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ANNALS OF GEOPHYSICS, 60, SUPPLEMENT TO 6, GM672, 2017; doi: 10.4401/ag-7481

A comparative study of sawtooth events and substorm onsets
triggered by interplanetary shocks
Tushar Andriyas1,*

1 DSK Post-Doctoral Fellow, Nehru Science Center, University of Allahabad, India - 211002

Article history
Received July 4, 2017; accepted November 20, 2017.
Subject classification:
Interplanetary Shocks; Geomagnetic Substorms; Sawteeth events; Auroral Oval.

GM672

ABSTRACT
A comparison of solar wind conditions, geomagnetic response, and au-
roral boundary movement, using a similar number of sawtooth events
and shock induced substorm triggers is carried out. 81% of the sawtooth
onsets were triggered at low latitudes compared to 33% for onsets dur-
ing shocks. Results of superposed epoch analysis indicated that the mean
interplanetary magnetic field (IMF) Bz remained strongly southward
during sawtooth events while it was southward during the loading pe-
riod and turned northward 20 minutes prior to the onset, during the shock
triggered events. During both the event types, the mean energy available
in the solar wind was found to be above the substorm threshold level. Rel-
atively high levels of solar wind density indicated to the magnetosphere
being compressed to a larger degree during substorms initiated by shocks.
The high latitude indices were elevated during sawtooth events with 2-
3 hr fluctuations was observed in the AL and PCN indices but AU in-
dex after the onset remained at similar levels. SYM-H remained below
storm time values during shock related onsets but was much stronger
and remained above the storm threshold during sawtooth events, that
they occurred embedded in a geomagnetic storm. Comparatively high-
er mean values of ASYM-H indicated to a much stronger asymmetric
ring current during sawtooth events. Mean boundary locations during
sawtooth events were located a few degrees equatorward of those dur-
ing the shock related onsets. Oval in the dawn, dusk, and midnight sec-
tors was much thicker during sawtooth events with clear widening around
the onset during both event groups but the thickness in noon sector was
similar and remained steady during the epoch window during both the
event groups. Elevated driving during sawtooth events was also indicative
in a greater stretching and relaxation of the magnetic field lines at geosyn-
chronous orbit in the midnight sector.

1. Introduction
A substorm is a short lived disturbance with a life

time of 2-4 hours [Tanskanen et al. 2002] in the mag-
netosphere-ionosphere coupled system that can de-
posit large quantity of energy into the auroral
ionosphere. Substorms occur at time scales that are

much faster when compared to a geomagnetic storm.
Three phases have been ascertained as occurring dur-
ing a substorm [McPherron et al. 1986]. The growth
phase is generally initiated after the southward turn-
ing of solar wind interplanetary magnetic field (IMF)
Bz. The expansion phase occurs when instabilities in-
herent to the magnetotail cause the diversion of the
cross-tail current through the polar regions. This is
recorded as a fluctuations in the local magnetic field
that can reach levels above 1000 nT. The magneto-
sphere then returns to a steady state during the recov-
ery phase. As such, substorms play an important role
in energy circulation through the magnetosphere
through dayside reconnection, energy storage in mag-
netotail, and release of this energy to relax the field
lines to a more dipolar shape.

Interplanetary shocks (IPS) are plasma and mag-
netic field structures that travel through the solar wind
and couple efficiently with planetary magnetospheres.
Increase in the dynamic pressure associated with a
shock structure leads to the compression of the mag-
netosphere indicated by large positive increases in the
north-south (H) component of the geomagnetic field.
If a geomagnetic storm occurs, such increases are
termed as sudden impulses (SI) or storm sudden com-
mencement (SSC) [Curto et al. 2007, Yao et al. 2009].
Sources of such structures lie in either coronal mass
ejections (CMEs) or corotating interaction regions
(CIRs) with the most of the shocks measured at Earth’s
orbit occurring due to CMEs. CIRs are flanked by for-
ward and reverse shocks which only develop sufficiently
after the Earth’s orbit (1 AU) [Gosling and Pizzo 1999].
Shocks induce complex processes in the Earth’s mag-
netosphere and as such are important in understanding
the solar wind-magnetospheric coupling. Long periods
of southward Bz that precede an IPS can lead to sub-

storm triggering and affect the amount of precipitated
power [Liou et al. 1998].

IPS have been shown to trigger substorms and en-
hance nightside precipitation with powers reaching
substorm levels [Meurant et al. 2005]. Zhou & Tsuru-
tani [2001] found that 43\% of magnetic bays induced
by shocks were associated with substorms. Using a
global magnetohydrodynamic simulation, Oliveira and
Raeder [2014] studied the effects of various types of IPS
on the nightside magnetosphere. They observed that
the frontal perpendicular shock led to a symmetric
compression of the magnetotail to trigger a substorm
and a large geomagnetic activity. Pressure pulses that
accompany an IPS leads to compression of the magne-
tosphere on the dayside and can trigger substorms
within minutes of arrival [Brittnacher et al. 2000]. Dur-
ing a period of southward IMF, an increase in the solar
wind dynamic pressure can lead to the triggering of
substorm expansion phase onset [Kokubun et al. 1977].

