The Solar Radius in the EUV during the Cycle XXIII
C. G. Gim\'enez de Castro (1), A. C. V. Saraiva (1,2), J. E. R. Costa, (2), C. L. Selhorst (1,2) ((1) CRAAM/EE, S\~ao Paulo, Brazil, (2) INPE,, S\~ao Jos\'e dos Campos, Brazil)
TL;DR
This study measures the solar transition region and coronal radius during Solar Cycle XXIII using EUV images, finding consistent radii across the cycle and comparing results with atmospheric and radio models.
Contribution
It provides the first detailed measurement of the solar radius at EUV wavelengths during a solar cycle, highlighting the stability of the radius over time.
Findings
Measured solar radius at 30.4 nm: 967.56''
Measured solar radius at 17.1 nm: 969.54''
No significant variation with solar cycle
Abstract
Aims. 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 +/- 0''.04) and (969''.54 +/- 0''.02) at 30.4 and 17.1 nm respectively. No significant correlation was found with the solar cycle at any of the two wavelengths. Conclusions. Since the temperature formation of the 30.4 nm line is between (60 - 80) 10^3 K (Transition Region), the obtained result is bigger than that derived from present atmospheric models. On the contrary this height is compatible with radio models.
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