The inverse problem consists of two parts: (i) To establish the form of the electron density fluctuation spectrum. (ii) To determine the strength of fluctuations in different regions of the ISM.
(i) Based on a variety of observations of scintillation‐related phenomena in radio pulsars and compact galactic and extragalactic radio sources, it appears that the density fluctuation spectrum for lengthscales shorter than ∼1014 cm is close to that predicted by Kolmogorov turbulence theory. Refractive scintillation is found to be unusually strong in several sources, suggesting either that there is enhanced fluctuation power at ∼1013–1014 cm or that the inner scale is larger than thought earlier, possibly as large as ∼1011–1012 cm.
(ii) Electron density fluctuations in the Galaxy consist of a strong clumpy component with a scale height ≤100 pc and a weaker nearly homogeneous component with a scale height ≥500 pc. The former component is particularly strong in the regions of the Galaxy interior to the sun and may be associated with HII regions and supernova remnants.
