Mirror-mode structures widely exist in various space plasma environments.
Here, we investigate a train of mirror-mode structures in the terrestrial
plasma sheet on 11 August 2017 based on the Magnetospheric Multiscale
mission. We find that bipolar current densities exist in the cross section
of two hole-like mirror-mode structures, referred to as magnetic dips. The
bipolar current density in the magnetic dip with a size of ∼2.2ρi (the ion gyro radius) is mainly contributed by variations
of the electron velocity, which is mainly formed by the magnetic
gradient–curvature drift. For another magnetic dip with a size of
∼6.6ρi, the bipolar current density is mainly
caused by an ion bipolar velocity, which can be explained by the collective
behaviors of the ion drift motions. The current density inside the mirror
dip contributes to the maintenance of the hole-like structure's stable. Our
observations suggest that the electrons and ions play different roles in the
formation of currents in magnetic dips with different sizes.
Introduction
Mirror modes are pressure-balanced and compressional magnetic structures
(Hasegawa, 1969; Tsurutani et al., 2011; Wang et al., 2016; Zhang et al.,
2018). They widely exist in many space plasma regions, such as solar wind
(Zhang et al., 2008, 2009; Russell et al., 2009), the planetary magnetosheath
(Volwerk et al., 2008; Schmid et al., 2014), the planetary magnetosphere
(Vaivads et al., 2001; Rae et al., 2007), and comets (Glassmeier et al.,
1993; Volwerk et al., 2016). These structures are believed to be generated
by the mirror instability excited in the mirror-unstable environment
(Hasegawa, 1969; Southwood and Kivelson, 1993). The plasma perpendicular
temperature anisotropy provides free energy to excite the mirror instability
(Kivelson and Southwood, 1996). Once the mirror-mode structures are
generated, they will be convected with the ambient flow since they are
non-propagating relative to the ambient flow (Tsurutani et al., 2011). Due
to gradients in the magnetic field and plasma density, the mirror-mode
structure may slowly propagate relative to the ambient plasma flow
(Hasegawa, 1969; Pokhotelov et al., 2003). It is expected that they will
stop to grow or decay when they move into the mirror-stable region.
Actually, they are reported to be able to survive in the mirror-stable
region in the solar wind and magnetosheath (Balikhin et al., 2009; Russell
et al., 2009).
Mirror-mode structures appear not only as quasi-periodic sinusoidal
oscillations, but also as local enhancements or decrease in the magnetic field
intensity, referred to as magnetic peaks or dips (Tsurutani et al., 2011).
Magnetic peaks can only exist in the mirror-unstable environments, while
magnetic dips are able to survive in the mirror-stable region (Kuznetsov et
al., 2007; Soucek et al., 2008). The typical scales of the mirror-mode
structures are 10 s ρi in the magnetosheath (Tsurutani et al.,
1982; Horbury and Lucek, 2009), where ρi is the ion gyro radius.
Based on observations of the four Cluster satellites, the longest scales of
the mirror-mode structures in the magnetosheath are found to be 2–6 times
the lengths of their shortest scales, and their shapes are approximately
cigar-like (Horbury and Lucek, 2009). By contrast, magnetic dips with a
scale less than 1 ρi also exist in the magnetosheath as well as
in the plasma sheet, and electron vortices are found inside the structure
(Ge et al., 2011; Huang et al., 2017, 2018, 2019; Yao et al., 2017).
In the terrestrial plasma sheet, there also exist mirror-mode structures
with several ion gyro radii (Vaivads et al., 2001; Zieger et al., 2011; Li
et al., 2014; Wang et al., 2016). The earthward fast flows can result in a
magnetic pileup in its leading area, and the ion perpendicular temperature
anisotropy in the pileup region is able to make the local plasma conditions
mirror-unstable to generate mirror-mode structures (Zieger et al., 2011).
