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## Detailed Regression Model of Plasma Sheet By

In a statistical model plasma sheet *B**y* primarily depends on interplanetary magnetic field (IMF) *B**y* and geodipole tilt *τ*. With 11 years of Geotail measurements we investigate a role of several other parameters with a linear regression model. Optimal averaging window of IMF input, maximizing correlation and regression coefficients, is found to be 2.25 hr. Influence of IMF and local *B**z* on IMF penetration (regression with regard to) and the deviations from the predefined warp deformation are at the level 5–10% relative to the primary model coefficients. The IMF penetration beyond 25 *R**E* is somewhat larger for northward IMF, while closer to the Earth it becomes somewhat larger for southward IMF. These smaller effects turned out to be rather uneven across the tail, making reliable quantification and physical interpretation not always possible. The major reasons of difficulties are uneven coverage and internal correlations in the input space ( *B**y*–*τ*–*Bz* ) due to combination of spacecraft orbit and neutral sheet dynamics, effects of coordinate transformations, etc. In particular, origins of extremely large IMF penetration (order of 30–50% above the average one) for some years and tail locations remain not fully clear. A larger multispacecraft data set covering all seasons in all spatial zones is necessary to further advance in this study.