High-latitude observations of ULF wave driven ion upflow

van Hazendonk, Charlotte M.; Baddeley, Lisa J.; Laundal, Karl M.; Partamies, Noora

doi:10.5194/egusphere-2025-5220

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https://doi.org/10.5194/egusphere-2025-5220

Preprints

04 Nov 2025

| 04 Nov 2025

High-latitude observations of ULF wave driven ion upflow

Charlotte M. van Hazendonk, Lisa J. Baddeley, Karl M. Laundal, and Noora Partamies

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.

Received: 21 Oct 2025 – Discussion started: 04 Nov 2025

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Journal article(s) based on this preprint

28 Jul 2026

High-latitude observations of ULF wave driven ion upflow

Charlotte M. van Hazendonk, Lisa J. Baddeley, Karl M. Laundal, and Noora Partamies

Ann. Geophys., 44, 697–714, https://doi.org/10.5194/angeo-44-697-2026,https://doi.org/10.5194/angeo-44-697-2026, 2026

Short summary

Charlotte M. van Hazendonk, Lisa J. Baddeley, Karl M. Laundal, and Noora Partamies

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-5220', Anonymous Referee #1, 04 Dec 2025

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

Citation: https://doi.org/10.5194/egusphere-2025-5220-RC1
- AC1: 'Reply on RC1', Charlotte van Hazendonk, 04 May 2026
  
  The authors thank referee 1 for careful reading of the manuscript and for the constructive comments. Below, are the point-to-point answers to the specific comments and the technical corrections.
  Comment 1: 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.
  We appreciate the referee’s request for a clearer definition of spatial scales. In the original manuscript, we avoided specifying azimuthal wave numbers, m, as our datasets do not allow for a direct determination of the m-number. However, we agree that explicit characterization improves clarity of the manuscript and have thus added this.
  For clarity, we have also specified that we are defining the azimuthal wave number in the ionosphere instead of in the equatorial plane and we have changed our equation for m accordingly, such that it now reads m=(2π*R_E*cos⁡(θ))/λ_az, where R_E is the Earth's radius, θ is the geographic latitude, and λ_az the azimuthal wave number. In our case, the event centers around Hornsund (θ=77°). This results in an azimuthal wavelength of λ_az>900 km for large scale (m<10) and λ_az<600 km for small-scale (m>15).
  The revised manuscript now reads (line 43 onward):
  
  "The azimuthal scale size of ULF waves is described by the m-number (m=(2π*R_E*cos⁡(θ))/λ_az, where R_E is the Earth’s radius, θ the geographic latitude, and λ_az the azimuthal wavelength in the ionosphere). Waves can be classified according to their m-number, where low-m waves have large spatial scales and are predominantly toroidally polarized. Although there are no strict definitions regarding scale sizes, low-m waves typically have m numbers between 1–10 and include FLRs, which are most effectively driven by fast mode waves when m ≈ 3 (Menk and Waters, 2013; Rubtsov et al., 2018). High-m waves, on the other hand, have small spatial scales and show poloidal polarization. Waves are generally considered high-m when m > 15 (Yeoman et al., 2010), but in other cases even higher values such as 50 < m < 150 are used for the high-m classification (Mager et al., 2019; Michael et al., 2024). In between low-m and high-m waves, there is a class of intermediate-m waves (10 ≲ m ≲ 15) which can show characteristics of either low-m or high-m waves (Mager et al., 2019). Above Svalbard, at θ = 77°, large-scale waves (m < 10) correspond to azimuthal length scales of λ_az>900 km, while small-scale waves (m > 15) are characterized by λ_az<600 km."
  Comment 2: 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.
  We thank the referee for raising this important point. We agree that the relationship between the Lompe time window and ULF wave periods requires clarification.
  Lompe performs a regularized least-squares inversion using all available measurements within the selected time window. Thus, while it does not explicitly compute time averages, multiple measurements within a window effectively contribute to a weighted fit, resulting in a solution that represents the system over that time interval.
  We have explored shorter time windows (1–2 minutes) to assess the robustness of the results. While these shorter windows preserve more of the temporal variability, they also lead to increased noise and less stable inversion results due to reduced data coverage. In particular, reliable conductance estimates require sufficient spatial sampling, which is strongly constrained by the availability of DMSP measurements. Since a full DMSP pass spans more than 5 minutes, shorter Lompe windows would introduce an inconsistency between the temporal resolution of the electrodynamic inversion and the conductance input. In addition, the longer time windows provide better spatial coverage for satellite-based magnetometer measurements, improving the inversion calculations.
  Based on these considerations, we retain the 5-minute Lompe window and clarify in the discussion of the revised manuscript that the resulting patterns should be interpreted as resulting an average over a part of the ULF wave cycle. The temporal limitation is now explicitly discussed as:
  "Lastly, it should be noted that the Lompe solutions represent the electrodynamic state over a 5-minute window, which is a significant part of the observed ULF wave periodicity of 15 minutes. As a result, the derived convection and current patterns correspond to an average over a fraction of the ULF wave cycle. This temporal smoothing may reduce the apparent amplitude of rapid variations and should be taken into account when interpreting the relationship between the electrodynamic response and ULF wave periodicities. Shorter time windows for the Lompe models are not feasible due to increased noise due to a reduction in mainly space based measurements and inconsistency of the temporal resolution of Lompe and the conductance as obtained using the DMSP spacecraft."
  
