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24 spotless days and counting — how does that compare historically?

Today marks the 24th consecutive spotless day. But which period holds the previous record?

Since mid-2016, the Sun has occasionally been devoid of sunspots. These blank days are becoming more and more common as solar cycle 24 continues its transitional minimum phase to cycle 25 (the bottom currently expected by 2020).

The number of spotless days can vary significantly from one solar cycle transit to another. For example, during the previous minimum (2008-09), no less than 817 spotless days were recorded, whereas the minimum period leading into solar cycle 23 (around 1996) counted only 309 such blemishless days.

The 2008-09 minimum was the deepest in nearly 90 years.

With our toes now firmly submerged in this next Grand Solar Minimum, it’s not impossible that the coming solar cycle (25) will be devoid of sunspots entirely — something that hasn’t happened since the previous GSM, the Maunder Minimum (1645-1715). Even the most optimistic predictions, from the mainstream, see solar cycle 25 having a 30% reduction over the previous, and already weak, cycle 24.

SILSO’s “Spotless Days page” provides us with a front-row seat on the current status of the Grand Solar Minimum and the number of spotless days.

Here are some of their very interesting historical charts:

 

Periods with spotless days (>30 days) since 1849

spotless days www.electroverse.net
[source: SILSO data/image, Royal Observatory of Belgium, Brussels]
The table above shows all periods with 30 or more consecutive spotless days. The period from 31 July 2009 till 31 August 2009 (SC23-24 transit) is one of longest since the beginning of daily solar observations in 1849.

An even longer period in recent history occurred during the SC22-SC23 minimum transition, from 13 September 1996 till 24 October 1996, when the Sun was spotless for 42 consecutive days.

One of the longest spotless periods (since 1818) is from 24 October 1822 till 12 March 1823 (140 days!), but unfortunately the series was broken on 29 December 1822 due to no observations being available for that day.

Top 25 of years with most number of spotless days since 1849

[source: SILSO data/image, Royal Observatory of Belgium, Brussels]
This graph shows the number of spotless days per year since 1849. There are 112 years (including 2014 and 2017) with at least 1 spotless day, of which 25 years with 150 or more spotless days.

1913 is the record holder with a staggering 311 days, while 2008 ranks fourth with a spotless run of 265 days.

With 262 days, 2009 falls just short of 2008 but still makes it into the top 5 years, testifying to the deep minimum prior to the onset of SC24.

During the first half of 2018, spotless days are already closing in on 2017’s yearly total (96).

 

For your own comparisons I’ve included a Solar Cycle chart:

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