A test of the active-day fraction method of sunspot group number calibration : dependence on the level of solar activity
|Author:||Willamo, T.1; Usoskin, I.G.2,3; Kovaltsov, G.A.4,2|
1Department of Physics, University of Helsinki, 00014 Helsinki, Finland
2Space Climate Research Unit, University of Oulu, Oulu, Finland
3Sodankylä Geophysical Observatory, University of Oulu, Oulu, Finland
4Ioffe Physical-Technical Institute, St. Petersburg, Russia
|Online Access:||PDF Full Text (PDF, 0.1 MB)|
|Persistent link:|| http://urn.fi/urn:nbn:fi-fe201804206748
|Publish Date:|| 2019-04-12
The method of active-day fraction (ADF) was proposed recently to calibrate different solar observers to standard observational conditions. The result of the calibration may depend on the overall level of solar activity during the observational period. This dependency is studied quantitatively using data of the Royal Greenwich Observatory by formally calibrating synthetic pseudo-observers to the full reference dataset. It is shown that the sunspot group number is precisely estimated by the ADF method for periods of moderate activity, may be slightly underestimated by 0.5 – 1.5 groups (≤10%) for strong and very strong activity, and is strongly overestimated by up to 2.5 groups (≤30%) for weak-to-moderate activity. The ADF method becomes inapplicable for the periods of grand minima of activity. In general, the ADF method tends to overestimate the overall level of activity and to reduce the long-term trends.
|Type of Publication:||
A1 Journal article – refereed
|Field of Science:||
115 Astronomy and space science
This work was carried out in the framework of the ReSoLVE Centre of Excellence (Academy of Finland, project no. 272157).
|Academy of Finland Grant Number:||
272157 (Academy of Finland Funding decision)
© Springer Science+Business Media B.V., part of Springer Nature 2018. This is a post-peer-review, pre-copyedit version of an article published in Solar Physics. The final authenticated version is available online at: http://dx.doi.org/10.1007/s11207-018-1292-7.