Sawteeth events have been classified as a separate
class of activations that can occur during periods of
moderately strong and steady solar wind [Pulkkinen et
al. 2007]. Large quasiperiodic oscillations in the ener-
getic particle fluxes, with a period of about 2-4 h [Hen-
derson et al. 2006a], particularly in proton channels, at
geosynchronous orbits were identified by Belian et al.
[1995]. Rapid increase and slow decrease of the fluxes
are a signature of such events. They generally occur
during periods of increased solar wind driving with a
continuous southward Bz. The saw blade like waveform
is measured in both the energetic proton and electron
fluxes. At geosynchronous orbit, the oscillation is ob-
served as periodic stretching and relaxation of magnetic
field lines [M. G. Henderson et al. 2006b]. This recon-
figuration from a highly stretched to relaxed state has
also been observed over a wide range of local time sec-
tors [Pulkkinen et al. 2006]. These events also accom-
pany a strongly enhanced partial ring current as
measured by the ASY-H index.

Pulkkinen et al. [2007] statistically compared the
solar wind conditions and geomagnetic activity during
sawtooth and other substorm like electrojet storm time
activations. They found that sawtooth events were
characterized by a 2-3 hour periodicity, strong stretch-
ing of the dusk sector field, and a strongly asymmetric
ring current. The level of driving seen during sawtooth
events was also found to be similar to that during other
types of storm-time activations, the only difference
being a weaker auroral (AL) response during sawtooth
events. Partamies et al. (2009) statistically compared
sawtooth events with periods of steady magnetospheric
convections and isolated substorms. They reported that

even though sawtooth events were strongly driven with
a strong geomagnetic response, the coupling efficiency
was the lowest. Solar wind speed also played a part in
the type of geomagnetic activity with sawtooth events
were triggered during times of elevated geomagnetic
activity.

Given that the passage of IPS through the magne-
tosphere can lead to large increases in geomagnetic ac-
tivity and that sawtooth events are found to occur
during elevated solar wind driving, the similarities and
differences between the two event types are investi-
gated. In this paper, shock related substorm onsets and
sawtooth events are compared with regards to the solar
wind driving and the geomagnetic response. Super-
posed epoch analysis is used to quantify the similarities
and differences between the two event groups using
multiple in situ and ground based data. The datasets
used in the analysis are discussed in section 2. The re-
sults report on the superposed epoch analysis of the
solar wind parameters in section 3a, the ensuing geo-
magnetic activity at polar and equatorial latitudes in
section 3b, and the auroral boundary movements with
the associated magnetic field conditions at geosyn-
chronous orbit in section 3c. The results of the analy-
sis are discussed further with previous literature in
section 4 and the conclusions from the study are pre-
sented in section 5.

2 Data and Procedure
The solar wind and the subsequent geomagnetic

response data at 1 minute (min) cadence, was obtained
from http://cdaweb.gsfc.nasa.gov/ [King and Papitashvili
2005], during the years 2000 to 2002. In this study, IMF
Bx, By, and Bz in nT (in Geocentric Solar Magneto-
spheric or GSM coordinates), solar wind flow speed (Vx
in km/s), and plasma ion density, Np in particles cm

-3

were used. Derived parameters such as the flow pres-
sure, P (nPa), the IMF magnitude, Btot=√Bx

2+By
2+Bz

(nT), solar wind electric field (Ey in mV/m), and the ep-
silon parameter (in TW), Θ=tan-1(By/Bz) being the
solar wind clock angle, were calculated from the pri-
mary parameters. Magnetospheric response was mea-
sured through AL, AU, AE (all in nT), PCN (mV/m),
SYM, and ASYM indices (nT). Substorm onset timings
and locations were derived from the Frey et al. [2004]
database.

Sawteeth events were searched for in a list com-
piled by R. L. McPherron, who identified the events vi-
sually from the particle injections as measured by the
Los Alamos Synchronous Orbit Particle Analyzer
(SOPA) onboard the geostationary 1990-095,1991-080,
1994-084, LANL-97A, LANL-01A, and LANL-02A satel-

ANDRIYAS

SAWTOOTH AND SHOCK RELATED SUBSTORM ONSET COMPARISON

lites at multiple magnetic local time sectors [Pulkkinen
et al. 2007]. The shock parameters were calculated
based on high resolution solar wind parameters (as ob-
tained from Solar Wind Electron Proton Alpha Moni-
tor (SWEPAM) instrument [McComas et al. 1998] and
the IMF from the MAGnetic field experiment (MAG)
[Smith et al. 1998] instrument, on board the Advanced
Composition Explorer (ACE) satellite). The time of the
shock interface as measured by the ACE satellite was
checked with the Harvard-Smithsonian shock database
located at https://www.cfa.harvard.edu/shocks/.

