Mirror mode structures ahead of dipolarization frontnear the neutral sheet observed by ClusterG. Q. Wang1,2,3, T. L. Zhang2,3, M. Volwerk3, D. Schmid3, W. Baumjohann3, R. Nakamura3, and Z. H. Pan1

1CAS Key Laboratory of Geospace Environment, University of Science and Technology of China, Hefei, China, 2HarbinInstitute of Technology, Shenzhen, China, 3Space Research Institute, Austrian Academy of Sciences, Graz, Austria

Abstract Magnetic compressional structures ahead of a dipolarization front (DF) on 30 August 2002 areinvestigated by using Cluster data. Our findings are as follows: (1) the structures, observed near the neutralsheet, are mainly compressional and dominant in BZ; (2) they are almost nonpropagating relative to the localion bulk flow and their lengths are several local proton gyroradius; (3) the ion density increases when BTdecreases; (4) ions are partially trapped by the structures with parallel and perpendicular velocities varying inantiphase; and (5) local conditions are favorable for excitation of the mirror instability, and we suggest thatthese structures are mirror mode-like. Our findings also suggest that local conditions ahead of the DF areviable for exciting the mirror instability to generate mirror mode waves or structures.

1. Introduction

The mirror instability is generated in the high-β plasma region [Hasegawa, 1969]. This instability can excitemirror mode waves or structures, which are found to occur in many space plasma regions from the solar wind[e.g., Zhang et al., 2008, 2009], to planetary magnetosheaths [e.g., Volwerk et al., 2008; Schmid et al., 2014],comets [e.g., Volwerk et al., 2016], the Earth's magnetosheath [e.g., Gary et al., 1993; Lucek et al., 1999;Soucek et al., 2008; Génot et al., 2009], and magnetotail [e.g., Rae et al., 2007; Ge et al., 2011a; Zieger et al.,2011]. Mirror mode structures (MMs) are compressional and nonpropagating structures with the plasma den-sity and the magnitude of the magnetic field being anticorrelated [e.g., Gary et al., 1993; Ge et al., 2011a].Zieger et al. [2011] reported that the interaction between the earthward moving fast plasma jet and thehigh-β plasma in the plasma sheet could result in magnetic pileup and compression ahead of the jet, andthey found that MMs at ion gyroradius scale can develop within the pileup region.

Dipolarization fronts (DFs), characterized by a sharp enhancement in the northward magnetic field BZ, havebeen widely studied in the last decade [Schmid et al., 2011, 2016; Ge et al., 2011b, 2012; Wu and Shay, 2012;Wu et al., 2013; Breuillard et al., 2016]. They are associated with bursty bulk flows [Nakamura et al., 2002;Takada et al., 2006; Ge et al., 2011b] and auroral activity [Ge et al., 2012]. The DF plays an important role inthe acceleration of ions [Wu and Shay, 2012] and electrons [Wu et al., 2013]. Ion can becomemore anisotropicafter the DF [Ge et al., 2011a]. And ions can be reflected and accelerated by the DF, resulting in an earthwardstreaming ion population; consequently, the streaming ions could excite various instabilities in the plasmasheet [Wu and Shay, 2012]. Similarly, ion reflection at the quasi-perpendicular bow shock can lead totemperature anisotropy to excite the mirror instability [e.g., Sckopke et al., 1983; Czaykowska et al., 1998].

In this study, we use Cluster data to investigate the magnetic compressional structures ahead of a DF. Ourobservations suggest that these structures are MMs.

2. Observation

We use the magnetic field data with full resolution (22Hz) obtained by the fluxgate magnetometer (FGM)[Balogh et al., 2001] and ion data with 4 s resolution in the energy range of 5 eV/e to 32 keV/e recorded bythe hot ion analyzer (HIA) of the Cluster Ion Spectrometer (CIS) experiment [Rème et al., 2001] in this paper.Cluster, launched in 2000, consists of four identical satellites, which set up to form a tetrahedron structurewith interspacecraft distances varying from 100 to 18,000 km [Escoubet et al., 2001].

