When an electromagnetic wave enters an ionized layer of ionosphere the elecctron cloud will oscillate in the electric field of wave with a phase retardation of 90° for a sinusoidal EM. Can u please explain why this happens?

Dear Student ,
Electromagnetic wave is a resultant wave diue to the magnetic field and electric field. When it enters the ionized layer of ionosphere the electrons and other ionized particles in the cloud present in the part of atmosphere will oscillate due to electric field of the wave with a phase retardation of 90⁰ for a sinusoidal electromagnetic wave.

Hope this explanation helps you.
Regards