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Comparing three solar downturns -- Dalton, "Late Victorian" and Recent

  • 11-11-2021 6:36am
    Registered Users Posts: 14,245 ✭✭✭✭

    When it comes to solar minima, the Maunder minimum (roughly 1645-1715) is of course the most noteworthy, and second to that would be the Sporer minimum (second half of 15th century) according to the lack of auroral sightings that are used to try to reconstruct sunspot activity before the observational age. There were probably a few other minima before those two, but none of them seem much different in duration or relative strength than the Dalton (1795 to 1835), the un-named "Late Victorian" (just named it) 1875-1915 and the current downturn.

    These three (Dalton, "Late Victorian" and recent, some call it the Gleissberg minimum) appear to be quite similar although we don't yet know when the current one will return to stronger levels of activity. Here in a graphical format are the comparative timing and intensity values of the three weaker cycles involved in each of them.

    SS PEAK ________ Dalton _______ Late Victorian ______ Recent (Gleissberg)

    Last strong ______ 1787-88 ______ 1870 ____________ 1989 ____

    first moderate ____ 1801-04 ______ 1883 ____________ 1999-2001

    second weak _____ 1815-16 ______ 1893 ____________ 2013

    third weak-mod ___ 1829-30 ______ 1905-07 _________ 2024?

    resumed strong ___ 1837-38 ______ 1917 ____________ ??


    These three solar downturns are not quite mirror image but close to that. The Dalton involved an interval of fifty years between sets of stronger or "regular" solar cycles. Some believe that the 1787-88 peak, which had a very long extinction period, was jammed together with a second rather modest peak around 1794-95 when there was a temporary uptick in sunspot numbers, but many solar cycles have secondary peaks at some point well after their main peaks. The peak in 1968 had a secondary in 1972 that is not generally seen as a separate event, nor was there any disruption to the Hale cycle (alternating polarity of sunspots which we don't know for sure happened before the mid-19th century).

    The "Late Victorian" had an interval of 47 years between two stronger or regular peaks. By 2036 this one would be going for 47 years also.

    The three weak maxima compare roughly also. 1801-04 was a flat-topped peak of quite modest sunspot numbers. 1883 had similar numbers from 1882 to 1885. The 1999-2001 peak was stronger although only considered moderate and not as strong as most of the peaks from 1917 to 1989 (it compared to perhaps 1917, 1937 and 1968). Thus one could postulate that 1999-2001 should be regarded as the last "strong" peak and that we have only seen two weaker peaks of the downturn so far.

    1815-16 was a very weak sunspot peak not much more impressive than the tiny fluctuations within the Maunder. It was preceded and followed by long intervals of absolute solar "quiet" lasting 2-3 years (around 1810-11 and 1822-23). Then the 1893 peak was somewhat stronger than the 1883 peak, while the 2013 peak was quite weak also. Note that the usual 10-11 year period of strong, regular cycles slows to an average of about 12 to 13 years in these downturns.

    1829-30 was also quite a weak sunspot cycle but had a brief quiet spell following before a very strong cycle hit in the late 1830s. That was one of the strongest on record. Then 1905-07 was a long-lasting but quite weak cycle that had two nearly equal peaks, and was followed by a more regular "solar quiet" around 1911-12 and the moderately strong 1917 peak. And the coming 2024 peak is widely expected to register as a "weak-moderate" on the scale developed by Schwabe (using the aforementioned long-term auroral data to build a data base going back continuously to around 290 AD).

    We can only speculate as to how this current downturn will evolve and whether this postulated 2024 peak will be the last reduced one, or whether it will go on for one or two more cycles. One reason that this is significant is that a resumption of stronger solar activity will very likely be accompanied by a faster warming trend as the natural signal combines with the more or less constant AGW signal. The longer this downturn lasts, the better it would be for our chances of avoiding severe temperature increases.

    As noted, only the Maunder and Sporer offer much hope of longer than three weaker cycle downturns, and their cycles turned very weak to nearly non-existent (the Sporer was postulated to be about mid-way between the Maunder and the Dalton in terms of weakness and longevity). Other postulated periods of weaker activity in the more distant historical past involved only two or three weak cycles. They do seem to have a rough period of about a century with a sub-period of 200 years.

    Here's a link to sunspot numbers throughout the observational period of record.

    The number of consecutive strong(er) cycles is not a constant either. The 20th century and two other periods in the 14th and 16th centuries had about seven or eight strong cycles with some (usually towards the end) that were rated very strong. There were some other similar strings of postulated strong cycles in earlier centuries too. However, in the cases that we know best, there were seven reasonably strong peaks from 1718 to 1787, then just four from 1837 to 1870, and eight from 1917 to 1989 (or nine from 1917 to 1999-2001). The Carrington event (Sept 2, 1859) was actually in a rather moderate peak, so there's no particular reason to correlate risks from solar flares of that magnitude with sunspot number. It is thought that there was in fact a similar flare in 2012 but luckily it was directed at right angles to the Sun-earth line so we missed the full impacts of it.

    As there is no really developed predictive science for solar variation, it is not possible to say with any confidence whether stronger activity will resume around some postulated peak in the period 2035-2040, assuming the usual slower frequency continues for a while. Nor are we absolutely sure that the predicted 2024 peak will not in fact resume stronger activity.