The International Association of Geomagnetism and Aeronomy (IAGA, Association Internationale de Géomagnétisme et d'Aéronomie – AIGA) is one of the eight associations of the International Union of Geodesy and Geophysics (IUGG). It is a non-governmental body funded through the subscriptions paid to IUGG by its Member Countries. IAGA is concerned with the understanding and knowledge that result from studies of the magnetic and electrical properties of the Earth's core, mantle and crust, the middle and upper atmosphere, the ionosphere and the magnetosphere, the Sun, the solar wind, the planets and interplanetary bodies. IAGA has a long history and can trace its origin to the Commission for Terrestrial Magnetism and Atmospheric Electricity, part of the International Meteorological Organisation (IMO), which was established in 1873. Following World War I, the International Research Council was established, and at meetings in London and Paris in 1918, the IUGG (Union Geodesique et Geophysique Internationale in French) was formed, with “Terrestrial Magnetism and Electricity (Magnétisme et Electricité Terrestres)” as section D, with leadership given to IMO. At the First IUGG General Assembly (Rome in 1922), the Section of Terrestrial Magnetism and Electricity became one of the constituent sections of the union. During the IVth IUGG General Assembly (Stockholm in 1930), the sections became associations, one of them being the International Association of Terrestrial Magnetism and Electricity (IATME). In 1951, the upper atmosphere scientists expressed their interest in being recognized in IATME. It was Sydney Chapman who suggested using “geomagnetism” instead of “terrestrial magnetism” and who created the term “aeronomy”, explained as “the science of the upper atmospheric regions where dissociation and ionization are important”. In 1954, again in Rome, during the 20th IUGG General Assembly, the newly created association adopted its present name, the International Association of Geomagnetism and Aeronomy. For more details on the history of IAGA, the reader is referred to a comprehensive review by Fukushima (1995).

Since 2015, IAGA has been organized into six divisions and four inter-divisional commissions, each led by a chair and a co-chair. Each division may form working groups in given specialized topics, and elects officers to run the business of the working groups. The working groups are the elementary cells of the association and at that level the main scientific activities of IAGA are designed. Division I deals with the theory of planetary magnetic fields, paleomagnetism, and rock and environmental magnetism. Division II aims at improving the understanding of the dynamics, chemistry, energetics and electrodynamics of the atmosphere–ionosphere system as well as the coupling processes. Division III is focused on understanding how energy input from the Sun and solar wind influence and drive Earth's magnetosphere and upper atmosphere. Division IV represents research fields related to solar wind, the heliosphere, and solar magnetism. Division V deals with quality standards in geomagnetic data acquisition, observatory and survey procedures, geomagnetic indices, data dissemination, and analyses of magnetic data for the purpose of understanding the various sources of the magnetic field. Finally, activities of Division VI involve the investigation of all theoretical and practical aspects of the spatial distribution of electrical properties within the Earth's and planetary interiors. Four inter-division commissions (on developing countries, history, education and outreach, and space weather) complete the internal IAGA structure. Moreover, IAGA is involved in several union and inter-association activities. The current activities of IAGA are due to efforts made during decades and even centuries by researchers involved in the Earth's magnetism, aeronomy and solar magnetic field. The breadth and complexity of research carried out within IAGA was reflected in five volumes of the IAGA Special Sopron Book Series published by Springer.

Besides the scientific activities, IAGA also plays a major role in the exchange and dissemination of scientific information between the various scientific communities in developing countries. The Interdivisional Commission on Developing Countries aims to increase the participation of developing countries in IAGA activities. Notable are the IAGA efforts for early career scientists. Since 2013, IAGA has organized the “IAGA School” (Fig. 1) during the week before the IAGA Scientific Assemblies and the IUGG General Assemblies with the aim of providing excellent early career scientists with a good basic understanding of a wide range of the scientific topics covered by IAGA. The IAGA-sponsored participants include the recipients of the IAGA Young Scientist Awards, and a number of PhD students or young post-docs, who are nominated by the IAGA Divisions and Working Groups. The Interdivisional Commission on Education and Outreach is deeply involved in these activities.

