Introduction
The substorm triggering mechanism is still one of the most controversial
issues in magnetospheric research. Substorm models proposed so far include
the near-Earth neutral line (NENL) model (e.g., Baker et al., 1996), the
current disruption (CD) model (e.g., Lui, 1996), and the catapult (slingshot)
current-sheet relaxation model (Machida et al., 2009, 2014). These models are
different in the physical process and location of initial action associated
with substorm onset and causal links between the substorm processes. That is,
in the NENL model, magnetic reconnection generates a tailward moving plasmoid
and a fast earthward flow at X∼-20 RE a few minutes before
substorm onset. This earthward flow leads to current disruption and
dipolarization at X∼-10 RE and auroral breakup. In the CD
model, current disruption and dipolarization first occur at X∼-10
RE, leading to auroral breakup and near-Earth reconnection. In
the catapult current-sheet relaxation model, an imbalance between the
J×B and pressure gradient forces generates a fast
earthward flow in the near-Earth magnetotail before onset. This effect leads
to near-Earth reconnection and current disruption just tailward and earthward
of the imbalance region, respectively.
Furthermore, a series of papers by Nishimura et al. (2010, 2011, 2013c) and
Lyons et al. (2010) proposed a new substorm model in which a new plasma
intrusion linked to preonset aurora plays an important role in substorm
triggering. That is, a fast earthward flow generated at the distant neutral
line typically located at X∼-130 RE (see Machida et al.,
2000), rather than at the near-Earth neutral line typically located at X∼-20 to -30 RE (see Nagai et al., 1998; Machida et al.,
1999; Miyashita et al., 2000, 2009; Imber et al., 2011), reaches the
near-Earth region at X∼-10 RE and triggers a substorm.
This fast flow in the magnetotail corresponds to a preonset north–south
auroral streamer or east–west auroral arc in the ionosphere propagating
equatorward from the auroral poleward boundary, which they call preonset
aurora. After the preonset aurora reaches the equatorward portion of the
auroral oval, the onset (initial brightening) occurs at this location for
about half of substorm events. For most of the rest of substorm events, the
preonset aurora changes direction to move azimuthally, and initial
brightening occurs when the preonset aurora reaches the initial brightening
site. According to Nishimura et al. (2010), such preonset auroras related to
initial brightening are observed frequently, i.e., for ∼90 % of
substorm events in total.
Many studies (e.g., Henderson, 2009; Rae et al., 2009; Mende et al., 2011;
Murphy et al., 2014b; Kepko, 2014), however, reported substorm events without
any auroral streamers and arcs moving to the auroral onset arc before initial
auroral brightening and even poleward expansion. This indicates that the
processes associated with a preonset aurora are not necessary conditions for
substorm triggering. Thus the preonset aurora scenario proposed by Nishimura
et al. (2010) is
controversial, and more detailed reexaminations of their events are required
to prove or disprove it.
In the present study, using the same auroral data as Nishimura et al. (2010,
2011), i.e., data from white-light ground-based observatory (GBO) all-sky
imagers (ASIs) (Donovan et al., 2006; Mende et al., 2008) of the Time History
of Events and Macroscale Interactions during Substorms (THEMIS) mission
(Angelopoulos, 2008) at 3 s resolution, we revisited three substorm events
that Nishimura et al. (2010, 2011) identified as isolated onset events
triggered by the processes associated with preonset auroras. These events
were discussed at a focus group, “Testing Proposed Links between Mesoscale
Auroral and Polar Cap Dynamics and Substorms”, of the summer and mini
workshops of the Geospace Environment Modeling (GEM) program held in
Snowmass, Colorado, and San Francisco, California, respectively, in 2015. Considering the
present results, we list a series of open questions for testing the preonset
aurora scenario further in future studies.
Here we performed more detailed analysis than Nishimura et al. (2010, 2011).
The main differences between the respective approaches are as follows. The
first is consideration of stepwise auroral onset arc development. Nishimura
et al. (2010, 2011) considered only either initial brightening or poleward
expansion as substorm onset. On the other hand, we attempted to distinguish
the three steps of initial brightening, enhancement of the wave-like
structure, and poleward expansion, as described below. This will make our
discussion about the arrival timing of the preonset aurora relative to the
three auroral steps and the role of the preonset aurora in the auroral steps
clearer. The second difference is the timing determination method. Nishimura
et al. (2010, 2011) determined the timings of the substorm onset and the
streamer arrival visually from substantial intensification. On the other
hand, we determined the timings of the auroral onset arc development and the
streamer arrival by a more quantitative method, paying attention to initial
faint auroras as well, as described in the next section. The third difference
is consideration of the spatial relationship between the auroral onset arc,
the auroral streamer, and the associated flow channel and flow vortices, as
described below. Nishimura et al. (2010, 2011) considered the flow channel, but we considered the flow
vortices as well.
Before describing the three events, we summarize stepwise development of the
auroral onset arc whose timings we determined for the present examination.
The auroral arc develops in four steps in association with substorm onset
(Mende et al., 2009): (1) preonset fading, (2) initial brightening,
(3) enhancement of the wave-like structure, and (4) poleward expansion.
