Relying on the expertise acquired from ESA's four-satellite Cluster mission, validation tests involving data provided by the Swarm constellation are proposed. The tests are based on techniques to estimate the spatial gradient of physical variables measured simultaneously by two or three satellites (see Vogt et al., 2009, 2013).
When the satellites form a close formation, i.e. when the satellites are sampling the same plasma environment, VFM data can be used to check if the divergence of the measured magnetic field, div(B), stays in the range of expected errors. The expected level of fluctuations is defined as part of the procedure by applying the spatial gradient estimation to synthetic data as well as to (well calibrated) data provided by the ESA Cluster satellites. Time-series data from selected intervals are used, when the contributions to div(B) from missing gradient components (e.g. variations normal to the spacecraft plane) can be determined by imposing suitable constraints, like time-stationarity or homogeneity along the East-West direction.
In the same way, since the electric field is expected to be close to curl-free at least during magnetically quiet conditions, the curl of the convection electric field (E = - V x B, with V measured by the EFI instrument) can be further checked. What 'close' means will is defined as part of the procedure. Based on the assumption of incompressible plasma flow during magnetically quiet conditions, a cross-check of the divergence of ion velocity is also foreseen on selected intervals, and its value as data validation procedure is assessed.
M. Dunlop, A. Balogh, K.-H. Glassmeier, and P. Robert, Four-point Cluster application of magnetic field analysis tools: The Curlometer , Journal of Geophysical Research (Space Physics), 107 , 23, 2002
Paschmann, G. and Daly, P. W. (editors), Analysis Methods for Multi-Spacecraft Data, ISSI Scienti?c Reports SR-001, ESA Publications Division, 1998
Paschmann, G. and Daly, P. W. (editors), Multi-Spacecraft Analysis Methods Revisited, ISSI Scienti?c Reports SR-008, ESA Publications Division, 2008
J. Vogt, A. Albert, and O. Marghitu, Analysis of three-spacecraft data using planar reciprocal vectors: methodological framework and spatial gradient estimation, Ann. Geophys. 27, 2009
J. Vogt, E. Sorbalo, M. He, and A. Blagau, Gradient estimation using configurations of two or three spacecraft, Ann. Geophys., 31, 1913-1927, 2013