Jan 27, 2020

Tuesday, Mar. 1, 2022

Something big just happened on the sun. Solar physicists Scott McIntosh (NCAR) and Bob Leamon (U. Maryland-Baltimore County) call it "The Termination Event."

"Old Solar Cycle 24 has finally died--it was terminated!" says McIntosh. "Now the new solar cycle, Solar Cycle 25, can really take off."

The "Termination Event" is a new idea in solar physics, outlined by McIntosh and Leamon in a December 2020 paper in the journal Solar Physics. Not everyone accepts it--yet. If Solar Cycle 25 unfolds as McIntosh and Leamon predict, the Termination Event will have to be taken seriously.

newprediction_strip sunspots.png
Predictions for Solar Cycle 25. Blue is the official prediction of a weak cycle. Red is a new prediction based on the Termination Event.

The basic idea is this: Solar Cycle 25 (SC25) started in Dec. 2019. However, old Solar Cycle 24 (SC24) refused to go away. It hung on for two more years, producing occasional old-cycle sunspots and clogging the sun's upper layers with its decaying magnetic field. During this time, the two cycles coexisted, SC25 struggling to break free while old SC24 held it back.

"Solar Cycle 24 was cramping Solar Cycle 25's style," says Leamon.

Researchers have long known that solar cycles can overlap. The twist added by McIntosh and Leamon is the realization that overlapping cycles can interact. This makes sense. In the early 20th century, George Ellery Hale discovered that the magnetic polarity of sunspot pairs reverses itself from one cycle to the next; indeed, the sun's entire global magnetic field flips every ~11 years. When adjacent, opposite-polarity solar cycles overlap, they naturally interfere.

Termination Events mark the end of interference, when a new cycle can break free of the old.

Bands of coronal bright points linked to old Solar Cycle 24 vanished in Dec. 2021, signalling a Termination Event. A Twitter thread from Scott McIntosh explains this in more detail.

The timing of the Termination Event can predict the intensity of the new cycle. In their Solar Physics paper, McIntosh and Leamon looked back over 270 years of sunspot data and found that Termination Events happen every 10 to 15 years.

"We noticed that the longer the time between terminators, the weaker the next cycle would be," explains Leamon. "Conversely, the shorter the time between terminators, the stronger the next solar cycle would be."

So when did the latest Termination Event happen? Dec. 2021. This yields a specific, testable prediction for Solar Cycle 25.

"We have finalized our forecast of SC25's amplitude," says McIntosh. "It will be just above the historical average with a monthly smoothed sunspot number of 190 ± 20."

"Above average" may not sound exciting, but this is in fact a sharp departure from NOAA's official forecast of a weak solar cycle. It could be just enough to catapult Terminators into the forefront of solar cycle prediction techniques.



Regarding the Sun's surface and sunspots, the following is of note from the article 'Triggering The Birth of New Cycle’s Sunspots by Solar Tsunami' by Dikpati, McIntosh, et al:

Following extensive studies of global hydrodynamics (HD) and Magneto Hydro Dynamics (MHD) of the solar tachocline* in quasi-3D MHD shallow-water model a global nonlinear MHD shallow-water tachocline model is used to simulate a solar tsunami.

Recently such a shallow-water tachocline model has demonstrated that tachocline nonlinear oscillations (TNO), generated by the oscillatory exchange of energies between tachocline differential rotation and magnetized Rossby waves there can successfully simulate the 6–18 months periodicity of the seasonal bursts of solar activity. We initialize the model by a toroidal magnetic configuration depicting the epoch of cessation of a cycle, during which the old cycle spot-producing toroidal magnetic band has just annihilated with its opposite hemisphere counterpart. The force-balance of the tachocline in this epoch is numerically computed and is presented in Fig. 3 in three different perspective views, namely in spherical polar coordinates, in latitude-longitude planform viewed along longitude, and viewed along latitude.

MHD tachocline.png
Equilibrium configuration of an MHD shallow-water tachocline top surface (20 times magnified) is presented in 3 perspective views: (a) tachocline fluid shell in spherical coordinate; top of gray inner, solid sphere represents bottom of tachocline and outer surface in semi-transparent color-shade the tachocline top-surface (red denotes bulging, blue depression). Two dams (two red bulging) on both sides of the equator arise due to supporting two oppositely-directed strong toroidal magnetic bands of 10-degree width at 5-degree latitudes on both sides of equator (about 150 kGauss peak-field). The two old-cycle’s toroidal bands are not explicitly shown in this picture in order to avoid complexity. (b,c) Display flattened global tachocline fluid shell respectively viewed along longitude (in panel b) and latitude (in panel c). Tachocline bottom is displayed in black and top-surface in color shades.

Two big dams on both sides of the equator in Fig. 3 appear from the plasma fluid that was supporting the strong, spot-producing toroidal bands at the equator until they were annihilated. Assuming that the annihilation of the old cycle’s band is a quick process, the two dams will be prone to create a solar tsunami. Theoretically there should be two more dams due to the presence of two high-latitude toroidal bands, representative of eventually new solar cycle, but these high-latitude dams are a few orders of magnitude weaker than that at the equator and hence almost invisible. In order to focus on the equilibrium structure of plasma fluid of the tachocline, we avoided showing the coexisting double toroidal magnetic bands (a strong band near the equator and a two-orders of magnitude weaker band at high-latitude) in each hemisphere. We first run the model with strong (150 kGauss peak field strength) low-latitude bands of 10-degree width at 5-degree latitude in each hemisphere and two weak (2 kGauss peak) bands at 40-degree latitudes to establish a fully developed solution for the nonlinear evolution of global MHD Rossby waves coupled with differential rotation. For this configuration, the high-latitude toroidal bands govern the dynamics, despite being a few orders of magnitude weaker than the bands at the equator, because they cause the global MHD Rossby wave instability while the system is stable to bands very close to the equator.

A physical mechanism, the solar tsunami, gives birth to the new cycle’s sunspots precisely within a few weeks from the cessation of old cycle’s spots. In no way does this specific tachocline dynamics replace or supersede any operating dynamo process, but instead adds to the precision with which it determines the onset timing. To include this mechanism in a full dynamo requires the inclusion of a realistic solar tachocline self-consistently with associated global MHD processes, something that has not been achieved in any dynamo model yet.


* Solar tachocline is a thin layer containing the strong radial differential rotation at the base of the convection zone has proven to be important for several reasons. The radial shear there is likely to generate the Sun’s strongest toroidal fields, which eventually erupt as bipolar spots at the surface, and the tachocline provides a good location for magnetic flux-storage. The subadiabatic stratification of this layer allows storage of strong toroidal field despite its magnetic buoyancy, while toroidal bands are held against poleward slip by either the prolateness of this layer, or by jet-like flows within the band, or both. Global hydrodynamics HD and magneto-hydrodynamics MHD instabilities that are theoretically predicted to occur in this layer produce two major results. One is the production of large-scale non-axisymmetries, by tipping or deforming the toroidal band, and the other is the generation of kinetic helicity. Both have important implications in solar dynamos. The former could be responsible for producing the Sun’s “active-longitudes”, while the latter produces the extended dipolar poloidal fields that are necessary for magnetically coupling the Sun’s N- and S-hemispheres.


It now appears that Solar Cycle 25 may be a bit above average. But, as the article above indicates, above average will mean a serious departure from NOAA's prediction of a weak Solar Sunspot Cycle and it may also result in some real trouble for us on Earth. And it may indicate that the sunspot cycle terminators will be a key forecasting element.
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