Ann. Geophys., 34, 943–959, 2016
www.ann-geophys.net/34/943/2016/
doi:10.5194/angeo-34-943-2016
© Author(s) 2016. CC Attribution 3.0 License.
Evidence for transient, local ion foreshocks caused by dayside
magnetopause reconnection
Yann Pfau-Kempf1,2, Heli Hietala3, Steve E. Milan4, Liisa Juusola1, Sanni Hoilijoki1,2, Urs Ganse5,1, Sebastian von
Alfthan6, and Minna Palmroth1
1Earth Observation, Finnish Meteorological Institute, Helsinki, Finland
2Department of Physics, University of Helsinki, Helsinki, Finland
3Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, USA
4Department of Physics and Astronomy, University of Leicester, Leicester, UK
5Department of Physics and Astronomy, University of Turku, Turku, Finland
6CSC, IT Center for Science, Espoo, Finland
Correspondence to: Yann Pfau-Kempf (yann.kempf@helsinki.fi)
Received: 20 May 2016 – Revised: 31 August 2016 – Accepted: 17 October 2016 – Published: 4 November 2016
Abstract. We present a scenario resulting in time-dependent
behaviour of the bow shock and transient, local ion re-
flection under unchanging solar wind conditions. Dayside
magnetopause reconnection produces flux transfer events
driving fast-mode wave fronts in the magnetosheath. These
fronts push out the bow shock surface due to their in-
creased downstream pressure. The resulting bow shock de-
formations lead to a configuration favourable to localized
ion reflection and thus the formation of transient, travelling
foreshock-like field-aligned ion beams. This is identified in
two-dimensional global magnetospheric hybrid-Vlasov sim-
ulations of the Earth’s magnetosphere performed using the
Vlasiator model (http://vlasiator.fmi.fi). We also present ob-
servational data showing the occurrence of dayside recon-
nection and flux transfer events at the same time as Geo-
tail observations of transient foreshock-like field-aligned ion
beams. The spacecraft is located well upstream of the fore-
shock edge and the bow shock, during a steady southward
interplanetary magnetic field and in the absence of any so-
lar wind or interplanetary magnetic field perturbations. This
indicates the formation of such localized ion foreshocks.
Keywords. Interplanetary physics (planetary bow shocks)
– magnetospheric physics (magnetosheath; solar wind–
magnetosphere interactions)
1 Introduction
The super-Alfvénic solar wind impinging upon the geomag-
netic field is slowed down and diverted around the Earth by
the bow shock which forms upstream of our planet. Most of
the plasma is abruptly compressed and heated by the shock
while being transported downstream into the magnetosheath.
There, it flows along the magnetopause surface, which delim-
its the magnetosphere, that is, the magnetic cavity in which
the Earth is situated.
Fluid theories such as ideal magnetohydrodynamics imply
that no wave or matter can travel upstream from a shock.
However, it is well-known observationally and explained
by kinetic plasma theory that, given a high enough Mach
number and a small enough angle θB−n between the up-
stream magnetic field (B) and the shock normal direction
(n) (θB−n/ 40–60◦, e.g. Greenstadt et al., 1980; Schwartz
et al., 1983), a fraction of the incoming ions is reflected by
the shock surface and streams back along the magnetic field
direction. The region where such a backstreaming ion pop-
ulation exists is called the ion foreshock. It is the stage for
a variety of plasma beam instabilities generating waves and
has been studied observationally as well as in simulations
for several decades (e.g. Bavassano-Cattaneo et al., 1983;
Sanderson et al., 1983; Thomsen et al., 1983; Fuselier et al.,
1987; Le and Russell, 1992; Eastwood et al., 2005b; Burgess
et al., 2012; Wilson et al., 2013). The value of θB−n allow-
ing the reflection of particles is dependent on several factors,
Published by Copernicus Publications on behalf of the European Geosciences Union.
944 Y. Pfau-Kempf et al.: Transient local foreshocks
among which most notably is the assumed reflection mecha-
nism, as the results of Greenstadt et al. (1980), for example,
show.
The bow shock under steady solar wind conditions is gen-
erally assumed to be a simple surface such as a paraboloid
with a dawn–dusk asymmetry primarily due to the Earth’s
orbital motion. This follows from fluid dynamical consid-
erations, numerical simulations as well as statistical stud-
ies of spacecraft observations. The inherently local nature of
spacecraft measurements compared to the spatial scale of the
bow shock, even in the case of spacecraft constellations such
as Cluster (Escoubet et al., 1997), Time History of Events
and Macroscale Interactions during Substorms (THEMIS;
Angelopoulos, 2008) or the Magnetospheric Multi-Scale
(MMS; Burch et al., 2016a) missions, does not allow us
to determine the global shape of the bow shock surface at
a given instant in time. Statistical studies are the method
of choice (e.g. Merka et al., 2005; Meziane et al., 2014).
Thus it is also usually assumed that under steady conditions,
the ion foreshock is located in the solar wind volume mag-
netically connected to the bow shock surface region where
θB−n/ 50◦. Depending on the orientation of the interplan-
etary magnetic field (IMF), this can be one or two compact
regions in space.
Another tenet of ideal magnetohydrodynamics is the con-
servation of magnetic field line topology, which is a good as-
sumption on large scales or in collisional plasma but breaks
down on smaller scales when strong magnetic field gradi-
ents are present. Through the kinetic process of magnetic re-
connection, antiparallel magnetic field lines rearrange their
topology while strongly accelerating inflowing plasma out
of the reconnection region (see, e.g., reviews by Zweibel
and Yamada, 2009, and Treumann and Baumjohann, 2013,
and references therein). A prime example of magnetic re-
connection in near-Earth space occurs at the magnetopause
in the subsolar region, when inflowing southward IMF re-
connects with the northward-oriented geomagnetic field lines
(e.g. Phan et al., 2000; Paschmann, 2008; Dunlop et al.,
2011). This phenomenon drives global magnetospheric dy-
namics as first proposed by Dungey (1961), and therefore it
is key to space weather studies (e.g. Cassak, 2016; Burch
et al., 2016b).
The topological reconfiguration of magnetic field lines at
the magnetopause can lead to the formation of well-delimited
structures called flux transfer events (FTEs). The classic pic-
ture of an FTE is that of a magnetic flux tube connected
both to the magnetosheath and the magnetosphere, but its
topology can be more complex. FTEs were first observed by
Russell and Elphic (1978, 1979) and Haerendel et al. (1978)
(who termed the process magnetic flux erosion) and subse-
quently much studied in space and ground observations as
well as simulations (e.g. Kawano and Russell, 1997; Wild
et al., 2001, 2003; McWilliams et al., 2004; Fear et al., 2009;
Eastwood et al., 2016; Hasegawa et al., 2016; Milan et al.,
2016). FTEs travel downstream along the magnetopause with
the magnetosheath plasma and are recognized by their signa-
ture in magnetic field measurements, such as the bipolar de-
flection of the component normal to their axis in the case of
a flux rope or magnetic island (e.g. Omidi and Sibeck, 2007;
Dorelli and Bhattacharjee, 2009; Sibeck and Omidi, 2012;
Eastwood et al., 2012). Their signatures also include pole-
ward moving auroral forms (PMAFs) and their equivalent
in radar observations named poleward moving radar auroral
forms (PMRAFs), which result from poleward ionospheric
flows. Oscillations can also be observed by ground magne-
tometers (e.g. Øieroset et al., 1996; Milan et al., 2000; Pang
et al., 2009). Although their role is crucial in the solar wind–
magnetosphere interaction, allowing plasma exchange along
the reconnected magnetic field lines, FTEs have not so far
been thought to be the direct cause of significant upstream
effects.