Figure 1 shows the overview of the event observed on 30 August 2002 between 15:35 and 15:38 UT when theCluster satellites are located at about (�18.4,�1.5, 1.8) RE in GSM (this coordinate system is used throughoutthe paper unless otherwise specified). There are no magnetic field data from C4 during this time interval, so

WANG ET AL. MIRROR MODE STRUCTURES AHEAD OF DF 8853

PUBLICATIONSGeophysical Research Letters

RESEARCH LETTER10.1002/2016GL070382

Key Points:• Compressional magnetic structuresare found ahead of the DF

• Properties of the compressionalstructures suggest that the structuresare mirror mode-like

• Local conditions ahead of the DFcould possibly excite the mirrorinstability

Correspondence to:T. L. Zhang,Tielong.Zhang@oeaw.ac.at

Citation:Wang, G. Q., T. L. Zhang, M. Volwerk,D. Schmid, W. Baumjohann,R. Nakamura, and Z. H. Pan (2016),Mirror mode structures ahead ofdipolarization front near the neutralsheet observed by Cluster, Geophys. Res.Lett., 43, 8853–8858, doi:10.1002/2016GL070382.

Received 10 JUL 2016Accepted 16 AUG 2016Accepted article online 17 AUG 2016Published online 2 SEP 2016

©2016. American Geophysical Union.All Rights Reserved.

no data from C4 are used in this study. As shown in Figure 1, BZ is dominant during the whole interval. BX fromC1 and C2 (marked as C1/2) is steady with a value between 0 and�3 nT before 15:36 UT, indicating that C1/2are located near the neutral sheet (BX=0), while BX from C3 is a bit more negative, indicating that C3 islocated at a bit more southward away from the neutral sheet. During this interval, BY and BZ from the threesatellites are steady with a value of ~0 for BY and ~10 nT for BZ. Obviously, two compressional structures areobserved by C1 between 15:36:10 and 15:36:30 UT and by C2/3 between 15:36 and 15:36:20 UT. The periodsof the structure on the left (right) side are ~8.7 (9), ~8.4 (13), and ~8.7 (9.7) s for C1, C2, and C3, respectively.Besides, the ion density increases when the magnitude of the magnetic field decreases during the structures'interval. Meanwhile, the ion thermal pressure increases accompanied by a decrease of themagnetic pressure,indicating that the structures are balanced in pressure between ion particles and the magnetic field. At15:36:30 UT, there is a clear dip in BZ from C1 and then BZ increases from 5.4 up to 13.7 nT within 1.4 s accom-panied by fast flow, a decrease of the ion density and an increase of the ion temperature, which reveals that it

Figure 1. A dipolarization front event observed on 30 August 2002. (from top to bottom) The three components of themagnetic field in GSM, magnitude of the magnetic field, ion density, temperature and velocity in GSM, and ion thermal(black), magnetic (red), and total (blue) pressures from C1. Black, red, and green colors in each panel except the last oneindicate data from C1, C2, and C3, respectively. Below each time tick the corresponding positions of C1 are given. Theshadow indicates the event interval.

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is a typical DF [Schmid et al., 2011]. TheDF is also observed by C2 and C3.Besides, the BZ profiles around the struc-tures and the DF observed by the threesatellites are very similar, which pro-vides a good opportunity to comparethe structures observed by differentsatellites.

The distances between C1 (C2) and C2(C3) are ~2.1 × 103 (19) km in the X direc-tion, ~3 × 103 (2.2 × 103) km in the Ydirection, and ~25 (2.9 × 103) km in theZ direction at 15:36:30 UT. The time dif-ferences between C1 and C2 are ~13 sfor the first structure and ~8 s for thesecond. Consequently, the velocities inthe X direction are estimated to be~162 and ~263 km/s for the first andsecond structures, respectively. Theplasma velocities in the X direction arebetween 163 and 253 km/s during the

first structure's interval and between 268 and 350 km/s during the second structure's interval observed byC1. Therefore, the velocities of the structures are comparable to the local ion bulk flow in the X direction,which is expected for MMs.

