A 60-Year Old Mystery About the Moon's Magnetosphere Is Finally Solved

One particularly well known fact about the Moon is that it doesn’t have much of a magnetosphere to speak of. There’s no blanket to protect it from the solar wind ravaging its surface, blowing away its atmosphere and charging the notoriously dangerous dust particles that make up its regolith. However, scientists have also known for around 60 years that some parts of the moon do experience sudden spikes in a magnetic field - some of which are up to 10 times stronger than the background magnetization. Since their discovery, these “lunar external magnetic enhancements” (LEMEs) have puzzled researchers - what was causing them, and why did they reach so high above the lunar surface that spacecraft could see them? A new paper published in The Astrophysical Journal Letters by Shu-Hua Lai and her colleagues at the National Central University in Taiwan explains for the first time what is likely causing these LEMEs - a novel type of the Kelvin-Helmholtz instability.

If you’ve ever seen rolling wave-like clouds in the sky, you’ve seen the Kelvin-Helmholtz instability (KHI) in action. It’s a fundamental physical process that happens whenever two fluids (or in space, two waves of plasma) are moving past each other at different speeds, creating what is known as a velocity shear. In the case of LEMEs, scientists knew the solar wind slammed into these “minimagnetospheres” created by surface anomalies of magnetic material in lunar regolith. But, they believed the KHI caused by this interaction would be localized to only the boundary where the two meet. It couldn’t explain why they were seeing magnetic fields on spacecraft hundreds of kilometers above the surface.

Like many physical phenomena, KHI requires a lot of complex math, and Dr. Shu-Hua and her colleagues realized that, in an effort to explain these LEMEs, scientists were using a simplified form of math for the calculation of how the KHI would appear at the boundary. Instead, they were interested in using what is known as a “nonlinear” branch of the same phenomena. Basically, they used some more advanced math to model the KHI in a way that better reflect what they believed was actually happening at the boundary between the minimagnetosphere caused by the magnetic material in the Moon’s surface and the solar wind.

Video describing the Lunar Vertex mission, which will explore more of the Moon’s magnetic anomalies. Credit - Taiwan Space Union YouTube Channel

To prove their idea, they turned to simulations - in this case, non-linear magnetohydrodynamic simulations. They created three “Cases” that each represented different solar wind speeds and that resulted in different types of KHI “regimes”. The two cases with higher wind speeds resulted in a “shock-dominated” KHI regime that produced fast, upward-propagating shock waves of magnetic fields - matching much of the data collected by spacecraft on LEMEs over the years. However, even at slow solar wind speeds, the “vortex-dominated” KHI regime that was created still locally amplified the magnetic field up to around 30-40 times the ambient level near the boundary layer. But, surprisingly, even in this regime, the waves from the near-surface vortex still propagate upwards into denser plasma, creating secondary waves at much higher altitudes.

Most importantly, the simulation data matched some real observations collected by Lunar Prospector back in 1998. It proved that the nonlinear version of the KHI instability was capable of producing the types of magnetic fields seen in the data. And the fact that all the different types of magnetic field shocks and vortices could be accounted for by this improvement in the mathematical modeling showcases how a simple tweak in how scientists look at a problem could break open a decades-long mystery.

It isn’t only the Moon this study could be applicable to, though. The researchers note that this exact same mechanism is likely happening on Mars. Recent observations from MAVEN have already confirmed that KHI can develop in the Martian plasma environment, and there are plenty of Martian crustal anomalies analogous to those on the Moon that could interact the same way. So in fact, in addition to explaining a 60 year old mystery, this new model of KHI plasma interactions could hold clues about the space environment on plenty of other weakly magnetized bodies across our solar system.

Discussion on how the solar wind interacts with lunar magnetic anomalies. Credit - Taiwan Space Union YouTube Channel

Learn More:

Taiwan National Central University - Taiwan NCU Researchers Solve a 60-Year-Old Lunar Magnetic Mystery

S.H. Lai, K. Wang & Y.H. Yang - Shock-like Magnetic Enhancements Generated by Kelvin–Helmholtz Instability above Weakly Magnetized Bodies

UT - Did a Large Impact on the Moon Make its Rocks Magnetic?

UT - Mysterious Swirls on the Moon Could Be Explained by Underground Magma