On the validity of application of the radial approximation for the photospheric field

The radiality approximation, introduced by Wang and Sheeley (1992), is rather frequently used to calculate – within the current-free approximation – the magnetic structure of the solar corona. In this paper it is shown that the data that are used as observational evidence of the `radiality' contradict neither the `radiality' nor the `non-radiality' of the measured magnetic field, while to ascertain the true character requires quantifying the standard deviation of the transverse component of the field. On the basis of a statistical processing of daily magnetograms, a method is suggested for quantifying the measure of non-radiality of the observed field. For Stanford magnetograms (Fei 5250) and NSO/Kitt Peak magnetograms (Fei 8688) it is shown that the measures of non-radiality corresponding to them are equal in magnitude and not small. The only difference between the two lines of measurement is the presence of a small mean azimuthal field in the line of Fei 5250, which may be interpreted as an indication of the non-potentiality at the level of measurement lying in deeper layers. Nevertheless, such a non-potentiality does not lead to magnetic field radialization.The final conclusion of this paper: the observational data indicate an essential non-radiality of the magnetic field at the level of measurements and, hence, the classical interpretation of magnetic measurements for extrapolating the magnetic field to the corona is preferred.