Estimated errors in magnetic cloud model fit-parameters with force free cylindrically symmetric assumptions
R. P. Lepping, D. B. Berdichevsky, and T. J. Ferguson
Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771
This paper briefly reviews our earlier study done on estimated error in various output fit-parameters resulting form a specific force free interplanetary magnetic cloud model [Lepping et al., 1990], applied to randomly noised-up fields of simulated clouds, and shows that such random-noise simulation studies are inadequate. It then proceeds to carry out an error study of the same model output parameters based on more realistic input magnetic field noise sets derived from actual clouds observed in WIND data over the years 1995 to 1998, inclusive. In this part of the study 1824 "noised-up" simulated clouds are produced for use in the same force free cloud model to study the spread in values for each of 7 model parameters, plus associated quantities, as a function of noise level. Four noise levels are used with rms's of 0.5, 2.0 (typical level), 3.0, and 4.0 nT per field component of input bias-free fluctuation fields. These are based on manipulation of difference-fields from 19 actual magnetic clouds, where a difference-field is defined as the vector difference between the observational field and that from the original model fit for each actual cloud. The averages and rms's of 240 output (model-fit) parameter-distributions are produced and discussed. The study determines how the model fit-parameters, and especially their distributions, vary as a function of various input noise levels and of the resulting least-squares (c2)1/2 values for various exact input parameters, such as the cloud's axial attitude and the closest-approach distance of the spacecraft. Practical goals were to better understand the fit-program's limitations and to provide a prescription for estimating future errors in specific magnetic cloud (and probably magnetotail flux rope) fittings. The degree of symmetry in the model-solution is used along with (c2)1/2 to judge the quality of the fit.
in press, J. Geophys. Res., 2003