# Identifying Magnetotail Jet Front Signatures in a 3D + 3V Global Hybrid-Vlasov Simulation

L. Pänkäläinen, G. Cozzani, M. Battarbee, U. Ganse, Y Pfau-kempf, J. Suni, M. Palmroth

► To cite this version:

L. Pänkäläinen, G. Cozzani, M. Battarbee, U. Ganse, Y Pfau-kempf, et al. Identifying Magnetotail Jet Front Signatures in a 3D + 3V Global Hybrid-Vlasov Simulation. Journal of Geophysical Research Space Physics, 2025, 130 (6), 10.1029/2025JA033892. insu-05121795

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Distributed under a Creative Commons Attribution 4.0 International License
# JGR Space Physics

## RESEARCH ARTICLE

10.1029/2025JA033892

**Key Points:**

* We investigate bursty bulk flows (BBFs) and dipolarization fronts (DFs) in the magnetotail of a global hybrid-Vlasov simulation

* Earthward DFs occur alongside BBFs, while anti-dipolarized magnetic field signatures are associated with tailward-propagating flux ropes

* Virtual satellite time series show velocity increases compatible with BBF criteria which are caused by X-line and current sheet motion

**Supporting Information:**

Supporting Information may be found in the online version of this article.

**Correspondence to:**

L. Pänkäläinen,
*lauri.pankalainen@helsinki.fi*

**Citation:**

Pänkäläinen, L., Cozzani, G., Battarbee, M., Ganse, U., Pfau-Kempf, Y., Suni, J., & Palmroth, M. (2025). Identifying magnetotail jet front signatures in a 3D + 3V global hybrid-Vlasov simulation. *Journal of Geophysical Research: Space Physics*, 130, e2025JA033892. https://doi.org/10.1029/2025JA033892

Received 3 MAR 2025
Accepted 20 MAY 2025

**Author Contributions:**

**Conceptualization:** L. Pänkäläinen,

G. Cozzani, M. Battarbee

**Data curation:** M. Battarbee, Y. Pfau-Kempf, J. Suni

**Formal analysis:** L. Pänkäläinen

**Funding acquisition:** L. Pänkäläinen, Y. Pfau-Kempf, M. Palmroth

**Investigation:** L. Pänkäläinen,

M. Battarbee, U. Ganse, Y. Pfau-Kempf, J. Suni

**Methodology:** L. Pänkäläinen, G. Cozzani, M. Battarbee

**Project administration:** M. Palmroth

**Resources:** M. Palmroth

**Software:** L. Pänkäläinen, M. Battarbee, U. Ganse, Y. Pfau-Kempf, J. Suni

©2025. The Author(s).

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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## Identifying Magnetotail Jet Front Signatures in a 3D + 3V Global Hybrid-Vlasov Simulation

L. Pänkäläinen<fnref n="1" /> ID, G. Cozzani<fnref n="1" />,² ID, M. Battarbee<fnref n="1" /> ID, U. Ganse<fnref n="1" /> ID, Y. Pfau-Kempf<fnref n="1" /> ID, J. Suni<fnref n="1" /> ID, and M. Palmroth<fnref n="1" />,³ ID

**Abstract** Magnetic reconnection in Earth's magnetotail is thought to create bursty bulk flows (BBFs), short-lived plasma bulk velocity enhancements in the magnetotail's central plasma sheet (CPS) region. Closely related to BBFs are dipolarization fronts (DFs), sudden increases in $B_z$, the magnetic field component aligned with Earth's magnetic dipole axis. BBFs are involved in, for example, transport of energy and mass, and DFs contribute to for instance energy conversion processes and plasma acceleration. Both phenomena increase magnetic flux transport in the magnetotail. We demonstrate novel methods of identifying BBFs and DFs in 3D global magnetospheric simulations and present results for multiple case studies. We search for BBFs and DFs in a simulation conducted using the 3D global magnetospheric hybrid-Vlasov code Vlasiator. DFs are identified using a $B_z$ time derivative threshold, whereas BBFs are defined based on a velocity threshold. The tailward DF (anti-DF) identification criteria are fulfilled by tailward-propagating flux ropes, while earthward-propagating DFs are mostly seen in finger-like structures of high earthward bulk velocity alongside BBFs. We show that detections of fast flows meeting the BBF criteria in virtual spacecraft time series also originate due to moving reconnection locations and movement of the current sheet within the CPS region, while the reconnection outflow stays roughly constant. The results show that rapid $B_z$ variations in the simulation have multiple sources, and similar satellite measurements of BBFs can arise from different physical phenomena. Our findings may help with interpreting satellite observations in the magnetotail.

**Plain Language Summary** A stream of charged particles called the solar wind is constantly originating from the Sun, and it stretches Earth's magnetic field to form a long magnetotail on Earth's nightside. A thin current sheet forms in the magnetotail between the oppositely directed magnetic fields. The current sheet hosts a process named magnetic reconnection, which converts energy stored in magnetic fields to plasma heat, particle acceleration and plasma kinetic energy. This is thought to create short-lived increases in plasma velocity in the magnetotail called bursty bulk flows (BBFs). The velocity increases are often linked to sudden jumps in magnetic field strength called dipolarization fronts (DFs). We identify and study the properties of BBFs and DFs using a large-scale computer simulation of the solar wind—Earth system. We find that DFs moving toward Earth happen due to BBFs, and twisted magnetic field structures cause sharp magnetic field variations moving tailward. We also find that simulated satellite measurements can show signs of BBFs even when the overall velocity stays constant, if the location of the magnetic reconnection shifts. This can also happen due to flapping motion of the current sheet. The results may help with analyzing satellite measurements of the magnetotail.

## 1. Introduction

Earth's magnetotail, the stretched nightside of Earth's magnetosphere, hosts numerous dynamic plasma processes. One of the much researched phenomena in the magnetotail is bursty bulk flows (BBFs), transient plasma flows in the central plasma sheet (CPS) region. BBFs are seen in satellite observations as short periods of enhanced plasma bulk velocity lasting from a few seconds to several minutes (Baumjohann et al., 1990). The flow bursts are often found together, in time periods spanning up to 10 min length (Angelopoulos et al., 1992) and they have been strongly associated with magnetic reconnection outflows (e.g., Ma et al., 2009). BBFs have been interpreted to be plasma bubbles which have a lower entropy and density compared to their surroundings, propagating earthward due to buoyancy forces (see the review by Wolf et al., 2009). In global MHD simulations of Earth's magnetosphere, BBFs appear as narrow, finger-like structures originating from magnetic reconnection sites and extending earthward (e.g., Merkin et al., 2019).

