The Contribution of Magnetic Flux Tubes to Brightness Fluctuations on the Extreme Limb of the Sun

Magnetic flux tubes are considered to be a possible source of observed brightness variations on the extreme limb of the Sun. A computer code to simulate the brightness fluctuations is developed. It follows from numerical modeling that a filling factor of f=4% is required to explain the observed fine structure. Such a value is typical for the boundaries of supergranules. Tubes produce a local maximum of intensity just inside the limb.     

CAN WE OBSERVE THE TEMPERATURE INHOMOGENEITIES AND MAGNETIC FLUX TUBES AS THE FINE STRUCTURE ON THE EXTREME LIMB OF THE SUN?

Fine structure of brightness inhomogeneities with I r.m.s. = 2.9% was discovered on the extreme limb of the Sun on the best quality white-light photographs obtained at the Soviet Stratospheric Solar Observatory. The concept is that temperature inhomogeneities are responsible for the limb structure. The real value T r.m.s. = 109K is required to explain our observations. Another possible explanation is that small-scale magnetic flux tubes with the realistic filling factor f* = 1% are the source of the limb brightness fluctuations.     

On the sunspot structure

The fine structure of a sunspot is studied on a series of photographs obtained during the third flight of the Soviet Stratospheric Solar Station. The main results are as follows:

1. The micro-photometer tracings on the frames show extremely high Rayleigh resolution of small elements, the smallest distances being near to the theoretical limit. The half-widths of the brighter elements are given in Tables III and VI. The corrected brightness of umbral dots has large dispersion.
2. The dimensions of the smallest dots are equal to the diffraction image of bright points. So the real radii of these objects are smaller than 150km, which is consistent with opaque models of sunspot umbra.
3. The penumbra and umbra structure (dark and bright objects) is in good agreement with the picture of magnetic field splitting in a system of magnetic ropes giving rise to the magnetic arcs in the chromosphere and corona. Only in the umbra do we meet the large scale continuities.

     

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) = 2k P(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%.