In Situ Observations of Magnetic Reconnection Caused by the Interactions of Two Dipolarization Fronts

Using high‐resolution data from the Magnetospheric Multiscale mission, an electron‐only reconnection current sheet is found between two successive dipolarization fronts (DFs). The electron‐only reconnection occurs between the northward component of the magnetic field of the flux pileup region (FPR) of the first DF (DF1) and the southward component of the magnetic dip of the second DF (DF2). The faster DF2 compresses the FPR of DF1, which constitutes an anti‐parallel topology and reduces the thickness of the current sheet. Further analysis shows that the current sheet is unstable to the electron tearing instability, which may power the onset of the reconnection. Our results suggest that these two DFs may merge into one by the reconnection, which sheds light on the evolution of DFs during their earthward propagation.


Introduction
Magnetic reconnection, which can effectively convert energy from the magnetic field to the particles, is an explosive physical process in space, astrophysical, and laboratory plasmas.And it is thought to be responsible for many explosive phenomena such as supernova ejections, solar flare, geomagnetic substorm, and so on.The terrestrial magnetosphere, capable of in situ observations, is an ideal laboratory to explore the dynamics of magnetic reconnection.Magnetic reconnection in the terrestrial magnetotail has been widely observed (Eastwood et al., 2010;Ergun et al., 2020;Huang et al., 2018Huang et al., , 2022Huang et al., , 2023;;Jiang et al., 2019Jiang et al., , 2022;;Øieroset et al., 2001;Runov et al., 2003;Torbert et al., 2018), and it is crucial for energy release and mass and magnetic flux transport in the magnetotail.
The onset of reconnection requires magnetic energy dissipation, which can be caused by the inverse electron Landau resonance due to electron tearing instability in the collisionless plasma (Coppi et al., 1966;Pritchett, 2015).For a current sheet with strictly anti-parallel field lines across the sheet, the growth rate for the collisionless tearing instability can significantly increase as the half-thickness of the current sheet approaches ion scales (Brittnacher et al., 1995;Pritchett, 2015;Pritchett et al., 1991;Schindler, 1974).However, a minor component (∼10% of the anti-parallel magnetic field) normal to the current sheet (B normal ) can restrain the electron Landau resonance and stabilize this instability (Brittnacher et al., 1995;Fairfield, 1979;Galeev & Zelenyi, 1976;Pellat et al., 1991;Schindler, 1974).In the terrestrial magnetotail, the curved topology of the magnetic field can give such a B normal component and prevent spontaneous reconnection.Thus, how to reduce the B normal and bypass the stabilization threshold is vital to the onset of reconnection in the magnetotail.
A magnetic dip is usually found ahead of a DF (e.g., Pan et al., 2015;Sun et al., 2014;Yao et al., 2013Yao et al., , 2015;;Zhao et al., 2016;Zhou, Angelopoulos, et al., 2014), and the generation mechanism of the magnetic dip is under debate.Yao et al. (2015) found that the redistribution of the cross-tail current caused by the plasma pressure gradient ahead of the DF combined with the duskward current at the DF can lead to the magnetic dip.Zhou, Angelopoulos, et al. (2014) simulated that earthward and dawnward secondary current carried by reflected ions can cause the magnetic dip prior to the arrival of DFs.Using 3D global hybrid simulation, Lu et al. (2015) explained the DFs as earthward propagating flux ropes.By reconnection between southward B z and northward geomagnetic field, the erosion of the southward magnetic flux of the flux ropes results in the DFs with magnetic dips (Slavin et al., 2003;Vogiatzis et al., 2015).
DFs can be generated by magnetic reconnection (Fu et al., 2013;Sitnov et al., 2009).As DFs approach the Earth, the magnitude of the magnetic field reduces, and the energy carried by the earthward plasma flow transfers into the thermal energy and compression of the magnetic field (Angelopoulos et al., 2008).However, how the DFs evolve during their earthward movement is rarely investigated.
