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| 14 Jul 2025
A Source or a Sink? How Trends in Particle Precipitation Dictate Electrodynamics in High-Latitude Ionosphere
Abstract. Fast, charged particles intermittently rain down into Earth's dense atmosphere. The kinetic energy of these particles are converted into heat and light, and it ionizes the atmospheric gas, providing a source of both free and bound energy for the ionosphere; this is the aurora borealis and australis. The specific kinetic energy of the constituent particles in the aurora dictates the atmospheric response to the ongoing particle precipitation, with hard (high-energy) particles penetrating deeper than those that are considered soft (possessing a low kinetic energy). In this paper, we analyze a large database of precipitating particle observations from the United States' Defense Meteorological Satellite Program, and aggregate the altitude-dependent response of the ionosphere at high-latitudes, using fast ionization rate parameterizations due to two important papers by Fang et al. (10.1029/2010GL045406 and 10.1002/jgra.50484). We explore a characteristic altitude-dependent pattern in space (magnetic latitude and longitude), and time (geomagnetic activity), pertaining to the shape of the northern hemisphere, high-latitude ionosphere during local winter. We briefly discuss the implied ratio of E- to F-region Pedersen conductance, and this ratio's ramifications for the growth & decay (and thus proliferation) of plasma turbulence in the high-latitude ionosphere.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-3068', Anonymous Referee #1, 13 Aug 2025
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RC2: 'Comment on egusphere-2025-3068', Anonymous Referee #2, 28 Aug 2025
"A Source or a Sink? How Trends in Particle Precipitation Dictate Electrodynamics in High-Latitude Ionosphere" by Magnus F. Ivarsen
Here the author uses DMSP and scintillation data together with OMNI solar wind and IMF measurements, the SuperMAG SME index, and parameterized, analytic expressions for ionization rates to examine how ionization rates and a handful of other measurements and derived quantities vary with altitude and local time during local winter.
While I find the idea behind the study appealing and interesting, there are some major holes that need to be filled — including literature gaps, correlation vs. causality issues, and qualitative descriptions (which I interpret as a desire to "keep the message simple") where a quantitative statement is needed. Major revisions are in order.
Major issuesThe choice of the word "dictate" in the title is far too strong. It implies a causal relationship between auroral particle precipitation and what happens in the ionosphere, without acknowledging that ionospheric conditions can feed back on the region from which the precipitation originates.
The limitations of the Fang et al relationships must be critically discussed. What caveats do they come with? The most obvious one for me is that they ignore a non-negligible amount of dayside auroral precipitation in that they are designed to represent monoenergetic precipitation, and (as far as I can tell) do not apply for energies below 100 eV. Meanwhile, the _majority_ of the energy and number flux carried by dayside electrons is in the form of broadband rather than monoenergetic precipitation, with a large fraction at energies less than 100 eV (classic example: Panel 6 of Figure 1 of Chaston et al, 1999, doi: 10.1029/1998GL900246). I would venture that the results presented here are missing a substantial contribution from such electrons, but this eventuality is not addressed in the manuscript.
There is also a great deal of emphasis put on particle precipitation, when the fact is that in addition to particle precipitation, a good deal of energy is carried by waves. This is especially true on the dayside, and is very well established in the literature (Neubert and Christiansen, 2003, doi: 10.1029/2003GL017808; Chaston et al, 2007, doi: 10.1029/2006GL029144; Dombeck et al, 2017, doi: 10.1029/2018JA025749). It therefore feels a little disingenuous to attribute so much of the regularities to particle precipitation; more than in any other MLT sector, dayside precipitation is driven by waves, and some would argue that it is at least as likely that these waves are the cause of both the irregularities _and_ the particle precipitation as it is that the precipitation causes irregularities. On the theory side, studies like Lotko and Zhang (2018, doi: 10.1029/2018JA025990) and Tu et al (2011, doi: 10.1029/2011JA016620) give good reason to believe that waves are an essential part of the picture.
