Ebihara Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-019-0273-2(2019) 6:24

Progress in Earth andPlanetary Science

REVIEW

Open Access

Simulation study of near-Earth space
disturbances: 2. Auroral substorms
Yusuke Ebihara

Abstract
A substorm is a transient phenomenon that lasts for only 1–2 h. One significant manifestation of the substorm is asudden brightening of the aurora on the nightside. Simultaneously, the auroral electrojets are abruptly intensified in
the ionosphere, disturbing the geomagnetic field in the polar region. The near-Earth space environment is highly
disturbed, which manifests as an earthward fast flow of plasma, a sudden change in the magnetic field, or asudden increase in hot plasma. Such disturbances are known to severely affect human society. The ultimate
cause of the disturbances is the solar wind, but an explanation of the chain of processes leading from solar
wind to the ionosphere is problematic. Here, the evolution of the auroral substorm is reviewed based on the
results of a global magnetohydrodynamics (MHD) simulation, called a REProduce Plasma Universe (REPPU)
code. The REPPU code is shown to reproduce many aspects of the auroral substorms and to be useful forunderstanding the primary chain processes from the solar wind to the ionosphere.
Keywords: Substorms, Aurora, Computer simulation, Magnetosphere, Ionosphere

Introduction
A substorm is a transient, complex phenomenon thattakes place in the magnetosphere and the ionosphere. It
starts with a growth phase, followed by an expansion
phase, and a recovery phase. Usually, the substorm lastsfor 1–2 h. The terms “polar magnetic substorm” and“auroral substorm” were first suggested by Sydney
Chapman (Akasofu, 1968) to represent polar magnetic
disturbances and the sudden activation of aurora.
Later, Neil M. Brice and Kinsey A. Anderson introduced
the term “magnetospheric substorm” to represent magnetospheric disturbances that coincide with both polar
magnetic and auroral substorms (Akasofu 1968). The
term “substorm” was originally intended to mean an element of a magnetic storm, which is a worldwide magnetic
disturbance lasting for a few days. This would mean that amagnetic storm develops when substorms occur
frequently. However, some counterexamples are provided(Kamide 1992; McPherron 1997), and the storm-substorm
relationship is still an unsolved problem.
One of the most striking and remarkable phenomenon
throughout the entirety of the substorm is an explosive
Correspondence: ebihara@rish.kyoto-u.ac.jp
Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho,Uji, Kyoto 611-0011, Japanbrightening of aurora followed by expansion (Akasofu
1964; Akasofu et al. 1965), known as auroral breakup.
Once the auroral breakup begins, a large amount ofenergy, exceeding 1011 W, is consumed in the polar ionosphere (Ahn et al. 1983; Sun et al. 1985; Kamide et al.
1986; Richmond et al. 1990; Palmroth et al. 2005). The
auroral substorm is a subset of the polar substorms, including polar magnetic substorms, ionospheric substorms,
X-ray substorms, proton aurora substorms, VLF emission
substorms, and micropulsation substorms (Akasofu 1968,
references therein). Substorm-associated magnetosphericdisturbances have also been identified such as hot plasma
injection into the inner magnetosphere (DeForest and
McIlwain 1971), high-speed flow in the plasma sheet(Hones and Schindler 1979), and a dipolarization front(Nakamura et al. 2002). Auroral kilometric radiation(Gurnett 1974) is also a notable feature. These disturbances are often used as proxies for the substorm onset,
but cannot provide consistent timing for it (Liou et al.
1999). Liou et al. (1999) suggested the use of auroral
breakups as a common reference time frame for substorm
onset. Therefore, the auroral substorm is of particularfocus in this paper.
Global magnetohydrodynamics (MHD) simulation studies
have been conducted to understand the evolution of© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

(2019) 6:24

Page 2 of 24

substorms. The following is a summary of previously
reproduced features and results obtained based on the
global MHD simulations: (1) an intensification of auroral electrojets in the ionosphere (Lyon et al. 1998;
Wiltberger et al. 2000b; Lopez et al. 2001; Raeder et
al. 2001; Tanaka et al. 2010), (2) a midlatitude positive bay of ground magnetic disturbance (Tanaka
2015), (3) Pi2 magnetic pulsation (Tanaka 2015), (4)ground magnetic disturbances from pole to equator
including overshielding conditions (Ebihara et al.
2014), (5) the global evolution of auroral patterns(Fedder et al. 1995; Palmroth et al. 2006; Tanaka
2015; Ebihara and Tanaka 2016), (6) localized auroral
patterns (Raeder et al. 2012), (7) the evolution of thepolar cap boundary (Lopez et al. 2001), (8) an earthward fast flow and associated enhancement of themagnetic fields in the plasma sheet (Hesse and Birn
1994; Tanaka 2000b; Slinker et al. 2001; Raeder et al.
2010; Birn et al. 2011), (9) the formation of a current
wedge (Hesse and Birn 1991; Birn et al. 1999; Birn
and Hesse 2014), (10) a tailward moving current
surge (Ohtani and Raeder 2004), (11) evolution ofmultiple near-Earth neutral lines (El-Alaoui et al.
2009), (12) energy transfer from solar winds(Papadopoulos et al. 1999; Ebihara and Tanaka 2017),(13) energy conversion in the tail region (Birn and Hesse
2005; Ebihara and Tanaka 2017, and (14) the injection of energetic particles into the inner magnetosphere with the aid of particle simulations, or
Fokker-Planck simulations (Birn et al. 1998; Kim et al.
2000; Fok et al. 2006; Ebihara and Tanaka 2013;
Nakayama et al. 2017).
In this paper, the simulation results obtained using the
REProduce Plasma Universe (REPPU) code (Tanaka 2015)
are used to demonstrate recent understanding of theauroral substorm. Detailed information about REPPU isdescribed in the Appendix.

200

General overview of simulated substorm
The simulation results overviewed in this paper are
obtained using the REPPU code with the following solar
wind and IMF parameters (Ebihara and Tanaka 2015a,
2016, 2018) unless otherwise mentioned. Initially, a northward IMF condition was imposed for 2 h to establish aquasi-steady state magnetosphere: a solar wind density of
5.0 cm−3, a solar wind speed of 372 km s−1, the Y-component of the interplanetary magnetic field (IMF B) at− 2.5 nT, and the IMF Bz at 4.3 nT. IMF Bx was held
at 0 throughout the calculation. IMF Bz was changedto − 3.0 nT after 2 h.
Figure 1 shows the H-component of magnetic disturbance on the ground at 48 magnetic local times (MLTs)
with an interval of 0.5 h at 70 magnetic latitude (MLAT).
The magnetic disturbances were calculated based on theionospheric Hall current. The upper envelope of the magnetic disturbance can be regarded as Auroral Upper (AU),and the lower as Auroral Lower (AL) indices (Davis andSugiura 1966). “T = 0” is referred to as the moment when
the southward IMF reached X = 40 Re. At T ~ 20 min, theAL index starts showing a gradual decrease. At T ~ 70 min,the AL starts decreasing abruptly, which can be regarded as
the beginning of the expansion phase of a substorm. Thesechanges are consistent with observation (McPherron 1970).
Rapid development of the westward electrojet as
manifested by the rapid decrease in AL is also reproduced in the other MHD simulations (Fedder et al. 1995;
Pulkkinen et al. 1998; Lyon et al. 1998; Wiltberger et al.
2000a; Lopez et al. 2001; Raeder et al. 2001). The westward electrojet (as seen by AL) peaks in the post-midnight
region, and the eastward electrojet (as seen by AU) peaksin the post-dusk region. This is consistent with observations (Silsbee and Vestine 1942; Allen and Kroehl 1975;
Kamide et al. 1994; Guo et al. 2014).
Figure 2 summarizes the field-aligned current (FAC) in
the ionosphere (Ebihara and Tanaka 2018). The simulation

AU

20

H (nT)

0
15

-200

10

AL

-400
-600
0

MLT (hour)

Ebihara Progress in Earth and Planetary Science

5
0

15

30

45

60
75
Time (minute)

90

105

Fig. 1 The H-component of the magnetic disturbance at 70 MLAT and 48 MLTs. The upper and lower envelops are regarded as AU and AL
indices, respectively

(2019) 6:24

T=1.6 min06

18

T=31.5 min

80

80
70

18

T=41.7 min

70

06

18

T=45.7 min

06
High-latitude aurora
80

80

70

70
18

06

T=69.4 min

06

18

T=71.6 min

80

06

0.4

0.2

80
0.0

Initial brightening 70

T=73.0 min

06

18

T=73.4 min

80
Bulge

18

80
Surge

70

T=74.5 min

06

18

80
Surge

06

00 min
T=77.2
Surge

70

70

-0.2
Upward

18

70

-0.4

Field-aligned current (10 -6A/m2)

18

Page 3 of 24

Downward

Ebihara Progress in Earth and Planetary Science

06

80
70

Fig. 2 Field-aligned current in the ionosphere (negative upward). The contour line indicates the ionospheric electric potential at an interval of 5
kV. The solid and dashed lines indicate the positive and negative potentials, respectively. The Sun is towards the top. The lower boundary is
located at 65 magnetic latitudes (MLATs). (Adapted from Ebihara and Tanaka (2018))

is the same as that used for Fig. 1. In the global MHD
simulation, the FAC J|| is given byJ∥ ¼

1ð∇  BÞ∥ ;μ0

ð1Þ

where μ0 is the magnetic constant. However, a global
MHD simulation based on the ideal MHD assumptions
cannot deal with the kinetic processes regarding electron
precipitation. The emission from excited atoms andmolecules in the thermosphere was also not calculated.
We used the upward FAC as a proxy of the brightaurora (discrete aurora) because the upward FAC issuitably correlated with the brightness of the discrete
aurora (Korth et al. 2014).

1. At T = 1.6 min, the magnetosphere is still exposedto the northward IMF. The distribution of the
upward FAC (hereinafter, simply referred to as
aurora) is complicated. N-S-aligned structures ofaurora are evident at high latitudes, which resemble
the sun-aligned arcs observed at high latitudes
when the IMF is northward, or close to zero (e.g.,
Ismail et al. 1977). The structure of the aurora(upward FAC) originates from the structured
plasma pressure in the high-latitude magnetosphereas explained later in this paper (Ebihara and Tanaka
2016). Detailed explanation for the sun-aligned arcs
is provided in terms of topology of the magnetic
field line (Tanaka et al. 2017b).
2. As the IMF turns southward, the two-cell patternof the ionospheric convection, known as the DP2

Ebihara Progress in Earth and Planetary Science(2019) 6:24

equivalent current system (Nishida 1968), starts
developing. The N-S aligned structures moveequatorward. For the sake of simplicity, we callthem “high-latitude aurorae,” which represent
structured aurorae inside the main auroral oval,
particularly on the nightside. The high-latitudeaurorae seem to originate from sun-aligned arcs,
and the direction of the high-latitude aurorae canchange to the E-W direction, and vice versa. A quiet
arc is also evident, which is caused by flow shear in
the high-latitude magnetosphere (Tanaka et al. 2017a).
3. At T ~ 69.4 min, the aurora starts becoming brightin the latitudinally narrow area of the equatorward
part of the preexisting aurora. This may be
regarded as an initial brightening of the aurora(Akasofu 1964). The brightening occurs at ~ 70
MLAT in the post-midnight sector for this particular simulation, which is slightly poleward and eastward of the median location of the expansion onset
observed by satellite (Liou 2010). Tanaka et al.
(2017a) used the same simulation code with different solar wind parameters, showing that the brightening starts at ~ 68 MLAT near midnight. Thus,
the location of the brightening is probably determined by solar wind parameters and simulation
settings, but the determinant factors that control

Page 4 of 24

the onset location are still unknown. The expansion onset predominantly takes place in the premidnight sector (Liou 2010). The eastward shift
in the location of the expansion onset may be due tothe following two factors. One is the skew of themagnetic field line depending on IMF By. The resultsof the REPPU code show that the skew is maximized
near the equatorial plane near the expansion onset(Saita et al. 2011). Observations show that thelocation of the expansion onset shifts eastward fornegative IMF By in the Northern Hemisphere (Liouand Newell 2010). The second factor is the eastward
shift of a near-Earth neutral line (NENL), which is
described below. The simulated location of the expansion onset is not far from that observed and iswithin the observational limit as shown in Fig. 3 ofLiou (2010). It should be emphasized that theprocesses behind the substorm are confirmed to be
independent of the location of the brightening.
4. After a few minutes, the bright aurora starts
expanding poleward, which may be referred to as abulge (Akasofu 1964).
5. As the bulge develops, an additional auroral
structure appears at T ~ 73.4 min, which propagates
westward. This may be referred to as the westward
traveling surge (Akasofu 1964). The calculated AL

Formation of
NENL
Onset

Arrival of SBZ
AU/AL (nT)

200
0
-200
-400

85

High-latitude aurora 22 MLT
Arrival of surge

MLAT (deg)

80

>0.08
0.05

75
0.00
70

Quiet arc

65

-0.05

60

<-0.08
00

15

30

45
60
75
Time (minutes)

90

105

Field-aligned current (10 -6 A/m 2)-600

Fig. 3 (Top) AU and AL indices and (bottom) field-aligned currents (negative upward) as a function of MLAT and time. (Adapted from Ebihara
and Tanaka 2016)

Ebihara Progress in Earth and Planetary Science(2019) 6:24

index shows an abrupt decrease (as shown in Fig. 1)
in association with the development of the brightaurora (Kamide and Akasofu 1975).
Figure 3 shows the FAC as a function of MLAT andtime at the meridian at 22 MLT (Ebihara and Tanaka
2016). After the arrival of the southward IMF (SBZ), theauroral structures, regarded as high-latitude aurora, startmoving equatorward. After some time, the aurora isintensified at T ~ 76 min due to the arrival of the westward traveling surge. In general, the temporal evolutionof the aurora is consistent with observations by satellite(e.g., Plate 2 of Brittnacher et al. 1999), although thesteady solar wind and IMF conditions are used to drive
the REPPU code.