Rankine-Hugonoit equations of conservation
were used to determine the speed of the shock along
the normal direction [Oliveira 2017]. This unit normal
vector was calculated based on the Abraham-Shrauner
mixed-mode analysis technique [Abraham-Shrauner
1972] as given by the following equation:

(1)

with subscripts 1 and 2 indicating upstream and down-
stream values and B

→
and V

→
representing the solar wind

magnetic field and velocity, respectively. Shock speed,
Vs, was calculated using the expression [Tsurutani and
Lin 1985]:

(2)

N being the plasma density. After calculating the angle
θ between the upstream magnetic field and the shock
normal, the other parameters such as magnetosonic
speed and Mach number can be calculated as

(3)

(4)

where VA and CS represent the upstream Alfvenic and
sound speeds, respectively.

Poleward and equatorward boundary data used to
study the oval movement during the onsets was ob-
tained from the British Antarctic Survey (BAS). These
boundaries were obtained from the images obtained
Imager for Magnetopause-to-Aurora Global Explo-
ration (IMAGE) satellite as described in Longden et al.
[2010]. Single and double Gaussians were used to fit to
the average intensity profiles in each of the 24 magnetic
local time (MLT) bins. Non-linear least square fitting
yielded parameters that were then used to estimate the
equatorward and poleward luminosity boundaries in
each MLT sector.

The auroral oval boundary locations were further
averaged into MLT bins based on the noon, dusk, mid-
night, dawn sectors. In this study, the noon sector was
chosen to be between 1000-1500 MLTs, dusk between
1600-2100 MLTs, midnight sector between 2000-0300
MLTs [Sandholt et al. 2014], and dawn sector between
0400-0900 MLTs, respectively. Additionally, magnetic
field at geosynchronous orbit as measured by GOES 8
and 10 (active during the years 2000-2002) magne-
tometers (Hp component in the local spacecraft frame),
was used to evaluate the amount of magnetotail
stretching and relaxation around the substorm onsets
[Singer et al. 1996]. Data from the two satellites were
used when they were within the MLTs around the mid-
night sector.

3. Results
The time at which a shock interface was measured

was compared against the substorm onset from the
database provided by Frey et al. [2004] that occurred
in a window of 120 mins after the shock. During the
years 2000-2002, 58 unique shocks were observed prior
to a substorm onset that could have triggered a sub-
storm. All the shocks measured were found to be fast
forward shocks. During the same years, 63 unique saw-
tooth events had onsets that matched with the Frey et
al. [2004] onset list. The shocks were either of the fast
forward or fast reverse type, with the majority (55) of
the onsets triggered by fast forward type shocks.
Within the shocks, the mean magnetic compression
was around 2 while the plasma was compressed by a
factor of around 2.3. A mean shock angle (of the mag-
netic field w.r.t. the shock normal) of 61o indicated
that, on average, the shocks were oblique.

The onset location (between 50-90o) in the alti-
tude-adjusted corrected geomagnetic coordinates or
AAGCM [K. B. Baker and Wing 1989], for both the
event classes were plotted. In Figure 1, the magnetic
latitude and MLT of the onsets associated with shocks
and sawtooth events are plotted in blue and red, re-
spectively. As seen in the figure, the onset locations
have a preference for premidnight sector. It can be
noted that majority (81%) of the sawtooth events
occur at low latitudes (below 63o) while only 33% of
the shock initiated onsets are triggered at such low lat-
itudes. This should indicate a high solar wind driving
associated with sawtooth events as measured by the
electrojet indices [Grocott et al. 2009]. A superposed
epoch analysis was conducted during a time window
of 720 mins around the onset with the reported su-
perposed epoch curves being the mean of the individ-
ual events, respectively.

n̂ =
(
→
B2 − B1) × ((V2 − V1) × B2)
(B2 − B1) × ((V2 − V1) × B2)

→ → → →→ → → → →

VS =
N2V2 − N1V1
N2 − N1

⎛

⎝
⎜

⎞

⎠
⎟⋅n̂

→ →

Vms =
(VA

2 + CS
2) ± (VA

2 + CS
2)2 − 4CS

2VA
2 cos2

Mms =
Vs
Vms

3.1 Mean solar wind conditions
Superposed epoch analysis is used in this section

to study the similarities and differences between the
two event types (blue curves for shocks and red for
sawtooth events) with regards to the solar wind driving
and the geomagnetic response. The mean IMF curves
resulting from the superposed epoch analysis are
shown in Figure 2. The values are plotted over an
epoch window of 720 mins around the substorm onset
(vertical black dashed line) as obtained from the Frey et
al. [2004] list. The mean IMF Bx, By, Bz, and Btot are

plotted in blue for shocks and in red line for sawtooth
events. The mean Bx and By do not exhibit any signifi-
cant changes during the epoch window with a prefer-
ence for a zero average value for most of the epoch
window, during shock related onsets. However, the
mean Bx and By during sawtooth events exhibit a
Parker like sector structure with the IMF pointing
away from the Sun [Partamies et al. 2009].