(1) Although in not all substorms, preonset auroral fading or dimming may be
seen a few minutes before initial auroral brightening or breakup (Pellinen
and Heikkila, 1978; Kauristie et al., 1997; Murphy et al., 2012, 2013). This
phenomenon does not necessarily extend to the entire arc, but is localized,
which is consistent with localized decrease in upward field-aligned currents
(Murphy et al., 2012, 2013; Coxon et al., 2017). We should note that
630.0 nm emission related to low-energy (<∼1 keV) electrons intensified
during fading of 557.7 nm emission primarily related to high-energy
(>∼1 keV) electrons (Deehr and Lummerzheim, 2001). Deehr and
Lummerzheim (2001) inferred that this difference between the emissions is a
signature of Alfvén waves, which is possibly supported by the result of
Chaston et al. (2002) that electrons accelerated by Alfvén waves are in a
lower energy range. (2) An auroral arc begins to intensify or newly appears
at initial brightening. The brightening segment is localized to a
longitudinal width of ∼30–60 km at the very early stage (Sakaguchi et
al., 2009a; see also Nishimura et al., 2016). It then extends westward and
eastward, and a wave-like or bead-like structure grows gradually (e.g.,
Davis, 1962; Elphinstone et al., 1995; Friedrich et al., 2001; Donovan et
al., 2007; Liang et al., 2008; Sakaguchi et al., 2009a, b; Henderson, 2009;
Rae et al., 2009, 2010; Kepko et al., 2009; Motoba et al., 2012, 2015; Chang
et al., 2012; Murphy et al., 2014b; Chang and Cheng, 2015; Kalmoni et al.,
2015, 2017; Motoba and Hirahara, 2016; Nishimura et al., 2016). Akasofu
(1964) described initial brightening as being sudden, but this
intensification seems to be gradual and can last for several minutes (Lyons
et al., 2002; Liang et al., 2008; Mende et al., 2009; Sakaguchi et al.,
2009b). This discrepancy of sudden or gradual intensification possibly comes
from the time resolution and sensitivity of cameras used. (3) Then the
luminosity of the arc is enhanced exponentially at some time and the
wave-like structure becomes clearer and grows further. Mende et al. (2009)
described this stage as appearance of a new rayed arc, but we call it
enhancement of the wave-like structure here. (4) Finally, poleward expansion
or auroral breakup begins.
In spite of stepwise auroral development, most previous studies marked only
one or at most two timings of the latter three auroral timings and chose one
as the substorm onset time. That is, different studies adopted different
definitions of the substorm onset. Some studies adopted initial brightening
as the substorm onset, while others adopted enhancement of the wave-like
structure or poleward expansion. Nishimura et al. (2010, 2011) intended to
mark only initial brightening as the substorm onset, while Nishimura et
al. (2016) marked initial brightening and poleward expansion. Thus, not to
lead to confusion, we will avoid using the term “onset” without caution
when we describe the timings of auroral onset arc development below.
Each step of auroral development probably corresponds to magnetotail substorm
signatures. That is, Miyashita et al. (2015) suggested that initial
brightening possibly corresponds to near-Earth magnetic reconnection,
enhancement of the wave-like structure corresponds to growth of the
ballooning instability in the near-Earth magnetotail, and poleward expansion
corresponds to near-Earth dipolarization. This indicates that determining and
distinguishing all of the three steps in particular are very important for
making our discussions about the timing issue and the magnetotail development
clear. In the case of the present study, examining the arrival timing of the
preonset aurora relative to the three auroral steps (initial brightening,
enhancement of the wave-like structure, and poleward expansion) is expected
to make the role of the preonset aurora in the auroral steps clearer.
In addition to the timing of each auroral step, we should pay attention to
the spatial relationship between the auroral streamer and the associated flow
channel and flow vortices. Nishimura et al. (2010, 2011) considered the flow
channel, but they do not seem to have considered the flow vortices. As shown
in the schematic of Fig. 1, the auroral streamer corresponds, not to the
central part of the flow channel, but to the western edge of the flow
channel, i.e., the western flow vortex and upward field-aligned current
(Kauristie et al., 2000; Nakamura et al., 2001). A typical scale of the flow
vortex is up to ∼1 h in magnetic local time (MLT) in the ionosphere
(Amm et al., 1999; Kauristie et al., 2000; Nakamura et al., 2001). Hence,
considering not only the auroral streamer but also the central part of the
flow and the flow vortices, the separation between the auroral streamer and
the initial brightening site should be within the sum of the scale of the
eastern flow vortex and the flow channel width if initial brightening occurs
east of the auroral streamer. On the other hand, it should be within the
scale of the western flow vortex if initial brightening occurs west of the
auroral streamer. If the separation is more than these scales, the processes
associated with the streamer would not affect initial brightening.
Furthermore, considering the angle between the auroral streamer and the onset
arc, if the auroral streamer moves purely equatorward, the auroral streamer,
the central part of the flow, and the flow vortices may arrive at the onset
arc simultaneously (Fig. 1a). If the auroral streamer moves equatorward and
westward, the central part of the flow and the eastern flow vortex may
contact the onset arc earlier than the auroral streamer (the western flow
vortex) (Fig. 1b). If the auroral streamer moves equatorward and eastward,
the auroral streamer may contact the onset arc earlier than the central part
of the flow and the eastern flow vortex (Fig. 1c).
Schematics showing the spatial relationship between the auroral
onset arc and the concurrent flow channel and flow vortices in the cases of
(a) a purely equatorward moving streamer, (b) an
equatorward and westward moving streamer, and
(c) an equatorward and eastward moving streamer.
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Timing determination method
To determine the timings of the auroral development associated with substorm
onsets, different previous studies adopted different methods. In studies
examining not only two-dimensional auroral images but also luminosity curves
from THEMIS ASI, for example, Mende et al. (2009) determined the three
timings of initial brightening, enhancement of the wave-like structure, and
poleward expansion from slope increase (breakpoint) in the total integrated
luminosity curve, although they seem to have identified visually.
Angelopoulos et al. (2008), Gabrielse et al. (2009), and Liu et al. (2011)
used a similar method to determine only auroral intensification time from the
breakpoint of the integrated auroral luminosity over the region of interest.
This intensification occurred before poleward expansion, so it most likely
corresponds to initial brightening or enhancement of the wave-like structure.
Murphy et al. (2014a) also used the total integrated luminosity, but
attempted automatic determination of the auroral breakup interval, which
corresponds to what Angelopoulos et al. (2008) determined. Nishimura et
al. (2016) determined initial brightening and poleward expansion. They
defined the former by the initial rise of the maximum luminosity along the
onset arc, which is simultaneous with the beginning of growth of the onset
arc's wave-like structure.
Kalmoni et al. (2015, 2017) determined two auroral timings by different
quantitative methods considering development of the wave-like (bead-like)
auroral structure. Kalmoni et al. (2015) first identified clear appearance of
the wave-like structure visually and then determined the beginning of growth
of the wave-like structure with linear fitting in log space for individual
wave numbers. Kalmoni et al. (2017) determined the beginning of growth of the
wave-like structure on the basis of the appearance of the wave-like structure
and also determined that of exponential growth of the total auroral
luminosity with linear fitting in log space. In spite of the different
methods, their total luminosity curve and north–south and east–west
keograms indicate that the first and second timings most likely correspond to
initial brightening and enhancement of the wave-like structure, respectively,
and poleward expansion occurred at a later time.
