The solar radius in the EUV during the cycle XXIII
Giménez de Castro, C. G.; Varela Saraiva, A. C.; Costa, J. E. R.; Selhorst, C. L.
Brazil
Abstract
Aims:We aim to determine the solar transition region and coronal radius at EUV wavelengths and its time evolution during Solar Cycle XXIII.
Methods: We use daily 30.4 and 17.1 nm images obtained by the Extreme Ultraviolet Imager (EIT) aboard the SoHO satellite and derive the solar radius by fitting a circle to the limb brightness ring.
Results: The weighted mean of the temporal series gives (967.56 arcsec ± 0.04 arcsec) and (969.54 arcsec ± 0.02 arcsec) at 30.4 and 17.1 nm respectively. No significant correlation was found with the solar cycle at either of the two wavelengths.
Conclusions: Since the temperature formation of the 30.4 nm line is between (60-80) 103 K (Transition Region), the obtained result is greater than that derived from present optical atmospheric models. On the contrary, this height is compatible with radio models.