A Comparison Between Global Proxies of the Sun’s Magnetic Activity Cycle: Inferences from Helioseismology
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The last solar minimum was, by recent standards, unusually deep and long. We are now close to the maximum of the subsequent solar cycle, which is relatively weak. In this article we make comparisons between different global (unresolved) measures of the Sun’s magnetic activity to investigate how they are responding to this weak-activity epoch. We focus on helioseismic data, which are sensitive to conditions, including the characteristics of the magnetic field, in the solar interior. Also considered are measures of the magnetic field in the photosphere (sunspot number and sunspot area), the chromosphere and corona (10.7 cm radio flux and 530.3 nm green coronal index), and two measures of the Sun’s magnetic activity closer to Earth (the interplanetary magnetic field and the galactic cosmic-ray intensity). Scaled versions of the activity proxies diverge from the helioseismic data around 2000, indicating a change in relationship between the proxies. The degree of divergence varies from proxy to proxy, with sunspot area and 10.7 cm flux showing only small deviations, while sunspot number, coronal index, and the two interplanetary proxies show much larger departures. In Cycle 24 the deviations in the solar proxies and the helioseismic data decrease, raising the possibility that the deviations observed in Cycle 23 are just symptomatic of a 22-year Hale cycle. However, the deviations in the helioseismic data and the interplanetary proxies increase in Cycle 24. Interestingly, the divergence in the solar proxies and the helioseismic data are not reflected in the shorter-term variations (often referred to as quasi-biennial oscillations) observed on top of the dominant 11-year solar cycle. However, despite being highly correlated in Cycle 22, the short-term variations in the interplanetary proxies show very little correlation with the helioseismic data during Cycles 23 and 24.
KeywordsHelioseismology, Observations Integrated Sun Observations Oscillations, Solar Solar Cycle, Observations
A.-M. Broomhall thanks the Institute of Advanced Study, University of Warwick for their support. V.M. Nakariakov: This work was supported by the European Research Council under the SeismoSun Research Project No. 321141, STFC consolidated grant ST/L000733/1, and the BK21 plus program through the National Research Foundation funded by the Ministry of Education of Korea. We thank the Birmingham Solar Oscillations Network, IZMIRAN Cosmic Ray Group, NOAA NGDC, OMNIWeb, and the Royal Observatory (Greenwich) for making their data freely available. We acknowledge use of NASA/GSFC’s Space Physics Data Facility’s OMNIWeb (or CDAWeb or ftp) service, and OMNI data. We acknowledge the Leverhulme Trust for funding the “Probing the Sun: inside and out” project upon which this research is based. The research leading to these results has received funding from the European Community’s Seventh Framework Programme ([FP7/2007 – 2013]) under grant agreement n° 312844 (see Article II.30. of the Grant Agreement).
Disclosure of Potential Conflicts of Interest
The authors declare that they have no conflicts of interest.
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