<footnote n="1">¹Department of Physics, University of Helsinki, Helsinki, Finland, ²LPC2E, CNRS, CNES, OSUC, University of Orléans, Orléans, France, ³Space and Earth Observation Centre, Finnish Meteorological Institute, Helsinki, Finland</footnote>
**Supervision:** G. Cozzani, M. Battarbee,
M. Palmroth

**Validation:** L. Pänkäläinen

**Visualization:** L. Pänkäläinen

**Writing – original draft:** L. Pänkäläinen

**Writing – review & editing:**
L. Pänkäläinen, G. Cozzani, M. Battarbee,
U. Ganse, Y. Pfau-Kempf, J. Suni,
M. Palmroth

BBFs have often been associated with dipolarization fronts (DFs), ion-scale structures characterized by a sharp increase in the z-component of the magnetic field in the Geocentric Solar Magnetic (GSM) coordinate system (e.g., Runov et al., 2009; Runov, Angelopoulos, Sitnov, et al., 2011). In spacecraft observations, the increase in $B_z$ is often preceded by a small dip (Ohtani et al., 2004) and it is succeeded by a dipolarizing flux bundle, a region of enhanced $B_z$ (Liu, Angelopoulos, Runov, & Zhou, 2013; Runov, Angelopoulos, Sitnov, et al., 2011). Typically, $B_z$ gradually returns to the ambient value within a minute (Liu, Angelopoulos, Runov, & Zhou, 2013). Recent research suggests that DFs can be divided into two distinct classes: class I with a slower return to the ambient value and a higher plasma velocity and class II with a faster $B_z$ decrease and a lower plasma velocity (Alqeeq et al., 2023).

DFs are often considered to be the leading edge of BBFs, and both event types are associated with an increase in density followed by particle depletion as well as an increase in temperature (Ohtani et al., 2004; Runov, Angelopoulos, Zhou, et al., 2011). These features as well as the increase in $B_z$ characteristic of DFs are predicted by the BBF plasma bubble model (Chen & Wolf, 1993). A recent observation of a dipolarization front by Hosner et al. (2024) was interpreted as a duskward-bent flux tube, consistent with the plasma bubble interpretation. However, the tight connection between BBFs and DFs has been questioned by for instance Richard et al. (2022), who found that 42% of BBF edges had strong $B_z$ variations and only 10% had solitary dipolarization fronts associated with them. Their data set consisted of 5 years of MMS (Magnetospheric Multiscale) data. DF-like $B_z$ signatures can also be produced by earthward-propagating flux ropes. Slavin et al. (2003) showed that magnetic reconnection produces such flux ropes embedded in high-velocity flow around O-lines, magnetic islands forming between X-lines. While the $B_z$ profile of a flux rope is quasi-sinusoidal, they hypothesized that earthward-propagating flux ropes reconnect with Earth's magnetic field lines to produce DF-like $B_z$ signatures, which was later demonstrated in simulations (Lu et al., 2015; Sitnov et al., 2009). Flux ropes with $B_z$ profiles practically indistinguishable from regular DFs have been detected by, for example, Vogiatzis et al. (2015). DFs have also been explained for instance through the kinetic ballooning/interchange instability (Pritchett et al., 2014). Hence, strong $B_z$ variations may not always be associated with BBFs.

The spatial extent of the magnetic and velocity structures of DFs and BBFs has been estimated in observational studies, and the general outcome is that they have similar sizes. Based on Cluster observations, Nakamura et al. (2004) estimated earthward BBFs to have a dawn-dusk extent of $2-3 R_E$ (Earth radii) and a north-south extent of $1.5-2 R_E$. Analysis of 506 DF events by Liu, Angelopoulos, Zhou, et al. (2013) reached the conclusion that DFs have a semicircular transverse ($YZ_{GSM}$) cross section with a radius typically below $1.5 R_E$. The sagittal ($XZ_{GSM}$) cross section is concave, resulting in a saddle-shaped DF structure (Liu, Angelopoulos, Runov, & Zhou, 2013). Despite their limited sizes and temporal extents, both BBFs and DFs are important for magnetotail dynamics: BBFs are responsible for the majority of the earthward transport of energy, magnetic flux and mass in the regions with the highest occurrence rates (close to the midnight sector, $15-22 R_E$ from Earth) (Angelopoulos et al., 1994), and DFs are involved in, for example, magnetic flux transport (Liu et al., 2014), energy conversion processes (Angelopoulos et al., 2013) and particle acceleration (Zhou et al., 2014). Most studies concerning BBFs and DFs have focused on earthward events, as they influence Earth's magnetosphere more directly due to the earthward transport of energy and magnetic flux. Earthward events are also more prevalent in some satellite observations (e.g., seen in the data set used by Richard et al., 2022). This is at least partially due to spacecraft orbit design, as for instance the MMS mission has an apogee near the expected reconnection sites where the flow changes from tailward to earthward (Burch et al., 2016).

While single- and multi-spacecraft observations have successfully been used to describe the characteristics of BBFs and DFs, including size and shape estimations, most in situ measurements are limited by the coverage of satellite orbits as well as lack of access to cross-scale or proper multi-scale measurements. This often leaves the physical context up for interpretation as temporal and spatial variations cannot be easily separated, especially in the case of single spacecraft observations or tetrahedral constellations with electron-scale interspacecraft separation. Global simulations are thus an important tool complementing the investigation of these processes.

This study concerns identifying BBFs and DFs and examining their properties in the magnetotail of a 3D global hybrid-Vlasov simulation. The coupled solar wind—magnetosphere—ionosphere system of Earth is simulated using the Vlasiator numerical code, which includes ion kinetic effects not present in MHD simulations and avoids the statistical noise inherent to hybrid-PIC models by directly evolving the ion distribution functions instead of
using representative macroparticles (Palmroth et al., 2018). We develop methods to locate both earthward and tailward BBF and DF events suitable for direct comparison with observations.

Our results show that transient, fast plasma bulk flows reminiscent of BBFs in satellite view originate also from dynamics of the magnetotail on a larger scale, such as movement of X-lines and flapping motion of the current sheet inside the CPS, without the presence of narrow flow channels. These detections correspond to measurements of high-velocity flows from different angles and locations. They might explain unexpected features in data sets using automatic BBF selection, where the physical context of each event is unavailable due to the combination of the sheer number of different events and the limited view of satellite missions.

The structure of the article is as follows: We describe the Vlasiator simulation model in Section 2, and present the identification methods for BBFs and DFs in Section 3. Section 4 concerns the results, including the state of the magnetotail in the simulation as well as closer inspections of two DF (Section 4.1) and two BBF case studies (Section 4.2). We discuss the results and their significance in Section 5 and summarize them in Section 6.

## 2. Simulation Description

We study the magnetotail using Vlasiator (Ganse et al., 2023; Palmroth et al., 2018), a global hybrid-Vlasov simulation code which calculates ion dynamics through directly solving the Vlasov equation. Electrons are treated as a cold, massless charge-neutralizing fluid. Electric and magnetic fields are derived from Maxwell's laws under the Darwin approximation to prevent purely electromagnetic wave modes (Schmitz & Grauer, 2006). The system of equations is brought into a closed form by using the generalized Ohm's law including the Hall term $(\mathbf{J} \times \mathbf{B})/e_n$ and the electron pressure gradient term $(\nabla \cdot \mathbf{P}_e)/e_n$, where $\mathbf{J}$ is the current density, $\mathbf{B}$ the magnetic flux density, $e$ the elementary charge, $n_e$ the electron number density and $\mathbf{P}_e$ the electron pressure tensor. The electron pressure is adiabatic, and resistivity and electron inertial effects are assumed to be negligible.

