ИСТИНА

In the sharp field reversal of the Earth's magnetotail, particles may not conserve their magnetic moment (first adiabatic invariant) due to large magnetic field variations on the length scale of the particle cyclotron turn. A well-known feature of particle dynamics in this region of space is the Speiser orbit where particles execute fast oscillations in the Z direction (due to the large BX component) in conjunction with a slow gyromotion in the X-Y plane (due to the small BZ component). Resonance between these two motions occur at specific particle energies (equivalently, specific scales of the particle motion with respect to magnetic field length scale) and may lead to the formation of narrow beams (or beamlets) of accelerated particles in the plasma sheet boundary layer. Although the motion of these particles is not adiabatic per say, quasi-adiabatic theory with the action integral as an approximate invariant of motion can be used to characterize the particle behavior. Here, we revisit the Speiser limit considering transient reconfigurations of the magnetotail, as is observed during substorm dipolarization. In addition to spatial nonadiabaticity, specific features due to temporal nonadiabaticity in the rapidly varying magnetic field may be obtained as is the case for heavy ions that have gyroperiods comparable to the field variation time scale. In averaged field reconfigurations, protons do not exhibit such a temporal nonadiabaticity but may still be subjected to prominent energization (up to the 100 keV range) due to meandering motion along the induced electric field. We show that the Speiser limit (equivalently, the quasi-adiabatic regime) is significantly affected by the rapidly changing magnetic field that favors particle injection onto trapped drift path into the inner magnetosphere.