Mercury Makes Waves Cruising through the Solar Wind

Mercury orbits the Sun in a unique regime. The solar wind is still fresh from the Sun, and the Sun’s magnetic field strength (which drops with the square of distance) is rapidly waning. Furthermore, Mercury’s highly elliptical orbit means the planet passes through a wider range of distances from the Sun 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.

These animations provide a conceptual schematic of the results of one such investigation as described in “Occurrence rate of ultra-low frequency waves in the foreshock of Mercury increases with heliocentric distance.” Using data from NASA’s MESSENGER spacecraft, the authors has detected Ultra Low Frequency (ULF) waves rebounding from Mercury’s foreshock, the turbulent area where solar wind particles collide with Mercury’s magnetosphere. These waves are caused by solar wind protons – the steady stream of particles escaping the Sun –collide with and reflect off of this foreshock against the stream of the solar wind.

The authors discovered that the ULF wave production rate varied throughout Mercury’s orbit. MESSENGER detected more ULF waves as Mercury moved farther from the Sun in its orbit, and fewer as it approached the Sun. The results support an existing theory that claimed that ULF waves are affected in part by the strength of the solar magnetic field, which is at its weakest when Mercury is farthest from the Sun.