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Power Density Dissipated by Field‐Aligned Currents in the Topside Ionosphere Derived From Swarm‐A Observations.

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Title: Power Density Dissipated by Field‐Aligned Currents in the Topside Ionosphere Derived From Swarm‐A Observations.
Authors: Giannattasio, F.1 (AUTHOR) fabio.giannattasio@ingv.it, Mestici, S.2 (AUTHOR), Pignalberi, A.1 (AUTHOR), De Michelis, P.1 (AUTHOR), Tozzi, R.1 (AUTHOR), Alberti, T.1 (AUTHOR), Pezzopane, M.1 (AUTHOR), Coco, I.1 (AUTHOR), Consolini, G.3 (AUTHOR)
Source: Journal of Geophysical Research. Space Physics. Nov2025, Vol. 130 Issue 11, p1-29. 29p.
Subject Terms: *Ionosphere, *Geomagnetism, *Solar activity, Power density, Magnetic coupling, Artificial satellites, Magnetic fields, Energy dissipation
Abstract: Magnetosphere‐ionosphere (MI) coupling is mainly achieved by field‐aligned currents (FACs) that inject energy and momentum into the high‐latitude ionosphere. A fraction of this energy is locally dissipated and converted into kinetic energy of the ionospheric plasma. Dissipation of FACs in the topside ionosphere has not yet been properly investigated because it is assumed to be negligible due to the expected very high electrical conductivity and weak electric fields in the direction parallel to the geomagnetic field. However, previous studies have shown that: (a) parallel electric fields are not null and may be significant under specific conditions; (b) parallel electrical conductivity, though high, is finite; (c) FACs exhibit distinctive dissipation patterns responding to the dynamics of the topside ionosphere. This study presents maps of local power density dissipated by FACs in both hemispheres by taking advantage of 8 years of data from the Swarm A satellite. It investigates the dependence of dissipation on magnetic latitude, magnetic local time, seasons and solar activity at around 450 km of altitude. The findings point out that while FAC dissipation is small compared to that of horizontal currents at 100–110 km of altitude, it is not negligible and exhibits significant spatial and time variations that are a function of sunlit conditions and solar activity. The obtained results are discussed in terms of the relation between electron density and temperature and particle precipitation, pointing out that dissipation is a clear consequence of MI coupling and the response of MI system to forcing of solar origin. Plain Language Summary: Field‐aligned currents (FACs) are electric currents that flow along Earth's magnetic field lines, linking the outer magnetic environment (magnetosphere) to the high‐latitude upper atmosphere (ionosphere). These currents deliver energy to the ionosphere, where it is partly turned into the movement of surrounding charged particles. Previous research has shown that the ability of the ionosphere to conduct electricity, although high, is not unlimited. This means that FACs can lose energy as they flow. In this study, we performed the first‐ever global maps (covering both Earth's hemispheres) that show where and how FACs lose energy in the upper ionosphere. We used 8 years of data collected every second by the European Space Agency's Swarm A satellite, which orbits about 450 km above the Earth's surface. Our findings reveal that the way FACs lose energy depends on factors like magnetic latitude, local time, seasons, and solar activity. Interestingly, the patterns of energy loss are slightly different in the Northern and Southern hemispheres, highlighting how Earth's upper atmosphere responds differently in each hemisphere to solar influences. Key Points: We analyze 8 years of Swarm A satellite data to study dissipation of field‐aligned currents in the topside ionospherePower density dissipated by field‐aligned currents varies with hemisphere, season, solar illumination, and solar activityA 90° hemispheric tilt and a six‐cell current system in the Northern hemisphere are consistently observed in all cases [ABSTRACT FROM AUTHOR]
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Abstract:Magnetosphere‐ionosphere (MI) coupling is mainly achieved by field‐aligned currents (FACs) that inject energy and momentum into the high‐latitude ionosphere. A fraction of this energy is locally dissipated and converted into kinetic energy of the ionospheric plasma. Dissipation of FACs in the topside ionosphere has not yet been properly investigated because it is assumed to be negligible due to the expected very high electrical conductivity and weak electric fields in the direction parallel to the geomagnetic field. However, previous studies have shown that: (a) parallel electric fields are not null and may be significant under specific conditions; (b) parallel electrical conductivity, though high, is finite; (c) FACs exhibit distinctive dissipation patterns responding to the dynamics of the topside ionosphere. This study presents maps of local power density dissipated by FACs in both hemispheres by taking advantage of 8 years of data from the Swarm A satellite. It investigates the dependence of dissipation on magnetic latitude, magnetic local time, seasons and solar activity at around 450 km of altitude. The findings point out that while FAC dissipation is small compared to that of horizontal currents at 100–110 km of altitude, it is not negligible and exhibits significant spatial and time variations that are a function of sunlit conditions and solar activity. The obtained results are discussed in terms of the relation between electron density and temperature and particle precipitation, pointing out that dissipation is a clear consequence of MI coupling and the response of MI system to forcing of solar origin. Plain Language Summary: Field‐aligned currents (FACs) are electric currents that flow along Earth's magnetic field lines, linking the outer magnetic environment (magnetosphere) to the high‐latitude upper atmosphere (ionosphere). These currents deliver energy to the ionosphere, where it is partly turned into the movement of surrounding charged particles. Previous research has shown that the ability of the ionosphere to conduct electricity, although high, is not unlimited. This means that FACs can lose energy as they flow. In this study, we performed the first‐ever global maps (covering both Earth's hemispheres) that show where and how FACs lose energy in the upper ionosphere. We used 8 years of data collected every second by the European Space Agency's Swarm A satellite, which orbits about 450 km above the Earth's surface. Our findings reveal that the way FACs lose energy depends on factors like magnetic latitude, local time, seasons, and solar activity. Interestingly, the patterns of energy loss are slightly different in the Northern and Southern hemispheres, highlighting how Earth's upper atmosphere responds differently in each hemisphere to solar influences. Key Points: We analyze 8 years of Swarm A satellite data to study dissipation of field‐aligned currents in the topside ionospherePower density dissipated by field‐aligned currents varies with hemisphere, season, solar illumination, and solar activityA 90° hemispheric tilt and a six‐cell current system in the Northern hemisphere are consistently observed in all cases [ABSTRACT FROM AUTHOR]
ISSN:21699380
DOI:10.1029/2025JA034057