Mirror-mode structures accompanied by electron dynamics and whistler waves
are also reported to occur during the dipolarization processes (Li et al.,
2014; Huang et al., 2018). Dipolarization fronts (DFs), characterized by a
sharp enhancement in BZ in GSM (geocentric solar
magnetospheric coordinates, used everywhere unless otherwise stated), are formed ahead of the earthward fast
flows (Ge et al., 2012; Wu et al., 2013; Schmid et al., 2016; Xiao et al.,
2017). They play an important role in the energy conversion, mass transport,
particle accelerations, and wave activities (Fu et al., 2012b; Huang et al.,
2012, 2015b). They are able to create a pressure pileup region ahead of the
DF when moving earthward (Schmid et al., 2011; Liu et al., 2013). Mirror-mode structures with a scale of ∼4ρi are reported
to occur in the pressure pileup region ahead of a DF, and the mirror
instability is suggested to be a potential mechanism to generate these
structures since local environments are mirror-unstable (Wang et al., 2016).
Within a mirror-mode structure there should be an electric current driven by
the magnetic gradient and curvature drifts of the ions and/or electrons in
order to sustain their stability (Constantinescu, 2002).
In this study, we investigate a train of ion-scale mirror-mode structures in
the terrestrial plasma sheet on 11 August 2017 using the Magnetospheric
Multiscale (MMS) mission data. Our aim is to figure out whether the main
contributor to the current density inside the ion-scale mirror-mode
structure is the electron or ion.
Observation
The MMS spacecraft consist of four identical satellites, which constitute a
tetrahedron with inter-spacecraft distances of tens of kilometers (Burch et al., 2015).
In the present study, we use the survey (a resolution of 16 Hz) of magnetic
field data obtained by the Fluxgate Magnetometer (Russell et al., 2016) and
the survey (4.5 s) of plasma data recorded by the Fast Plasma Instrument
(Pollock et al., 2016).
Overview of a DF event
Figure 1 shows that BZ sharply increases ∼8 nT within 7 s accompanied by a fast earthward flow with a maximum speed of
∼397 km s-1 at ∼20:38 UT on 11 August 2017.
Also, the local ion beta, the ratio of the ion thermal pressure to the
magnetic pressure, is ∼4, and the elevation angle
(θ=arctan(BZBX2+BY2)) changes ∼50∘ with a maximum angle of
64∘ (not shown). These observations satisfy the criteria of the DF
from Fu et al. (2012a), indicating that it is a DF event shown as the
vertical dotted line at around 20:38 UT in Fig. 1. At 20:40 UT, the MMS
spacecraft are located near (-18, 14.6, 2) RE in GSM. The
normal direction of the DF is (0.34, 0.82, -0.46) determined by the minimum
variance analysis (MVA) (Sonnerup and Scheible, 1998) using the data in the
interval between 20:37:33 and 20:37:42 UT. The ratio of the intermediate to
minimum eigenvalues (λ2/λ3) is ∼15, indicating that the estimated normal direction is reliable (Volwerk,
2006; Wang et al., 2014). The estimated normal direction suggests that the
MMS spacecraft are located at the duskward side of the DF based on the
semi-circle assumption of the DF (Huang et al., 2015a).
Observations of a DF event by MMS1 on 11 August 2017. From top to
bottom: three components of the magnetic field in GSM (a), the total
magnetic field (b), ion density (c), ion perpendicular (red) and parallel
(black) temperatures (d), ion perpendicular temperature anisotropy (e), the
threshold of the mirror instability (f), the magnetic, ion thermal, and total
pressures (g), and three components of the ion velocity in GSM (h). The gray
shadow indicates several compressional structures. The vertical dotted line
indicates the DF, and the dashed lines indicate the trough of two hole-like
structures.
Several quasi-periodic compressional magnetic oscillations with a period of
∼2 min are observed in the interval between 20:51 and 21:04 UT shown as the gray region in Fig. 1. The total magnetic field varies in
anti-phase with the ion number density during this interval. In addition,
the total pressure and sum of the magnetic and ion thermal pressures are almost
constant, indicating that they are pressure-balanced structures. The
threshold of the ion mirror instability Ki is shown in Fig. 1f, where
K=T⊥T∥-1-1β⊥, T⊥,
T∥, and β⊥ are perpendicular and parallel ion
temperatures and perpendicular ion beta, respectively (Southwood and
Kivelson, 1993). Local plasma environments become mirror-unstable and can
excite ion mirror instabilities when Ki>0. The maximum
Ki in each compressional structure reaches over 0.2, and it tends to
decrease to near or below 0 from the center of each structure to its edge.