In this work, we propose a scenario by which dayside mag-
netopause reconnection generates FTEs, which in turn cause
steepening fast magnetosonic bow and stern waves to prop-
agate throughout the magnetosheath. The increased pressure
behind the steepened wave fronts causes the bow shock to
bulge outward in an area travelling along the bow shock sur-
face. The geometry at the edge of such a bulge can lead θB−n
to become smaller than∼ 50◦ in a well-delimited region, de-
tached from the “regular” foreshock, upstream of which a
beam of reflected ions generates a local, transient and travel-
ling foreshock. This chain of processes has first been identi-
fied in a two-dimensional polar-plane hybrid-Vlasov simula-
tion of steady southward IMF interacting with an Earth-like
dipolar magnetic field. The simulation was performed using
the Vlasiator model (http://vlasiator.fmi.fi). We also present
observational data supporting the scenario. Geotail space-
craft observations show the existence of short foreshock-like
ion beams during steady southward IMF, in a region well-
detached from both the bow shock and the regular ion fore-
shock region and in the absence of any IMF fluctuations, thus
excluding a momentary transit of the spacecraft through the
regular foreshock due to a change in IMF orientation. Simul-
taneously, the signatures of FTEs moving poleward are found
in ground magnetometer and SuperDARN (Super Dual Au-
roral Radar Network) radar data.
Section 2 describes the simulation and the observa-
tional methods. The scenario of magnetopause–bow-shock–
foreshock interaction is detailed in Sect. 3, while the ground
and spacecraft observations are presented in Sect. 4. The re-
sults are then discussed in Sect. 5 before the conclusions are
given in Sect. 6.
2 Methods
2.1 Hybrid-Vlasov simulation
The hybrid-Vlasov model Vlasiator has been developed with
the aim of producing global magnetospheric simulations
Ann. Geophys., 34, 943–959, 2016 www.ann-geophys.net/34/943/2016/
Y. Pfau-Kempf et al.: Transient local foreshocks 945
of the Earth’s magnetosphere including kinetic physics be-
yond magnetohydrodynamics while avoiding the limitations
due to the statistical sampling inherent to particle-in-cell
approaches (von Alfthan et al., 2014). Vlasiator has been
used to study amongst other things the magnetosheath and
the foreshock in equatorial plane simulations of the terres-
trial magnetosphere (Pokhotelov et al., 2013; Kempf et al.,
2015; Palmroth et al., 2015; Hoilijoki et al., 2016). It solves
Vlasov’s equation to propagate the ion (proton) velocity dis-
tribution function in up to three spatial and three velocity di-
mensions. The equation system is closed via Ampère’s and
Faraday’s laws as well as a generalized Ohm’s law includ-
ing the Hall term (see von Alfthan et al., 2014, and Palmroth
et al., 2015, for more details).
The simulation used in this study is two-dimensional in
the polar x–z plane and three-dimensional in velocity space.
It covers both the dayside and the nightside magnetosphere.
The spatial coordinates are similar to the Geocentric Solar
Magnetospheric (GSM) coordinate system with the x axis
pointing from the Earth towards the Sun and the z axis or-
thogonal to the x axis and parallel to the geomagnetic dipole
field axis (no dipole tilt). We use a two-dimensional line
dipole centred at the origin and scaled to match the geomag-
netic dipole strength in the same way as is done by Daldorff
et al. (2014). The steady solar wind has a proton density of
1× 106 m−3, an inflow temperature of 0.5 MK and a veloc-
ity of −750 km s−1 purely along the x axis. The constant
and purely southward IMF has an intensity of 5 nT. The up-
stream boundary maintains a constant field and a Maxwellian
velocity distribution; the three other outer boundaries have
copy conditions ensuring proper outflow. The inner bound-
ary, which is set at a distance of 30 000 km (∼ 5 Earth radii,
RE) around the origin, enforces a static Maxwellian veloc-
ity distribution and perfect conductor field boundary condi-
tions. The out-of-plane direction is treated periodically. The
boundaries are located at 47RE from the origin in each di-
rection. Since this study concentrates on dayside phenomena,
the nightside is not shown in this work. The spatial resolution
is 300 km or 0.047RE or 1.3 solar wind ion inertial lengths
and the velocity space extends from−4000 to+4000 km s−1
in all three dimensions with a resolution of 30 km s−1 or 0.33
solar wind ion thermal speeds. The phase space density spar-
sity threshold is 10−15 m−6 s3 (see von Alfthan et al., 2014,
and Kempf et al., 2015, for details on the sparse phase space
strategy used in Vlasiator).
The simulation has been run for over 1850 s or 140 so-
lar wind proton gyroperiods, and it reaches a steady state on
the dayside after less than 900s or 70 gyroperiods. The bow
shock and the magnetopause form as expected and there is a
foreshock at high latitudes both in the Northern and Southern
hemispheres. The magnetosheath is pervaded by anisotropy-
driven waves, most notably mirror-mode waves as has been
demonstrated by Hoilijoki et al. (2016). Figure 1 shows an
overview of the simulation setup after 1150 s of simulated
time. The magnetopause–magnetosheath–bow-shock struc-
Figure 1. Colour code: plasma number density (protons m−3) after
1150s of simulation time. Contour lines: magnetic field lines. A
large magnetic island is prominent at (+6,−7)RE; another one is
in the southern cusp region and a series of smaller magnetic islands
is visible at the dayside magnetopause northward of the equator.
ture is clear, and a large magnetic island can be recognized
due to its high density at the position (+6,−7)RE. A smaller
magnetic island is in the southern polar cusp region, while a
series of even smaller islands is also visible along the day-
side magnetopause boundary northward of the equator. The
animation provided in the Supplement to this work shows
the time evolution of the ion number density and the parallel
temperature for the same spatial extents as Fig. 1 and with
the same colour scales as Figs. 1 and 5a.
2.2 Spacecraft and ground measurements
We first use solar wind densities, velocities and the IMF
one-minute averaged data from NASA/GSFC’s OMNI data
www.ann-geophys.net/34/943/2016/ Ann. Geophys., 34, 943–959, 2016
946 Y. Pfau-Kempf et al.: Transient local foreshocks
set accessed through CDAweb (Coordinated Data Analysis
Web) to identify suitable intervals of stable solar wind and
steady southward IMF conditions.
