Solar minimum

For broader coverage of this topic, see Solar cycle.

Solar minimum is the regular period of least solar activity in the Sun's 11-year solar cycle. During solar minimum, sunspot and solar flare activity diminishes, and often does not occur for days at a time. On average, the solar cycle takes about 11 years to go from one solar minimum to the next, with duration observed varying from 9 to 14 years. The date of the minimum is described by a smoothed average over 12 months of sunspot activity, so identifying the date of the solar minimum usually can only happen 6 months after the minimum takes place.

Three recent solar cycles

Solar minimum is contrasted with the solar maximum, when hundreds of sunspots may occur.

Solar minimum and solar maximum

Solar minima and maxima are the two extremes of the Sun's 11-year and 400-year activity cycle.^[1] At a maximum, the Sun is peppered with sunspots, solar flares erupt, and the Sun hurls billion-ton clouds of electrified gas into space. Sky watchers may see more auroras, and space agencies must monitor radiation storms for astronaut protection. Power outages, satellite malfunctions, communication disruptions, and GPS receiver malfunctions are just a few of the things that can happen during a solar maximum.^[2]

At a solar minimum, there are fewer sunspots and solar flares subside. Sometimes, days or weeks go by without a spot.

Predicting solar minimum cycles

Their non-linear character makes predictions of solar activity very difficult.^[3] The solar minimum is characterized by a period of decreased solar activity with few, if any, sunspots. Scientists from the National Center for Atmospheric Research (NCAR) also developed a computer model of solar dynamics (Solar dynamo) for more accurate predictions and have confidence in the forecast based upon a series of test runs with the newly developed model simulating the strength of the past eight solar cycles with more than 98% accuracy.^[4] In hindsight the prediction proved to be wildly inaccurate and not representative of the observed sunspot numbers.^[5]

During 2008–09 NASA scientists noted that the Sun is undergoing a "deep solar minimum," stating: "There were no sunspots observed on 266 of [2008's] 366 days (73%). Prompted by these numbers, some observers suggested that the solar cycle had hit bottom in 2008. Sunspot counts for 2009 dropped even lower. As of September 14, 2009 there were no sunspots on 206 of the year's 257 days (80%). Solar physicist Dean Pesnell of the Goddard Space Flight Center came to the following conclusion: "We're experiencing a very deep solar minimum." His statement was confirmed by other specialists in the field. "This is the quietest sun we've seen in almost a century," agreed sunspot expert David Hathaway of the National Space Science and Technology Center NASA/Marshall Space Flight Center.^[6] However, the activity is still at a higher level than at a grand solar minimum.

Another index of solar activity allowing predicting solar cycles on a larger timescale than one cycle ahead was carried out using Principle Component Analysis (PCA) (Zharkova et al, 2015) by calculating eigen vectors (EVs) of solar background magnetic field (SBMF)using magnetic synoptic maps of Wilcox Solar Observatory (Stanford, US) for cycle 21-23 (Zharkova et al, 2015 https://www.nature.com/articles/srep15689) and 21-24 (Zharkova and Shepherd, 2022 https://solargsm.com/wp-content/uploads/2022/04/zharkova_shepherd_mnras22.pdf.

The first two PCs, or EVs, derived from PCA reflect the two primary waves of the solar magnetic dynamo produced by dipole magnetic sources (Zharkova et al. 2015) as shown in their Figure 1 for cycles 21-26 and Figure 3 for 2000 thousand years (1200-3200).

The authors derived the analytical formula for these two magnetic waves (eigen vector) and managed to reproduced with this formula the vast majority of sunspot cycles in the past as shown in Figures 6 and 9. The comparison of sunspot and modulus summary curves of two eigen vectors with scatter plots shown high 67% correlation (Figure 8) with sunspot numbers for the cycles from 1900 to 2020 shown in the paper Zharkova et al, 2023a https://solargsm.com/wp-content/uploads/2023/04/zharkova_etal_mnras2023.pdf.

Furthermore, the inclusion of a second set of PCs, or magnetic waves, produced by quadruple magnetic waves (Zharkova and Shepherd, 2022) revealed the their summary curve fit very well the soft X-ray index of solar flares for cycles 21-24. Hence the joint effect of dipole, quadruple, octuple and sextuple summary curves can cover a wider range of solar activity indices including flares and active regions.

Grand solar minima and maxima

Main article: Solar cycle

Grand solar minima occur when several solar cycles exhibit lesser than average activity for decades or centuries. Solar cycles still occur during these grand solar minimum periods but are at a lower intensity than usual. The grand minima form a special mode of the solar dynamo operation.^[7]

The Grand Solar Minima are nicely seen in the summary curve of two eigen vectors, or two principal components (PCs), derived with Principle Components Analysis, from the full disk magnetograms of solar background magnetic field in Zharkova et al, 2015 https://www.nature.com/articles/srep15689), which presents the solar activity backward 1200 years and forward 3200 years from the current time. This curve reveals the very distinct periodic variations of the 11y cycle amplitudes in every 330–380 yr, or grand solar cycles (GSCs) (see Figure 3 in Zharkova et al, 2015). These GSCs are separated by the Grand Solar Minima (GSMs) when the 11 yr cycle amplitudes become very small, similar to those reported for Maunder, Wolf, Oort, and other GSMs. The summary curve has also shown the modern GSM to occur in the cycles 25–27, or in 2020–2053 (Zharkova et al. 2015; Zharkova 2020 https://www.tandfonline.com/doi/full/10.1080/23328940.2020.1796243).

The expansion of this summary curve of two PCs, or EVs, up to 3000 years backwards can also reveal that the summary curve hit the main GSM down to Homeric minimum about 800 BC (Fig. 1 in Zharkova et al, 2018a https://solargsm.com/wp-content/uploads/2020/03/zharkova_iau335_paper1.pdf with all the objections by Usoskin answered in Zharkova et al, 2018b https://solargsm.com/wp-content/uploads/2020/03/zharkova_reply2usoskin_jastp17.pdf).

400 year history of sunspot numbers.

Solar minimum and maximum events with approximate dates

Event	Start	End
Homeric Minimum[8]	950 BC	800 BC
Roman Warm Period	250 BC	AD 400
Medieval maximum 1 (see Medieval Warm Period)	950	1040
Oort minimum (see Medieval Warm Period)	1040	1080
Medieval maximum 2 (see Medieval Warm Period)	1100	1250
Wolf minimum	1280	1350
Spörer Minimum	1450	1550
Maunder Minimum	1645	1715
Dalton Minimum	1790	1820
Modern Maximum	1914	2008

A list of historical Grand minima of solar activity^[9] includes also Grand minima ca. 690 AD, 360 BC, 770 BC, 1390 BC, 2860 BC, 3340 BC, 3500 BC, 3630 BC, 3940 BC, 4230 BC, 4330 BC, 5260 BC, 5460 BC, 5620 BC, 5710 BC, 5990 BC, 6220 BC, 6400 BC, 7040 BC, 7310 BC, 7520 BC, 8220 BC, 9170 BC.

See also

• Active region
• List of solar cycles
• Maunder Minimum
• Solar cycle
• Solar cycle 24
• Solar maximum