Earth's magnetic field is modeled as a dipole with the axis aligned with the $Z_{GSE}$ direction, which makes the GSM and GSE (Geocentric Solar Ecliptic) coordinate systems interchangeable within the simulation. The coordinates in this article are expressed in the GSM coordinate system unless specified otherwise. The dipole strength corresponds to the realistic strength of the geomagnetic field. The simulation domain is a rectangular box of extents $X \in [-110.5, 50.2] R_E$, $Y \in [-57.8, 57.8] R_E$ and $Z \in [-57.8, 57.8] R_E$. The solar wind within the simulation is initialized to be consistent with fast solar wind conditions, with a speed of 750 km/s in the negative x-direction and a temperature of 0.5 MK. The solar wind ions are assumed to consist of exclusively protons with a density of $n_p = 1 \text{ cm}^{-3}$. The interplanetary magnetic field is purely southward with a magnitude of 5 nT. The solar wind is therefore super-Alfvénic with an Alfvén Mach number of $M_A = 6.9$, and it has a dynamic pressure of $P_{dyn} \approx 0.9 \text{ nPa}$ and a plasma beta of $\beta \approx 0.7$.

The simulation contains three separate numerical grids. The ion distribution function is evaluated on the Vlasov grid, a Cartesian grid with three spatial and velocity dimensions (3D + 3V). In order to save computational resources, Adaptive Mesh Refinement with a fixed grid refinement is used to lower the spatial resolution of simulation cells with less scientific importance and simpler dynamics (Ganse et al., 2023). The cells with the highest levels of refinement have a grid spacing of $\Delta x = 1000 \text{ km}$ and they are concentrated on the magnetotail current sheet and the magnetopause region. The most refined grid spacing is comparable to the average ion inertial length in the simulated magnetotail, $d_i \approx 900 \text{ km}$. The lowest spatial resolution found, for example, in the solar wind is 8,000 km. Further optimizations are achieved using a sparse velocity space (von Alfthan et al., 2014). Electromagnetic fields are processed on a field solver grid operating on the highest refinement level (Papadakis et al., 2022) and propagated using the constrained transport method by Londrillo and Del Zanna (2004). A recent addition to the simulation model is an ionospheric solver operating on a third, triangular mesh grid on a spherical shell (Ganse et al., 2025). The ionosphere grid is coupled to a spherical inner boundary with a radius of $4.7 R_E$, enveloping Earth. The inflow boundary at $X = 50.2 R_E$ is set to have values equal to the solar wind conditions. The outflow boundaries at the other outer limits are homogeneous Neumann boundaries, where derivatives are set to zero and the values are copied from neighboring cells. The simulation state is exported to an output file format once per second.

Initial conditions in the simulation include Earth's dipole and the previously discussed values of the solar wind parameters. Earth's magnetosphere is formed by the simulation during an initialization period. Therefore the simulation results are comparable to the realistic conditions in the magnetotail only after the magnetotail has
formed and reconnection has initiated. To ensure that the magnetotail has had enough time to form, the analysis is restricted to the final 500 saved time steps, corresponding to the time interval of $t \in [1113, 1612]$ s.

3D + 3V Vlasiator simulations have previously been used to study for instance magnetotail plasma eruptions (Palmroth et al., 2023), auroral proton precipitation (Grandin et al., 2023), the magnetospheric response to a pressure pulse (Horaites et al., 2023) and dayside ULF waves (Tesema et al., 2024). Analytical asymmetric magnetic reconnection rates have been reproduced in 2D + 3V Vlasiator simulations (Hoilijoki et al., 2017).

### 3. Identification Methods

Several methods have been developed to identify DFs and BBFs from in situ measurements (e.g., the methods discussed by Angelopoulos et al. (1999) and Fu et al. (2012), to name a few). However, these methods are designed to identify events using spacecraft time series. Utilizing the capabilities of global magnetospheric simulations, we have developed novel methods that are suitable for use with a 3D data set and allow for direct comparison with observations. BBFs and DFs are searched for in the vicinity of the current sheet, where magnetotail reconnection is expected to occur. The surface of the current sheet center is found using the proxy $B_x = 0$ with magnetic field values between the grid cells calculated using radial basis function interpolation. The same interpolation technique is used to estimate quantities on the $B_x = 0$ surface. X- and O-lines, which give important information about magnetic reconnection sites and magnetic field topology, are found using the methodology developed by Alho et al. (2024).

Identification criteria for dipolarization fronts are adapted from Liu, Angelopoulos, Runov, and Zhou (2013) and Li et al. (2014). DFs are identified in the Vlasiator simulation data as areas where the $V_x$ component of the plasma velocity is positive and $dB_z/dt > 0.35$ nT/s (earthward fronts) or where $V_x < 0$ and $dB_z/dt < -0.35$ nT/s (tailward or anti-dipolarization fronts). The threshold originally used by Liu, Angelopoulos, Runov, and Zhou (2013) is slightly higher (+0.5 nT/s), but it was found to be too restrictive for the simulation data. $dB_z/dt$ is calculated using a second-order finite central difference at the interpolated current sheet position, which can smooth peaks unless the current sheet aligns exactly with a grid position. The choice for the threshold is discussed further in Section 5.

The DF event selection method used by Liu, Angelopoulos, Runov, and Zhou (2013) includes additional criteria to remove noise and to only select the first front when multiple events are detected in quick succession. However, due to the time period consisting of 500 output files and the global view offered by Vlasiator, all events matching the criteria can be analyzed individually. Additional criteria for preventing noise or separating fronts are therefore not needed.

BBFs in the simulation are selected with criteria adapted from Richard et al. (2022). To be classified as a BBF, an event must have a peak bulk velocity of $|V_x| > 300$ km/s. The boundaries of the event are located at the points where $|V_x| = 100$ km/s. The average magnetotail lobe velocity is subtracted from the velocity values; this is explained further in Section 4. The studied volume is limited by $|Y| < 12 R_E$ in order to avoid magnetosheath velocity signatures. Furthermore, only the events intersecting with the current sheet are studied to ensure that they are within the CPS. Explicit criteria depending on the ion plasma beta $\beta_i$ (the ratio between plasma and magnetic pressures), which is often used to position spacecraft in the CPS, are therefore not needed.

