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THOR: A next generation spacecraft to study plasma heating in the universe
The THOR spacecraft is proposed to be the first space mission dedicated to fundamental problem of plasma heating. The Universe is permeated by hot, turbulent magnetized plasmas. We believe that energy dissipation of turbulent fluctuations in plasmas play a key role in plasma heating and energization. Understanding these processes  is fundamental for understanding of much of the matter in the Universe. THOR is planned to orbit Earth and study plasma energization in turbulence in our solar system. THOR, together with astrophysical observations and numerical simulations, will enable us to understand a key interaction between planets and their hosts stars.
The European Space Agency (ESA) has selected THOR for further consideration as the fourth medium-class mission in ESA’s Cosmic Vision science programme. THOR (the Turbulence Heating ObserveR) is among the three candidate concepts (of an original 27) recommended for further study. If chosen it would be launched in 2025.
Crab nebula

We posed a few questions to the happy project scientist, Dr Andris Vaivads (Swedish Institute of Space Physics, Uppsala):
What will THOR do?
“THOR is really a study of fundamental physics. With a spacecraft orbiting in near-Earth space we can study and understand how space plasma is heated to very high temperatures. Plasma is very common in the whole galaxy, and beyond.”
What is space plasma?
“Most of the visible Universe is in the plasma state, meaning that the particles are charged. This includes stars, stellar winds and the upper atmospheres of planets (including Earth). This also includes far away objects such as active galactic nuclei, supernova remnants and the interstellar and intergalactic medium filling the space between stars and between galaxies.”
How is space plasma heated?

“As particles are charged, they interact with electric and magnetic fields permeating the Universe, and as a result can gain or lose energy. When the majority of particles gain energy, plasma gets hotter.  The strongest heating occurs when the electric and magnetic fields are very irregular — one often says turbulent. Thus turbulence is a key ingredient when one needs to heat plasma.” 

Why is this important to understand?
“This is a chance to understand fundamental processes in the Universe. When I look at stars at night, I know that there is plasma out there and that turbulence is heating particles everywhere. This hot plasma is producing radiation (the light that we see). Yet we do not know how this happens because we cannot measure plasma so far away. On the other hand there is plasma around the Earth, and this plasma we can study directly using spacecraft. But until now we have not had sufficiently detailed measurements to understand how exactly the heating works. THOR will have state of the art instruments allowing precise measurements down to the smallest scales. Thus THOR is a kind of a plasma microscope looking at cosmic plasmas. Remarkably, it allows us not only to see details of things near us, but also to better understand plasmas many light years away.”
Can knowledge of plasma heating and turbulence be of any practical everyday use?
“For us earthlings, understanding plasma heating might have very practical consequences in the future. Energetic plasma particles and turbulence are some of the fundamental processes determining space weather. Our advanced society is becoming sensitive to extreme space weather events that can damage spacecraft, and disrupt GPS signals and power lines. To make predictions of possible disruptions, we need to understand turbulence and plasma heating. In addition, plasma heating is a practical problem for attempts to produce electrical power by confining man-made plasmas in fusion reactors. If we understand turbulence, we can minimize it.”  
How can you study far away stars and galaxies without sending the THOR spacecraft there?
“Physics is the same everywhere. We will understand a lot more about plasma processes around Earth. We will understand a lot more about how particles are heated and accelerated. In far away stars and galaxies, the heated particles generate electromagnetic radiation such as visible light and X-rays. We will work with astronomers who observe this radiation. Parameters such as density and magnetic field strength can be very different in other parts of the galaxy. We will work with numerical experts who use computer simulations to verify our findings for objects where we cannot go with a spacecraft.”

See also:
2015-06-10, ESA press release
2015-06-17, IRF press release, (in swedish)