Before 20:51 UT or after 21:04 UT, Ki is near or below 0; i.e., the
background environment for these structures is marginally mirror-stable.
The above properties of the compressional structures indicate that they are
likely to be mirror-mode structures (Tsurutani et al., 2011). Mirror-mode
structures are supposed to be non-propagating structures relative to the
ambient flow if there are no significant gradients in the magnetic field and
plasma density (Pokhotelov et al., 2003). Burst magnetic field data (a
resolution of 128 Hz) are available only between 20:51 and 20:54 UT; thus,
we perform timing analysis (Harvey, 1998) to calculate the propagating
velocity of the hole-like structure between 20:51:55 and 20:52:56 UT to
verify whether these compressional structures are non-propagating. Figure 2a
shows the positions of the MMS spacecraft relative to MMS1 at 20:52 UT. The
inter-spacecraft distances are ∼13 to 21 km. Before
performing the timing, the magnetic field data have been low-pass filtered
with a cutoff period of 30 s to reduce the effect of high-frequency
fluctuations. Figure 2b shows the cross-correlations between MMS1 and the
three other satellites by using BZ. The maximum correlation
coefficients are all almost 1 between MMS1 and MMS2/3/4, with lag times of
-0.312, -0.164, and -0.039 s, respectively. The estimated velocity is
(71.3, 11.7∘, -28∘) in spherical coordinates (r,θ, φ) transferred from the GSM coordinate system, where θ and φ are the longitude and latitude, respectively. By contrast,
the average ion velocity is (71.6, 37.8∘, -28.4∘) in
this interval. Comparing these two velocities, one can find that the
compressional structures in Fig. 1 are approximately stationary; i.e., they
are mirror-mode structures.
(a) Positions of the MMS spacecraft relative to MMS1 at 20:52 UT
in the x–z (left) and y–z (right) planes. (b) The cross-correlations between
MMS1 and the three other MMS satellites are calculated by using BZ in the
interval 20:52:55–20:52:56 UT.
The first and last mirror-mode structures as the dashed lines shown in
Fig. 1 are hole-like, which are referred to as magnetic dips. We will
focus on these two magnetic dips in the rest of the paper, and we mark them as MM1
(20:51:55–20:52:56 UT) and MM2 (21:02:26–21:03:34 UT).
Plasma properties in MM1
To further look at the plasma properties in the magnetic dips, we transform
the ion and electron velocities as well as the magnetic field and current
density into the principal axis (LMN) coordinate system as shown in Fig. 3. The principal axis vectors are calculated by MVA using the magnetic field
data obtained from MMS1 in the interval between 20:51:55 and 20:52:56 UT. To
reduce the effect of the high-frequency fluctuations, the magnetic field
data have been low-pass filtered with a cutoff period of 30 s before
performing the MVA analysis. The L, M, and N directions are (0.46, 0.27,
0.85), (0.28, 0.86, -0.42), and (-0.84, 0.43, 0.32) in GSM, respectively. The
eigenvalue ratio λ2/λ3 is ∼9.
From top to bottom: three components of the magnetic field, ion
and electron velocities in LMN, the ion and electron perpendicular thermal
pressures, the current density in LMN, and the ratio of |∇⋅B|/|∇×B| between
20:51 and 20:53:30 UT. The black, red, green, and blue colors indicate data
obtained from MMS1, MMS2, MMS3, and MMS4, respectively. The current density
is calculated by the curlometer technique. The gray region indicates the
interval of the magnetic dip.
