extreme values

Abstract. With 11 years of Geotail measurements we con-struct a model of plasma sheet By, depending on IMF By, coordinates X, Y, and geodipole tilt angle. At midnight and pre-midnight local times By is positively correlated with tilt (positive in summer). Thus in summer By is shifted towards positive values and in winter towards negative values, so that up to several nT could be added to the IMF influence. The dawn side plasma sheet By generally does not exhibit any tilt dependence, but within 15RE the weaker negative correla-tion with tilt was revealed. The tilt dependence is just a use-ful parametrization and several mechanisms actually affect-ing plasma sheetBy were previously suggested. In particular, similar coupling between tilt and IMF By was earlier found in the ionospheric convection patterns. Besides this average response, extreme By (|By |>5 nT, By>IMF By) were often observed (up to 20–25 % of cases during solar maximum and in the pre-midnight sector within 20RE). They can not be explained by our statistical model and are preliminary inter-preted as an “over-reaction ” of the magnetosphere in some individual events. Large By field radically changes dynamics of the current sheet and has to be taken into account during substorm-related studies