Electric Moon jolts the solar wind
The solar wind is a thin stream of electrically conducting gas called plasma thats 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 Earths magnetic field, it can generate 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.
Unlike Earth, the Moon is not 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, says Dr. Andrew Poppe of the University of California, Berkeley. Recently, however, an international fleet of lunar-orbiting spacecraft has detected signs of the Moons presence upstream in the solar wind. Weve seen electron beams and ion fountains over the Moons day side, says Dr. Jasper Halekas, also of the University of California, Berkeley.
These phenomena have been seen as far as 10,000 kilometers (6,214 miles) above the Moon and generate a kind of turbulence in the solar wind ahead of the Moon, causing subtle changes in the solar winds direction and density. The electron beams were first seen by NASAs Lunar Prospector mission, while the Japanese Kaguya mission, the Chinese Change mission, and the Indian Chandrayaan mission all saw ion plumes at low altitudes. NASAs ARTEMIS mission has now also seen 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, says Halekas. ARTEMIS stands for Acceleration, Reconnection, Turbulence and Electrodynamics of the Moons Interaction with the Sun.
An upstream turbulent region called the foreshock has long been known to exist ahead of the Earths bow shock, but the discovery of a similar turbulent layer at the Moon is a surprise, said Dr. William Farrell of NASAs Goddard Space Flight Center in Greenbelt, Md. Farrell is lead of the NASA Lunar Science Institutes Dynamic Response of the Environment At the Moon (DREAM) team, which contributed to the research.
Computer simulations help explain these observations by showing 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 can generate electron beams by accelerating electrons blasted from surface material by solar ultraviolet light. Also, related simulations show 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 centers (nuclei) of hydrogen atoms, the most common element in the solar wind.
Its 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, says Poppe. Other moons and asteroids in the solar system should have this turbulent layer over their day sides as well, according to the team.
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 its actually far away, said Halekas.