the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Jun 2026
Bursty bulk flow ionospheric impacts from triple-conjunctive observations
Abstract. Substorms are the regular reconfiguration of the Earth's geomagnetic space environment, and magnetosphere-ionosphere coupling is integral to the substorm chain. Bursty bulk flows (BBFs) are mesoscale fast plasma flows that are released from magnetic reconnection sites in the magnetotail during substorms, and they couple to the ionosphere through field aligned current (FAC) systems. BBFs may drive localised changes to the ionospheric plasma, such as velocity flows or changes to the local temperature, density and conductivity, but determining the specific ionospheric effects of a given BBF and their associated FAC is a complex undertaking. In this study we identify triple-conjugate observations of BBF detections within the magnetotail, FAC observations in low Earth orbit and ground-based radar stations that observe the local ionospheric plasma in order to evaluate the different components of the coupled BBF-FAC-ionosphere system. We evaluate over 11000 BBFs and identify only 13 events that satisfy our conjunctive criteria. We perform detailed analysis of one of these BBFs, which occurred in the recovery phase of a relatively weak substorm, to evaluate the FAC associated with this BBF and its ionospheric impacts. Multiple FAC signatures were observed that are associated with the BBF, in addition to variations in the ionospheric velocity and temperature. There is an ongoing need for coordinated observations of the coupled BBF-FAC-ionosphere system to further distinguish the ionospheric impacts of BBFs from broader substorm dynamics.
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RC1: 'Comment on egusphere-2026-3108', Anonymous Referee #1, 31 Jul 2026
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AC1: 'Reply on RC1', Harriet George, 24 Sep 2026
We thank the reviewer for their feedback on the manuscript. The BBF evaluated in this study was detected at MLT 2.5 (2.0) by THEMIS-A (-E), which is within the statistical spatial distribution of BBFs (midnight ± 3 hours MLT). The greatest occurrence rate of magnetic reconnection site detections is in the premidnight sector, but reconnection sites have also been observed in the postmidnight sector. This BBF is therefore further dawnward than the majority of BBFs and and magnetic reconnection sites tend to occur, but not unusually so.
Changes in manuscript: We have added the MLT of the BBF observation by THEMIS to Section 4.2 and compared it to the typical MLT of BBF detections
Citation: https://doi.org/10.5194/egusphere-2026-3108-AC1
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AC1: 'Reply on RC1', Harriet George, 24 Sep 2026
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RC2: 'Comment on egusphere-2026-3108', Anonymous Referee #2, 06 Sep 2026
This manuscript presents a valuable multi-instrument case study combining THEMIS, Swarm, PFISR, and auroral observations to investigate the possible ionospheric signatures of a BBF. The coordinated dataset is interesting and potentially useful for improving our understanding of magnetosphere-ionosphere coupling. However, several interpretations are currently stated with greater certainty than is supported by the reported temporal and spatial correspondence, particularly regarding the association of the Swarm FAC signatures and the transient auroral feature with the specific BBF. The novelty claim should also be more carefully qualified in light of closely related studies, and several inconsistencies in the event count, plasma-density description, figures, and terminology should be corrected. I recommend publication after minor revision.
One general omission which could be considered concerns key work by Wei et al. This and some related references ought to be included and addressed in the Introduction. I mention a couple below which I am sure the authors are aware of…
Wei, D., W. Dunlop, J-Y. Yang, X-C. Dong, Y.-Q Yu, T-Y. Wang (2021), Intense dB/dt variations driven by near-Earth Bursty Bulk Flows (BBFs): A case study, Geophys. Res. Letts, doi:10.1029/2020GL091781.
Zhang, C-M., M. W. Dunlop, -Y. Yang, X. Tan, O. Marghitu, A. Blagau, C. Xiong, X-C. Dong, D. Wei, V. Constantinescu, G Kervalishvili (2026), Joint Analysis with Swarm and Ground Stations: Ionospheric Current System and Geomagnetically Induced Currents, J, Geophys. Res., http://dx.doi.org/10.1029/2025JA034698.
1. Line 347: "There is a decrease in Ne at altitude of 150 - 200 km..."Line 390-391: "An increase in electron density and velocity flow reversal was observed during and shortly after the BBF observation by THEMIS."
There is a direct contradiction of 'decrease/increase' in the interpretation of Figure 6. Please verify the data and correct the text.
2. A Bz enhancement and plasma density compression may provide useful signatures of a dipolarization front associated with a BBF. It will be good to compare these data of THEMIS-A and -E during the BBF to additionally assess whether they observe the same BBF structure.
3. . Line 310-311: "In panels b-f of Figure 5, a transient auroral brightening develops around magnetic latitude of 71°, near the north Alaskan coastline, and propagates westward before disappearing."
