A recent study by an international team of astronomers has determined that Earth's moon does indeed have a far-reaching, invisible influence on the Sun's solar wind.
Solar wind can best be described as a thin stream of electrically conducting gas called plasma that's constantly blown off the surface of the Sun in all directions at around a million miles per hour.
When a particularly fast, dense or turbulent solar wind strikes Earth's magnetic field, it often generates magnetic and radiation storms that are capable of disrupting satellites, power grids, and communication systems.
The magnetic "bubble" surrounding Earth also pushes back on the solar wind, creating a bow shock tens of thousands of miles across over the day side of Earth where the solar wind slams into the magnetic field and abruptly slows from supersonic to subsonic speed. However, unlike Earth, the moon isn't surrounded by a global magnetic field.
"It was thought that the solar wind crashes into the lunar surface without any warning or 'push back' on the solar wind," explained Dr. Andrew Poppe of the University of California, Berkeley. "Recently, however, an international fleet of lunar-orbiting spacecraft has detected signs of the moon's presence 'upstream' in the solar wind. We've [definitely] seen electron beams and ion fountains over the moon's day side."
To be sure, such phenomena have been observed as far as 10,000 kilometers (6,214 miles) above the moon - generating a kind of turbulence in the solar wind ahead of the moon and causing subtle changes in the solar wind's direction and density.
The electron beams were first identified by NASA's Lunar Prospector mission, while the Japanese Kaguya mission, the Chinese Chang'e mission, and the Indian Chandrayaan mission all confirmed ion plumes at low altitudes. NASA's ARTEMIS mission has also observed both the electron beams and the ion plumes, plus newly identified electromagnetic and electrostatic waves in the plasma ahead of the moon, at much greater distances from the moon.
"With ARTEMIS, we can see the plasma ring and wiggle a bit, surprisingly far away from the moon," said Dr. Jasper Halekas, also of the University of California, Berkeley.
And as Dr. William Farrell of NASA's Goddard Space Flight Center in Greenbelt notes, an upstream turbulent region called the 'foreshock' has long been known to exist ahead of the Earth's bow shock.
"[Still], the discovery of a similar turbulent layer at the moon is a surprise," he added.
According to Farrell, computer simulations help explain these observations by demonstrating that a complex electric field near the lunar surface is generated by sunlight and the flow of the solar wind. The simulation reveals this electric field is capable of generating electron beams by accelerating electrons blasted from surface material by solar ultraviolet light.
In addition, related simulations indicate that when ions in the solar wind collide with ancient, "fossil" magnetic fields in certain areas on the lunar surface, they are reflected back into space in a diffuse, fountain-shaped pattern. These ions are mostly the positively charged ions (protons) of hydrogen atoms, the most common element in the solar wind.
"It's remarkable that electric and magnetic fields within just a few meters (yards) of the lunar surface can cause the turbulence we see thousands of kilometers away. When exposed to solar winds, other moons and asteroids in the solar system should have this turbulent layer over their day sides as well," says Poppe.
"Discovering more about this layer will enhance our understanding of the moon and potentially other bodies because it allows information about conditions very near the surface to propagate to great distances, so a spacecraft will gain the ability to virtually explore close to these objects when it's actually far away."