Focusing of Fast Magnetosonic Waves in the Dayside Magnetosphere

Abstract

The spatial structure of ultra‐low‐frequency (ULF) fast magnetosonic (FMS) waves is calculated in the dayside magnetosphere. A balanced model is developed of a dipole magnetosphere with a plasma rotating in the azimuthal direction. The magnetopause is in the form of a smooth transition layer. It is shown that the largest‐scale FMS eigen harmonics are either weakly unstable or neutrally stable. The calculations show that the eigen oscillation amplitudes increase greatly from the magnetopause into the magnetosphere. Presumably, the dayside magnetosphere plays the role of a sort of focusing lens for ULF FMS waves. Hence, one can expect a significant increase in the magnetosonic oscillation amplitudes in the inner part of the magnetosphere when the FMS oscillations (determining the magnetopause eigen oscillation amplitudes) are amplified in the magnetosheath.