the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
How does auroral electron precipitation near the open–closed field line boundary compare to that within the auroral oval during substorm onset?
Abstract. Auroral electron precipitation during a substorm exhibits complex spatiotemporal variations which are still not fully understood, especially during the very dynamic phase immediately following the onset. Since during disturbed times the auroral oval typically extends across several hundreds of kilometers in the latitudinal direction, one may expect that precipitating electron spectra differ at locations close to the open–closed field line boundary (OCB) compared to the central part of the auroral oval. We carry out a statistical study based on 57 auroral breakups associated with substorm onsets observed above Tromsø (66.7° N geomagnetic latitude, i.e. central oval) and 25 onsets occurring above Svalbard (75.4° N geomagnetic latitude, i.e. poleward boundary) between 2015 and 2022. The events were selected based on the availability of both optical observations and field-aligned incoherent scatter radar measurements. Those are two sets of different substorms; hence we compare solar wind driving conditions and geomagnetic indices for the two event lists in the statistical sense. Using the ELSPEC method (based on the inversion of the electron density profile) on the radar data, we retrieve precipitating electron fluxes within 1–100 keV around each onset time, and we apply the superposed epoch analysis method on the electron spectra at each location. We compare the statistical precipitation characteristics above both sites in terms of peak differential flux, energy of the peak, integrated energy flux, and their time evolution during the minutes following the onset. We find that the integrated energy flux associated with events occurring in the central part of the auroral oval (Tromsø) exhibit a sharp peak up to 25 mW m-2 in the first two minutes following the auroral breakup, before decreasing and reaching stable values around 7 mW m-2 for at least 20 min. In turn, no initial peak is seen near the open–closed field line boundary (Svalbard), and values remain low throughout (1–2 mW m-2). A comparison of the median spectra indicates that the precipitating flux of > 10 keV electrons is lower above Svalbard than above Tromsø by a factor of at least 10, which may partly explain the differences. However, it proves difficult to conclude whether the differences originate from the latitude at which the auroral breakup takes place or from the fact that the breakups seen from Svalbard occur equatorward from the radar beam, which only sees expansion-phase precipitation after a few minutes.
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RC1: 'Comment on egusphere-2024-483', Anonymous Referee #1, 18 Mar 2024
The paper presents a comparative study of precipitating electron characteristics during auroral breakups from two different latitudes, one close to the open-closed magnetic field line boundary and the other site approximately residing inside the auroral oval. 57 events from the auroral oval and 25 events closer to the open-closed field line boundary were selected based on a mix of machine learning-aided labelling and visual inspection approaches. By using an electron density inversion method called ELSPEC, the paper characterizes the peak energy, the number and energy flux of the precipitating electron profiles. Further applying a superposed epoch analysis, the paper demonstrates and compares the before and after of the poleward expansion into the radar field of view. The paper conducts a well-defined, coherent, and attainable study to analyze properties of precipitation associated with auroral breakup phenomena, however, the low number of events identified at the open-closed magnetic field line boundary and simplifying assumptions about the vertical profile of precipitation raises concerns that authors are advised to further discuss. It is evident that the instrument-based limitations pose difficulties for the authors to address, therefore strengthening these points with results from prior literature is suggested.
Specific Comments
- The paper discusses a lack of events to decisively conclude the differences between the two cameras due to the limited number of events detected by the Svalbard all-sky camera. However, it is not clear why the Svalbard all-sky camera data set is not further extended to further differentiate the properties in a statistically significant manner. Furthermore, the paper doesn’t adequately discuss the lack of significant energy variations before and after the break-up events at the Svalbard location. I suggest authors either explain in more detail why only 25 events were available or extend the Svalbard data set to be comparable to Tromsø. In addition, I suggest the authors further discuss why energy levels were higher at a higher latitude during pre-breakup events.
- The methodology for detecting break-up aurora events from discrete labels is not well explained. As the auroral arc brightens and expands northward, wouldn’t it make more sense to find events where initial arc labels are followed by consistent discrete labels? I suggest the authors further justify why arc labels were not used in the event selection.
- The paper doesn’t adequately justify the reasoning behind obtaining the median of a wide altitude range which can significantly impact the energy obtained from the inversion method. In the literature, precipitation due to breakup aurora could be observed at altitudes as low as 65 km. Using the median between 85 to 125 km could significantly mischaracterize the precipitation characteristics. I suggest the authors provide a range for these characteristics, especially in Figures 3 and 5.
Technical Corrections
- The title can be improved as currently, it implies the precipitation profile from two different cameras for the same event are being used to compare and contrast the precipitation characteristics, however, events are not related.
