Abstract
Abstract Long-term changes in the magnetic environment of the Earth are of interest to those studying space weather and climate change, particularly in the upper atmosphere. In this paper we examine long-term changes in daily variation as derived from hourly mean values from 14 geomagnetic observatories around the world. Their time series date back to the beginning of the 20th century. We find that there are similar features in all the records, with peaks in the amplitudes of the daily variation occurring in the 1950s and 1980s, and a small upward trend of 1.3 nT/century corresponding to an increase of over 10%. The extrema coincide with those seen in solar irradiance proxy data, in particular the F10.7 flux density dataset which starts in 1947.
Highlights
The geomagnetic field has a regular variation with a fundamental period of 24 hours
In this paper we look for trends in magnetic data at periods longer than the solar cycle, but this time concentrating on the regular variation which is related to, though not exclusively, the electromagnetic radiation from the Sun
Le Mouël et al (2005) allude to geomagnetic jerks being a possible cause for changes in the geometry of the Earth’s magnetosphere but as these jerks are impulse-like changes in the magnetic field and for the 20th century are known to occur at 1925, 1969, 1978, 1991 and 1999 (Mandea et al, 2000), it does not seem likely that they can be connected to the extrema observed in the daily variations in the 1950s and 1980s
Summary
The geomagnetic field has a regular variation with a fundamental period of 24 hours. This regular variation is dependent on local time, latitude, season and solar cycle. The magnetic effect of these current systems is observed on the ground at observatories at mid-latitudes as solar quiet-day variation, or Sq. The variation of Sq range with sunspot number has been known for some time (eg Olsen, 1993 and references therein) but what is less well known is any variation at periods longer than the 11-year solar cycle.
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