Growth phase
The term “growth phase” is used to describe a precursor
or pre-conditioning stage that occurs before the explosive auroral brightening (McPherron 1970; Akasofu andSnyder 1972). Some auroral structures, such as the quietarc, move equatorward gradually. Gradual changes in
the horizontal component of the magnetic field (ΔH) are
observed in the auroral oval. In the magnetosphere, thestrength of the magnetic field in the lobe is graduallyenhanced (Fairfield and Ness 1970), and the magnetic
field lines are gradually stretched in the magnetotail(Nishida and Fujii 1976). The magnetic energy is stored
in the tail region, which is released in the expansion
phase (Atkinson 1966; Siscoe and Cummings 1969).
The quiet arc is one of the auroral activities that isoften observed during a growth phase (Akasofu 1964;
Meng 1976; Lassen et al. 1977; Johnson et al. 1998;
Lessard et al. 2007; Haerendel et al. 2012). The quiet arc
is also known as a growth phase arc (Haerendel 2010;
Lyons et al. 2011; Nishimura et al. 2012), a preexistingarc (Jiang et al. 2012), and a quiet evening arc (Pritchettet al. 2014). On the basis of satellite observation, thethickness of the quiet arcs are estimated to be ~ 100 km(Johnson et al. 1998), which may be an upper limit
because of the relatively coarse resolution ofsatellite-borne imaging. Ground-based optical cameras
have captured an auroral arc with a thickness of 3.5 km
at 200 km altitude (Dahlgren et al. 2009). Precipitating
electrons with a peak energy of ~ 1–5 keV are thought
to cause the quiet arc, suggesting that the electrons
are accelerated by the field-aligned electric field(Meng 1976; Lassen et al. 1977; Haerendel et al. 2012)
in the region of the upward field-aligned current(FAC) (Haerendel et al. 2012; Jiang et al. 2012). The
upward FAC associated with the quiet arc is thought
to originate from an azimuthal gradient in the plasmapressure near midnight (Lyons and Samson 1992;
Antonova 1993; Stepanova et al. 2002; Haerendel 2008;

Page 5 of 24

Coroniti and Pritchett 2014). Using the REPPU code,
Tanaka (2015) proposed that the quiet arc is generated by
the FAC associated with flow shear in the Y-direction of
the high-latitude magnetosphere. Convective flow fromthe high-latitude lobe towards the equatorial plane isdeflected and divided into the westward and eastward
flows off the equator. Another interpretation of the formation of the quiet arc is the precipitation of electrons
caused by fieldline curvature scattering (Pulkkinen et al.
1992; Yahnin et al. 1997; Sergeev et al. 2011). The contribution of scattered particles to the formation of thequiet arc should be investigated in the future.
The high-latitude aurorae, which appear within themain auroral oval as shown in Fig. 2, resemble theauroral structures propagating equatorward during the
growth phase (Akasofu 1964; Kadokura et al. 2002;
Kepko et al. 2009; Nishimura et al. 2010; Mende et al.
2011). The high-latitude aurora is elongated in the N-S
or E-W directions. The N-S-aligned high-latitude aurora
can switch to the E-W, and vice versa. This is consistent
with observational results showing that 34% of the arcs
are east-west (E-W) aligned, 48% of them arenorth-south (N-S) aligned, and 19% switch from N-S to
E-W alignments over the course of the equatorward
movement (Nishimura et al. 2010). Ebihara and Tanaka(2016) suggested that the high-latitude aurorae areclosely associated with the flow vorticity near the complicated structure of the plasma pressure region of thelobe off the equator. Figure 4 shows the evolution ofplasma pressure in the Y-Z plane at X = − 7.0 Re and in
the X-Z plane at Y = − 1.5 Re. At T = 3.7 min, small protuberances of the plasma pressure distribution appear in
the lobe as indicated by the arrow in Fig. 4a. During theperiod from the quiet to the early growth phase (T = 3.7,
14.6 and 29.6 min), protuberances are clearly identified
in the Y-Z plane and are shown to evolve away from theequatorial plane. Consequently, the plasma pressure distribution becomes complicated off the equator. Eachcomplicated structure moves away from the equatorial
plane and towards the east in the Northern Hemisphereand the west in the Southern Hemisphere. The distribution of the plasma pressure seen in the Y-Z plane is similar to the Rayleigh-Taylor type instability.
Figure 5 is a sketch for the evolution of a protuberance, as the structured plasma pressure distribution(Ebihara and Tanaka 2016). A protuberance of thehigh-pressure region appears due to some reason (indicated by yellow color in Fig. 5). As the protuberance
moves away from the equatorial plane, it induces flow
vorticities. Counterclockwise vorticity (viewed in the
direction of the field lines) appears at the west of the
protuberance, generating downward FAC. Clockwise
vorticity appears in the east, generating upward FAC.
When these FACs are connected to the ionosphere, the(2019) 6:24

Ebihara Progress in Earth and Planetary Science(a) t=3.7 min
20(b) t=14.6 min

Page 6 of 24

20

20

10

10

10

-10

-20 10

-10

-10

-10

-20

-20

-20

-30

-40

-50

(c) t=29.5 min

0.3

0.2

(d) t=43.7 min

0.1

Pressure (nPa)

>0.4

0.0

(e) t=55.7 min
NENL(f) t=69.5 min(onset)Onset-related enhancement

Z
Y(X=-7 Re)Z
X
(Y=-1.5 Re)

Fig. 4 Plasma pressure in the Y-Z plane at X = − 7.0 Re (left), and in the X-Z plane at Y = − 1.5 Re (right) at a 3.7, b 14.6, c 29.5, d 43.7, e 55.7, andf 69.5 min. (Adapted from Ebihara and Tanaka 2016)

(2019) 6:24

Ebihara Progress in Earth and Planetary Science

Page 7 of 24

Z
X
Field-aligned current
Eastward
electric
field

Y

||

||

Flow
Protuberance of high-pressure region

Fig. 5 A possible generation mechanism for the structured distribution of plasma pressure, which is associated with the high-latitude aurorae
often observed in the growth phase. (Adapted from Ebihara and Tanaka 2016)

eastward electric field develops to satisfy current continuity
in the ionosphere. The eastward electric field propagates
into the magnetosphere, causing further development of
the protuberance. Because of the non-linear nature, theprotuberance (the structure of the high-pressure region)
becomes complicated as shown in Fig. 4.
As the IMF turns southward, the magnetospheric convection is enhanced. The structured high-pressure
region that appeared off the equator moves towards theequatorial plane due to enhanced convection. Consequently, the high-latitude aurora moves equatorward in
the ionosphere (T = 29.5, 43.7, and 55.7 min). The
high-latitude aurora is elongated in the N-S direction, or
the E-W direction, depending on the structure of thehigh-pressure region. The uncertainty of the direction isconsistent with observation (Nishimura et al. 2010).
Figure 6 shows the calculated FACs together with theionospheric flow (Ebihara and Tanaka 2016). The
high-latitude aurorae, elongated in the E-W and N-S
directions, resemble observations as shown in Fig. 7(Nishimura et al. 2010). The high-latitude aurora islocated near the westward edge of the equatorward flow
channel in the ionosphere. However, a downward FAC islocated near the eastward edge of the channel. The equatorward flow channel is caused by the westward electricfield arising from the current closure brought by a pair ofthe localized FACs. These localized FACs are closely associated with the protuberance of the high-pressure region
off the equator (Ebihara and Tanaka 2016), and not theplasma sheet, as shown in Fig. 8. One may speculate thatthe equatorward flow channel is a projection of the earthward flow channel in the plasma sheet and that a pair of
the FACs are connected to both sides of the earthward
flow channel as illustrated by Sergeev et al. (2000). The
simulation result shows that most of the high-latitudeaurorae are located in open magnetic field lines as shownin Fig. 8. The white lines in Fig. 8 indicate the magnetic
field line extending from the high-latitude aurorae. The

magnetic field lines extending from aurorae 1 and 2 are
open, whereas the magnetic field line extending from aurora 3 is closed. This means that some of these lines cannot
be mapped to the plasma sheet.
The existence of the high-pressure region in the lobe
is not confirmed. One possibility is the highly structuredions observed by the Cluster satellite in the lobe region(Shi et al. 2013). These are observed in the northward
IMF, with typical energies in the kiloelectron volt range.
Shi et al. (2013) explained the ions in terms of the entry
of solar wind ions into the high-latitude magnetosphere.
However, there is a possibility that the structured ionsare responsible for the generation of the high-latitudeaurora (Ebihara and Tanaka, 2016). In the global MHD
simulation, the structured high-pressure region develops in
the northward IMF. If the IMF remained northward over along time period, the high-latitude magnetosphere would
become filled with the high-pressure region as observed by(Zhang et al. 2009). This will be investigated in the near
future in terms of the evolution of the high-latitude aurora
and the structure of the magnetosphere.

Onset of the expansion phase
One of the longitudinally elongated auroral arcs, usually
the equatorward arc, starts showing a sudden brightening followed by a rapid poleward motion (Akasofu
1964). This is called an auroral breakup. The trigger
process leading to auroral breakup is a controversial
issue. Observations show that the NENL is closely associated with substorms (Hones et al. 1973; Nishida andNagayama 1973). It is natural to consider that intense
FACs appear in the vicinity of the NENL and initiate theauroral brightening (Baker et al. 1993; Treumann et al.
2006). If this is the case, auroral breakups will start near
the poleward boundary of the auroral oval because thisis thought to be the open-closed boundary of the magnetic field line. However, observations show that auroral
breakups start near the equatorward boundary of the(2019) 6:24

Page 8 of 24(a) FAC
22

00
MLT

02

>0.08

80
MLAT

0.05

High-latitude aurora

0.00

Quiet arc
70

FAC (10-6A/m2)

Ebihara Progress in Earth and Planetary Science-0.05<-0.08

22

00
MLT

02

>400

80
MLAT

200

0

70

-200
<-400

Northward flow velocity (m/s)(b) Northward ionospheric flow velocity(t = 43.7 min)Fig. 6 a Simulated high-latitude aurora and b northward ionospheric flow velocity during the growth phase (at T = 43.7 min). The contour line
indicates the ionospheric electric potential at an interval of 2 kV. (Adapted from Ebihara and Tanaka 2016)

auroral oval (Lui and Burrows 1978; Elphinstone et al.
1991). To solve this problem, a current disruption model
has been suggested (Lui 1996). Current disruption isthought to occur in the near-Earth plasma sheet at a radial distance of ~ 8 Re and is supported by observations
that the magnetic field oscillates in large-amplitude
wavelengths, breaking down the frozen-in condition(Takahashi et al. 1987; Lui 2011). Ballooning mode instability is one of the plausible mechanisms leading to
the current disruption (Roux et al. 1991; Cheng and Lui
1998; Liu et al. 2012). Once the current disruption
occurs, the current wedge (CW) appears as a diversionof the cross-tail current (Lui 1996). Considering earthward flow in the plasma sheet (Hones 1979), one canreasonably regard the formation of the NENL as a trigger of the auroral breakups starting near the equatorward boundary of the auroral oval. Examples include theintrusion of low entropy plasma (Chen and Wolf 1993),
inertial current associated with flow braking (Haerendel
1992; Shiokawa et al. 1997), and flow vorticity (Birn and
Hesse 1991; Birn et al. 2004b; Keiling et al. 2009). Both the
NENL and current disruption models can explain auroral
breakups starting near the equatorward boundary.

