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.

PhD in IAGA #3

IAGA has a lot of different scientists working on various topics. In this series of blogs, we will introduce some topics that are being worked on by PhD students. Hopefully this will give a better picture of the work being done in the field and encourage more early career researchers.

Samuel Fielding, a PhD candidate at the University of Edinburgh, says:

My research topic is looking at the current forecasting capabilities within the field of space weather, and trying to improve them and find new avenues of research within the field using machine learning. With the large amounts of data being collected on space weather from the many satellites currently in orbit around the Earth or at the L1 Lagrange point, there is a lot of data to train machine learning algorithms on, and the Earth-Sun system is currently not well modelled by current physical models because of the complexity of interactions within the system. This means that the field is a perfect place to explore machine learning models, and it is a very active field with a lot of research on optimising current prediction models. Can these models be optimised further, and looking forwards, is machine learning the right way for us to predict space weather events?

 

False-colour image of a solar eclipse from 21 August 2017. Copyright Miroslay Druckmuller. As published in SciTechDaily, 21 June 2021 and Habbal et al. 2021.

Magnetic Field : Sun

Just like a planetary dynamo generates a magnetic field inside the planet, solar dynamo is responsible for the solar magnetic field. The Sun is composed of plasma, i.e, charged particles, which in motion produce magnetic fields. This field is carried away from the sun towards the planets in all directions by the solar wind, and is called the interplanetary magnetic field (IMF). Two interesting phenomena related to the field are its variation and shape.  

Interplanetary magnetic field extending out from the Sun. Credit: Vallée, J 1998

The solar magnetic field acts as a bar magnet with two poles. These poles are observed to flip regularly every 11 years. This variation is termed as a solar cycle. The solar minimum, when the field is weak, is considered the start of a solar cycle. The sunspots are lowest during this time. Sunspots are dark spots observed on the surface where the magnetic flux is very high (over 0.2 Teslas).

Number of sunspots vs time depicting solar cycles. Credit: ESA/NOAA.

The IMF extending out from the sun with the solar wind travels as a rotating spiral due to the spinning of the sun. This shape is known as the Parker spiral. The sun rotates around every 24 days at the equator and every 35 days at the pole. On average, this is taken to be 27 days and is known as the Carrington rotation.

Image Credit: NASA

                                                  
The solar magnetic field and its related phenomena are active topics of research. There is still a lot to find and understand! 

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Shivangi Sharan is a second year PhD student at the Laboratory of Planetology and Geodynamics in France. Her research focusses on the study of the magnetic field of Mars and to infer its internal structure from it. She is an active member of the IAGA Blog Team and can be contacted via e-mail here.

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Aurorae Events of November 3-4, 2021 : A Summary

People up and down the United Kingdom have been able to see the northern lights this month- a rare occurrence, considering that the aurora borealis are only usually visible at higher latitudes (think Iceland, or northern Finland!)

So, what has been happening up there in space? For those of you readers that do not know about the origins of the northern lights, what causes them is what is commonly known as the solar wind- a string of charged particles which stream outwards from the sun constantly in all directions. These particles then interact with the Earth’s magnetic field, causing them to curve to higher latitudes where the magnetic field lines pass through the Earth’s atmosphere. As these particles enter the upper atmosphere, they excite and ionise the upper layers of gases, which in turn causes light emission- the aurora borealis in northern latitudes, or aurora australis in southern latitudes. This is not unique to our planet- Jupiter has aurora as well, for example.

Demonstrating how the aurora form. Particles from the Sun are deflected towards the poles. The early November event also saw enhanced aurora in the southern hemisphere, including sightings in New Zealand. Credit: NOAA

The widespread aurora seen on the week of the 1st of November was caused by exciting solar activity. The sun released an X class flare (the highest class) on Thursday 28th October, and we saw some geomagnetic activity here on Earth on the 30th-31st October (though, according to the British Geological Survey, most of the effects of the flare and the associated coronal mass ejection missed the Earth to the south). The 03-04 November event was mostly due to a coronal mass ejection and M-class flares (the second highest class) which occurred on the 2nd of November on the surface of the sun. Though the speed of propagation through the solar system is very fast (hundreds of km per second, in fact), it still takes several days for the effects to reach us on Earth. The effects were strong and did interact with the Earth’s magnetic field rather than missing it (in 3D space, it is very likely that any given event will not interact with us, considering the Earth is so small). In addition, the direction of the IMF (interplanetary magnetic field) was in the direction of the Earth’s magnetic field, leading to a strong connection between the two and stronger space weather impacts.

