the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 May 2026
Estimating the source altitude of auroral precipitation from dispersed Alfvén waves in the dayside ionosphere
Abstract. The VISIONS-2 sounding rockets performed in-situ measurements of the active dayside auroral region. Numerous broadband dispersed signatures up to keV energies are visible in the electron electrostatic analyser data, typical of Alfvénic precipitation. In order to characterize the region where the particle are accelerated, we estimate source altitudes based on different fits of the observed energy–time dispersions. Additionally, a method based on pitch-angle–time dispersions is developed, which allows relaxing the assumption of simultaneous injection at all distances. Both approaches are found to yield similar source altitudes. For most of the analysed dispersed precipitation structures, these are found to lie between 1,000 and 3,000 km, and increase in height for larger electron energies. Further, variations are observed across events, suggesting different conditions in the acceleration region. Finally, a comparison with previous observational studies and theoretical predictions is performed, and our estimated source altitudes are found to be generally consistent with some of the inertial Alfvén wave velocity profiles, especially those related to lower plasma densities environments. Overall, the results presented here provide further detail about the Alfvénic auroral acceleration region on the dayside. The developed method also opens the possibility of inferring the plasma density profiles and essential wave parameters above the spacecraft.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-2126', Anonymous Referee #1, 08 Jun 2026
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RC2: 'Comment on egusphere-2026-2126', Anonymous Referee #2, 11 Jun 2026
Review:
This article covers an interesting area of research. The investigation uses a high-quality dataset, discusses various methodologies for analyzing it, and applies them. It assesses different methodologies and demonstrates which method most likely provides the best information for describing the ionosphere density profile along the magnetic field line. It is a well-conducted scientific investigation that advances science and should be available to the community.
There are no major issues, and the articles can be published as is. These are suggested changes that can make the paper more impactful:
Figure 1: In text or at the bottom of the figure, indicate the change in the geomagnetic lat-long between 262s and 536s (this is the time period covered in Figure 5).
Line 145 and in the discussion: It is suggested that an instrument with high energy-time resolution is the best option for identifying the density altitude profile. However, when an observation is made close to the source region, the signature is easiest to see in the pitch-angle information. The text should acknowledge the importance of pitch-angle information so that future missions are designed with it in mind.
Line 145 and in the discussion: From an observational point of view, it is better to be far from the source to see DAW. The article does a great job of describing how to increase the accuracy when describing the source region (Figure 7); however, it could be valuable to balance the text by acknowledge having a standard instrument (with a similar resolution that was used in this study) at a slightly higher altitude on a satellite (which would degrade the accuracy) would instead provide more observations form any more different conditions. That is, this technique could be used to explore the altitude profiles of the ionosphere earthen (a) highly accurately with state-of-the-art instrumentation or (b) to get a sufficiently well-designed instrument that can observe the changes in the ionosphere over a long time. Both are valuable contributions to science.
Line 175, 255: Up-going particles are not used. This can be important information, improving the accuracy of the pitch-angle-time investigation. The manuscript should mention that and, hopefully, use it in future studies.
Line 205: remove 'despise'
Line 125: The first sentence indicates contrasting events, and the second sentence provides an example. However, the contrasting example appears several sentences later, making the text difficult to follow. Rearrange the paragraph so it is clear in the first or second sentence what the contrasting events/features are.
Figure 5 and line 300: Looking at the figure, a 30/60 second periodicity might be present. This periodicity could be of solar wind conditions outside the bow shock, which should be visible in magnetometer data, but it could be something else. Since the time periods in Figures 1 and 5 differ, it would be great if Figure 5 could add a second panel showing the energy spectrogram from 260s to 540s on top of the existing panel. It will also provide information on how easy/common the analyzed 29 events were. Adding the extra panel in Figure 5 would help to understand line 300.
Line 260-265: In this discussion, remind the reader of the altitude the rocket is moving at so that that information can be contrasted with the other observed speeds and altitudes. It is unclear if the results from the other investigations just reflect observations at different Geomagnetic latitudes and altitudes.
Line 320: This paragraph is good with an impact sentence at the end. However, it feels that this paragraph could benefit from a second impact sentence. Something, 'This is one of the few observation techniques that can provide density profiles of the ionosphere close to the critical inflection of the oxygen and hydrogen-dominant ionosphere, and allows the dynamics of the ionosphere to be investigated at aurora latitudes.'
Line 345: When describing the density profiles, rephrase it from 'low plasma densities' to describe the oxygen ionosphere profile with a small or a large scale height.
Line 350: When discussing cavities in the aurora region, it could be misinterpreted by readers. When satellites encounter a cavity, they observe a low-density region surrounded horizontally by higher densities. In Figure 6a, the vertical change in the density for the cavity profile is the same as the others. In the body of the text, it is therefore recommended not to use the word cavity and only use the word together with the reference. It is the combination of the small oxygen population's scale height and the small hydrogen component's scale height, relative to the magnetic field profile, that establishes the required Alfven speed profile for DAW waves to efficiently couple to the plasma, thereby creating the dispersed features as a result, as Figure 6 demonstrates.
Figure 7: The text does not mention the two bumps observed in Figure 7a. It is recommended that the manuscript address this, as it could be used for future high-accuracy observational investigations. It is a cool result.