In this work, we present in situ observation of an electron-only reconnection by the Magnetospheric Multiscale (MMS) mission between the northward flux pileup region (FPR) of the former DF and the southward magnetic dip of the latter DF.The latter DF moves faster than the former DF, compressing the current sheet between them.The electron tearing instability is found to be unstable in this current sheet, which may power the reconnection between these two DFs.We deduce that these two DFs may merge through the reconnection.Our observations reveal the possible evolution of DFs during their propagation in the magnetotail.

Observations
The data used in this work are from the MMS.The Fluxgate Magnetometer (Russell et al., 2016) records the 3-D magnetic field of 128 Hz in burst mode.The Electric Double Probes (Ergun et al., 2016;Lindqvist et al., 2016) give the 3-D electric field of 8,192 Hz in burst mode.The Fast Plasma Investigation (Pollock et al., 2016) provides the plasma moments and 3-D plasma distribution functions, where electrons' data are sampled once every 30 ms in burst mode, and ions' data are sampled once every 150 ms in burst mode.
Figure 1 shows an overview of two successive DFs observed by MMS2 in the terrestrial magnetotail from 10:03:03 to 10:03:55 UT on 2 June 2017, when MMS2 was located at [ 17.0, 5.5, 2.0] Earth radii (R E ) in geocentric solar magnetospheric (GSM) coordinate system.During this interval, the spectra of ions and electrons are mainly concentrated on the energy range from several to tens keV (Figures 1a and 1b).Combined with the high temperature of electrons (Figure 1g) and large plasma β (larger than 0.5, red dashed line in Figure 1i), one can deduce that MMS was in the plasma sheet (Cao et al., 2006).From ∼10:03:17 to ∼10:03:24.9UT, B z increases sharply (Figure 1c), N e decreases (Figure 1e), and plasmas flow earthward (Figures 1d and 1f), which are features of a DF (dubbed DF1, Fu et al., 2012;Nakamura et al., 2002;Runov et al., 2009).Then, B z continuously increases (Figure 1c) and N e decreases (Figure 1e) from ∼10:03:43.4 to ∼10:03:48.9UT, which suggests another DF (entitled DF2).T e shows clear enhancement at DF1 and DF2, and the maximum of T e at the FPR (also called dipolarizing flux bundles) of DF1 is larger than that of DF2 and FPR2 (Figure 1g), which means that DF1 and DF2 are not one single DF.DF2 has a negative B z dip at the leading edge (Figure 1c).Timing analysis (Russell et al., 1983) are performed on B z from 10:03:22.03 to 10:03:24.05UT and from 10:03:43.47 to 10:03:48.41UT, and the average motion velocity of DF1 and DF2 are V DF1 = 233 × [0.95, 0.24, 0.22] km/s and V DF2 = 513 × [0.76, 0.49, 0.43] km/s (GSM), which indicates that DF2 moves faster than DF1.At the end of the FPR of the DF1, an unambiguous current sheet is detected (Figure 1h, as marked by the red shade), in which the current is mainly contributed by electrons (Figure 1f).
Figure 2 shows the details of the current sheet in a local boundary normal (LMN) coordinate system.The LMN coordinate system is determined by the minimum variance analysis (MVA, Sonnerup & Scheible, 1998) on the magnetic field from 10:03:40.76 to 10:03:43.04UT, and the results are L = [ 0.13, 0.14, 0.98], M = [0.48,0.86, 0.18], and N = [0.87,0.50, 0.05] in GSM coordinates.The ratio of the maximum eigenvalue to the median one is 12.3, and the ratio of the median eigenvalue to the minimum one is 18.3, implying that the LMN coordinate system is reliable.A current sheet (the maximum of J L and J M are 30 nA/m 2 and 73 nA/m 2 , respectively) is observed from ∼10:03:41.44 to 10:03:42.14UT (Figure 2f).Timing analysis is performed on B z from 10:03:41.29The bars on the top mark different parts, in which "BG" stands for the background plasmas, "DF1" stands for the first dipolarization front, "FPR1" stands for the flux pileup region of the first dipolarization front, "DF2" stands for the second dipolarization front, and "FPR2" stands for the flux pileup region of the second dipolarization front.The red shade stands for the current sheet.All data are from MMS2 and presented in geocentric solar magnetospheric coordinates.