The current structure of IAGA is presented at http://www.iaga-aiga.org/ (last access: 18 January 2019). On the same website, information on how IAGA is administered by an Executive Committee on behalf of IUGG Member Countries, in accordance with the Association's Statutes and By-Laws, can be found. The website provides a wide range of information about the association's activities, from the meetings, products and services to awards and honors. The full list of IAGA resolutions is also available, as well as all IAGA Newsletters.

The IAGA Presidents and Secretaries General have been pioneers and well-known scientists in the modern world of geomagnetism and aeronomy, e.g. Aikitsu Tanakadate (Fig. 2). The full list of those who served IAGA can be found on the IAGA website. However, we have to mention here two other great names: Sydney Chapman, who acted as President of the International Association of Meteorology and Atmospheric Sciences (IAMAS) from 1936 to 1948, President of IAGA from 1948 to 1951, President of IUGG from 1951 to 1954, and President of the Special Committee for the International Geophysical Year, and Valery Troitskaya from the Soviet Union, who was the first woman president of any IUGG association (IAGA President between 1971 and 1975).

In order to acknowledge significant scientific achievements as well as service to the community, IAGA presents several medals and awards. The most prestigious is the Shen Kuo Award, introduced in 2006 (IAGA News 43). Outstanding long-term service to the IAGA community in technical or managerial positions is acknowledged by the Long Service Award (introduced in 1988, IAGA News 27). The Young Scientist Award (introduced in 2005, IAGA News 43) is addressed to young scientists for outstanding contributions at meetings and workshops for which IAGA is a major sponsor. Finally, since 1980 (IAGA News 19), a person who has given outstanding service to IAGA may be elected an Honorary Member of IAGA.

Information on the IAGA activities has been published regularly since 1966 through the annual IAGA News (Fig. 3; all issues of the IAGA News are available on the IAGA website). The following statement can be found in the first issue of the IAGA News: “One should express a wish that all the investigators carrying on this work publish the information quickly. This would give an opportunity to coordinate the work, made by different investigators in different countries in a better way.” The role of IAGA is clearly noted.

During the last century IAGA provided official (via resolutions) and organizational support to several high-profile scientific programmes, including the Second International Polar Year (IPY) in 1932–1933, the International Geophysical Year (IGY) in 1957–1958, the International Years of the Quiet Sun (IQSY) in 1964–1965, and more recently, two international efforts in 2007–2008 to commemorate the 50th anniversary of the IGY: International Heliophysical Year (IHY) and Electronic Geophysical Year (eGY).

In the following, the scientific landscape of the association is reviewed from its foundation until the present. The review is complicated by the diverse studies carried out within IAGA (from geodynamo up to the Sun and outer space), as well as a huge number of facts and achievements, either accomplished with IAGA involvement, or representing the most important benchmarks of geomagnetism studies. Therefore, in this paper, only a flavor of the IAGA heritage is presented.

The history of IAGA is linked to the history of the Earth's magnetic field, the origin of it being the most challenging scientific question for many centuries. In fact, Albert Einstein once ranked the source of the Earth's magnetism among the most important unsolved problems in classical physics.

The magnetic compass was used already in the 4th century BC in China. However, the magnetic declination, denoting the difference between the magnetic and geographic north, was first recognized by Shen Kuo in 1088 (to recognize this notable person, IAGA named its highest award after him). In Europe, one of the first key names in geomagnetism remains Pierre de Maricourt, known as Petrus Peregrinus (“the pilgrim”). In his “Epistola de Magnete”, written in 1269 and translated into English during the 20th century (Arnold, 1904), de Maricourt describes the pivoted compass he carefully devised and the concept of magnetic poles.

At the beginning of the 17th century a debate on local versus global departures of the field from that of an axial dipole pitted William Gilbert (with his “De Magnete”, published in 1600) against Guillaume le Nautonier (with his “Mecometrie de l'eymant”, published in 1601), as shown in Mandea and Mayaud (2004). Gilbert's work resolved long-lasting discussion and experiments concerning magnetism and measurements by the magnetic needle, and magnetism became the first property to be attributed to the body of the Earth as a whole. During this century, the number of measurements of magnetic directional elements increased. The Royal Society in London and the Académie des Sciences in Paris supported the building of astronomical observatories and also fostered magnetic research.

