the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 May 2026
The composite radar-GNSS spectrum of auroral plasma turbulence
Abstract. In the auroral ionosphere, plasma turbulence acts as an important dissipation mechanism for magnetospheric energy and the primary cause of radio wave scintillation. Characterizing auroral plasma turbulence across its full spatial extent has historically been limited by the narrow bandwidths of individual instruments. Our investigation approaches the problem of obtaining accurate, scale-dependent information using the physics of the Farley-Buneman (FB) instability, a modified two-stream plasma instability. In this study, we construct a composite spatial powerspectrum of plasma turbulence in the auroral electrojets spanning roughly four orders of magnitude in scale (from ~100 km down to ~20 m). This is achieved by combining a recent Monte-Carlo-based method of spatial clustering of very-high-frequency (VHF) radar echoes, with phase screen information derived from global navigation satellite system (GNSS) signals, using ground-based instrumentation in Canada. Through multi-instrument conjunctions with the European Swarm and Japanese Arase missions, we observe that the clustering of electrojet turbulence matches the structuring of field-aligned currents, and correlates with magnetospheric electron fluxes. Statistical analysis of the composite spectra, as well as a very large database of radar clustering spectra only, reveals a consistently steep decay of spectral power in the auroral electrojets, with the most probable spectral index being near −8/3. The observations suggest a continuous, scale-invariant cascade that frequently preserves the spatial signature of its magnetospheric drivers, where we outline a way for Alfvén waves to structure the turbulent E-region. Furthermore, we demonstrate that the plasma structures guilty of causing GPS scintillations (~270 meters in size) were moving at the ion acoustic speed, implying that those structures were, in fact, FB waves, and we thereby establish an observational basis for low-frequency electrojet turbulence. The method that we present, the composite radar-GNSS spectra, will on both counts offer useful empirical constraints for future efforts seeking to simulate the "sub-grid" turbulence that complicates the magnetosphere-ionosphere coupling around aurorae.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Annales Geophysicae.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 23 Jul 2026)
- RC1: 'Comment on egusphere-2026-2344', Anonymous Referee #1, 26 Jun 2026 reply
Data sets
Icebear data Hussey, Ivarsen https://doi.org/10.5281/zenodo.7509022
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- 1
This manuscript presents an interesting observational study of auroral E-region plasma turbulence. The main new point, in my view, is the construction of a composite radar–GNSS spectrum by combining ICEBEAR radar echo-clustering spectra with GNSS/CHAIN phase-screen or TEC spectra. I think this is a useful and nice idea, because one instrument alone cannot cover such a wide range of spatial scales. The comparison with Swarm FAC observations and Arase particle measurements also gives the paper a useful magnetosphere-ionosphere coupling context.
I think the strongest part of the paper is observational and methodological. The composite spectra, the comparison between radar Doppler speeds, radar target motions, and GPS phase-screen speeds, and the attempt to connect radar-scale turbulence with scintillation-producing structures are all interesting. These results can be valuable for the community.
However, in several places I feel there is a sharp jump from the observations to a broader physical picture. The data show interesting spectral similarities and velocity relations, but some of the physical interpretations are stated stronger than what the evidence can support at this stage. In particular, the claims about a universal or near-universal −8/3 spectrum, long-wavelength Farley–Buneman waves at about 270 m, and direct Alfvénic structuring of the E-region need either stronger support or more careful language.
Major comments
1. The physical meaning of the composite spectrum should be clarified.
ICEBEAR echo-clustering spectra and GNSS phase/TEC spectra are related, but they are not exactly the same observable. One is based on coherent scatter echo locations, while the other comes from integrated density structure and phase-screen assumptions. The authors should explain more clearly what the final composite spectrum physically represents. Is it a density-irregularity spectrum, an echo-clustering spectrum, a scintillation proxy, or a combined observational spectrum? This is important, because the later physical interpretation depends on treating the composite spectrum as one meaningful physical object.
2. The −8/3 spectral slope needs stronger uncertainty and robustness analysis.
The spectral slope near −8/3 is used several times to support the physical interpretation of the paper. Because of this, the authors should show more clearly how well constrained this slope actually is. Please report the slope statistics more explicitly, including mean or median, standard deviation, confidence intervals, and event-to-event variability. The standard deviation is particularly important. If the slope distribution is broad, then a most-probable value near −8/3 may not be enough to support a strong interpretation based on that number.
