**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

**Abstract:**

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 (c^{2})^{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 (c^{2})^{1/2} to judge the quality of the fit.

in press, *J. Geophys. Res.*, 2003