Correction of artificial jumps in the historical geomagnetic measurements of Coimbra Observatory, Portugal

Abstract

Abstract. The Coimbra Magnetic Observatory (International Association of Geomagnetism and Aeronomy code COI) inPortugal has a long history of observation of the geomagnetic field, spanning almost 150 yr since the first geomagnetic measurements in 1866. These long instrumental geomagnetic records provide very important information about variability of geomagnetic elements and indices, their trends and cycles, and can be used to improve our knowledge on the sources that drive variations of the geomagnetic field: liquid core dynamics (internal) and solar forcing (external).However, during the long life of the Coimbra Observatory, some inevitable changes in station location, instrument's park and electromagnetic environment have taken place. These changes affected the quality of the data collected at COI causing breaks and jumps in the series of geomagnetic field components and local K index. Clearly, these inhomogeneities, typically shift-like (step-like) or trend-like, have to be corrected or, at least, minimized in order for the data to be used in scientific studies or to be submitted to international databases.In this study, the series of local K index and declination of the geomagnetic field are analysed: the former because it allows direct application of standard homogenization methods and the latter because it is the longest continuous series produced at COI. For the homogenization, visual and statistical tests (e.g. standard normal homogeneity test) have been applied directly to the local geomagnetic K index series (from 1951 to 2012). The homogenization of the monthly averages of declination (from 1867 to 2012) has been done using visual analysis and statistical tests applied to the time series of the first differences of declination values, as an approximation to the first time derivative. This allowed not only estimating the level of inhomogeneity of the studied series but also detecting the highly probable homogeneity break points. These points have been cross-checked with the metadata, and the COI series have been compared with reference series from the nearest geomagnetic stations and, in the case of declination series, from the recent geomagnetic field model COV-OBS to set up the required correction factors. As a result, the homogenized series measured in COI are considered to be essentially free of artificial shifts starting from the second half of the 20th century, and ready to be used by the scientific community.

Homogenization of the historical series from the Coimbra Magnetic Observatory, Portugal

“…In previous work by Morozova et al (2014), the series of monthly values of the declination of the geomagnetic field (D) measured at the Coimbra Magnetic Observatory (COI) between 1866 and 2006 were analyzed for the presence of artificial homogeneity breaks (HBs). A number of HBs related to the changes to the observatory location and instrument changes or repairs were found and, when possible, corrected.…”

“…The analysis is supported by metadata about the instruments used to measure the H , Z and I components between 1866 and 2015. More details about the history of the Coimbra Magnetic Observatory and metadata can be also found in Pais and Miranda (1995) and Morozova et al (2014).…”

“…Those related to the measurements of H , Z and I are described below and summarized in Tables 1 (H ), 2 (I ) and 3 (Z). The information on the D component can be found in Morozova et al (2014) and is also summarized in Table S1 in the Supplement.…”

Early geomagnetic data from the Astronomical Observatory of Madrid (1879–1901)

“…Thus, for example, we today have long-term geomagnetic records of some European cities including London (Malin and Bullard, 1981;Barraclough et al, 2000) and Paris (Alexandrescu et al, 1996), as well as records of the great geomagnetic storms that occurred more than 100 years ago (Tsurutani, 2003;Vaquero et al, 2008;Ribeiro et al, 2011Ribeiro et al, , 2016. In the Iberian Peninsula, several geomagnetic observatories were found in the second half of the 19th century including Madrid, Lisbon, Coimbra, San Fernando, and Casa Balaguer (Cubillo, 1949;González, 1992;Pais and Miranda, 1995;Batlló, 2005;Morozova et al, 2014;Ribeiro et al, 2016). In particular, we can mention the "magnetic crusade" of Sabine and the "magnetic union" of Gauss.…”

“…In particular, we can mention the "magnetic crusade" of Sabine and the "magnetic union" of Gauss. In the Iberian Peninsula, several geomagnetic observatories were found in the second half of the 19th century including Madrid, Lisbon, Coimbra, San Fernando, and Casa Balaguer (Cubillo, 1949;González, 1992;Pais and Miranda, 1995;Batlló, 2005;Morozova et al, 2014;Ribeiro et al, 2016). Note that the unique observatory in Spain with continuous geomagnetic record is San Fernando, in the South coast.…”