Abstract
Equinoctial asymmetry of the range of the solar quiet day variation (Sq) of the horizontal geomagnetic field (H) has been found in some low latitude geomagnetic observatories. This study conducted an investigation of its latitude distribution and the relationship with the solar cycle by using the H field measurements from six observatories along the 120° E meridian chain in the years 1957–2013. Results illustrate a significant equinoctial asymmetry of the SqH range at all observatories. Three main features were identified. First, the signature of the equinoctial asymmetry of the SqH range is opposite for observatories located at the northern and southern sides of the Northern Hemisphere Sq current focus. It shows larger values around spring than autumn equinox at southern observatories, and the converse is seen at northern observatories. Second, the asymmetry increases with the distance from the Sq current focus, suggesting the stronger sensitivity of the distant observatories than observatories around the focus. The result of linear fitting presents a positive dependence of the asymmetry coefficient on geographic latitude, with a reversal of the asymmetry occurring at 28.1° N near the focus of the average Sq current. Third, there is no obvious dependence of the equinoctial asymmetry of the SqH range on solar activity, suggesting a possible cause from some regional factors related to the ionospheric dynamo process.
Highlights
During geomagnetic quiet days, a regular diurnal variation—with an amplitude of a few tens of nanotesla—of the Earth’s geomagnetic field, known as the solar quiet day variation (Sq), is observed at middle and low latitudes
We found that the sign of the equinoctial asymmetry is generally stable at a given observatory—positive north of the Sq current focus and negative at southern observatories
Measurements of the H component of the geomagnetic field acquired at six magnetometer observatories along the 120◦ E meridian chain in the years 1957–2013 were used to investigate the Sq range equinoctial asymmetry
Summary
A regular diurnal variation—with an amplitude of a few tens of nanotesla (nT)—of the Earth’s geomagnetic field, known as the solar quiet day variation (Sq), is observed at middle and low latitudes. The purpose of this study was to investigate the latitudinal distribution of the equinoctial asymmetry of the range of SqH by using hourly observations of the horizontal component of the geomagnetic field, acquired at six geomagnetic observatories located along the 120◦ E meridian chain. Their proximity locations allow us to examine more detailed characteristics of the equinoctial asymmetry of the SqH at the Sq current region of the Northern Hemisphere, located in East Asia, than previous studies. We investigated the relationship between the SqH and the solar activity using data from the years 1957–2013
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