Digitized Continuous Magnetic Recordings for the August/September 1859 Storms From London, UK

• Published in: Space weather 2024-03, Vol.22 (3), p.n/a
• Main Authors: Beggan, C. D., Clarke, E., Lawrence, E., Eaton, E., Williamson, J., Matsumoto, K., Hayakawa, H.
• Format: Article
• Language: English
• Subjects: Carrington; digitization; historical events; magnetic storms; magnetograms
• ISSN: 1542-7390; 1542-7390
• DOI: 10.1029/2023SW003807

Dedicated scientific measurements of the strength and direction of the Earth's magnetic field began at Greenwich and Kew observatories in London, United Kingdom, in the middle of the nineteenth century. Using advanced techniques for the time, collimated light was focussed onto mirrors mounted on free‐swinging magnetized needles which reflected onto photographic paper, allowing continuous analog magnetograms to be recorded. By good fortune, both observatories were in full operation during the so‐called Carrington storm in early September 1859 and its precursor storm in late August 1859. Based on digital images of the magnetograms and information from the observatory yearbooks and scientific papers, it is possible to scale the measurements to International System of Units (SI units) and extract quasi‐minute cadence spot values. However, due to the magnitude of the storms, the periods of the greatest magnetic field variation were lost as the traces moved off‐page. We present the most complete digitized magnetic records to date of the 10‐day period from 25 August to 5 September 1859 encompassing the Carrington storm and its lesser recognized precursor on 28 August. We demonstrate the good correlation between observatories and estimate the instantaneous rate of change of the magnetic field. Plain Language Summary The Carrington storm of September 1859 is one of the largest known geomagnetic storms in the historic record. Two observatories in London were operating at the time and by good fortune both recorded the extreme geomagnetic storm on paper records. These are held at the British Geological Survey and have been made available online as digital images. The next step is to digitally trace over the magnetograms to produce useful digital values. However, scaling the values of digital pixels to International System of Units (SI units) of degrees of angle and nanoTesla is not easy as the original scaling factors are not available. We use a mixture of written reports at the time and notes from the observatory yearbooks to track down plausible scaling factors and to explain the process of digitization of very old records. The data for 10 days covering 25 August to 5 September 1859 are now available for other researchers to use. Key Points Continuous magnetogram traces are available in London, United Kingdom for August and September 1959 Serendipitously, two observatories recorded the Carrington flare and the subsequent storm We digitize the archive paper records t