I think two robustness checks are needed here. First, the authors should test their spectral-estimation pipeline using synthetic data with known input slopes. For example, they could generate spectra with slopes such as −2.4, −2.6, −8/3, and −3.0, add realistic noise/sampling effects, and then apply the same processing pipeline. This would show whether the method can recover the true slope, or whether the processing tends to push the result toward a value close to −8/3.
Second, the authors should report what slope is obtained with less processing. For example, what are the slopes from radar-only spectra, GNSS-only spectra, minimally smoothed spectra, and spectra closer to a raw FFT estimate? How much does the slope change after smoothing, normalization, truncation, splicing, binning, fitting-range selection, and automatic break-point detection?
These processing steps can reduce noise and make the spectra cleaner, but they can also introduce method-dependent bias or change the apparent slope and break points. If the −8/3 slope is a robust physical result, it should remain visible, within uncertainty, across reasonable levels of processing. Since the physical interpretation relies strongly on this value, this analysis is needed before making a strong claim about universality or kinetic-Alfvén-like scaling.
3. The 270 m Farley–Buneman interpretation is suggestive, but too strong as currently written.
The comparison between GPS phase-screen speed, radar Doppler speed, and ICEBEAR target motion is one of the most interesting parts of the paper. However, the conclusion that the ~270 m Fresnel-scale structures are FB waves should be written more carefully. The argument depends strongly on interpreting the GPS phase-screen speed as comparable to the radar Doppler speed and close to the ion-acoustic speed, while the target motion reflects a faster E×B/source-region motion.
The assumptions in this velocity comparison should be made more explicit. For example, how sensitive is the GPS-derived speed to scan geometry, anisotropy, pierce-point motion, layer thickness, and propagation direction? Since standard linear FB growth at 270 m is expected to be weak, the authors should also discuss other possible explanations more carefully, such as GDI, mixed GDI/FB processes, nonlinear coupling, or selection effects. If stronger support cannot be provided, the result should be phrased as “consistent with long-wavelength FB-like structures” rather than as a firm identification.
4. The Alfvénic-driver interpretation should be toned down or better supported.
The Swarm FAC comparison and impedance analysis are valuable, and they support an Alfvénic connection. However, similar spectral slopes do not by themselves prove that Alfvén waves directly structure the E-region turbulence down to all observed scales. The manuscript should avoid language implying a fully demonstrated causal chain from magnetospheric Alfvénic structure to the full E-region turbulent spectrum, unless more quantitative evidence is added.
A safer interpretation is that the observations are consistent with Alfvénic structuring and MI coupling, but do not yet prove direct scale-by-scale transfer.
5. Event-selection effects should be discussed more clearly.
The GNSS cases appear to be selected because they show clear scintillation or amplitude fluctuations. This is reasonable, but it may bias the sample toward Fresnel-scale structures and strong events. Since the 200–300 m knee is central to the paper, the authors should explain how the events were selected, how many candidate events were excluded, and whether radar-active but GNSS-quiet or weak-scintillation cases show similar spectra.
6. The discussion should stay closer to what is demonstrated.
The broader discussion includes interesting ideas about Alfvén-wave energy extraction, impedance matching, preferred auroral altitude, polar-cap potential saturation, self-organized criticality, and the companion RG interpretation. These may be useful future directions, but they are not demonstrated by this paper. The manuscript would be stronger if these parts were shortened or clearly marked as possible implications.
Minor comments
Please soften phrases such as “only reasonable explanation.” The proposed interpretation may be the most plausible one, but other mechanisms should not be ruled out too quickly.
Terms such as “universal,” “continuous scale-invariant cascade,” “preserves the magnetospheric driver,” and “directly structures” should be used more carefully.
The thin-layer phase-screen assumption should be mentioned more clearly in the main text, not only in the appendix.
The automatic slope and break-point detection method should be described more clearly, since the spectral index results depend strongly on it.
Please fix typos, formatting issues, and incomplete references, including the “?” placeholder in the discussion.
Overall assessment
This is a valuable observational paper with a nice methodological novelty. The composite radar–GNSS spectrum can be a useful contribution to auroral turbulence and space-weather studies. However, the manuscript should be more careful not to overclaim physical mechanisms based on limited conjunctions, spectral similarity, and fitted slopes. The main revisions should focus on clarifying the physical meaning of the composite spectrum, quantifying the robustness of the −8/3 slope, supporting or softening the 270 m FB interpretation, addressing selection bias, and reducing the strongest causal language about Alfvénic structuring.