We also use in situ spacecraft measurements from Geotail
in this study. The ion velocity distribution measurements are
taken from the Low Energy Particle instrument (LEP; Mukai
et al., 1994). During the event presented in Sect. 4.3, LEP
was in the EA (energy-per-charge analyser) mode, which is
not well-suited to properly measure the cold core solar wind
ion population but does not impact the quality of suprather-
mal ion measurements. Editor-B data are available for that
event, meaning that only two-dimensional velocity distri-
butions are available. These projected distributions are pro-
duced using data from all three-dimensional channels (LEP
instrument team, personal communication, 26 August 2016).
They are provided in Geocentric Solar Ecliptic (GSE) co-
ordinates (xGSE-axis pointing from the Earth towards the
Sun, yGSE-axis in the ecliptic plane pointing towards dusk
and zGSE-axis perpendicular to the ecliptic plane). Magnetic
field measurements are from the Magnetic Field Measure-
ment fluxgate magnetometers (MGF; Kokubun et al., 1994).
The following measurements from the Wind spacecraft are
used: solar wind data from its Solar Wind Experiment (SWE;
Ogilvie et al., 1995), IMF data from its Magnetic Fields
Investigation (MFI; Lepping et al., 1995), moments from
its 3-D Plasma and Energetic Particle Analyzer (3-DP; Lin
et al., 1995) as well as densities retrieved from the electron
plasma frequency measured by the radio and plasma wave
instrument (WAVES; Bougeret et al., 1995). The following
datasets from the Advanced Composition Explorer (ACE)
spacecraft are used: IMF measurements from the Magnetic
Fields Experiment (MAG; Smith et al., 1998) and ion mo-
ments from the Solar Wind Electron Proton Alpha Monitor
(SWEPAM; McComas et al., 1998).
Ground-based ionospheric backscatter data from Super-
DARN (Greenwald et al., 1995) as well as ground magne-
tometer data from the International Monitor for Auroral Ge-
omagnetic Effects (IMAGE, http://space.fmi.fi/image, Tan-
skanen, 2009) are used. Additionally, we use the electrojet
activity auroral electrojet (AE) indices provided by the Uni-
versity of Kyoto through the World Data Center for Geomag-
netism (Davis and Sugiura, 1966).
3 Magnetopause–bow-shock–foreshock interaction
scenario
The scenario proposed in this work has been identified in the
simulation presented in Sect. 2.1. We describe the scenario
here in Sect. 3 in a narrative fashion and present the corre-
sponding observations in Sect. 4. Limitations are discussed
in Sect. 5.
-40
-30
-20
-10
0
10
20
30
40
B
 (n
T)
(b) BX
BY
BZ
-40
-30
-20
-10
0
10
20
30
40
-30 -20 -10 0 10 20
B
 (n
T)
Time (s)
(c)
BT
BY
BN
Figure 2. (a) Close-up view of the large magnetic island from Fig. 1
travelling tailward along the magnetopause. Colour code: plasma
number density, protons m−3. Contour lines: magnetic field lines.
Arrows: rotated coordinate system (N,Y,T ) with N normal to the
magnetopause and T parallel to it. (b) Magnetic field evolution at
a virtual spacecraft located at the white cross in panel (a), in sim-
ulation (GSM) and (c) rotated coordinates. The grey vertical bar
indicates the time of panel (a) and Fig. 1. The characteristic bipo-
lar signature of the passing magnetic island is obvious in the BN
component.
3.1 Magnetopause reconnection
Under steady southward IMF, magnetic reconnection occurs
typically along a line at the equator on the magnetopause
(e.g. Trattner et al., 2007; Dunlop et al., 2011; Hoilijoki et al.,
2014). In the present simulation, the position of the X-line is
not stable in time and multiple reconnection sites can coex-
ist at any given time on the magnetopause. Reconnected field
lines form magnetic islands in the exhaust regions of recon-
nection sites, which grow and travel downstream (poleward)
along the magnetopause. This continuously ongoing process
is prominent in the animation provided in the Supplement.
The magnetic islands can be seen as the two-dimensional
equivalents of FTEs, that is, cuts through an out-of-plane
flux rope. A more detailed analysis of the propagation of the
magnetic islands and the location and intensity of magnetic
reconnection is the subject of a separate study.
Figure 2 shows such a magnetic island and time series
of the magnetic field components seen at a virtual space-
craft over which the magnetic island flows. The magnetic
Ann. Geophys., 34, 943–959, 2016 www.ann-geophys.net/34/943/2016/
Y. Pfau-Kempf et al.: Transient local foreshocks 947
(a)
+
Core
Core
Beam
Beam
V
z 
(1
00
0 
km
 s
  )-1
Vx (1000 km s  )-1
Figure 3. (a) Example of bow (black dashed) and stern (white dash–dotted) fast wave fronts driven by a magnetic island (density peak at
(2,−9)RE; colour code: plasma number density; protons m−3; simulation time 1340 s). The bow wave accelerates particles ahead of it, as
can be seen in the (b) two-dimensional projected isocontour and (c) three-dimensional isocontour plots of the ion velocity distribution (phase
space density in s3 m−6; 3-D isocontour at 1× 10−15 s3 m−6) taken at the location of the white cross. The core population with very low
drift velocity (blue, pink and grey isocontours, centre and top right part of the 3-D isocontour) is preceded by an accelerated population in
the −Vx and −Vz direction. The white arrow shows the location of the profiles shown in Fig. 4.
field components are shown both in the simulation coordi-
nates and in a coordinate system (N,Y,T ) rotated by 150◦ in
the plane of the simulation so that N points in the direction
normal to the magnetopause and T points along the magne-
topause. The strong bipolar fluctuation in the BN component
is characteristic of the passage of a magnetic island.
3.2 Magnetosheath waves and bow shock perturbations
Figure 3a shows a magnetic island in the southern cusp re-
gion. The increased dynamic pressure of the magnetic is-
lands with respect to the surrounding magnetosheath plasma
drives bow waves ahead of the islands. These fast magne-
tosonic waves propagate throughout the magnetosheath and
steepen to almost form fast forward shocks. In some cases,
strong magnetic islands can also be followed by a fast re-
verse wave front, but these stern waves are less steep than
the bow wave fronts. Both the bow and stern fast mode waves
are visible in Fig. 3a. The profiles of plasma density, velocity
and temperature perpendicular to the magnetic field as well
as the magnetic field intensity show clearly the steep corre-
lated increase corresponding to the fast forward wave front in
Fig. 4a–d. The forward wave fronts are steep enough to re-
flect ions much in the way a shock can accelerate ions to gen-
erate upstream foreshock populations. Figure 3b and c shows
the two- and three-dimensional velocity distribution function
isocontours at the location of the white cross in Fig. 3a. The
accelerated ions are clearly visible ahead of the core popu-
lation in the −Vx and −Vz direction. The structure is also
readily visible in the profile of the temperature parallel to the
2x106
2.5x106
3x106
(a)
N
o.