to 10:03:42.24UT, and the average result is 470 × [0.84, 0.50, 0.24] km/s (GSM).Then, the thickness of the current sheet is estimated as 329 km, that is, ∼0.6 d i or ∼27.7 d e (where d i ∼ 509 km and d e ∼ 11.9 km are ion and electron inertial length calculated with the average N i = 0.2 cm 3 at the two sides of the current sheet from 10:03:41.2 to 10:03:41.44UT and from 10:03:42.14 to 10:03:42.5UT), indicating that this current sheet belongs to a sub-ion-scale structure.Ion flow is large but nearly unchanged during the crossing of the current sheet (the maximum change of V i is less than 0.1 V A , V A ∼ 288 km/s is the ion Alfvén speed derived from the average plasma parameters |B L | = 5.9 nT and N i = 0.2 cm 3 at the two sides of the current sheet from 10:03:41.2 to 10:03:41.44UT and from 10:03:42.14 to 10:03:42.5UT, Figure 2c).Considering that the background flow is strong, it is simpler and clearer to remove the background flow in the electron velocity and convective term in the electric field to investigate the local processes.V eL and V eM are mainly negative in the current sheet, and the maximum of V eL and V eM are 849 km/s and 2,025 km/s (Figure 2e), which are much larger than the ion Alfvén speed V A .Thus, they are super-ion-Alfvénic electron jets.Along with the negative V eL , B L changes from positive to negative (Figure 2a).B M stays positive and it has a negative to positive perturbation relative to a guide field ∼6.8 nT (estimated by B M at B L reversal) ∼0.8 B 0 (B 0 ∼ 8.7 nT is the average asymptotic magnetic field at the two sides of the current sheet from 10:03:41.2 to 10:03:41.44UT and from 10:03:42.14 to 10:03:42.5UT, Figures 2a  and 2b), which is consistent with Hall magnetic field of the guide field reconnection (e.g., Pritchett, 2001).B N negatively enhances (Figure 2a), which is consistent with the crossing of one side of the outflow.N component of the electric field in the ion frame changes from negative to positive (Figure 2g), which is the Hall electric field.Therefore, the current sheet is reconnecting.L components of V i⊥ and V e⊥ deviate significantly from that of V E×B (Figure 2h), which implies that both ions and electrons demagnetized.Besides, there is intense energy conversion from the fields to the plasmas in the current sheet (J • E′ with a maximum of 68 pW/m 3 , Figure 2i), which is on the same order as the reconnections in the magnetotail (Huang et al., 2018;Man et al., 2018).Two negative J • E′ peaks are found next to the positive one, which is consistent with the observations and simulations with a N-direction dominated, one-side crossing of the reconnection current sheet (Huang et al., 2018;Pucci et al., 2018).According to the super-ion-Alfvénic electron jets, electron demagnetization, and noteworthy J • E′, one can deduce that MMS detected an electron diffusion region (EDR, e.g., Zhou et al., 2019).It is worth noting that V i is steady in the current sheet (Figure 2c).Neither ion outflow nor ion inflow is observed, indicating ions do not respond to the reconnection process.Thus, the reconnection observed here is an electron-only reconnection, as reported in the magnetosheath and magnetotail (Huang et al., 2018;Lu, Wang, et al., 2020;Phan et al., 2018).
