Abstract
Electric currents flowing in the terrestrial ionosphere have conventionally been diagnosed by low-earth-orbit (LEO) satellites equipped with science-grade magnetometers and long booms on magnetically clean satellites. In recent years, there are a variety of endeavors to incorporate platform magnetometers, which are initially designed for navigation purposes, to study ionospheric currents. Because of the suboptimal resolution and significant noise of the platform magnetometers, however, most of the studies were confined to high-latitude auroral regions, where magnetic field deflections from ionospheric currents easily exceed 100 nT. This study aims to demonstrate the possibility of diagnosing weak low-/mid-latitude ionospheric currents based on platform magnetometers. We use navigation magnetometer data from two satellites, CryoSat-2 and the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO), both of which have been intensively calibrated based on housekeeping data and a high-precision geomagnetic field model. Analyses based on 8 years of CryoSat-2 data as well as ~ 1.5 years of GRACE-FO data reproduce well-known climatology of inter-hemispheric field-aligned currents (IHFACs), as reported by previous satellite missions dedicated to precise magnetic observations. Also, our results show that C-shaped structures appearing in noontime IHFAC distributions conform to the shape of the South Atlantic Anomaly. The F-region dynamo currents are only partially identified in the platform magnetometer data, possibly because the currents are weaker than IHFACs in general and depend significantly on altitude and solar activity. Still, this study evidences noontime F-region dynamo currents at the highest altitude (717 km) ever reported. We expect that further data accumulation from continuously operating missions may reveal the dynamo currents more clearly during the next solar maximum.
Highlights
There exist many different systems of electric currents in the terrestrial ionosphere
The purpose of this study is to demonstrate the ability of platform magnetometers, e.g., on CryoSat-2 and Gravity Recovery and Climate Experiment Follow-On (GRACE-FO), after extensive calibration effort during post-processing, to diagnose weaker ionospheric currents flowing at low-/mid-latitude regions, which typically amounts only to a few nA/m2 at LEO: approximately 100 times weaker than typical high-latitude Field-aligned current (FAC) density
Inter-hemispheric field-aligned current (IHFAC): reproducing science‐grade magnetometer observations The platform magnetometer data used in this study can reproduce low-/mid-latitude IHFAC climatology reported previously by science-grade magnetometers
Summary
There exist many different systems of electric currents in the terrestrial ionosphere. Ionospheric current systems have conventionally been diagnosed by combining the Ampere’s induction law and data from science-grade magnetometers onboard LEO satellites: especially suites equipped with long booms (to avoid spacecraft bus noise), vector field magnetometers (to get full 3-dimensional vectors), and absolute scalar magnetometers (for continuous inflight calibration and corrections for sensor axis distortion) Those systems include, since the seminal mission of Magsat in the 1970s (e.g., Maeda et al 1982), Ørsted (e.g., Neubert et al 2001), Satélite de Aplicaciones Científicas-C (SAC-C; Colomb and Varotto 2003), Challenging Minisatellite Payload (CHAMP; Reigber et al 2005), and the European Space Agency (ESA)’s Swarm constellation (Friis-Christensen et al 2006). A review of scientific studies from satellite-based highprecision magnetometers is provided in Olsen and Stolle (2012)
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