moon a negative charge.

On the Moon's dayside this effect is counteracted to a degree by sunlight: UV photons knock electrons back off the surface, keeping the build-up of charge at relatively low levels. But on the nightside, in the cold lunar dark, electrons accumulate and voltages can climb to hundreds or thousands of volts.

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.

"It is important to note," says Halekas, "that the plasma sheet (where all the electrons come from) is a very dynamic structure. The plasma sheet is in a constant state of motion, flapping up and down all the time. So as the Moon orbits through the magnetotail, the plasma sheet can sweep across it over and over again. Depending on how dynamic things are, we can encounter the plasma sheet many times during a single pass through the magnetotail with encounters lasting anywhere from minutes to hours or even days."

"As a result, you can imagine how dynamic the charging environment on the Moon is. The Moon can be just sitting there in a quiet region of the magnetotail and then suddenly all this hot plasma goes sweeping by causing the nightside potential to spike to a kilovolt. Then it drops back again just as quickly."

The roller coaster of charge would be at its most dizzying during solar and geomagnetic storms....

Earth's magnetotail isn't the only source of plasma to charge the Moon. Solar wind can provide charged particles, too; indeed, most of the time, the solar wind is the primary source. But when the Moon enters the magnetotail, the solar wind is pushed back and the plasma sheet takes over. The plasma sheet is about 10 times hotter than the solar wind and that gives it more "punch" when it comes to altering the charge balance of the Moon's surface. Two million degree electrons in the plasma sheet race around like crazy and many of them hit the Moon's surface. Solar wind electrons are relatively cool at only 140 thousand degrees, and fewer of them zip all the way down to the shadowed surface of the Moon's nightside."

The Lunar Phases

Perhaps this is another of several reasons for the high-emotional and sometimes erratic energy people often feel around the Full Moon, and even why some Full Moons are more intense than others. Solar storms and solar flairs coinciding with the Full Moon would certainly intensify our lunacy.

Earth's Plasma Tail

A schematic diagram of Earth's magnetosphere. Earth is the circle near the middle
and the plasma tail is denoted in yellow. Credit: Larry Lyons/UCLA


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