the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 May 2025
Properties of large-amplitude kilometer-scale field-aligned currents at auroral latitudes, as derived from Swarm satellites
Abstract. High-resolution magnetic field recordings by the Swarm A and C spacecraft have been used to investigate the properties of field-aligned currents (FACs) at auroral latitudes down to their smallest scales (<1 km). Particularly suitable for that purpose are the magnetic field recordings, taken at a rate of 50 Hz, during the two weeks around the quasi-coplanar orbit configuration on 1 October 2021. We have split the recorded signal caused by FACs of along-track wavelengths from 0.3 km to 40 km into 8 quasi-logarithmically spaced ranges. Our investigations revealed that the kilometer-scale FACs (0.3–5 km) show quite different characteristics from those of the small-scale FACs (10–40 km). The kilometer-scale FACs exhibit short-lived (<1 s) randomly appearing large current spikes. They are confined to certain latitude ranges, which depend on local time. Small-scale FAC structures last for longer times (>10 s) and are distributed over larger latitude ranges. Their largest amplitudes are achieved at latitudes that overlap with the kilometer-scale FACs. The small-scale FACs have earlier been identified as Alfvén waves that are partly reflected at the ionosphere, and they can oscillate within the ionospheric Alfvén resonator. When at the same time additional Alfvén waves are launched from the magnetosphere they will interfere with the reflected. We suggest that the interaction between oppositely travelling Alfvén waves, when continuing sufficiently long, is generating the large-amplitude and short-lived kilometer-scale FACs.
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RC1: 'Comment on egusphere-2025-1961', W. Lotko, 02 Jun 2025
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Review of “Properties of large-amplitude kilometer-scale field-aligned currents at auroral latitudes, as derived from Swarm satellites” by Y-L Zhou and H. Lühr
This study takes a major step in advancing knowledge of small-scale field-aligned currents (FAC) found at cusp and auroral latitudes with variations on 0.04 to 5 second time scales or 0.3-20 km spatial scales. The results are based on high-resolution, fluxgate magnetometer measurements from a special, two-week campaign in which the SWARM A and C satellites maintain separations of < 2.5 km cross-track and about 2 seconds along-track.
The study is well-motivated. The methodology is clear. The authors present new results pertaining to observed characteristics of small-scale field-aligned currents. Their presentation is well-organized and informative. The discussion of results offers interesting and plausible interpretations that suggest future research directions.
The manuscript will be of interest to the broader scientific community and is publishable. Before accepting it for publication in AG, I recommend implementing minor revisions to include some descriptive clarifications and improvements in language expression and syntax and in the font size in two figures. To facilitate revisions, I have sent a markup of the manuscript directly to the authors via email with embedded comments for their consideration (also attached with referee report).
New and noteworthy scientific methodology and results include:
- Novel and rigorous analysis of two-point, high-resolution time-series from SWARM A and C, including cross-correlation, spectral distributions and polarization characteristics.
- Resolution of spatiotemporal ambiguities using two-point measurements.
- Determination of the preferred length scales for very large-amplitude FACs.
- Characterization of waveform and amplitude persistence as a function of fluctuation length and time scales.
- Analysis of relationships between fast and slowly varying fluctuations.
- Determination of signal polarization, with more slowly varying signals exhibiting mostly elliptical polarization and faster varying signals exhibiting mostly (near) linear polarization.
- MLAT–MLT distributions of signal characteristics.
An intriguing aspect of the study is its plausible interpretation of the data analysis in terms of Alfvénic turbulence. Building on a previous investigation of CHAMP satellite data (Rother et al., 2007) and appealing to results from previous theoretical and modeling studies, the authors assert that the observed magnetic fluctuations and attendant FACs on the dayside are a consequence of magnetopause disturbances that launch Alfvén waves earthward and become trapped in an F-region ionospheric Alfvén resonator (IAR). The guided waves achieve 5-20 km field-perpendicular scales upon reaching the ionosphere, i.e., the longer duration fluctuations in the data. With ongoing magnetopause stimulation of Alfvén waves flowing into the IAR, the resonator modes intensify until their dissipation within it balances the power flowing into it. The authors presume that nonlinear interaction between counterpropagating Alfvén resonator modes produces a turbulent cascade to smaller scales – the short duration, km-scale FACs identified in the data. The dissipation range of the cascade may be attributable to ionospheric Ohmic dissipation of sub-km-scale FACs according to cited modeling studies. Nightside Alfven wave activity originates from magnetotail processes and is more episodic than dayside activity, so its statistical properties differ from those on the dayside. However, the Alfven wave dynamics within the nightside ionosphere should be similar.
It is customary in turbulence analysis to determine the power spectral density of the fluctuations and identify a power law spectral index if one exists. An evaluation of the power spectral density and the energetics of the fluctuations across the spectral range might be a useful addition to the paper.
The paper concludes with some unresolved questions for future study. What are the effects of the km-scale Alfven waves on ionosphere-thermosphere heating and neutral gas winds? What is the nature of the electric fields accompanying km-scale FACs? What are the effects on charged-particles, e.g., transverse acceleration of ions and field-aligned electron acceleration?
I would add to this list a key question posed by the authors’ interpretations: If small-scale (5-20 km) and km-scale FACs are causally related, how is the elliptical polarization of small-scale FACs transformed into the linear polarization of km-scale FACs, and by what means do the km-scale FACs achieve much larger amplitudes than the presumed energy-containing population of 5-20 km-scale FACs?
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