Thus quantitative timing determination in the previous studies is based on
mainly the breakpoint (trend increase) of the luminosity curve and growth of
the wave-like structure. In the present study, we adopted the former for the
following reasons, developing the previous method as described below. Our
method can determine all the timings of the three auroral steps of initial
brightening, enhancement of the wave-like structure, and poleward expansion
that Mende et al. (2009) proposed, although it is simple. As Nishimura et
al. (2016) and Kalmoni et al. (2017) showed, the auroral luminosity begins to
increase at the same time as growth of the onset arc's wave-like structure.
Although growth of the wave-like structure is an important feature of the
auroral onset arc, determining the trend increases in the luminosity is
adequate for our purpose of timing discussion.
Our method of timing determination is as follows (the results of each event
are described in detail in the next section). We first determined the timing
and location of the three auroral steps visually from two-dimensional ASI
images (as Fig. 2 and Supplement Movie S1) by changing the black–white scale
to pay attention to initial faint auroras as well and looking at the series
of the images back and forth. To see the variations in the luminosity of the
auroral arc and determine the timings of auroral onset arc development
quantitatively, we then examined the variations of average counts for areas
of 0.5∘ longitudinal width including the auroral onset arc (the
magenta boxes in Fig. 2), as shown in Fig. 3. To determine the trend increase
in the luminosity, we applied a segmented (piecewise) linear fitting with one
breakpoint (cf. Tomé and Miranda, 2004, 2005) to the time-series count data
for each area at and near the location of each step around its beginning.
Here we used the Levenberg–Marquardt algorithm for the fitting (Moré,
1978; Markwardt, 2009). Whether two lines with one breakpoint or a single
line are better fitting was judged from the values of χ2. We took
fitting intervals manually. Unless they are too short and include the
previous and next large changes, different intervals did not make large
differences in the fitting results. In Fig. 3, the thick line for each area
indicates a result of the fitting. The short vertical bar indicates the
breakpoint of the line, while it is not drawn if a single straight line is
better in fitting. The horizontal bar at the breakpoint indicates the error
of the time of the breakpoint. For each auroral step, the earliest breakpoint
where the slope increased is regarded as the beginning of the step. If two
auroral steps are close in time to each other, as in Fig. 3b, the two
fitting intervals are allowed to overlap with each other to make the fitting
intervals not too short. Furthermore, we tried this method on both linear and
logarithmic scales. We found that the method worked and the results were
nearly the same between the linear and logarithmic scales for initial
brightening and enhancement of the wave-like structure, but the method seems
to have worked only for the logarithmic scale for poleward expansion (not
shown). Hence we use the logarithmic scale for timing determination.
Selected auroral images obtained from the THEMIS GBO all-sky imager
at GILL between 05:22 and 05:32 UT on 25 February 2008, along with
footprints of THEMIS D (light blue) and E (blue) calculated using the T96
magnetic field model (Tsyganenko, 1995). White arrows and circles indicate
approximate locations of auroral phenomena. The maximum count values of the
black–white scale are 1800 for 05:22:39 to 05:29:51 UT and 3000 for the
later times.
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Variations of the averages of the logarithm of count for areas of
0.5∘ longitudinal width in the region indicated in the top of this
figure, or the magenta boxes in the (a) 05:22:39 and
(b) 05:28:00 UT panels of Fig. 2, including the central and eastern
parts of the auroral onset arc obtained from the THEMIS GBO all-sky imager at
GILL on 25 February 2008. The data from the east to the west are plotted from
the top to the bottom. The vertical lines with labels of IB, EW, and PE
indicate the times of initial auroral brightening, enhancement of the
wave-like structure, and poleward expansion, respectively. The thick line for
each area at and near the site of each auroral step indicates a result of
segmented (piecewise) linear fitting with one breakpoint. The short vertical
bar indicates the breakpoint of the line, while it is not drawn if a single
straight line is better in fitting. The horizontal bar at the breakpoint
indicates the error of the time of the breakpoint. For each auroral step, the
earliest breakpoint where the slope increased is regarded as the beginning of
the step.
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In addition, we adopted the same method as auroral onset arc development to
auroral streamers. After determining the timing and location of auroral
streamers visually from two-dimensional ASI images (as Fig. 2 and Supplement
Movie S1) in a similar way to auroral onset arc development, we applied the
segmented linear fitting with one breakpoint to each area of 0.05∘
latitudinal width in the magenta box indicated in two-dimensional images
including the streamer's path from the poleward arc or a few degrees poleward
of the auroral onset arc to the poleward part of the auroral onset arc
(Fig. 4). The count should increase when a streamer enters an area, and this
increase should propagate equatorward until the streamer arrives just
poleward of or at the auroral onset arc.
Variations of the averages of the logarithm of count for areas of
0.05∘ latitudinal width in the region indicated in the top of this
figure, or the (a) western and (b) eastern halves of the magenta box
in the 05:24:54 UT panel of Fig. 2, including the path of the southeastward
moving auroral streamer obtained from the THEMIS GBO all-sky imager at GILL
on 25 February 2008. The data from the north to the south are plotted from
the top to the bottom. The left and right vertical dashed lines indicate the
times of the arrival of the streamer just poleward of and at the auroral
onset arc, respectively. The thick line for each area where the streamer
reached indicates a result of segmented (piecewise) linear fitting with one
breakpoint, which indicates the time of the arrival of the streamer at this
area. The short vertical bar indicates the breakpoint of the line, while it
is not drawn if a single straight line is better in fitting. The horizontal
bar at the breakpoint indicates the error of the time of the breakpoint.
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Observations of auroral streamers
We show only the results of ASI data analysis for the three events in this section, since they
are not necessarily good conjunction events in terms of the locations of the THEMIS spacecraft
in the magnetotail. We first describe the three timings of auroral onset arc development and
then describe equatorward moving auroral streamers observed by ASIs.