Figure 1 shows the state of the magnetotail at $t = 1500$ s. Each of the three panels displays the x-component of the proton bulk velocity with the lobe velocity subtracted ("lobe-relative bulk velocity"), $V'_x$, inside $X \in [-30, -5] R_E$, $|Y| < 12.5 R_E$ from a different perspective. In panel (a), the bulk velocity is plotted along the $B_x = 0$ isosurface, which corresponds to the approximate location of the center of the magnetotail current sheet. Panels (b) and (c) contain 2D mesh plots of the bulk velocity along with contours corresponding to BBF peak velocity thresholds of $|V'_x| = 300$ km/s as well as X- and O-lines projected onto the XY-plane. In panel (b) the velocity is plotted along the equatorial plane $Z = 0$, while in panel (c) the velocities correspond to a 2D projection of the values along the $B_x = 0$ isosurface from panel (a). As visible in panel (a), the current sheet center (corresponding to the $B_x = 0$ isosurface) is perturbed by waves that cause it to flap. In some regions, these disturbances can lead to the current sheet folding onto itself, creating multiple layers with $B_x = 0$. When this occurs, only the northernmost layer is shown in the projection displayed in panel (c). Potential BBFs are identified via visual inspection from the current sheet, using plots following the format of panels (a) and (c).
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Lobe-relative proton bulk velocity $V'_x$ along (a) the $B_x = 0$ isosurface, (b) the $Z = 0$ plane and (c) 2D projection of the $B_x = 0$ isosurface viewed from the north. Blue ($V'_x = 300$ km/s) and red ($V'_x = -300$ km/s) contours in panels (b) and (c) correspond to bursty bulk flow peak thresholds. X- and O-lines projected onto the XY-plane are shown in magenta and green, respectively.</figcaption></figure>

The visual inspection focuses on areas where sources of BBF signatures are expected to be found. The magnetotail is searched for locations where the magnitude of $V'_x$ increases above the BBF peak threshold or decreases below the boundary threshold. As seen from Figure 1, $|V'_x| > 300$ km/s for most of the $B_x = 0$ isosurface. Locations with $|V'_x| < 100$ km/s are found near the major X-line located at around $X = -15 R_E$ and away from the $B_x = 0$ isosurface, which is showcased by the fact that low velocities far from the primary X-line are found in panel (b) of Figure 1 but not in panel (c). As the plot in panel (c) follows the shape of the current sheet but the plot in panel (b) does not, the lower velocities visible in panel (b) occur due to an increased distance to the center of the current sheet.

We therefore posit that BBF signatures are found in places the isosurface passes through due to flapping of the current sheet, bringing high-velocity flow with it, and in places where the velocity contours change near the X-lines, possibly due to the motion of an X-line. Apparent velocity increases due to current sheet flapping are searched for in areas where the Z-coordinate of the $B_x = 0$ isosurface seems to change quickly in plots following the format of panel (a) in Figure 1. Temporary flow enhancements due to X-line motion are expected to occur near X-lines that exhibit back-and-forth motion along the x-direction in plots following the format of panel (c).

Promising locations are inspected further using Virtual Spacecraft (VSC), which return time series of different variables at a given location. Data from multiple virtual spacecraft along a straight line can be studied at once by
combining the time series into a “tower plot,” where the vertical axis represents the location of each VSC and time is shown on the horizontal axis. We utilize VSC towers aligning with the z-direction to capture the north-south extent and characteristics of BBF and DF events.

## 4. Results

Reconnecting X-lines on the current sheet can be found throughout the studied time interval, typically between $X = -10 R_E$ and $X = -20 R_E$. Figures 1b and 1c have a primary reconnecting X-line at $X \approx -17 R_E$ extending from $Y \approx -11 R_E$ to $Y \approx 7 R_E$. The active reconnection is evidenced by the diverging plasma flow on different sides of the X-line. As a result of the ongoing magnetic reconnection, the current sheet constantly exhibits flows with $|V'_x| > 1000$ km/s. The flow velocity in the tail lobes is also significant, and averages $V_x \approx -200$ km/s over the studied time period. As the magnitude of the flow velocity is greater than that of the tailward BBF boundary threshold ($|V'_x| = 100$ km/s), the background flow is removed to avoid contaminating BBF results.

The current sheet exhibits flapping motion, which is visible in Figure 1a. Current sheet flapping has also been present in earlier Vlasiator simulations and has been studied by Zaitsev et al. (2025) and Cozzani et al. (2025). The amplitude of the motion in the z-direction increases with distance from Earth and it often exceeds $1 R_E$ when $X < -20 R_E$. The intense kinking occasionally causes the CS to fold, leading to the sheet occupying multiple Z-coordinates at the same $(X, Y)$ location in the magnetotail. The high plasma velocities created by magnetic reconnection follow the kinking of the CS. As a result, an equatorial cut of the magnetotail shows lower velocities at locations where the flapping motion causes the sheet to deviate from the $Z = 0$ plane. The flow is more uniform along the CS, which can be seen from Figure 1: The red and blue velocity contours form multiple separate structures in panel (b), which are not visible in panel (c). Analyzing the magnetotail using planar cuts along the XY-plane can therefore lead to misleading conclusions regarding relevant dynamical structures and their locations. Consequently, for the rest of the study we will only show projections of the current sheet surface instead of planar cuts.

As the magnetotail conditions are quite dynamic, presence of BBFs and DFs is expected. In the following sections we will identify and analyze case studies of both event types using the methodologies described in Section 3. Statistical analysis of all the detected events will be the subject of a follow-up study.

### 4.1. Dipolarization Front Case Studies

More than 25 events match the dipolarization front criteria in the simulated magnetotail. We present two of the events as case studies, each representative of a different kind of front seen in the simulation. The first case study is an earthward DF located at the edge of a finger-like structure of enhanced velocity along the current sheet, while the second one is a tailward-propagating flux rope that forms between two X-lines and contains $B_z$ variations sharp enough to meet the anti-DF identification criteria.

The first event is shown from two different perspectives. The context along the current sheet is shown in Figure 2, which contains a plot of the lobe-relative proton bulk velocity $V'_x$ along the $B_x = 0$ isosurface, similarly to panel (c) in Figure 1. The areas where the DF threshold $dB_z/dt = 0.35$ nT/s is exceeded, marked with a black outline, coincide with the edges of narrow high-velocity structures. In order to capture the extent and shape of the event across the current sheet and compare it to observational studies, we utilize a tower of virtual spacecraft located at the same X- and Y-coordinates but with different Z-coordinates, covering the entire thickness of the current sheet. The tower is placed at $(X, Y) = (-8.3, 3.9) R_E$ (shown using a blue cross in Figure 2), as the DF passes through that location at $t \approx 1240$ s.

Time series from the VSC tower are shown using tower plots in Figure 3. Each row in panels (a), (c), (e), and (g) corresponds to the time series measured by one virtual spacecraft. Three time series from the VSC tower are chosen to be plotted in panels (b), (d), (f) and (h) in order to make interpreting the results more intuitive. The Z-coordinates of the chosen VSCs are marked with colored arrows in panels (a), (c), (e) and (g). The individual time series are equivalent to in situ spacecraft time series.