 
de
ns
ity
 
(m
)
-3
350
400
450
500(b)
V
 (m
s-
1 )
5
10
15
(c)
T 
(M
K
)
T//T⊥
25
30
35
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
(d)
B
 (n
T)
Distance (RE)
Figure 4. Cut across the bow wave front along the white arrow in
Fig. 3 showing (a) the plasma density and (b) velocity, (c) the tem-
perature parallel and perpendicular to the magnetic field, and (d) the
magnetic field intensity. The correlated jump in all these parameters
at the abscissa 2RE characterizes the steep fast forward wave front.
Note the fast wave signature in the perpendicular temperature while
the parallel temperature is much more sensitive to the particle beam
accelerated ahead of the wave front.
www.ann-geophys.net/34/943/2016/ Ann. Geophys., 34, 943–959, 2016
948 Y. Pfau-Kempf et al.: Transient local foreshocks
+
(a)
Core
Core
Beam
Beam
V
z  
( 1
0
0
0
 k
m
 s
  
)
- 1
Vx (1000 km s  )
-1
Figure 5. (a) Colour code: temperature (K) parallel to the magnetic field in a region presenting a local foreshock at 1548.5s simulation
time. This variable is sensitive to the presence of an ion beam, hence the choice to bring out the local foreshock. The continuous white
isocontour curve shows where the ion density is 2× 106 m−3 (twice the solar wind density), thus indicating the bow shock location. The
dashed white curve would show the approximate position of the bow shock were it not for the increased pressure due to a fast wave front
in the magnetosheath. The continuous and dashed segments indicate the normal direction for each of these curves. θB−n: 41◦ and 54◦;
thus the angle between both is 13◦. (b) Two-dimensional projected isocontour and (c) three-dimensional isocontour plots of the ion velocity
distribution (phase space density in s3 m−6; 3-D isocontour at 1× 10−15 s3 m−6) at the location of the white cross. The field-aligned beam
is prominent and directly comparable to Figs. 2 and 6 in the work by Kempf et al. (2015).
magnetic field in Fig. 4c; the steep increase in the parallel
temperature from 7 to 14 MK is the direct signature of the
presence of an accelerated ion population upstream of the
wave front in addition to the background magnetosheath ion
population. The increased parallel temperature ahead of the
wave fronts is visible too in the right panel of the animation
provided in the Supplement.
Downstream of the fast magnetosonic waves, the mag-
netosheath plasma has higher thermal, dynamic and mag-
netic pressure. The straightforward consequence of this phe-
nomenon is – considering the pressure balance when the
wave fronts reach the bow shock – that the bow shock is
pushed outwards against the solar wind, forming a local-
ized bulge corresponding to the region of enhanced magne-
tosheath pressure.
3.3 Local foreshocks
Figure 5a shows the detailed view of a small region of the
bow shock south of the subsolar point. When the angle be-
tween the shock normal and the upstream magnetic field
θB−n is lower than ∼ 50◦, incoming particles with suffi-
cient energy can be accelerated back upstream and form a
foreshock. Consequently, when the bow shock bulge caused
by the fast-wave-mediated pressure increase is pronounced
enough, the region of the bulge with θB−n/ 50◦ is the source
of a separate ion beam propagating upstream along the IMF
direction. This is of course only the case when the bulge has
not yet travelled into the region where the mean θB−n is al-
lowing ion reflection anyway. The spatial extent in the di-
rection parallel to the bow shock surface is limited to the
corresponding patch of favourable θB−n, and this localized
foreshock travels along with the driving magnetosheath wave
front until it merges with the regular foreshock further down-
stream. Due to its being a travelling and transient ion beam,
the expected beam instabilities do not have time to grow to
form ultra low-frequency (ULF) waves as in the regular fore-
shock (see, e.g., von Alfthan et al., 2014; Palmroth et al.,
2015). This means that the typical ULF wave signature char-
acteristic of the regular foreshock is absent from this struc-
ture.
Figure 5a shows an example of a localized foreshock
driven by a magnetosheath wave. The colour code in the
figure shows the temperature parallel to the magnetic field,
which is sensitive to the presence of a field-aligned ion
beam. The white contour curve is set at a plasma density of
2×106 m−3 (twice the solar wind density), which highlights
the position and shape of the bow shock. The dashed curve
would indicate the approximate location of the bow shock
were it not for the pressure increase in the magnetosheath
Ann. Geophys., 34, 943–959, 2016 www.ann-geophys.net/34/943/2016/
Y. Pfau-Kempf et al.: Transient local foreshocks 949
Figure 6. Energy–time spectrogram of the simulated local fore-
shock crossing event at the location marked in Fig. 5. The grey
vertical bar indicates the time of Fig. 5. The velocity distribution
has been split into four sectors (sunward, southward, tailward and
northward in the simulation/polar plane, ±45◦ from the +x, −z,
−x and +z direction respectively), two of which are plotted here.
(a) The dense tailward population at an energy of a few kiloelec-
tronvolt is the solar wind core. (b) The local foreshock beam forms
the population at a few tens of kiloelectronvolt in the southward sec-
tor and lasts almost 70s. Thus the expected observational signature
is that of a field-aligned beam with a duration of the order of 1 min.
after the passage of a fast forward wave front. The continu-
ous and dashed segments indicate the local direction normal
to the respective curve. Their θB−n is respectively 41◦ and
54◦. The ion beam is generated by solar wind ions reflected
at the foot of the bow shock bulge where θB−n is favourable.
Beyond −3RE(xGSM), the regular foreshock is also visible
upstream of the bow shock as a region of increased parallel
temperature. The fact that the density and parallel tempera-
ture increases due to the bow shock do not coincide, illus-
trates that the shock primarily heats plasma in the perpen-
dicular direction. Isotropization of the velocity distribution
happens further downstream in the magnetosheath.
The animation provided in the Supplement to this work
shows that such local foreshocks occur both north and south
of the ecliptic whenever favourable θB−n conditions are met
at the foot of a bow shock perturbation.
Figure 5b–c shows the ion velocity distribution function
corresponding to a field-aligned beam population, which is
similar to the distribution expected at the edge of the regular
foreshock upstream of the ULF foreshock boundary (Kempf
et al., 2015). The density of the beam is of the order of 1 % of
the inflowing density as in the regular foreshock (not shown).
Figure 6 shows an energy–time spectrogram for the tailward
and southward sectors of the velocity distribution. The tail-
ward part contains the cold solar wind core population, while
the southward sector, in the direction of the field-aligned
beam, indicates the typical signature expected when a local
foreshock crosses an upstream spacecraft. The timescale of a
local foreshock crossing is on the order of 1 min in the simu-
lation (almost 70s in Fig. 6), but this value can vary depend-
ing on the geometry of the event. Other factors affecting the
observational signatures are discussed in Sect. 5.