Figure 3 shows that BL is dominant, while BM and BN vary
around 0. The angles between the average magnetic field in this interval and
the L, M, and N directions are ∼18, 108,
and 87∘, respectively. It indicates that the cross section of MM1
is approximately parallel to the M–N plane and is approximately
perpendicular to the ambient magnetic field. The N direction is supposed to
be parallel to the above-estimated velocity by timing; however, the angle
between these two directions is ∼37∘. The MVA
technique can be affected by waves or noises superimposed on the
discontinuity surface (Lepping and Behannon, 1980; Schmid et al., 2019),
while the inter-spacecraft distances and configuration of the MMS spacecraft
can affect the accuracy of the calculation (Harvey, 1998), which might be a
possible explanation for the large difference between the two estimated
normal directions. The ion velocity is mainly in the M–N plane during the
whole interval, and there are no significant changes in both ViM and
ViN. By contrast, the N component of the electron velocity VeN
shows a bipolar variation with an amplitude of ∼40 km s-1. To
reduce the effect of the high-frequency noise, the electron data have been
smoothed within a 30 s window in Fig. 3 as well as in Fig. 4.
Interestingly, an enhancement of (a decrease in) VeN occurs on the left-hand
(right-hand) side of MM1; i.e., a bipolar feature appears in VeN.
The current density in Fig. 3 is calculated by the curlometer technique
(Dunlop et al., 2002) using the magnetic field data low-pass filtered with a
cutoff period of 30 s. The current density can be regarded as reliable when
the ratio |∇⋅B|/|∇×B| is less than 0.2 (e.g., Wang et al., 2017, 2019). The N
component of the current density jN shows a bipolar variation similar
to VeN with an opposite trend of change. The correlation coefficient
between jN and VeN inside MM1 is -0.97. By comparing the
variations in the ion and electron velocities, one can note that the bipolar
current density inside MM1 is mainly associated with the electron velocity.
The peak and trough of the bipolar VeN tend to occur near the maximum
gradient of BL, while there is no significant change in Pe⊥.
Since the magnetic dips are stationary in the ambient flow, we can estimate
their scale in the cross section by
∫t1t2VMdt2+∫t1t2VNdt2,
where VM and VN are the M and N components of the ion velocity,
and t1 and t2 are the start and end times of each magnetic dip. The
scale of MM1 is estimated to be ∼4.1×103 km,
or ∼2.2ρi, where ρi is the local ion
gyro radius calculated by the average ion perpendicular temperature and the
average BT in MM1 between 20:51:55 and 20:52:56 UT. Since the spacecraft
may not cross the center of the magnetic dip, the estimated scale is the
lower limit.
From top to bottom: three components of the magnetic field, ion
and electron velocities in LMN, the ion and electron perpendicular thermal
pressures, the current density in LMN, and the ratio of |∇⋅B|/|∇×B| between
21:01 and 21:05 UT. The black, red, green, and blue colors indicate data
obtained from MMS1, MMS2, MMS3, and MMS4, respectively. The current density
is calculated by the curlometer technique. The gray region indicates the
interval of the magnetic dip.
Plasma properties in MM2
Figure 4 shows the magnetic field, ion velocity, electron velocity, and
current density in LMN between 21:01 and 21:05 UT. The magnetic field data
between 21:02:26 and 21:03:34 UT are used to calculate the principal axis
vectors by MVA. The ratio λ2/λ3 is
∼6, and the L, M, and N directions are (0.26, 0.1, 0.96),
(-0.44, 0.89, 0.02), and (-0.86, -0.43, 0.28), respectively. The angles
between the average magnetic field in this interval and the L, M, and N
directions are ∼1.5, 89, and
89∘, respectively. BL is dominant during the whole interval,
while BM and BN are very small. Thus, the cross section of MM2
is also approximately parallel to the M–N plane and almost perpendicular to
the ambient magnetic field. No large-amplitude fluctuations appear in MM2
compared to MM1. The ion velocities ViM and ViN are dominant, while
ViL varies around 0. Interestingly, a bipolar feature in ViN with
a variation up to 80 km s-1 (peak minus trough) can be distinctly found inside
the dip, while ViM tends to increase compared to the ambient flow.
ViN is smaller (larger) than the ambient value on the left-hand (right-hand) side
of the dip. The peak and trough of the bipolar ViN appear when there
are significant gradients in the magnetic field and the ion perpendicular
thermal pressures. This indicates that the bipolar ViN could be
associated with the magnetic gradient and diamagnetic drifts. The length of
MM2 in the cross section is estimated to be ∼6.4×103 km, or ∼6.6ρi.