Electron-only reconnection is a novel type of reconnection in which ions do not respond to the process of reconnection, which is very different from the traditional reconnection where both ions and electrons participate in the reconnection.Electron-only reconnection, first reported in the magnetosheath (Phan et al., 2018), has been observed in the magnetotail (Huang et al., 2018;Lu, Wang, et al., 2020).However, the properties of the electrononly reconnection in the magnetotail are different from those in the magnetosheath.In the magnetosheath, electron-only reconnection occurs in the electron-scale current sheet and can persist relatively long.The spatial scale of the reconnection is confined to a small region so that ions do not respond to the reconnection process (Phan et al., 2018;Pyakurel et al., 2021;Stawarz et al., 2022).However, observations and simulations have proved that the electron-only reconnection in the magnetotail is a short-lived transition phase from the quiet current sheet to the traditional reconnection, and it can only exist for several ion cyclotron periods (Hubbert et al., 2021(Hubbert et al., , 2022;;Lu, Wang, et al., 2020;Lu et al., 2022;Wang et al., 2020).Besides, the thickness of the electron-only reconnection current sheet in the magnetotail can vary from electron scale to ion scale (Hubbert et al., 2022;Lu, Wang, et al., 2020), which can be larger than that in the magnetosheath.N e enhances in the current sheet (Figure 2d), which is consistent with the electron-only reconnection reported in the magnetotail and simulations (Hubbert et al., 2022;Lu, Wang, et al., 2020).On the contrary, N e is depleted in the traditional reconnection (Torbert et al., 2018).In general, we propose that MMS encountered a guide field electron-only reconnection event without ion coupling (Huang et al., 2021;Phan et al., 2018) between two DFs.The inferred trajectory of MMS crossing the reconnection current sheet is shown in Figure 2j.
The normal direction of the electron-only reconnection current sheet is earthward and duskward directed ([0.87, 0.50, 0.05], GSM), and the electron outflow is mainly in the negative Z direction in GSM coordinates, which is very different from the standard magnetic reconnection expected in the magnetotail (normal direction of the reconnection current sheet is oriented in the ±Z direction and outflow is oriented in the ±X direction in GSM coordinates, e.g., Torbert et al., 2018).This difference may indicate the dynamics in this reconnection current sheet and the onset of the reconnection is particular.

Discussions
3D simulations suggest that the DF can be a place for secondary reconnection (Lapenta et al., 2015).However, how can the secondary reconnection be initiated?The magnetotail's neutral sheet can be unstable to electron tearing instability, in which the electron Landau resonance can provide the dissipation required to initiate reconnection (Coppi et al., 1966).However, a normal component (i.e., B z in GSM coordinates) caused by the curved magnetic field lines in the magnetotail can stabilize the electron tearing instability in a current sheet in the x-y plane (Galeev & Zelenyi, 1976) and prevent the onset of reconnection.For the event observed here, the current sheet lies mainly in the y-z plane.Thus, the current sheet is a tilted current sheet.The tilted current sheet makes B z a component in the plane of the current sheet and may allow electron tearing instability to occur spontaneously.The growth rate of the tearing mode can significantly enhance when the current sheet thins (e.g., Brittnacher et al., 1995;Pritchett, 2015;Pritchett et al., 1991;Schindler, 1974), and the orientation of the X-line is tended to be the direction of the fastest growth rate of the tearing mode (e.g., Liu et al., 2018).However, the reconnection is 3-D.The magnetic fields on two sides of the boundary layer can shear at an arbitrary angle ϕ (see details in Liu et al., 2018).As long as the antiparallel magnetic components can be found in a plane, reconnection can occur on this plane.Thus, the choice of the plane is not only (Liu et al., 2018).Considering that the X-line is perpendicular to the reconnection plane, determining the reconnection plane is equivalent to determining the orientation of the X-line.Based on the simulations (e.g., Liu et al., 2018), if the orientation of the X-line is in the range determined by the magnetic field on the current sheet's two sides, there is always a plane where antiparallel components can be found, and the growth rate of the tearing mode can be positive (Liu et al., 2018).In other words, determining