the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Jul 2025
Ionospheric signatures of Bursty Bulk Flows in the 6D Vlasiator simulations
Abstract. Bursty Bulk Flows (BBFs) are transient plasma flows in the Earth's magnetotail plasma sheet. These short-lived, high-speed flows play a key role in the magnetosphere-ionosphere coupling. Currently, most insights into the ionospheric signatures of BBFs come from individual case studies that include conjugate observations of BBFs in the magnetotail and field-aligned currents (FACs) in the nightside ionosphere. In this study, we utilise the 6D hybrid-Vlasov simulations to study the ionospheric signatures of BBFs in the near-Earth magnetotail. We show that a BBF with Vx ≥ 400 km/s emerges shortly after magnetic reconnection occurs on the duskside at a radial distance between 11 and 14 RE (where RE = 6371 km is the radius of the Earth) in the current sheet. As the BBF moves Earthward, clockwise (counterclockwise) flow vortices are induced on its dawn (dusk) sides. These vortical flows generate FACs flowing upward (out of the current sheet) on the dawnside and downward (into the current sheet) on the duskside flank, respectively. The mapping of BBF structures onto the ionosphere shows that the structure is primarily aligned in the east-west direction, with its ionospheric signatures appearing as enhancements in FACs, ionospheric conductances, horizontal ionospheric currents, energies of precipitating electrons and protons, and the formation of localised plasma flow channels. The upward and downward FACs associated with BBFs in the magnetotail consistently map to enhanced Region 2 (R2) and Region 1 (R1) FAC structures at ionospheric altitude, which are then closed in the ionosphere by north-west flowing Pedersen currents. The ionospheric counterpart of the Earthward plasma flow of the BBF is a channel of equatorward plasma flow, while the westward drift of these enhanced structures corresponds to the duskward motion of the BBF in the magnetotail.
Competing interests: A co-author is a member of the Annales Geophysicae editorial board.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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RC1: 'Comment on egusphere-2025-2282', Anonymous Referee #1, 28 Jul 2025
General comments:
In this paper, numerical simulation results from the 6D hybrid-Vlasov code Vlasiator are utilised to study the ionospheric signatures of a bursty bulk flow (BBF). This is the first time when an ionospheric model enabling a two-way magnetosphere-ionosphere coupling is used with Vlasiator. The magnetospheric signatures of the BBF include a flow channel of earthward fast plasma flow, which has a significant azimuthal orientation at the farther parts, and an appearance of oppositely directed vorticity on the flanks of the BBF channel. The flow vortices/vorticity induces field-aligned currents flowing into the ionosphere on the dawnside and flowing out from the ionosphere on the duskside of the BBF channel. The ionospheric signatures include a localised enhanced equatorward plasma flow corresponding to the earthward part of the magnetospheric BBF and signatures of enhanced FACs and vortical plasma flow on the flanks of the enhanced flow channel consistent with the magnetospheric counterpart. In addition, observable signatures in ionospheric conductances, precipitation energy flux and both Pedersen and Hall currents can be seen in association with the BBF.
This is the first time when ionospheric signatures of BBFs obtained from the Vlasiator are presented. Although the BBF in the simulation is generated rather close to Earth, the simulation produce many signatures reported in previous observational and simulation studies. This suggests that the simulation codes used can be utilised to study dynamical magnetosphere-ionosphere coupling and the results are reasonably well and can be used in comparison with observations. The manuscript is clearly worth publishing, but before I could recommend publication, the authors could address a few comments, which are presented below.
Specific comments:
The authors present only one BBF case. The authors could consider modifying the manuscript title to “Ionospheric signatures of a bursty bulk flow in the 6D Vlasiator simulation”.
When discussing the FAC pair, the authors focus now on the earthwardmost part of the BBF. However, the signatures of weaker upward FAC on the duskside flank and downward FAC on the dawnside flank of the BBF are visible both in the magnetosphere and in the ionosphere also in the tailward (more dusk-dawn oriented) part the BBF (see e.g, the vorticities and FACs in Figure 3). The authors could consider adding some discussion about that.
It seems that the vortical flows on the flanks of the BBF structure do not form complete vortices. The authors might want to point that out in the text.
Line 19 and elsewhere: The use of brackets for the reference is a bit weird: Angelopoulos et al. (1992). Should it be written (Angelopoulos et al., 1992) here? Compare the citation style on line 24.
