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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. Ciclo solar XXIV. Alternative index.

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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.



  • 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.


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


Nobeyama Radioheliograph



Selhorst et al. 2004


Selhorst et al. 2011


Selhorst et al. 2010


  • Svalgaard & Cliver (2006): the bright patches are locations of strong magnetic fields.


Selhorst et al. 2011


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.


  • 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.


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