candidate: The root mean square ( RMS ) intensity fluctuations are often expressed relative to the mean intensity from the source , in a term called the scintillation index , which is written asformula_1This can be related to the phase deviation caused by turbulence in the solar wind by considering the incident electromagnetic plane wave , and yieldsformula_2The next step , relating the phase change to the density structure of the solar wind , can be made more simple by assuming that the density of the plasma is highest towards the sun , which allows the " thin screen approximation . "