Bifurcation of drift shells near the dayside magnetopause

Close to the dayside magnetopause, there is a region of space where each field line has two magnetic field minima, one near each cusp. That region is located around local noon, and extends about 1–2 R e from the magnetopause. Particles that enter this region with equatorial pitch angles sufficiently close to 90° will cross the dayside not along an equatorial path, but along one of the two branches on either side of the equatorial plane. The two branches are joined again past local noon. This process of drift‐shell bifurcation (DSB) is nonadiabatic even under static conditions. Two physical mechanisms can cause this nonadiabaticity: one that is operative for nearly all magnetospheric magnetic field configurations and another that depends on a particular combination of north‐south and east‐west asymmetry in the magnetic field. This paper deals only with the first mechanism. For configurations with north‐south and east‐west symmetry, DSB changes the second invariant I of the motion by a small amount that is of the order of the gyroradius (the first invariant is intact). For near‐equatorial particles ( I ≈ 0) the change can be significantly larger. Assuming north‐south and dawn‐dusk symmetry, we present general theoretical expressions for the second‐invariant jump Δ I , which can be applied to a variety of magnetic field models. The results show that Δ I is sensitively dependent on the bounce phase of the particle at the bifurcation line. The RMS value of Δ I over a bounce‐phase ensemble increases with decreasing mirror field and with increasing kinetic energy. We verify these results with test‐particle simulations using model magnetic fields.