At the end of the 17th and the beginning of the 18th centuries, significant progress was made in the fields of electricity and magnetism. One of the major discoveries to be recalled, relevant to some areas of geomagnetism, was associated with the activity of Edmund Halley, the leader of the first global magnetic survey (1698–1700) on the Paramore (e.g. Thrower, 1981). The classical charts of lines of equal values of declination obtained during the survey (in 1701 for the Atlantic and in 1702 for the world) are still relevant. The first half of the next century paved the way from laboratory instruments to magnetic observatories. Von Humboldt, Gauss and Weber were deeply involved in running observatories, which formed the Göttingen Magnetic Union (Magnetischer Verein). As early as in 1836, Gauss advocated measuring the full magnetic vector and not only directional values. In the same epoch, the simultaneity of magnetic disturbances over large areas was confirmed, and Gauss developed his general theory of geomagnetism and showed that almost all of the magnetic field observed at the Earth's surface originated inside the Earth (Gauss, 1839).

The number of highlights to be traced back to the 18th and 19th centuries is large, and it is far beyond the scope of this work to name all of them. The interested reader is referred to Courtillot and le Mouel (2007), Gubbins and Herrero-Bervera (2007), and Cliver and Petrovsky (2019). In the following, we focus on a few of the achievements of the scientists from the IAGA community, in order to highlight the importance of the magnetic field observation and modelling, as well as processes linked to the origin of the Earth's magnetic field and solar–terrestrial relationship.

The term aeronomy was introduced by Chapman (1946) in a letter to Nature, suggesting that aeronomy should replace meteorology as the “meteor is now irrelevant and misleading”. This proposal was apparently not received with much support. Thus, in his short note Chapman (1953) wrote that “If, despite its obvious convenience of brevity in itself and its derivatives, it does not commend itself to aeronomers, I think there is a case for modifying my proposal so that instead of the word being used to signify the study of the atmosphere in general, it should be adopted with the restricted sense of the science of the upper atmosphere, for which there is no convenient short word.” In 1960, Champan noted that “Aeronomy is the science of the upper region of the atmosphere, where dissociation and ionization are important”. Today it includes the science of the corresponding regions of the atmospheres and ionospheres of the Earth and other planets.

Research in aeronomy requires, in addition to ground-based observations, access to observations obtained from rockets or satellites. IAGA has largely contributed in collaborations with other associations and in coordinating some of the worldwide activities in aeronomy during the international years starting with IGY. In those early days, IAGA Commission VIII was concerned with the “Upper atmospheric structure dealing with electrodynamics, involving aeronomic processes on neutral and ionized particles”. Several continuous efforts were aimed at encouraging research in new areas. For instance, in a joint resolution issued in 1963, IAGA-IAMAP recommended global studies of lunar-induced geophysical variations, including those arising from lunar atmospheric tides, which had not been studied as much as solar tides. During the IQSY (1964–1965). Four of the 10 IQSY reporters for various disciplines (drawn from different unions, associations, and committees of ICSU) were from IAGA. In addition to the reporter for geomagnetism, the other three IQSY reporters from IAGA were for aurora, airglow, and aeronomy (Beynon, 1964). The URSI/IAGA Joint Working Group on “Structure and Dynamics of the Thermosphere, Ionosphere, and Exosphere” was established in 1974. At its 6th Scientific Assembly in 1989, IAGA decided to designate the period from September 1991 to March 1993 as the International Equatorial Electrojet Year (IEEY). An IAGA/URSI joint working group on “VLF/ELF Remote Sensing of the ionosphere and magnetosphere” (VERSIM) was set up in 1975 (originally with a different name), and this group continues to be very active, holding a biennial international workshop. In recent years, IAGA has collaborated with the International Commission on Middle Atmosphere (ICMA) of IAMAS to hold a series of workshops on “Long term changes and trends in the Atmosphere”, a topic of current interest in view of the anthropogenic contribution to changes in Earth's atmosphere.

IAGA scientific assemblies have provided a platform for reporting the progress achieved in recent years in the fields of mesosphere–lower thermosphere dynamics and chemistry; vertical coupling by upward-propagating waves; ionospheric electrodynamics and structuring; thermosphere–ionosphere coupling, dynamics, and trends; and ionosphere–thermosphere disturbances and modelling (Abdu et al., 2011). All these advanced studies dramatically contributed to the understanding of the variability of Earth's ionosphere, an important component of space weather, and represent the core of activities of Division II.