Line 313-315: "This auroral feature began before the THEMIS BBF detection and maps poleward of the THEMIS magnetic footpoints, so it is possible that this may be an auroral streamer that was driven by the BBF when it was located further away in the magnetotail, before it encountered the THEMIS satellites."
Line 315-317: "The THEMIS BBF observations show a significant −vy component (Fig. 2a, b) which would correspond to eastward motion of the BBFs ionospheric footpoint and associated auroral streamer for this event, rather than the more common north-south alignment of auroral streamers."
The transient auroral feature is described as propagating westward and is subsequently interpreted as a possible BBF-driven streamer. However, THEMIS observed a −vy component corresponding to eastward motion of the BBF footprint and its associated streamer. Please explain the opposite propagation directions.
4. Line 11: "Multiple FAC signatures were observed that are associated with the BBF, ..."
Line 365-366: "..., the FAC associated with that BBF in low Earth orbit, ..."
The expression 'associated with' should be substantially toned down here. The conclusions appropriately state that the FAC structures 'may be associated' with the BBF and that separating BBF-driven ionospheric variations from the broader substorm dynamics remains an open question. The degree of certainty should be consistent across the abstract, discussion, and conclusions.
5. Line 62-63: "..., the first time that the three distinct components of this coupled system have been analysed with observational data for the same event."
Line 373-374: "This analysis represents the first time that the three distinct components of the coupled BBF-FAC-ionosphere system have been analysed using observational data during the same event."
One of the BBF intervals in Lanabere et al. (2026) titled "Ionospheric currents and auroral signatures during successive earthward bursty bulk flows and dipolarization: a 7 December 2023 case study" is also a study on BBF-FAC-ionosphere system. There are other similar cases like Wei et al. (2021) titled "Intense dB/dt Variations Driven by Near-Earth Bursty Bulk Flows (BBFs): A Case Study" and Zhang et al. (2026) titled "Joint Analysis With Swarm and Ground Stations: Ionospheric Current System and Geomagnetically Induced Currents". The authors should substantially qualify the 'first time' claim and specify precisely which scientific finding is reported here for the first time.
6. The two entries on Nov 03 2023 in Table A1 share nearly identical observation intervals. Please confirm if they may represent multi-spacecraft detections of the same physical BBF. If so, they should be combined into a single item. The two Nov 11 2023 entries have the same issue. Please note that this also affects the reported count of 13 events in Line 9, 368 and 406.
7. Line 378-379: "..., although the overpass occurred slightly later than the estimated propagation time..."
The expression 'slightly later' should be substantially toned down here. The Swarm-B FAC signature was observed approximately 6-9 min after the THEMIS-A BBF detection, about one order of magnitude longer than the estimated 46 s propagation time.
8. Please add a colour bar to Figures 5 and B2 and clarify whether a common intensity scale is used across all panels. If each panel is normalized independently, this should be stated explicitly.
9. Please make the following corrections:
Line 51: 'equivilant' -> 'equivalent'
Line 96: 'phase-array' -> 'phased-array'
Line 116: 'Kenia' -> 'Kenai'
Line 331: 'PSFIR' -> 'PFISR'
Citation: https://doi.org/10.5194/egusphere-2026-3108-RC2 -
AC2: 'Reply on RC2', Harriet George, 24 Sep 2026
We thank the reviewer for their feedback on the manuscript, and have responded to each of their points below.
General omission of Wei and coauthors
We thank the reviewer for bringing this omission to our attention.
Changes in manuscript: We have reevaluated the literature, including the works by Wei and coauthors, and incorporated selected additional references into our introduction section to more fully represent the previous research on this topic. We note that our manuscript focuses on the ionospheric response to BBFs: Zhang (2026) primarily addresses the possible GIC associated with BBFs, which is beyond the scope of this study and so has not been incorporated.
Comment 1
We thank the reviewer for noting this inconsistency: the text was referring to ionospheric dynamics occurring at different altitudes.
Changes in manuscript: we have revised the text at Line 390 - 391 to clarify this point.
Comment 2
We thank the reviewer for this useful suggestion. The possible presence of dipolarisation fronts were evaluated for the THEMIS-A and -E observations of the BBF, but were not identified according to the criteria of Richard (2022). Therefore, the suggested evaluation of dipolarisation front signatures is unfortunately unable to provide further evidence for whether or not the two THEMIS satellites observed the same BBF. We note that only 10% of BBFs contain dipolarisation fronts (Richard, 2022), so the lack of a dipolarisation front embedded in this BBF is not unusual.