Mean Bz for the shocks turns southward from a
zero average value to around -6 nT around the sub-
storm onset and turns northward around the onset,
which may be indicative of magnetotail loading/un-
loading [D. N. Baker et al. 1996]. The total IMF
strength becomes stronger around 100 mins and
strongest 20 mins prior to the onset related to shocks.
During sawtooth events, the mean Bz remains negative
throughout the epoch window (~ -4 nT) and starts to
decrease to more negative values 120 mins prior to the
onset. It can be seen that there is a reduction in the
mean Bz values to around -7 nT, 20 mins prior to the
substorm onset and a return to less negative values
post onset. However, during the sawtooth events, the
total field remains much stronger (double) compared
to that for the shock related onsets outside a window
±100 mins around the onset.

The mean solar wind plasma parameters are plot-
ted in Figure 3. The shocks on an average compressed
the solar wind by a factor of two, with the density in-
creasing from around 6 particles cm-3 to 13 particles
cm-3 around the onset and remained around 10 parti-
cles cm-3 through the epoch window after the onset.

ANDRIYAS

0012

Figure 1. Substorm onset locations associated with the 58 shock
and 63 sawtooth events indicated by blue and red filled circles.
Midnight (00 hrs), dawn (06 hrs), day (12 hrs), and dusk (18 hrs)
MLTs are indicated along with the magnetic latitude.

−2
0
2

Epoch [mins]

B
x

Shock
Sawteeth

−2
0
2
4

Epoch [mins]

B
y

−8
−6
−4
−2

0
2

Epoch [mins]

B
z

−300 −200 −100 0 100 200 300
0

Epoch [mins]

B
to

Figure 2. Mean IMF components with Bx, By, Bz, and the magnitude of the IMF, Btot plotted in blue and red lines for the shocks and saw-
tooth events, respectively, over an epoch window of 720 minutes around the substorm onset (zero epoch). The vertical and horizontal
dashed black lines indicate the zero mean and zero epoch, respectively.

SAWTOOTH AND SHOCK RELATED SUBSTORM ONSET COMPARISON

The mean dynamic pressure, being a function of both
the solar wind velocity and density, also increased from
around 2 nPa to 7 nPa around the onset and remained
steady at around 6 nPa post onset. Elevated mean tem-
perature prior to the substorm onset as a result of
shock compression can be seen with average tempera-
ture increasing from around 1×105 K to around 2.5×105

K around the onset. During the sawtooth events, mean
Np remained steady at around 6-8 particles cm

-3 with a
mean dynamic pressure around 3 nPa and a mean tem-
perature of around 1×105 K. For the shock related on-
sets, the mean solar wind velocity, Vx, increased across
the shock interface before the substorm onset. The

mean velocity became faster around the substorm
onset from around -410 km/s prior to and -520 km/s
after the substorm onset. During the sawtooth events,
it increased marginally from -460 km/s to around -480
km/s after the onset.

During both the event groups, the epsilon param-
eter lies below 300 GW, with the shock related onsets
experiencing a large increase in the mean epsilon pa-
rameter, 90 mins prior to the substorm onset (upper
panel in Figure 4). As indicated in Figure 2, mean IMF
Bz turns southward 90 mins prior to the onset during
shock related onsets. As a result, this increased Bz con-
tributes to an increased value of the mean epsilon. Dur-

Epoch [mins]

N
p
[

c
m

−
3
]

2
4
6
8

Epoch [mins]

P
[

n
P

a
]

1
2
3

x 10
5

Epoch [mins]

T
[

K
]

−300 −200 −100 0 100 200 300

−500
−450
−400

Epoch [mins]

V
x
[

k
m

/s
]

Figure 3. Mean solar wind plasma parameters for the shock (blue) and sawtooth (red) events. Mean density (particles cm-3), dynamic
pressure (nPa), temperature (K), and solar wind velocity (Vx km/s) are plotted from top to bottom.

0.1

0.2

Epoch [mins]

ε
p

a
ra

m
e
te

r
[T

W
]

−300 −200 −100 0 100 200 300
−2

Epoch [mins]

E
y
[

m
V

/m
]

Figure 4. Mean epsilon parameter (TW) and solar wind electric field (mV/m) in the dawn-dusk direction for the 58 shock (blue) and 63
sawtooth (red) events.

ing the sawtooth events, a steady southward IMF Bz im-
plies that the mean epsilon parameter remains elevated
throughout the epoch window. The mean epsilon pa-
rameter increases to values around 200 GW close to the
substorm onset for the shock related events while it re-
mains between 100-200 GW before increasing to val-
ues close to 300 GW around the onset for the sawtooth
events. These values are above the substorm loading
threshold of 100 GW [Akasofu 1981]. However, in both
the cases the storm threshold of 1 TW is not exceeded.

