University of Oulu

Takalo, J. Analysis of the Solar Flare Index for Solar Cycles 18 – 24: Extremely Deep Gnevyshev Gap in the Chromosphere. Sol Phys 298, 86 (2023).

Analysis of the Solar Flare Index for Solar Cycles 8–24: : Extremely Deep Gnevyshev Gap in the Chromosphere

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Author: Takalo, Jouni1
Organizations: 1Space Physics and Astronomy Research Unit, University of Oulu, POB 3000, FIN-90014, Oulu, Finland
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 5.1 MB)
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Language: English
Published: Springer Nature, 2023
Publish Date: 2023-09-27


We study the solar flare index (SFI) for the Solar Cycles 18 – 24. We find that SFI has deeper Gnevyshev gap (GG) in its first principal component than other atmospheric parameters. The GG is extremely clear especially in the even cycles.

The GG of the SFI appears about a half year later as a drop in the interplanetary magnetic field near the Earth and in the geomagnetic Ap-index. The instantaneous response of the magnetic field to solar flares, however, shows about two to three days after the eruption as a high, sharp peak in the cross-correlation of the SFI and Ap-index and as a lower peak in SFI vs. IMF B cross-correlation. We confirm these rapid responses using superposed-epoch analysis.

The most active flare cycles during 1944 – 2020 are Cycles 19 and 21. Cycle 18 has very strong SFI days as many as Cycle 22, but it has the least nonzero SFI days in the whole interval. Interestingly, Cycle 20 can be compared to Cycles 23 and 24 in its low flare activity, although it is located between the most active SFI cycles.

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Series: Solar physics
ISSN: 0038-0938
ISSN-E: 1573-093X
ISSN-L: 0038-0938
Volume: 298
Issue: 7
Article number: 86
DOI: 10.1007/s11207-023-02177-8
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Funding: Open Access funding provided by University of Oulu including Oulu University Hospital.
Dataset Reference: The Solar Flare index data were obtained from Part of the Flare Index Data used were calculated by T. Atac and A. Ozguc from Bogazici University Kandilli Observatory, Istanbul, Turkey. The dates of cycle minima were obtained from the National Geophysical Data Center, Boulder, Colorado, USA ( The newly reconstructed corona indices were obtained from The corona index for Solar Cycle 24 was downloaded from The SSN2 data has been downloaded from, and the solar radio flux data from Definitive values of Ap are provided by GeoForschungs Zentrum (GFZ) Potsdam. The OMNI2 data are downloaded from Daily plage area data are from
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