The event meets the dipolarization front threshold and is first visible on the current sheet at $t = 1236$ s at $(X, Y) \approx (-8.4) R_E$. The front advances roughly earthward until disappearing from the current sheet view at $t = 1254$ s close to $X = -7.5 R_E$. The tower plots of the magnetic field in panel (a) of Figure 3 show clear signs of a concave shape in the XZ-plane, in agreement with the description by Liu, Angelopoulos, Runov, and
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Lobe-relative proton bulk velocity $V'_x$, shown along the 2D projection of the $B_x = 0$ isosurface. Areas inside the black contours surpass the dipolarization front threshold of $dB_z/dt = 0.35$ nT/s. Location of the VSC tower in Figure 3 is marked by a blue cross.</figcaption></figure>

Zhou (2013). However, the dipolarization front threshold of 0.35 nT/s is only surpassed near the center inside a convex region.

The change in the magnetic field seen in panel (a) is accompanied by an increase in earthward velocity in panel (c). The density at the location of the highest $dB_z/dt$ increases at the arrival of the front. After the passing of the front the density decreases to a value below the ambient current sheet density. Bulk velocity at the same location increases over 300 km/s. These features are in good agreement with in situ measurements of BBFs and DFs (Ohtani et al., 2004). However, as the proton velocity is already above 300 km/s before the arrival of the front, the dipolarization is not associated with an edge of a BBF, although the flow as a whole could be classified as a BBF. Panels (g) and (h) serve to illustrate the fact that as the $B_x$ values stay roughly constant, the current sheet does not exhibit major flapping motion during the event.

The individual VSC time series confirm the conclusions made based on the tower plots. All virtual spacecraft detect an increase in the magnetic field, but VSCs away from the center of the front (in red and green) detect the increase earlier than the central virtual spacecraft (in black). The same pattern was also visible in the time series for $V'_x$, though less clearly. All three VSCs detect a peak in the number density before a significant decrease. With every studied

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Virtual spacecraft plots from $(X, Y) = (-8.3, 3.9) R_E$ of the z-component of the magnetic field $B_z$ (panels (a) and (b)), lobe-relative bulk velocity $V'_x$ (panels (c) and (d)), proton number density $n_p$ (panels (e) and (f)) and $B_x$ (panels (g) and (h)), showing an earthward dipolarization front. The panels on the left are tower plots, where each vertical row corresponds to a VSC with a different Z-coordinate. The red ($Z = 1.0 R_E$), green ($Z = 0.5 R_E$) and black ($Z = 0.1 R_E$) arrows correspond to locations of the individual VSC time series on the plots on the right. The lines in panels (a) and (c) mark the locations of $dB_z/dt$ and $V'_x$ contours, respectively, with the solid lines corresponding to DF and bursty bulk flow (BBF) peak thresholds. The dashed lines in panel (c) signify BBF boundary thresholds and in panel (a) serve as visual aid. The lack of a $dB_z/dt = -0.35$ nT/s contour in panel (a) implies $dB_z/dt > -0.35$ nT/s for all VSCs and time steps.</figcaption></figure>
<figure><img src="image_6.png" /></figure>

**Figure 4.** Virtual spacecraft plots from (X, Y) = (-16.9, -8.6) $R_E$ of the z-component of the magnetic field $B_z$ (panels (a) and (b)), lobe-relative sunward bulk velocity $V'_x$ (panels (c) and (d)), proton number density $n_p$ (panels (e) and (f)) and $B_x$ (panels (g) and (h)), showing a tailward event matching the DF criteria. The panels on the left are tower plots, where each vertical row corresponds to a VSC with a different Z-coordinate. The red ($Z$ = 1.0 $R_E$), green ($Z$ = 0.7 $R_E$) and black ($Z$ = 0.4 $R_E$) arrows correspond to locations of the individual VSC time series on the plots on the right. Green dots mark the locations of O-lines. The lines in panels (a) and (c) mark the locations of $dB_z/dt$ and $V'_x$ contours, respectively, with the solid lines corresponding to DF and bursty bulk flow (BBF) peak thresholds. The dashed lines in panel (c) signify BBF boundary thresholds and in panel (a) serve as visual aid. The lack of a $dB_z/dt$ = 0.35 nT/s contour in panel (a) implies $dB_z/dt < 0.35$ nT/s for all VSCs and time steps.

variable, the strength of the features decreases with distance from the center of the current sheet, but the detections are made earlier due to the convex shape of the front. The z-component of the magnetic field decreases slightly before the arrival of the front, but not strongly enough to resemble the dip seen in situ studies (Fu et al., 2012). The increase in $B_z$ is accompanied by a ~1 keV increase in temperature (not shown).

The second case study concerns a typical example of a tailward event seen in the simulation. The event emerges around $t$ ≈ 1490 s near (X, Y) = (-15, -10) $R_E$. It reaches $X$ = -30 $R_E$ at $t$ = 1598 s, exiting the studied volume. The event is connected to an O-line throughout its entire lifetime. Figure 4 shows tower plots and singular VSC observations of $B_z$, $V'_x$, $n_p$ and $B_x$ from (X, Y) = (-16.9, -8.6) $R_E$, following the format of Figure 3. The chosen coordinates correspond to the location the front passes through at $t$ ≈ 1511 s. As seen from the tower plot of $B_z$ in Figure 4a, the dipolarization front threshold is surpassed in a convex region extending approximately 1 $R_E$ in the z-direction. The bulk velocity in Figure 4c deviates from the background value inside a region with the same Z-extent. Compared to the bulk velocity, the number density exceeds the background value within a narrower strip in Figure 4e. The lack of time-dependent features of $B_x$ in Figure 4g is a sign of little to no flapping motion of the magnetotail during the measurements.

In panels 4(a) and 4(b), small increases in $B_z$ can be seen immediately before and after the rapid decrease that surpasses -0.35 nT/s. The magnetic field returns close to the pre-event state after the second increase. The profile is therefore quasi-sinusoidal and thus more flux-rope-like than dipolarization-front-like (Runov et al., 2009; Slavin et al., 2003). The decrease in $B_z$ corresponds to increases in magnitude in $V'_x$ and $n_p$, most clearly seen by the green and black VSCs in panels (d) and (f). The density in the middle of the current sheet (detected by the VSC
<figure><img src="image_7.png" /><figcaption>**Figure 5.** A cut of the magnetotail along the YZ-plane at *t* = 1435 s (panel (a)) and *t* = 1475 s (panel (b)), showcasing current sheet flapping increasing the velocity detected by virtual spacecraft. Lobe-relative proton bulk velocity in the *x*-direction (*V*'<sub>*x*</sub>) is shown in color. The tailward bursty bulk flow peak threshold of −300 km/s is marked by a red contour. The light green line shows the current sheet proxy *B*<sub>*x*</sub> = 0. Locations of three virtual spacecraft are shown as red, green, and black crosses, corresponding to the similarly colored arrows in Figure 6.</figcaption></figure>

shown in black in panel (f)) increases from $n_p \approx 0.09 \text{ cm}^{-3}$ to $n_p \approx 0.20 \text{ cm}^{-3}$ and returns back to the ambient value after the front has passed. The velocity signatures in panel (d) vary depending on the Z-coordinate, but the magnitude of $V'_x$ along the current sheet stays above 600 km/s after $B_z$ and $n_p$ have returned to their pre-front values. The magnetic field change happens over a larger z-extent than the changes in $V'_x$ and $n_p$, as the red VSC (at $Z = 1.0 R_E$) detects the same quasi-sinusoidal pattern in $B_z$ in panel (b), but the velocity and density values remain relatively constant in panels (d) and (f).

