| 11 Mar 2026
Electric fields in and around an auroral arc and the inferred current system from the BROR sounding rocket experiment
Abstract. Magnetosphere-ionosphere interactions play a crucial role in the dynamics of near-Earth space, and the electric field in the vicinity of the auroral arc is one of the major links in these interactions. The electric field in the auroral ionosphere has been measured using various techniques: coherent and incoherent radars from the ground, and in situ measurements using rockets and satellites. The effective approach to studying the auroral electric field is to determine it from observations of the motion of artificial ion clouds released into the ionosphere by a sounding rocket. On 23 March 2023, the Barium Radio and Optical Rocket (BROR) experiment was conducted at the Esrange rocket range, near Kiruna, Sweden. In the experiment, 8 canisters containing a barium-strontium-thermite mixture were released at altitudes between 130 and 240 km. A novelty of the experiment is multi-station narrow-band optical observations of emissions with ALIS_4D. This allows us to reconstruct the 3D distribution of optical phenomena in the ionosphere using a tomography-like technique with spatial and temporal resolutions of ∼500 m and 0.1 s, respectively. The active auroral arc developed inside the area occupied by the barium clouds and intersected with the one of clouds at an altitude of ≈230 km for quite a long time. During this time, the ion cloud experienced various deformations, which we observed and used to determine the electric field as a function of position relative to the auroral arc.
Review of „Electric fields in and around an auroral arc and the inferred current system from the BROR sounding rocket experiment” by Taki et al.
This paper investigates a recent rocket experiment in which barium was released into the auroral ionosphere to measure electric fields by tracking the motion of the ion cloud using optical observations. Optical measurements of an auroral arc at 120 km altitude and of the barium ion cloud at 230 km were used to infer the surrounding electric field system.
The objective of the experiment is interesting, and the data visualization is well presented. However, the applicability of the data fusion between the two optical measurements and the novelty of the results are not clearly demonstrated. In addition, both the abstract and the introduction require improvement to more clearly convey the study’s background and the linkage to main findings. For example, the altitude variation of the electric fields may be of significant importance for this study.
A major concern on the dataset is that the barium cloud is observed at 230 km, whereas the auroral arc is located at 120 km. The authors should explain more clearly how this large altitude separation is compensated for in their analysis. After this can be done convincingly, the novelty of the electric‑field measurements should also be presented more clearly. Another aspect is whether the derived electric fields represent the background field and/or the short‑term fluctuations. Since the method should have high time resolution, it would theoretically capture fluctuations, but the text does not make this clear.
Below are specific comments referenced by line numbers (L). These points should be addressed before the manuscript can be considered for possible publication.
Abstract
L1 and Abstract: The current abstract does not include any direct results or new findings of this paper itself. It describes the novelty of the experiment, but not the novelty of the findings.
L4 “The effective approach”: Could the authors clarify in what sense this approach is considered effective?
L7 “A novelty of the experiment is…”: At L94, the authors state that the difference between ALIS and ALIS_4D is the use of an EMCCD instead of a CCD. Combining these two sentences, does this mean that the only novelty of the experiment, compared to previous studies, is the use of an EMCCD?
L11 “quite a long time”: How long is this period?
Introduction
L15 “current systems”: Could they also explain more why the current system is important for understanding this coupled ionosphere – magnetosphere system?
L15” Various observational approaches”: The introduction discusses rocket, satellite, and ground‑based measurement approaches. These techniques have different horizontal and vertical observation ranges, but the current text does not describe these differences and their implications.
L20 “were small, less than 10 mV/m”: It may be useful to mention how small these values are compared to the background.
L21 “altitude dependent … were reported”: This sentence appears inconsistent with the statement at L22 that “no significant altitude variation was observed.”
L24 “On the other hand, charge oscillations were observed”: The meaning of this sentence and relation to the previous sentence are unclear. Please rephrase it.
L26 “spacecraft”: This literature appears in the middle of a paragraph introducing rocket studies. How is it related to that context? Spacecraft studies are introduced starting at L37.
L37” attempted”: The authors could add more information to this sentence explaining why satellite measurements were attempted.
