Oscillating relic magnetic field in the Sun can explain solar long-term evolution and systematic hemispheric asymmetry

Sun is a magnetic star whose magnetic field is generated in the upper third of solar interior by the motion of charged particles called solar plasma. Upward transport of hot plasma and differential rotation form a system of electric currents that produce magnetic fields. This is called the solar dynamo mechanism. Magnetic fields can be seen on the solar surface, occasionally even by naked eye, as sunspots, whose variable occurrence has been followed during several hundred years. Sunspots vax and vane according to a roughly 11-year cycle, commonly called the sunspot cycle. However, the height and length of sunspot cycles also vary in a roughly 100-year cyclicity called the Gleissberg cycle. The maximum of the last Gleissberg cycle was during cycle 19 (in the late 1950s), which is the highest solar cycle so far. This activity has declined now and, since cycle 24, solar activity is on a much lower level.

The heights of the past solar cycles have alternated so that an odd cycle is higher than the previous even cycle. This is called the Gnevyshev-Ohl (G-O) rule according to its finders. Since cycle properties vary randomly in dynamo models, this systematic alternation of cycle heights cannot be explained by dynamo theory. On the other hand, a relic or fossil magnetic field prevailing in the solar interior from the times of solar system formation can, together with the dynamo mechanism, naturally explain the G-O rule. Relic electric currents producing a relic magnetic field can exist during billions of years because currents weaken very slowly in the Sun due to high electric conductivity. Relic currents must flow in the direction of solar rotation in order to agree with the G-O rule. This creates a relic magnetic field which is northward oriented.

Solar northern and southern hemispheres depict very often somewhat different levels of activity. It was recently shown (Mursula, 2023) that solar hemispheres are systematically asymmetric so that maximum activity is stronger in the northern than southern hemisphere in odd cycles, while it is stronger in the southern hemisphere in even cycles. Again, such a systematic alternation cannot be explained by the dynamo alone. However, it can be explained by a relic magnetic field which is shifted slightly northward from the solar equator (see Figure). It was also found there that cycle height and asymmetry are correlated. Cycle 19 was not only the highest but also most strongly dominated by activity of the northern hemisphere. The relic field had its largest shift to the north during this cycle. Accordingly, the Gleissberg cycle can be explained as an excursion of the location of the relic field to the north (or south) and back to the solar equator during a roughly 100-year oscillation. A full oscillation of relic consists of two Gleissberg cycles, with one shift to the north and one to the south. This also gives a new interpretation for the 210-year Suess/deVries cycle as the full relic oscillation cycle and connects Gleissberg cyclicity and Suess/deVries cyclicity under the same new paradigm of an oscillating relic field.

Left part of vertical line depicts the schematic operation of solar dynamo from solar minimum (plots of first column) to solar maximum (second column) by the action of differential rotation (depicted by capital omega). Minimum-time poloidal (vertical) magnetic field lines are transformed to maximum-time toroidal (horizontal) field lines. In the upper plots, poloidal lines are upward (so-called positive minimum), in the lower plots they are oriented downward. Right part of vertical line depicts how the existence of a northward oriented relic field (thick upward arrow) modifies the (pure) dynamo field. During a positive minimum (upper row), relic field and dynamo poloidal field are oriented in the same direction. As a result, the toroidal field due to relic (thick horizontal arrow) and dynamo toroidal field strengthen each other during th emaximum. This effect is stronger in the northern than southern hemisphere, leading to northern dominance in sunspot activity during odd maxima. In the negative minimum (lower row), relic and dynamo fields are opposite, which decreases the toroidal field in both hemispheres. However, the decrease is more effective in the northern hemisphere, which implies southern dominance during even maxima.

Oscillating relic magnetic field allows to make long-term forecasting for several cycles into the future, contrary to the one-cycle limit of pure dynamo theories. Cycle 25 will become slightly larger than cycle 24 because it is G-O favored but it will remain only moderately high because the relic shift is still quite small. Further on into the 21st century, cycle heights tend to increase since the relic shift is increasing, but cycle 26 is G-O disfavored and will remain still rather small. However, with increasing relic shift the G-O favored cycle 27 will already be a lot higher, maybe above 200 in annual sunspot numbers. Relic field will reach its maximum shift to the south in cycle 29, which will be the highest cycle in the 21st century, in analogy with cycle 19, which was the highest cycle of the 20th century. Thereafter, cycle heights will again start decreasing, with relic location returning to the solar equator. Cycle 29, as all odd cycles of the 21st century will be south-dominated, while even cycles will be north-dominated. Accordingly, the hemispheric dominance in the 21st century will alternate oppositely to that in the 20th century, because of the southern shift of the relic field.

Oscillating relic magnetic field will become the new paradigm of space climate, the study of long-term changes in the Sun and the solar-terrestrial environment, in the coming decades. Hopefully helioseismic methods and dynamo models amended by relic fields will soon be improved to allow them to possibly find more direct evidence for relic fields. Eventually, the above predictions on future cycles will test the new paradigm in the coming decades.

Kalevi Mursula is an active professor emeritus from the University of Oulu, Finland, where he was faculty professor for nearly 30 years and leader of the Center of Excellence of the Academy of Finland on space climate in 2014-2019. He is the originator of the concept of space climate and main organizer of a series of ten space climate symposia and schools in 2004-2023.


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