Photo of Northern Lights over Derwent Water, Cumbria, early November 2021. Image Credit: Owen Humphries (PA).

On Earth, we experienced large differences in the geomagnetic field compared to quiet periods, which produced a high value of the Kp index (which we use to quantify the changes in the magnetic field).

The northern lights are, however, hit and miss. If there is cloud cover, or you are looking at the wrong time or in an area with a lot of light pollution, you may not see the aurora even if they are directly overhead. I found this, to my chagrin, when I went looking for this particular event with another colleague working in geomagnetism. However, when this happens to you, you simply need to come back another time. Aurora are rare at lower latitudes, but there will be another opportunity. Sometime in the future, you may yet see a faint, translucent green light on the northern horizon.

 

 

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Samuel Fielding is a first year PhD candidate at the University of Edinburgh, working on the real-time forecasting of space weather using machine learning and satellite data. He can be contacted via e-mail here.

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Magnetospheric Phenomena : IAGA Division III

Dr. Simon Wing is the Division Chair for IAGA Division III : Magnetospheric Phenomena. Here, he answers some of our questions about himself and his division.

1) Could you please tell us something about yourself?

I am a space physicist at the Johns Hopkins University.  I study solar wind entry into the magnetosphere, plasma transport in the planetary (Earth, Jupiter, Saturn) magnetospheres, magnetosphere-ionosphere coupling, radiation belts, solar dynamo, and space weather. Recently, I have been applying information theory to space data to establish linear and nonlinear relationships and causalities. 

2) What are the basic research questions of the IAGA division you head?

There are many, perhaps too many to list here.  IAGA Div III research questions include how the Sun transfers mass and energy to the planetary (including Earth’s) magnetospheres and ionospheres?; what are the magnetospheric responses to the solar wind driving, e.g., storms and substorms?; how do electromagnetic waves interact with ions and electrons?; what are the roles of plasma turbulence in magnetospheric dynamics?; where and how does magnetic reconnection occur and what are its impacts in the magnetosphere? etc. This is by far an incomplete list and a rather simplistic description of very complex research questions and problems that Div III tries to solve.  

Flow of plasma energy around Earth's magnetosphere. Solar energy absorbed through the magnetopause circulates in the magnetosphere and becomes energy that generates the radiation belt and auroras. (from JAXA)

3) Who are your main collaborators within IAGA and outside?

In the last five years, I have collaborated with about 20-30 scientists within and outside IAGA.

4) What are the past important results of this division?

This is a difficult question to answer because there are too many. It would be difficult to come up with a complete list. Rather than trying to enumerate them all, I would say that Div III has made a lot of progresses in the list of questions listed above. 

At each IAGA conference, Div III presents a rather unique session called Reporter Review.  This session highlights the progresses in the fields in the last few years. So, the readers who are interested to learn about recent important results in the Div III are encouraged to attend this session. 

( IAGA-IASPEI 2021 conference can be followed here : http://iaga-iaspei-india2021.in )

5) What do you think would be the future applications or impacts through this research?

Space weather can adversely affect our technological infrastructures and lives on Earth and in space. Div III research has huge potential impacts on and applications for space weather.   

IAGA... What's that?

 

You must have seen a number of sci-fi movies and wondered what so many people shown sitting in the cubicles do! Wonder no more, for I’m about to explain the basic topics some of those scientists deal with.