Many research efforts aimed at understanding thewhole process from trigger to auroral evolution havebeen conducted. However, it was difficult to construct a
synthesis model without contradictions due to thelimited number of observations. Here, the capability of
the MHD model in reproducing the auroral substorm
expansion is briefly overviewed. First of all, it should be
noted that the MHD model cannot deal with the kinetic
processes involved by the current disruption (Lui et al.
1990; Cheng and Lui 1998). Despite this limitation, theMHD model has successively reproduced many aspectsof substorm expansion (Tanaka, 2015). In the MHD
model, the substorm expansion is triggered by the formation of the NENL. Figure 9 shows an example of the
X-component of the plasma flow velocity perpendicular to
the magnetic field in the equatorial plane at T = 55.7 min.
The simulation settings are the same as those used for
Figs. 1, 2, 3, 4, and 6. The NENL is identified at ~ 40 Re
downtail. The NENL appears due to an increase in theanomalous resistivity that is a function of the local magnetic field and the current density (Tanaka et al. 2010).
For this particular simulation, the location of the NENL is
tailward of that typically observed (20–30 Re downtail)Ebihara Progress in Earth and Planetary Science(2019) 6:24

Page 9 of 24

Fig. 7 a–h Observed high-latitude aurora moving equatorward. Auroral images observed at different sites are mapped to the ionosphere altitude.
(Adapted from Nishimura et al. 2010)

Ebihara Progress in Earth and Planetary Science(2019) 6:24

(a) Plasma pressure

Page 10 of 24(b) Parallel current

Y-Z plane at X=-7 Re

3

1
2
Z

X

Y

0.05

0

>0.1

0

<-0.5

>0.5

J ||(nA/m2)

Pressure (nPa)(c) Parallel current
Sphere at 3 Re

1
Arc 1
Arc 2

2

3

Arc 3

Y-Z plane at X=-7 Re
Z

X

Y

<-0.5

0
J ||(nA/m2)

>0.5

<-2

0

>2

J ||(nA/m2)

Fig. 8 a Plasma pressure and b, c parallel current density in the Y-Z plane at X = − 7 Re. In c, parallel current is also drawn on the surface at 3 Re.
The yellow and white lines indicate the current line and the magnetic field line, respectively, extending from Arcs 1, 2 (high-latitude aurora), and
3 (quiet arc) at T = 43.7 min. The color codes indicate the parallel current density (negative upward in the Northern Hemisphere). (Adapted from
Ebihara and Tanaka 2016)(Nishida and Nagayama 1973; Angelopoulos et al. 1994;
Baker et al. 1996; Nagai et al. 1998; Machida et al. 2009).
Using the same simulation code with different solar windparameters, Tanaka et al. (2017a) suggested that the NENL
appears at ~ 17 Re downtail. The reason why the NENL
starts to appear here is not well known and requires further investigation.
An immediate consequence of the formation of the
NENL is the earthward and tailward flows near theequatorial plane. The earthward flow, known as a bursty
bulk flow (Angelopoulos et al. 1994), is diverted westward and eastward as it approaches the Earth, generating flow shear (vorticity). The flow shear near theequatorial plane is observed near the expansion onset(Keiling et al. 2009). A pair of FACs (Birn and Hesse 1991;
Birn et al. 2004a) are generated near the flow shear(vorticity). Another type of flow shear appears off theequator (Ebihara and Tanaka 2015b). When the magnetic
reconnection proceeds, the plasma originating in the lobe
traverses the separatrix (the open-closed boundary ofmagnetic field lines) and moves into the closed field line
region. As the plasma traverses the separatrix on the duskside (dawnside), it is immediately deflected westward(eastward), generating flow shear and upward (downward)
FACs at off-equator. The equatorial shear and theoff-equatorial shear are schematically drawn in Fig. 10.
The spatiotemporal evolution of the magnetospheric flowis reviewed by Tanaka et al. (2017a) in detail.
The red lines in Fig. 10 schematically indicate thecurrent line on the basis of result of the line integral of
the current density vector (Ebihara and Tanaka 2015b;
Tanaka, 2015; Tanaka et al. 2017a). In the global MHD

(2019) 6:24

Ebihara Progress in Earth and Planetary Science

Page 11 of 24

t=55.7 min

06 MLT

150

50
1012 MLT

20

30

40

50

NENL

0
-50

Vx, (km/s)

100

-100
-150

18 MLT
Fig. 9 X-component of the perpendicular plasma flow velocity in the equatorial plane at T = 55.7 min

simulation, three types of current can be defined:diamagnetic current, inertial current, and FAC. The
diamagnetic current and the inertial current are given by
Jd ¼

ð2Þ

B  ∇P;
B2

andJi ¼ ρ

B dV⊥

;
dt
B2

ð3Þ

where ρ, V⊥, and P are the mass density, perpendicularvelocity, and the plasma pressure, respectively. The sumof the three currents (=Jd + Ji + J||) is taken into accountto draw the current line (Tanaka et al. 2010; Ebihara andTanaka 2015b). It should be emphasized that current
carriers, such as electrons and ions, do not necessarily
move along the current line. The diamagnetic current
is associated with the cyclotron motion of charged particles (Parker 1957), and the FAC is associated withfield-aligned motion of the particles. Two types of

Plasma flow at off-equator

Current line
Off-equatorial shear

Z
X
Last-closed
field line

Earth
Y

Current line

Plasma flownear equatorial plane
Equatorial shear

Flow shear (vorticity)

Near-Earthneutral line

Plasma flow

Fig. 10 A schematic of plasma flow (blue line), flow shear (triple circles), and current lines (red line) at the expansion onset

Ebihara Progress in Earth and Planetary Science(2019) 6:24

Page 12 of 24

current line are drawn in Fig. 10, which are closely associated with the expansion onset. One is found near
the equatorial plane, which is wedge-like in form. The
cross-tail current is diverted from the equatorial plane
due to the FACs associated with the earthward plasma
flow (Birn et al. 1999; Keiling et al. 2009). However, theleg of the wedge is too short to reach the ionosphere.
The reason why the wedge-like current is incomplete isthat the perpendicular current (mostly the diamagnetic
current) overcomes the FAC, so that the current line is
no longer field-aligned (Ebihara and Tanaka 2015b;
Ebihara and Tanaka 2017). The intensification of the
diamagnetic current is directly associated with anabrupt increase in the plasma pressure, which is often
observed by geosynchronous satellites around the
expansion onset (Birn et al. 1997). The abrupt increasein the plasma pressure is also observed in the nearEarth plasma sheet near the equatorial plane and off
the equator (Yao et al. 2015a; Yao et al. 2015b). An
observer can detect FACs near the equatorial plane(Nagai et al. 1987), but it does not always mean thatthe total current is parallel to the magnetic field line.
Observations are required to confirm whether or not
the current wedge is complete. The direction between
the current density J and the magnetic field B will
provide a significant clue in evaluating the dominanceof the perpendicular current. Interestingly, the shortlegged incomplete current wedge is connected to the poleward half of the Region 2-like FAC that is newly developed
during the substorm expansion (Ebihara et al. 2014). This
current line is consistent with the theoretically obtainedone (Roelof et al. 2004).
The other type of current is drawn in Fig. 10. This
type of current is connected to the ionosphericlocation and is more directly related to the expansion
onset for the following reasons (Ebihara and Tanaka
2015b; Ebihara and Tanaka 2017):
1. The rate of change in the FAC, which is given byð∇  ∇  EÞ∥
∂J ∥
;
¼−
∂t
μ0

The last two items are direct evidence supporting theidea that the FACs are generated off the equator, rather
than via the propagation of J|| from the equatorial plane
as an Alfvén wave. For better understanding of Eq. (4),the following approximation is suggested (Song andLysak 2001a; Song and Lysak 2001b) as∂J ∥B  ∇Ω∥;≈∂t
μ0where Ω|| (=(∇ × V)‖) is the vorticity of flow in the
direction parallel to the magnetic field and V is the flow
velocity. Equation (5) is confirmed to be valid for the
expansion onset (Ebihara and Tanaka 2015b; Ebihara
and Tanaka 2017) and implies that the gradient of thevorticity along the field line generates the FAC. This vorticity gradient is the flow coming from the lobe region
towards the equatorial plane as schematically drawn in
Fig. 10. The flow is diverted to westward (eastward) off
the equator, so as to generate the flow shear (vorticity),
and the upward (downward) FAC. The off-equatorial flow
shear in the Y-Z plane has been observed by the Cluster
satellite during dipolarization (Nakamura et al. 2014).
Figure 11 shows a schematic of the stream lines projected onto the meridional plane. Two types of streamline are shown. One travels near the equator (the equatorial path), which is accompanied by bursty bulk flow
and flow braking, and is associated with the equatorial
flow shear shown in Fig. 10. The other travels off theequator (the off-equatorial path). The off-equatorial path
is associated with the off-equatorial shear. Both thestream lines participate in the generation of shear and
FACs, but the off-equatorial path is more directly related
to the expansion onset in the ionosphere as illustrated
above, according to the REPPU code.
When the strong upward FAC reaches the ionosphere,
the auroral substorm expansion begins. It begins near
the equatorward border of the preexisting aurora as
shown in Fig. 2 (T = 69.4 min). This is consistent withobservation (Akasofu 1964). The reason why the upward
FAC starts to be intensified near the equatorward borderof the preexisting aurora is that the lobe plasma

ð4Þ

is maximized at off-equator along the magnetic field line
extending from the location of the expansion onset.
2. The generation of the FAC as expressed by Eq. (4)coexists with the dynamo region where J·E < 0.
3. The upward FAC region propagates towards the
Earth along the onset field line with a significantly
slower speed than the local Alfvén speed.

ð5ÞOff-equatorial path
Onset

Equatorial path

Near-Earth neutral line
Fig. 11 A schematic of stream lines of plasma at expansion onset.
The blue line indicates the stream line flowing near the equatorial
plane. The red line indicates the stream line flowing at off-equatorEbihara Progress in Earth and Planetary Science(2019) 6:24

traverses the separatrix (open-closed boundary) and penetrates deep into the inner magnetosphere (Ebihara
and Tanaka 2015a; Ebihara and Tanaka 2015b) as schematically drawn in Figs. 10 and 11. The simulated initialbrightening is longitudinally elongated. This is also consistent with observation (Akasofu 1964). However,fine-scale structures such as a distinct ray structure(Akasofu 1964), a quasi-periodic series of bright spots, orbeads (Elphinstone et al. 1995; Henderson 2009; Rae et al.
2010) are not present in the simulation results. The ballooning mode instability is the most plausible mechanism
for the quasi-periodic series of bright spots, or beads(Elphinstone et al. 1995; Liang et al. 2008). An interesting
result was obtained from the global MHD simulation, withballoon-like fine-scale structures appearing in the magnetotail (Raeder et al. 2012). Further investigations arenecessary to investigate these structures.

Expansion phase
After the initial brightening of the aurora, a bulge forms
around midnight, which expands poleward, westward,
and eastward (Akasofu, 1964). Following the formationof the bulge, folds of aurora appear, moving westward
rapidly. This is called a westward traveling surge (WTS).
It is pointed out that the bulge dominantly expands
westward, or eastward, depending on the polarity of the
IMF By (Liou et al. 2006; Liou and Ruohoniemi 2006).
The surge is clearly identified in the bulge that is
expanding westward, whereas it is not clear in the eastward expanding bulge. Satellite observations show that
the WTS is caused by precipitation of electrons downwardly accelerated to ~ 10 keV (Rème and Bosqued
1973; Meng et al. 1978). Intense upward FACs accompany the WTS (Kamide and Akasofu 1975; Fujii et al.
1994). The ionospheric counterpart of the intense FACs
is thought to be connected to the auroral electrojet near
the region where the ionospheric conductivity gradientis high (Inhester et al. 1981; Baumjohann et al. 1981).
Partial blockage of the ionospheric Hall current is suggested as the cause of the buildup of excess charge near
the surge, which distorts the ionospheric electric fieldnear the surge (Kan et al. 1984). Electron-ion recombination effects may also play a role in modulating theintensity of the FACs (Rothwell et al. 1984). The magnetospheric counterpart of the intense FACs is suggested
to be connected to the current wedge (CW) system(Fujii et al. 1994; Birn and Hesse 1996; Keiling et al.
2009). The WTS is also suggested to be a projection of
the structure in the vicinity of the boundary between the
tail lobe and the plasma sheet (Akasofu et al. 1971;
Bythrow and Potemra 1987). Shear instability grows in
the plasma sheet boundary layer, resulting in the intrusion of the plasma sheet into the tail lobe.

