Mercury is special. As the closest planet to the Sun, it occupies a region where the Sun’s influence is changing dramatically. The Sun’s magnetic field, which dominates space close to the Sun, is rapidly waning. And Mercury’s orbit – more elliptical or “oval-shaped” than any other planet – allows it to experience a wider range of solar magnetic field conditions than any other planet. As a result, Mercury provides a unique opportunity to study how the Sun’s influence on a planet varies with distance.
In a new study published in Nature Communications, Goddard scientists Norberto Romanelli and Gina DiBraccio used data from NASA’s MESSENGER spacecraft to study the Sun’s changing interaction with Mercury. As Mercury moves through the solar wind, the steady stream of particles escaping the Sun, some of them strike Mercury’s magnetosphere and bounce back towards the Sun. These rebounding solar wind particles generate low-frequency waves that reverberate through space, traveling “upstream” in the solar wind towards the Sun.
Romanelli and DiBraccio observed these waves emanating from Mercury and discovered that the rate of wave production varied throughout Mercury’s orbit. As Mercury moved farther from the Sun it generated more waves; as it got closer, the rate of wave production dropped. The results provide key evidence for a theory that these waves are affected, in part, by the strength of the Sun’s magnetic field, which grows weaker with distance.
So Mercury is special not just because it’s the closest planet to the Sun but also because the distance it is from the Sun varies more than any other planet in our solar system.
Mercury actually has a very oval-shaped orbit compared to the rest.
Mercury is a natural laboratory for running experiments. We can observe the planet’s surroundings as it gets closer and farther away from the Sun and see what changes.
When solar wind particles bounce off this magnetospheric boundary, they generate low-frequency waves. They’re like ripples traveling upstream in the solar wind.
Farther from the Sun, the Sun’s magnetic field is weaker. We found that this is the key parameter that allows more waves to be generated.
This is something we could only have measured with Mercury, thanks to how close it is to the Sun and the shape of its orbit.
Reference: “Occurrence rate of ultra-low frequency waves in the foreshock of Mercury increases with heliocentric distance” by N. Romanelli and G. A. DiBraccio, 19 November 2021, Nature Communications.