We rotate the magnetic field data into a mean field-aligned (MFA) coordinate system. The MFA is defined asfollows: z axis is parallel to the low-pass filtered magnetic field data with a shortest period of 30 s, y axis isperpendicular to the mean magnetic field and the X direction, and x axis completes the right-handed coor-dinate system. As shown in Figure 2, both structures are mainly compressional, while there is a small trans-verse component in By during the interval of the second structure. During the interval of both structures,T⊥/T∥ decreases from ~1.3 to ~1.2, while the perpendicular ion beta (β⊥) increases from ~2 up to 10 or larger.In addition, the threshold of the mirror instability for cold electrons K is> 0, where

K ¼ T⊥T∥

� 1� 1β⊥

; (1)

And T⊥ and T∥ are ion perpendicular and parallel temperatures, respectively [see Génot et al., 2009; Ge et al.,2011a]. The value of T⊥/T∥ increases after the DF, which is in agreement with the findings of Ge et al. [2011a].After the DF, K is< 0 indicating that local conditions are mirror stable.

Figure 3 shows four ion pitch angle distributions of the differential particle flux (DPF) near the DF. We definethe perpendicular direction (relative to the ambient magnetic field) quantified by the pitch angle between60° and 120° and the field-aligned direction qualified by the pitch angle between 0° and 40° or between140° and 180° [see, Fu et al., 2012]. The pitch angle distributions have a distinct peak (energy range

Figure 2. (from top to bottom) The three components of the magneticfield data in the mean field-aligned coordinate system, ion perpendicu-lar beta, ratio of T⊥ to T∥, and the threshold of the mirror instability K ¼ T⊥

T∥�1� 1β⊥from C1. The shadows indicate the intervals selected to do mini-

mum variance analysis.

Figure 3. Differential particle flux distributions as a function of pitch angle and kinetic energy observed by C1 at four spintime intervals.

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1–4 keV) in the perpendicular direction accompanied by weaker peaks in the field-aligned direction at15:36:13 and 15:36:25 UT, while there is a strong peak in the field-aligned direction at 15:36:37 UT. The ionphase space density distributions in Figure 4 show that there are distinct peaks near the E×B drift velocitiesat 15:36:13, 15:36:25, and 15:36:37 UT, where the E×B drift velocities are approximately equal to the local ionbulk flow velocities. It is asymmetric in the phase space density distributions in the perpendicular direction,corresponding to the earthward moving ion bulk flow. There is a curved peak with parallel velocity between100 and 300 km/s at 15:36:13 UT, while a curved peak with a parallel velocity between 200 and 400 km/s canbe found at 15:36:25 UT. Both peaks have the following properties: (1) the maximum parallel velocity islocated near the perpendicular velocity with a value of VE× B and (2) the parallel velocity decreases accom-panied by an increasing perpendicular velocity.

Next, we determine the normal directions on each side of the structures by minimum variance analysis (MVA)[Sonnerup and Scheible, 1998]. Five intervals with a sharp change in BZ from C1, the shadow regions includingthe DF as shown in Figure 2, are selected for MVA. The results can be found in Table 1. The ratios of the inter-mediate to minimum eigenvalues are larger than 18 except for that of the first time interval in Table 1 with avalue of ~3.4, indicating that the estimated normal directions are reliable. These results show that the normaldirections on each side of the structures are almost perpendicular to the ambient magnetic field, and the nor-mal direction of the DF is almost earthward.