4 Spacecraft and ground observations
In Sect. 3 we present a scenario based on a global hybrid-
Vlasov simulation, in which dayside reconnection eventually
leads to the formation of local, transient foreshock-like struc-
tures upstream of the terrestrial bow shock and outside of
the region where the angle between the shock normal and
the IMF (θB−n) would normally be favourable for ion re-
flection. In this section, we present observational data from
an event on 30 August 2004 which supports the interpre-
tation of the simulation. While Geotail observed transient
field-aligned ion beams in the solar wind upstream of the
bow shock and the foreshock between 08:09 and 08:24 UT,
ground-based SuperDARN radar data and IMAGE magne-
tometer data indicate that dayside reconnection was active
and producing FTEs.
4.1 Upstream pristine solar wind conditions
The OMNI data set (from ACE) containing the upstream
magnetic field, ion velocity, ion density and ion tempera-
ture on 30 August 2004 is plotted in Fig. 7. It shows that
the IMF turned south at about 05:00 UT and Bz remained
strongly negative around −10nT for most of the day until
about 22:00 UT. The velocity and temperature of the solar
wind remained stable around 480–490 km s−1 and 104 K re-
spectively between 08:00 and 19:00 UT, while Bz slowly de-
creased from −8 to −11nT and the density slowly increased
from about 5 to 10cm−3.
4.2 Ground observations
The strong southward Bz component of the IMF is the
cause of strong magnetic reconnection at the dayside mag-
netopause, which in turn is known to produce numerous
FTEs (e.g. Kawano and Russell, 1997). Global activity in-
dices clearly indicate ongoing magnetic reconnection during
the event. The prolonged period of southward IMF triggered
a geomagnetic storm and the increased levels of magnetic
reconnection both on the dayside and the nightside are re-
flected in the AE indices, which started picking up between
05:00 and 06:00 UT and reached levels above AE= 500nT
after 08:00 UT.
Evidence for continuous FTE activity during the period
06:00 to 10:00 UT is observed by two SuperDARN radars
in the Southern Hemisphere, presented in Fig. 8. The Ker-
guelen and Syowa East radars were observing backscatter
from the pre-noon and noon region during this period. Fig-
ure 8a–c show the Kerguelen line-of-sight velocity data at
07:08, 07:32 and 08:22 UT. Three regions of backscatter are
labelled A to C. In the polar cap (A), 1 kms−1 flows away
from the radar (antisunwards) are observed, 700ms−1 flows
away from the radar (polewards) are seen entering the polar
cap near noon (B), and 700ms−1 flows towards the radar
(sunwards) are seen in the return flow region (C). As the
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950 Y. Pfau-Kempf et al.: Transient local foreshocks
Figure 7. Upstream solar wind observations between 04:00 and 22:00 UT on 30 August 2004, from the OMNI dataset with 1min time
resolution. (a) Magnetic field. (b) Velocity. (c) Ion number density. (d) Ion temperature. The grey box shows the interval of the event
presented in Fig. 11 and Sect. 4.3.
Earth rotates the look direction of the radar changes, but
these backscatter regions continue to be observed. Range–
time plots show that these backscatter regions are quasi-
periodically pulsed with periods near 10–15 min, the ex-
pected signature of pulsed reconnection (e.g. Provan et al.,
1998; Milan et al., 1999a, 2000; Wild et al., 2001). This is
seen as poleward-moving enhancements in the backscatter
power in the polar cap flows of region A in Fig. 8f. It is
also observed as pulses of backscatter and flow in the re-
turn flow region, as first discussed by Milan et al. (1999b),
that is, pulsed flows observed moving towards the Kergue-
len radar (i.e. sunwards) in the return flow backscatter re-
gion C (Fig. 8e) and pulsing moving away from the Syowa
East radar (also sunwards), in backscatter collocated with re-
gion C (Fig. 8i).
IMAGE magnetometers also observed signatures that
could be interpreted as FTE activity. Figure 9a shows the
ionospheric equivalent current density at 110 km altitude in
the Northern Hemisphere at 08:15 UT. The equivalent current
density was derived from 10s IMAGE magnetometer data
using spherical elementary current systems (SECS; Amm,
1997; Amm and Viljanen, 1999). Before applying the SECS
method, a baseline was subtracted from the variometer data
following van de Kamp (2013). The Jeq data are presented as
a function of Altitude Adjusted Corrected Geogmagnetic Co-
ordinates (AACGM; Shepherd, 2014) latitude and longitude,
which at the given UT correspond to 09:05–11:09 magnetic
local time (MLT). The plot has been rotated such that local
noon is at the top. The plot shows eastward and equatorward
equivalent current density vectors in the poleward part of the
IMAGE field of view. If gradients of the ionospheric conduc-
tances are vanishingly small or aligned with the electric field
in a large enough area, the equivalent current equals the Hall
current, which flows antiparallel to the ionospheric E×B
drift. According to Weygand et al. (2012), this is often a good
approximation. Thus, the equatorward equivalent current in
Fig. 9a may indicate poleward plasma flow entering the polar
cap.
Figure 9b shows a
∣∣Jeq∣∣ keogram, that is, latitude profiles
of |Jeq| along 105◦ longitude presented as a function of time
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Y. Pfau-Kempf et al.: Transient local foreshocks 951
(  ) (  ) (  )
(  )
(  )
(  )
(  )
(  )
(  )
Figure 8. (a–c) Line-of-sight velocities (blue towards the radar, red away from the radar) from the Kerguelen SuperDARN radar in the
Southern Hemisphere, at 07:08, 07:32 and 08:22 UT. Grey regions are ground scatter. The data are presented in geomagnetic latitude and
magnetic local time coordinates, with dotted circles indicating geomagnetic latitudes of 60, 70 and 80◦ and dotted lines showing local time
meridians with local noon at the top. The outline of the radar field of view is shown by dashed lines, as is the field of view of the Syowa East
radar. Grey circles indicated the expected locations of the poleward and equatorward edges of the auroral oval. Three regions of backscatter
are indicated by the letters A, B and C. (d–i) Backscatter power and line-of-sight velocity from beams 0 and 13 of the Kerguelen radar and
beam 9 of the Syowa East radar. Regions of backscatter are also labelled A to C.
between 06:00 and 10:00 UT. The vertical lines indicate the
interval 08:09–08:24 UT during which Geotail observed the
ion beam signature. The occurrence of the
∣∣Jeq∣∣ intensifica-
tions observed by IMAGE between 70 and 75◦ latitude be-
fore about 09:00 UT roughly agrees in time with the South-
ern Hemisphere FTE signatures observed by SuperDARN.
One of the intensifications occurred during the interval when
Geotail observed the ion beam signature.