The current density in Fig. 4 is also determined by the curlometer
technique. Before performing the curlometer analysis, the magnetic field
data have been low-pass filtered with a cutoff period of 20 s to
reduce the effect of the high-frequency fluctuations. One can find that
jN shows a similar bipolar feature to ViN. The correlation
coefficient between ViN and jN is 0.92 in the whole interval of
MM2, indicating that both parameters have a strong relation. The peak minus
the trough of jN during MM2 is ∼5.6 nA m-2. By
contrast, jL and jM have no such clear bipolar feature. The
electron velocities show variations with periods larger than 1 min, but
no clear bipolar feature appears in any component of the electron velocity
during MM2, indicating that the bipolar jN is mainly determined by
ViN.
To look at the variations of the ion flow in MM2, we assume that the ion
velocity observed during MM2 consists of Vi_a and
Vi_md, where Vi_a is the ambient ion
velocity and Vi_md is the ion velocity inside MM2
relative to the ambient flow. The average velocity 30 s before and
after MM2 is selected to be regarded as Vi_a with a
value of (-2.6, 51.4, 33.4) km s-1 in LMN. Figure 5 shows the deflection of
Vi_md in the M–N plane. The arrows indicate the
direction of the ion velocity, and their lengths indicate the magnitude of
Vi_md in the M–N plane. The direction of
Vi_md gradually changes from around -60 to
50∘ in the M–N plane. Also, the strength of Vi_md in this plane gradually increases and then decreases from the left-hand side
of the magnetic dip to the right-hand side. In addition, the N component of
Vi_md changes from negative to positive at just around
the center of the structure.
Ion velocities Vi_md in the M–N plane during
MM2. The arrows indicate the direction of the ion velocities, and their
lengths indicate the amplitudes of the ion velocities. The gray line
indicates the total magnetic field of MM2.
Discussion
Since mirror-mode structures are stationary in the ambient flow, we can
estimate the distance of the structures relative to the DF in the Y
direction using the average VY∼30 km s-1 during the
structures. Thus, they are likely to occur dawnside of the MMS spacecraft,
with a distance of ∼4RE in the Y direction when the
spacecraft are crossing the DF at around 20:38 UT. Comparing this distance
with the typical size of the DF (∼3RE) (Huang et al.,
2015a) and the size of the magnetic dips in Fig. 1, the mirror-mode
structures might come from the dawnside flank of the DF. Since the DF is
considered to be a tangential discontinuity (Schmid et al., 2019) which
pushes the background plasma to its flanks (Fu et al., 2012a, b; Liu et
al., 2013; Birn et al., 2015), the plasma near the flank is expected to come
from the pressure pileup region ahead of DFs. In addition, mirror-mode
structures have been reported to be potentially generated in such a pressure
pileup region (Zieger et al., 2011; Wang et al., 2016). Thus, the mirror-mode structures in Fig. 1 might originate from the pressure pileup region
ahead of the DF.
Based on Ampère's law, there should exist a current in the magnetic dip
to sustain the structure's stability (see Constantinescu, 2002). Figures 3
and 4 show that a bipolar current density is observed in both MM1 and MM2.
BL changes ∼5 nT in MM1 between 20:52:30 and 20:52:56 UT, and half of the estimated length of MM1 is 2.05×103 km
in the cross section. Assuming that BM and BN are 0 and that BL
changes just along the trajectory of MMS, a current density jB with a
value of ∼2 nA m-2 in the cross section is necessary to
be self-consistent with the magnetic field depression. The amplitude of the
bipolar jN is ∼2 nA m-2 between 20:52:30 and
20:52:56 UT, almost equal to jB, indicating that MM1 is a stable
structure (Constantinescu, 2002). Similarly, MM2 is also a stable structure.