For the shock related onsets, an increase in both
the mean solar wind velocity and more negative Bz
translated to an increase in the solar wind electric field
Ey = Vx Bz), plotted in blue in Figure 4. The electric field

recovered to preonset values around 80 mins after the
onset as seen in the figure. Compared to shocks related
onsets, the mean electric field during sawtooth events
remained elevated throughout the epoch window at
values between 1-2 mV/m with a slight increase close
to the onset owing to an increase in the solar wind
speed and Bz becoming more negative.

3.2 Magnetospheric response
Mean geomagnetic activity at high latitudes as

measured by the AE, AL, AU, and PCN indices, are
plotted in Figure 5. Sawteeth events produce a
stronger response in the indices the mean AE, AL, AU,
and PCN reaching peak values close to 800 nT, -600

ANDRIYAS

200
400
600
800

Epoch [mins]

A
E

[
n

T
]

−600

−400

−200

Epoch [mins]

A
L

[
n

T
]

100

200

300

Epoch [mins]

A
U

[
n

T
]

−300 −200 −100 0 100 200 300
1
2
3
4

Epoch [mins] P
C

N
[

m
V

/m
]

−300 −200 −100 0 100 200 300
−100

−50

100

Epoch [mins]

M
e
a
n

[
n

T
]

Figure 6. Mean SYM-H (continuous lines) and ASYM-H (dotted lines) indices during the shock related onsets (indicated in blue) and saw-
tooth events (indicated in red).

Figure 5. Mean high latitude geomagnetic response as measured by the AE, AL, AU, and PCN indices (top to bottom) for the shock re-
lated (blue) and sawtooth (red) events.

nT, 250 nT, and 4 mV/m, when compared to those for
the onsets driven by shocks (700 nT, -450 nT, 250 nT,
and 3.5 mV/m). Expansion phase can be seen to occur
for almost the same duration for both the event types
(100 mins). A recovery to pre onset values can be seen
120 mins after the onset for both the classes. The
mean values of AL and as a result AE indices remain el-
evated during the entire epoch window during the
sawtooth events. AU index, on the other hand, experi-
ences an almost equal increase after the onset during
the two event groups. PCN increases by 1.5 mV/m for
the sawtooth events compared to 1 mV/m for the
shock related onsets.

Equatorial activity indices follow the same trend
as for the high latitude indices, with much stronger re-
sponse to the sawtooth related solar wind driving as
seen in Figure 6. Shock related onsets lead to a de-
crease in the mean SYM-H of around -40 nT where
during the sawtooth events, the index dips to values
close to -70 nT and remains elevated throughout the
epoch window. Variations in the mean ASYM-H index
are seen to be more quasi-periodic and fairly steady
during the sawtooth events compared to the shock re-
lated onsets, during which the ASYM-H experiences a
sudden increase from values around 25 nT to 55 nT
around the onset as seen in Figure 6. Higher mean val-
ues of ASYM-H during sawtooth events indicate to a
much stronger asymmetric ring current compared to
that during shock related onsets.

3.3 Auroral oval and field line stretching
As indicated in section 2, the auroral oval bound-

aries were averaged into MLT bins based on the day-
side, dusk, midnight, dawn sectors to investigate the
differences in auroral movement between the two
event groups. Figure 7a are plots of the mean equa-
torward and poleward boundaries in the four sectors,
while the plots in Figure 7b represent the respective
mean oval thickness in the sectors. Both the mean
boundaries during the sawtooth events are located
equatorward of the oval boundaries during shock ini-
tiated onsets indicating a stronger stretching of the
field lines during sawtooth events. Before the sub-
storm onset, an equatorward descent in both the
boundaries can be seen in the sectors, with the pole-
ward boundary moving faster than the equatorward
boundary in any given sector. A sudden equatorward
descent is observed in the mean equatorward bound-
aries during the shock related onsets, especially in the
dawn, dusk, and midnight sectors. Due to stronger
solar wind forcing during sawtooth events, this equa-
torward advancement in the mean equatorward

boundaries is barely noticeable in the already strongly
stretched field lines.

The top left panel in Figure 7a indicates that in the
dusk sector (1600-2100 MLT), the mean equatorward
boundary starts to retreat equatorward around 40
mins prior to the onset, during the shock induced on-
sets (blue continuous line) compared to the sawtooth
events, during which the boundary gradually moved
equatorwards. The mean boundary recovers to preon-
set latitudes during sawtooth events but remains at
lower latitudes for the shock related onsets. Clear pre-
onset equatorward departure and poleward descent
after the onset can be seen in the mean poleward
boundary for the sawtooth events and is observed to
be much stronger compared to shock related onsets.
In the noon sector (1000-1500 MLT), both the mean
equatorward and poleward boundaries tend to move
equatorward at the same rate after the onset, as seen in
the top right panel of Figure 7a. This trend is similar
for both sawtooth events and shock related onsets with
the only difference being that the boundaries during
the sawtooth events are located equatorward of the
mean oval boundaries during the shock related onsets.

