High‐latitude irregularity spectra deduced from scintillation measurements

High‐latitude scintillation data show that the strength and spectral index of intensity scintillation are dependent on the propagation geometry. It will be shown that anisotropic irregularity spectra, with different indices along and across the magnetic field, lead to geometrical effects similar to those observed. In general, the spectrum along the magnetic field is steeper than that across the field, and the difference is more pronounced for nighttime conditions. Spectral anisotropy can be interpreted as a size‐dependent irregularity anisotropy. Our data indicate that large‐scale irregularities in the daytime and nighttime ionosphere are almost isotropic, while small‐scale irregularities are anisotropic and considerably more so at night than during the day. It will be shown that anisotropic irregularity spectra could account for the observed scintillation and in situ temporal spectra with frequency‐dependent slope. The effect depends strongly on the geometry.