25 February 2008 event
According to Nishimura et al. (2010, 2011), this event was an isolated
substorm with poleward expansion. Figure 2 presents selected ASI images from
Gillam (GILL), Canada (66.00∘ geomagnetic latitude, 333.19∘
geomagnetic longitude) (see also Supplement Movie S1). Figure 3 presents the
variations of auroral counts along the auroral onset arc in the magenta boxes
in the 05:22:39 and 05:28:00 UT panels of Fig. 2. Initial brightening and later development
were observed in the field of view of this camera. Before initial
brightening, the luminosity was increased gradually (Fig. 3a); such
gradual intensification of the growth phase arc was also reported by Lessard
et al. (2007). Preonset auroral fading was not seen just before initial
brightening in the present event as well as in the other two events shown
below. Although there was a bright arc extending from the eastern edge of the
field of view toward the central part of the field of view, initial
brightening did not occur on this arc, but instead occurred near the central
part of the field of view at ∼68∘ magnetic latitude and
∼23 h MLT at 05:23:50 UT. Figure 3a shows that the breakpoint
due to slight increase in the trend appeared first at a localized segment (on
yellow and orange lines) at this time and then appeared at the westward and
eastward segments (on neighbors) successively. That is, the onset arc
gradually grew brighter, although very faint, and it extended mainly westward
and slightly eastward. This localization and gradual progress of initial
brightening is consistent with Mende et al. (2009) and Sakaguchi et
al. (2009a, b), as mentioned above. Figure 3a also shows that the
luminosity oscillated after initial brightening by increasing the trend, due
to gradual growth of the wave-like structure of the auroral onset arc. At
05:28:47 UT, ∼5 min after initial brightening, the onset arc as well
as the wave-like or bead-like structure were further enhanced to the west of
the initial brightening site. The breakpoint due to larger increase appeared
first on a green line and then on neighbors in Fig. 3b. At
05:29:34 UT, another ∼50 s later, poleward expansion began nearly at
the same place as enhancement of the wave-like structure. The breakpoint due
to explosive increase appeared first on a blue line and then on neighbors in
Fig. 3b. The times of auroral onset arc development, along with those
of the preonset aurora shown below, are summarized in Table 1.
A few previous papers studied this event. Nishimura et al. (2010, 2011)
determined only one timing of 05:29 UT, ∼5 min later than our initial
brightening time, for auroral onset arc development. Their timing is based on
visual inspection of substantial intensification of the auroral onset arc, so
it possibly corresponds to what we identified as enhancement of the wave-like
structure. Kepko et al. (2009) also analyzed this event using multiple
emission data. They determined the times of brightening of equatorward
boundary, formation of auroral ray, auroral beading, and poleward expansion
as 05:29:11, 05:29:43, 05:30:07, and 05:30:31 UT, respectively. Their first
step possibly corresponds to initial brightening, while the second or third
step corresponds to enhancement of the wave-like structure. Their timings are
all later than our determination, possibly because they determined the
timings from sufficiently intensified signatures, as also pointed out by
Lui (2011). That is, Lui (2011), who revisited this event using the same data
as Kepko et al. (2009), paid attention to faint aurora and pointed out that
the luminosity was increasing from 05:26:50 to 05:29:14 UT. Although it is
not clear whether he examined the data for earlier times, he determined the
times of initial brightening and poleward expansion as between 05:26:37 and
05:27:01, and 05:29:35 UT, respectively. The latter agrees with our
determination. Our interpretations of these times are summarized in Table 1.
The white-light images in Fig. 2 and Movie S1 show that the auroral streamer
of this event was very faint. It seems to have appeared in the arc in the
northern part of the field of view at ∼05:25 UT and then propagated in
the equatorward and slightly eastward directions. The auroral count
variations along the path of this streamer in Fig. 4 show that this streamer
arrived just poleward of the onset arc at 05:26:50 UT, or 3 min after
initial brightening and ∼2 min before enhancement of the wave-like
structure (green lines in Fig. 4b). It then slowed down and arrived
at the onset arc at 05:27:15 UT, or ∼3.5 min after initial brightening
and ∼1.5 min before enhancement of the wave-like structure (blue lines
in Fig. 4b). The arrival point was ∼0.3 h in MLT to the west of
the initial brightening site as well as enhancement of the wave-like
structure and poleward expansion. Note that after the auroral streamer
arrived at the onset arc, it did not move along the onset arc. We also could
not identify another preonset aurora moving along the onset arc from the
outside of the field of view toward the initial brightening site.
UT of auroral onset arc development and preonset aurora that the
present study determined by fitting, our interpretations of the times that
previous studies determined, and the arrival location of the preonset aurora
relative to the initial brightening or poleward expansion site (ΔMLT)
for the 25 February 2008 substorm event.
25 Feb 2008
The present study
Previous studies
Auroral onset arc development
Initial brightening
05:23:50
05:26:37–05:27:01 (Lui, 2011)
05:29:11 (Kepko et al., 2009)
Enhancement of the wave-like structure
05:28:47
05:29 (Nishimura et al., 2010, 2011)
05:29:43 or 05:30:07 (Kepko et al., 2009)
Poleward expansion
05:29:34
05:29:35 (Lui, 2011)
05:30:31 (Kepko et al., 2009)
Preonset aurora
Emergence
∼05:25
diffuse patch, 05:23:15 (Kepko et al., 2009)
Arrival just poleward of/at the onset arc
05:26:50/05:27:15
arrived (Nishimura et al., 2010, 2011, 2013a)
∼05:27 (Lui, 2011)
diffuse patch, 05:29:14 (Kepko et al., 2009)
ΔMLT, h
∼0.3, west
According to Kepko et al. (2009), a diffuse auroral patch or streamer moving
equatorward, which was seen clearly in their auroral images of 630.0 nm
emission, emerged near the middle of the field of view at 05:23:15 UT and
arrived at the onset arc at 05:29:14 UT, ∼30 s after enhancement of
the wave-like structure and 20 s before poleward expansion. The western edge
of this diffuse patch corresponds to a narrow discrete form seen in 557.7 and
427.8 nm emissions or the streamer mentioned above. As also pointed out by
Lui (2011), Kepko et al. (2009) did not discuss that this streamer already
connected with the onset arc at ∼05:27 UT. Furthermore, Nishimura et
al. (2013a) also examined this event. Although they did not describe the
specific timings, they concluded that a polar cap patch was generated on the
dayside, propagating across the polar cap and the nightside auroral poleward
boundary, and then became the auroral streamer and/or diffuse patch that we
and Kepko et al. (2009) discussed.