4.3 Geotail observations
On 30 August 2004, Geotail was located on the dayside
of the Earth and upstream of the bow shock in the so-
lar wind. Between about 08:00 and 08:30 UT, Geotail was
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952 Y. Pfau-Kempf et al.: Transient local foreshocks
30 Aug 2004 08:15:00 UT (09:05−11:09 MLT)
55o
60o
65o
70o
75o
90o
100o
110o
Ionospheric J [90 A km  ]
AACGM lat
AACGM long
(a)
|J
eq
| [
A 
km
 ]
0
10
20
30
40
50
60
70
80
90
06:00 06:30 07:00 07:30 08:00 08:30 09:00 09:30 10:00
55
60
65
70
75
AA
C
G
M
 la
t [
de
g]
 (l
on
g 
=1
05
.0
 d
eg
)
(b)
|J
eq
| [
A 
km
 ]
0
20
40
60
80
100
120
-1
-1
-1
UT of 30 Aug 2004
Figure 9. (a) Ionospheric equivalent current density at 110 km alti-
tude (Jeq, arrows;
∣∣Jeq∣∣, colour) in the Northern Hemisphere on 30
August 2004 at 08:15 UT, derived from 10s IMAGE magnetometer
(squares) data. The data are presented as a function of geomagnetic
(AACGM) latitude and longitude, which at the given time corre-
spond to 09:05–11:09 MLT. The plot has been rotated such that lo-
cal noon is at the top. (b)
∣∣Jeq∣∣ keogram (latitude profiles along
105◦ longitude presented as a function of time between 06:00 and
10:00 UT). The vertical lines indicate the interval (08:09–08:24 UT)
during which Geotail observed the ion beam signature.
located at (17.6,23.0,−9.2)RE in GSM coordinates and
(17.6,24.5,3.6)RE in GSE coordinates. The location of
Geotail with respect to a model bow shock and ion foreshock
edge is shown in Fig. 10 (details of the bow shock and fore-
shock models are given in Sect. 5.1). Geotail/MGF observed
stable IMF withBGSM = (7,5,−9)nT, as shown in Fig. 11c.
No perturbations of the magnetic field are seen which would
modify its orientation, thus altering the magnetic connection
to the bow shock and the location of the foreshock. Small-
amplitude regular fluctuations are visible throughout, which
(a) (b)
Figure 10. (a) Plot showing the location of Geotail at 08:16:10 UT
on 30 August 2004 with respect to the bow shock following the
model by Merka et al. (2005) and the foreshock edge assuming that
the maximum θB−n for ion reflection is 60◦ and the reflected ions
have twice the solar wind speed (in the solar wind frame). (b) x–z
slice at the y coordinate of Geotail showing the spacecraft and the
bow shock location. (Plots in GSE coordinates.)
coincide with Geotail’s nominal spin rate of 20 rpm. They are
therefore likely a residual from the data calibration process.
The energy–time spectrograms from the Geotail/LEP in-
strument for the ions flying in the tailward and duskward di-
rections are shown in Fig. 11a and b. Only two-dimensional
projected ion velocity distributions in the ecliptic plane are
available from LEP for this event, hence the choice of sec-
tors. The tailward sector is dominated by the steady cold
and dense solar wind core population just below 1keV. Be-
tween about 08:09 and 08:24 UT (time delimited by the black
dotted lines in Fig. 11) the duskward sector exhibits several
occurrences of an ion population at energies of a few kilo-
electronvolt reminiscent of the signature of foreshock field-
aligned beam ions. The presence of a beam in this sector is
consistent with the significant By component of the IMF.
In contrast to regular foreshock field-aligned beams, this
beam is transient and appears several times after 08:10 UT
for one to a few minutes without correlation with changes
in the magnetic field direction, as can be seen by comparing
panels a–c in Fig. 11. Panel d shows the velocity and number
density of the ions from the OMNI data set with a 1min time
resolution. The velocity is stable and varies only by about
1 %, while the density fluctuates between 4.5 and 5.5cm−3
but without correlating with the transient beam event. We
choose the OMNI density and velocity data because the den-
sities reported by Geotail/LEP do not seem to be consistent.
This is likely due to the fact that LEP is in EA mode and
not in SW (solar wind analyser) mode, which would have
ensured a better measurement of the solar wind core popu-
lation. To ensure that the choice of the OMNI data is sensi-
ble, we compare shifted ACE and Wind magnetic field mea-
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Y. Pfau-Kempf et al.: Transient local foreshocks 953
Figure 11. Geotail/LEP energy–time spectrogram on 30 August 2004 between 07:48 and 08:47 UT, for the (a) tailward and (b) duskward
sectors (±45◦ from the +x and +y direction respectively), extracted from the two-dimensional reduced velocity distribution in the GSE
ecliptic plane. The tailward sector shows the cold and dense solar wind core population just below 1keV. The duskward sector shows the
signature of a transient beam whose density peaks several times between 08:09 and 08:24 UT (time delimited by the black dotted lines).
(c) Geotail/MGF magnetic field in GSM coordinates. The magnetic field components and thus its orientation are stable during the event.
The short-period oscillations coincide with Geotail’s nominal spin rate of 20 rpm. (d) OMNI plasma number density and velocity. The grey
continuous bars indicate the time at which the velocity distribution shown in Fig. 12 is measured.
surements to the Geotail/MGF data to check that the OMNI
propagation algorithm is successful. We then check that the
densities and velocities from ACE and Wind are similar to
each other and to the OMNI dataset. Since this is the case
and the OMNI values are similarly stable to the ACE (which
was used to produce the OMNI dataset) and Wind data at a
higher time resolution, we conclude that the OMNI dataset is
reliable and sufficient here. LEP being in EA mode instead
of SW affects the quality of the measurement for the core
population but not for the beam, which is of prime interest
here.
Figure 12 shows the projected two-dimensional veloc-
ity distribution in the Vx–Vy GSE plane measured by the
Geotail/LEP between 08:16:10 and 08:16:22 UT (time de-
limited by the grey continuous bars in Fig. 11). The so-
lar wind core population is prominent in the lower left at
Vx ∼−400kms−1, while a typical field-aligned beam flows
back upstream along the magnetic field with positive Vx and
Vy components. The black arrow points in the average direc-
tion of the magnetic field in the x–y GSE plane during the
time interval, and the grey dots indicate all measurements
taken at an 8Hz cadence by the MGF instrument during the
same time. Their close grouping once more indicates the sta-
bility of the magnetic field direction.
It is worth noting that while exactly similar magnetic
field and solar wind conditions prevail in the 10 min pre-
ceding the event, no such field-aligned beam is seen before
08:10 UT. Additional ion beams are visible between 08:24
and 08:40 UT, but in their case the influence of magnetic
field perturbations observed simultaneously cannot be con-
clusively ruled out.
5 Discussion
In Sect. 4 we present Geotail observations of transient field-
aligned ion beams upstream of the Earth’s bow shock, while
ground-based SuperDARN radar data and IMAGE magne-
tometer data show that pulsed dayside reconnection produc-
ing FTEs was occurring at the same time. This matches the
observational signatures expected from the scenario drawn in
Sect. 3 based on a global magnetospheric simulation. In this
Sect. 5 we first investigate the position of Geotail with re-
spect to the regular foreshock, and we then discuss the more
general factors which might affect the interpretation of the
simulation and the measurements.
