Discovering mysterious plasma waves around Mercury

If you listen with the right instruments, you can hear the whistling, almost like birdsong at dawn and dusk. So-called choral waves have been recorded on Earth, Jupiter and Saturn. It was observed on Uranus and Neptune.

Now, led by astronomer Mitsunori Ozaki of Kanazawa University, scientists from Japan and France have discovered those planets orbiting Mercury, barren and lonely, as it orbits the sun.

This is interesting, because these other planets have some things that Mercury doesn’t: thick, lush atmospheres and permanent radiation belts where the Sun’s particles are trapped in the planet’s magnetic field.

Scientists say it’s a discovery that could shed some light on the magnetic environment around Mercury and how planetary magnetic fields are generally shaped by the solar wind.

Mercury does not have a large magnetic field. It’s a fairly bare piece of rock, with an almost non-existent atmosphere, and too close to the sun for comfort. It is constantly exposed to radiation as well as to the solar wind.

But this scattered and distorted world holds secrets. Just this year, scientists finally discovered that Mercury, even with its pathetic magnetic field and atmosphere, has a strange kind of aurora.

But long before this discovery, scientists thought that Mercury might have chorus waves. This happens when energetic electrons become trapped in the planet’s magnetosphere, spiraling along magnetic field lines and generating waves in the plasma.

These waves can be recorded and converted into sounds that vary depending on how and where the electrons move. You can hear, for example, hissing waves recorded on Earth in the video below.

Exploration of Mercury has been sporadic and scattered, meaning that our understanding of its space environment is incomplete. We’ve known about its magnetic field since Mariner 10 made observations in the 1970s.

But scientists are trying to make up for the deficiency. Part of the Mercury BepiColombo mission, launched in 2018, is an instrument called MIO dedicated to studying Mercury’s magnetosphere.

This device has not yet reached orbit; The Sun’s gravity makes orbital insertion complicated. But the spacecraft conducted flybys around Mercury in 2021 and 2022, which recorded observations of Mercury’s magnetic field.

There, in the data collected by MIO, researchers found clear evidence of whistle waves in Mercury’s magnetosphere. But, because it was Mercury, there was something strange about them: they only appeared in a small part of Mercury’s magnetosphere, in a wedge known as the dawn sector.

This suggests a physical mechanism that enhances chorus waves in that region or suppresses them elsewhere. The team performed modeling and simulation and determined that the transfer of energy from electrons to electromagnetic waves is most efficient in the early morning sector, generating whistles.

A greater understanding and characterization of these chorus waves will require further observations and further analysis. These first discoveries will allow researchers to plan their investigations in detail, before the orbital insertion of MIO in 2025.

“At this time, we still do not know whether Earth and Mercury have similar spatiotemporal properties of their electron-driven chorus,” the researchers wrote.

“The current study paves the way for challenging future investigations that will reveal how magnetized planetary environments are shaped by solar winds in our solar system, with possible extrapolations to exoplanets and their interactions with stellar winds.”

Translated by Matthews Lineker from Science Alert