Origins of plasma sheet B_y

Abstract

With 11 years of Geotail measurements we analyzed sources of plasma sheet B_y and constructed a model, depending on IMF B_y, coordinates X, Y, and geodipole tilt angle. B_y dependence on dipole tilt due to the neutral sheet warping and hinging has an odd (antisymmetric) profile with respect to Y. In addition, a new, even with respect to Y, B_y component was discovered, which is positively correlated with dipole tilt with the maximal amplitude ±1-2 nT. In the postmidnight sector the dipole tilt effects in B_y almost cancel each other, while at the premidnight sector they are summed up and are comparable with the IMF penetration. Such season-dependent net B_y creates a principal azimuthal asymmetry of the magnetotail and is consistent with some polar convection and aurora observations. Plasma sheet B_y is often substantially larger than the statistically expected value. This effect can be understood as “amplification” due to internal plasma sheet dynamics. As a result an asymmetric tail of the B_y distribution forms, causing certain overestimation of the regression coefficients in statistical models.