- Paragraphs in the introduction section, especially after paragraph four, are disconnected, hence they do not adequately motivate the study. Authors can add a transitionary sentence at the end of paragraphs highlighting how the study differs.
- In the paragraph indicated with 220, the claim that “at energies greater than 20 keV, for which those two curves are almost one order of magnitude higher than the subsequent ones.” seems to be misleading as at 20 keV the difference seems to be twice to that of later times.
- Further comparisons with literature where auroral breakup characteristics were provided (using satellite data) can improve the discussion of the paper. (Kataoka et al., 2019 and referencing literature)
Citation: https://doi.org/10.5194/egusphere-2024-483-RC1 -
RC2: 'Comment on egusphere-2024-483', Anonymous Referee #2, 02 Apr 2024
In their submission the authors discuss the electron precipitation during magnetic substorms at two different locations: central auroral oval (Tromsoe) and further North (Svalbard). At first different drivers have been investigated, which did not how significant differences between onsets observed in the central oval region and those near the open-closed field line boundary. In a second step the precipitation is investigated using all-sky cameras and incoherent scatter radars. Electron flux spectra are derived based on the radar signal. Further processing of the spectra lead to statements on the peak differential flux, peak energies, and the integrated precipitating energy flux and their temporal evolution during the substorms. Here differences between the two locations have been identified.
In general the paper is well written and understandable. It addresses a topic that is of interest and the used methods seem to be reasonable with some caveats concerning the limited differentiation between temporal and spatial evolution at Svalbard. However I have one major point of criticism that may be of interest for the whole study.
Major issue:
-The complete second part of the study deals with details of the electron spectrum that is derived from the radar signal using the ELSPEC method.However the reader has no idea how exact such a derived spectrum might be. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JA025636 for example states that "When the distributions have two maxima (D, F, and G), the maximum at higher energy is well reproduced, but the secondary maximum at lower energy is not as well modeled." Fig. 1b shows that double peak structures (e.g. at approx. 17:48, 17:52) and thus known periods of enhanced uncertainty appear. I don't want to enlarge on double peak structures here, but the reader should get an impression of the uncertainty of the derived spectra and how reliable the statements on the results (e.g. peak flux) are.
Minor issues:
-ELSPEC is an important tool in this study thus please add enough information to Section 2.3 to allow a reader to follow you. For example why do the resulting spectra all (or mostly) look Maxwellian? Is that an assumption in the model or does it appear naturally? What about the fine structure of the curves, does that depend in the internal energy resolution of ELSPEC (or EISCAT)? Also ELSPEC is an acronym that is not introduced.-Fig. 1: please check units: 10^12 m^-3 appears in many publications as typical peak electron density at 100km, you show 10^12 cm^-3.
-For better understanding of the "valid data points" described in l. 188 the according periods when data from in Fig. 1 contributes should be marked there. Also it is not ideal to have a full paragraph on "data points" but the information that a data point last for 30s (and not as initially described in the EISCAT section 5-6s) follows in the next paragraph.
-l. 190: which median? of all altitudes?
-l. 195: do I understand this right that this method is similar to a selection of the periods when the all-sky camera shows strong illumination above the radar? So why not simply using a threshold of the magenta graph in 1c?
-l. 197: A spectrum is accepted when it exceeds background level by factor 3, OK, that should select periods with increased particle precipitation. But doesn't this introduce a major offset, especially during the period before the onset? I mean the background "reference" value is eliminated, so this should not be very representative.
-l. 234: "zero epoch" should be clarified here. Due to the different locations of onset and radar at Svalbard (and thus the time delay of the observation) it helps to remember the reader that the onset time is meant here.
-l. 295: It is probably easier to label the two options with a) and b) and refer to that in the following paragraph(s).
-l. 317: Shouldn't be a big deal to exclude those 14 events and check if impacts 3 and 4, isn't it?
Typos and similar:
-l. 80: compare substorm characteristics-Fig. 1 l. 2: "the the"
-Fig. 1 l. 4: the arbitrary unit is linearly scaled? Show be clarified because its drawn on a log graph.
-l. 207: in the order
-l. 226: can be seen in the first 4~min, as
-l. 339: include
Note:
Currently temporal evolution and distance to the onset impacts the measurements at Svalbard in a probably similar and indistinguishable manner.I don't know if that could be successful, but you may try to plot a version of Fig. 5b with all individual events and a color coded distance to the onset. In that way the currently cloaked distance to the onset may give a more comprehensive picture. Well, maybe.
Citation: https://doi.org/10.5194/egusphere-2024-483-RC2
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