Page 13 of 24

In the global MHD simulation, the auroral structuresthat resemble the bulge and the WTS appear as shownin Fig. 2. Based on the simulated WTS, Ebihara andTanaka (2015a, 2018) suggest that the WTS is caused by
the mutual interaction between the magnetosphere andthe ionosphere. Figure 12 summarizes the FAC in theionosphere, the nondiagonal component of the ionospheric conductivity, and the magnetospheric plasmapressure mapped to the ionosphere. A surge head (asmanifested by the upward FAC) is situated in the
westward-most area of the strong upward FAC region
that is elongated in the east-west direction. In the
REPPU code, the ionospheric conductivity is intensified
in the regions where the upward FAC flows and where
the magnetospheric plasma pressure is high. The former
is a proxy of the discrete aurora, and the latter is a proxyof the diffuse aurora. It can clearly be seen in Fig. 12bthat ionospheric conductivity is intensified near thesurge head (where the upward FAC is high as shown in
Fig. 12a), and the main body of the auroral bulge where
the magnetospheric plasma pressure is high (as shownin Fig. 12c, as in the diffuse aurora). This is consistent
with satellite observation (Meng et al. 1978).
The yellow contour in Fig. 12a indicates the region
where the divergence of the electric field is positive(∇·E > 0, where E is electric field), which coincides withthe surge head. The divergence of the electric field isnegative (∇·E < 0) on the backside of the surge as indicated by the red contour. The positive divergence andthe negative divergence give rise to the development of
positive and negative space charge, respectively. Due tothe space charge represented by ∇·E, the pattern of theconvection is skewed near the surge. The convection
flow tends to be counterclockwise near the surge head due
to a positive space charge, whereas it tends to be clockwise
on the backside of the surge due to a negative one. The
skewed pattern of the convection around the surge head issimilar to that suggested by Weimer et al. (1994).
Figure 13 shows ∇·JH, ∇·JP and (∇·JH+∇·JP)/|∇·JH|,
where JP and JH are the ionospheric Pedersen and theHall currents, respectively. (The periodic structures at
67–68 MLATs come from a numerical error.) The solid
contour line indicates the upward FAC, and the dashed
one indicates the downward one. ∇·JH is negative near
the surge head, whereas ∇·JH is positive on the backsideof the surge. This means that the Hall current overflows
near the surge head. ∇·JP is positive near the surge head,
meaning that a part of the Hall current is connected to
the Pedersen current. The bottom panel of Fig. 13 indicates the ratio of −∇·J|| to |∇·JH| (because −∇·J|| = ∇·JH+∇·JP), suggesting that most of the Hall current (~ 90%)
is connected to the upward FAC. This condition can be
regarded as a partial blockage of the Hall current as suggested by Kan et al. (1984). A small amount of theEbihara Progress in Earth and Planetary Science(2019) 6:24

Page 14 of 24(a) Field-aligned current
Surge

div E>0

LAT

72 M

T

LA
70 M

div E<0

LAT

68 M

0.4
0.2
0.0
-0.2
-0.4

FAC (10-6A/m2)

T=74.5 min

LAT

72 M

LAT

70 M

LAT

68 M
23

00 MLT(c) Pressure

01

LAT

72 M

14
12
10
8
6
4
2
0
1.0
0.8

LAT

70 M

68
23

00 MLT

T
MLA

0.6
0.4
0.2
0.0

Pressure (nPa)(b) Ionospheric conductivity

Conductivity (mho)00 MLT

01

Fig. 12 a. Close-up views of the FACs, b. the ionospheric conductivity, and c. the magnetospheric pressure mapped to the ionosphere at T =74.5 min. In a., the yellow and red contour lines indicate the region where ∇·E is positive (0.3 μV/m2 for thick line and 0.15 μV/m2 for thin line) and∇·E is negative (− 0.3 μV/m2 for thick line and − 0.15 μV/m2 for thin line), respectively. E is the ionospheric electric field. The contour line in a andb indicates the ionospheric electric potential at interval of 1 kV. The contour line in c indicates the FACs. (Adapted from Ebihara and Tanaka 2018)

positive charge remains in the ionosphere, which is responsible for generating the additional FAC. This meansthat the WTS will not develop if the blockage is perfect(∇·J|| = −∇·JH). And what about the magnetosphericcounterpart of the WTS? The upward FAC associatedwith the WTS is connected to the inertial current in
the low-altitude magnetosphere, according to the
REPPU code (Ebihara and Tanaka 2015a).
Figure 14 presents an illustration as an explanation.
Let us suppose that a bright aurora occurs and that a
Hall current traverses the bright aurora from right to left
in the Northern Hemisphere. The intensity of the Hall
current is larger in the bright aurora than in the generalambience because ionospheric conductivity is high inthe bright aurora (Fig. 12b). Due to the gradient of thisconductivity, the Hall current overflows in the redregion where JH·∇ΣH < 0, where ΣH is the Hall conductivity. Positive space charge deposits (due to the partial
blockage of the Hall current), giving rise to the condition
that ∇·E > 0. The divergent electric field generates a
counterclockwise flow in the low-altitude magnetosphere when the Earth is viewed from space. The counterclockwise flow gives rise to further intensification of
the upward FAC, which results in a bright aurora near
the surge head. The opposite processes take place on the
trailing side of the surge where JH·∇ΣH > 0. Lack of theHall current results in a negative charge (∇·E < 0). The
resulting clockwise flow weakens the upward FACs that

Region1-sense FAC

LAT

72 M

T

LA
70 M

LAT

-0.5
-1.0

00 MLT01
LAT(b) div JP

72 M

T

LA
70 M

LAT

68 M

23

0.5
0.0

68 M

23

1.0

div JH (10-6A/m2)

Page 15 of 24

1.0
0.5
0.0

div JP (10-6A/m2)(a) div JH(2019) 6:24

-0.5
-1.0

00 MLT01(c) (div JH+div JP)/|div JH|

LAT

72 M

70

T
MLA

LAT

68 M

1.0
0.5
0.0

Ratio

Ebihara Progress in Earth and Planetary Science

-0.5
-1.0

23

00 MLT01(t = 74.5 min)Fig. 13 a Divergence of the Hall current JH, b divergence of the Pedersen current JP, and c the ratio (div JH + div JP)/|div JH| (= − div J||/|div JH|).
The contour lines indicate the field-aligned current with an interval of 0.05 μA/m2. (Adapted from Ebihara and Tanaka 2015a)

had been intensified in the surge head. By repeatingthese processes, the WTS moves towards the directionof the Hall current. When one views the auroral breakup
from space, it looks as if flames have spread rapidly overa sheet of paper. It can be said that ignition (near thesurge head) and digestion (near the backside of thesurge) take place simultaneously as the surge moves.
The direction of the Hall current can be locally skewed
by the space charge that deposits near the leading/trailing edges of the surge as a consequence of the partial
blockage. The ratio of −∇·J|| to |∇·JH| (shown in Fig. 13c)
could determine the degree of the skew, but it is unclear what determines the ratio.
In reality, the bright aurora is caused by the precipitation of electrons accelerating downwards in the parallel

electric field. The global MHD simulation cannot deal
with electron kinetic processes, such as the formation ofa parallel electric field. A complete explanation for theWTS will be obtained when the electron kinetic processes embedded in the global processes taking place inthe magnetosphere are taken into consideration.

Energy flow and conversion
The Joule dissipation rate exceeds 1011 W in theionosphere during substorm expansion (Ahn et al. 1983;
Sun et al. 1985; Kamide et al. 1986; Richmond et al.
1990; Palmroth et al. 2005). We know that the ultimate
source of the energy is solar wind. A number of studies
have been conducted to relate the Joule dissipation rate(2019) 6:24

Ebihara Progress in Earth and Planetary Science

Page 16 of 24(J||< 0)
Upward FAC
originating in magnetosphere

0)
( ||/
Intensification of upward FAC

Shear( ||/ > 0)
Reduction of upward FAC

Conductivity
Appearance of aurora

Disappearance of aurora

Movement of surge

Secondary E-field

Poleward

Ionospheric Hall current
Primary E-field
Overflow of Hall current(positive space charge)

Lack of Hall current(negative space charge)
Surge(bright aurora)Fig. 14 Schematic drawing for the formation of the westward traveling surge. (Ebihara and Tanaka 2018)

to the solar wind energy input (Perreault and Akasofu
1978; Akasofu 1979; Baker et al. 1997; Koskinen andTanskanen 2002). However, details of the pathway andthe conversion of energy from the solar wind to the
Earth are still problematic.

Ebihara and Tanaka (2017) suggested a way to represent the pathway of the electromagnetic energy fromthe solar wind to the ionosphere. The basis of thisprinciple is to trace the Poynting flux S (=E × B/μ0). The
result is provided in Fig. 15. (The solar wind condition isFig. 15 The principal pathway of electromagnetic energy in the magnetosphere (as shown by the white line) at 15 min before onset (left panel)and at expansion onset (right panel). The blue shade indicates the region where J•E < 0 (dynamo region). The light blue line indicates themagnetic field line. The plane indicates the equatorial plane with color given by the X-component of the plasma flow. The yellow line stands forthe magnetic field line extending from the onset location in the ionosphere. (Adapted from Ebihara and Tanaka 2017)

Ebihara Progress in Earth and Planetary Science(2019) 6:24

Page 17 of 24

different from that used in the other figures. After thesouthward turning of the IMF, the following parameters
were used: a solar wind density of 5.0 cm−3, a solar windspeed of 500 km s−1, an IMF By of − 2.5 nT, and an IMF
Bz of − 5 nT.) The integral curve of the Poynting flux isgiven by ∫S ⋅ ds/S, where s is a line element. This iscalled an S-curve (Ebihara and Tanaka 2017). The color
on the S-curve represents J•E. The S-curve is a snapshot; it does not always represent propagation of theelectromagnetic energy. The S-curve comes from thesolar wind. As the S-curve passes through the bow
shock, it meets the dynamo region located in the cusp
and mantle. The cusp/mantle dynamo is responsible fordriving the magnetospheric convection and the Region 1
FACs (Siscoe et al. 2000; Tanaka 2000a). The S-curve
further travels tailward and towards the equatorial plane.
It turns westward on the nightside and continues toextend sunward. Finally, it becomes a helix, which is likea cylindrical coil spring. The center of the helix moves
towards the Earth. The earthward propagation of the
S-curve is due to a twist of the magnetic field line in thepresence of the large-scale FAC known as the Region 1
FAC (Iijima and Potemra 1976). The circular curvearound the center of the helix is a manifestation of magnetospheric convection. At the expansion onset, an
additional dynamo region appears in the near-Earthregion (Birn and Hesse 2005; Ebihara and Tanaka 2015b;
Tanaka 2015). This near-Earth dynamo is responsible forthe generation of the FACs both at the onset and afterwards (Ebihara and Tanaka 2015b; Tanaka 2015; Ebihara
and Tanaka 2017). The S-curve shows a sharp kink in
the near-Earth region, which is a manifestation of theflow shear generating the upward FAC which thereby
causes the onset of the expansion.
In the ideal MHD, three types of energy can be defined as follows (Birn and Hesse 2005). The first one isthe electromagnetic energy represented by the Poynting
flux S as
 
∂ B2
∇Sþ
¼ −J  E∂t 2μ0¼ −V⊥  ð J  BÞ:

ð6Þ

The second one is the kinetic energyρ∂ ρ 2 V þ ∇  V 2 V ¼ V  ð J  B−∇PÞ:
∂t 2
2

ð7Þ

The third one is the internal energy∂uþ ∇  ðγuVÞ ¼ V  ∇P;∂tð8Þ

where γ is the ratio of specific heat (=5/3). The kinetic
energy flux K and the enthalpy flux H are given by1
K ¼ ρV 2 V;
2

ð9Þ

H ¼ γuV;

ð10Þ

and

respectively. Ebihara and Tanaka (2017) investigated theprincipal conversion of energy associated with the expansion onset. Figure 16 summarizes the principal
energy flow and conversion among S, K, and H.
1. S and K are accompanied by the solar wind.
2. In the magnetosheath, a part of K that originated in
the solar wind is converted to H. A small fraction of
it is converted to S (Tanaka et al. 2016).
3. When the IMF is southward, the cusp/mantle
dynamo appears (Siscoe et al. 2000; Tanaka 2000a),which converts a part of K to S.
4. In the lobe, a part of S is stored in association witha partial stagnation of the convection as representedby ∇∙V⊥ < 0 (Ebihara and Tanaka 2017).
5. After the formation of the NENL, S stored in thelobe is released together with the continuously
supplied S from the cusp/mantle dynamo and thesolar wind (Ebihara and Tanaka 2017).
6. In the vicinity of the NENL, S originating in thelobe splits to K and H (Birn et al. 2010).
7. In the plasma sheet, K associated with earthward
flow is converted to H and S due to thecompression associated with flow braking (Birn and
Hesse 2005; Ebihara and Tanaka 2017).
8. In the plasma sheet, H propagates earthward along
a field line (Tanaka et al. 2010).
9. Off the equator, S originating in the lobe is converted
to H due to compression. Then, H is converted to K,
followed by S (Ebihara and Tanaka 2017).
10. Off the equator, S propagates earthward along afield line due to the FACs associated with the nearEarth dynamo.
Traditionally, two processes have been suggested forthe energy transfer. One is the directly driven process(Perreault and Akasofu 1978; Akasofu 1979), and theother is the loading-unloading process (McPherron
1970; Mcpherron et al. 1973; Hones 1979). In the context of electromagnetic energy, the directly driven
process is associated with the energy originating fromthe solar wind and that released from the cusp/mantle
dynamo. The loading-unloading process is the energy
storage and release in the lobe. The simulation resultshows that both the energy sources merge together inthe lobe during the expansion phase (Ebihara andTanaka 2017). This means that the directly driven
process also participates in the generation of the FACs,

(2019) 6:24

Ebihara Progress in Earth and Planetary Science

Page 18 of 24

Sun

S : Poynting flux
K : Kinetic energy flux
H : Enthalpy flux

K

S

S

Magnetosheath
H
K

S

K

Near equatorial plane

Off-equator

Cusp/mantle dynamo

S
Directly driven
component

S

Compression

Loading and unloading component

S
S

S

Storage and release
H

Near-earth reconnection

H

K
S

H
Flow braking

H

Inflation

H

?
S

S

Near near-earth dynamo

K

Ionosphere (auroral breakup)

