Toward more reliable long‐term indices of geomagnetic activity: Correcting a new inhomogeneity problem in early geomagnetic data

• Published in: Journal of geophysical research. Space physics Vol. 120; no. 10; pp. 8288 - 8297
• Main Authors: Holappa, L., Mursula, K.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-10-2015
• Subjects: Astronomical instruments; Evolution; geomagnetic activity; Geomagnetism; Geophysics; Inhomogeneity; long‐term variations; Magnetic fields; magnetic measurements; Observatories; Solar physics; Solar wind; space climate; Strength; Trends; Wind speed; British Isles, Scotland, Shetland, Lerwick
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1002/2015JA021752

For the time before the space era, our knowledge of the centennial evolution of solar wind (SW) and interplanetary magnetic field (IMF) is based on proxies derived from geomagnetic indices. The reliability of these proxies is dependent on the homogeneity of magnetic field data. In this paper, we study the interhourly (IHV) and interdiurnal (IDV1d) variability indices calculated from the data of two British observatories, Eskdalemuir and Lerwick, and compare them to the corresponding indices of the German Niemegk observatory. We find an excess of about 14 ± 4% (5.8 ± 2%) and 27 ± 10% (15 ± 6%) in the IHV (IDV1d) in the indices of Eskdalemuir and Lerwick in 1935–1969. The timing of this excess accurately coincides with instrument changes made in these observatories, strongly supporting the interpretation that the excess is indeed caused by instrument related inhomogeneities in the data of Eskdalemuir and Lerwick. We show that the detected excess notably modifies the long‐term trend of geomagnetic activity and the centennial evolution of IMF strength and solar wind speed estimated using these indices. We note that the detected inhomogeneity problem may not be limited to the data of the two studied observatories but may be quite common to long series of geomagnetic measurements. These results question the reliability of the present measures of the centennial change in solar wind speed and IMF.
Key Points
A new inhomogeneity was found in two long‐running magnetic observatories from 1930s until 1960s
We quantified this inhomogeneity and corrected the related geomagnetic activity indices
This inhomogeneity can modify the long‐term trends of SW speed by 20–30% and IMF strength by 10–15%