L42 “strong”: How strong are these?
L44 “relatively large”: Relative to what, and how large? Please specify.
L45 “… However…”: The reviewer does not understand the implication of this “however” or the logical connection between the preceding and following sentences. Please clarify the intended contrast.
L50 “Strong”: Please explain how strong, and relative to what.
L51 “3 s,”: Is this about optical or EISCAT observation ?
L53 “the two”: Please clarify what “the two” refers to.
L53 “response”: Does this imply any interaction between the radar and the electric field?
L57 “downward field-aligned currents”: Could the authors describe how these currents are related to the electric field?
L58 “background electric field was…canceled by a polarization electric field”: Please explain the mechanism responsible for this.
L 59 “this study”: Please consider rephrasing “this” to avoid possible ambiguity.
L62 “but have relatively limited resolution. However,…”: The reviewer does not understand how these sentences about Clayton et. al., are connected by “but” and “however.” Please rephrase.
L66 “As a result, it was difficult to separate temporal variations from spatial variations based on the observations alone.”: The reviewer does not understand this as a conclusion from the preceding sentence.
L67 “temporal and spatial scales smaller than… not be detected”: This limitation applies to any observation technique. What are intended to mean here?
L68 “barium clouds”: Active ionospheric experiments using chemical releases, including barium, have been conducted for many years. The authors cite Fahleson et al. in the beginning, but additional previous studies and results relevant to the present work could be introduced if there are any.
Experiment
L75 “23 March 2023”: The storm events in March 2023 including March 23 have been studied by several authors already. Can authors find any relationship between their study to previous studies? e.g,
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2023JA032145
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2023SW003728
As seen in these papers, the EISCAT radars were operating on March 23, and one of their objectives should have been to support the BROR experiment. Were there any EISCAT results that could be relevant to the current work? Also, with regard to previous EISCAT–rocket studies, such as the cited works by Lanchester et al. or Aikio et al.? In particular, ion‑drift measurements or electric field/FAC estimates derived from the radar data would be highly relevant.
L84 “moved under influence of the electric field”: Similar to the comment above, strong thermospheric winds were reported on 23-24 March 2024 storm (see Oyama et al 2024 above). Did the authors estimate the possible influence of thermospheric winds?
L96 Figure 2: What does the yellow point on the map represent? No description is provided.
2.2 Instruments and Data
L104 “strontium”: Why was only the camera at Esrange used to measure strontium? In addition, were any results from these strontium measurements considered in the analysis from Esrange in Figure 3?
L112 “too weak to be detected”: Is this conclusion based on experimentally established results from previous studies, or is it derived from the observations in the present study? In Figure 4, both cameras show isolated round structures near the arc (for the Kiruna camera, one appears near the center of the image). What are these features?
3 Data processing and analysis
L118 “Figure 5”: Please indicate where the fourth cloud is located in all of the Ba‑related sub‑figures in Figure 5, and specify how many clouds are present in total.
L123 Figure 6: This reconstruction figure seems that some of the early ion clouds reached lower altitudes, close to the auroral altitude around 120 km. If this interpretation is correct, these clouds may be more suitable for analyzing the electric field in the vicinity of the aurora.
L125 “a vertical range of ±10 km around the altitude at which the cloud intensity reaches its maximum.”: Please explain in more detail how the altitude of maximum Ba intensity is determined. Is this done by comparing mapped images using altitude as a parameter from all four stations?
L 133 “we shifted the cloud data to the height at an altitude of 120 km”:
The objective of this projection needs more explanation.
This reconstruction procedure appears to assume that electric field variations can be ignored over the 100‑km altitude range between approximately 120 and 230 km. How do the authors justify this assumption in the case of an auroral arc?
Moreover, this projection gives the impression that the analysis does not fully utilize the capabilities of the experimental design. If the reviewer interpreted it correctly, as the tomographic results in Figure 6 show, the total set of ion clouds spans a much larger altitude and horizontal range than the fourth cloud alone, with some clouds extending down toward auroral altitudes. In principle, this experiment could provide not only horizontal motion but also information on vertical deformation/motion.