Fig. 16 Possible energy conversion from the Sun to the ionosphere at the expansion onset. S, K, and H represent the Poynting flux, the bulk
kinetic energy flux, and the enthalpy flux, respectively. The filled blocks indicate the cusp/mantle dynamo (Tanaka 2000a) and the near-Earthdynamo (Tanaka 2015, Ebihara and Tanaka 2015b)

which deliver electromagnetic energy to the ionosphere
during substorm expansion. The energy consumed in
the ionosphere is not fully sustained by the energy released from the lobe. This is consistent with the observation showing that the substorm size is largely controlled
by the directly driven processes (Tanskanen et al. 2002).
Akasofu (2013) pointed out that the energy stored in
the lobe is insufficient to sustain the energy consumed in the ionosphere. The simulation result may
settle this argument.
As inspired by the illustration of Akasofu (2015), theconcept of the energy transfer from the solar wind to
the ionosphere is schematically shown in Fig. 17. The
water (energy) is supplied through a tube. The rate offlow in the tube is regulated by a valve. The valve iswidely opened when the southward component of the
IMF is strong, and the solar wind speed is high. After
passing through the valve, some of the water flow turns
the first watermill (cusp/mantle dynamo) that generates
electromagnetic energy to light bulb 1 (quiet arc). The
Region 1 FACs (associated with the cusp/mantle
dynamo) are responsible for guiding the electromagnetic
energy to the ionosphere. After passing through the firstwatermill (cusp/mantle dynamo), some of the water goes
into a water bucket. Water coming directly from thetube also goes into the water bucket. When the bucket
is filled up with water, it is tilted so as to release thewater. The released water flows out, turning the second
watermill (near-Earth dynamo). It is emphasized that thewater directly coming from the first watermill and thetube also turn the second watermill. The second

watermill generates electromagnetic energy to light bulb
2 (initial brightening). The Region-1 sense FACs (associated with the near-Earth dynamo) are responsible for
guiding the electromagnetic energy to the ionosphere.
Two photoresistors are serially connected to the bulbs 3
and 4. The photoresistor is an electronic device in which
resistance decreases with the increasing intensity of anincident light. When the photoresistor serially connected
to bulb 3 detects light coming from bulb 2, its resistivity
decreases, lighting bulb 3. Subsequently, when thephotoresistor serially connected to bulb 4 detects lightcoming from bulb 3, its resistivity decreases, lighting
bulb 4. The final chain reaction (bulbs 2, 3, and 4) isanalogous to the surge traveling westward.
Table 1 summarizes characteristics of the typicalauroral structures that can be seen during a substorm
from the growth phase to the expansion phase, together
with possible causes of the auroral structure assuggested by the global MHD simulation. It seems thatthese auroral structures can be reasonably explained by
the MHD processes coupled with the ionosphere. To
date, fine-scale structures, such as distinct ray structures(Akasofu 1964) or quasi-periodic series of bright spots
or beads (Elphinstone et al. 1995; Henderson 2009; Raeet al. 2010) are not resolved by the MHD model. Kinetic
simulations will be needed to explain the fine-scale
structures of the aurora.

Conclusion
Large- and meso-scale structures of the aurora that evolve
during the substorm can be reasonably reproduced using

(2019) 6:24

Ebihara Progress in Earth and Planetary Science

Page 19 of 24

Valve regulated by
IMF Bz and solar wind speed

Cusp/mantle dynamo
bulb 1
Quiet arc
Downtail(Photoresistor)
bulb 2
3
4

Partial storage
and release

Breakup aurora

Surge

Near-earth dynamo

Fig. 17 Conceptual illustration of the possible chain reaction of auroral substorm in terms of energy transfer from the solar wind to the ionosphere.
The quiet arc (bulb 1) is associated with the cusp/mantle dynamo. The breakup aurorae (bulbs 2, 3, and 4) are connected to the near-Earth dynamo.
Illustration is based on Akasofu (2015)

the global MHD simulation (Tanaka 2015; Ebihara andTanaka 2015a, 2015b, 2016; Tanaka et al., 2017a, b).
During the growth phase, a quiet arc and high-latitudeaurorae (found in the main auroral oval) appear in thesimulation. The quiet arc is attributed to the flow shear
off the equator. The high-latitude aurorae are attributed
to fine structures in the plasma pressure distribution of
the lobe region (open field line region). The expansion onset is triggered by the formation of a near-Earth neutralline (NENL). Plasma flow off the equator seems to be responsible for the generation of a field-aligned current thatcauses the onset of expansion. The initial brightening
starts near the equatorward boundary of the preexistingaurora, which is consistent with observation. During the
expansion phase, an excess of space charge near the brightaurora generates an additional field-aligned current, which

manifests the westward traveling surge (WTS). Coupling
between the magnetosphere and the ionosphere is essential for formation of the WTS. The energy consumed in
the ionosphere during the expansion phase is not fully
supplied by the lobe. The continuous supply of energy
from the solar wind and the cusp/mantle dynamo also
participates in the energy input into the ionosphere. Most
importantly, the primary chain reactions from the solar
wind to the WTS are reasonably explained by the MHD
processes. Of course, contribution from non-MHD processes to the evolution of auroral substorms should be
investigated in the future.
Fundamental properties and observable effects of thesubstorms came to be understood by numerical simulations. The next step is to understand the storm-substorm
relationship by considering the comprehensive processes

Table 1 Cause and effect relationship for substorm-associated aurora as suggested by the global MHD simulation
Substorm
phase

Characteristics of aurora

Possible cause

Growthphase

Quiet arc

Flow shear at off-equator (Tanaka et al., 2017a)

High-latitude aurora within main
auroral oval

Non-linear interaction between magnetosphere and ionosphere (Ebihara and Tanaka 2016)

Onset

Initial brightening

Enhanced flow shear at off-equator (Ebihara and Tanaka 2015b)

Expansion

Westward traveling surge

Coupling of partial blockage of Hall current and MHD magnetosphere (Ebihara and Tanaka 2015a;
Ebihara and Tanaka 2018)

Ebihara Progress in Earth and Planetary Science(2019) 6:24

occurring from the ionosphere to the solar wind. Suchefforts have already begun (Fok et al. 2006; Welling et al.
2011; Glocer et al. 2011; Ouellette et al. 2013; Lotko et al.
2014; Nakayama et al. 2017). Substorms should influence
the development of magnetic storms, and vice versa. In
this sense, the magnetic storms and substorms are manifestations of magnetospheric and the ionospheric disturbances. Our goal should be to understand the overallsystem from the solar wind to the Earth. Obviously, thenon-linear and complex system makes it difficult tounderstand the storm-substorm relationship quantitatively. Key processes include ionospheric ion outflow, thekinetics of auroral electron precipitation, global magnetospheric processes with multispecies plasmas, andnon-adiabatic transport and acceleration. The numericalsimulations, which are verified to be consistent withobservations, are a powerful tool to help us. Mutual connections among the processes involved by the substorms
are becoming clear. Some of them are probably applicableto the space environment near planets with an inherent
magnetic field.

Appendix
The global magnetohydrodynamics (MHD) simulation
developed by Prof. Takashi Tanaka, called a REPPU(REProduce Plasma Universe) code, solves a set of ideal
MHD equations using the finite volume total variation
diminishing (TVD) scheme (Tanaka 1994). The innerboundary is located on the sphere at a geocentric distance of ~ 3 Re. Although the MHD equations are notsolved in the region between the inner boundary and theionosphere, the magnetosphere-ionosphere coupling istaken into consideration by exchanging quantities along
the dipole magnetic field line. The following calculationis performed at every time step (~ 0.03 s). First, thefield-aligned current (FAC) and the plasma pressure at
the inner boundary are mapped from the inner boundaryof the magnetospheric domain to the ionosphere.
Secondly, the ionospheric conductivities (Pedersen andHall conductivities) are calculated on the basis of the
mapped values (the FAC and the plasma pressure), andthe solar zenith angle. The ionospheric conductivity is
increased where the upward FACs flow and the plasmapressure is high. Thirdly, to satisfy the current continuity
conditions in the ionosphere, an elliptic partial differential equation is solved for the given FACs and the ionospheric conductivity. The conjugate residual method wasused to solve the partial differential equation. Fourthly,
the electric field is mapped from the ionosphere to theinner boundary of the magnetospheric domain along the
dipole magnetic field line. Fifthly, the velocity corresponding to the electric field is imposed on the innerboundary as one of the boundary conditions. For
detailed explanations concerning this calculation, readers

Page 20 of 24

may refer to Tanaka 1994; Tanaka 1995; Tanaka, 2000a, b;
Tanaka 2015).
The simulation employs a grid system having no singularity. A sphere with a radius of ~ 3 Re (where theinner boundary of the magnetospheric domain is located) is divided into 12 pentagons. Each pentagon isdivided into five triangles. The number of the triangles
is 60. This is called Level 1. Each triangle is further divided into 4 triangles. This is called Level 2, in which
the number of the triangles is 240. We used Level 6(61,440 triangles), or Level 7 (245,760 triangles). The
triangular prisms are stacked outward from the innerboundary to the outer boundary located at 300 Re. Fora detailed explanation of the grid system, readers may
refer to Moriguchi et al. (2008).
The coordinate system is essentially the same as the
Solar Magnetospheric coordinates. The X-axis points
towards the Sun, and the Z-axis is anti-parallel to Earth’s
dipole moment. The Y-axis is defined as the cross-product of the Z- and X-axes.
Abbreviations
CW: Current wedge; FAC: Field-aligned current;
MHD: Magnetohydrodynamics; MLAT: Magnetic latitude; MLT: Magnetic local
time; NENL: Near-Earth neutral line; REPPU: REProduce Plasma Universe
Acknowledgements
The REPPU code was developed by Dr. Takashi Tanaka. A part of thecomputer simulation was performed on the KDK computer system at
Research Institute for Sustainable Humanosphere, Kyoto University. The
author thanks Drs. Atsuhiro Nishida, Takashi Tanaka, Takashi Kikuchi and ShinIchi Ohtani for fruitful discussion and valuable suggestion.
Funding
This work was supported by JSPS KAKENHI Grant Number 15H03732 and
15H05815, as well as Flagship Collaborative Research and Research Mission 3“Sustainable Space Environments for Humankind” at RISH, Kyoto University.
Availability of data and materials
This is a review paper. All data used here are in the original papers cited in
the text except for Figs. 1 and 9. Data files for Figs. 1 and 9 are available at
http://www2.rish.kyoto-u.ac.jp/~ebihara/2018-peps/.
Author’s contributions
The author read and approved the final manuscript
Competing interests
The author declares that he has no competing interest.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Received: 5 October 2018 Accepted: 8 February 2019

References
Ahn B-H, Robinson RM, Kamide Y, Akasofu S-I (1983) Electric conductivities,
electric fields and auroral particle energy injection rate in the auroral
ionosphere and their empirical relations to the horizontal magnetic
disturbances. Planet Space Sci 31:641–653. https://doi.org/10.1016/00320633(83)90005-3
Akasofu S (2015) Auroral substorms as an electrical discharge phenomenon. Prog
Earth Planet Sci 2:20. https://doi.org/10.1186/s40645-015-0050-9

