the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 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.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-5220', Anonymous Referee #1, 04 Dec 2025
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RC2: 'Comment on egusphere-2025-5220', Anonymous Referee #2, 07 Apr 2026
Comments on 5he manuscript “High-latitude observations of ULF wave driven ion upflow”
By Charlotte M. van Hazendonk et al.This work was triggered by the combined observation of ULF waves and ion upflows. The event of about 1.5 hours duration was covered centrally by the EISCAT ESR radar at Svalbard, a meridian scanning photometer, the magnetometer chain IMAGE, and crossings of two DMSP spacecraft. The conversion of this excellent set of data into meaningful physical parameters, such as electron and ion temperatures and density, particle and energy fluxes, and ion upflows, serve as basis for the derivation of field-aligned current density and differential energy flux by the ELSEC method. The incorporation of the 2D data of three DMSP overflights allowed producing maps of FACs, convection velocities, Pedersen conductance and Joule heating rates by the Lompe method. The ground magnetometer data show where the spectral power of the ULF wave is concentrated in frequency and latitude. In the regions between alternating FAC directions, where Pedersen currents must flow, clearly higher dissipation rates are found. Taken together, this constitutes an exemplary piece of data reduction and conversion.
The derived physical quantities fall into the range of those pertaining to field line resonances. However, the authors conclude that the wave power of the magnetic fields does not have a sufficiently concentrated frequency peak for being classified as field line resonances. In the discussion, the authors concentrate on the ion upflow, which is of type 2, i.e. caused by enhanced electron heating and density, not by enhanced ion temperature due to transverse electric fields. The conclusion, “the UAA event provides a strong coupling between the ionosphere and magnetosphere” is certainly correct. The lack of Ti enhancements suggests that most of the UAA energy is deposited via kinetic processes rather than Joule and/or frictional heating as expected for FLRs. However, the question, what drives the ULF wave, is not being addressed. Overall, the discussed event is very interesting and the effort invested by the authors adequate thus producing an excellent preprint, worth publication as is.
I believe there enough information to go a bit further. It is the geomagnetic location of the event. The authors think that it occurred at closed magnetic field lines, but close to the border of open fields. Secondly, it is concentrated around a local time of 16:00. So, the solar wind or mantle flow is passing by the high-altitude extension of the involved field lines. Although it is a magnetically quiet period, spatially concentrated perturbations may be passing by or are excited by shear flows, feeding enough energy into the closed flux tubes to excite ULF waves. Since the energy of the auroral electrons ranges up to 8 keV, there is enough ambipolar field generated to generate the ion upflow of type 2. I tend to think that the auroral event is the primary item and the ULF wave the framework, not the driver of the ion upflow. At least, one may consider this idea as an item for discussion.
Citation: https://doi.org/10.5194/egusphere-2025-5220-RC2
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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