Professor Dr. Mike Lockwood, School of Mathematical and Physical Sciences, University of Reading, UK, gave this year's Birkeland Lecture at the Norwegian Academy of Science and Letters on the 26th of September. The title of his lecture was: "What auroral and geomagnetic observations tell us about long term variations of the sun".
Photo: Stephane Vetter
Aurorae have been reported throughout history and the associated fluctuations in the Earth's magnetic field were first detected in 1701. Both are linked to currents flowing in near-Earth space, including the Birkeland currents, driven by the variable solar wind. Using spacecraft in concert with ground-based radars, auroral imagers and magnetometers, we have gained considerable understanding of these currents and how they vary with the solar wind flow and the weak magnetic field in interplanetary space that is carried with it.
A major challenge has been to interpret century-scale variations in the Sun's behaviour using our modern understanding with four sources of information that are available to us: sunspot observations, geomagnetic activity data, auroral sightings and the abundances of "cosmogenic isotopes".
Naked-eye sunspot observations are rare and depend on specific meteorological conditions such as dust storms. However, the telescope made regular observations possible, starting with the measurements made by Galileo in 1612. The first reliable magnetometer was developed by Gauss in 1835 and a worldwide network was established soon after. Auroral sightings need careful verification but a good dataset is available from northern Europe from about 1780. The fourth source of information is different as it is stored not by human observers but in layered reservoirs such as ice sheets and tree trunks, into which we can drill cores and make measurements that go back in time as we drill deeper. Cosmic rays are accelerated to close to the speed of light in explosive events such as super-novae. On impacting Earth's atmosphere they generate cosmogenic isotopes, such as Berillium-10 or Carbon-14, which are stored in the reservoirs where their abundance is measured using drilled cores. They tell us about the Sun because the interplanetary magnetic field shields Earth from cosmic rays such that the abundances fall when solar activity is high. By removing other influences, we can gain a record of solar variability that extends back over millennia.
Using these four sources and our space-age understanding, a coherent picture of past variations is emerging from which we can make analogue forecasts of how the Sun is likely to behave in the future and what consequences are for modern technology. It may even give us better forecasts of cold European winters. All this is based on the work of the great innovators in the study of space, such as Kristian Birkeland who once wrote: "A very few lonely pioneers make their way to high places never before visited. . . they create the living conditions of mankind and the majority are living on their work".
Mike Lockwood is the Director of Research of the School of Mathematical and Physical Sciences of the University of Reading, UK where he is a Professor of Space Environment Physics. He has been
researching space, aurora and the Sun for over 30 years, having gained his PhD fromExeter University in 1980. He has worked at Auckland University, the Royal Aircraft Establishment, NASA's Marshall Space Flight Centre, Rutherford Appleton Laboratory and Southampton University. He has served on a great many national and international committees, including Chair of the EISCAT Council, President of Solar-Terrestrial sciences of the European Geophysical Society and Chair of Division 3 (Magnetospheres) of the International Association of
Geomagnetism and Aeronomy.
He is the author of over 300 journal publications and one book.