3. Discussion

The structures with periods between 8 and 13 s ahead of the DF are found to be mainly compressional andcompanied by DPF in the field-aligned direction in this study. Their velocities are comparable to the local ionbulk velocities in the X direction, indicating that these structures are almost frozen in the ion bulk flow. Weuse the ion bulk flow velocities and the durations of the structures to estimate their lengths in the X direction.Consequently, the lengths of the first and second structures in the X direction observed by C1 (C3) are~3.6 × 103 (3.4 × 103) and ~5.3 × 103 (5 × 103) km, respectively, on the order of the local proton gyroradius(~1.4 × 103 km). Although the periods of the second structure observed by C1 and C3 are a little different,the estimated lengths are almost equal. Figure 1 shows that the ion bulk flow velocities observed by C1 dur-ing the structures' interval are different from those observed by C3. These results suggest that the differenceof the structure durations or the magnetic profiles observed by C1 and C3 could be caused by the differenceof the local ion bulk flow velocities or by the different crossing positions of the spacecraft. The ion data

Figure 4. Ion phase space density distributions in field-aligned coordinates observed by C1 at four spin time intervals. The horizontal axis represents a cut of thedistribution perpendicular to the magnetic field and the vertical axis parallel to the field. The blue vertical line indicates the E × B drift velocity.

Table 1. Average Magnetic Field, Normal Direction, Ratio of the Intermediate to Minimum Eigenvalues Calculated byMinimum Variable Analysis, and Angle Between the Average Magnetic Field and Normal Direction During Each Intervalfrom C1

Time (hh:mm:ss) B (X, Y, Z) n (X, Y, Z) λint/λmin θ (deg)

15:36:11.8–15:36:13.5 (�0.72, �0.34, 8.47) (0.998, 0.008, 0.069) 3.4 90.915:36:18.6–15:36:19.6 (�0.82, 0.29, 6.3) (0.963, �0.233, 0.138) 43.9 89.915:36:19.6–15:36:21.8 (�1.18, �1.11, 6.93) (0.976, 0.035, 0.216) 19.4 87.515:36:25.0–15:36:28.0 (�0.43, �0.47, 6.65) (0.884, �0.459, �0.092) 18.8 96.715:36:29.7–15:36:31.0 (0.8, 4.9, 9.3) (0.97, �0.214, 0.117) 49.1 85.6

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resolution is ~4 s, which means that there are only two or three data points during each structure's interval.Thus, we cannot get an unambiguous result that whether the magnetic field amplitude and plasma densityare exactly anticorrelated or not. In spite of this, Figure 1 still shows that the ion density increases during thestructures, which indicates that the magnetic field amplitude and the ion density variations could be in anti-phase. The ion phase space density distributions in Figure 4 show that there are curved peaks with the max-imum parallel velocity located near the perpendicular velocity with a value of VE× B. VE× B is approximatelyequal to the ion bulk flow velocity; therefore, these peaks are almost fixed in the ion bulk flow. The peaks alsoshow that parallel and perpendicular velocities vary in antiphase. The properties of these peaks suggest thatparticles are partially trapped by the compressional structures ahead of the DF. All the above results are inagreement with the properties of the MMs [e.g., Gary et al., 1993; Rae et al., 2007; Soucek et al., 2008; Génotet al., 2009; Ge et al., 2011a]. Therefore, we suggest that these two compressional structures ahead of theDF are MMs.

Both mirror instability and ion cyclotron instability can develop in the presence of the ion perpendiculartemperature anisotropy (T⊥> T∥). Ion-cyclotron-like fluctuations of the magnetic field with a wide rangeof frequencies up to the proton cyclotron frequency are left-hand polarized and predominantly transverseto the ambient magnetic field [Gary et al., 1993; Génot et al., 2009]. Magnetosheath observations foundthat mirror-like fluctuations occur without proton-cyclotron-like waves if β ≥ 1 and T⊥/T∥ ≤ 2, whileproton-cyclotron-like fluctuations occur without mirror-like waves under the opposite conditions [seeGary et al., 1993]. Our results show that one structure ahead of the DF is almost purely compressional,and another has a dominant compressional component of the magnetic field with small transverse com-ponents. Besides, both structures occur when β⊥ ≥ 10 and 1.2< T⊥/T∥< 1.4. Therefore, local conditionsahead of the DF in this study are viable to mirror mode but not ion cyclotron mode [Gary et al., 1993;Soucek et al., 2008; Génot et al., 2009]. We infer that the small transverse components of the magnetic fieldduring the second structure's interval could be associated with the trajectories of the Cluster satellitescrossing the structure.