A similar trend is seen in the mean oval bound-
aries around the dawn sector (0400-0900 MLT), with
equatorward advancement prior to the onset and
poleward retreat after the substorm onset. Equator-
ward movement is seen to be much weaker in the
equatorward boundary but strong relaxation can be
seen in the poleward retreat of the poleward bound-
ary, during the sawtooth events. Conversely, the mean
equatorward boundary experiences a much stronger
equatorward descent in the equatorward boundary
and slight variations in the poleward boundary loca-
tion during the shock related onsets. Mean boundaries
in the midnight sector (2000-0300 MLT) for the two
event groups are plotted in the bottom right panel of
Figure 7a, which can be seen to have the most dy-
namics. Equatorward movement of the boundaries
can be seen prior to the onset caused by the stretching
of field lines in the magnetotail. After the onset, the
equatorward boundaries gradually move poleward for
both the event groups while the poleward departure
of the poleward boundaries occurs at a comparatively
faster rate. After the onset, the poleward boundary
also experienced a large poleward retreat from around
68o to 73o, 30 mins after the onset during sawtooth
events while a change from 70oto 73o was observed
in 15 mins during shock related onsets. However, dur-
ing the shock related onsets, the poleward boundary
could not recover to its preonset location for the re-
maining epoch.

SAWTOOTH AND SHOCK RELATED SUBSTORM ONSET COMPARISON

The mean oval thickness (Figure 7b) during the
sawtooth events is observed to be greater than during
the shock related onsets in the dawn, dusk, and mid-
night sectors. The mean oval around the dusk sector
can be seen to be much thicker for the sawtooth events,
but the oval around the onset time is more dynamic
during the shock related onsets. Given that both the re-
spective mean boundaries in both the event groups
moved at the same rate in the noon sector, the mean
oval thickness was around the same for both the event
types with the mean width fluctuating between 6-8o
latitude. Clear thinning prior to the onset can be seen
during the sawtooth events which is absent during the
shock related onsets in the dawn sector. Rapid widen-
ing of the oval can be seen in the dawn sector after the
onset compared to the dusk sector, in both the event

groups. This thinning prior to the onset and widening
post onset for the event groups is attributed due to fast
poleward retreat of the mean poleward boundary com-
bined with a relatively gradual poleward retreat of the
equatorward boundary. Rapid thinning before and
widening after the onset can be seen in the oval thick-
ness in the midnight sector during both the event groups.
This process is more pronounced for the sawtooth events
due to the poleward boundary changing rapidly around
the onset (Figure 7a) in the midnight sector.

In order to study the magnetic field in the mid-
night sector at geosynchronous orbit during substorm
onsets [Singer et al. 1996], superposed means for the
magnetic field, Hp (in the local spacecraft coordinates,
measured in nT) were generated for GOES 8 (blue and
red continuous lines for shocks and sawtooth events,

ANDRIYAS

−200 0 200

Epoch [mins]

D
u

s
k

−200 0 200
65

Epoch [mins]

N
o

o
n

−200 0 200

Epoch [mins]

D
a
w

−200 0 200
55

Epoch [mins]

M
id

n
ig

h
t

Epoch [mins]

D
u

s
k

6
7
8
9

Epoch [mins]

N
o

o
n

Epoch [mins]

D
a
w

−300 −200 −100 0 100 200 300
8

10
12
14
16
18

Epoch [mins]
M

id
n

ig
h

t
Figure 7. Mean oval boundaries (continuous lines for equatorward locations and dotted lines for poleward locations) during sawtooth
events (red) and shock triggered (blue) onsets in Figure 7a in the four oval sectors as discussed in the text. Figure 7b is the mean oval thick-
ness during the two event groups.

(a)

(b)

respectively) and 10 (blue and red dashed lines for
shocks and sawtooth events, respectively). Field
strength weakening prior to the onset led to the equa-
torward descent of both the mean boundaries, as noted
in Figure 7a. Mean field values increased after the onset,
indicative of a poleward retreat of the boundaries. The
mean stretching and relaxation signature is weaker for
the onsets triggered by shocks compared to those dur-
ing sawtooth events. The mean poleward and equator-
ward boundaries during sawtooth events were found to
be equatorward of those found during shock triggered
substorms. As seen in Figure 8, the mean Hp at GOES
8 and 10 location was found to be much stronger (dipo-
lar) during shock related onsets as indicated by values
between 85 and 95 nT. Weak stretching prior to the
onset (to values around 80 nT) and relaxation post
onset (to values close to 90 nT) was observed in the
mean Hp during shock related onsets. Comparatively,
during sawtooth events, the magnetic field was much
weaker (or stretched more) prior to the onset with a
much stronger relaxation at both GOES 8 and 10 loca-
tions. This is consistent with the movement of mean
poleward boundary during the sawtooth events in the
midnight sector (bottom right panel of Figure 7a),
which retreated poleward to a much greater degree
when compared to the onsets triggered by shocks.