Lines 37-40: Discussion of Sergeev et al. (2020) on these line is not accurate. Sergeev et al. (2020) do not present any BBF observations in their paper. Also the last statement about the arcs tending to align with the direction of the electric field appears not to be based on Sergeev et al. (2020). Please, check again Sergeev et al. (2020) and rewrite this paragraph if you want to introduce Sergeev et al. (2020) work here.
Figure 3 (and Figure 7e): Is the length of the Vxy vector shown on the top of the Figure 3 on the right hand side of the magenta text “Vx = 400 km/s” 400 km/s?
Line 223: Should the clockwise vortex be indicated by X and counterclockwise by O? Compare e.g. to lines 224-225. Double-check and correct the text if necessary.
Line 305: Do you mean here southwest-directed plasma flow channel in the ionosphere? You write southeast-directed plasma flow channel.
Lines 355-356: On lines 149-151 the authors describe the general evolution of the BBF structure in the magnetosphere. Double-check if your statements on lines 355-356 agree with the description on lines 149-151.
Lines 366-367: Do you mean Figure 7b instead of Figure 7d? The enhanced ionospheric flow channel seems to correspond to the earthwardmost part of the BBF. Maybe specify that on these lines. Actually, the authors could discuss somewhere in section 3.x that one cannot see the enhanced ionospheric flows for the entire magnetospheric BBF channel. Could the authors say anything for the possible reason for that?
Finally, I encourage the authors to continue to carry out studies related to BBFs and their ionospheric signatures using Vlasiator in the future, for instance, when BBFs are observed in the different parts of the magnetotail, such as in the postmidnight region, for comparison. And using different simulation runs.
Citation: https://doi.org/10.5194/egusphere-2025-2282-RC1 -
RC2: 'Comment on egusphere-2025-2282', Anonymous Referee #2, 04 Aug 2025
This paper presents numerical simulation results from the 6D hybrid-Vlasov code Vlasiator, coupled with an electrostatic ionospheric model, to investigate the ionospheric signatures of bursty bulk flows (BBFs). The study's core contribution lies in the detailed mapping of BBF-induced vortical flows to specific field-aligned current (FAC) systems and their associated ionospheric responses. These include enhancements in FACs, ionospheric conductances, and precipitating particle energies. The manuscript is well-written, and the results are clearly presented. The manuscript is clearly worth publishing, but before I could recommend publication, the authors could address a few minor comments, which are listed below.
- The simulation uses steady and extreme solar wind conditions (V_SW = 750 km/s, B_z = −5 nT). How representative are these results of more typical solar wind conditions? Please justify this choice and, importantly, discuss how the results might change under more typical or variable solar wind conditions. For example, would a weaker IMF or lower solar wind velocity still produce such distinct ionospheric signatures?
- The BBF criterion (Vx ≥ 400 km/s) is standard, but duration criteria are not discussed. How long does this BBF persist?
- The study focuses on a single BBF event within the simulation. Is it a typical or ideal BBF produced by the Vlasiator model? Have the authors observed similar signatures for other BBFs in their simulations?
- The duskside preference of BBFs is noted. How does this asymmetry influence the ionospheric signatures compared to dawnside BBFs?
- The simulation assumes a 0° dipole tilt, which is a simplification. Could including a more realistic dipole tilt affect ionospheric coupling?
6. The thin-shell approximation at 100 km altitude is justified, but how might altitude-dependent effects influence the results?
7. A recent study by Kumar et al. (2025) (https://doi.org/10.1029/2024JA032953), using THEMIS and MMS observations, reported a dawn–dusk asymmetry in flows and a significant deceleration of these flows earthward of X < –15 RE. Please comment on whether the Vlasiator simulation reproduces a comparable braking effect in the near-Earth region.
8. On page 2, line 45, "breaking region" should likely be "braking region". Please check for this consistency throughout the manuscript.
9. Line 199-200: At t = 450 s, a BBF is active, yet the paper says there are “no significant ionospheric signatures.” What is the expected delay between BBF arrival and ionospheric response in this setup?
10. The use of Bz = 0 as a proxy for reconnection lines is an oversimplification, as it does not inherently confirm the presence of active reconnection. Could the authors elaborate on how additional reconnection indicators—such as flow reversals, Hall magnetic field signatures, or localized energy conversion— align with the Bz = 0 regions in their simulation?
Citation: https://doi.org/10.5194/egusphere-2025-2282-RC2
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