Ebihara Progress in Earth and Planetary Science(2019) 6:24

Akasofu SI (1968) Polar and magnetospheric substorms. D. Reidel Publ. Co.,
Dordrecht
Akasofu S-I (1964) The development of the auroral substorm. Planet Space Sci 12:
273–282. https://doi.org/10.1016/0032-0633(64)90151-5
Akasofu S-I (1979) Interplanetary energy flux associated with magnetosphericsubstorms. Planet Space Sci 27:425–431. https://doi.org/10.1016/00320633(79)90119-3
Akasofu S-I (2013) Where is the magnetic energy for the expansion phase ofauroral substorms accumulated? J Geophys Res Sp Phys 118:7219–7225.
https://doi.org/10.1002/2013JA019042
Akasofu S-I, Hones EW, Montgomery MD, Bame SJ, Singer S (1971) Association ofmagnetotail phenomena with visible auroral features. J Geophys Res 76:
5985–6003. https://doi.org/10.1029/JA076i025p05985
Akasofu S-I, Kimball DS, Meng C-I (1965) The dynamics of the aurora—II
westward traveling surges. J Atmos Terr Phys 27:173–187. https://doi.org/10.
1016/0021-9169(65)90114-5
Akasofu S-I, Snyder AL (1972) Comments on the growth phase ofmagnetospheric substorms. J Geophys Res 77:6275. https://doi.org/10.1029/
JA077i031p06275
Allen JH, Kroehl HW (1975) Spatial and temporal distributions of magnetic effects
of auroral electrojets as derived from AE indices. J Geophys Res 80:3667–
3677. https://doi.org/10.1029/JA080i025p03667
Angelopoulos V, Kennel CF, Coroniti F V, Pellat R, Kivelson MG, Walker RJ, Russell
CT, Baumjohann W, Feldman WC, Gosling JT (1994) Statistical characteristics
of bursty bulk flow events density above the ion density They showed thatfiir Extraterrestrische regardless activity [ Huang and Alamos National
Laboratory , New The CPS was defined by Huang andAntonova EE (1993) The development of initial substorm expansive phase
disturbance due to generation of localized electric fields in the region of
maximum upward field-aligned current. Adv Sp Res 13:261–264. https://doi.
org/10.1016/0273-1177(93)90343-A
Atkinson G (1966) A theory of polar substorms. J Geophys Res 71:5157–5164.
https://doi.org/10.1029/JZ071i021p05157
Baker DN, Pulkkinen TI, Angelopoulos V, Baumjohann W, McPherron RL (1996)
Neutral line model of substorms: past results and present view. J Geophys
Res 101:12975. https://doi.org/10.1029/95JA03753
Baker DN, Pulkkinen TI, Hesse M, McPherron RL (1997) A quantitative assessmentof energy storage and release in the Earth’s magnetotail. J Geophys Res Sp
Phys 102:7159–7168. https://doi.org/10.1029/96JA03961
Baker DN, Pulkkinen TI, McPherron RL, Craven JD, Frank LA, Elphinstone RD,
Murphree JS, Fennell JF, Lopez RE, Nagai T (1993) CDAW 9 analysis ofmagnetospheric events on May 3, 1986: Event C. J Geophys Res Sp Phys 98:
3815–3834. https://doi.org/10.1029/92JA02475
Baumjohann W, Pelunen RJ, Opgenoorth HJ, Nielsen E (1981) Joint twodimensional observations of ground magnetic and ionospheric electric fields
associated with auroral zone currents: current systems associated with local
auroral break-ups. Planet Space Sci 29:431–447. https://doi.org/10.1016/00320633(81)90087-8
Birn J, Borovsky JE, Hesse M, Schindler K (2010) Scaling of asymmetric reconnection
in compressible plasmas. Phys Plasmas. https://doi.org/10.1063/1.3429676
Birn J, Hesse M (1991) The substorm current wedge and field-aligned currents in
MHD simulations of magnetotail reconnection. J Geophys Res 96:1611–1618.
https://doi.org/10.1029/90JA01762
Birn J, Hesse M (1996) Details of current disruption and diversion in simulationsof magnetotail dynamics. J Geophys Res 101:15345. https://doi.org/10.1029/
96JA00887
Birn J, Hesse M (2005) Energy release and conversion by reconnection in themagnetotail. Ann Geophys 23:3365–3373. https://doi.org/10.5194/angeo-233365-2005
Birn J, Hesse M (2014) The substorm current wedge: further insights from MHD
simulations. J Geophys Res Sp Phys 119:3503–3513. https://doi.org/10.1002/
2014JA019863
Birn J, Hesse M, Haerendel G, Baumjohann W, Shiokawa K (1999) Flow braking
and the substorm current wedge. J Geophys Res 104:19895. https://doi.org/
10.1029/1999JA900173
Birn J, Nakamura R, Panov EV, Hesse M (2011) Bursty bulk flows anddipolarization in MHD simulations of magnetotail reconnection. J Geophys
Res Sp Phys 116:A01210. https://doi.org/10.1029/2010JA016083
Birn J, Raeder J, Wang YL, Wolf RA, Hesse M (2004a) On the propagation of
bubbles in the geomagnetic tail. Ann Geophys 22:1773–1786. https://doi.org/
10.5194/angeo-22-1773-2004

Page 21 of 24

Birn J, Thomsen MF, Borovsky JE, Reeves GD, McComas DJ, Belian RD (1997)
Characteristic plasma properties during dispersionless substorm injections at
geosynchronous orbit. J Geophys Res 102:2309. https://doi.org/10.1029/
96JA02870
Birn J, Thomsen MF, Borovsky JE, Reeves GD, McComas DJ, Belian RD, Hesse M(1998) Substorm electron injections: geosynchronous observations and test
particle simulations. J Geophys Res Sp Phys 103:9235–9248. https://doi.org/
10.1029/97JA02635
Birn J, Thomsen MF, Hesse M (2004b) Electron acceleration in the dynamic
magnetotail: test particle orbits in three-dimensional magnetohydrodynamic
simulation fields. Phys Plasmas 11:1825–1833. https://doi.org/10.1063/1.
1704641
Brittnacher M, Fillingim M, Parks G, Germany G, Spann J (1999) Polar cap areaand boundary motion during substorms. J Geophys Res 104:12251. https://doi.org/10.1029/1998JA900097
Bythrow PF, Potemra TA (1987) Birkeland currents and energetic particles
associated with optical auroral signatures of a westward traveling surge. J
Geophys Res 92:8691–8699. https://doi.org/10.1029/JA092iA08p08691
Chen CX, Wolf RA (1993) Interpretation of high-speed flows in the plasma sheet.
J Geophys Res 98:21409–21419. https://doi.org/10.1029/93JA02080
Cheng CZ, Lui ATY (1998) Kinetic ballooning instability for substorm onset andcurrent disruption observed by AMPTE/CCE. Geophys Res Lett 25:4091–4094.
https://doi.org/10.1029/1998GL900093
Coroniti FV, Pritchett PL (2014) The quiet evening auroral arc and the structure of
the growth phase near-Earth plasma sheet. J Geophys Res Sp Phys 119:
1827–1836. https://doi.org/10.1002/2013JA01943
Dahlgren H, Ivchenko N, Lanchester BS, Ashrafi M, Whiter D, Marklund G, Sullivan
J (2009) First direct optical observations of plasma flows using afterglow ofin discrete aurora. J Atmos Solar-Terrestrial Phys 71:228–238. https://doi.org/
10.1016/j.jastp.2008.11.015
Davis TN, Sugiura M (1966) Auroral electrojet activity index AE and its universal
time variations. J Geophys Res 71:785–801. https://doi.org/10.1029/
JZ071i003p00785
DeForest SE, McIlwain CE (1971) Plasma clouds in the magnetosphere. J Geophys
Res 76:3587–3611. https://doi.org/10.1029/JA076i016p03587
Ebihara Y, Tanaka T (2013) Fundamental properties of substorm time energetic
electrons in the inner magnetosphere. J Geophys Res Sp Phys 118:1589–
1603. https://doi.org/10.1002/jgra.50115
Ebihara Y, Tanaka T (2015a) Substorm simulation: formation of westward
traveling surge. J Geophys Res Sp Phys 120:10,466–10,484. https://doi.org/10.
1002/2015JA021697
Ebihara Y, Tanaka T (2015b) Substorm simulation: insight into the mechanisms ofinitial brightening. J Geophys Res Sp Phys 120:7270–7288. https://doi.org/10.
1002/2015JA021516
Ebihara Y, Tanaka T (2016) Substorm simulation: quiet and N-S arcs precedingauroral breakup. J Geophys Res Sp Phys 121:1201–1218. https://doi.org/10.
1002/2015JA021831
Ebihara Y, Tanaka T (2017) Energy flow exciting field-aligned current at substorm
expansion onset. J Geophys Res Sp Phys. https://doi.org/10.1002/
2017JA024294
Ebihara Y, Tanaka T (2018) Why does substorm-associated auroral surge travel
westward? Plasma Phys Control Fusion. https://doi.org/10.1088/1361-6587/aa89fd
Ebihara Y, Tanaka T, Kikuchi T (2014) Counter equatorial electrojet andovershielding after substorm onset: Global MHD simulation study. J Geophys
Res Sp Phys 119:7281–7296. https://doi.org/10.1002/2014JA020065
El-Alaoui M, Ashour-Abdalla M, Walker RJ, Peroomian V, Richard RL, Angelopoulos
V, Runov A (2009) Substorm evolution as revealed by THEMIS satellites and a
global MHD simulation. J Geophys Res Sp Phys 114:n/a-n/a. doi: https://doi.
org/10.1029/2009JA014133
Elphinstone RD, Hearn D, Murphree JS, Cogger LL (1991) Mapping Using the
Tsyganenko Long Magnetospheric Model we relate the the model on thenightsid • This was chosen to 96:1467–1480
Elphinstone RD, Hearn DJ, Cogger LL, Murphree JS, Singer H, Sergeev V, Mursula
K, Klumpar DM, Reeves GD, Johnson M (1995) Observations in the vicinity of
substorm onset: implications for the substrom process. J Geophys Res 100:
7937–7969. https://doi.org/10.1029/94JA02938
Fairfield DH, Ness NF (1970) Configuration of the geomagnetic tail during
substorms. J Geophys Res 75:7032–7047. https://doi.org/10.1029/
JA075i034p07032
Fedder JA, Slinker SP, Lyon JG, Elphinstone RD (1995) Global numericalsimulation of the growth phase and the expansion onset for a substormEbihara Progress in Earth and Planetary Science(2019) 6:24

observed by Viking. J Geophys Res 100:19083. https://doi.org/10.1029/
95JA01524
Fok MC, Moore TE, Brandt PC, Delcourt DC, Slinker SP, J a F (2006) Impulsive
enhancements of oxygen ions during substorms. J Geophys Res Sp Phys 111:
A10222. https://doi.org/10.1029/2006JA011839
Fujii R, Hoffman RA, Anderson PC, Craven JD, Sugiura M, Frank LA, Maynard NC(1994) Electrodynamic parameters in the nighttime sector during auroral
substorms. J Geophys Res 99:6093. https://doi.org/10.1029/93JA02210
Glocer A, Fok M-C, Nagai T, Tóth G, Guild T, Blake J (2011) Rapid rebuilding of theouter radiation belt. J Geophys Res Sp Phys 116:n/a-n/a. doi: https://doi.org/
10.1029/2011JA016516
Guo J, Liu H, Feng X, Pulkkinen TI, Tanskanen EI, Liu C, Zhong D, Wang Y (2014)
MLT and seasonal dependence of auroral electrojets: IMAGE magnetometer
network observations. J Geophys Res Sp Phys 119:3179–3188. https://doi.org/
10.1002/2014JA019843
Gurnett DA (1974) The Earth as a radio source: terrestrial kilometric radiation. J
Geophys Res 79:4227–4238. https://doi.org/10.1029/JA079i028p04227
Haerendel G (1992) Disruption, ballooning or auroral avalanche-on the cause of
substorms. In: Proceedings of the First International Conference on
Substorms. ESA Publications Division, Kiruna, pp 417–420
Haerendel G (2008) Auroral arcs as current transformers. J Geophys Res Sp Phys
113:1–16. https://doi.org/10.1029/2007JA012947
Haerendel G (2010) Equatorward moving arcs and substorm onset. J Geophys
Res 115:1–9. https://doi.org/10.1029/2009JA015117
Haerendel G, Frey HU, Chaston CC, Amm O, Juusola L, Nakamura R, Seran E,
Weygand JM (2012) Birth and life of auroral arcs embedded in the evening
auroral oval convection: a critical comparison of observations with theory. J
Geophys Res Sp Phys 117:1–20. https://doi.org/10.1029/2012JA018128
Henderson MG (2009) Observational evidence for an inside-out substorm
onset scenario. Ann Geophys 27:2129–2140. https://doi.org/10.5194/
angeo-27-2129-2009
Hesse M, Birn J (1991) On dipolarization and its relation to the substorm current
wedge. J Geophys Res 96:19417. https://doi.org/10.1029/91JA01953
Hesse M, Birn J (1994) MHD modeling of magnetotail instability for localized
resistivity. J Geophys Res 99:8565–8576. https://doi.org/10.1029/94JA00441
Hones EW (1979) Transient phenomena in the magnetotail and their relation to
substorms. Space Sci Rev 23:393–410. https://doi.org/10.1007/BF00172247
Hones EW, Asbridge JR, Bame SJ, Singer S (1973) Substorm variations of themagnetotail plasma sheet from X SM ≈ −6 R E to X SM ≈ −60 R E. J
Geophys Res 78:109–132. https://doi.org/10.1029/JA078i001p00109
Hones EW, Schindler K (1979) Magnetotail plasma flow during substorms: asurvey with imp 6 and imp 8 satellites. J Geophys Res 84:7155. https://doi.
org/10.1029/JA084iA12p07155
Iijima T, Potemra TA (1976) The amplitude distribution of field-aligned currents at
northern high latitudes observed by Triad. J Geophys Res 81:2165. https://doi.org/10.1029/JA081i013p02165
Inhester B, Baumjohann W, Greenwald RA, Nielsen E (1981) Joint twodimensional observations of ground magnetic and ionospheric electric fields
associated with auroral zone currents. III - auroral zone currents during thepassage of a westward travelling surge. J Geophys 49:155–162
Ismail S, Wallis DD, Cogger LL (1977) Characteristics of polar cap Sun-aligned
arcs. J Geophys Res 82:4741–4749. https://doi.org/10.1029/JA082i029p04741
Jiang F, Strangeway RJ, Kivelson MG, Weygand JM, Walker RJ, Khurana KK,
Nishimura Y, Angelopoulos V, Donovan E (2012) In situ observations of the“preexisting auroral arc” by THEMIS all sky imagers and the FAST spacecraft. J
Geophys Res Sp Phys 117:1–17. https://doi.org/10.1029/2011JA017128
Johnson ML, Murphree JS, Marklund GT, Karlsson T (1998) Progress on relating
optical auroral forms and electric field patterns. J Geophys Res 103:4271.
https://doi.org/10.1029/97JA00854
Kadokura A, Yukimatu AS, Ejiri M, Oguti T, Pinnock M, Hairston MR (2002)
Detailed analysis of a substorm event on 6 and 7 June 1989 1. Growth phaseevolution of nightside auroral activities and ionospheric convection toward
expansion phase onset. J Geophys Res Sp Phys. doi: https://doi.org/10.1029/
2001JA009127
Kamide Y (1992) Is substorm occurrence a necessary condition for a magnetic
storm? J Geomagn Geoelectr 44:109–117. https://doi.org/10.5636/jgg.44.109
Kamide Y, Akasofu S-I (1975) The auroral electrojet and global auroral features. J
Geophys Res 80:3585. https://doi.org/10.1029/JA080i025p03585
Kamide Y, Craven JD, Frank LA, Ahn B-H, Akasofu S-I (1986) Modeling substorm
current systems using conductivity distributions inferred from DE auroral
images. J Geophys Res 91:11235. https://doi.org/10.1029/JA091iA10p11235