I therefore cannot get on board with the hypothesis that particle precipitation might be the cause of the irregularities that the author reports here and elsewhere, since there is almost no discussion of cusp-region physics beyond observing the ionization rates peak at higher altitudes. As mentioned above, this reflects the fact that reconnection occurs on the dayside, reconnection drives waves, and waves drive broadband electron precipitation. (For completeness, Dombeck et al, 2017, also point out that isotropic precipitation peaks directly in the cusp; see their Figure 14b.) Beyond this, waves and broadband precipitation have a strong connection with heavy-ion outflows that themselves could drive instabilities (e.g., Chaston et al, 2006, doi: 10.1029/2005JA011367).
An issue throughout is that the reader is given the impression that the results are novel, whereas at least some of them follow naturally from what one already finds in the literature from the past two or three decades. For example, in Figure 3 and the description of it in the main text, a lot of emphasis is given to MLT differences in the altitude profiles of the inferred ionization rate. However, that the dayside ionization rate from auroral precipitation peaks at altitudes above those of the peak altitude ionization rate on the nightside is a simple reflection of the fact that dayside precipitation tends to have much lower energies — say, hundreds of eV — than nightside precipitation, which routinely reaches energies of several keV. With this in mind, and the fact that the more energetic the precipitation the deeper the penetration into the ionosphere, results in Figure 3b seem intuitive, or maybe even obvious.
Other comments
L48–50: The mechanism mentioned here (a conducting ionosphere short-circuiting an electric field that drives instabilities) seems suspect on the surface; how is it supposed to work? The manuscript seems to philosophically embrace the so-called E, J paradigm in which electric fields and currents are treated as the primary variables. One hallmark of this paradigm is using language that suggests the coupled magnetosphere-ionosphere system can be represented via a lumped-element circuit model or transmission-line theory. This paradigm has a number of problems, among which the most significant is perhaps that it gets causality wrong. (I have come to believe this in the course of the past few years by taking seriously the arguments of, among others, Parker (1996, doi: 10.1029/95JA02866; 2007, doi: 10.2307/j.ctt2111gdt) and Vasyliunas (2001, doi: 10.1029/2001GL013014; 2012. doi: 10.5194/angeo-30-357-2012), which I urge the author to gain a basic familiarity with. Another easily digestible intro to this is Laundal et al, 2024, doi: 10.1029/2024GL108695.) As Mannucci et al (2022, doi: 10.1029/2021JA030009) have pointed out, the E, J paradigm also violates Galilean relativity. For a great variety of phenomena, and certainly on large scales, magnetosphere-ionosphere interactions can be understood in terms of the magnetic field and the motion of plasma (the "B, v" paradigm).L70: Need to say which DMSP satellites.
Figure 2: The description of what constitutes a GNSS phase scintillation is too brief; how are they defined, quantitatively, and are they predefined in the datasets you reference, or do you derive them yourself? Basic details are lacking in the main text about the number of receivers, data availability / coverage, etc., which makes it hard to evaluate the presence of possible biases.
L78–79: "how the above basic mechanism evolves in time". There is no time dependence, so it should not be possible to speak of evolution in time. What is shown is statistical differences related to particular states of the solar wind and geomagnetic activity. That is, I would say that panels c and d indicate how the height-resolved ionization rate varies from active to quiet times and southward to northward IMF orientations, respectively.
L82: "(this time using the median value of 150 nT as the deliminator)". As opposed to what? Is a different value used elsewhere? Figure 3 caption also says 150 nT, so this statement needs to be clarified.
L91–92: "In fact, for the most extreme geomagnetic activity bin (top quintile in the SME-index), ionization below 150 km is almost completely absent in the noon-sector." This again reflects that broadband, low-energy (but not necessarily low energy flux!) precipitation dominates on the dayside, and I am not sure whether such precipitation is represented at all in the author's methodology.
L96–97: What does it mean to infer the cusp? Need to state how the cusp is identified.