28 February 2008 event
Nishimura et al. (2011) showed this isolated substorm with poleward expansion
in detail. The Dst and AE indices indicate that this substorm occurred during
active time, or between two weak storms under enhanced convection. Figure 5
shows selected ASI images from McGrath (MCGR), Alaska (61.74∘
magnetic latitude, 260.25∘ magnetic longitude) (see also Supplement
Movie S2). The average count variations along the auroral onset arc (the
magenta box in the 10:57:24 UT panel of Fig. 5) are shown in Fig. 6. Before
initial brightening, the luminosity was increased gradually (Fig. 6a),
similarly to the events of Lessard et al. (2007). Initial brightening
occurred in the east of the field of view at ∼63∘ magnetic
latitude and ∼0 h MLT at 10:59:22 UT. Figure 6a shows that the
breakpoint due to slight increase in the trend appeared first at a localized
segment (on a green line) at this time and then appeared at the westward
segments (on neighbors) successively. That is, the onset arc gradually grew
brighter and extended mainly westward. Here the luminosity enhancements west
of the initial brightening site (in the west of the magenta box in Fig. 5)
between ∼10:58 and 11:01 UT (blue to light violet lines in Fig. 6a) and between ∼11:00 and 11:02 UT (dark violet to black lines in
Fig. 6a) were due to auroral streamers coming from the poleward region.
Figure 6a also shows that the luminosity oscillated after initial
brightening by increasing the trend, due to gradual growth of the wave-like
structure of the auroral onset arc. At 11:07:19 UT, ∼8 min after
initial brightening, the onset arc as well as the wave-like structure were
further enhanced in the middle of the field of view. The breakpoint due to
larger increase appeared in this part of the onset arc (on a blue line and
then on neighbors) in Fig. 6a and b. At 11:10:19 UT, another
3 min later, poleward expansion began to the east of enhancement of the
wave-like structure. The breakpoint due to explosive increase appeared first
on a green line and then on neighbors in Fig. 6b. The times of auroral
onset arc development are summarized in Table 2.
Selected auroral images obtained from the THEMIS GBO all-sky imager
at MCGR between 10:57 and 11:12 UT on 28 February 2008. The maximum count
value of the black–white scale is 3000, except that it is 4000 for the last
image.
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Variations of the averages of the logarithm of count for areas of
0.5∘ longitudinal width in the region indicated in the top of this
figure, or the magenta box in the 10:57:24 UT panel of Fig. 5, including the
auroral onset arc obtained from the THEMIS GBO all-sky imager at MCGR on 28 February 2008. The format is the same as Fig. 3.
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UT of auroral onset arc development and preonset aurora, and the arrival location of
the preonset aurora relative to the initial brightening or poleward expansion site (ΔMLT)
for the 28 February 2008 substorm event.
28 Feb 2008
The present study
Previous studies
Auroral onset arc development
Initial brightening
10:59:22
Enhancement of the wave-like structure
11:07:19
Poleward expansion
11:10:19
11:10 (Nishimura et al., 2010, 2011)
Preonset aurora
Emergence
∼11:07
Arrival just poleward of/at the onset arc
11:07:50/11:09:09
11:10? (Nishimura et al., 2010, 2011)
ΔMLT, h
∼0.5, west
Nishimura et al. (2010, 2011) marked 11:10 UT from substantial
intensification of the auroral onset arc. This timing is ∼10.5 and
3 min after our times of initial brightening and enhancement of the
wave-like structure, respectively, or rather, near our poleward expansion
time. Hence we surmise that the time marked by Nishimura et al. (2010, 2011)
was that of poleward expansion. Here note that another sudden auroral
enhancement did not occur between initial brightening and enhancement of the
wave-like structure.
A clear equatorward moving auroral streamer appeared between the
poleward and equatorward arcs (∼1∘ poleward of the equatorward
arc) in the middle of the field of view at 10:57:30 UT, although Nishimura
et al. (2011) did not mention it. While this aurora was growing bright, it
extended westward, or another aurora appeared on the westward side. These
auroras moved equatorward, and the first auroral streamer arrived at the
equatorward arc at ∼10:58 UT, ∼1 min before initial brightening.
The arrival point was ∼0.6 h in MLT west of the initial brightening
site, but it may be within the expected flow vortex scale. Since this aurora
appears to have been generated in the auroral oval, we may conclude that it
was not related to Nishimura et al.'s (2010) scenario in which a preonset
aurora should be generated at the auroral poleward boundary corresponding to
the distant neutral line. We would like to report on this streamer in detail
elsewhere.
After that, another very clear auroral streamer that Nishimura et al. (2011)
regarded as being related to the processes leading to initial brightening
appeared in the poleward part of the auroral oval after initial brightening
at ∼11:07 UT and then extended toward the onset arc. The auroral count
variations along the path of this streamer in Fig. 7 show that this streamer
arrived just poleward of the onset arc near the site of enhancement of the
wave-like structure at 11:07:50 UT, or ∼8.5 min after initial
brightening and ∼30 s after enhancement of the wave-like structure (a
light blue line). It then slowed down and arrived at the onset arc at
11:09:09 UT, or ∼2 min after enhancement of the wave-like structure
and ∼1 min before poleward expansion (blue lines). The arrival point
was ∼0.5 h or less in MLT to the west of the initial brightening site
as well as enhancement of the wave-like structure and poleward expansion.
Note that the streamer did not move eastward or toward the poleward expansion
site after arriving at the onset arc and before poleward expansion. We also
could not identify another preonset aurora moving along the onset arc from
the outside of the field of view toward the initial brightening site.
Variations of the averages of the logarithm of count for areas of
0.05∘ latitudinal width in the region indicated in the top of this
figure, or the magenta box in the 11:05:06 UT panel of Fig. 5, including the
path of the equatorward moving auroral streamer obtained from the THEMIS GBO
all-sky imager at MCGR on 25 February 2008. The format is the same as Fig. 4.
