The 17 GHz active region number

Abstract

We report the statistics of the number of active regions (NAR) observed at 17 GHz with the Nobeyama Radioheliograph between 1992, near the maximum of cycle 22, and 2013, that also includes the maximum of cycle 24, and we compare with other activity indexes. We find that NAR minima are shorter than those of the sunspot number (SSN) and radio flux at 10.7 cm (F10.7). This shorter NAR minima could reflect the presence of active regions generated by faint magnetic fields or spotless regions, which were a considerable fraction of the counted active regions. The ratio between the solar radio indexes F10.7/NAR shows a similar reduction during the two minima analyzed, which contrasts with the increase of the ratio of both radio indexes in relation to the SSN during the minimum of cycle 23-24. These results indicate that the radio indexes are more sensitive to weaker magnetic fields than those necessary to form sunspots, of the order of 1500 G. The analysis of the monthly averages of the active region brightness temperatures shows that its long term variation mimics the solar cycle, although, due to the gyro-resonance emission, a great number of intense spikes are observed in the maximum temperature study. The decrease, in number, of these spikes is also evident during the current cycle 24, a consequence of the sunspot magnetic field weakening in the last years.

arXiv commentAccepted for publication in The Astrophysical Journal

References in corpus (3)

• Behavior of Solar Cycles 23 and 24 Revealed by Microwave Observations
• Structural Invariance of Sunspot Umbrae Over the Solar Cycle: 1993-2004
• Obscuration of Flare Emission by an Eruptive Prominence

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• Solar observations with single-dish INAF radio telescopes: continuum imaging in the 18-26 GHz range
• Comparison of solar radio and EUV synoptic limb charts during the present solar maximum
• The Submillimeter Active Region Excess Brightness Temperature during Solar Cycles 23 and 24
• The behavior of the spotless active regions during the solar minimum 23-24