Temperature anisotropy can provide free energy for exciting the mirror instability [Hasegawa, 1969]. We findthat both MMs occur near the neutral sheet with high β⊥ value (>10) and K is somewhat larger than 0, whichmeans that local conditions can excite the mirror instability. These two structures display as a train of holes.Mirror modes in the magnetosheath are often observed to be displayed as trains of holes or peaks [Souceket al., 2008; Génot et al., 2009]. Génot et al. [2009] statistically found that deep holes are observed for mirrorstable conditions (K< 0); sinusoidal mirror modes and moderate holes and peaks are encountered undermarginally mirror unstable conditions, and large peaks are observed far from threshold, i.e., larger positiveK value. The MMs in this study are observed with a small positive K value, which indicate that local conditionsare marginally mirror unstable. MMs structures can be found ahead of some DFs; the evolution of these MMswill be the subject of future work.

The perpendicular temperature anisotropy does not increase during the structures' interval, while β⊥increases up to 10. In addition, local conditions satisfy the mirror instability. Figure 1 shows that the ion den-sity, velocity, and thermal pressure increase ahead of the DF. And the ion pitch angle distributions ahead ofthe DF also show clear peaks. These phenomena could be associated with ion particles reflected and accel-erated by the DF [Wu and Shay, 2012; Zhou et al., 2012]. Zhou et al. [2012] reported that an enhancement ofearthward moving ion fluxes can occur near the neutral sheet about 30 s before the DF arrival. Therefore, weinfer that ion particles reflected and accelerated by the DF and other phenomena associated with DF, such asthe compression of magnetic field and plasma ahead of the DF [Zieger et al., 2011; Liu et al., 2013], couldcontribute to the excitation of the mirror instability.

4. Summary

Magnetic compressional structures with periods between 8 and 13 s ahead of a DF have been investigated byusing Cluster data. We summarize our findings as follows: (1) the structures occur near the neutral sheet andare dominant in the Z component of the magnetic field; (2) the lengths of the structures in the X direction areseveral local proton gyroradius, and the normal directions on each side of the structures are almost perpen-dicular to the ambient magnetic field; (3) the structures are almost frozen in the ion bulk flow; the ion densityincreases when the magnetic field magnitude decreases; and the structures are balanced in pressure; (4) ions

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are partially trapped by the structures with parallel and perpendicular velocities varying in antiphase; (5) localconditions are marginally mirror unstable. The above findings suggest that the structures ahead of the DF aremirror mode, and local conditions ahead of the DF could possibly generate mirror mode waves or structuresvia exciting the mirror instability.

ReferencesBalogh, A., et al. (2001), Cluster magnetic field investigation: Overview of in-flight performance and initial results, Ann. Geophys., 19,

1207–1217.Breuillard, H., et al. (2016), Multispacecraft analysis of dipolarization fronts and associated whistler wave emissions using MMS data, Geophys.

Res. Lett., 43, 7279–7286, doi:10.1002/2016GL069188.Czaykowska, A., T. M. Bauer, R. A. Treumann, and W. Baumjohann (1998), Mirror waves downstream of the quasi-perpendicular bow shock,

