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.

中文翻译：

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使用平台磁力计诊断低/中纬度电离层电流：CryoSat-2 和 GRACE-FO

在地面电离层中流动的电流通常由配备科学级磁力计和磁清洁卫星上的长臂的低地球轨道 (LEO) 卫星进行诊断。近年来，有多种努力将平台磁力计纳入研究电离层电流，平台磁力计最初是为导航目的而设计的。然而，由于平台磁力计的次优分辨率和显着噪声，大多数研究仅限于高纬度极光区域，在那里电离层电流的磁场偏转很容易超过 100 nT。本研究旨在证明基于平台磁力计诊断低/中纬度微弱电离层电流的可能性。我们使用来自两颗卫星的导航磁力计数据，CryoSat-2 和重力恢复和气候实验后续 (GRACE-FO)，两者都已根据内务数据和高精度地磁场模型进行了密集校准。基于 8 年的 CryoSat-2 数据以及 ~ 1.5 年的 GRACE-FO 数据的分析再现了众所周知的半球间场对齐流 (IHFAC) 气候学，正如之前致力于精确磁观测的卫星任务所报告的那样。此外，我们的结果表明，出现在中午 IHFAC 分布中的 C 形结构符合南大西洋异常的形状。F 区发电机电流在平台磁力计数据中仅部分识别，可能是因为电流通常比 IHFAC 弱，并且在很大程度上取决于高度和太阳活动。仍然，这项研究证明了有史以来最高海拔（717 公里）的正午 F 区发电机电流。我们预计，持续运行任务的进一步数据积累可能会更清楚地揭示下一个太阳活动极大期期间的发电机电流。