Significant changes can be found in electron velocities in MM1, while the
three components of the ion velocity are almost constant. Therefore, the
current density in MM1 is mainly contributed by electrons. The amplitude of
the bipolar electron velocity in VeN is ∼40 km s-1 (see
Fig. 3). Three kinds of the electron drift motions are expected to create
the current density, i.e., the magnetic gradient drift, the magnetic
curvature drift, and the diamagnetic drift. The electron perpendicular
thermal pressure Pe⊥ changes ∼0.002 nPa in MM1, the
average electron number density is ∼0.4 cm-3, and the
average total magnetic field is ∼3 nT. Consequently, the
estimated electron diamagnetic drift velocity is ∼4 km s-1,
much smaller than the amplitude of the bipolar VeN. The peak of the
bipolar VeN occurs in the time interval between 20:52:40 and 20:52:50 UT, during which there are no significant magnetic field fluctuations. We
select this time interval to estimate the velocities of the magnetic
gradient and curvature drifts. The total magnetic field changes
∼1.1 nT, and the median total magnetic field is
∼2.2 nT in this interval. The median electron perpendicular
and parallel temperatures are ∼680 and 650 eV. The length
scale of MM1 is ∼4.1×103 km in the M–N plane
and its duration is ∼61 s; thus, the length for the time
interval between 20:52:40 and 20:52:50 UT is ∼680 km. Using
the data from all four MMS satellites, we can determine the curvature of MM1
by
ρcj=B-2Bi∇iBj-B-4BjBiBl∇iBl,
where the indices i, j, and l indicate the three components of the magnetic
field, and B=|B| (Shen et al., 2003). The curvature
radius RC is 1/ρc. Before performing the calculation, the
magnetic field data have been low-pass filtered with a cutoff period of 1 s to reduce the effect of the high-frequency noise. The median RC
in this interval is 1.1×103 km. Thus, the velocities of the
electron magnetic gradient and curvature drifts are ∼209
and 262 km s-1, respectively. Since the magnetic curvature drift in MM1 is in
the opposite direction of the magnetic gradient drift, the collective
velocity of these two velocities is ∼53 km s-1, which is close
to the amplitude of the bipolar VeN. It suggests that the bipolar
electron velocity in MM1 is mainly formed by the electron magnetic gradient
and curvature drifts.
The size of MM1 is ∼2.2ρi, and its central
magnetic field strength is almost 0. Thus, the ion gyro radius is expected
to significantly change within one orbit, and ions would randomly jump
between neighboring magnetic dips. These ions are referred to as chaotic
particles (Büchner and Zelenyi, 1989), which could be one reason why
ions do not seem to contribute to the formation of the current in MM1.
No significant changes occur in the electron velocity in MM2; thus, the
bipolar current density is mainly contributed by the variations of the ion
velocity (see Fig. 4). The size of MM2 is ∼6.6ρi, larger than that of MM1. The trough of the bipolar ViN is
observed at around 21:02:45 UT; meanwhile, ViM increases
∼50 km s-1 compared to the ambient flow on the left-hand side of
MM2. The amplitude of the bipolar ViN is ∼50 km s-1, i.e.,
the ion velocity inside MM2 ∼70 km s-1 relative to the ambient
ion flow. The ion perpendicular thermal pressure tends to be larger from the
edge of MM2 towards its center (see Fig. 4); therefore, an ion diamagnetic
drift is expected to be formed (Baumjohann and Treumann, 1997). We use the
data in the time interval between 21:02:30 and 21:02:50 UT to estimate the
ion thermal pressure and magnetic gradients. Also, the average ion
perpendicular and parallel temperatures, average total magnetic field, and
average curvature radius in this interval are used to estimate the
velocities of the ion drift motions. Consequently, the velocities of the ion
diamagnetic, magnetic gradient, and curvature drift motions are
∼17, 33, and 79 km s-1, respectively. By contrast, the
velocities of the electron diamagnetic, magnetic gradient, and curvature
drifts are ∼5, 14, and 36 km s-1, since the ion
diamagnetic and magnetic curvature drifts move almost in the same direction
in the M–N plane, while the ion magnetic gradient drift moves in the
opposite direction. Thus, the collective drift velocity is ∼63 km s-1, very close to the ion velocity inside MM2 with a speed of 70 km s-1.
Thus, one can expect that the bipolar ViN in Fig. 4 is the collective
behaviors of the ion drift motions in MM2.