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Nishimura et al. (2011) seem to have taken the arrival time of the streamer
as the time of poleward expansion (11:10 UT), which is ∼2 or 1 min
later than our determination. In any case, the arrival time was later than
the times of initial brightening and enhancement of the wave-like structure.
5 March 2008 event
According to Nishimura et al.'s (2010, 2011) list, this event was an isolated
one without the subsequent significant poleward expansion. Figure 8 shows
selected ASI images from GILL (see also Supplement Movie S3). The average
count variations along the auroral onset arc (the magenta boxes in the
06:01:33 and 06:02:33 UT panels of Fig. 8) are shown in Fig. 9. The
luminosity was almost constant before initial brightening in this event,
unlike the other events shown above. Lessard et al. (2007) showed an event in
which only 630.0 nm emission was intensified while 557.7 and 486.1 nm
emissions were unchanged. We surmise that this was the case with our event
shown here. Since 557.7 nm emission is often more intense than 630.0 and
486.1 nm emissions, the white-light images from THEMIS ASI possibly
reflected the unchanged 557.7 nm emission in the present event. Then initial
brightening occurred near the middle of the field of view at
∼67∘ magnetic latitude and ∼0 h MLT at 06:02:15 UT.
Figure 9a shows that the breakpoint due to slight increase in the trend
appeared first at a localized segment (on a green line) at this time and then
appeared at the westward and eastward segments (on neighbors) successively.
That is, the onset arc gradually grew brighter and extended westward and
eastward. Figure 9a also shows that the luminosity oscillated after
initial brightening with increasing the trend, due to gradual growth of the
wave-like structure of the auroral onset arc. At 06:03:15 UT, 1 min after
initial brightening, the onset arc as well as the wave-like structure were
further enhanced somewhat eastward of the initial brightening site. The
breakpoint due to larger increase appeared in this part of the onset arc (on
a yellow green line and then mainly on western neighbors) in Fig. 9a.
At 06:06:04 UT, another ∼3 min later, poleward expansion began to the
east of the initial brightening site. The breakpoint due to explosive
increase appeared first on green lines and then on neighbors in Fig. 9b. The times of auroral onset arc development are summarized in
Table 3.
Selected auroral images obtained from the THEMIS GBO all-sky imager
at GILL between 05:59 and 06:09 UT on 5 March 2008. The maximum count values
of the black–white scale are 1000 for 05:59:15 to 06:06:06 UT and 2000 for
the later times.
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Nishimura et al. (2010, 2011), Liu et al. (2008), and Rae et al. (2012, 2017)
analyzed this event and determined the breakup arc formation time as
06:04 UT. Nishimura et al. (2016) revised the times of auroral development,
determining the initial brightening time as 06:03:25 UT and the poleward
expansion time as 06:06:18 UT. The breakup arc formation and initial
brightening times determined by these previous studies nearly agree with
∼1 min after our time of enhancement of the wave-like structure, rather
than our initial brightening time. This is possibly because they determined
the time from substantial intensification, while we paid attention to initial
faint aurora as well. The poleward expansion time of Nishimura et al. (2016)
roughly agrees with our determination.
UT of auroral onset arc development and preonset aurora for the 5 March 2008 substorm event.
5 Mar 2008
The present study
Previous studies
Auroral onset arc development
Initial brightening
06:02:15
Enhancement of the wave-like structure
06:03:15
06:03:25 (Nishimura et al., 2016)
06:04 (Nishimura et al., 2010, 2011)
06:04 (Liu et al., 2008)
06:04 (Rae et al., 2012, 2017)
Poleward expansion
06:06:04
06:06:18 (Nishimura et al., 2016)
Preonset aurora
Emergence
∼05:53, 06:00:05
Arrival just poleward of/at the onset arc
not arrived
arrived (Nishimura et al., 2010, 2011)
For this event, we could not find any preonset auroras propagating from the
auroral poleward boundary to the onset arc, as also pointed out by Rae et
al. (2017). A bright east–west-aligned arc appeared at ∼05:53 UT at a
few degrees higher latitude than the onset arc. Another bright
east–west-aligned arc appeared further poleward of this arc at 06:00:05 UT
(green to red lines of auroral count variations in Fig. 10a). These two
arcs did not approach the onset arc. Furthermore, a faint east–west-aligned
arc appeared between the prior arc and the onset arc at 05:59:43 UT (blue
lines in Fig. 10a). Although it slightly moved equatorward, it stopped
just poleward of the onset arc soon. In any case, this faint arc was not the
preonset aurora propagating from the poleward boundary as proposed by
Nishimura et al. (2010).
It is not clear which poleward auroral arc Nishimura et al. (2010, 2011)
identified as the preonset aurora, but they seem to have mistakenly counted
the aurora unrelated to auroral onset arc development associated with
substorm.
Variations of the averages of the logarithm of count for areas of
0.5∘ longitudinal width in the region indicated in the top of this
figure, or the magenta boxes in the (a) 06:01:33 and (b) 06:02:33 UT
panels of Fig. 8, including the auroral onset arc obtained from the THEMIS
GBO all-sky imager at GILL on 5 March 2008. The format is the same as Fig. 3.
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Discussion
Effect of flow and flow vortices
In the previous section we showed the relative timings of the three steps of
auroral onset arc development and arrival of the auroral streamers at the
onset arc on the basis of ASI data. As we mentioned in the introduction and
as depicted in Fig. 1, we should also pay attention to the effect of flow and
flow vortices related to the auroral streamer, which can be observed by
ground-based radars and magnetometers, not by ASIs. We checked the SuperDARN
data (Greenwald et al., 1995), but unfortunately there were too few echoes to
identify the flow vortices and their spatial scale. Ground magnetometer data
were also available, but the ground stations were too sparsely distributed to
identify the small-scale flow vortices associated with the auroral streamer
for the present events. Hence we just discuss the flow vortex effect for the
first two streamer events shown in Sects. 3.1 and 3.2 by considering that a
typical spatial scale of the vortex is ∼1 h in MLT.