  Comment 3: 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.
  We thank the referee for this insightful suggestion. We agree that phase mixing may contribute to the observed non-field line resonance (non-FLR) signatures. Due to the lack of longitudinal spaced data, we cannot derive the azimuthal propagation direction, thus making it impossible to provide further evidence as to an internal or external driving mechanism. We have, however, extended the discussion to include the possibility of phase mixing. The added paragraph now reads:
  "In addition to internal generation mechanisms, phase mixing could contribute to the observed non-FLR nature of the ULF wave event. Characteristically, a FLR shows a narrow resonant frequency peak on the resonant field line that matches eigenfrequency of the wave. Wave growth on adjacent field lines is slower, causing broadening of the peak and phase mixing (Mann et al., 1995; Rankin et al., 2021). Phase mixing can thus redistribute wave energy and change its signature."
  In addition, a sentence about the possibility of phase mixing has also be added to the conclusion:
  
  “Phase mixing may play a role in the apparent conflicting observations, in which case the frequency peak of an initial, externally generated FLR is broadened and the wave exhibits characteristics more aligned with internally driven small-scale wave signatures.”
  Technical corrections:
  
  Technical correction 1: 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.
  Thank you for this suggestion. We fully agree with this comment and have changed it accordingly in the manuscript.
  Technical correction 2: 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.
  We have chosen to add an extra panel to Figure 4 showing the median filtered X- and Y-components of the magnetometer data.
  Technical correction 3: 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
  Thank you for pointing this out. Something had gone wrong in the manuscript file which we have now corrected. The paper is now cited correctly.
  
  Citation: https://doi.org/10.5194/egusphere-2025-5220-AC1
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 T_i 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
- AC2: 'Reply on RC2', Charlotte van Hazendonk, 04 May 2026
  
  We thank the referee for the positive assessment of our manuscript and for the thoughtful and physically insightful comments regarding the possible drivers of the observed ULF wave activity and their relationship to the auroral precipitation and ion upflow.
  We agree that the geomagnetic location of the event (near the open–closed field line boundary in the afternoon sector) makes external driving mechanisms, such as magnetosheath or mantle flow interactions and shear-driven processes, plausible contributors to the observed ULF wave activity. We also appreciate the referee’s suggestion that the auroral precipitation may represent the primary driver of the ion upflow, with the ULF wave acting more as a framework than the direct driver.
  Possible sources of ULF wave activity are briefly introduced in the manuscript (lines 45–46), followed by a discussion of field line resonance formation (lines 49–50). To point the reader towards additional information sources, we have now added two references in the discussion (line 288) regarding the Kelvin–Helmholtz instability as a potential source of ULF wave activity. Since a detailed investigation of wave generation mechanisms would require additional data and analysis that are not available within the current dataset, we consider this level of detail to be sufficient for the scope of the present study, which focuses on the ionospheric response and energy dissipation associated with the observed event. This, after careful consideration, we have therefore decided not to expand the discussion of potential ULF wave drivers further.
  We note that the relationship between ULF waves, electron acceleration, and auroral precipitation is already partly addressed in the manuscript (lines 243–254). There, we discuss that wave energy is likely partitioned into particle acceleration, leading to periodic electron precipitation into the ionosphere. In this interpretation, the ULF wave modulates the acceleration of electrons, which in turn produces the observed periodic auroral features.
  At the same time, we agree that the auroral precipitation itself is the most likely driver of the observed type 2 ion upflow, as it provides the necessary enhancement in electron temperature and density to generate ambipolar electric fields. Thus, while the particles are required to drive the upflow, the ULF wave may play an important role in controlling the temporal modulation of the precipitation and associated auroral dynamics.
  We believe that this interpretation, already outlined in the manuscript, is consistent with the referee’s suggestion and adequately reflects the coupling between ULF wave activity, auroral processes, and ion upflow without extending beyond the scope of the present work.
  