4. Discussion
The response of Earth’s magnetosphere to the

solar wind driving during substorm onsets related to
shocks and sawtooth oscillations is investigated. With
almost equal number of events (58 shock related trig-
gers and 63 sawtooth events) in the two groups, a su-

perposed epoch analysis was performed based on
merged OMNI data and GOES observations, during
2000-2002. It was found that around 81% of the saw-
tooth events and 33% of shock driven triggers were at
latitudes below 63o latitude. Previously, Grocott et al.
[2009] had reported that onsets at low latitudes lead to
intense auroral activity, and a similar observation was
made in the case of sawtooth events. Superposed epoch
analysis was performed on these event groups with
onset timing derived from Frey et al. [2004], over an
epoch window of 720 mins.

Mean negative values of IMF Bx and positive val-
ues of IMF By during sawtooth events indicated to a
sector structure with IMF pointing away from the Sun
in the equatorial plane. Comparatively, during shock re-
lated onsets, the mean IMF Bx and By did not have any
preference for either positive or negative values and av-
eraged to zero over the epoch window. Mean IMF Bz
remained strongly negative during sawtooth events
(stronger than that during the shock related onsets)
throughout the epoch window while it turned south-
ward around a 100 mins before the onset and turned
northward 20 mins before the onset. The total field
strength was also found to be much stronger for saw-
tooth events compared to the shock related triggers.

For shocks that occurred in a window of 120 mins
prior to the onset, the mean velocity jumped from
about 400 to 600 km/s. This was accompanied by
jumps in particle density (from around 6 to 13 particles
cm-3) and a jump in the mean dynamic pressure from 2
to around 7 nPa around the onset. The mean solar
wind values during sawtooth events were stable around
460 km/s, 7 particles cm-3, and 3 nPa during the whole

SAWTOOTH AND SHOCK RELATED SUBSTORM ONSET COMPARISON

−300 −200 −100 0 100 200 300

100

S
u

p
e
rp

o
s
e
d

M
e
a
n

[
n

T
]

Figure 8. Mean GOES 8 (continuous lines) and 10 (dashed lines) Hp magnetic field during the two event groups. Red color indicates the
mean values during sawtooth events while plots in blue indicate the mean Hp values during shock related onsets.

epoch window. The dayside compression was there-
fore, much stronger during shock triggered onsets.

The mean epsilon parameter increased to values
around 200 GW around the substorm onset for the
shock related events while it remained between 100-
200 GW before increasing to values close to 300 GW
around the onset for the sawtooth events. These val-
ues are above the substorm loading threshold of 100
GW [Akasofu 1981]. However, in both the cases the
storm threshold of 1 TW was not exceeded. Mean Ey
close to 3.5 mV/m around the onset and elevated val-
ues during the whole epoch indicated to the sawtooth
events being driven strongly. Relatively, during the
shock related triggers, the mean Ey reached peak values
close to 2.5 mV/m and did not remain at these values
given the nature of mean IMF Bz which turned north-
ward 20 mins prior to the onset.

High latitude indices were also found to be ele-
vated during sawtooth events compared to shock re-
lated triggers. Given that majority of the sawtooth
onsets were triggered at latitudes below 63o, they were
associated with a strong electrojet response as is the
case with low latitude substorm onsets. Mean AL index
reduced by almost 100 nT more during sawtooth
events compared to shock related triggers. Although
the AU index was lower prior to the onset, during
shock related triggers, the mean values reached the
same levels as found during sawtooth events. This dif-
ference in behavior of the AL and AU indices could be
attributed to how AU and AL indices describe the east-
ward and westward electrojets [Kamide and Kokubun
1996, Kamide and Rostoker 2004]. This was also the
case with the PCN index with values during both event
groups reaching peak values around 3.5-3.8 mV/m.
However, these peak values were reached 20 and 50
mins after the onset during shock related and sawtooth
events, respectively. Both AL and PCN indices indicated
to a 2-3 hour (hr) periodicity as also reported in Pulkki-
nen et al. [2007].

