Solar flare activity, 1937-2024: Introducing the new hemispheric Solar Flare Index (hSFI) in the context of 2024’s major solar storm events

A new daily composite of the solar flare index (SFI) and the hemispherically-resolved versions (hSFI) is presented for 1937 to 2024. We offer this new dataset to the space weather community in daily, monthly, and annual resolutions. The highest SFI value occurred during the superstorms of May 2024. The dataset confirms that the northern hemisphere dominated solar flare activity during Solar Cycles 17 to 21, but that the southern hemisphere has dominated from Solar Cycle 22 to present. In sunspot activity, the “GnevyshevOhl rule” shows that the sum of sunspot numbers during even-numbered cycles is greater than that of the subsequent odd-numbered cycle. However, for SFI, the equivalent rule seems to be different: SFI activity during even-numbered cycles is greater than that of the preceding odd-numbered cycle. This suggests different pairing rules apply for the photosphere and chromosphere. This cross-cycle interplay might offer a potential mechanism for multi-decadal changes in solar activity. The “Gnevyshev gap” phenomenon where solar maximum activity sometimes has two peaks separated by up to 1-2 years of a gap is confirmed for SFI. A theoretical explanation for this phenomenon is discussed in terms of the asymmetry between the two hemispheres and slight differences in the peaks and intensities of the “Mid-term Periodicities” (MTP). Although our dataset represents the longest continuous daily dataset for solar flare activity to-date, it is known that stronger solar flare events occurred before 1937. Therefore, a brief discussion of earlier solar flare events in the historical record is also provided for context.