Dipole tilt effects in plasma sheet B_y: statistical model and extreme values

Abstract. With 11 years of Geotail measurements we construct a model of plasma sheet B_y, depending on IMF B_y, coordinates X,Y and geodipole tilt angle. At midnight and pre-midnight local times B_y is positively correlated with tilt (positive in summer). Thus in summer B_y 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 15 R_E the weaker negative correlation with tilt was revealed. The tilt dependence is just a useful parametrization and several mechanisms actually affecting plasma sheet B_y were previously suggested. In particular, similar coupling between tilt and IMF B_y was earlier found in the ionospheric convection patterns. Besides this average response, extreme B_y (|B_y|>5 nT, B_y>IMF B_y) were often observed (up to 20–25% of cases during solar maximum and in the pre-midnight sector within 20 R_E). They can not be explained by our statistical model and are preliminary interpreted as an "over-reaction" of the magnetosphere in some individual events. Large B_y field radically changes dynamics of the current sheet and has to be taken into account during substorm-related studies.