## 4.2. BBF Case Studies

The dipolarization front event presented in Figures 2 and 3 also exhibits many of the features expected of BBFs, such as a transient increase in density followed by a sharp decrease (Ohtani et al., 2004) and a narrow flow channel, seen as the finger-like structure of high velocity visible in Figure 2. Apart from the velocity constantly above the BBF thresholds (exacerbated by the removing of the background velocity), the event matches the typical description of a BBF well. We therefore treat the event as a validation case showing that Vlasiator is capable of reproducing BBFs seen in observations.

In addition to the signatures originating from finger-like structures and flux ropes, BBF-like velocity variations in the simulated magnetotail are also found for instance near reconnecting X-lines and when moving from the current sheet to the magnetic lobes. Therefore we posit that moving X-lines and significant current sheet flapping can function as sources of BBF signatures that differ from the typical model of BBFs. We present two different sets of measurements to demonstrate the origins of the alternative BBF detections.

As the magnitude of the bulk velocity is constantly higher on the CS compared to the tail lobes, the shape of the current sheet determines where high-velocity flows can be detected. This is demonstrated in Figure 5 in which the flapping motion is investigated along the YZ-plane at $X = -29 R_E$ at $t = 1435$ s (panel (a)) and $t = 1475$ s (panel (b)). Three virtual spacecraft at $Y = 4.9 R_E$ are placed at different Z-coordinates and marked using colored crosses. In panel (a), the current sheet center (the light green $B_x = 0$ contour) resides near $Z = 0$, and all three VSCs measure velocity magnitudes of $|V'_x| < 300$ km/s. In panel (b), the current sheet center has shifted noticeably, enveloping two of the three VSCs in high velocity flow even though the velocity along the current sheet has not changed significantly.

The time series from a VSC tower at $(X, Y) = (-29, -4.9) R_E$ including individual time series from the three highlighted VSCs are shown in Figure 6. The Z-coordinate of the current sheet at the location of the VSC tower is visible as the $B_x \approx 0$ region in panel (g). As expected, the shapes of the bulk velocity and proton density signatures follow the CS closely: The Z-coordinates of the high velocity and density regions in panels (c) and (e) increase as
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Virtual spacecraft plots from (X, Y) = (-29.0, -4.9) $R_E$ of the z-component of the magnetic field $B_z$ (panels (a) and (b)), lobe-relative sunward bulk velocity $V'_x$ (panels (c) and (d)), proton number density $n_p$ (panels (e) and (f)) and x-component of the magnetic field $B_x$ (panels (g) and (h)), showing the flapping of the current sheet. The panels on the left are tower plots, where each vertical row corresponds to a VSC with a different Z-coordinate. The red ($Z = 1.7 R_E$), green ($Z = 1.3 R_E$) and black ($Z = 0.9 R_E$) arrows correspond to locations of the individual VSC time series on the plots on the right. The lines in panels (a) and (c) mark the locations of $dB_z/dt$ and $V'_x$ contours, respectively, with the solid lines corresponding to DF and bursty bulk flow (BBF) peak thresholds. The dashed lines in panel (c) signify BBF boundary thresholds and in panel (a) serve as visual aid. The lack of $dB_z/dt \pm 0.35$ nT/s contours in panel (a) implies $|dB_z/dt| < 0.35$ nT/s for all VSCs and time steps.</figcaption></figure>

the current sheet flaps northward. This is seen as an increase in $V_x$ and $n_p$, as can be seen from the individual time series in panels (d) and (f). The VSC at $Z = 1.7 R_E$ (in red) stays the furthest from the CS and does not detect significant changes in the variables. The current sheet passes through the VSC at $Z = 0.9 R_E$ (shown in black), characterized by $B_x$ changing sign after $t = 1460$ s in panel (h). The change in $B_x$ detected by the VSC is associated with changes in $V_x$ and $n_p$, as $|V'_x|$ increases from 50 km/s to 1,200 km/s, while $n_p$ increases from 0.01 $cm^{-3}$ to 0.04 $cm^{-3}$. Both quantities return to pre-event values as the current sheet moves away, seen as the increase in $B_x$ in panel (h). Similar features are seen by the VSC at $Z = 1.3 R_E$ (in green) even though $B_x$ stays above zero and thus the center of the CS does not pass through the VSC. Both the green and black VSCs detect velocities surpassing the BBF peak threshold of -300 km/s. The flapping of the current sheet is accompanied by variations in $B_z$ that stay below the DF threshold of $|dB_z/dt| = 0.35$ nT/s (panels (a) and (b)).

Current sheet flapping is not the only source of non-conventional detections of BBFs. Based on Figure 1c, the bulk velocity is relatively small in the vicinity of X-lines and increases with distance from them. Part of the velocity variation detected by a virtual spacecraft may therefore be a result of the X-lines changing location instead of changes in reconnection rates or formation of new flow channels. This corresponds to the detection of early reconnection outflow instead of the developed bubbles or finger-like structures typically associated with BBFs.

The current sheet context of such an event is shown in Figure 7. An animated version of the figure is available in Supporting Information S1. At the time step $t = 1180$ s a tower of VSCs (marked with a cross) is located near the X-line and detects bulk velocity values below the BBF peak threshold value highlighted using a blue contour. At
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Lobe-relative proton bulk velocity in the x-direction, shown along the 2D projection of the $B_x = 0$ isosurface at three different time steps. Blue (300 km/s) and red (−300 km/s) contours correspond to bursty bulk flow peak thresholds. X- and O-lines are shown in magenta and green, respectively. Location of the VSC tower in Figure 8 is marked by a black cross.</figcaption></figure>

$t = 1215$ s is part of the X-line has moved farther down the magnetotail, and as a result the VSCs are embedded in a high-velocity flow, surpassing the peak threshold. At $t = 1250$ s, the X-line has returned close to the virtual spacecraft, and the velocity falls below the BBF thresholds.