The Earth's magnetosphere is the region of space surrounding our planet where its own magnetic field, rather than of the solar wind, is dominant. Although the idea of the Earth's magnetosphere can be found already in the work of Gilbert (1600), the term “magnetosphere” was first proposed by Gold (1959). The magnetosphere is controlled by the interaction of the solar wind with the Earth's magnetic field, with short-term disturbances known as geomagnetic storms. These have significant technological and societal impacts on the ground as well as on orbiting objects.

The great solar storm of September 1859 (Fig. 5), generally referred to as the Carrington Event (Carrington, 1860; Hodgson, 1860; Stewart, 1861), was related to large sunspot groups during solar cycle 10. In retrospect, we know that this solar flare (the first ever reported) was associated with a major coronal mass ejection (CME) that travelled directly to Earth in 17.6 h, to cause the great storm (Bartels, 1937). Aurorae were seen around the world, those in the Northern Hemisphere as far south as the Caribbean; those over the Rocky Mountains were so bright that their glow awoke gold miners, who began preparing breakfast because they thought it was morning. People who happened to be awake in the northeastern US could read a newspaper by the aurora's light. Telegraph systems all over Europe and North America failed, in some cases giving telegraph operators electric shocks. Telegraph pylons threw sparks. Some telegraph systems continued to send and receive messages despite having been disconnected from their power supplies.

Studies have shown that a solar storm of this magnitude occurring today would likely cause widespread problems for modern civilization, mainly because of its impact on the power grid through geomagnetically induced currents or on the performance of satellite-based communication and navigation systems such as GNSS. There is an estimated 12 % chance of a similar event occurring between 2012 and 2022 (Riley, 2012). For example, it is assumed that the major solar eruptive event of July 2012 was at least as strong as that in September 1859 and it was fortunate it did not occur a week earlier when the CME would have been directed to Earth (Baker et al., 2013, 2014). Divisions II, III and IV are deeply involved in space weather research. Moreover, IAGA established an Interdivisional Commission on Space Weather, which encourages research into space weather, geomagnetism and aeronomy, and on space weather impacts on society.

IAGA also provides several products, tools, and standards, which are developed by its Divisions and Working Groups. These efforts of all Divisions and Interdivisional Commissions are remarkable, and here we summarize a few of them.

IAGA is and will remain a strong association in its field of activity. During the upcoming years, IAGA will play a major role in our research to understand our magnetic planet and other planetary bodies. From continuous observations to sophisticated models, the scientists of IAGA will bring new inputs in a better knowledge of how the geodynamo works, of the magnetic lithosphere, and the Earth–Sun environment and interactions. The magnetism of our solar system will continue to be observed and interpreted: the planetary magnetism will bring, undoubtedly, more surprises.

Despite all the technological progress, it is obvious that ground-based observations remain crucial for reliable recording of the geomagnetic field and its variations. Although we do have comparatively good coverage by land observatories, oceans still represent a large blank spot on the observatory map. Therefore, IAGA strongly encourages deployment of seafloor magnetic observatories, which would reduce the gap in the Earth's observation coverage and would lead to significant improvements of the geomagnetic field models and yield associated technical and societal benefits. Seafloor geomagnetic observatory programmes already exist and their installation and operation represent one of the major challenges in geomagnetism. On the other side of future IAGA activities, we can note that new satellites and planet rovers will represent major steps towards a better understanding of planetary fields and space-related events. The ESA Swarm trio of satellites, complemented by Canada's Cassiope satellite, all launched in 2013, have already brought new data needed for our understanding of how the Earth's magnetic field is generated and how it protects us from the intervention of harmful charged particles from outer space. New ideas for a constellation of NanoMagSat are under development (Hulot et al., 2018). Progress in geodynamo, paleomagnetism or magnetotellurics will be mostly determined by experimental, technological and computational facilities. These will also shape the future progress of new discipline – data assimilation, which combines the observations, models, and governing physical laws in order to identify the initial conditions and/or to obtain reliable forecasts of the system evolution. It is very likely that a number of minor steps in these fields will result in achieving more reliable and more complex knowledge of the processes in the interior and on the surface of the Earth and other planetary bodies, including their history.