List of solar cycles

The following is a list of solar cycles (sometimes called sunspot cycles), tracked since 1755 following the original numbering proposed by Rudolf Wolf in the mid-19th century. The source data are the revised International Sunspot Numbers (ISN v2.0), as available at SILSO.^[1] Sunspot counts exist since 1610^[2] but the cycle numbering is not well defined during the Maunder minimum.^[3] It was proposed that one cycle might have been lost in the late 18th century,^[4] but this still remains not fully confirmed.

Solar cycles can be reconstructed indirectly, using the radiocarbon 14C proxy, for the last millennium.^[5]

The smoothing is done using the traditional SIDC smoothing algorithm.^[6] Using this algorithm, if the month in question is notated month 0, a weighted average is formed of months -6 to 6, where months -5 to 5 are given weightings of 1, and months -6 and 6 are given weightings of 0.5. Other smoothing formulas exist, and they usually give slightly different values for the amplitude and timings of the solar cycles. An example is the Meeus smoothing formula,^[7] with related solar cycles characteristics available in this STCE news item.^[8]

The start of solar cycle 25 was declared by SIDC on September 15 2020 as being in December 2019.^[9] This makes cycle 24 the only "11-year solar cycle" to have lasted precisely 11 years.

The following table is instead divided into (unofficial) cycles starting and ending with a maximum, to give a better feel for the number of spotless days associated with each minimum:

Following is a comparison of the growth of cycle 25 versus cycle 24, using the 13-month sunspot averages, beginning with the months of the respective minimums.
Numbers in brackets for cycle 25 indicate the minimum possible value for that month, assuming there are no more sunspots between now (20 Jan 2022) and six months after the end of the month in question.

The following table gives the number of days so far in cycle 25 (to Jan 20 2022), and up to the same point in cycle 24 (to Jan 20 2011), which have passed various thresholds for the numbers of sunspots:

As at Jan 20 2022, solar cycle 25 is averaging 33% more spots per day than solar cycle 24 at the same point in the cycle (Jan 20 2011).

References[]

External links[]

• Solar Influences Data Analysis Center (SIDC)
• Graphics of historic solar cycles at the SIDC page
• Near realtime solar images from SOHO
• SORCE Total Solar Irradiance (TSI) Data