ESA's quartet of satellites studying Earth's magnetosphere（磁气圈）, Cluster, has discovered that our protective magnetic bubble lets the solar wind in under a wider range of conditions than previously believed. Earth's magnetic field is our planet's first line of defence against the bombardment of the solar wind. This stream of plasma is launched by the Sun and travels across the Solar System, carrying its own magnetic field with it.

Depending on how the solar wind's interplanetary magnetic field -- IMF -- is aligned with Earth's magnetic field, different phenomena can arise in Earth'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.

Under certain circumstances this can drive 'space weather', generating spectacular aurorae, interrupting GPS signals and affecting terrestrial（陆地的） power systems.

In 2006, Cluster made the surprising discovery that huge, 40 000 km swirls of plasma along the boundary of the magnetosphere -- the magnetopause -- could allow the solar wind to enter, even when Earth's magnetic field and the IMF are aligned.

These swirls were found at low, equatorial latitudes, where the magnetic fields were most closely aligned.

These giant vortices are driven by a process known as the Kelvin-Helmholtz (KH) effect, which can occur anywhere in nature when two adjacent（邻近的） flows slip past each other at different speeds.

Examples include waves whipped up by wind sliding across the surface of the ocean, or in atmospheric clouds.

Analysis of Cluster data has now found that KH waves can also occur at a wider range of magnetopause locations and when the IMF is arranged in a number of other configurations, providing a mechanism for the continuous transport of the solar wind into Earth's magnetosphere.

"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," says Kyoung-Joo Hwang of NASA's Goddard Space Flight Center and lead author of the paper published in the Journal of Geophysical Research.