L103: "onset" carries connotations of time dependence, as does "superposed epoch analysis" as mentioned earlier. But there is no time dependence in Figure 5. This needs to be rewritten to avoid confusing the reader.
Figure 3 caption: Need to state what constitutes the "bottom and top thirds of the ensemble". This is an example of a semi-qualitative statement (no numbers are provided), where what is wanted is a qualitative statement. What if someone wished to reproduce this study? I think it would be very hard or impossible to do given the sparsity of detail.
Figure 3 caption: The description of panel c is wrong; the E- to F-region ionization rate is not shown here. (If it is, I have completely misunderstood what the panel shows. To me it looks like height-resolved ionization rates.
Figure 4a–e: Please spell out how MLT sectors were defined, and give some indication of the amount of data that contributed to the profiles that are shown. Is the author showing the median profile, or the mean, or something else? Consider showing the variability at each altitude as well.
Figure 4f–j: I appreciate howthe author gives qualitative categories of geomagnetic activity, but it would be better to spell out somewhere how the data were grouped to arrive at these results — if nothing else, for the sake of reproducibility.
Figure 4f: I can't help but think about the lack of representation of other types of precipitation when
Figure 5a–c: Please define what constitutes a cusp observation.
Figure 5: Similar to the problem on Lines 78 and 79 that I describe above, panels d–g are referred to as a superposed epoch analysis without there being any actual dependence on time. I tried several times to understand these panels, but I just cannot put together how the author derives the lines that are shown. Therefore, I follow neither the description of these panels nor the argumentation that the author makes based on them. Why not just make the x axis of panels d–g Δdeg relative to scintillations? Or is the stretching of the cusp that is mentioned in the caption nonlinear? (I don't understand what stretching means here.)
L110: "rates of eta". According to Equation 1, eta is not a rate.
L110–111: The mechanism described here should be described in more detail. The discussion seems to lack balance, because no alternative explanations are given. Is there really only one candidate mechanism in the literature? Has it been verified? If it has been verified in the studies that the author references here, that verification should be described in careful detail since it is central to the point the author is trying to make here.
L116: In what sense are electric fields the strongest driver of irregularities? From the B, v perspective, the electric field is a simple manifestation of plasma motion, while from the perspective of the ionospheric Ohm's law and the definition of current, it is the manifestation of differences in electron and ion motion. Neither of these are immediately clear as drivers of irregularities. On the other hand, Alfvén waves and the fragmented, small-scale currents they carry as well as variable broadband precipitation — two of the hallmarks of the cusp — would seem to be more natural explanations of the irregularities one observes near the cusp.
L136: "interesting and somewhat surprising results". As pointed out earlier, I don't find the results generally surprising based on the conditions that exist in the dayside cusp region. But at any rate, it would be helpful if the author pointed to specific results that seems surprising, and motivate why they are counterintuitive and surprising. At present, I really don't know which results exactly the author is referring to.
L144–145: "the trends uncovered are clear and convincing". As with my previous comment, in isolation, I don't know which trends the author is referring to.
L117: "The irregularities generated here convect poleward …" Do they?
L148: "the absence of energetic particle precipitation consistently lead to the occurrence of F-region irregularities". The author has suggested a mechanism (electric fields and a lack of precipitation), but nothing in this study comes close to a demonstration of causality. This statement therefore feels like a very big leap to me.
Editorial comments
L32: Preclude means to make something impossible, which I don't think is the intended meaning here or elsewhere in the paper.L80: Are the changes absolute? I would have thought they are absolute if the absolute value were taken.
Citation: https://doi.org/10.5194/egusphere-2025-3068-RC2
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General Comments
The preprint "A Source or a Sink? How Trends in Particle Precipitation Dictate Electrodynamics in High-Latitude Ionosphere" by Magnus F. Ivarsen is a well-structured and scientifically sound piece of research within the field of space physics, specifically ionospheric electrodynamics. The paper addresses a pertinent question regarding the role of particle precipitation in the high-latitude ionosphere, offering new insights derived from a substantial dataset.
