During the past century of exploration of the Universe, we have learned that normal matter in the Universe is primarily in the plasma state. It is the hot dilute plasma (ionized gas) between galaxies and galaxy clusters, and not stars, that dominates baryonic matter. Furthermore most of the baryonic matter in the Universe is not detectable in the visible light, but instead becomes apparent only in X-rays that are generated by hot plasmas. Hot dilute plasma can also be found within galaxies, such as interstellar medium, outer atmospheres and stellar winds of stars, coronas of accretions disks. These hot plasmas may well be heated by the dissipation of the turbulence driven by large scale shear motions, shock waves, jets, and other large-scale instabilities and processes. Astrophysical plasmas are turbulent, and dissipation of turbulent fluctuations leads to continuous plasma heating and to acceleration of charged particles. Understanding basic plasma processes of plasma heating and energization in turbulent magnetized plasmas is of fundamental importance if we are ever to understand the evolution of the Universe.
Turbulent fluctuations in astrophysical plasmas reach up to scales as large as stars, bubbles or "clouds" blown out by stellar winds, or even entire galaxies. However, most of the irreversible dissipation of energy within turbulent fluctuations occurs at the very small scales - kinetic scales, where the plasma no longer behaves as a fluid and the properties of individual plasma species (electrons, protons, and other ions) become important. The efficiency of plasma heating, the partition of energy transferred to different particle species, the acceleration of particles to high energies - all are strongly governed by kinetic processes that determine how the turbulent electromagnetic fluctuations dissipate. Thus, plasma processes at kinetic scales will directly affect the large-scale properties of plasma.
Turbulence Heating ObserveR (THOR) is the first mission ever flown in space dedicated to plasma turbulence. It will explore the kinetic plasma processes that determine the fundamental behavior of the majority of baryonic matter in the universe. THOR will lead to an understanding of the basic plasma heating and particle energization processes, of their efficiency for different plasma species and of their relative importance in different turbulent regimes. THOR will provide closure of these fundamental questions by making detailed in situ measurements of the closest available dilute and turbulent magnetized plasmas at unprecedented temporal and spatial resolution. THOR focuses on particular regions - pristine solar wind, Earth's bow shock and interplanetary shocks, and compressed solar wind regions downstream of shocks. These regions are selected because of their differing turbulent fluctuation characteristics, and reflect similar astrophysical environments. In addition, both spatial and temporal characteristic plasma scales in the key science regions are sufficiently large, so that the particle instruments are able to resolve the kinetic scales. The THOR spacecraft will carry, for the first time, a comprehensive payload tailored to explore plasma energization in turbulence, with both fields and particle instrumentation that will allow the simultaneous resolution of both the turbulent fluctuations and the signature of the resultant plasma energization. The payload consists of mature instruments with recent flight heritage. THOR will also open new paths by providing measurements that go beyond our current theoretical expectations, thus allowing the exploration of new physics and challenging our theories.
THOR science directly addresses the Cosmic Vision question "How does the Solar System work?" by studying basic processes occurring "From the Sun to the edge of the Solar System". By quantifying the fundamental processes involved, the advances made by the THOR mission will extend beyond the Solar System to plasmas elsewhere in the Universe. THOR will provide understanding of fundamental plasma processes with applications to very different astrophysical, solar system and laboratory plasma environments. Due to studies involving a variety of space missions, including Cluster and THEMIS (and in the near future, missions such as Magnetospheric Multiscale, Solar Orbiter and Solar Probe Plus) we now understand many aspects of plasma turbulence, such as 3D properties of plasma turbulence owing to multi-spacecraft observations. However, how the turbulence dissipates and heats the surrounding medium and energizes particles is not at all well understood. That is the unique mission of THOR. THOR will provide the understanding of fundamental processes underlying the measurements of exciting future missions such as the L2 X-ray astronomy mission with science theme: "The Hot and Energetic Universe''.
Our local space environment, near Earth's space, provide a unique opportunity for in situ study plasma turbulence under a wide range of physical parameters that reflect conditions in other astrophysical locales. The plasma turbulence community is one of the largest cross-disciplinary science communities. Carefully designed laboratory plasma experiments, as well as increasingly sophisticated numerical simulations will complement space experiments such as we propose to conduct with THOR. The totality of those efforts will lead to a paradigm shift in our understanding of turbulence and energization mechanisms in astrophysical plasmas and will open new horizons for studying the fundamental physics of visible matter.