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Abstract

Dedicated scientific measurements of the strength and direction of the Earth's magnetic field began at Greenwich and Kew observatories in London, UK, in the middle of the 19th century. Using advanced techniques for the time, light-sensitive photographic paper and light-levered reflections from magnetized needles, allowed continuous analogue magnetograms to be recorded. By good fortune, both observatories (which were located around 20 km apart) were in full operation around the so-called Carrington storm in the early September 1859 and the precursor storm in late August, providing as complete a record as possible. Based on digital images of the magnetograms and information from the observatory yearbooks and subsequent scientific papers, we extract and scale the measurements to SI units, allowing us to extract minute cadence values. The magnetogram records have several missing periods, lost as the traces moved off-page due to the magnitude of the storm. We present the most complete digitized magnetic records to date of the ten-day period from 25th August to 5th September 1859 encompassing the Carrington storm and its lesser recognized precursor which may have been just as large. We discuss some of the issues encountered in deciphering the old records and methods for cross checking and validating the output values.