Also, similar to a comment before, the EISCAT Tromsø radar was operational on that day and should provide plasma parameters, and likely most relevant for this study, altitude variation of plasma parameters including ion‑drift.
L138 “…green points. A blue line…”: The blue lines in Figure 7 do not appear to represent the observed auroral arc accurately, as they differ noticeably from the grayscale structure. Fitting a straight line to an auroral arc does not seem physically justified or effective, especially at 18:30 UT. At 18:25, no auroral arc is visible in the image, but only the blue lines are shown.
L139 “The fourth cloud projected”: The reviewer assumes that the red tracings in Figure 7 represent the projected cloud structures from Figure 6. However, at 18:25 these appear only as very small red points in Figure 7. Based on Figures 6 and 7 alone, it is difficult for the reader to verify whether the red circles in Figure 7 correctly correspond to the fourth cloud shown in Figure 6.
To improve clarity, the authors should consider adding the auroral arc outline also in Figure 6 for comparison, and/or provide the geolocation (latitude, longitude, and altitude) of the fourth cloud. This would allow readers to follow the temporal and spatial development of the fourth cloud more easily.
In addition, as noted earlier, the bright round structures in the auroral images create confusion if they are not related to the barium clouds. Are these bright features indeed unrelated to the BROR cloud releases? A clear explanation is needed to avoid misinterpretation.
L147 “We selected four points to characterize the cloud deformation”. The reviewer assumes that this paragraph is intended to describe Figure 8, although Figure 8 is never mentioned in the main text.
L162: “7-s moving average”: High‑resolution imaging was a key novelty of this experiment. Please explain whether this novelty is preserved when the data are averaged.
L164: “60s, 100 s”: Please specify the times in UT so that readers can compare them directly with the optical figures.
L172 “Watec images also include BROR clouds”: This sentence seems to contradict the statement in line 112, creating confusion for the readers. Please explain this point more clearly.
L178 “distance of each point from”: Related to earlier comments, is the projection from 230 km down to 120 km include any geometric effects? For example, can the spatial distance between two points become larger or smaller after the projection? Any such distortion could directly affect the distance/velocity calculation.
L188 “vertical velocity along the z-axis”. Please clarify whether this computation is performed before or after the tomographic reconstruction shown in Figure 6. If it is derived after tomography, then the vertical resolution is limited to approximately 20 km, the it could impact the interpretation of Figure 11.
A more essential question concerns the vertical motion of the barium clouds: how much of the observed descent can be explained by natural gravitational settling, and how much is attributable to electric‑field–driven motion?
Aslo, in Sergienko et al., 2024, a vertical motion of ions was reported as an unexpected finding. Is there any connection to the current study?
4 Results and discussion
L193: This section 4 is titled “Results,” but in practice the material presented in Section 3 also contains results. The distinction between the two sections is therefore unclear.
L199 “for the altitude of approximately 230 km treated in this study,”: One of the most significant methodological questions in this study concerns how the lower‑F‑region electric field is being characterized when the mapped auroral structures are located at E‑region heights. The manuscript does not clearly explain the physical justification for interpreting an electric field at 230 km based on features mapped to ~120 km. The E‑region is highly collisional compared to lower F-region, so the electrodynamic behavior, and therefore the inferred electric field may be sustainably different.
L 213 “a function of distance from the auroral arc”:
One of the central viewpoints of this manuscript is the characterization of the electric field as a function of distance from the auroral arc, then compared with modeled fields derived from IMAGE data. The manuscript would rather benefit from a clearer explanation of the importance and novelty of measurements themselves relative to previous work. A brief comparison to Marklund (1984) is provided at the end, but this seems insufficient, unless there has been no significant progress in electric‑field measurements over the past 40 years prior to the BROR experiment.
L220/ L238 “Figure 13/14”. How can one interpret the values of this equivalent current in terms of electric field and altitudes? In addition, it would be helpful to show the location of Esrange as well as the auroral arc on the map to aid interpretation. Ideally , Figure 14 can be also overlaid there to provide a complete visualizing of the results in order to support conclusion of the manuscript.