Figure 8 shows the observations by the VSC tower at $(X, Y) = (-13, 3.9) R_E$, close to the moving X-line. Similarly to earlier tower plots, three individual VSC time series are shown separately in panels (b), (d), (f) and (h)

<figure><img src="image_10.png" /><figcaption>**Figure 8.** Virtual spacecraft plots from $(X, Y) = (-13.0, 3.9) R_E$ of the z-component of the magnetic field $B_z$ (panels (a) and (b)), lobe-relative sunward bulk velocity $V'_x$ (panels (c) and (d)), proton number density $n_p$ (panels (e) and (f)) and $B_x$ (panels (g) and (h)), showing a velocity increase due to a moving X-line. The panels on the left are tower plots, where each vertical row corresponds to a VSC with a different Z-coordinate. The red ($Z = 0.1 R_E$), green ($Z = -0.2 R_E$) and black ($Z = -0.5 R_E$) arrows mark the locations of the individual VSC time series on the plots on the right. The lines in panels (a) and (c) mark the locations of $dB_z/dt$ and $V'_x$ contours, respectively, with the solid lines corresponding to DF and bursty bulk flow (BBF) peak thresholds. The dashed lines in panel (c) signify BBF boundary thresholds and in panel (a) serve as visual aid. The lack of $dB_z/dt = \pm 0.15$ nT/s contours in panel (a) implies $|dB_z/dt| < 0.15$ nT/s for all VSCs and time steps.</figcaption></figure>
in order to make interpreting the results easier. A velocity increase from $V'_x < 100$ km/s to $V'_x > 300$ km/s is visible in panel (c) as the X-line moves farther away from the spacecraft. The VSC at $Z = -0.2 R_E$, shown in green in panel (d), detects velocity signatures consistent with the BBF criteria from $t \approx 1190$ s to $t \approx 1240$ s. The velocity increase is accompanied by a rise in $B_z$ of roughly 1 nT in panel (b). $dB_z/dt$ is too small to reach the dipolarization front threshold, indicated by the lack of the corresponding contour in panel (a). The VSC also detects a very slight increase in proton density in panel (f). The density then drops due to the VSC exiting the current sheet, demonstrated by the movement of the $B_x \approx 0$ region in panel (g). However, $B_x$ stays roughly constant during the detection of the high $V'_x$, indicating that the feature is not a result of current sheet flapping.

The VSC at $Z = -0.5 R_E$ (shown in black) detects similar features as the VSC at $Z = -0.2 R_E$, but with smaller magnitudes in every variable. As the bulk velocity stays below 300 km/s, the spacecraft does not register the BBF signatures. The VSC at $Z = 0.1 R_E$ (shown in red) only detects a minor increase in $V'_x$ and no density enhancement until it crosses the current sheet plane starting at $t = 1230$ s.

## 5. Discussion

In this study, we have demonstrated dipolarization front and BBF identification methods and shown the results for several case studies. The DF identification method successfully detected an earthward dipolarization front that had the expected observational features, including the peaking and dropping of density and a velocity increase (Runov, Angelopoulos, Sitnov, et al., 2011). The event in the earthward case study (Figures 2 and 3) is also located at the edge of a finger-like structure of earthward high velocity, indicating that the DF is associated with a BBF. The event lacks the common observational feature of the $B_z$ dip before the dipolarization, which may be due to the lack of electron physics in the simulation (e.g., Huang et al., 2022). The event in the tailward case study (Figure 4) is flux-rope-like both in its quasi-sinusoidal $B_z$ profile and its location near an O-line. Both of these features, along with the embedment in high-velocity flow, are consistent with the observations presented by Slavin et al. (2003). The lack of clear asymmetric dipolarization signatures in the tailward case study is also consistent with the fact that only earthward flux ropes are expected to evolve to have asymmetric dipolarizations preceded by a dip (Vogiatzis et al., 2015). The small level of asymmetry in panel 4(b) is meaningfully different from the highly asymmetric anti-DFs described by for example, Li et al. (2014), even though the $B_z$ variation is sharp enough to satisfy the anti-DF identification threshold.

The method of identifying BBFs only considered the proton bulk velocity and reflected observational studies (e.g., Richard et al., 2022). This criterion allowed us to identify fast flows, including finger-like regions of fast earthward flows compatible with the standard BBF picture and associated with magnetic field dipolarization and density variation (Ohtani et al., 2004). In addition to the narrow channels of fast bulk flow, the method captured flows that do not fit the typical BBF description. As shown in Figures 6 and 8, velocity profiles consistent with the observational BBF criteria can originate from flapping of the current sheet or shifting of a reconnecting X-line leading to the spatial reconfiguration of the outflow region at large scales. In these two cases, the outflow velocity is rather stable and reconnection is not bursty, but the flapping motion of the current sheet or the shift of the X-line leads to the presence of transient enhancement of bulk velocity in the time series of the virtual spacecraft. The apparent velocity increases due to these phenomena are not dependent on the VSC crossing the current sheet center or the X-line: Instead, the velocity increases even as the distance between the VSC and the CS center only decreases (green VSC in Figure 6) or when the X-line shifts relative to the VSC (Figure 7).

The results for the BBF case studies suggest that the fast flow enhancements caused by the reconfigurations of the outflow region can be distinguished from standard BBFs, if other quantities in addition to $V_x$ are considered in the identification process. Specifically, variations in $B_x$ can indicate the presence of current sheet flapping (see Figure 6), while a depletion in plasma density following the velocity increase is expected in standard BBFs but absent in the case driven by X-line motion (see Figure 8). The signatures associated with each high-velocity flow source can likely be recognized from single spacecraft measurements, but issues may arise when creating large databases of events based on solely $V_x$. If different fast flow sources are not correctly identified, mixing multiple event types may hide statistical trends in each category. Our results indicate that incorporating $B_x$ and density variations into BBF identification algorithms could help separate standard BBFs from other transient flow events, improving the reliability of observational data sets.
The magnetotail in the simulation is active, with multiple different phenomena operating at the same time (including e.g. wave activity, plasmoid formation and continuous reconnection). The simulation also contained quick $B_z$ changes and bulk flows with unclear origins or several interacting elements, such as movement of multiple reconnecting X-lines. In order to properly identify and investigate BBFs and DFs, we focused on cases where it was possible to disentangle the factors contributing to each event.

A noticeable difference between the simulation and typical measured conditions in Earth's magnetotail is the location of the primary reconnection X-line. A review of Geotail measurements by Nagai (2005) placed the near-Earth X-line between 20 and 30 $R_E$ from Earth, farther than the primary X-lines at $X \sim -15 R_E$ seen in the simulation. However, in extreme cases X-lines have been seen as close as $X \approx -8 R_E$ (Angelopoulos et al., 2020) and their radial distance to Earth is expected to decrease as solar wind driving increases (Nagai et al., 2005). While the location of the X-line in the simulation can therefore be considered realistic, the relatively short radial distance to the Earth can affect the development of BBFs and DFs. One of the consequences is that the location of the X-line likely limits the size of the DF events, as dipolarization fronts expand in the y-direction as they travel earthward (Liu et al., 2015). Some of the DF events may therefore be less developed than their observational counterparts, but the overall characteristics of the case studies still match the expectations.