the range of the growth of the mode is equivalent to determining the orientation of the X-line.Thus, based on the theory, simulations, and observations (Daughton et al., 2011;Liu et al., 2013Liu et al., , 2018;;Zhong et al., 2022), the unstable range of the tearing mode instability can be approximately determined with the magnetic field on the current sheet's two sides (the cyan and magenta arrows in Figure 3).As long as the orientation of the X-line lies in the unstable range, the tearing mode can be unstable, and reconnection may occur.In simulations, the orientation of the X-line can be determined by the magnitude of the current density with all information in the simulation box known (Liu et al., 2018).However, it is impossible to do so in observations, where only a slice of the 3-D reconnection is observed.In a rigorous LMN coordinate system where the reconnection plane is perfectly in the LN plane, the orientation of the X-line is in the M direction and the current is only in the M direction at the X-line.However, the orientation of the X-line can be tilted from the M direction in observations due to the difficulty of determining the perfect reconnection plane.In the EDR, which contains the X-line, V eM is the dominant component, V eL is weaker, and V eN is negligible compared to V eL and V eM (e.g., Torbert et al., 2018), which is also the case in our event.Thus, for a reconnection in a LMN coordinates with M direction tilted from the X-line orientation, we can approximately use current density in the LM plane to infer the orientation of the X-line (i.e., the orientation of the fastest growth of the tearing mode).In our event, the magnetic field used to determine the unstable range of the tearing instability are the average values on the current sheet's two sides.B L1 and B M1 are the average values from 10:03:40 to 10:03:40.9UT, and B L2 and B M2 are the average values from 10:03:42.2 to 10:03:43.2UT.The currents in Figure 3 are acquired from 10:03:41.44 to 10:03:42.14UT.Most of the currents in the current sheet observed by MMS are well inside the unstable range (coral shade in Figure 3), suggesting that this current sheet is dominated by tearing instability.Moreover, the thickness of the current sheet is sub-ion-scale.Hence, under the compression of DF2, electron tearing instability may develop in the current sheet and trigger the electron-only reconnection therein.Besides, this electron-only reconnection may not develop into an ion-coupled reconnection due to the compression of DFs limiting the scale of the current sheet.However, it is interesting to discuss what will happen if the electron-only reconnection evolves into an ion-coupled one.The electron-only reconnection is regarded as the early stage of the ion-coupled reconnection (e.g., Hubbert et al., 2021Hubbert et al., , 2022;;Lu, Wang, et al., 2020;Lu et al., 2022;Wang et al., 2020).Thus, electron-only reconnection will occur first if two DFs reconnect.If the electron-only reconnection evolves into an ion-coupled one, the reconnection between DFs can be faster due to the much larger energy conversion rate (J • E′) of the ion-coupled reconnection than the electrononly reconnection (e.g., Lu et al., 2022).However, the total energy released by the reconnection will not change because the magnetic energy stored in the magnetic dip is the same.Besides, ions will be energized if the reconnection between two DFs is an ion-coupled reconnection.
Both DFs have negative dips ahead of them (Figure 1c).It is said that the magnetic dip ahead of a DF can be formed by reconnection between a flux rope and the geomagnetic field (e.g., Lu et al., 2015;Man et al., 2018;Slavin et al., 2003;Vogiatzis et al., 2015) or the dawnward current (Pan et al., 2015;Yao et al., 2013;Zhou, Angelopoulos, et al., 2014).Dawnward currents are found near the dip regions of DF1 and DF2 (Figure 1h).But the duskward currents dominate.Besides, the dawnward currents are mainly carried by electrons rather than ions (Figures 1d, 1f, and 1h, e.g., Pan et al., 2015).And the precursor signature such as increases of V ix ahead of DFs (e.g., Pan et al., 2015) are not found (Figure 1d).Thus, the magnetic dips are not caused by the dawnward current here (e.g., Pan et al., 2015;Yao et al., 2013;Zhou, Angelopoulos, et al., 2014).B y component is not dominated at both DFs (Figure 1c).However, the N e minorly increases at the reversals of B z from the dips to the DFs (Figure 1e), which means the magnetic dips may be a remnant signature of the erosion of a plasmoid without a strong core field (e.g., Lu et al., 2015;Lu, Angelopoulos, et al., 2020).
