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.
Mercury (black and white circle; black indicating the nightside) orbits the Sun. The Sun’s magnetic field is rendered in the gradient blue background, where dark blue (close to the Sun) indicates the strongest magnetic field and light blue (far from the Sun) indicates weakest. Mercury’s orbit is shown with a dotted white line. Mercury’s magnetosphere, the region of space influenced by the planet’s magnetic field, is rendered as a peach-colored parabola around the planet. The solar wind is modeled as tiny rays escaping from the Sun. As Mercury moves along its orbit, solar wind particles strike the front boundary of its magnetosphere, or foreshock. Some number of solar wind protons rebound from the foreshock, generating low-frequency plasma waves (green curved lines) that issue from the leading edge of Mercury’s foreshock. As Mercury moves along to the portion of its orbit farthest from the Sun, the rate of ULF waves increases. This far away portion is where the solar magnetic field is weakest.