For the 25 February 2008 event shown in Sect. 3.1, the separation of the
arrival point of the auroral streamer and the sites of auroral onset arc
development was ∼0.3 h in MLT, which may be within the expected eastern
flow vortex scale. (There were too few SuperDARN echoes on the eastern side
of the auroral streamer to identify the eastern flow vortex and its spatial
scale, while the western flow vortex was possibly identified to have a
spatial scale of ∼1 h in MLT (not shown).) However, the auroral diffuse
patch was still poleward of the onset arc at initial brightening, and the
auroral streamer appeared after initial brightening, as shown above. Taking
the equatorward direction of the diffuse patch and the expected flow vortex
scale into account (Fig. 1a), the central part of the flow and the flow
vortices should have been still poleward of the onset arc at initial
brightening as well. Hence it is unlikely that they directly affected initial
brightening. Thus, even if the flow vortex is considered, we can conclude
that the present results are inconsistent with Nishimura et al.'s (2010)
scenario in which the preonset aurora should reach the initial brightening
site before initial brightening. Here we cannot deny the possibility that the
auroral streamer affected enhancement of the wave-like structure and the
subsequent poleward expansion, since the corresponding eastern flow vortex
may contact the site of enhancement of the wave-like structure and poleward
expansion before these auroral developments, but there are insufficient data
to confirm this.
For the 28 February 2008 event shown in Sect. 3.2, the separation of the
arrival point of the auroral streamer and the sites of auroral onset arc
development was ∼0.5 h or less in MLT, probably within the expected
eastern flow vortex scale, although the auroral streamer reached near the
onset arc between the times of enhancement of the wave-like structure and
poleward expansion. (There were too few SuperDARN echoes on either side of
the auroral streamer to identify a flow vortex.) Considering the direction of
the auroral streamer and the expected flow vortex scale, the auroral streamer
was directed equatorward and eastward and was still poleward of the onset arc
at enhancement of the wave-like structure. Since the central part of the flow
and the eastern flow vortex should have been poleward of the onset arc as
well (Fig. 1c), it is unlikely that the flow and the eastern flow vortex
affected the onset arc at this time. The auroral streamer then turned in the
equatorward and westward direction at ∼11:08 UT (∼1 min after
enhancement of the wave-like structure, but ∼2 min before poleward
expansion), so the eastern flow vortex may have contacted the poleward
expansion site just before the auroral streamer arrived at the onset arc
(Fig. 1b). That is, there may be a possibility that the eastern flow vortex
affected poleward expansion. Nevertheless, the appearance timing indicates
that this auroral streamer cannot be related to initial brightening, which is
inconsistent with Nishimura et al.'s (2010) scenario.
Summary and open questions
In the present paper, based on THEMIS GBO ASI data, we revisited three
substorm events, which Nishimura et al. (2010, 2011) identified as being
triggered by the processes associated with preonset auroras. Unlike most
previous studies that determined only one or two timings of auroral onset arc
development, we attempted to determine the three timings of initial
brightening, enhancement of the wave-like structure, and poleward expansion
to make our discussion about the arrival timing of the preonset aurora
relative to the three auroral steps and the role of the preonset aurora in
the auroral steps clearer. Here we adopted a more quantitative method for
timing determination, paying attention to not only substantial
intensification but also initial faint auroras. We also discussed the spatial
relationship between the auroral onset arc, the auroral streamer, and the
associated flow channel and flow vortices.
Variations of the averages of the logarithm of count for areas of
0.05∘ latitudinal width in the region indicated in the top of this
figure, or the (a) western and (b) eastern halves of the
magenta box in the 05:59:15 UT panel of Fig. 8, including the two east–west
arcs obtained from the THEMIS GBO all-sky imager at GILL on 5 March 2008. The
format is the same as Fig. 4.
![]()
The 3 s resolution ion velocity and the 0.25 s resolution magnetic
field in GSM coordinates obtained from the electrostatic analyzer (ESA)
(McFadden et al., 2008) and solid state telescope (SST) (Angelopoulos, 2008)
and from the fluxgate magnetometer (FGM) (Auster et al., 2008), respectively,
onboard THEMIS D (THD) and E (THE) from 05:10 to 05:40 UT on 25 February 2008.
The left-hand, middle, and right-hand vertical lines indicate the times of
initial auroral brightening (IB), enhancement of the wave-like structure
(EW), and poleward expansion (PE), respectively.
![]()
Our more detailed analysis showed that preonset auroral streamers reached the
auroral onset arc but away from the initial brightening site after initial
brightening for two events, while no preonset aurora reaching the initial
brightening site could be identified for the other event. This result
suggests that the processes associated with the auroral streamers are
unlikely to affect at least initial brightening, even if we consider not only
the presence and arrival timing and location of the auroral streamers but
also the scale of the corresponding flow and flow vortices. Although we
examined only the three events, these results possibly suggest that the
preonset aurora scenario is questionable, and at least the processes
associated with the preonset aurora are not necessary conditions for initial
brightening. Murphy et al. (2014b), for example, reached the same conclusion.
Furthermore, Nishimura et al. (2010) showed that not all events were
accompanied by preonset auroras; this result itself implies that conclusion.
Thus careful, detailed reexamination of other Nishimura et al. (2010, 2011) events
is required to prove or disprove the preonset aurora scenario in future
studies.
For testing the preonset aurora scenario further in future studies, we list a series of
open questions that arose from the present and previous studies. We suggest the issues
not only from the viewpoint of existence or nonexistence of the preonset aurora but also
from different viewpoints.
First of all, it is necessary to reexamine the relative timings of
initial auroral brightening and arrival of the preonset aurora or the
corresponding flow and flow vortex at the site of auroral onset arc
development carefully and exactly, paying attention to faint auroras as well.
Furthermore, to clarify the causal relationship, the timings of enhancement
of the wave-like auroral structure and poleward expansion should be
determined as well. As shown above, the auroral streamer arrived at the
auroral onset arc after, not before, initial brightening, i.e., between
initial brightening and enhancement of the wave-like structure for the
present first event and between enhancement of the wave-like structure and
poleward expansion for the second event. Hence it should be reexamined
whether a preonset aurora or the corresponding flow and flow vortex really
arrived at the onset arc before initial brightening for other events of
Nishimura et al. (2010, 2011). It should also be clarified what role the
preonset aurora plays in onset arc development.
