The magnetic compass was invented in China sometime around 100 AD and was first described in European texts by Guyot de Provins and Alexander Neckam in 1180. Using it, inclination (the angle between the geomagnetic field vector and the horizontal) was discovered in 1576 by Robert Norman. In 1600, William Gilbert (later personal doctor to Elizabeth I of England) published his book “De Magnete” (the full title translates from the latin as “On Loadstone, Magnetic Bodies, and on the Great Magnet of the Earth”), in which he noted that the variation of inclination with latitude was the same as that around a sphere of loadstone and deduced for the first time that Earth had a magnetic field. From observations of the field declination (the angle between the local geographic and geomagnetic northward directions) in London, Henry Gellibrand concluded in 1634 that the geomagnetic field changes over time (i.e., he had discovered the secular variation) and in 1701 Edmund Halley published the first geomagnetic field map, showing the declination throughout the Atlantic ocean. By 1722, the London watchmaker George Graham had developed a compass that was sufficiently sensitive for him to observe the fluctuations that we now call “geomagnetic activity” (Graham, 1724a,b). The diurnal variation in that activity was first noted in 1740 by Olof Hiorter, Anders Celsius’ student (and brother-in-law), in Uppsala. On the night of 1 March, 1741 Hiorter also observed large magnetic variations that were connected to local auroral displays and, on the same night, magnetic variations were also recorded in London by Graham, making these the first multi-point observations of geomagnetic activity.
“Who could have thought that the northern lights would have a connection and a sympathy
with a magnet?”
– Olof Peter Hiorter, Swedish Scientist (1696 – 1750)
The relative intensity of the geomagnetic field at a number of locations was measured in 1798 by Alexander von Humboldt, who was the first to describe the disturbances he observed as “geomagnetic storms”. His work attracted the interest of his house guest, Carl Friedrich Gauss, when visiting Berlin to attend a conference in 1828. Gauss subsequently developed the first magnetometer that could reliably measure the field strength and/or its horizontal component, establishing the first magnetic observatory in Göttingen in 1832. He also established the mathematics of how to separate the internal and external components of Earth’s magnetic field.
The year after the establishment of the Göttingen observatory, Gauss and Wilhelm Weber founded the “Magnetischer Verein” (Magnetic Union) which from 1834 to 1841 initiated the growth of a network of observatories throughout Europe making measurements at 5-minute intervals. Magnetometers were soon established at sites such as Berlin (1836), Dublin (1838), Greenwich (1838), Prague (1839), and Munich (1841). Gauss and Weber began to organize a global magnetic survey, an idea strongly supported by the scientific adviser to the British Admiralty, Edward Sabine. As a result, the British Navy set up more stations in Toronto, St. Helena, Cape of Good Hope, and Tasmania and the British East India Company established four more in India and Singapore. Russia established ten stations in its own territory (which at the time included Helsinki) and one in Beijing, and by 1841 a world-wide network of 53 stations was operating. Using data from these observatories, Sabine was the first to realize that geomagnetic activity could be divided into a regular diurnal cycle and irregular variations which correlated very closely with sunspot number (Sabine, 1851, 1852), the decadal-scale cycle of which had been discovered in 1844 by the German astronomer Heinrich Schwabe. The subsequent development of solar-terrestrial science using the geomagnetic activity observations has been detailed in three excellent reviews by Cliver (1994a,b, 1995).
The number of available magnetic observatories subsequently grew gradually over the next century, helped by international campaigns such as the Polar Year (1882 – 1883) and the Second International Polar Year (1932 – 1933), so that by 1955 about 100 stations worldwide were supplying regular routine observations. This number rose rapidly because of the International Geophysical Year, IGY (1957 – 1958), reaching of order 170 by 1960 (Jankowski and Sucksdorff, 1996). Figure 1 shows the global distribution of stations known to be operating in 1996.
Modern understanding of geomagnetic activity relies heavily on in-situ spacecraft observations of the solar wind, shortly before it impacts on the Earth. Such measurements were first made routinely in 1963 but the monitoring was not close to continuous until about 1966. After a few years the length and number of data gaps in this vital space science resource began to increase (Finch and Lockwood, 2007), driven by factors such as telemetry limitations and a shortage of available tracking stations. In this respect, 1995 is a significant date in that (almost completely) continuous solar wind monitoring began with the WIND spacecraft and has continued with the ACE spacecraft to the present day. Covering almost two solar cycles, these continuous data constitute the most valuable resource we have for understanding how solar wind properties, including its bulk flow speed, , and the interplanetary magnetic field (IMF) embedded within it, , drive geomagnetic activity. The near-Earth interplanetary data have been collected by NASA’s Goddard Space Flight Centre (the Space Physics Data Facility) into the OMNI and OMNI2 datasets (Couzens and King, 1986; King and Papitashvili, 2005).