The use of 17 GHz radio emission to characterize the solar minimum

1. The use of 17 GHz radio emission to characterize the solar minimum Selhorst, Caius L., Svalgaard, L., Giménez de Castro, C. G., Válio, A., Costa, J. E. R., Shibasaki, K.

2. Ciclo solar XXIV

3. Alternative index • Tlatov (2009) suggested to employ alternative index in the solar minimum periods. # Polar faculae # Sunpots • Sheeley (2008) pointed that the number of faculae in 2006 was smaller than that observed in previous minima.

4. Alternative index • Wang et al. (2009): polar magnetic field 40% smaller during this minimum in comparison with the previous cycle.

5. The Sun at 17 GHz Nobeyama Radioheliograph

6. The Sun at 17 GHz

7. Solar radius Selhorst et al. 2004

8. Solar radius Selhorst et al. 2011

9. Polar limb brightening Selhorst et al. 2010

10. The Sun at 17 GHz • Svalgaard & Cliver (2006): the bright patches are locations of strong magnetic fields.

11. Polar limb brightening Selhorst et al. 2011

12. Polar limb brightening Selhorst et al. 2010 • Gyro-resonance emission at 17 GHz originates from the 3rd harmonic, i.e., ~2000 G (?); • Bremsstrahlung can produce the observed due to the heating of the local atmosphere.

13. Discussion • The results above show a remarkable decrease of the solar radius and polar limb brightening at 17 GHz during this unusual solar minimum. • It is necessary to take into account that measurement of the radius at 17 GHz may represent the mean emission of many small chromospheric features joined with the emission originating from sunspots/active regions. From this perspective, our results can be interpreted as a reduction in the activity related to the 17 GHz atmospheric. • Since the present analysis shows a clear reduction in the polar limb brightening intensities during the current minimum, the observed decreases in faculae number (Sheeley 2008) and polar magnetic field intensities (Wang et al. 2009) support the theory that the magnetic features are the origin of the limb brightening.