  Citation: https://doi.org/10.5194/egusphere-2025-5220-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-5220', Anonymous Referee #1, 04 Dec 2025

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

Citation: https://doi.org/10.5194/egusphere-2025-5220-RC1
- AC1: 'Reply on RC1', Charlotte van Hazendonk, 04 May 2026
  
  The authors thank referee 1 for careful reading of the manuscript and for the constructive comments. Below, are the point-to-point answers to the specific comments and the technical corrections.
  Comment 1: 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.
  We appreciate the referee’s request for a clearer definition of spatial scales. In the original manuscript, we avoided specifying azimuthal wave numbers, m, as our datasets do not allow for a direct determination of the m-number. However, we agree that explicit characterization improves clarity of the manuscript and have thus added this.
  For clarity, we have also specified that we are defining the azimuthal wave number in the ionosphere instead of in the equatorial plane and we have changed our equation for m accordingly, such that it now reads m=(2π*R_E*cos⁡(θ))/λ_az, where R_E is the Earth's radius, θ is the geographic latitude, and λ_az the azimuthal wave number. In our case, the event centers around Hornsund (θ=77°). This results in an azimuthal wavelength of λ_az>900 km for large scale (m<10) and λ_az<600 km for small-scale (m>15).
  The revised manuscript now reads (line 43 onward):
  
  "The azimuthal scale size of ULF waves is described by the m-number (m=(2π*R_E*cos⁡(θ))/λ_az, where R_E is the Earth’s radius, θ the geographic latitude, and λ_az the azimuthal wavelength in the ionosphere). Waves can be classified according to their m-number, where low-m waves have large spatial scales and are predominantly toroidally polarized. Although there are no strict definitions regarding scale sizes, low-m waves typically have m numbers between 1–10 and include FLRs, which are most effectively driven by fast mode waves when m ≈ 3 (Menk and Waters, 2013; Rubtsov et al., 2018). High-m waves, on the other hand, have small spatial scales and show poloidal polarization. Waves are generally considered high-m when m > 15 (Yeoman et al., 2010), but in other cases even higher values such as 50 < m < 150 are used for the high-m classification (Mager et al., 2019; Michael et al., 2024). In between low-m and high-m waves, there is a class of intermediate-m waves (10 ≲ m ≲ 15) which can show characteristics of either low-m or high-m waves (Mager et al., 2019). Above Svalbard, at θ = 77°, large-scale waves (m < 10) correspond to azimuthal length scales of λ_az>900 km, while small-scale waves (m > 15) are characterized by λ_az<600 km."
  Comment 2: 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.
  We thank the referee for raising this important point. We agree that the relationship between the Lompe time window and ULF wave periods requires clarification.
  Lompe performs a regularized least-squares inversion using all available measurements within the selected time window. Thus, while it does not explicitly compute time averages, multiple measurements within a window effectively contribute to a weighted fit, resulting in a solution that represents the system over that time interval.
  We have explored shorter time windows (1–2 minutes) to assess the robustness of the results. While these shorter windows preserve more of the temporal variability, they also lead to increased noise and less stable inversion results due to reduced data coverage. In particular, reliable conductance estimates require sufficient spatial sampling, which is strongly constrained by the availability of DMSP measurements. Since a full DMSP pass spans more than 5 minutes, shorter Lompe windows would introduce an inconsistency between the temporal resolution of the electrodynamic inversion and the conductance input. In addition, the longer time windows provide better spatial coverage for satellite-based magnetometer measurements, improving the inversion calculations.
  Based on these considerations, we retain the 5-minute Lompe window and clarify in the discussion of the revised manuscript that the resulting patterns should be interpreted as resulting an average over a part of the ULF wave cycle. The temporal limitation is now explicitly discussed as:
  "Lastly, it should be noted that the Lompe solutions represent the electrodynamic state over a 5-minute window, which is a significant part of the observed ULF wave periodicity of 15 minutes. As a result, the derived convection and current patterns correspond to an average over a fraction of the ULF wave cycle. This temporal smoothing may reduce the apparent amplitude of rapid variations and should be taken into account when interpreting the relationship between the electrodynamic response and ULF wave periodicities. Shorter time windows for the Lompe models are not feasible due to increased noise due to a reduction in mainly space based measurements and inconsistency of the temporal resolution of Lompe and the conductance as obtained using the DMSP spacecraft."
  
