Instruments

Q: What are fgs and efs data?
A: On-board spin-fit magnetic field (FGM) and electric field (EFI) data. They have one-spin (approximately 3 sec) time resolution.

Q: Why do fgs data contain saw-tooth noise and spikes, in distinction from fgl and fgh data?
A: The causes of spikes and noise in the fgs data are currently being investigated. The process for obtaining fgs data is different than those for fgh and fgl. The fgs data are averaged on-board using an algorithm shown in the spin-fit processing document.
The fgl and fgh data are not fit using this algorithm; instead, they are "despun" SSL to DSL coordinates during the calibration process. For definitions and diagrams relating to the different THEMIS coordinate systems see the Coordinate Systems documentation.

Q: What are efs_dot0 data?
A: The efs_dot0 data are calculated from EFI spin-averaged efs data using the two spin-plane components of the electric field E and the magnetic field B (from FGM) and the orthogonality condition E dot B = 0. Thus the efs_dot0 data give the components of E in the appropriate coordinate system (the default is DSL), under the assumption that E is perpendicular to the magnetic field B.

Q: What are efs_0 data?
A: The efs_0 data are calculated from EFI spin-averaged efs data for the electric field E with the z component (i.e., along the spin axis, in DSL coordinates) of the electric field set to zero. Thus the efs_0 data give the components of E perpendicular to the spin axis.