DFs and flux ropes can be generated by magnetic reconnection.As the counterpart of the DFs, flux ropes can host complicated evolution during their movement, such as coalescing with other flux ropes by reconnection (e.g., Wang et al., 2016;Zhou et al., 2017).Then, it is reasonable to ask whether DFs have similar processes as flux ropes In our event, two successive DFs interact with each other through reconnection between the southward component of the magnetic dip of DF2 and the northward component of the FPR of DF1.With the proceeding of reconnection, the southward component of the magnetic dip of DF2 will be dissipated.When the dip is totally dissipated, the DF2 may integrate into DF1 due to the compression of the faster plasma flow behind DF2 (Figure 1d).Magnetic dips are usually accompanied with DFs (e.g., Pan et al., 2015) and multiple DFs have also been reported (e.g., Zhou et al., 2009).Thus, the reconnections between DFs may be common.A single DF in the magnetotail can be the result of the coalescence of two or even more DFs.And the multiple DFs can be coalescing.However, more data are needed to be surveyed because the reconnections between DFs are transient compared to the time of the motion of the DFs.

Summary
In summary, an electron-only reconnection is identified in a tilted current sheet between two successive DFs.Both DFs have negative magnetic dips ahead of them, and the reconnection happens between the magnetic field of the FPR of DF1 and the magnetic field of the dip region of DF2.The faster DF2 compresses the FPR of DF1, which constitutes an anti-parallel topology and reduces the thickness of the tilted current sheet.In the tilted current sheet, the B z caused by the curved topology of the magnetotail is a component in the plane, which can circumvent the stabilization effect of electron tearing instability.The electron tearing instability is unstable in the tilted current sheet, which may power the onset of the electron-only reconnection between two DFs.The successive DFs may merge into one DF eventually due to the reconnection and the faster plasma flow behind DF2.Our observations fill the gap of the evolution of the DFs between their generation and their ultimate fate of integrating into the geomagnetic field.Central Universities (2042023kf0097), the China National Postdoctoral Program for Innovative Talents (BX20220238), and the Hubei Provincial Natural Science Foundation of China (2023AFB046).SYH acknowledges the project supported by Special Fund of Hubei Luojia Laboratory.KJ acknowledges the project supported by the Open Fund of Hubei Luojia Laboratory (230100005).We thank the MMS team and instrument leads for data access and support.

Figure 1 .
Figure 1.MMS2's observations of dipolarization fronts (DFs) in the terrestrial magnetotail.(a) Ion and (b) electron omnidirectional differential flux; (c) magnetic field; (d) ion velocity; (e) electron density; (f) electron velocity; (g) electron temperature; (h) current density calculated by plasma moments J = ne(V i V e ); (i) plasma beta, the red dashed line is 0.5.The bars on the top mark different parts, in which "BG" stands for the background plasmas, "DF1" stands for the first dipolarization front, "FPR1" stands for the flux pileup region of the first dipolarization front, "DF2" stands for the second dipolarization front, and "FPR2" stands for the flux pileup region of the second dipolarization front.The red shade stands for the current sheet.All data are from MMS2 and presented in geocentric solar magnetospheric coordinates.

Figure 2 .
Figure 2. Detailed observations of the electron-only reconnection in the thin current sheet by MMS2 in LMN coordinates.(a) L, N components, and the magnitude of the magnetic field; (b) M component of the magnetic field; (c) ion velocity; (d) electron density; (e) electron velocity with background flow removed (V e V i ); (f) current density calculated by plasma moments; (g) electric field in the ion frame; (h) L components of ion perpendicular velocity, E × B drift velocity, and electron perpendicular velocity; (i) energy conversion rate J • E′, where E′ = E + V e × B; (j) a sketch of the crossing of the reconnection current sheet.

Figure 3 .
Figure 3.The current density of the reconnection current sheet in the L-M plane.The colorful dots represent the current density observed by different Magnetospheric Multiscale mission.The magenta and cyan arrows are the average magnetic field on the two sides of the reconnecting current sheet, which define the unstable range of the tearing instability predicted by the theory (as shown by the coral shade).