Magnetic dipolarization can be observed at or inside geosynchronous orbit during intense substorms with high Auroral Electrojet (AE) indices (AE > 500 nT) (e.g., Dai et al., 2015; Nagai, 1982; Nosé et al., 2014; Ohtani et al., 2018). Dipolarizations are marked by the magnetic elevation angle increase with the decrease in the radial components of Bx and By and the increase in the Bz component (Liu and Liang, 2009; Duan et al., 2011; Dai et al., 2014, 2015). Ohtani et al. (2018) presented the statistical characteristics of magnetic dipolarizations inside geosynchronous orbit. They reported that the dipolarization region expanded in the azimuthal direction with a speed of 60 km s-1 at 5.5 RE. Using multiple satellite conjunction observations at or inside geosynchronous orbit, Dai et al. (2015) reported that the large dipolarization electric field was associated with a substorm injection of MeV electrons into the inner magnetosphere (r<6.6RE).

Magnetic dipolarizations are accompanied with complex ion bulk flows in the near-Earth plasma sheet (NEPS) (e.g., Duan et al., 2008; Liang et al., 2009). Especially, it is more complex in the inner edge of NEPS. Usually, the substorm-associated dipolarizations in the NEPS are accompanied with earthward ion bulk flows (e.g., Angelopoulos et al., 1992; Baumjohann et al., 1999; Duan et al., 2011; Liang et al., 2009; Liu et al., 2008; Nakamura et al., 2009; Shiokawa et al., 1998). Using the conjunction observations of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) multiple probes in the NEPS, Duan et al. (2011) pointed out that the dipolarization at inner edge of the near-Earth plasma sheet had no one-to-one relationship with the earthward ion bulk flow. Lui et al. (1999) pointed out that dipolarization at X∼10RE was detected with tailward flows. Inside geosynchronous orbit, magnetic dipolarizations were detected with the earthward ion bulk flow (Dai et al., 2015).

Near-Earth dipolarizations with low-frequency waves are detected with thermal ions and electron energization (e.g., Dai et al., 2015; Liang et al., 2009; Nosé et al., 2014; Ohtani et al., 2018). These energetic particles are the main source of inner magnetosphere during substorms and storms. Nosé et al. (2014) proposed that the dipolarizations associated with low-frequency fluctuations were observed in the inner magnetosphere during the storm main phase. These low-frequency electromagnetic waves can accelerate O+ ions in the perpendicular direction. The low-frequency waves can accelerate particles crossing the magnetic field with a large perpendicular electric field (e.g., Dai et al., 2014, 2015; Duan et al., 2016; Nosé et al., 2014). Usually, dipolarization associated dispersionless energetic particle injections are accompanied with earthward ion bulk flows in the NEPS (Dai et al., 2015). But few reports show dipolarizations with a sharp increase in the energy flux of energetic particles are associated with tailward ion flows at or inside geosynchronous orbit.

The ballooning mode which occurred in the near-Earth plasma sheet is associated with a tailward expansion of the plasma sheet during substorms (Liu, 1997; Liu et al., 2008; Liu and Liang, 2009; Liang et al., 2009; Saito et al., 2008). Liu et al. (2008) pointed out that the ballooning mode could excite a quasi-electrostatic field a few minutes before local current disruption and that the perturbations associated with the ballooning instability propagated downtail.

In this paper we present dipolarizations with tailward ion flows inside geosynchronous orbit during an intense substorm expansion phase. The observations of an intense substorm on 27 August 2014 by THEMIS-D (TH-D) and THEMIS-E (TH-E) are presented in detail in Sect. 2. Discussions and conclusions of our observations are displayed in the last section.

The OMNI data of the solar wind, interplanetary magnetic field (IMF) and the Disturbance Storm Time (Dst) and AE geomagnetic field indices, during a storm on 27 August 2014, are presented in Fig. 1. The minimum value of SYM-H index is about -90 nT, as shown in Fig. 1f and implies that a moderate storm had taken place. During the main phase of this moderate storm, there was an intense substorm with the AE maximum value 700 nT around 10:10 UT. The beginning of this intense substorm expansion phase was around 09:31 UT with decrease in AL index. A significant substorm enhancement occurred around 09:48 UT with a sharp decrease in AL index and an increase in AE index.

During this intense substorm, THEMIS probes (Angelopoulos, 2008) TH-D and TH-E were both located in the near-Earth magnetotail. Figure 2 displays the orbits of TH-D and TH-E from 09:20 to 10:00 UT in the solar magnetic (SM) coordinate system. At 09:30 UT, locations of these two spacecraft in SM coordinates were (-6.10, -0.06, 0.43) RE for TH-D and (-8.26, -2.28, 0.99) RE for TH-E, respectively. The TH-D orbit plot shows that it was located inside geosynchronous orbit at the beginning of this intense substorm expansion phase. On the other hand, TH-E was located outside geosynchronous orbit. These two spacecraft presented good conjunction observations during this intense substorm expansion phase. The instruments adopted in our investigations were a fluxgate magnetometer (FGM) (Auster et al., 2008), an electrostatic analyzer (ESA) (McFadden et al., 2008), an electric field instrument (EFI) (Bonnell et al., 2008) and a solid state telescope (SST) on-board the THEMIS probes.

