Modeling the Longitudinal Asymmetry in Sunspot Emergence -- the Role of the Wilson Depression

The distributions of sunspot longitude at first appearance and at disappearance display an east-west asymmetry that results from a reduction in visibility as one moves from disk centre to the limb. To first order, this is explicable in terms of simple geometrical foreshortening. However, the centre-to-limb visibility variation is much larger than that predicted by foreshortening. Sunspot visibility is also known to be affected by the Wilson effect: the apparent dish shape of the sunspot photosphere caused by the temperature-dependent variation of the geometrical position of the tau=1 layer. In this article we investigate the role of the Wilson effect on the sunspot appearance distributions, deducing a mean depth for the umbral tau=1 layer of 500 to 1500 km. This is based on the comparison of observations of sunspot longitude distribution and Monte Carlo simulations of sunspot appearance using different models for spot growth rate, growth time and depth of Wilson depression.Comment: 18 pages, 10 figures, in press (Solar Physics

Heights of solar tracers observed at 8 mm and an interpretation of their radiation

Context. At the wavelength of 8 mm, emissive features (high brightness-temperatrue regions, HTRs) and absorptive features (low brightness-temperature regions, LTRs) can be traced for the determining the solar rotation. From earlier studies it is known that about two thirds of LTRs are associated with Hα filaments. Aims. Thermal bremsstrahlung and gyromagnetic (cyclotron) radiation mechanism can be important for explaining the observed phenomena, so we determine the heights of solar structures and interpret their radiation mechanism(s). Methods. We use the method of simultaneous determination of the solar synodic rotation velocity and the height of tracers. The rotation velocities were determined by the linear least-square fit of their central meridian distance as a function of time. We used a procedure for calculating the brightness temperature for a given wavelength and model atmosphere, which integrates the radiative transfer equation for the thermal bremsstrahlung. Results. The mean value of the low brightness-temperature regions' heights is about 45 600 km. This height was used as input for constructing prominence and coronal condensation models, which, when assuming thermal bremsstrahlung as the radiation mechanism, yield a decrease in the brightness temperature of 2–14%, in agreement with observations. If the same radiation mechanism is considered, the models of the solar corona above active regions give an increase in the brightness temperature of 5–19%, also in agreement with observations. In this case an indirect indication (from the rotational analysis) that the HTRs are located higher in the solar atmosphere than the LTRs was taken into account. Conclusions. The method for simultaneously determining the solar synodic rotation velocity and the height of tracers could have only been properly applied on LTRs, since a homogeneous distribution over latitudes and central meridian distances of a large enough data set is necessary. Thermal bremsstrahlung can explain both the LTR (prominences and coronal condensations) and HTR (ordinary active regions) phenomena observed at 8 mm. At this wavelength, thermal gyromagnetic emission is almost surely excluded as a possible radiation mechanism

Height correction in the measurement of solar differential rotation determined by coronal bright points

Full-disc solar images obtained with the Extreme Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) are used to analyse solar differential rotation by tracing coronal bright points for the period June 4, 1998 to May 22, 1999. A method for the simultaneous determination of the true solar synodic rotation velocity and the height of the tracers is applied to data sets analysed with interactive and automatic methods. The calculated height of coronal bright points is on average 8000–12000 km above the photosphere. Corrected rotation velocities are transformed into sidereal ones and compared with results from the literature, obtained with various methods and tracers. The differential rotation profile determined by coronal bright points with the interactive method corresponds roughly to the profile obtained by correlating photospheric magnetic fields and the profile obtained from the automatic method corresponds roughly to the rotation of sunspot groups. This result is interpreted in terms of the differences obtained in the latitudinal distribution of coronal bright points using the two methods