We should examine how far from the initial brightening site the path
and final arrival point of each preonset aurora are. In the present study,
the final arrival point was ∼0.3–0.5 h in MLT away from the initial
brightening or poleward expansion site for the first and second events. The
auroral streamers did not move toward the initial brightening site after they
reached the auroral onset arc, although Nishimura et al. (2010) proposed that
the preonset aurora reaches the initial brightening site. Here, as mentioned
in the introduction, we should consider the spatial scales of the central
part of the flow and the flow vortices. There may be a possibility that the
central part of the flow or the flow vortex affects the auroral onset arc if
the auroral streamer is separated by less than ∼1 h in MLT. We should
study, however, whether the effect of the flow vortex can really lead to
initial auroral brightening.
There are a few types of preonset aurora, but is it valid to mix them?
Mende et al. (2011) showed that the preonset aurora events identified by
Nishimura et al. (2010) can be categorized as having a north–south-aligned
equatorward moving streamer, an east–west-aligned equatorward moving arc, or
no equatorward moving aurora at longitudes of initial brightening. Hence it
should be validated whether these types can be regarded as being the same
phenomenon in spite of different appearances.
It should also be proved whether both north–south and east–west auroras
correspond to fast earthward flows in the magnetotail and whether the width,
direction, and position of the preonset auroras are consistent with those of
fast earthward flows in the magnetotail. Regarding the direction of the fast
earthward flow in the near-Earth magnetotail, for the 25 February 2008 event
described in Sect. 3.1, THEMIS D, located in the plasma sheet at (X,Y)∼(-11.1,3.1) RE, observed a fast earthward flow at 05:28:20 UT,
∼4.5 min after initial brightening and ∼30 s before enhancement
of the wave-like auroral structure (Fig. 11). THEMIS E, located in the plasma
sheet at (X,Y)∼(-10.6,4.0) RE or ∼1 RE
earthward and duskward of THEMIS D, also observed a fast earthward flow at
05:28:42 UT, ∼20 s after the THEMIS D observation and just before
enhancement of the wave-like structure. These fast flows had a small duskward
component in the front part. Hence the fast flow propagated from the tailward
region in the earthward and slightly duskward direction. If this observed
magnetotail fast flow is mapped to the ionosphere and unless the magnetic
field line is extremely distorted by a field-aligned current, the auroral
streamer should have moved southwestward. This expected direction, however,
seems to be inconsistent with that of the observed auroral streamer and
diffuse patch, which propagated southeastward and nearly equatorward,
respectively, in the premidnight sector. If the observed auroral streamer and
diffuse patch are mapped to the magnetic equator in the magnetotail, the
magnetotail flow should be directed earthward and dawnward. Hence it is
possibly questionable whether the auroral streamer and diffuse patch really
correspond to the fast earthward flow observed by THEMIS D and E. Xing et
al. (2010) also examined earthward flows in the magnetotail corresponding to
preonset streamers. Considering the dawn–dusk component together, however,
not all magnetotail flows seem to be consistent with the propagation
direction of the streamers (see their Figs. 2 and 9).
In addition, determining the origin of the fast earthward flow in the
magnetotail is a clue. If the auroral streamer appears at the auroral
poleward boundary and between the auroral poleward and equatorward
boundaries, the corresponding magnetotail flow should have originated at the
distant neutral line at X∼-130 RE and at the near-Earth
neutral line at X∼-20 RE, respectively, on average. Some
previous studies attempted to determine the location and timing of magnetic
reconnection from flow or beam observations in the magnetotail (e.g., Elphic
et al., 1995; Kazama and Mukai, 2003; Nagata et al., 2006; Nishimura et al.,
2013b), but it seems challenging to determine them without spatial and
temporal ambiguity.
There is a possibility that some preonset auroras are too weak to be
observed by THEMIS imagers. As shown by Kepko et al. (2009), high-sensitivity
imagers for 630.0 nm emission may be able to detect weak auroral streamers or
patches moving toward the auroral onset arc. Otherwise, it may be possible
that particle precipitation is too weak to cause any aurora. In this case,
ionospheric flow observations are needed to detect streamers.
Nishimura et al.'s (2010, 2011) event list included events without significant
poleward expansion, i.e., pseudobreakups. They did not distinguish between
pseudobreakups and fully fledged substorms in their analysis. If the preonset
aurora scenario mentions not only initial brightening but also later
enhancement of the wave-like structure and poleward expansion, pseudobreakup
events should be separated in discussing the role of the preonset aurora in
auroral onset arc development. Furthermore, according to Frey (2010), some
events of Nishimura et al. (2010) were mistakenly identified as substorm
events and should be categorized as pseudobreakups or substorm
intensifications. Hence it would be necessary to reexamine the category of
each of Nishimura et al.'s (2010, 2011) events.
It should be discussed whether the arrival time of the fast earthward
flow in the near-Earth magnetotail corresponding to the preonset aurora is
consistent with the growth time of an instability leading to initial auroral
brightening. The preonset aurora scenario suggests that the fast earthward
flow evolves an onset instability, such as ballooning instability, leading to
initial auroral brightening (Nishimura et al., 2014). Ballooning mode waves
may be excited a few minutes before initial brightening (as determined with a
spacecraft-borne auroral imager) (Saito et al., 2008). The time difference
between the arrival of the preonset aurora at the onset arc and initial
brightening that Nishimura et al. (2010) showed seems consistent with the
growth time of an instability. As mentioned above, however, the relative
timing of streamer arrival and initial brightening need to be reexamined, so
this test should be done on the basis of the revised timings.
Finally, even if the processes associated with a preonset aurora are not
necessary conditions for initial brightening, it may still be possible that
for some substorms, a preonset aurora and the corresponding fast earthward
flow in the magnetotail make a seed of substorm instabilities leading to
initial auroral brightening, magnetic reconnection or current
disruption/dipolarization, and later auroral development (enhancement of the
wave-like structure and poleward expansion) and magnetotail development, as
mentioned in the discussion of the first and second events of the present
study. Testing this possibility may be important for full understanding of
substorm development.