THE BETA-DEPENDENCE OF THE COLLISIONLESS TEARING INSTABILITY AT THE DAYSIDE MAGNETOPAUSE

The beta-dependence of the collisionless tearing instability at the dayside magnetopause current sheet is investigated analytically for various current sheet models, where beta is the ratio of plasma pressure to magnetic pressure. In the present models the magnetosheath field (B(s)) is separated from the magnetospheric field (B(m)) by a current sheet with an antiparallel magnetic field component in the z direction and a common guiding magnetic field component (B(y)) in the y direction. Four different current sheet models are considered. For a symmetric current sheet with equal magnetic field strength (\B(m)\ = \B(s)\) and plasma density on its two sides and B(y) = 0 (case A), it is found that the collisionless tearing instability has a strong beta-dependence. An increase in beta leads to a significant decrease in the tearing growth rate. For an asymmetric current sheet with fixed \B(m)\ and beta(m) on the magnetospheric side and B(y) = 0 (case B) or B(y) = const not-equal 0 (case C), it is found that the collisionless tearing growth rate is not very sensitive to the values of beta(s), where beta(s) is the ratio of plasma pressure to magnetic pressure in the magnetosheath. An increase of beta(s) only slightly changes the tearing growth rate. For an asymmetric current sheet with fixed \B(m)\ and beta(m) and a large, nonconstant B(y) (case D), it is found that the collisionless tearing growth rate displays a strong beta(s) dependence. The present study shows that the beta-dependence of the collisionless tearing instability at the dayside magnetopause is controlled by the magnetic field profile across the magnetopause current sheet.