J. Geophys. Res., 103, 4747–4753, doi:10.1029/97JA03245.Escoubet, C. P., M. Fehringer, and M. Goldstein (2001), The Cluster mission, Ann. Geophys., 19, 1197–1200.Fu, H. S., Y. V. Khotyaintsev, A. Vaivads, M. André, V. A. Sergeev, S. Y. Huang, E. A. Kronberg, and P. W. Daly (2012), Pitch angle distribution of

suprathermal electrons behind dipolarization fronts: A statistical overview, J. Geophys. Res., 117, A12221, doi:10.1029/2012JA018141.Gary, S. P., S. A. Fuselier, and B. J. Anderson (1993), Ion anisotropy instabilities in the magnetosheath, J. Geophys. Res., 98, 1481–1488,

doi:10.1029/92JA01844.Ge, Y. S., J. P. McFadden, J. Raeder, V. Angelopoulos, D. Larson, and O. D. Constantinescu (2011a), Case studies of mirror-mode structures

observed by THEMIS in the near-Earth tail during substorms, J. Geophys. Res., 116, A01209, doi:10.1029/2010JA015546.Ge, Y. S., J. Raeder, V. Angelopoulos, M. L. Gilson, and A. Runov (2011b), Interaction of dipolarization fronts within multiple bursty bulk flows

in global MHD simulations of a substorm on 27 February 2009, J. Geophys. Res., 116, A00I23, doi:10.1029/2010JA015758.Ge, Y. S., X.-Z. Zhou, J. Liang, J. Raeder, M. L. Gilson, E. Donovan, V. Angelopoulos, and A. Runov (2012), Dipolarization fronts and associated

auroral activities: 1. Conjugate observations and perspectives from global MHD simulations, J. Geophys. Res., 117, A10226, doi:10.1029/2012JA017676.

Génot, V., E. Budnik, P. Hellinger, T. Passot, G. Belmont, P. M. Trávníček, P. L. Sulem, E. Lucek, and L. Dandouras (2009), Mirror structures aboveand below the linear instability threshold: Cluster observations, fluid model, and hybrid simulations, Ann. Geophys., 27, 601–615.

Hasegawa, A. (1969), Drift mirror instability in the magnetosphere, Phys. Fluids, 12, 2642–2650.Liu, J., V. Angelopoulos, A. Runov, and X.-Z. Zhou (2013), On the current sheets surrounding dipolarizing flux bundles in the magnetotail: The

case for wedgelets, J. Geophys. Res. Space Physics, 118, 2000–2020, doi:10.1002/jgra.50092.Lucek, E. A., W. M. Dunlop, A. Balogh, P. Cargill, W. Baumjohann, E. Georgescu, G. Haerendel, and K.-H. Fornacon (1999), Mirror mode

structures observed in the dawnside magnetosheath by Equator-S, Geophys. Res. Lett., 26, 2159–2162, doi:10.1029/1999GL900490.Nakamura, R., et al. (2002), Motion of the dipolarization front during a flow burst event observed by cluster, Geophys. Res. Lett., 29(20), 1942,

doi:10.1029/2002GL015763.Rae, I. J., I. R. Mann, C. E. J. Watt, L. M. Kistler, and W. Baumjohann (2007), Equators observations of drift mirror mode waves in the dawnside

magnetosphere, J. Geophys. Res., 112, A11203, doi:10.1029/2006JA012064.Rème, H., et al. (2001), First multi-spacecraft ion measurements in and near the Earth's magnetosphere with the identical Cluster ion

spectrometry (CIS) experiment, Ann. Geophys., 19, 1303–1354.Schmid, D., M. Volwerk, R. Nakamura, W. Baumjohann, and M. Heyn (2011), A statistical and event study of magnetotail depolarization fronts,

Ann. Geophys., 29(9), 1537–1547, doi:10.5194/angeo-29-1537-2011.Schmid, D., M. Volwerk, F. Plaschke, Z. Vörös, T. L. Zhang, W. Baumjohann, and Y. Narita (2014), Mirror mode structures near Venus and Comet

P/Halley, Ann. Geophys., 32, 651–657, doi:10.5194/angeo-32-651-2014.Schmid, D., et al. (2016), A comparative study of dipolarization fronts at MMS and Cluster, Geophys. Res. Lett., 43, 6012–6019, doi:10.1002/

