Two-dimensional spatial spectrum of the photospheric brightness field near to the solar disc center

From high-quality direct frames taken by the Soviet Stratospheric Solar Observatory, using coherent optical methods, the two-dimensional spatial spectrum of the photospheric brightness field was obtained (Figure 1). This spectrum is isotropic and continuous. Spectral densities P(k) and A(k) = 2kP(k), where k is the radial wavenumber, were estimated for the solar disc centre, and their statistical uncertainty calculated. P(k) has a maximum near k = 10^–3 km^–1 and then it tends to fall after that to zero frequency. The k dependence of A(k) cannot be satisfactorily approximated by a power law. For the highest frequencies studied, the spectrum falls as k^–9. The measured statistical uncertainty of the spectra of individual domains for k 125 × 10^–4 km^–1 is in agreement with that calculated for a gaussian homogeneous field. But for a higher k the uncertainty may essentially exceed that of a gaussian homogeneous field. The true rms value for 4650 is equal to about 29%.