Sawteeth events occurring usually during geo-
magnetic storms (Dst ≤ -50 nT) [Kallio et al. 2000, Par-
tamies et al. 2009], can be seen in comparatively large
values of the SYM-H index. The quasi-periodic fluctu-
ations during sawtooth events in the high latitude in-
dices was also seen as the partial recovery of the
SYM-H index corresponding to the individual teeth
[M. G. Henderson et al. 2006]. Large quasi-periodic
variations, specially prior to the onset were observed in
the ASYM-H index during sawtooth events. The initial
decrease and increase in ASYM-H response to quasi-
periodic flux injections at geosynchronous orbit was
delayed by around 40 mins compared to that in the

SYM-H index [Pulkkinen et al. 2006].
Stronger mean solar wind driving within saw-

tooth events was also seen in the oval boundary loca-
tions and dynamics. The mean auroral boundaries
during sawtooth events were found to be located equa-
torward in all the sectors when compared to the
boundary locations during shock triggered substorms.
This indicated that the magnetosphere was stretched
to a greater degree during sawtooth events (as also
seen in the strength of geosynchronous magnetic field
measured by the GOES satellites) and that the oval was
thicker by at least 2-4o in the dawn, dusk, and midnight
sectors. The poleward retreat of the mean poleward
boundary was much stronger during the sawtooth
events in the dawn and midnight sectors. It was ob-
served that the both the mean equatorward and pole-
ward boundaries in the day sector proceeded
equatorward at the same rate around the onset and
this meant that the the oval thickness during the two
event groups was almost the same.

Equatorward movement of the boundaries seen
prior to the onset caused by the stretching of field lines
in the magnetotail during the growth phase of a sub-
storm [Kozelova and Kozelov 2013]. As seen in Figure
8, the mean field is much weaker (stretching) during
sawtooth events compared to shock related onsets as
also indicated in Figure 7a, where the oval boundaries,
during sawtooth events, were located equatorward of
the boundaries during shock related onsets. This is a
result of much stronger and persistent southward IMF
Bz during sawtooth events. During sawtooth events,
the magnetic field was much weaker (or stretched
more) prior to the onset with a much stronger relax-
ation at both GOES 8 and 10 locations. This is consis-
tent with the movement of mean poleward boundary
during the sawtooth events in the midnight sector (bot-
tom right panel of Figure 7a), which retreated pole-
ward to a much greater degree when compared to the
onsets triggered by shocks.

5. Conclusions
As shocks lead to elevated geomagnetic activity

and sawtooth events are found to occur during ele-
vated solar wind driving, a comparative study between
sawtooth events and substorm onsets triggered by
shocks was carried out. Superposed epoch analysis was
used to examine the similarities and differences be-
tween 58 shock related onsets and 63 sawtooth events
during the years 2000-2002.

• During both the event types, the mean energy
available in the solar wind (as determined by the
epsilon parameter) was found to be above the

ANDRIYAS

substorm threshold level of 100 GW [Akasofu
1981]. During sawtooth events, the mean epsilon
parameter was around 300 GW around the onset
compared to 200 GW during shock related on-
sets. The sudden increase in mean epsilon pa-
rameter around the onset during shock related
events indicates to a result previously found by
Akasofu and Chao [1980], who indicated that a
substorm could not only be triggered by a storm
sudden commencement but was a direct conse-
quence of increased power of the solar wind-
magnetosphere dynamo.

• Both the high latitude indices were elevated dur-
ing sawtooth events with 2-3 hr fluctuations ob-
served in AL and PCN indices. Post onset, the AU
index reached similar levels during sawtooth
events and shock initiated onsets.

• Ring current intensity as measured by the SYM-
H was found to be around the storm levels dur-
ing sawtooth events throughout the epoch
window with similar quasi-periodic fluctuations
observed in the partial recovery of the index. On
average, the ASYM-H response to quasi-periodic
flux injections at geosynchronous orbit was de-
layed by around 40 mins compared to fluctua-
tions in the SYM-H index.

• In both the event groups, the latitudinal thickness
of the oval in the noon sector did not show any
significant variation during the epoch window
due to both the boundaries moving equatorward
before the onset and poleward after the onset. It
was also seen that the oval thickness was around
the same during both the event groups. The
dawn sector experienced a clear thinning and
widening of the oval around the onset during
both sawtooth events and shock related onsets.
This sequence was however, not observed in the
dusk sector and could point to a stronger sub-
storm activity in the dawn MLTs compared to
dusk MLTs [Mende et al. 2003].

• Sawteeth events were characterized by large vari-
ations of the magnetic field at the geosyn-
chronous orbit. The tail field strength became
weak compared to shock related onsets due to
magnetotail stretching and rapidly relaxed to
much stronger values after the onset.

Acknowledgements. The author wishes to acknowledge
CDAWeb for the solar wind and geomagnetic activity data. This re-
search was supported by the Dr. D. S. Kothari post doctoral fellow-
ship (EN/14-15/0025).

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*Corresponding author: Tushar Andriyas,
Nehru Science Center, University of Allahabad, India;
email: tushar.andriyas@aggiemail.usu.edu

SAWTOOTH AND SHOCK RELATED SUBSTORM ONSET COMPARISON