Figure 3 shows the plasma parameters and the electromagnetic field detected by TH-D mostly inside geosynchronous orbit at around midnight in the interval from 09:25 to 09:55 UT. The solar magnetic (SM) coordinate system is adopted. The panels from top to bottom represent the following: the total magnetic field value Bt and the Bx component; the By and Bz components; the magnetic field elevation angle defined by θ=tan⁡-1(Bz/(Bx2+By2)1/2); the ion and electron density and temperature; the plasma value β (β=2μ0nT/B2) which determines the location of the satellite (Miyashita et al., 2000); three components of ion bulk flow velocity parallel (black line) and perpendicular (red line) to the magnetic field, Vx, Vy and Vz; three components of the electric field, Ex (red), Ey (black) and Ez (blue); three components of convection electric field from -V×B, Ecx (red), Ecy (black) and Ecz (blue), respectively. Figure 3 displays the distinct fluctuations of the magnetic field and plasma density and velocity at around 09:30 and 09:36 UT, respectively. The magnetic elevation angle has two step enhancements as displayed in Fig. 3c. The first increase in elevation angle is from about 15 to 25∘ during the interval from 09:30:34 to 09:30:54 UT, which are marked by the left two vertical dashed lines in Fig. 3. The total magnetic field value and the Bx component both decrease. The Bz component increases weakly from about 35 to 45 nT. The By component has obvious fluctuations around 0 nT. These magnetic signatures indicate a magnetic dipolarization had taken place inside geosynchronous orbit around (-6.10, -0.06, 0.43) RE. During this weak magnetic field dipolarization, the plasma beta value, β, increases from around 0.5 to 1.0. The electron density and temperature both increase. The ion density also increases, but the ion temperature decreases. This dipolarization accompanied by tailward ion bulk flows, V//x∼-100 km s-1, and the perpendicular component to the magnetic field in the X direction, V⊥x∼-50 km s-1, is detected by TH-D, as shown in Fig. 3g. The perpendicular velocity in the Y direction is mainly dawnward at the beginning of this dipolarization, V⊥y∼-30 km s-1. The electric field detected by TH-D also has large fluctuations with negative Ey values during the first depolarization as shown in Fig. 3j. During the intervals from 09:30:34 to 09:30:54 UT, the convection electric field direction is dawnward with a large magnitude, Ecy∼-12 mV m-1, as presented in Fig. 3k. The second magnetic field elevation angle increases sharply at around 09:36 as displayed in Fig. 3c marked by the right two vertical dashed lines. The elevation angle increases from about 25 to 45∘ during the interval from 09:36:06 to 09:36:21 UT. The magnetic field has similar variations to the first dipolarization signatures. Especially, the second dipolarization has larger elevation angle maximum value, ∼45∘, as marked by the fourth vertical dashed line in Fig. 3c. During the second dipolarization, the tailward ion bulk flows perpendicular to the magnetic field is also detected by TH-D, V⊥x∼-70 km s-1, as presented in Fig. 3g. Also the significant negative Ey component is accompanied by this intense dipolarization in Fig. 3j and k.

During the intervals of magnetic dipolarizations with tailward ion bulk flows detected by TH-D inside geosynchronous orbit, TH-E observed a very weak increase in the magnetic field elevation angle and the Bz component around 09:35 and 09:41 UT, as shown in Fig. 4b and c, about 5 min after the two dipolarizations detected by TH-D. The ion and electron densities and temperature increase weakly from very low values as displayed in Fig. 4d and e. Outside geosynchronous orbit, TH-E observed very low beta values, as shown in Fig. 4f, β∼0.01 and β∼0.2 around 09:35 and 09:41 UT, respectively. An interesting phenomenon, where the weak dipolarization was with tailward ion bulk flows, V//x∼-180 km s-1, was also detected by TH-E around 09:35 UT, as shown in Fig. 4g. The perpendicular velocity is dominated in the negative Y direction, V⊥y∼-50 km s-1.

The energy fluxes of energetic electrons associated with the intense electric field inside geosynchronous orbit during the two dipolarizations, as shown in the middle panel of Fig. 5, with energies of 31 (blue), 41 (gray), 52 (red), 65.5 (black), 93 (brown) and 139 keV (purple), all simultaneously increase at 09:30:38 and 09:36:09 UT as detected by the SST on-board TH-D. These energetic electrons have a quasi-perpendicular pitch angle distribution, as presented in the bottom panel of Fig. 5.

