Abstract
Characteristics of long-term variation in the amplitude of solar quiet (Sq) geomagnetic field daily variation have been investigated using 1-h geomagnetic field data obtained from 69 geomagnetic observation stations within the period of 1947 to 2013. The Sq amplitude observed at these geomagnetic stations showed a clear dependence on the 10- to 12-year solar activity cycle and tended to be enhanced during each solar maximum phase. The Sq amplitude was the smallest around the minimum of solar cycle 23/24 in 2008 to 2009. The relationship between the solar F10.7 index and Sq amplitude was approximately linear but about 53% of geomagnetic stations showed a weak nonlinear relation to the solar F10.7 index. In order to remove the effect of solar activity seen in the long-term variation of the Sq amplitude, we calculated a linear or second-order fitting curve between the solar F10.7 index and Sq amplitude during 1947 to 2013 and examined the residual Sq amplitude, which is defined as the deviation from the fitting curve. As a result, the majority of trends in the residual Sq amplitude that passed through a trend test showed negative values over a wide region. This tendency was relatively strong in Europe, India, the eastern part of Canada, and New Zealand. The relationship between the magnetic field intensity at 100-km altitude and residual Sq amplitude showed an anti-correlation for about 71% of the geomagnetic stations. Furthermore, the residual Sq amplitude at the equatorial station (Addis Ababa) was anti-correlated with the absolute value of the magnetic field inclination. This implies movement of the equatorial electrojet due to the secular variation of the ambient magnetic field.
Highlights
The geomagnetic field on the ground shows regular variation with a fundamental period of 24 h during a solar quiet day
In order to confirm whether the changes in both the magnetic field intensity and inclination are strongly related to the long-term variation in the residual solar quiet (Sq) amplitude or not, we investigated the relationship using the long-term magnetic field data calculated with the International Geomagnetic Reference Field (IGRF)-11 model [Finlay et al 2010]
The Sq amplitude observed at these geomagnetic stations showed a clear dependence on the 10- to 12-year solar activity, and it tended to increase during each solar maximum phase
Summary
The geomagnetic field on the ground shows regular variation with a fundamental period of 24 h during a solar quiet day. Using long-term observation data of the geomagnetic field obtained at three low- and middlelatitude stations (Apia, Fredericksburg, and Hermanus), Elias et al (2010) showed that significant long-term trends exist in the Sq amplitude after the removal of seasonal and solar cycle effects and that the Sq amplitude tends to increase over time. They explained more than 50% of their trend values with the secular variation in the ambient magnetic field and less than 10% with the increase in greenhouse gases. Because Elias et al (2010) did not show the relationship between the Sq amplitude and solar activity index (sunspot numbers), it is unknown whether the assumption of the linear relationship used for the deviation of the residual Sq amplitude is correct or not
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