IAGA is the International Association of Geomagnetism and Aeronomy, one of the eight semi-autonomous Associations of the International Union of Geodesy and Geophysics (IUGG), which in turn is one of the forty unions of the International Science Council. IUGG is an international scientific organisation established in 1919 and dedicated to advancing, promoting and communicating knowledge about the Earth and its environment in space. The ‘AIGA’ part attached is the French name abbreviation (because we like things fancy).

It is a non-governmental body that focusses on the study of the magnetic and electrical properties of the Earth and other planets, the Sun and its phenomena, and interplanetary bodies. It is funded through the subscriptions paid to IUGG by its Member Countries. It encourages free exchange of scientific information and facilitates international collaboration. There is a General/Scientific Assembly every two years, with the IAGA-IASPEI Joint Scientific Assembly 2021 happening virtually at http://iaga-iaspei-india2021.in/index.html from the 21st-27th August.

There are six Divisions- each dealing with a different science, with some forming their own Working Groups (WG) in specific topics- and four Interdivisional Commissions on Developing Countries, History, Education and Outreach, and Space Weather. Each Division and Commission is led by a Chair and a Co-Chair, and has an Early Career Liaison Person. 

IAGA also has two Inter-Association Activities and six Union Commissions. It is administered by an Executive Committee (EC) - consisting of a President, Vice-President, Secretary and Treasurer - on behalf of the IUGG Member Countries. 

The different divisions of IAGA:

Division I is primarily concerned with the internal magnetic field of planetary bodies, using measurements from satellites and observatories, and analysis of magnetic field from rocks as well as seismic, gravitational and other data. Theory of conducting fluid cores is studied to better comprehend the core field of planets, and palaeomagnetic data is analysed to understand the geomagnetic field. But in layman terms, if you like rocks, you like Division I.

Division II deals with the understanding of the lower atmosphere of the Earth, other planets and their satellites. This includes the dynamics of the middle atmosphere, the ionospheric waves, fields, its meteorological effects and interactions in polar regions, and the long term trends of the different layers of the atmosphere. It aims at improving the atmosphere-ionosphere understanding. Anyone fascinated by the Aurora Borealis and Australis should definitely check out the research of Division II.

Division III also deals with the atmosphere, but the upper atmosphere and magnetosphere through satellite and ground based data. It answers how the Sun and the solar wind influence the Earth’s and other planet's magnetospheres. It also researches on the magnetosphere-ionosphere coupling, the radiation belts, ring currents, plasmasphere and magnetic storms. If solar storms are something that interests you, this is for you. 

Division IV focusses on all phenomena related to the Solar Wind and the Interplanetary Magnetic Field. This is the study of enormous energy produced and released by the Sun due to motion inside it, the plasma that is emitted and the resulting consequences of these phenomena within the Solar System. Do you like everything about the sun, including its turbulent motions, and all physics related to it? Look no more.

Division V analyses magnetic observation data to identify its sources and produces indices and models from it. It takes care of the geomagnetic observatories all over the world, sets standards for data measurement, processing and exchange. Everyone’s interested about setting up instruments and sending satellites to space, but that’s half the picture. The other half happens after that. This Division takes care of the data that are observed from those and processes it.

Division VI research involves the investigation of theoretical and practical aspects of distribution of electrical properties within the Earth's and planetary interiors. In particular, it works on electrical conductivity and its relationship with other physical parameters to understand geological structures and processes ranging from meter to mantle scale. This Division makes basic classroom physics concepts like electromagnetic induction come alive in gigantic planetary bodies.

If you are interested in the workings and research of any specific Division, look out for the blogs of the coming weeks!

IAGA invites young scientists and scientists throughout the world to participate in the Association’s activities. There are no membership formalities or fees. It serves scientists and decision-makers in research establishments, government and intergovernmental bodies, universities and private enterprises.

For more information on IAGA, please visit http://www.iaga-aiga.org/.

Images: (1) Lunar and Planetary Institute, USRA. (2),(3) NASA. (4) ESA.

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Shivangi Sharan is a second year PhD student at the Laboratory of Planetology and Geodynamics in France. Her research focusses on the study of the magnetic field of Mars and to infer its internal structure from it. She is an active member of the IAGA Blog Team and can be contacted via e-mail here.

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