We have always believed that we are safe, but it doesn't mean that the situation will remain unchanged.
The Earth's magnetosphere is leaking, the solar wind penetrates deeper. We are not entirely safe because
weakened field could leave our planet vulnerable to solar winds.
The Cluster quartet - Rumba, Salsa, Samba and Tango
Credits: Dornier Satelliten Systeme GmbH
Data delivered by ESA’s Cluster quartet of satellites shows that it is easier for the solar wind
to penetrate Earth’s magnetic environment, the magnetosphere, than had previously been thought.
Using Rumba, Salsa, Samba and Tango satellites as a space plasma microscope, scientists have zoomed in on the
solar wind to reveal tiny turbulent swirls that could play a big role in heating it.
Also scientists from NASA's Goddard Space Flight Center in Greenbelt, Md. have, for the first time, directly observed
the presence of certain waves in the solar wind—called Kelvin-Helmholtz waves that can help transfer energy into near-Earth
space—under circumstances when previous theories predicted they were not expected.
Turbulence is highly complex and all around us, evident in water flowing from a tap, around an aircraft wing, in experimental
fusion reactors on Earth, and also in space.
In the stream of charged particles emitted by the Sun – the solar wind – turbulence is thought to play a key part in
maintaining its heat as it streams away and races across the Solar System.
Click on image to enlarge
Tiny turbulent swirls in the solar wind
A 2D vision of the solar wind turbulence at the smallest scale seen yet, thanks to observations by Cluster
satellites. The approximate location of the measurements are indicated on a graphic illustrating features of Earth’s
magnetic environment. The inset shows conditions as would be seen facing the solar wind, with current sheets forming at
the border of turbulent eddies. The trajectory of the cluster spacecraft is marked on the inset by the black line and
the colour gradients represent the magnetic field strength intensity from 4.8 nT (darkest shades) to 5.2 nT (white).
Credits: ESA/ATG Medialab; inset: J. Dorelli (NASA).
As the solar wind expands, it cools down, but to a much smaller extent than would be expected if the flow were smooth.
Turbulence arises from irregularities in the flow of particles and magnetic field lines, but understanding how this
energy is transferred from the large scales where it originates, to the small scales where it is dissipated, is like
trying to trace energy as it is transferred from the smooth, laminar flow of a river down to the small turbulent eddies
formed at the bottom of a waterfall.
Two of the four Cluster satellites have also made extremely detailed observations of plasma turbulence in
the solar wind.
Separated by just 20 km along the direction of the plasma flow, the satellites operated in ‘burst mode’ to take
450 measurements per second.
By comparing the results with computer simulations, scientists confirmed the existence of sheets of electric current just
20 km across, on the borders of turbulent swirls.
“This shows for the first time that the solar wind plasma is extremely structured at this high resolution,” says Silvia
Perri of the Universita della Calabria, Italy, and lead author of the paper reporting the result.
Cluster previously detected current sheets on much larger scales of 100 km in the magnetosheath, the region sandwiched
between Earth’s magnetic bubble – the magnetosphere – and the bow shock that is created as it meets the solar wind.
At the borders of these turbulent eddies the process of ‘magnetic reconnection’ was detected, whereby oppositely directed field lines spontaneously break and reconnect with other nearby field lines, thus releasing their energy.
Artist’s impression of the four Cluster spacecraft flying through the thin layer of Earth’s bow shock. Credit: ESA/AOES Medialab
“Although we haven’t yet detected reconnection occurring at these new, smaller scales, it is clear that we are seeing a cascade of energy which may contribute to the overall heating of the solar wind,” said Dr Perri.
Future missions such as ESA’s Solar Orbiter and NASA’s Solar Probe Plus will be able to determine whether similar processes are also in play closer to the Sun, while NASA’s Magnetospheric Multiscale mission will specifically probe the small-scale regions where reconnection can occur.
“This Cluster result demonstrates the mission’s unique capability to probe universal physical phenomena, in this case pushing the mission’s instrument measurement capabilities to their limit to unlock features at small scales,” comments Matt Taylor, ESA’s Cluster Project Scientist.
“Future multi-spacecraft missions will make very detailed studies of these small-scale plasma phenomena and provide further context to our Cluster measurements.”