Schematic of MMS1 crossing the magnetic dip in the M–N plane. The
colors changing from the center (red) of the magnetic dip to its edge (blue)
indicate the decrease in the ion perpendicular thermal pressure as shown by
the color bar. The black arrows in the magnetic dip indicate the direction of
the ion velocity. The red arrow indicates a possible trajectory of MMS1.
Except for the bipolar ViN, there is an enhancement of ViM in MM2.
To figure out the variations of ViM and ViN in MM2, we analyze the
possible trajectory of the MMS spacecraft crossing MM2. Mirror-mode
structures in the magnetosheath are found to be cigar-like structures
instead of sheets or tubes (Constantinescu et al., 2003; Horbury and Lucek,
2009). To simplify our analysis, we assume that the cross section of MM2 is
a circle. To be self-consistent with the magnetic field depression, the ion
flow as well as the current are supposed to be clockwise like the black arrows
shown in Fig. 6. Based on the normal directions of both half sides of
the structure along the spacecraft trajectory and the ambient flow
direction, we can get the possible trajectory of the MMS spacecraft in the
M–N plane. We calculate the normal directions of the two sides of MM2 by
MVA, and the values are (0.03, 0.79, 0.61) and (-0.05, -0.65, 0.76) in LMN
for the intervals 21:02:30–21:03 and 21:03:10–21:03:25 UT,
respectively. The ratios of the intermediate to minimum eigenvalues λ2/λ3 are 6.4 and 8.5, respectively. The normal
directions are almost orthogonal to each other; thus, the maximum length of
MM2 in the cross section could be 1.4 times the estimated length (6.6 ρi) based on the assumption of a circle. The velocity of the ambient
ion flow is (-2.6, 51.4, 33.4) km s-1 in LMN. Thus, a possible trajectory of
MMS in the M–N plane can be drawn based on the ambient flow and the above-normal directions like the red arrow shown in Fig. 6. Since the
inter-spacecraft distances are very small compared to the scale of MM2, only
the possible trajectory of MM1 is shown in Fig. 6. Along the trajectory,
ViN changes from negative to positive from one to another side of MM2,
while ViM is positive, which is in agreement with the deflection of the
ion flow shown in Fig. 5. Thus, the variations of ViM and ViN
are consistent with the prediction of the ion vortex in the cross section.
Such a ring-like flow might play an important role in the evolution of the
mirror-mode structure or in maintaining the stability of the magnetic dip.
Summary
We have studied the ion-scale mirror-mode structures in the plasma sheet on
11 August 2017. We find that a bipolar current density in the magnetic dip
with a size of ∼2.2ρi is mainly contributed by an
electron bipolar velocity in the cross section. The electron bipolar
velocity mainly results from the magnetic gradient and curvature drifts. The
chaotic motion of ions might be one significant reason that ions have almost
no contribution to the formation of the bipolar current in this magnetic
dip. For another magnetic dip with a size of 6.6 ρi, the bipolar
current is mainly contributed by the ion bipolar velocity, which can be
explained by the collective behavior of the ion drift motions. And the
variations of the ion velocity in the cross section suggest the potential
existence of the ion vortex. We suggest that the scale as well as the
magnetic geometry of the magnetic dip are significant for determining the roles
of electrons and ions in the formation of the current inside the dip.
Code and data availability
The FPI and FGM data used in the present paper are stored at the MMS Science Data Center (https://lasp.colorado.edu/mms/sdc/, MMS, 2019) and are publicly available.
Author contributions
GW and TZ designed the main idea of this study, and
the data analysis was mainly performed by GW. GW
prepared the manuscript with contributions from all the co-authors.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We acknowledge the data from the NASA MMS mission. We also acknowledge the MMS project FGM and FPI teams. The authors also acknowledge the financial supported by the grant from the Key Laboratory of Lunar and Deep Space Exploration, CAS.
Financial support
This research has been supported by the National Natural Science Foundation of China (NSFC) (grant nos. 41804157, 41774171, 41974205, 41774167, and 41904156), a grant from the Key Laboratory of Lunar and Deep Space Exploration, CAS,
the Shenzhen Science and Technology Research Program (grant no. JCYJ20170811154933612), and the 111 project (grant no. B18017).
Review statement
This paper was edited by Nick Sergis and reviewed by two anonymous referees.
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