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.

Carrington Event 1859 – Reconstructing Historical Space Weather Events

What is Space Weather and how does it affect us?

Space weather is the result of particles emitted by the Sun, termed the solar wind, interacting with the Earth's magnetic field. Large eruptions from the Sun, called Coronal Mass Ejections (CMEs), pose a hazard to the ground infrastructure, such as the high-voltage power network, in the form of Geomagnetically induced currents (GIC). GICs are excess currents in the power network that are created by the rapid variations of the magnetic field and in extreme cases have caused damage to transformers causing the power grid to temporarily shut down. These space weather events are the same natural phenomenon as those responsible for the Aurora becoming visible at mid-latitudes.

There are important questions about the hazard posed by space weather that are unanswered. These include:

• How big can geomagnetic storms be?

• How often do storms of this size occur?

• What is the series of events that can lead to the largest geomagnetic storms?

• What are the effects on modern technology?

  
  

What records do we have of historical geomagnetic storms?

The British Geological Survey (BGS) holds records for eight geomagnetic observatories operating in the UK going back to 1847. The digital era of geomagnetic observations began in the 1980s providing us with high-quality recordings of the Earth’s magnetic field but prior to this all data were recorded on photographic paper. The problem with the digital dataset is the dearth of very large geomagnetic storms. As shown in Figure 1, we have enjoyed an unusually quiescent era of solar activity since the 1960s.

Figure 1: Geomagnetic activity from 1880 to 2020, expressed by the daily Aa index. Figure produced as part of the British Geological Survey and the ESA Space Safety Programme (https://swe.bgs.ac.uk/bgs/indices.shtml?index=Aaindexdaily).

What is the Carrington Event?

These historical records contain some of the largest known geomagnetic storms including the famous Carrington event of 1st-2nd September 1859 – one of the large storms on record. The event and its lesser-known precursor were recorded at two rival observatories both operating in London at the time, Kew and Greenwich. This provides a great opportunity to cross-compare the two observatories only 20km apart.
The paper magnetograms provide a unique example of near-continuous measurements for the Carrington event and pre-cursor storm. We manually extracted the digital time series of Kew and Greenwich records of three components of vertical, horizontal and declination of magnetic field from 25-Aug to 05-Sep-1859 by digitizing the historic records. To assist with scaling the magnetograms into accurate time and magnetic units, we use published journal papers from the period to benchmark our interpretation and spot values recorded at Greenwich. This isn’t without its own problems, including:

• the poor quality of parts of the recorded traces

• overlap or missing traces

• Issues with instrumentation at the time of recording

• Lack of metadata to scale recorded traces to modern SI units of magnetic field.

Figure 2: Paper magnetograms recorded on photographic paper for observatories in London, UK. (a, b) Declination angle at Kew from 10:20UT 02-Sep-1859 to 12:05UT 05-Sep-1859 (c, d) Declination angle and Horizontal Force at Greenwich from 12:00UT 01-Sep-1859 to 12:00UT 03-Sep

Figure 3 shows the reconstructed time series of the magnetic field at both the Greenwich and Kew observatories. Reconstructing the days prior to the famous 1-2 September storms enables us to see the series of events that led to such an extreme geomagnetic storm. When storms occur in close succession, the first storm essentially clears the interplanetary space of solar wind and enables the subsequent storms to have greater impact on Earth. We can see evidence for a large storm during the 28th-30th storm for example. The period around August and September 1859 was unusually stormy compared to the modern era.

Figure 3: Digitized magnetograms of (a) horizontal magnetic field strength, (b) declination, (c) vertical magnetic field strength, at Kew (orange) and Greenwich (blue) observatories from 25- Aug to 05-Sep-1859. Highlighted green is the Carrington-observed flare at ~11:15-11:23 01 September 1859. Highlighted in grey are evidence for suspected earlier solar flares.

The analogue record is a rich and yet untapped source of information about geomagnetic activity in the past. With a more focussed effort and new digitisation tools, the community may be able to find better answers to the great unknowns of space weather hazards in future.

BGS Magnetograms: 
https://www.bgs.ac.uk/information-hub/scanned-records/magnetograms/

Authors:
E. Eaton¹, C. Beggan¹, E. Lawrence¹, E. Clarke¹, K. Matsumoto², H. Hayakawa^2
1British Geological Survey, ²Nagoya University

Eliot Eaton is a magnetotelluric field technician at the British Geological Survey, UK. His primary research focus is completing a magnetotelluric survey of England, Wales, and Southern Scotland to improve understanding of how geomagnetic storms influence the UK’s grounded infrastructure, such as the high-voltage power grid. During the COVID-19 pandemic, all fieldwork was postponed so he had the opportunity to dig into the historical geomagnetic archives of the UKs observatories.

