Abstract
The Earth’s ionosphere presents long-term trends that have been of interest since a pioneering study in 1989 suggesting that greenhouse gases increasing due to anthropogenic activity will produce not only a troposphere global warming, but a cooling in the upper atmosphere as well. Since then, long-term changes in the upper atmosphere, and particularly in the ionosphere, have become a significant topic in global change studies with many results already published. There are also other ionospheric long-term change forcings of natural origin, such as the Earth’s magnetic field secular variation with very special characteristics at equatorial and low latitudes. The ionosphere, as a part of the space weather environment, plays a crucial role to the point that it could certainly be said that space weather cannot be understood without reference to it. In this work, theoretical and experimental results on equatorial and low-latitude ionospheric trends linked to the geomagnetic field secular variation are reviewed and analyzed. Controversies and gaps in existing knowledge are identified together with important areas for future study. These trends, although weak when compared to other ionospheric variations, are steady and may become significant in the future and important even now for long-term space weather forecasts.
Highlights
Introduction published maps and institutional affilThe Earth’s ionosphere at low latitudes depends strongly on solar radiation, the geomagnetic field, and atmospheric conditions [1], all of which present variabilities in different time scales
Due to the geomagnetic field being horizontal at the dip equator and to the existence of an east–west electric field, another feature of the equatorial region is an enhancement of the Cowling conductivity, which results in the enhanced eastward current that is the EEJ flowing along the dip equator at a height of ~100 km [20,21,22]
Soares et al [66], noting that the magnetic equator in the Brazilian region has moved over 1100 km northward since 1957, passing the geomagnetic observatory Tatuoca (1.2◦ S, 311.5◦ E) in northern Brazil, analyzed ∆H from 1957 until 2019, detecting the trend expected from the main field secular variation and corroborated by the Ionosphere Electrodynamic General Circulation Model (TIE-GCM) simulations
Summary
We present below a brief description of some equatorial and low-latitude ionosphere distinctive characteristics, followed by the Earth’s magnetic field’s main configuration in present days and long-term variability. Their connection is analyzed through a simple theoretical analysis using first approximations, in order to convince the reader that the equatorial and low latitude ionosphere is very sensitive to Earth’s magnetic field variations and to anticipate in an intuitive manner the results which are later reviewed based on experimental data and complex modeling analysis
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