2016GL069520.Sckopke, N., G. Paschmann, S. J. Bame, J. T. Gosling, and C. T. Russell (1983), Evolution of ion distributions across the nearly perpendicular

bow shock: Specularly and nonspecularly reflected ions, J. Geophys. Res., 88, 6121–6136, doi:10.1029/JA088iA08p06121.Sonnerup, B. U. Ö., and M. Scheible (1998), Minimum and maximum variance analysis, ISSI Sci. Rep. Ser., 1, 185–220.Soucek, J., E. Lucek, and I. Dandouras (2008), Properties of magnetosheath mirror modes observed by Cluster and their response to changes

in plasma parameters, J. Geophys. Res., 113, A04203, doi:10.1029/2007JA012649.Takada, T., R. Nakamura, W. Baumjohann, Y. Asano, M. Volwerk, T. L. Zhang, B. Klecker, H. Rème, E. A. Lucek, and C. Carr (2006), Do BBFs

contribute to inner magnetosphere dipolarizations: Concurrent Cluster and Double Star observations, Geophys. Res. Lett., 33, L21109,doi:10.1029/2006GL027440.

Volwerk, M., T. L. Zhang, M. Delva, Z. Vörös, W. Baumjohann, and K.-H. Glassmeier (2008), Mirror-mode-like structures in Venus' inducedmagnetosphere, J. Geophys. Res., 113, E00B16, doi:10.1029/2008JE003154.

Volwerk, M., et al. (2016), Mass-loading, pile-up, and mirror-mode waves at comet 67P/Churyumov-Gerasimenko, Ann. Geophys., 34, 1–15,doi:10.5194/angeo-34-1-2016.

Wu, M. Y., Q. M. Lu, M. Volwerk, Z. Vörös, T. L. Zhang, L. C. Shan, and C. Huang (2013), A statistical study of electron acceleration behind thedipolarization fronts in the magnetotail, J. Geophys. Res. Space Physics, 118, 4804–4810, doi:10.1002/jgra.50456.

Wu, P., and M. A. Shay (2012), Magnetotail dipolarization front and associated ion reflection: Particle-in-cell simulations, Geophys. Res. Lett.,39, L08107, doi:10.1029/2012GL051486.

Zhang, T. L., et al. (2008), Characteristic size and shape of the mirror mode structures in the solar wind at 0.72 AU, Geophys. Res. Lett., 35,L10106, doi:10.1029/2008GL033793.

Zhang, T. L., W. Baumjohann, C. T. Russell, L. K. Jian, C. Wang, J. B. Cao, M. Balikhin, X. Blanco-Cano, M. Delva, and M. Volwerk (2009), Mirrormode structures in the solar wind at 0.72 AU, J. Geophys. Res., 114, A10107, doi:10.1029/2009JA014103.

Zhou, X.-Z., Y. S. Ge, V. Angelopoulos, A. Runov, J. Liang, X. Xing, J. Raeder, and Q.-G. Zong (2012), Dipolarization fronts and associated auroralactivities: 2. Acceleration of ions and their subsequent behavior, J. Geophys. Res., 117, A10227, doi:10.1029/2012JA017677.

Zieger, B., A. Retinò, R. Nakamura, W. Baumjohann, A. Vaivads, and Y. Khotyaintsev (2011), Jet front-driven mirror modes and shocklets in thenear-Earth flow-braking region, Geophys. Res. Lett., 38, L22103, doi:10.1029/2011GL049746.

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AcknowledgmentsThis work in China was supported byNSFC grants 41574173 and 41304145and supported by the Science andTechnology Development Fund ofMacao SAR (039/2013/A2 and082/2015/A3). The work by D.S. wassupported by the Austrian Science FundFWF under grant P25257-N27 andP23862-N16. We thank Ali Varsani forhelp with the ion phase space densitydistributions. We appreciate the ClusterActive Archive at http://caa.estec.esa.int/ and Cluster project FGM and CISteams. We also appreciate theGSFC/SPDF OMNIWeb at http://omni-web.gsfc.nasa.gov to provide the solarwind, IMF, and AE data.