Page 22 of 24

Kamide Y, Richmond AD, Emery BA, Hutchins CF, Ahn B-H, de la Beaujardiere O,
Foster JC, Heelis RA, Kroehl HW, Rich FJ, Slavin JA (1994) Ground-based
studies of ionospheric convection associated with substorm expansion. J
Geophys Res 99:19451. https://doi.org/10.1029/94JA01625
Kan JR, Williams RL, Akasofu S-I (1984) A mechanism for the westward travelingsurge during substorms. J Geophys Res 89:2211. https://doi.org/10.1029/
JA089iA04p02211
Keiling A, Angelopoulos V, Runov A, Weygand J, Apatenkov SV, Mende S,
McFadden J, Larson D, Amm O, Glassmeier KH, Auster HU (2009) Substorm
current wedge driven by plasma flow vortices: THEMIS observations. J
Geophys Res Sp Phys. https://doi.org/10.1029/2009JA014114
Kepko L, Spanswick E, Angelopoulos V, Donovan E, McFadden J, Glassmeier K-H,
Raeder J, Singer HJ (2009) Equatorward moving auroral signatures of a flow
burst observed prior to auroral onset. Geophys Res Lett 36:L24104. https://doi.org/10.1029/2009GL041476
Kim H-J, Chan AA, Wolf RA, Birn J (2000) Can substorms produce relativistic outerbelt electrons? J Geophys Res 105:7721. https://doi.org/10.1029/
1999JA900465
Korth H, Zhang Y, Anderson BJ, Sotirelis T, Waters CL (2014) Statistical relationshipbetween large-scale upward field-aligned currents and electron precipitation.
J Geophys Res Sp Phys 119:6715–6731. https://doi.org/10.1002/2014JA019961
Koskinen HEJ, Tanskanen EI (2002) Magnetospheric energy budget and theepsilon parameter. J Geophys Res 107:1415. https://doi.org/10.1029/
2002JA009283
Lassen K, Sharber JR, Winningham JD (1977) The development of auroral andgeomagnetic substorm activity after a southward turning of theinterplanetary magnetic field following several hours of magnetic calm.
https://doi.org/10.1029/JA082i032p05031
Lessard MR, Lotko W, LaBelle J, Peria W, Carlson CW, Creutzberg F, Wallis DD(2007) Ground and satellite observations of the evolution of growth phaseauroral arcs. J Geophys Res Sp Phys 112:1–17. https://doi.org/10.1029/
2006JA011794
Liang J, Donovan EF, Liu WW, Jackel B, Syrjäsuo M, Mende SB, Frey HU,
Angelopoulos V, Connors M (2008) Intensification of preexisting auroral arc
at substorm expansion phase onset: wave-like disruption during the first tensof seconds. Geophys Res Lett 35:1–6. https://doi.org/10.1029/2008GL033666
Liou K (2010) Polar ultraviolet imager observation of auroral breakup. J Geophys
Res 115:A12219. https://doi.org/10.1029/2010ja015578
Liou K, Meng C-I, Lui TY, Newell PT, Brittnacher M, Parks G, Reeves GD, Anderson
RR, Yumoto K (1999) On relative timing in substorm onset signatures. J
Geophys Res Sp Phys 104:22807–22817. https://doi.org/10.1029/
1999JA900206
Liou K, Meng CI, Wu CC (2006) On the interplanetary magnetic field by control ofsubstorm bulge expansion. J Geophys Res Sp Phys 111:1–8. doi: https://doi.
org/10.1029/2005JA011556
Liou K, Newell PT (2010) On the azimuthal location of auroral breakup:
hemispheric asymmetry. Geophys Res Lett. https://doi.org/10.1029/
2010GL045537
Liou K, Ruohoniemi JM (2006) A case study of relationship between substorm
expansion and global plasma convection. Geophys Res Lett 33:1–5. https://doi.org/10.1029/2005GL024736
Liu WW, Liang J, Donovan EF, Spanswick E (2012) If substorm onset triggers tailreconnection, what triggers substorm onset? J Geophys Res Sp Phys 117:1–
12. https://doi.org/10.1029/2012JA018161
Lopez RE, Lyon JG, Wiltberger MJ, Goodrich CC (2001) Comparison of global
MHD simulation results with actual storm and substorm events. Adv Sp Res
28:1701–2001. https://doi.org/10.1016/S0273-1177(01)00535-X
Lotko W, Smith RH, Zhang B, Ouellette JE, Brambles OJ, Lyon JG (2014)
Ionospheric control of magnetotail reconnection. Science (80- ) 345:184–187.
https://doi.org/10.1126/science.1252907
Lui ATY (1996) Current disruption in the Earth’s magnetosphere: observations
and models. J Geophys Res Sp Phys 101:13067–13088. https://doi.org/10.
1029/96JA00079
Lui ATY (2011) Reduction of the cross-tail current during near-Earth dipolarization
with multisatellite observations. J Geophys Res Sp Phys. https://doi.org/10.
1029/2011JA017107
Lui ATY, Burrows JR (1978) On the location of auroral arcs near substorm onsets.
J Geophys Res Sp Phys 83:3342–3348. https://doi.org/10.1029/
JA083iA07p03342
Lui ATY, Mankofsky A, Chang C-L, Papadopoulos K, Wu CS (1990) A current
disruption mechanism in the neutral sheet: a possible trigger for substormEbihara Progress in Earth and Planetary Science(2019) 6:24

expansions. Geophys Res Lett 17:745–748. https://doi.org/10.1029/
GL017i006p00745
Lyon JG, Lopez RE, Goodrich CC, Wiltberger M, Papadopoulos K (1998) Simulationof the March 9, 1995, substorm: Auroral brightening and the onset of lobe
reconnection. Geophys Res Lett 25:3039–3042. https://doi.org/10.1029/
98GL00662
Lyons LR, Nishimura Y, Kim H-J, Donovan E, Angelopoulos V, Sofko G, Nicolls M,
Heinselman C, Ruohoniemi JM, Nishitani N (2011) Possible connection ofpolar cap flows to pre- and post-substorm onset PBIs and streamers. J
Geophys Res 116:A12225. https://doi.org/10.1029/2011JA016850
Lyons LR, Samson JC (1992) Formation of the stable auroral arc that intensifies atsubstorm onset. Geophys Res Lett 19:2171–2174. https://doi.org/10.1029/
92GL02494
Machida S, Miyashita Y, Ieda A, Nosé M, Nagata D, Liou K, Obara T, Nishida A,
Saito Y, Mukai T (2009) Statistical visualization of the Earth’s magnetotail
based on Geotail data and the implied substorm model. Ann Geophys 27:1035–1046. https://doi.org/10.5194/angeo-27-1035-2009
McPherron RL (1970) Growth phase of magnetospheric substorms. J Geophys
Res 75:5592–5599. https://doi.org/10.1029/JA075i028p05592
McPherron RL (1997) The role of substorms in the generation of magnetic
storms. In: Tsurutani BT, Gonzalez DW, Kamide Y, Arballo JK (eds) Magnetic
storms. American Geophysical Union, Washington, D. C, pp 131–147
Mcpherron RL, Russell CT, Aubry MP (1973) Satellite studies of magnetosphericsubstorms on August 15, 1968. IX - phenomenological model for substorms.
J Geophys Res 78:3131–3149. https://doi.org/10.1029/JA078i016p03131
Mende SB, Frey HU, Angelopoulos V, Nishimura Y (2011) Substorm triggering bypoleward boundary intensification and related equatorward propagation. J
Geophys Res Sp Phys 116:A00I31. https://doi.org/10.1029/2010JA015733
Meng C-I (1976) Simultaneous observations of low-energy electron precipitationand optical auroral arcs in the evening sector by the DMSP 32 satellite. J
Geophys Res 81:2771–2785. https://doi.org/10.1029/JA081i016p02771
Meng C-I, Snyder AL, Kroehl HW (1978) Observations of auroral westward
traveling surges and electron precipitations. J Geophys Res 83:575. https://doi.org/10.1029/JA083iA02p00575
Moriguchi T, Nakamizo A, Tanaka T, Obara T, Shimazu H (2008) Current systems
in the Jovian magnetosphere. J Geophys Res 113:A05204. https://doi.org/10.
1029/2007JA012751
Nagai T, Fujimoto M, Saito Y, Machida S, Terasawa T, Nakamura R, Yamamoto T,
Mukai T, Nishida A, Kokubun S (1998) Structure and dynamics of magnetic
reconnection for substorm onsets with Geotail observations. J Geophys Res
Sp Phys 103:4419–4440. https://doi.org/10.1029/97JA02190
Nagai T, Singer HJ, Ledley BG, Olsen RC (1987) Field-aligned currents associatedwith substorms in the vicinity of synchronous orbit. I - the July 5, 1979,
substorm observed by SCATHA, GOES 3, and GOES 2. J Geophys Res 92:
2432–2446. https://doi.org/10.1029/JA092iA03p02432
Nakamura R, Baumjohann W, Klecker B, Bogdanova Y, Balogh A, Reme H,
Bosqued J, Dandouras I, Sauvaud J, Glassmeier K, Kistler L, Mouikis C, Zhang
T, Eichelberger H, Runov A (2002) Motion of the dipolarization front during aflow burst event observed by Cluster. 29:2–5. doi: https://doi.org/10.1029/
2002GL015763
Nakamura R, Karlsson T, Hamrin M, Nilsson H, Marghitu O, Amm O, Bunescu C,
Constantinescu V, Frey HU, Keiling A, Semeter J, Sorbalo E, Vogt J, Forsyth C,
Kubyshkina MV (2014) Low-altitude electron acceleration due to multiple
flow bursts in the magnetotail. Geophys Res Lett 41:777–784. https://doi.org/
10.1002/2013GL058982
Nakayama Y, Ebihara Y, Fok MC, Tanaka T (2017) Impact of substorm time
O+outflow on ring current enhancement. J Geophys Res Sp Phys. https://doi.
org/10.1002/2016JA023766
Nishida A (1968) Coherence of geomagnetic DP 2 fluctuations withinterplanetary magnetic variations. J Geophys Res 73:5549–5559. https://doi.
org/10.1029/JA073i017p05549
Nishida A, Fujii K (1976) Thinning of the near-earth (10∼15 RE) plasma sheet
preceding the substorm expansion phase. Planet Space Sci 24:849–853.
https://doi.org/10.1016/0032-0633(76)90075-1
Nishida A, Nagayama N (1973) Synoptic survey for the neutral line in themagnetotail during the substorm expansion phase. J Geophys Res 78:3782–
3798. https://doi.org/10.1029/JA078i019p03782
Nishimura Y, Lyons L, Zou S, Angelopoulos V, Mende S, Sciences O, Angeles L,
Physics P, Sciences O, Sciences S, Arbor A (2010) Substorm triggering by new
plasma intrusion : THEMIS all-sky imager observations. J Geophys Res 115:
A07222. https://doi.org/10.1029/2009JA015166

