Diagnosing low-/mid-latitude ionospheric currents using platform magnetometers: CryoSat-2 and GRACE-FO

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.

Observing Earth’s magnetic environment with the GRACE-FO mission

“…The flip of sign towards positive towards north in B E with a few nanotesla amplitude during noon is also consistent with the earlier CHAMP results and reflects inter-hemispheric field-aligned currents. These signatures relate to a statistical analysis for inter-hemispheric field-aligned currents and F-region dynamo currents conducted by Park et al (2020) based on GRACE-FO data. GF2 observations are less clear for these low-latitude ionospheric signatures, a fact which is also supported by Park et al (2020).…”

“…These signatures relate to a statistical analysis for inter-hemispheric field-aligned currents and F-region dynamo currents conducted by Park et al (2020) based on GRACE-FO data. GF2 observations are less clear for these low-latitude ionospheric signatures, a fact which is also supported by Park et al (2020). On the night side, the GF1 residuals are very low which is expected due to the absence of strong ionospheric currents.…”

“…These plots show that also small amplitudes near the noise level of GRACE-FO data are well accessible when they are applied in a statistical approach. This capability was also demonstrated by Park et al (2020) who used CryoSat-2 and GRACE-FO data and successfully characterised interhemispheric field-aligned currents which have statistical amplitudes of as low as few nA/m 2 .…”

“…CASSIOPE e-POP data were used to derive fine structures of auroral arcs (Miles et al 2018), and observations from the low-inclination satellite C/NOFS added in describing the local time dependence of the geomagnetic signal due to the magnetospheric ring currents during geomagnetic active times (Le et al 2011). However, recently, Park et al (2020) demonstrated that statistical analyses of multiple years of field-aligned current data derived from magnetometer observations of the CryoSat-2 and GRACE-FO missions successfully revealed mid-latitude inter-hemispheric currents. This finding is especially interesting, because it proves the capability of data from non-dedicated magnetometers to be sensitive to small amplitude currents, as well.…”

Special issue “Characterization of the geomagnetic field and its dynamic environment using data from space-based magnetometers”

“…Furthermore, during conjunction events, the DMSP field-aligned currents were proven to be similar in amplitude with Swarm-derived results further supporting their value for space science. Park et al (2020) demonstrate that calibrated platform magnetic data can also be used in statistical investigations of low amplitude ionospheric currents at non-polar latitudes. From vertical currents detected during 8 years at CryoSat-2 and during nearly 2 years at GRACE-FO the authors successfully derive global pattern of interhemispheric FACs, the distribution of which is dominated by seasonal variations and tidal forcing from the lower atmosphere.…”