Abstract
Low Earth orbiting geomagnetic satellite missions, such as the Swarm satellite mission, are the only means to monitor and investigate ionospheric currents on a global scale and to make in situ measurements of F region currents. High-precision geomagnetic satellite missions are also able to detect ionospheric currents during quiet-time geomagnetic conditions that only have few nanotesla amplitudes in the magnetic field. An efficient method to isolate the ionospheric signals from satellite magnetic field measurements has been the use of residuals between the observations and predictions from empirical geomagnetic models for other geomagnetic sources, such as the core and lithospheric field or signals from the quiet-time magnetospheric currents. This study aims at highlighting the importance of high-resolution magnetic field models that are able to predict the lithospheric field and that consider the quiet-time magnetosphere for reliably isolating signatures from ionospheric currents during geomagnetically quiet times. The effects on the detection of ionospheric currents arising from neglecting the lithospheric and magnetospheric sources are discussed on the example of four Swarm orbits during very quiet times. The respective orbits show a broad range of typical scenarios, such as strong and weak ionospheric signal (during day- and nighttime, respectively) superimposed over strong and weak lithospheric signals. If predictions from the lithosphere or magnetosphere are not properly considered, the amplitude of the ionospheric currents, such as the midlatitude Sq currents or the equatorial electrojet (EEJ), is modulated by 10–15 % in the examples shown. An analysis from several orbits above the African sector, where the lithospheric field is significant, showed that the peak value of the signatures of the EEJ is in error by 5 % in average when lithospheric contributions are not considered, which is in the range of uncertainties of present empirical models of the EEJ.
Highlights
The Earth’s magnetic field results from different sources in the Earth’s interior, which are moving liquid iron in the outer core, magnetization of the lithosphere and induced currents in the Earth’s electrically conductive crust, mantle and ocean, and from electric currents in the ionosphere and magnetosphere
High-precision geomagnetic field observations at scientific low Earth orbit (LEO) satellite missions have brought up observational evidence for current systems in the ionospheric F region such as diamagnetic and gravity-driven, and field-aligned currents in the low- and midlatitudes
High-precision geomagnetic field observations at scientific LEO satellite missions have successfully been applied for global studies of E region currents, such as Sq currents, the equatorial electrojet and quiet-time polar electrojet and field-aligned currents
Summary
The Earth’s magnetic field results from different sources in the Earth’s interior, which are moving liquid iron in the outer core, magnetization of the lithosphere and induced currents in the Earth’s electrically conductive crust, mantle and ocean, and from electric currents in the ionosphere and magnetosphere. A very efficient approach to studying these currents is to analyze the difference between observations and predictions of effects from non-ionospheric sources derived from geomagnetic field modeling. A representative empirical model of the core field is the International Geomagnetic Reference Field, IGRF, that provides predictions up to a spherical harmonic degree 13 (Thébault et al 2015a), corresponding to about 3000km spatial resolution on ground and low Earth orbit. High-resolution empirical models exist that have the capability to predict the lithospheric field and the quiet-time magnetospheric field dominated by the magnetospheric ring current. These are the CHAOS, GRIMM and POMME model families. This paper does not aim at discussing the role of high-resolution lithospheric field models for regional and global geological investigations, but it highlights their relevance for reliably monitoring ionospheric currents at satellite altitudes
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