| 04 Nov 2025
High-latitude observations of ULF wave driven ion upflow
Abstract. We present a comprehensive study of the first observations of ionospheric ion upflow generated by ultra-low frequency (ULF) wave driven auroral arcs (UAAs). Ground- and space-based instrumentation, together with inversion models, allow us to study the event at different length scales. This shows the complex dynamics of UAAs and their role in the ionosphere-magnetosphere coupling via ion upflow, field-aligned currents (FACs), and energy dissipation. The UAA event was observed as a series of six poleward moving arcs, primarily in the 630.0 nm emission line. At the northern extent of the arcs incoherent scatter radar (ISR) data indicated that the UAAs have driven type 2 ion upflow with low to medium fluxes of around 3.3 × 1013 particles m-2 s-1. Data from the ISR, spacecraft, and models, result in FAC magnitudes up to 6 μA m-2, total energy fluxes up to 12 mW m-2, and Joule heating rates up to 11 mW m-2 associated with the arcs. These values mostly correspond to localized measurements, while at large-scale the values are up to 50 % smaller. In addition, ground-based magnetometers suggested that the UAA is driven by small-scale ULF waves, while energy dissipation rates and FAC magnitudes are significant and comparable to previously reported large-scale wave events, indicating the importance of using a multi-instrument approach when investigating energy dissipation associated with ULF waves. This event thus shows that even small-scale ULF waves can drive ion upflow in the ionosphere.
In their manuscript, "High-latitude observations of ULF wave driven ion upflow," van Hazendonk et al. use observations and modeling to identify ionospheric upflow associated with ULF wave auroral arcs. In particular, they use a combination of ground (ISR, magnetometers, meridian scanning photometer), satellite (DMSP, Iridium, Swarm), and data-driven modeling (Elspec, Lompe) to detect a ULF wave auroral arc (UAA) using an automatic algorithm, measure key ionospheric parameters and wave properties in the region of the arc and just outside it, and place the measurements in context with ionospheric flows, currents, and heating over a broader region. They find that the auroral arc event is likely driven by small scale ULF waves, with energy dissipation rates and field-aligned current (FAC) magnitudes comparable to previous large-scale wave events. The manuscript presents a comprehensive set of observations needed to understand the complex magnetosphere-ionosphere coupling processes associated with ULF waves. The results are both novel (first reported upflow related to UAA, along with key parameters such as current intensity and heating rates) and important as they advance our understanding of ULF waves and point to several areas where future research and observations are needed, such as the need to accurately determine the scale size of wave activity and the need for multi-instrument investigations. The manuscript is well written and logically organized, and there are only a few comments and technical corrections that should be addressed before publication (see below).
Specific Comments:
Line 10 and elsewhere – "small-scale ULF waves..." Here or somewhere early in the manuscript, please further clarify the definitions of small-scale versus large-scale (e.g., m-number below or above 10, small scale is ~<100 km in ionosphere in north-south or east-west sense, >~500 km for large scale, etc.). There is always confusion in the literature on scale size definitions (small-, meso-, large-), and clarifying this would help place the results in context with other studies. Lines 45-46 propose that m-number can be used to divide large from small-scales, but no threshold for large versus small m-number is given and there are no m-number measurements in the manuscript.
Line 181 – “The Lompe analysis are based on 5 min windows…” 5-min windows are an appreciable fraction of the 10- and 15-minute wave periods (1.1-1.67 mHz), thus this approach could smooth and/or average out contributions of the waves to the overall results. If the window is using a snapshot rather than averages (e.g., Lompe calculations based on instantaneous ground magnetometer values every 5-min rather than a 5-min average), this issue could be mitigated, but other issues may arise if there are timing offsets between ground magnetometer snapshots, DMSP snapshots, etc. Please add some discussion on how the Lompe calculation window does (or doesn’t) affect the wave properties. This is not a major issue because the study validates Lompe results by making direct comparisons with individual Iridium satellites and cross-comparisons with other datasets, but it's still important to discuss whether the 5-minute window could affect e.g., the absolute numbers for FAC intensity, heating rate, etc.
Line 286 – “This would indicate a generation mechanism internal to the Earth’s magnetosphere” Externally driven ULF waves can also exhibit a non-FLR nature due to, for example, phase-mixing after the external driving concludes (e.g., the transition from large to small scale waves via phase mixing can occur in an MHD approximation, Rankin et al., 2021 Figure 5), non-stationary driving more generally, presence of multiple drivers/wave modes, etc. Is it possible that these scenarios could fit the non-FLR/small-scale wave observations, in addition to the internal driving mechanism? If so, it would help to mention some alternate possibilities in the text.
Technical Corrections
Line 40 – “ULF waves are magnetohydrodynamic oscillations…” It might be better to say something like “ULF waves are oscillations that can often be described with a magnetohydrodynamic approximation…” since later in the paragraph parallel electric fields (not an MHD feature) associated with FLR are mentioned.
Line 150 – “The UAA event does not have a classic FLR nature” Would it be possible to include the original magnetometer time series as a Figure in the Appendix? This would be useful for future studies to investigate the possibility of ground conductivity effects, presence of other wave activity, etc., as well as for future studies seeking to model this event.
Line 244 – Rankin et al. 2021 is cited but missing from the reference list. I think this refers to the following reference (which is also mentioned in comments above): Rankin, R., Gillies, D.M. & Degeling, A.W. On the Relationship Between Shear Alfvén Waves, Auroral Electron Acceleration, and Field Line Resonances. Space Sci Rev 217, 60 (2021). https://doi.org/10.1007/s11214-021-00830-x