Opportunities and challenges with palaeomagnetic data sources

In our quest to understand the geomagnetic field of past millennia, we rely on two primary sources of data. The first source is archaeomagnetic data, which provides us with valuable information about the geomagnetic field based on ancient artefacts. However, one major challenge we encounter is the uneven distribution of this data in both space and time. It's like trying to put together a historical jigsaw puzzle without all the pieces.

But don't worry, there is another data source that helps overcome these limitations and provides a more comprehensive understanding of the past geomagnetic field. Sedimentary records, a treasure trove of information spanning vast time periods and offering improved spatial coverage. Imagine these records as growing bars, each representing a time series of the geomagnetic field at a specific location. In contrast, archaeomagnetic data appears as dots, providing snapshots of the geomagnetic field at specific locations. As we venture further back in time, sediment data becomes increasingly essential since archaeomagnetic data becomes sparser.

The spatial and temporal data coverage.

Now let's unravel the different magnetization mysteries within these sediments. Archaeomagnetic data captures the geomagnetic field through a fascinating process known as thermoremanent magnetization (TRM). Picture this: ancient artifacts, like pottery and kiln structures but also lava flows are heated and then left to cool down. But here's the magical part - during this process, they become magnetized, preserving a snapshot of the geomagnetic field at that exact moment. It's like capturing a piece of geomagnetic field's history in a magnetic time capsule.

The magnetization process in sediments is known as detrital remanent magnetization (DRM). During the sedimentation process, magnetic particles settle in such a way that their magnetic moments tend to point in the direction of the geomagnetic field. It's like they have an ancient compass within them, pointing in the direction of the geomagnetic forces. With the accumulation of additional sediment material, the magnetic particles become mechanically fixed within the sediment structure, preserving their magnetic orientations. It's as if they have been frozen in time, capturing the magnetic field's influence at the lock-in moment.

The magnetization in sediments is influenced by several factors, such as the interaction between the magnetic particles and the substrate at the sediment-water interface (depositional DRM), as well as the consolidation and dewatering process of the sediment (post-depositional DRM). Within depositional DRM, various effects come into play. For instance, there is the inclination error, which arises when non-spherical particles settle flat on the sediment-water interface. This leads to a distortion of the inclination, resulting in smaller inclination values than expected.

In our investigation, we're particularly interested in unraveling the secrets of post-depositional DRM. Initially, only the larger sediment particles become mechanically fixed shortly after deposition. Smaller particles, on the other hand, enjoy a freer journey, moving within water-filled voids and pore spaces for a longer duration. However, as the sediment consolidates and dries out, these smaller particles slowly become locked in too. It's a mesmerizing process, like witnessing magnetic particles tell stories of the ever-changing geomagnetic field.

Check out the figure below that illustrates the journey of magnetic particles during the lock-in or pDRM process.

The sedimentation process.

A The lock-in adventure begins when the particles settle on the sediment-water interface. Sediments are composed of a mix of magnetic and non-magnetic particles, creating a vibrant playground. During the early stages of the lock-in process, particles rotate freely and align with the geomagnetic field. It's like a magnetic ballroom dance conducted by the geomagnetic field forces.

B As time passes and sedimentation continues, the surrounding material consolidates. Larger magnetic particles begin to lose their mobility and get locked in. They find their forever spots, holding onto the memories of the geomagnetic field at that time. But what about the smaller particles? They're still lively and free, closely following the twists and turns of the geomagnetic field.

C After ample sedimentation and consolidation, the lock-in process reaches its grand finale. Each particle becomes a storyteller, carrying a piece of the geomagnetic field's history within it. The sediment layer becomes a mosaic of magnetic moments, depicting diverse states of the geomagnetic field throughout the entire lock-in period. It's like a magnetic symphony composed of the melodies of the geomagnetic field.

The magnetic moment of a whole sediment layer represents a weighted average of the geomagnetic field over the lock-in period. This is where the concept of a lock-in function comes into play. The lock-in function assigns weights to the different geomagnetic field values, reflecting their significance during the lock-in process.

The investigation of the lock-in process and the development of a modeling concept to estimate the lock-in functions for individual core samples is the primary goal of our studies. Our research outcomes provide in-depth details, methodologies, and findings that shed light on the fascinating world of sediment records and post-depositional DRM. Visit https://sec23.git-pages.gfz-potsdam.de/korte/pdrm/ for more information. With our results we make sediment data a more reliable data source for modeling the geomagnetic field. It's time to unlock the secrets of the geomagnetic past!

Lukas Bohsung is a second year PhD student at the University of Potsdam and the Helmholtz Centre Potsdam — GFZ German Research Centre for Geosciences in Germany. His main focus is on investigating and modeling magnetization processes in sediments to make sediment records a more reliable data source for geomagnetic field reconstructions. He can be contacted via email here.