Citation: https://doi.org/10.5194/egusphere-2026-2126-RC2
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- 1
--- General Comments ---
This paper presents a dataset from the VISION-2 sounding rocket mission dealing with a pair of payloads that traveled southward into the dayside ionosphere. The analysis focuses on the many electron time dispersions observed by the low flyer's electrostatic analyzer, where historical time-of-flight methods are employed to estimate their source altitudes. The authors compare their predictions to the literature and those altitudes predicted by the inertial Alfven wave-particle acceleration process, developing a fitting method between the two in the process. The paper does not present a simulation/numerical model of the wave-particle acceleration process (which is known to occur many altitudes and depends upon the properties of the source plasma as well as accelerating waves) for their events, yet the authors acknowledges the limitations of their analysis while remaining reasonable. Although the electron dispersions, time of flight analysis and source mechanism for the dispersions are not new, the method of reverse-fitting dispersions to build up density profiles in a form of remote sensing is more novel and the succinct tabularization of the literature on this topic offer useful tools for further community discussion. I find this paper's core science and conclusions compelling for publication and believe it would be a great addition to the broader literature. However, minor caveats or considerations should be added to the paper to improve clarity regarding the limitations of the methods involved. In particular, the possible effects of the accelerating waves being different across their events as well as the accepted interpretation that resonant particles travel significant distances during IAW acceleration (as opposed to instantaneous release) complicate the interpretation presented, and should be acknowledged further.
--- Specific Comments ---
L51/L115
While the limitations are known, the introduction or discussion section does not mention them. Such complications as variability in wave properties (poynting flux, wave scale, frequency, wave parallel potential), wave resonance bands or differences in source populations are not discussed/mentioned throughout.
L79
The author references another paper to explain the particular pitch angle scheme which is supposed to explain how a 360deg/20bin = 18 deg/bin resolution detector can resolve 0-7.5degs in Figure 1. Unless the other referenced work is in hand, it is quite unclear how that is achieved. A very brief explanation of the scheme should be given in this manuscript.
Fig 1
Some notes on this figure:
(1) Suggestion: consider adding the altitude of the payload to the x-axis of this figure to help contextualize measurements. This can be helpful for thinking about IAR's impact.
(2) Provide details for the filter used in the DC components.
(3) Suggestion: Separate out the E/B Field components into pairs on another panel. It's difficult to read on printed page otherwise.
(4) Should be differential energy flux instead of energy flux
L96
The mere existence of field aligned Poynting flux is insufficient to claim these are Alfven wave observations, and it is surprising no analysis using the wave data from Figure 1 was used for this purpose. For mostly downward-propagating, field-aligned Alfven waves near the ionosphere, the E/B ratio is expected to match the Alfven speed reasonably well. There even appears to be in-phase behavior between E_north and B_east in the data presented, which is typical. While a thorough analysis may be inappropriate for this paper, reporting the frequency range of the associated waves along with a brief E/B calculation adds necessary credibility.
Fig 5
For the events in Fig 5 which are not shown in Fig 1, are they coincident with any other types of precipitation? Should they be thought of in the same way as those in Figure 1? This information is not given. Since the authors refer to the differences in plasma conditions along magnetic field lines throughout the paper often, this information would provide context on that point.
L292 to L298, L308, L321, L409
For a linear IAW interpretation, variations in the plasma density along field lines or resonant source populations certainly affect the IAW phase speed and thereby the TOF fits, which is alluded to in this work. However, the variation in the wave perpendicular scales below 3km has also been shown by Tanaka 2005b (Fig 14 of that paper) to meaningfully modify the TOF polynomial fits. Additionally, in fig 1 of this paper the Poynting flux is well correlated with the flux intensity/peak energy of the electron dispersion (also consistent with Tanaka 2005b) and emphasizes that the waves themselves vary on the ~10 second spatial/temporal scales observed by this rocket. The author then considers dispersions over much of the several minutes of rocket flight in Fig 5, which increases the likelihood of the accelerating IAWs having different properties unless otherwise shown. Some words in the discussion or section 5.2 should acknowledge that variation in wave parameters between events can also modify TOF source altitude estimation instead of just differences of the plasma density, say.
L371-393
While the assumption of matching IAW phase velocity with particle energy is understandable given the fitting method employed, it is important to specify sources of error by expanding upon L391-L393. Similiar to my previous comments, this fitting doesn't account for the parallel electrostatic potential of the IAW which accelerates electrons that are below V_A in parallel energy (but within the resonance window) and carries them over significant distances before being released. This point, at least, should be stated.
--- Minor Comments/Technical Corrections ---
L30 Kletzing 1994 specifies the maximum energy of an IAW accelerated electrons in the laboratory frame as 2V_A - v_e, where v_e is the initial velocity of a resonant electron moving parallel to the wave.
L31 There are multiple Kletzing 1994 references without identifiers, e.g. (a)/(b) between the two papers.
L58 "with the aim to help improving" is difficult to read.
L80, L395 It is not explicitly stated which flyer (low or high) data for Figure 1 comes only from. The text suggests its only the low flyer.
L234 "...for some event to switch..." is awkward
L254 "to help" is awkward.