The dipolarizations with tailward ion bulk flows inside geosynchronous orbit are investigated in our present paper. The energy fluxes of energetic electrons accompanied these dipolarizations with energies between 31 and 139 keV simultaneously increase inside geosynchronous orbit. In accordance with these energetic electron pitch angle distributions, it was found that high-energy electrons were mainly in the quasi-perpendicular direction to the magnetic field, as shown in Fig. 5. On the other hand, the inductive electric field during these two magnetic dipolarizations was in the dawnward direction as displayed in Fig. 3j and k. Previous research work reported that the inductive electric field associated with substorm dipolarization can accelerate particles in the near-Earth plasma sheet (e.g., Dai et al., 2014, 2015; Duan et al., 2016; Fu et al., 2011; Fok et al., 2001; Liu et al., 2010; Lui et al., 1988, 1999; Nakamura et al., 2009; Nosé et al., 2014). As shown in Fig. 3j, at around 09:36:30 UT the inductive electric fields in the second dipolarization are dominated in the Ey component with a large negative value, Ey∼-25 mV m-1, and the X component also increases with a negative value Ex∼-6 mV m-1. This intense electric field can drive ions moving into the tailward–dawnward direction. On the other hand, we can calculate the energy quantity relationship between the electric field and energetic electrons. The energy of such an intense Ey in the distance of ∼1000 km is about ∼10-15 J. The energetic electrons with energy range from 31 to 139 keV are of the same energy order ∼10-15 J. It is inferred that the intense Ey can perpendicularly accelerate electrons to a state of tens of kiloelectronvolts.

Dipolarizations occurring at the inner edge of plasma sheet are complicated with disturbances of ion bulk flows and the electromagnetic field. Lui et al. (1999) pointed out that near-Earth dipolarization was a non-magnetohydrodynamic (non-MHD) process and was also accompanied with tailward ion flows. Our observations of dipolarizations inside geosynchronous orbit are also associated with tailward ion flows. This result is consistent with the report proposed by Liu et al. (2008) that the perturbations associated with the ballooning mode in the near-Earth plasma sheet propagate tailward. Based on the statistical studies, Nosé et al. (2016) proposed that the occurrence probability of the dipolarizations in the inner magnetosphere had a peak at 21:00–00:00  magnetic local time (MLT). Our observations show that two distinct dipolarizations with tailward flows inside geosynchronous orbit are detected by TH-D around 00:02 and 00:05 MLT, respectively.

According to the distance between TH-D and TH-E, (-2.23, -2.30, 0.56 RE) and the delay time of dipolarization from inside to outside geosynchronous orbit, ∼5 min, the dipolarization propagating speeds or the plasma sheet expanding speeds can be estimated as Vx∼-47, Vy∼-48 and Vz∼12 km s-1, respectively. Liou et al. (2002) proposed that the dipolarization region expanding speed was ∼60 km s-1 westward at geosynchronous orbit. By comparing observations between TH-D and TH-E in our investigations, the azimuth speed of dipolarization region is obtained ∼48 km s-1. These two observational results are consistent with each other. The dipolarization associated with the current disruption propagated tailward with a speed of Vx∼-100 km s-1 detected by THEMIS satellites in the near-Earth plasma sheet X∼-11RE (Liu et al., 2008). It is larger than the dipolarization propagating speed from inside to outside geosynchronous orbit Vx∼-47 km s-1. The different speeds of dipolarizations propagating tailward imply that the magnitude of the dipolarization speed may be associated with its beginning location in magnetotail plasma sheet.

On the other hand, Lui (1991) reported that substorm disturbance propagated tailward through a rarefaction wave front accompanied by earthward flows during the early substorm expansion phase period. Chao et al. (1977) proposed that the rarefaction wave propagating tailward was accompanied by the thinning of the plasma sheet and an earthward plasma flow. This earthward flow is possibly a convection flow or outflow of magnetic reconnection from the middle magnetotail.

Based on the above observation analysis, we can draw the results as follows. Two distinct magnetic dipolarizations with tailward ion flows are observed by TH-D inside geosynchronous orbit on 27 August 2014 during the intense substorm with AEmax∼1000 nT. TH-D was located inside geosynchronous orbit around midnight in the interval from 09:20 to 10:00 UT. The first dipolarization is displayed by the magnetic elevation angle increase from 15 to 25∘ around 09:30:40 UT. The second one is presented by the elevation angle increase from 25 to 45∘ around 09:36 UT. These two significant dipolarizations are accompanied by the energy flux of energetic electrons which simultaneously increase inside geosynchronous orbit. After a 5 min expansion tailward of near-Earth plasma sheet, TH-E located outside geosynchronous orbit also detects this tailward expanding plasma sheet with an ion flow of -150 km s-1. The dipolarization propagates tailward with a speed of -45 km s-1 along a 2RE distance in the X direction between TH-D and TH-E within 5 min. These dipolarizations with tailward ion flows observed inside geosynchronous orbit indicate a new energy transfer path in the inner magnetosphere during substorms.