  Comment 3: 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.
  We thank the referee for this insightful suggestion. We agree that phase mixing may contribute to the observed non-field line resonance (non-FLR) signatures. Due to the lack of longitudinal spaced data, we cannot derive the azimuthal propagation direction, thus making it impossible to provide further evidence as to an internal or external driving mechanism. We have, however, extended the discussion to include the possibility of phase mixing. The added paragraph now reads:
  "In addition to internal generation mechanisms, phase mixing could contribute to the observed non-FLR nature of the ULF wave event. Characteristically, a FLR shows a narrow resonant frequency peak on the resonant field line that matches eigenfrequency of the wave. Wave growth on adjacent field lines is slower, causing broadening of the peak and phase mixing (Mann et al., 1995; Rankin et al., 2021). Phase mixing can thus redistribute wave energy and change its signature."
  In addition, a sentence about the possibility of phase mixing has also be added to the conclusion:
  
  “Phase mixing may play a role in the apparent conflicting observations, in which case the frequency peak of an initial, externally generated FLR is broadened and the wave exhibits characteristics more aligned with internally driven small-scale wave signatures.”
  Technical corrections:
  
  Technical correction 1: 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.
  Thank you for this suggestion. We fully agree with this comment and have changed it accordingly in the manuscript.
  Technical correction 2: 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.
  We have chosen to add an extra panel to Figure 4 showing the median filtered X- and Y-components of the magnetometer data.
  Technical correction 3: 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
  Thank you for pointing this out. Something had gone wrong in the manuscript file which we have now corrected. The paper is now cited correctly.
  
  Citation: https://doi.org/10.5194/egusphere-2025-5220-AC1
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 T_i 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
- AC2: 'Reply on RC2', Charlotte van Hazendonk, 04 May 2026
  
  We thank the referee for the positive assessment of our manuscript and for the thoughtful and physically insightful comments regarding the possible drivers of the observed ULF wave activity and their relationship to the auroral precipitation and ion upflow.
  We agree that the geomagnetic location of the event (near the open–closed field line boundary in the afternoon sector) makes external driving mechanisms, such as magnetosheath or mantle flow interactions and shear-driven processes, plausible contributors to the observed ULF wave activity. We also appreciate the referee’s suggestion that the auroral precipitation may represent the primary driver of the ion upflow, with the ULF wave acting more as a framework than the direct driver.
  Possible sources of ULF wave activity are briefly introduced in the manuscript (lines 45–46), followed by a discussion of field line resonance formation (lines 49–50). To point the reader towards additional information sources, we have now added two references in the discussion (line 288) regarding the Kelvin–Helmholtz instability as a potential source of ULF wave activity. Since a detailed investigation of wave generation mechanisms would require additional data and analysis that are not available within the current dataset, we consider this level of detail to be sufficient for the scope of the present study, which focuses on the ionospheric response and energy dissipation associated with the observed event. This, after careful consideration, we have therefore decided not to expand the discussion of potential ULF wave drivers further.
  We note that the relationship between ULF waves, electron acceleration, and auroral precipitation is already partly addressed in the manuscript (lines 243–254). There, we discuss that wave energy is likely partitioned into particle acceleration, leading to periodic electron precipitation into the ionosphere. In this interpretation, the ULF wave modulates the acceleration of electrons, which in turn produces the observed periodic auroral features.
  At the same time, we agree that the auroral precipitation itself is the most likely driver of the observed type 2 ion upflow, as it provides the necessary enhancement in electron temperature and density to generate ambipolar electric fields. Thus, while the particles are required to drive the upflow, the ULF wave may play an important role in controlling the temporal modulation of the precipitation and associated auroral dynamics.
  We believe that this interpretation, already outlined in the manuscript, is consistent with the referee’s suggestion and adequately reflects the coupling between ULF wave activity, auroral processes, and ion upflow without extending beyond the scope of the present work.
  
  Citation: https://doi.org/10.5194/egusphere-2025-5220-AC2

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

ED: Publish subject to minor revisions (review by editor) (21 May 2026) by Dalia Buresova

AR by Charlotte van Hazendonk on behalf of the Authors (31 May 2026) Author's response Author's tracked changes Manuscript

ED: Publish as is (04 Jun 2026) by Dalia Buresova

AR by Charlotte van Hazendonk on behalf of the Authors (13 Jun 2026)

Journal article(s) based on this preprint

28 Jul 2026

High-latitude observations of ULF wave driven ion upflow

Charlotte M. van Hazendonk, Lisa J. Baddeley, Karl M. Laundal, and Noora Partamies

Ann. Geophys., 44, 697–714, https://doi.org/10.5194/angeo-44-697-2026,https://doi.org/10.5194/angeo-44-697-2026, 2026

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Charlotte M. van Hazendonk, Lisa J. Baddeley, Karl M. Laundal, and Noora Partamies

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This study shows the first observations of the upflow of ions in the Earth's ionosphere generated by ultra-low frequency waves. These waves are visible as auroral arcs. Using various instruments and models, their complex dynamics and the coupling between the ionosphere and magnetosphere were highlighted. Results show significant energy dissipation and currents, even from small-scale waves, highlighting the importance of a multi-instrument approach to understanding such phenomena.


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