Richard, L., Khotyaintsev, Y. V., Graham, D. B., & Russell, C. T. (2022). Are dipolarization fronts a typical feature of magnetotail plasma jets fronts? Geophysical Research Letters, 49, e2022GL101693. https://doi.org/10.1029/2022GL101693
Changes in manuscript: No changes made to the manuscript.
Comment 3
We thank the reviewer for raising this important point. This auroral feature was observed several minutes before the in-situ THEMIS detection, corresponding to the auroral streamer being driven by the “younger” BBF when it was more distant in the magnetotail. It is possible that the dominant propagation of the BBF in the Y direction varied over its lifetime - snaking Earthward rather than propagating directly radially inward. This interpretation is consistent with simulation studies that show significant and temporally varying motion in Y over the lifetime of the BBF, which would necessarily require changes in vy. It is therefore possible that at the time of the auroral streamer, the BBF had dominant motion in the +Y (westward) direction and then twisted during its Earthward propagation to travel in the -Y (eastward) direction by the time it reached the THEMIS satellites. This interpretation of temporal variations in the vy component of the BBF over its lifetime is supported by the THEMIS-A observations: THEMIS-A observed -vy for the first ∼2/3 of the BBF detection, which then rotated to +vy for the final ∼ ⅓ of the BBF.
Changes to manuscript: We have substantially revised the text in Section 4.4 that compares the orientation of the auroral streamer to the THEMIS velocity observations.
Comment 4
We thank the reviewer for this feedback.
Changes in manuscript: we have revised the text throughout the manuscript to ensure consistency in the description of FAC that may be associated with the BBF.
Comment 5
We thank the reviewer for raising this point. This manuscript is indeed complementary to Lanabare (2026): Lanabare (2026) had not been accepted for publication at the time that this manuscript was submitted, so we did not initially incorporate it into this manuscript. Lanabare (2026) evaluated ionospheric current systems from magnetometer data over a relatively broad area (Fennoscandia) during a highly geomagnetically active period with six successive BBFs. By contrast, this manuscript evaluates radar data in the immediate vicinity of a BBFs ionospheric footprint to evaluate the local ionospheric changes during a single, isolated BBF.
The Wei (2021) and Zhang (2026) studies center on the ground-based magnetic field response to BBFs, with Zhang (2026) specifically evaluating the geomagnetically induced currents (GIC) that may be associated with BBFs. This manuscript does not evaluate dBdt data but focuses on evaluating the ionospheric response to BBFs. This manuscript is therefore scientifically distinct from the Wei and Zhang manuscripts despite also addressing cross-region coupling of BBFs.
Changes in manuscript: We have incorporated Lanabare (2026) into the introduction and contrasted our study to Lanabare (2026) to motivate the novelty of this analysis. We have also rephrased the text where relevant when specifying the novelty of the study.
Comment 6
The events on Nov 3 and Nov 11 2023 are indeed two THEMIS spacecraft detecting the same BBF, where the footpoint of the two THEMIS detections are both identified as a triple conjunction according to our algorithm. We report these separately to achieve the 13 triple-conjunctions mentioned throughout our manuscript, which correspond to 11 unique BBFs.
Changes in the manuscript: we have specified throughout the manuscript that the 13 triple conjunctions correspond to 11 unique BBFs.
Comment 7
Changes in manuscript: ‘slightly later’ has been changed to ‘minutes later’.
Comment 8
Changes in manuscript: we have added colourbars to these figures, with a single colourbar (intensity scale) used for all subplots in these figures.
Comment 9
Changes in manuscript: these have been corrected as suggested.
Citation: https://doi.org/10.5194/egusphere-2026-3108-AC2
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AC2: 'Reply on RC2', Harriet George, 24 Sep 2026
Data sets
Conjunctions: BBF-radar and Swarm-radar H. George, A. P. Dimmock, V. Lanabare, H. Vanhamaki, I. Virtanen, and A. Aikio https://doi.org/10.5281/zenodo.20448703
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Bursty bulk flow ionospheric impacts from triple-conjunctive observations
George et al.
This study combines observations from THEMIS, Swarm, the Poker Flat incoherent scatter radar, a co-located all-sky camera, and ground magnetometer stations to study the ionospheric signatures of a bursty bulk flow. Due to the requirement to have conjunctions between so many instruments, very few suitable events were found, and only one was studied in detail. The observations show that it is difficult to determine a one-to-one correspondence between the BBF observed in the tail by THEMIS and ionospheric signatures; rather, the large-scale response of the magnetosphere/ionosphere to the substorm dominates the measurements. Despite this, the paper is a useful and interesting addition to the literature on BBFs and I recommend it for publication in Annales Geophysicae. I have only a minor comment, which is that it would be useful if the paper gave some more emphasis to the MLT of the BBF footprint. This is, I believe, in the dawn sector, so away from the typical onset region of substorms