Kinetic Alfvén Waves at the Magnetopause -- Mode Conversion, Transport and Formation of LLBL
Authors: Jay R. Johnson and C.Z. Cheng
Date of PPPL Report: May 2002
To be published in: the proceedings of the Chapman Conference on Low-Latitude Boundary Layer and Its Dynamic Interaction with the Solar Wind and Magnetosphere, April 16-20, 2001, in New Orleans, LA. Edited by Patrick Newell and Terry Onsager.
At the magnetopause, large amplitude, low-frequency (ULF), transverse MHD waves are nearly always observed. These waves likely result from mode conversion of compressional MHD waves observed in the magnetosheath to kinetic Alfvén waves at the magnetopause where there is a steep gradient in the Alfvén velocity [Johnson and Cheng, Geophys. Res. Lett. 24 (1997) 1423]. The mode-conversion process can explain the following wave observations typically found during satellite crossings of the magnetopause: (1) a dramatic change in wave polarization from compressional in the magnetosheath to transverse at the magnetopause, (2) an amplification of wave amplitude at the magnetopause, (3) a change in Poynting flux from cross-field in the magnetosheath to field-aligned at the magnetopause, and (4) a steepening in the wave power spectrum at the magnetopause. We examine magnetic field data from a set of ISEE1, ISEE2, and WIND magnetopause crossings and compare with the predictions of theoretical wave solutions based on the kinetic-fluid model with particular attention to the role of magnetic field rotation across the magnetopause. The results of the study suggest a good qualitative agreement between the observations and the theory of mode conversion to kinetic Alfvén waves. Because mode converted kinetic Alfvén waves readily decouple particles from the magnetic field lines, efficient quasilinear transport (D ~ 109m2/s) can occur. Moreover, if the wave amplitude is sufficiently large (Bwave/B0 > 0.2) stochastic particle transport also occurs. This wave-induced transport can lead to significant heating and particle entry into the low latitude boundary layer across closed field lines.