have now received a closer look at how protons
in solar winds are heated, since solar wind is much hotter than
expected, and there are several theories explaining why with no real
evidence to back them up. But an expedition to the Sun has provided
the evidence needed.
Sahraoui, lead author of the study and a scientist at the CNRS-Ecole
Polytechnique-UPMC in France, along with co-author Melvyn Goldstein,
chief of the Geospace Physics Laboratory at NASA's Goddard Space
Flight Center in Greenbelt, Maryland, have made it possible to
observe and understand how the solar wind is millions of degrees
This solar wind
surrounds the Sun and is made up of electrons and protons that are
constantly turbulent. This turbulence can result in long streaming
jets, or much smaller movements where charged particles create
miniature orbits. In the midst of this chaos, electric currents and
magnetic waves pass through disturbing the particles further. The
solar wind can move quickly at 750 kilometers per second. Combined
with its temperatures at millions of degrees Celsius, the solar wind
defies all theories - until now.
further observe the solar wind, the ESA/NASA Cluster
was sent into space. The Cluster consists of four identical
spacecraft flying together in formation in order to obtain
3-dimensional snapshots of the particles surrounding the Sun.
had a perfect window of 50 minutes," said Goldstein. "It
was a time when the four Cluster spacecraft were so close together
they could watch movements
in the solar wind at a scale small enough that it was
possible to observe the heating of protons through turbulence
directly for the first time."
heating process works similarly to ocean waves, where turbulence
"cascades" into smaller turbulence. The energy in ocean
wave cascades adds a small amount
of heat from the friction of particles passing one another,
which warms the water a bit. In the case of the solar wind, charged
particles heat up in a similar way but do not encounter this type of
the usual fluids of everyday life, plasmas possess electric and
magnetic fields generated by the motions of protons and electrons,"
said Sahraoui. "This changes much of the intuitive images that
we get from observing conventional fluids."
the exact nature of the waves doing the heating and the exact methods
of energy transfer are not "completely established,"
scientists did find that electric and magnetic fields in the plasma
play major roles in the particles' heating. With the help of previous
solar wind data indicating the effects and length of the magnetic
waves, scientists were able to conclude that the magnetic waves begin
as long wavelength fluctuations, eventually losing energy as they get
shorter over a certain time period. This loss of energy transfers
energy to solar wind particles, which heats them up.
can see that not all the energy is dissipated by protons," said
Sahraoui. "The remaining energy in the wave continues its
journey toward smaller scales, wavelengths of about two kilometers
long. At that point, electrons in turn get heated."
study was published in Physical