Sen— A flotilla of European spacecraft called Cluster has, for more than a decade, been helping scientists learn more about a natural shield that protects Earth from harmful radiation from space.
But this forcefield called the magnetosphere is not a perfect barrier. The latest finding by the four probes is that it lets in the solar wind under a wider range of conditions than had been thought.
Knowing just what is happening is important because this stream of hot, ionised gas from the Sun not only produces those dramatic and beautiful light shows called the aurora. They also create "space weather" that can threaten power grids, electronics aboard satellites in orbit and even astronauts' lives.
The four identical Cluster probes, nicknamed Rumba, Salsa, Samba and Tango, were launched in 2000 by two Soyuz-Fregat rockets and placed in a triangular pyramid formation in a large elliptical polar orbit that took them from a distance of 19,000km to 119,000km above the Earth every 57 hours. The inner point of their orbit has subsequently been lowered.
Their mission for the European Space Agency (ESA) was to study the vast magnetic bubble of the magnetosphere which is thought to be generated by a dynamo process in our planet's liquid core. Just like a bar magnet, field lines in the magnetosphere channel some of the stream of charged particles from the Sun towards the magnetic poles.
The plasma from the Sun carries its own magnetic field with it and when the particles collide with atoms and molecules in the ionosphere, they produce energy that causes them to glow brightly, between 100 and 1,000km above the Earth.
Depending on how the solar wind's interplanetary magnetic field is aligned with Earth's magnetic field, different phenomena can arise in our planet's immediate environment.
One well-known process is magnetic reconnection, where magnetic field lines pointing in opposite directions spontaneously break and reconnect with other nearby field lines. This redirects their plasma load into the magnetosphere, opening the door to the solar wind and allowing it to reach Earth.
In 2006, scientists were surprised when Cluster discovered that huge, 40,000 km swirls of plasma along the boundary of the magnetosphere – called the magnetopause – could allow the solar wind to enter, even when Earth's magnetic field and the IMF are aligned.
An impression of the Cluster probes flying through a region of the magnetosphere called the magnetotail because it is formed behind the Earth. Credit: ESA / AOES Medialab
These giant vortices were found at low, equatorial latitudes, where the magnetic fields were most closely aligned and are driven by a well-known process in nature known as the Kelvin–Helmholtz (KH) effect.
Now analysis of Cluster data has revealed that KH waves can also occur at a wider range of locations in the magnetopause, and that in a number of other configurations of the interplanetary magnetic field, it can allow the solar wind entry into Earth's magnetosphere.
Kyoung-Joo Hwang, of NASA's Goddard Space Flight Center, is lead author of a paper published in the Journal of Geophysical Research. He said: "We found that when the interplanetary magnetic field is westward or eastward, magnetopause boundary layers at higher latitude become most subject to KH instabilities, regions quite distant from previous observations of these waves.
"In fact, it's very hard to imagine a situation where solar wind plasma could not leak into the magnetosphere, since it is not a perfect magnetic bubble."
Co-author Melvyn Goldstein, also from Goddard, said: "The solar wind can enter the magnetosphere at different locations and under different magnetic field conditions that we hadn't known about before. That suggests there is a 'sieve-like' property of the magnetopause in allowing the solar wind to continuously flow into the magnetosphere."
ESA's Cluster project scientist Matt Taylor said: "Cluster's observations of these boundary waves have provided a great advance on our understanding of solar wind."