The provided results reveal a broader understanding of plasma instabilities and irregularities. The concept of the cusp as an "instability factory" is an engaging and insightful analogy. The paper also acknowledges limitations and caveats, such as the simplifications made and the need for more thorough investigations using holistic models.
Overall, the preprint exhibits a high level of scientific rigor and makes a valuable contribution to the understanding of high-latitude ionospheric electrodynamics.
Specific Comments
Lines at 45. How did you calculate the occurrence of “phase scintillation”? Additionally, when the traditional (again Butterworth filtering with 0.1 Hz cutoff frequency) phase detrending is adopted, it is not fair to call them phase “scintillation”, rather phase “fluctuations”, as they are mostly due to refraction and tend to vanish if Ionosphere Free Linear Combination between two signals is considered. This has been clearly demonstrated in a series of a recent works (Conroy et al., 2022 https://doi.org/10.1029/2021RS007259; Ghobadi et al., 2020 https://doi.org/10.1007/s10291-020-01001-1; McCaffrey & Jayachandran https://doi.org/10.1029/2018JA025759, 2019; Spogli et al., 2021 https://doi.org/10.1007/s10291-020-01001-1; Wang et al., 2018 https://doi.org/10.1002/2017JA024805, 2022 https://doi.org/10.1029/2022JA030870). I also don’t understand the use of the word defraction: it should be refraction. Please, check.
It is also not specified the length of the dataset used to produce the climatological scenario in Figure 2. Please, specify.
What IPP altitude did you assume to place the GNSS data in Figure 5. Is it again at 130 km, as in Figure 2?
Between 195 and 200 you state that “E-region conductance is dominated by ionization at the nominal Pedersen conductivity peak, which we take as near 140 km, with falling conductivity towards 170 km” while in the main body you assume 130 km for the same value. Please, check.
Technical corrections
Being a European journal, I would suggest using the British English. Please, check the text thoroughly
Lines at 5: “soft” in italic, in agreement with the previous italic for the word “hard”
Lines at 5: Fang et al. (10.1029/2010GL045406 and 10.1002/jgra.50484): don’t use the doi but the year of the publications
Lines at 25: “Figure 1 shows an implementation of those equations” -- > “Figure 1 shows an implementation of those models”
Caption of Figure 1: observed by the DMSP F18 satellite on 24 October 2014. -- > observed by the DMSP F18 satellite at 00:05:16 UT on 24 October 2014.
Caption of Figure 2: just for sake of historical reasons and no need to change the text: the magnetic cusp as the prominent hotspot for increased occurrence of phase scintillation (calculated in the traditional Butterworth filtering with 0.1 Hz cutoff frequency) has been introduced in 2009 by Spogli et al. (https://doi.org/10.5194/angeo-27-3429-2009), while the term “hot spot” for scintillation occurrence was introduced in 2013 by Spogli et al. (https://doi.org/10.4401/ag-6240).
Lines at 45: the references to Yeh and Liu (1982); Kintner P. M. et al. (2007); Song et al. (2023); Meziane et al. (2023) must be put in parentheses.
Lines at 55: explode the SSJ acronym. It is here used for the first time.
Figure 5c: the definition of occurrence is hidden in the label of your colorbar. You use 0.15 radians (units are missing in each panel of your figure). In any case, I encourage you to provide further details on the occurrence calculation in the text.
Lines at 105: the role of the tongue of ionisation in creating phase fluctuations has been highlighted even before in a work by De Franceschi et al. (2008 - https://doi.org/10.1016/j.jastp.2007.05.018) based on the Halloween and November 2023 Storm. Again, it is just to provide an historical background and there is no need to change the text.
Lines at 120: Whereas This -- > This
Lines at 185. You can refer to IRI with a recent review by Bilitza et al. (2022 - https://doi.org/10.1029/2022RG000792)