How are the EUV and radio polar limb-brightenings correlated?

Abstract

Aims. We correlate the polar limb brightening time evolution observed with pass-band filters centered at the EUV 17.1 nm (Fe ix,x) and 30.4 nm (He ii) lines with radio continuum images obtained at 17 GHz (∼1.76 cm) during solar cycle 23. Methods. We determine the limb brightening in units of the quiet Sun from daily maps at 17.1 and 30.4 nm obtained by the Extreme Ultraviolet Imager (EIT) aboard the SOHO satellite between 1997 and 2007. The limb brightness at 17 GHz is obtained from daily maps taken by the Nobeyama Radioheliograph (NoRH) since 1992. Results. The variation in the limb brightening observed at coronal heights (17.1 nm) is correlated positively with the 11 year cycle. However, the observation at chromospheric/transition region heights (17 GHz / 30.4 nm) shows a clear negative correlation with the solar cycle. Conclusions. The limb brightening measurements at 17.1 nm reproduce the emission measure clearly in the solar corona during a solar cycle in which coronal holes are constantly present at the poles during the minimum. On the other hand, the negative correlation of the polar brightening at 17 GHz and 30.4 nm with the solar cycle are shown to depend upon polar features in the lower atmosphere (chromosphere/transition region). Moreover, the polar brightening variation at these frequencies is similar to that of the photospheric faculae observed at the poles.