5.1 Position of Geotail relative to the regular foreshock
It is important to ascertain that Geotail is not too close to
the bow shock or to the foreshock. Indeed if it were in the
vicinity of either, it could observe for example shock foot
ion populations or the edge of field-aligned beam populations
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954 Y. Pfau-Kempf et al.: Transient local foreshocks
Beam
Core
Figure 12. Coloured dots: Two-dimensional reduced ion velocity
distribution in GSE coordinates measured by Geotail/LEP between
08:16:10 and 08:16:22 UT. The cold solar wind core flows with
Vx ∼−400kms−1, and the hotter field-aligned beam propagates
in the opposite direction. Black arrow: averaged magnetic field di-
rection in the x–y GSE plane measured by Geotail/MGF during the
interval. Grey dots: all individual Geotail/MGF measurements taken
at 8 Hz cadence. The stability of the orientation of the magnetic field
is remarkable.
from the foreshock, which would look very similar to the
signature expected from a local foreshock.
We use the bow shock model from Merka et al. (2005)
with the OMNI solar wind parameters and the Geotail/MGF
observed magnetic field as inputs to determine the expected
bow shock shape and position. As a first approach we deter-
mine the position of the expected ion foreshock edge. We
trace the observed magnetic field to find the locus on the
bow shock surface where θB−n = 60◦, and then we trace the
trajectories of ions reflected from there with twice the so-
lar wind inflow speed in the solar wind rest frame. This is
typical of field-aligned beams in the foreshock (e.g. Green-
stadt et al., 1980; Eastwood et al., 2005a; Kis et al., 2007).
From this modelling we determine that Geotail is 3–5RE
away from the foreshock edge and 9–10RE clear of the bow
shock during the event between 08:09 and 08:24 UT, whence
we could conclude that the spacecraft is well beyond reach
of bow shock foot or foreshock edge ions. The result of this
analysis is what is presented in Fig. 10.
However, as can readily be estimated from the observed
velocity distribution shown in Fig. 12, the beam is signifi-
cantly faster in the solar wind rest frame than twice the solar
wind inflow velocity in the spacecraft frame. To get a bet-
ter estimate of whether Geotail is within reach of the regular
foreshock, we consider the trajectory of foreshock ions as-
suming adiabatic reflection at the bow shock (Schwartz et al.,
1983; Liu et al., 2016). Adiabatic reflection and not specular
reflection is assumed because it yields higher beam speeds
and would thus be more likely to reach the spacecraft. In-
coming ions at velocity V impinge on the bow shock, which
has a local normal vector n, and are reflected with a velocity
V r =−V + 2V HT, (1)
where
V HT = n× (V ×B)
n ·B (2)
is the de Hoffmann–Teller velocity of the bow shock
(De Hoffmann and Teller, 1950) and B is the IMF.
The validity of the assumption that ions are reflected adia-
batically can be checked against the simulation. In Fig. 5a
we have θB−n = 41◦ at the foot of the local foreshock,
the solar wind velocity is −750kms−1 purely along the
x axis, and the IMF is constant and purely southward at
5nT. With those parameters, Eqs. (1) and (2) yield V simr =
(−750,0,1725.6)kms−1. This does indeed correspond to
the beam velocity as shown by the projected velocity distri-
bution function in Fig. 5b, confirming the assumed adiabatic
reflection process.
To check whether Geotail observed adiabatically reflected
ions from the regular foreshock, we trace the observed beam
from Geotail back to the bow shock along the IMF direc-
tion and invert Eq. (1) to reconstruct the solar wind veloc-
ity vector V ′ that would yield the measured beam velocity
with the assumed model bow shock. For the observed beam
(Fig. 12), the resulting solar wind velocity vector would have
the components V ′ = (−557,−602,317)kms−1 in GSE co-
ordinates, which is obviously not in accordance with the ob-
served solar wind (Figs. 7b and 11d). Having thus ascertained
that in these solar wind and IMF conditions the observed
beam cannot have been reflected adiabatically from the mod-
elled bow shock, we perturb the model bow shock until the
adiabatically reflected ion trajectory matches the observed
beam. When n is rotated towards B by an angle of 15◦, the
adiabatically reflected ion beam does indeed hit Geotail. This
value is remarkably close to the angle of 13◦ between the un-
perturbed and perturbed bow shock normals at the foot of the
local foreshock in the global simulation (Fig. 5a). It has to
be noted though that the beam velocities obtained with this
approach do not agree well with the observed ones (recon-
structed velocity 688kms−1, observed velocity 885kms−1
in the spacecraft frame), which means that the reflection and
acceleration process and their geometry are probably more
complex than the simple adiabatic reflection we assume here.
In summary this analysis demonstrates that Geotail is out
of reach for adiabatically reflected field-aligned beam ions
originating from the unperturbed foreshock under the pre-
vailing solar wind and IMF conditions. By introducing an ad
hoc local perturbation of the bow shock normal of 15◦, we
recover a beam direction consistent with the Geotail obser-
vations, which is similar to the bow shock perturbation seen
in the global simulation.
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Y. Pfau-Kempf et al.: Transient local foreshocks 955
5.2 Estimating the propagation direction of the
perturbation
Assuming that three-dimensional velocity distribution mea-
surements were available with a high cadence, it would be
possible to estimate the direction and speed of propaga-
tion of the field-aligned ion beam. Indeed at the edge of an
ion beam, non-gyrotropic partial ring or beam distributions
are observed in a region of one gyrodiameter width (e.g.
Schwartz et al., 2000; Kempf et al., 2015). Timing the tran-
sition from a partial to a full gyrotropic beam and back on
each side of the beam passage across the spacecraft yields an
approximate beam propagation speed since the gyrodiame-
ter of the ions is known. Furthermore, the gyrophase of the
ions at the very edge of the beam indicates on which side
of the spacecraft the beam is located, so that the incoming
and outgoing edges yield an estimate of the beam edge ori-
entation. However the lack of knowledge of the shape of the
beam complicates the matter to some extent. For the event
presented in Sect. 4, such estimates are not feasible with the
Geotail data available and the comparison of the event tim-
ings in the absence of a more detailed knowledge of the shape
of the bow shock perturbation and beam is of no use.
5.3 Simulation model limitations
The main limitation of the simulation presented is its two-
dimensionality. Due to this configuration, all inflowing mag-
netic flux is forced to reconnect at the magnetopause and can-
not flow past without reconnecting, unlike in three dimen-
sions. This forces magnetic reconnection to be strong and
occur all the time at the magnetopause. Further, this likely
means that the magnetic islands carry more momentum and
thus drive stronger bow and stern waves into the magne-
tosheath than they would in three dimensions.
Additionally, the steady solar wind conditions in the sim-
ulation preclude any upstream turbulence, yielding a smooth
bow shock and no more downstream turbulence than the
anisotropy-driven wave activity in the magnetosheath. There-
fore the magnetic-island-driven fast waves can propagate rel-
atively unhindered in the magnetosheath and the localized
field-aligned beam is also very prominent in the solar wind.
More realistic turbulent conditions would certainly yield less
conspicuous signatures.
Nevertheless, none of these limitations mean that the fully
three-dimensional and turbulent case could not exhibit tran-
sient local foreshocks, they might only be more difficult to
detect and distinguish from other sources of bow shock and
foreshock perturbations or ion beams.