Page 23 of 24

Nishimura Y, Lyons LR, Kikuchi T, Angelopoulos V, Donovan E, Mende S, Chi PJ,
Nagatsuma T (2012) Formation of substorm Pi2: a coherent response toauroral streamers and currents. J Geophys Res 117:A09218. https://doi.org/10.
1029/2012JA017889
Ohtani S-I, Raeder J (2004) Tail current surge: new insights from a global MHD
simulation and comparison with satellite observations. J Geophys Res 109:
1207. https://doi.org/10.1029/2002JA009750
Ouellette JE, Brambles OJ, Lyon JG, Lotko W, Rogers BN (2013) Properties ofoutflow-driven sawtooth substorms. J Geophys Res Sp Phys 118:3223–3232.
https://doi.org/10.1002/jgra.50309
Palmroth M, Janhunen P, Germany G, Lummerzheim D, Liou K, Baker DN, Barth C,
Weatherwax AT, Watermann J (2006) Precipitation and total power
consumption in the ionosphere: global MHD simulation results comparedwith polar and SNOE observations. Ann Geophys 24:861–872. https://doi.org/
10.5194/angeo-24-861-2006
Palmroth M, Janhunen P, Pulkkinen TI, Aksnes A, Lu G, Østgaard N, Watermann J,
Reeves GD, Germany GA (2005) Assessment of ionospheric Joule heating by
GUMICS-4 MHD simulation, AMIE, and satellite-based statistics: towards
a synthesis. Ann Geophys 23:2051–2068. https://doi.org/10.5194/angeo23-2051-2005
Papadopoulos K, Goodrich C, Wiltberger M, Lopez R, Lyon JG (1999) The physics
of substorms as revealed by the ISTP. Phys Chem Earth, Part C Solar, Terr
Planet Sci 24:189–202. https://doi.org/10.1016/S1464-1917(98)00028-2
Parker EN (1957) Newtonian development of the dynamical properties of ionizedgases of low density. Phys Rev 107:924–933. https://doi.org/10.1103/PhysRev.
107.924
Perreault P, Akasofu S-I (1978) A study of geomagnetic storms. Geophys J Int 54:
547–573. https://doi.org/10.1111/j.1365-246X.1978.tb05494.x
Pritchett PL, Coroniti FV, Nishimura Y (2014) The kinetic ballooning/interchange
instability as a source of dipolarization fronts and auroral streamers. J
Geophys Res Sp Phys 119:4723–4739. https://doi.org/10.1002/2014JA019890
Pulkkinen TI, Baker DN, Pellinen RJ, Büchner J, Koskinen HEJ, Lopez RE, Dyson RL,Frank LA (1992) Particle scattering and current sheet stability in thegeomagnetic tail during the substorm growth phase. J Geophys Res 97:
19283. https://doi.org/10.1029/92JA01189
Pulkkinen TI, Baker DN, Wiltberger M, Goodrich C, Lopez RE, Lyon JG (1998)
Pseudobreakup and substorm onset: observations and MHD simulations
compared. J Geophys Res Sp Phys 103:14847–14854. https://doi.org/10.1029/
97JA03244
Rae IJ, Watt CEJ, Mann IR, Murphy KR, Samson JC, Kabin K, Angelopoulos V (2010)
Optical characterization of the growth and spatial structure of a substorm
onset arc. J Geophys Res Sp Phys 115:1–10. https://doi.org/10.1029/
2010JA015376
Raeder J, McPherron RL, Frank LA, Kokubun S, Lu G, Mukai T, Paterson WR,
Sigwarth JB, Singer HJ, Slavin JA (2001) Global simulation of the Geospaceenvironment modeling substorm challenge event. J Geophys Res Sp Phys
106:381–395. https://doi.org/10.1029/2000JA000605
Raeder J, Zhu P, Ge Y, Siscoe G (2010) Open geospace general circulation model
simulation of a substorm: axial tail instability and ballooning mode
preceding substorm onset. J Geophys Res Sp Phys 115:n/a-n/a. doi: https://doi.org/10.1029/2010JA015876
Raeder J, Zhu P, Ge Y, Siscoe G (2012) Auroral signatures of ballooning modenear substorm onset: open geospace general circulation model simulations.
In: Keiling A, Donovan EF, Bagenal F, Karlsson T (eds) Auroral
phenomenology and Magnetospheric processes: {earth} and other planets.
American Geophysical Union, Washington, D. C, pp 389–395
Rème H, Bosqued JM (1973) Rocket observations of electron precipitation in a
westward-traveling surge. J Geophys Res 78:5553–5558. https://doi.org/10.
1029/JA078i025p05553
Richmond AD, Kamide Y, Akasofu S-I, Alcaydé D, Blanc M, de la Beaujardière O,
Evans DS, Foster JC, Friis-Christensen E, Holt JM, Pellinen RJ, Senior C, Zaitzev
AN (1990) Global measures of ionospheric electrodynamic activity inferredfrom combined incoherent scatter radar and ground magnetometer
observations. J Geophys Res 95:1061. https://doi.org/10.1029/
JA095iA02p01061
Roelof EC, Brandt PC, Mitchell DG (2004) Derivation of currents and diamagnetic
effects from global plasma pressure distributions obtained by IMAGE/HENA.
Adv Sp Res 33:747–751. https://doi.org/10.1016/S0273-1177(03)00638-0
Rothwell PL, Silevitch MB, Block LP (1984) A model for the propagation of the
westward traveling surge. J Geophys Res 89:8941. https://doi.org/10.1029/
JA089iA10p08941

Ebihara Progress in Earth and Planetary Science(2019) 6:24

Roux A, Perraut S, Robert P, Morane A, Pedersen A, Korth A, Kremser G, Aparicio
B, Rodgers D, Pellinen R (1991) Plasma sheet instability related to thewestward traveling surge. J Geophys Res 96:17697. https://doi.org/10.1029/
91JA01106
Saita S, Kadokura A, Sato N, Fujita S, Tanaka T, Ebihara Y, Ohtani S, Ueno G,
Murata K, Matsuoka D, Kitamoto A, Higuchi T (2011) Displacement ofconjugate points during a substorm in a global magnetohydrodynamic
simulation. J Geophys Res Phys 116: doi: https://doi.org/10.1029/
2010ja016155
Sergeev V, Angelopoulos V, Kubyshkina M, Donovan E, Zhou XZ, Runov A, Singer
H, McFadden J, Nakamura R (2011) Substorm growth and expansion onset as
observed with ideal ground-spacecraft THEMIS coverage. J Geophys Res Sp
Phys 116:A00I26. https://doi.org/10.1029/2010JA015689
Sergeev VA, Sauvaud J-A, Popescu D, Kovrazhkin RA, Liou K, Newell PT,
Brittnacher M, Parks G, Nakamura R, Mukai T, Reeves GD (2000) Multiplespacecraft observation of a narrow transient plasma jet in the Earth’s plasma
sheet. Geophys Res Lett 27:851–854. https://doi.org/10.1029/1999GL010729
Shi QQ, Zong Q-G, Fu SY, Dunlop MW, Pu ZY, Parks GK, Wei Y, Li WH, Zhang H,
Nowada M, Wang YB, Sun WJ, Xiao T, Reme H, Carr C, Fazakerley a N, Lucek
E (2013) Solar wind entry into the high-latitude terrestrial magnetosphere
during geomagnetically quiet times. Nat Commun 4:1466. https://doi.org/10.
1038/ncomms2476
Shiokawa K, Baumjohann W, Haerendel G (1997) Braking of high-speed flows in thenear-Earth tail. Geophys Res Lett 24:1179. https://doi.org/10.1029/97GL01062
Silsbee HC, Vestine EH (1942) Geomagnetic bays, their frequency and currentsystems. J Geophys Res 47:195. https://doi.org/10.1029/TE047i003p00195
Siscoe GL, Crooker NU, Erickson GM, Sonnerup BU, Ouml SKD, Weimer DR, White
WW, Maynard NC (2000) Global geometry of magnetospheric currents
inferred from MHD simulations. Magnetos Curr Syst 118:41–52. https://doi.
org/10.1029/GM118p0041
Siscoe GL, Cummings WD (1969) On the cause of geomagnetic bays. Planet
Space Sci 17:1795–1802. https://doi.org/10.1016/0032-0633(69)90055-5
Slinker SP, Fedder JA, Ruohoniemi JM, Lyon JG (2001) Global MHD simulation of
the magnetosphere for November 24, 1996. J Geophys Res Sp Phys 106:361–
380. https://doi.org/10.1029/2000JA000603
Song Y, Lysak RL (2001a) The physics in the auroral dynamo regions and auroral
particle acceleration. Phys Chem Earth, Part C Solar, Terr Planet Sci 26:33–42.
https://doi.org/10.1016/S1464-1917(00)00087-8
Song Y, Lysak RL (2001b) Towards a new paradigm: from a quasi-steadydescription to a dynamical description of the magnetosphere. Space Sci Rev
95:273–292. https://doi.org/10.1023/A:1005288420253
Stepanova MV, Antonova EE, Bosqued JM, Kovrazhkin RA, Aubel KR (2002)
Asymmetry of auroral electron precipitations and its relationship to thesubstorm expansion phase onset. J Geophys Res Sp Phys. https://doi.org/10.
1029/2001JA003503
Sun W, Ahn BH, Akasofu SI (1985) The global joule heat production rate and the
AE index. Planet Space Sci 33:279–281. https://doi.org/10.1016/00320633(85)90059-5
Takahashi K, Zanetti LJ, Lopez RE, McEntire RW, Potemra TA, Yumoto K (1987)
Disruption of the magnetotail current sheet observed by AMPTE/CCE.
Geophys Res Lett 14:1019–1022. https://doi.org/10.1029/GL014i010p01019
Tanaka T (1994) Finite volume TVD scheme on an unstructured grid system forthree-dimensional MHD simulation of inhomogeneous systems including
strong background Potential fields. J Comput Phys 111:381–389
Tanaka T (1995) Generation mechanisms for magnetosphere-ionosphere current
systems deduced from a three-dimensional MHD simulation of the solar
wind-magnetosphere-ionosphere coupling processes. J Geophys Res 100:
12057–12074. https://doi.org/10.1029/95JA00419
Tanaka T (2000a) Field-aligned-current systems in the numerically simulated
magnetosphere. Magnetos Curr Syst Geophys Monogr Ser 118:53–59
Tanaka T (2000b) The state transition model of the substorm onset. J Geophys
Res Sp Phys 105:21081–21096. https://doi.org/10.1029/2000JA900061
Tanaka T (2015) Substorm auroral dynamics reproduced by advanced global
magnetosphere−ionosphere (M-I) coupling simulation. In: Auroral dynamics
and space weather. Wiley, Hoboken, pp 177–190
Tanaka T, Ebihara Y, Watanabe M, Den M, Fujita S, Kikuchi T, Hashimoto KK,
Kataoka R (2017a) Global simulation study for the time sequence of events
leading to the subsorm onset. J Geophys Res Sp Phys:6210–6239. https://doi.
org/10.1002/2017JA024102
Tanaka T, Nakamizo A, Yoshikawa A, Fujita S, Shinagawa H, Shimazu H, Kikuchi T,
Hashimoto KK (2010) Substorm convection and current system deduced

Page 24 of 24

from the global simulation. J Geophys Res Sp Phys 115:A05220. https://doi.
org/10.1029/2009JA014676
Tanaka T, Obara T, Watanabe M, Fujita S, Ebihara Y, Kataoka R (2017b) Formationof the Sun-aligned arc region and the void (polar slot) under the nullseparator structure. J Geophys Res Sp Phys 122:4102–4116. https://doi.org/10.
1002/2016JA023584
Tanaka T, Watanabe M, Den M, Fujita S, Ebihara Y, Kikuchi T, Hashimoto KK,
Kataoka R (2016) Generation of field-aligned current (FAC) and convection
through the formation of pressure regimes: correction for the concept ofDungey’s convection. J Geophys Res A Sp Phys. https://doi.org/10.1002/
2016JA022822
Tanskanen E, Pulkkinen TI, Koskinen HEJ, Slavin JA (2002) Substorm energy
budget during low and high solar activity: 1997 and 1999 compared. J
Geophys Res 107:1086. https://doi.org/10.1029/2001JA900153
Treumann RA, Jaroschek CH, Nakamura R, Runov A, Scholer M (2006) The role ofthe Hall effect in collisionless magnetic reconnection. Adv Sp Res 38:101–111. https://doi.org/10.1016/j.asr.2004.11.045
Weimer DR, Craven JD, Frank LA, Hanson WB, Maynard NC, Hoffman RA, Slavin
JA (1994) Satellite measurements through the center of a substorm surge. J
Geophys Res 99:23639. https://doi.org/10.1029/94JA01976
Welling DT, Jordanova VK, Zaharia SG, Glocer A, Toth G (2011) The effects ofdynamic ionospheric outflow on the ring current. J Geophys Res Sp Phys
116:n/a-n/a. doi: https://doi.org/10.1029/2010JA015642
Wiltberger M, Pulkkinen TI, Lyon JG, Goodrich CC (2000a) MHD simulation of themagnetotail during the December 10, 1996, substorm. J Geophys Res 105:27649. https://doi.org/10.1029/1999JA000251
Yahnin AG, Sergeev VA, Gvozdevsky BB, Vennerstrøm S (1997) Magnetosphericsource region of discrete auroras inferred from their relationship withisotropy boundaries of energetic particles. Ann Geophys 15:943–958. https://doi.org/10.1007/s00585-997-0943-z
Yao Y, Ebihara Y, Tanaka T (2015a) Formation and evolution of high-plasmapressure region in the near-Earth plasma sheet: precursor and postcursor of
substorm expansion onset. J Geophys Res Sp Phys 120:6427–6435. https://doi.org/10.1002/2015JA021187
Yao Y, Ebihara Y, Tanaka T (2015b) Sudden pressure enhancement and tailward
retreat in the near-Earth plasma sheet: THEMIS observation and MHD
simulation. J Geophys Res Sp Phys 120:201–211. https://doi.org/10.1002/
2014JA020482
Zhang Y, Paxton LJ, Newell PT, Meng CI (2009) Does the polar cap disappear
under an extended strong northward IMF? J Atmos Solar-Terrestrial Phys 71:
2006–2012. https://doi.org/10.1016/j.jastp.2009.09.005