The instruments onboard THOR take the next technological steps to further increase the time resolution over existing magnetospheric satellite missions. In comparison to earlier and other upcoming missions key major improvements include:
Particle heating and acceleration happen both in quasi-monochromatic waves and in turbulent fluctuations. While the quasi-monochromatic waves have discrete frequencies and wavevectors and appear as clear peaks in the energy spectrum, the turbulent fields no longer have clear spatial or temporal structures and appear as a continuous spectrum.
Properties of the quasi-monochromatic waves are studied using a variety of analysis methods. Using the magnetic field data, one may determine the energy spectra for different fluctuation components, the compressibility of magnetic field, the field rotation sense around the mean magnetic field, and the wavevector direction by minimum variance analysis. When combining with the electric field and the plasma data, one may determine the phase speed, the Poynting flux, the helicity quantities, the wave distribution function, and the wavevectors.
For the studies of turbulent fluctuations, the slope and the shape (flattening or steepening) of the energy spectrum are determined not only in the frequency domain (in the spacecraft frame) but also in the streamwise wavenumber domain using Taylor's frozen-in flow hypothesis. The spectra of the helicity quantities can also be determined. Wave-wave and wave-particle interactions are studied, e.g., for the detection of Landau and cyclotron resonances, pitch angle scattering, and three-wave couplings (bispectrum). Statistical behavior, in particular non-Gaussian nature of the turbulent fluctuations are studied using the method of phase coherence, probability density function, and local intermittency measure.
Turbulent fields are closely associated with coherent structures and discontinuities. Shock waves, for example, are a major driver of turbulence. In order to investigate one dimensional plasma structures such as current sheets or two and tree dimensional structures such as magnetic islands and flux ropes, it is often useful to transform them into a proper, co-moving reference frame. Often, it is also necessary to establish the orientation of plasma structures. Experience based on multi-spacecraft missions like Cluster and MMS has shown that a well equipped single spacecraft platform, like the proposed THOR mission, is capable of providing the required resolution and can resolve the inherent space-time ambiguities without the need for a full constellation of spacecraft.
To achieve this, a number of methods are available. In particular, variance analysis and residue methods have proven to be very robust and useful for this purpose. Residue methods are typically based on simple conservation laws for example conservation of energy, mass or flux. The most commonly used method is probably minimum variance of the magnetic field to establish the orientation of a one-dimensional current sheet [Sonnerup & Scheible, (1998)]. A unified approach to variance analysis and residue methods, applicable to any measured quantity (both vector fields and scalars) which obeys classical conservation laws was presented in Sonnerup et al, (2006). In its simplest form, the unified approach only takes the B-field as input. More refined variants take the electric field, density, plasma flow or higher order moments such as pressure or heat flux as inputs. An advantage of residue methods is that they can provide error estimates both for the frame velocity and for the boundary normal of the structure. An overview of the methods and their application can be found in Paschmann and Daly (2008).
During the last decade, various methods to reconstruct the topology of the magnetic field and plasma properties in a region around the spacecraft have been developed. Plasma and field parameters can be visualized by two dimensional maps as shown in the figure to the right. The reconstruction is based on MHD equations and under the assumption that structures can be treated as 2D structures and are nearly time-independent in their proper reference frame. The proper frame and invariant axis are typically established using one of the structure analysis methods mentioned above.
Reconstruction has been tested extensively in a number of plasma regimes and scale sizes ranging large flux ropes in the solar wind to the electron diffusion region in a reconnection region. The high time resolution of THOR opens for new possibilities to investigate the topology of small scale structures.
project that investigates solar system plasma turbulence through a thorough survey of the existing data bases (e.g. Ulysses, Cluster, Venus Express). One of the goals of the project and one of its major deliverables is to build and test an integrated software library for nonlinear data analysis (INA) that cumulates analysis methods able to reveal the structure and topology of turbulent fluctuations. The library is versatile enough to ingest data from various missions and to apply a full package of analysis methods, ranging from lower order analysis, like the Power Spectral Density analysis, to higher order analyses like the Probability Distribution Functions and their moments (e.g. flatness), wavelet analysis and multifractals. The integrated library is equipped with a graphical user interface that enables an interactive analysis and a full control of the analysis parameters. The library is extensively tested with ESA missions data (e.g. Cluster, Ulysses, Venus Express). More detailed description is in the attachment.
IRFU-MATLAB (https://sites.google.com/site/irfumatlab/) This is a MATLAB package that includes many different type of routines allowing to analyse both multi-spacecraft and single spacecraft data. Some of the important parts include:
) is a powerful tool for manipulating, analyzing and plotting data from spacecraft missions. QSAS can read a variety of data files (cdf,cef, ASCII etc) and should be applicable to THOR data without any special adaptations. Many of the structure analysis methods above are implemented in QSAS and have been extensively tested on Cluster and MMS data.
rbit Visualization Tool - OVT (http://ovt.irfu.se) is a software for visualization of satellite orbits in the Earth's and interplanetarymagnetic field. The program can display satellite orbits in five coordinate systems (GEI, GEO, GSM, SMC, GSE), satellite footprints projected on the Earth's surface and shown in either geographic (GEO) or geomagnetic (SMC) coordinates. In addition to satellite orbits the software computes and displays various models of magnetospheric structures, magnetopause, bow shock and foreshock, and interplanetary field conditions. The models are time-dependent on the solar wind, IMF and geomagnetic activity levels. OVT will be particularly useful for planning of THOR operations.