5.4 Observational limitations
The long chain of phenomena from the magnetopause
through the magnetosheath and bow shock to the foreshock,
constituting the scenario presented in this work, makes it
daunting to observe the whole cascade of a single event in
space and time. This would require the fortuitous availability
of adequate measurement data firstly at the magnetopause to
identify FTEs, secondly in the magnetosheath to single out
steepened fast wave fronts, and thirdly upstream in a narrow
region close to but definitively more than one ion gyrora-
dius away form both the bow shock foot and the regular fore-
shock edge, all of this during a stable southward IMF stretch
and in the absence of any magnetic field fluctuations which
could either drive an ion beam or produce a regular fore-
shock crossing at the upstream spacecraft instead. No suit-
able spacecraft were located in the magnetosheath or at the
magnetopause during the Geotail event presented above so
that a direct observation of fast-mode magnetosheath wave
fronts is not possible in this case.
As shown in Sect. 4, transient foreshock-like ion beams
upstream but well-separated from both the bow shock and
the foreshock are observed. At the same time, ground-based
measurements confirm that dayside reconnection was oc-
curring and producing FTEs propagating towards the poles.
Without adequate magnetosheath observations, it is not pos-
sible to claim with certainty that the complete scenario
mapped in Sect. 3 holds. Yet the observations are consistent
with the first and the last part of the story, namely that while
dayside reconnection is active and pulsed, a localized change
in the bow shock shape causes localized ion reflection and
the formation of a transient, local foreshock. It cannot be ex-
cluded that sources other than FTE-driven fast waves exist,
but it is unlikely there would be distinctive features helping
to tell them apart purely based on the observation of the ion
beam without other measurements, from within the magne-
tosheath for example. Such putative sources could include
unpredicted magnetosheath waves interacting with the bow
shock or very localized solar wind transients not observed by
the upstream spacecraft. Finally, we note that the presented
scenario offers an alternative mechanism to explain transient
foreshock encounters that may have been interpreted previ-
ously as foreshock skimmings due to a change in the mag-
netic connection of the spacecraft to the bow shock.
6 Conclusions
Global hybrid-Vlasov simulations of the terrestrial magneto-
sphere in the polar plane under constant southward IMF show
that the two-dimensional equivalents of flux transfer events,
formed by dayside magnetopause reconnection, drive steep-
ening bow and stern fast-mode waves in the magnetosheath.
The increased pressure behind the wave fronts causes the
bow shock to bulge out, inducing favourable ion reflection
conditions which can result in the generation of local, tran-
sient foreshock-like field-aligned ion beams upstream of the
bow shock. The two-dimensionality of the simulation exac-
erbates the phenomena, but the scenario would be entirely
similar in three dimensions.
www.ann-geophys.net/34/943/2016/ Ann. Geophys., 34, 943–959, 2016
956 Y. Pfau-Kempf et al.: Transient local foreshocks
Ground-based and spacecraft observations support this
scenario. During an extended period of stable southward
IMF, we observe ionospheric signatures of dayside reconnec-
tion and flux transfer events in SuperDARN radar data and
IMAGE magnetometer data. Simultaneously, using Geotail
magnetic field and ion velocity distribution measurements we
observe the expected signature of an ion beam detached both
from the bow shock and the regular foreshock and not linked
to any upstream magnetic field fluctuation. Further observa-
tions especially in the magnetosheath are needed though to
confirm that indeed fast-mode waves lead to bow shock de-
formations generating localized, transient field-aligned ion
beams.
7 Data availability
The simulation dataset is available on request from the
Vlasiator team (http://vlasiator.fmi.fi, von Alfthan et al.,
2014). IMAGE magnetometer data are available from http:
//www.space.fmi.fi/image (Tanskanen, 2009). The AACGM
software is available from http://engineering.dartmouth.edu/
superdarn/aacgm.html (Shepherd, 2014). The SuperDARN
data can be accessed from the SuperDARN data portal hosted
by Virginia Tech at http://vt.superdarn.org (Greenwald et al.,
1995).
The Supplement related to this article is available online
at doi:10.5194/angeo-34-943-2016-supplement.
Author contributions. Sebastian von Alfthan and Yann Pfau-Kempf
designed and ran the simulation presented in this work. Heli Hietala
helped in the analysis of the Geotail observations. Steve E. Milan
provided the analysis and figure of the SuperDARN observations.
Liisa Juusola provided the analysis and figure of the IMAGE obser-
vations. Yann Pfau-Kempf, Heli Hietala, Sanni Hoilijoki, Urs Ganse
and Sebastian von Alfthan developed analysis tools used in this
study. Minna Palmroth is the Principal Investigator of the Vlasia-
tor team at the Finnish Meteorological Institute. Yann Pfau-Kempf
led the analysis and prepared the manuscript and figures with the
help of all co-authors.
Acknowledgements. The simulation was run on the Sisu supercom-
puter at the CSC – IT Center for Science, Espoo, Finland.
We thank T. Mukai at ISAS, JAXA in Japan for providing Geo-
tail/LEP data; S. Kokubun at STELAB, Nagoya University, Japan
for providing Geotail/MGF data; A. Szabo at NASA/GSFC for pro-
viding Wind/MFI data; K. Ogilvie at NASA/GSFC for providing
Wind/SWE data; R. Lin and S. Bale at UC Berkeley for providing
Wind/3DP data; M. L. Kaiser at GSFC for providing Wind/WAVES
data; N. Ness at Bartol Research Institute for providing ACE/MAG
data; D. J. McComas at SWRI for providing ACE/SWEPAM data;
and J. H. King and N. Papatashvili at AdnetSystems and GSFC for
providing OMNI data, all through CDAweb. We also thank T. Na-
gai and Y. Saito for providing Geotail/MGF and Geotail/LEP data
through DARTS at ISAS, JAXA in Japan. We thank the University
of Kyoto for providing the AE electrojet indices through the World
Data Center for Geomagnetism.
We acknowledge the use of SuperDARN data. SuperDARN is a
collection of radars funded by national scientific funding agencies
of Australia, Canada, China, France, Japan, South Africa, United
Kingdom and United States of America.
We thank the institutes who maintain the IMAGE magnetometer
array.
Yann Pfau-Kempf, Sanni Hoilijoki, Sebastian von Alfthan
and Minna Palmroth acknowledge financial support from the
Academy of Finland under the project 267144/Vlasov. Part of this
study was done by Yann Pfau-Kempf, Liisa Juusola, Urs Ganse,
Sanni Hoilijoki and Minna Palmroth under the ERC CoG-682068-
PRESTISSIMO project, a Consolidator grant to Minna Palmroth
from the European Research Council. The work of Heli Hietala is
funded by NASA contract NAS5-02099. Steve E. Milan was sup-
ported by the Science and Technology Facilities Council (STFC),
UK, grant no. ST/N000749/1. Urs Ganse acknowledges funding
from the German Research Foundation Grant GA1968/1 and the
Academy of Finland project 267186.
The topical editor, C. Owen, thanks two anonymous referees for
help in evaluating this paper.
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