The magnetotail conditions differ from observations also in other ways. The simulation exhibits continuous reconnection in the magnetotail, resulting in high proton bulk speeds along the current sheet throughout the studied time period. This is in contrast to the observed average conditions in the CPS, where the proton flow speeds are typically below 100 km/s (Baumjohann et al., 1989). Some of this discrepancy can be attributed to the driving conditions used in the simulation: Purely southward IMF and a solar wind speed of 750 km/s are realistic, but rare compared to the typical conditions with slower velocities and other IMF orientations (Wilson et al., 2021; Zhang et al., 2019). Thus, direct comparison with observations during times with average solar wind parameters may not be fruitful. Another feature present in the simulation but not in observational studies is the background tailward plasma flow in the magnetic lobes. While removing the background flow is necessary for constraining the size of tailward BBFs to reasonable sizes, the decision to take it into account is expected to have a minimal impact on the analysis: The DF identification criterion is independent of the velocity, and offsetting the velocities by 200 km/s only affects the location of the BBF thresholds contours, not their dependence of the CS and X-line motion. The virtual spacecraft can always be repositioned near the new BBF threshold contour to capture the bulk velocity increases due to X-line movement and current sheet flapping.

The search for BBFs and DFs focuses on the current sheet, which may introduce slight limitations to the identifiable events. Reducing the three-dimensional magnetotail to the two-dimensional current sheet center will inevitably lead to missed detections of events that occur above or below the CS center. In the context of this study, this is not expected to lead to major problems or misrepresentations as the fast flows and changes in the magnetic field are concentrated in the vicinity of the CS. However, individual events could still be affected, and the choice of the current sheet proxy (for instance, instead of $B_x = 0$ using $B_L = 0$ in a local LMN coordinate system (Alho et al., 2024)) could have a small impact on the interpretation of the events. The method for locating DFs does not necessarily require the reduction to 2D data sets, which can be used to avoid this issue entirely.

The DFs identification method used in our analysis is based on the time derivative of the magnetic field, $dB_z/dt$, adapting the criteria used by Liu, Angelopoulos, Runov, and Zhou (2013). However, we reduce the observational threshold by 30% to $|dB_z/dt| = 0.35$ nT/s. This adjustment is motivated by the $B_z$ variations in the simulation being systematically smaller than those observed in situ (see e.g. Runov, Angelopoulos, Sitnov, et al., 2011), which in turn reduces the peak $dB_z/dt$ values attainable. This difference arises from several factors, including the spatial resolution of the simulation, which limits the steepness and magnitude of magnetic field variations. The process of interpolating $dB_z/dt$ values to the current sheet position could also reduce the peak values. It is expected that increasing the spatial resolution of the simulation would likely lead to steeper and larger $B_z$ variations. Another element to be considered is that the relatively close-to-Earth location of the X-line could hinder the development of earthward DFs, reducing both $dB_z/dt$ and the magnitude of $B_z$ variations. Nevertheless, the DFs identified in the simulation exhibit a good qualitative agreement with in situ observations. The lower threshold of $|dB_z/dt| = 0.35$ nT/s allows us to capture more events matching the qualitative description of DFs that could be used for future statistical studies. Lowering the threshold even further would increase the number of events as well as the sizes of the dipolarization fronts found, but it would also likely introduce non-DF-associated $dB_z/dt$ variations to the data set.
The results for the BBF and DF case studies will be supplemented by a follow-up study that focuses on statistical analysis of a larger simulation data set. Creating the data set should entail a semi-automatic or fully automatic procedure for locating BBFs in the simulation. Additionally, the detections of BBF-like velocity signatures from X-line movement and current sheet flapping in the simulation need to be compared with satellite observations in order to verify whether the phenomena reflect the in situ magnetotail conditions.

## 6. Summary

We utilized the 3D hybrid-Vlasov simulation code Vlasiator to identify and study the properties of BBFs and dipolarization fronts in Earth's magnetotail. BBFs were identified based on a velocity threshold and DFs based on the time derivative of $B_z$, making the identified events directly comparable to satellite observations. Both BBFs and DFs were searched for along the current sheet center, automatically placing them in the CPS. Flapping motion of the current sheet made the view along the equatorial plane misleading.

DFs and BBFs were studied on the basis of case studies. An earthward DF case study was consistent with satellite observations and coincided with a finger-like structure of high earthward bulk flow, typically associated with BBFs. A case study of an event passing the anti-dipolarization front threshold was a flux rope created by multiple X-line reconnection. These structures have been observed by, for example, Slavin et al. (2003), so the events identified using the newly developed methodology had properties expected based on observational studies.

In addition to the finger-like structures of earthward velocity, BBF-like velocity signatures were detected in the virtual spacecraft view due to current sheet flapping and X-line motion. The velocity measurements were transient even though the reconnection outflow was relatively stable; the velocity varied with distance to the current sheet center or X-line. As a result, the velocity variations due to current sheet flapping were accompanied by similar changes in $B_x$. Only the measurements from the finger-like structure had the density profile expected of BBFs.

The developed identification methods can be used to locate BBF and DF events in global 3D simulations. The results for the DF case studies suggest that Vlasiator is capable of reproducing $B_z$ variations seen in observational studies of Earth's magnetotail. These results are expected to be beneficial to observational studies: As velocity signatures consistent with commonly used BBF selection criteria were found from multiple sources, the identification criteria used for creating observational BBF data sets might need to be adjusted to include only one type of event.

## Data Availability Statement

Vlasiator version 5.2 (Pfau-Kempf et al., 2024) is distributed under the GPL-2 open-source license and hosted at GitHub (https://github.com/fmihpc/vlasiator). The output of Vlasiator simulations are stored in a custom.vlsv file format (see github.com/fmihpc/vlsv) which is readable with the Analysator software (Battarbee et al., 2021), which is also used to produce the presented figures. The data used for this study are available for download from Suni and Horaites (2024).

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## Acknowledgments

The work of LP and JS was made possible by doctoral researcher positions at the Doctoral Programme in Particle Physics and Universe Sciences funded by the University of Helsinki. MP acknowledges the Research Council of Finland Grant 361901. MP, MB and LP acknowledge the Research Council of Finland for the Finnish Centre of Excellence in Research of Sustainable Space (Grant 352846). YP acknowledges the Research Council of Finland Grant 339756 (KIMCHI). The work of MB and UG are supported by the EuroHPC “Plasma-PEPSC” Centre of Excellence (Grant 4100455) and the Research Council of Finland matching funding (Grant 359806). GC is supported by the Integration Fellowship of Le Studium Loire Valley Institute for Advanced Studies. The Scientific color maps by Crameri (2023) are used in this study to prevent visual distortion of the data and exclusion of readers with color-vision deficiencies (Crameri et al., 2020). LP thanks D. Schmid, R. Nakamura, E. Panov and M. Hosner for enlightening discussions. The simulation presented in this work was run on the LUMI-C supercomputer through the EuroHPC project Magnetosphere-Ionosphere Coupling in Kinetic 6D (MICK, project number EHPC-REG-2022R02-238). The authors thank the Finnish Computing Competence Infrastructure (FCCI), the Finnish Grid and Cloud Infrastructure (FGCI) and the University of Helsinki IT4SCI team for supporting this project with computational and data storage resources. Open access was funded by the Helsinki University Library. Open access publishing facilitated by Helsingin yliopisto, as part of the Wiley - FinELib agreement.
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