Controlling Poloidal Alfven Waves in the Magnetosphere

Magnetospheric poloidal mode wave with high azimuthal wave number (m) is of great interest due to its capability of interacting with the ring current particles. It is generally believed that poloidal mode waves decay within several wave periods (< ~ 1 hour) with the lifetime proportional to the azimuthal wavenumber. However, there are observational evidence of long-lasting (from several hours to days) poloidal mode waves, yet the mechanism is not understood. We conduct MHD wave simulations in both box and dipolar geometry to examine under which condition the poloidal mode can be persistent. The waves are assumed to be excited by particles with different energies, that determine the radial extent of the initial source region. Our results show that the poloidal mode remains steady when local conditions prohibit the propagation of the compressional mode. In particular, the poloidal mode is persistent when m is extremely high, or radial width of the source region (gyroradius of the source particle) is very narrow. Observations clearly show the occurrence of coherent wave-particle interaction only in the region where the compressional cutoff condition is satisfied and thus the compressional mode becomes evanescent, that confirms our scenario.