Abstract
A variety of indices have been proposed in order to represent the many different observables modulated by the solar cycle. Most of these indices are highly correlated with each other owing to their intrinsic link with the solar magnetism and the dominant eleven year cycle, but their variations may differ in fine details, as well as on short- and long-term trends. In this paper we present an overview of the indices that are often employed to describe the many features of the solar cycle, moving from the ones referring to direct observations of the inner solar atmosphere, the photosphere and chromosphere, to those deriving from measurements of the transition region and solar corona. For each index, we summarize existing measurements and typical use, and for those that quantify physical observables, we describe the underlying physics.
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Notes
Faculae is the name given to brightenings seen in photospheric radiation mainly near the solar limb and in the general vicinity of sunspots. Find more information in e.g. Solanki and Krivova (2009).
The term white-light indicates the sum of all visible wavelengths of solar radiation from 400 to 700 nm, so that all colors are blended to appear white to the eye.
Plage is the name given to the brightening seen in chromospheric radiation corresponding to photospheric faculae. In contrast to faculae, plage are seen over the whole disk, in active regions and in the quiet sun, on the network pattern formed at the borders of supergranular cells. Find more information in e.g. Solanki and Krivova (2009).
Flare is the name given to a sudden, rapid, and intense brightening observed over the solar disk or at the solar limb, due to a release of magnetic energy (up to 1032 erg on the timescale of hours), followed by ejection of solar plasma through the corona into the heliosphere. Find more information e.g. in Benz (2008).
Coronal holes are areas where the Sun’s corona is darker, and colder, and has lower-density plasma than average. They are associated with rapidly expanding open magnetic fields and the acceleration of the high-speed solar wind. Find more information in e.g. Potgieter (2013).
Conversion: 1×1016 W sr−1=4.5×10−7 W m−2=1.2×photons cm−2s−1.
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Acknowledgements
The authors are grateful to the International Space Science Institute, Bern, for the organization of the workshop “The Solar Activity Cycle: Physical Causes and Consequences”, the invitation to contribute to it, and the kind support received to the purpose. The authors thank Fabrizio Giorgi for preparing Figs. 1 to 8. This study received funding from the European Union’s Seventh Programme for Research, Technological Development and Demonstration, under the Grant Agreements of the eHEROES (No. 284461, www.eheroes.eu), SOLARNET (No. 312495, www.solarnet-east.eu), and SOLID (No. 313188, projects.pmodwrc.ch/solid/) projects. It was also supported by COST Action ES1005 “TOSCA” (www.tosca-cost.eu). LvDG’s work was supported by the Hungarian Research grants OTKA K-081421 and K-109276, and by the STFC Consolidated Grant ST/H00260/1.
Final acknowledgements go to the many observers and astronomers, both amateur and professional, for performing the regular observations of the solar atmosphere and creating the databases of solar indices described in this paper.
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Ermolli, I., Shibasaki, K., Tlatov, A. et al. Solar Cycle Indices from the Photosphere to the Corona: Measurements and Underlying Physics. Space Sci Rev 186, 105–135 (2014). https://doi.org/10.1007/s11214-014-0089-8
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DOI: https://doi.org/10.1007/s11214-014-0089-8