 Effect of Earth Magnetic Field on Refractive Index of the Ionosphere: The theory which deals with the propagation of Radio wave through Ionosphere in presence of earth magnetic field is known as Magneto-Ionic-theory. 
 The phenomenon of propagation of radio waves through Ionosphere in the presence of earth magnetic field is changed.
 because is the presence of earth magnetic field, the formula of refractive Index u is changed,
 u=sqrt(1-81N/f^2)
 i.e.,
 u^2=sqrt(1-(2/(2a-(yt^2/a-1)+sqrt(yt^2/(a-1)^2+4yL^2))))
 where
   a=(EoMw^2)/(Ne^2)=d^2/dc^2
   yt=aBt.e/wm
   yL=aBL.e/wm and y=sqrt(yt^2+yL^2)
   BL=component of earth magnetic field intensity B along the direction of propagation.
   Bt=component of earth magnetic field intensity traverse to the direction of propagation.
   B=uo.H
   M=mass of electron=9.1*10^-31 kg
   e=charge of electron=1.6*10^-19c
   w=2*3.14*d=angular frequency
   N=electron density
   Eo=dielectric constant=8.854*10^-12 F/M
   u=refractive index of Ionosphere.