ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-36-1171-2018Magnetosheath jet properties and evolution as determined by a global hybrid-Vlasov simulationMagnetosheath jetsPalmrothMinnaminna.palmroth@helsinki.fihttps://orcid.org/0000-0003-4857-1227HietalaHelihttps://orcid.org/0000-0002-3039-1255PlaschkeFerdinandhttps://orcid.org/0000-0002-5104-6282ArcherMartinhttps://orcid.org/0000-0003-1556-4573KarlssonTomasBlanco-CanoXóchitlhttps://orcid.org/0000-0001-7171-0673SibeckDavidKajdičPrimožGanseUrsPfau-KempfYannhttps://orcid.org/0000-0001-5793-7070BattarbeeMarkushttps://orcid.org/0000-0001-7055-551XTurcLucilehttps://orcid.org/0000-0002-7576-3251Department of Physics, University of Helsinki, Helsinki, FinlandSpace and Earth Observation Centre, Finnish Meteorological Institute, Helsinki, FinlandDepartment of Physics and Astronomy, University of Turku, Turku, FinlandDepartment of Earth, Planetary, and Space Sciences, University of California, Los Angeles, USAInstitute of Physics, University of Graz, Graz, AustriaSpace Research Institute, Austrian Academy of Sciences, Graz, AustriaThe Blackett Laboratory, Imperial College London, London, UKSchool of Physics and Astronomy, Queen Mary University of London, London, UKSchool of Electrical Engineering and Computer Science, KTH Royal Institute of Technology, Stockholm, SwedenInstituto de Geofísica, Universidad Nacional Autónoma de México, Mexico City, MexicoCode 674, NASA Goddard Space Flight Center, Greenbelt, MD, USAMinna Palmroth (minna.palmroth@helsinki.fi)7September2018365117111822March20188March201813June201828August2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://angeo.copernicus.org/articles/36/1171/2018/angeo-36-1171-2018.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/36/1171/2018/angeo-36-1171-2018.pdf
We use a global hybrid-Vlasov simulation for the magnetosphere, Vlasiator, to
investigate magnetosheath high-speed jets. Unlike many other hybrid-kinetic
simulations, Vlasiator includes an unscaled geomagnetic dipole, indicating
that the simulation spatial and temporal dimensions can be given in SI units
without scaling. Thus, for the first time, this allows investigating the
magnetosheath jet properties and comparing them directly with the observed
jets within the Earth's magnetosheath. In the run shown in this paper, the
interplanetary magnetic field (IMF) cone angle is 30∘, and a
foreshock develops upstream of the quasi-parallel magnetosheath. We visually
detect a structure with high dynamic pressure propagating from the bow shock
through the magnetosheath. The structure is confirmed as a jet using three
different criteria, which have been adopted in previous observational
studies. We compare these criteria against the simulation results. We find
that the magnetosheath jet is an elongated structure extending earthward from
the bow shock by ∼2.6RE, while its size perpendicular to
the direction of propagation is ∼0.5RE. We also
investigate the jet evolution and find that the jet originates due to the
interaction of the bow shock with a high-dynamic-pressure structure that
reproduces observational features associated with a short, large-amplitude
magnetic structure (SLAMS). The simulation shows that magnetosheath jets can
develop also under steady IMF, as inferred by observational studies. To our
knowledge, this paper therefore shows the first global kinetic simulation of
a magnetosheath jet, which is in accordance with three observational jet
criteria and is caused by a SLAMS advecting towards the bow shock.
Introduction
Earth's magnetosphere is surrounded by the magnetosheath, which consists of
shocked and turbulent plasma of solar wind origin. The sunward boundary of
this region is the bow shock through which the solar wind plasma flows into
the magnetosheath. The earthward boundary of the magnetosheath is the
magnetopause, the outer edge of Earth's magnetosphere. The bow shock and
magnetosheath plasma properties relative to those in the upstream pristine
solar wind depend broadly on the interplanetary magnetic field (IMF)
direction. One of the most important defining conditions within the
magnetosheath is the angle between the bow shock normal and the IMF. In
portions of the bow shock, where the bow shock normal lies more or less
parallel to the IMF direction, the bow shock is said to be
quasi-parallel. At the quasi-parallel shock, part of the solar wind
particles reflect back towards the Sun , causing
instabilities and waves upstream, and forming a so-called foreshock. The
region downstream from the quasi-parallel shock is called the quasi-parallel
magnetosheath, where the plasma properties are highly turbulent (e.g.,
). On the other hand, the region downstream from the
quasi-perpendicular side is less turbulent. There is no foreshock upstream
from the quasi-perpendicular bow shock because IMF lines keep the reflected
particles close to the bow shock and the waves do not have time to grow.
Nevertheless, the magnetosheath downstream from the quasi-perpendicular bow
shock hosts a variety of locally generated waves, e.g., mirror mode waves
.
reported observations of peaks in the ion fluxes within the
quasi-parallel magnetosheath, which they termed transient flux enhancements.
Several studies have since (i) investigated the properties of these high-speed
structures that have been termed as magnetosheath jets and (ii) demonstrated
their importance in terms of geoefficiency. They can for example distort the
magnetopause and drive magnetospheric dynamics because
they can trigger magnetopause reconnection . Statistical
investigations of the jets show that they are clearly associated with the
foreshock and the quasi-parallel magnetosheath (e.g., ). therefore suggested that foreshock waves may be
related to the origin of the jets. proposed a mechanism to
produce the jets by a rippled bow shock, which collimates particles into a
high-speed structure. suggested that the jets could be
associated with foreshock a short, large amplitude magnetic structure (SLAMS,
) originating from steepening foreshock waves and
traveling through the bow shock.
Originally, the jets were observationally identified by high velocities
e.g.,. In recent years the vast majority of
observational studies have used dynamic pressure and not velocity as the key
quantity, although a level of agreement is expected due to the quadratic
dependence of the velocity in the dynamic pressure. devised a
criterion CP, defined as the ratio of the magnetosheath dynamic pressure
in the X direction to the upstream solar wind dynamic pressure.
defined that, in order to represent jets, CP had to fulfil
the condition
CP=ρvX2ρswvsw2>0.25,
where ρ is the density, vX is the velocity component in the -X
direction, the numerator refers to the conditions in the magnetosheath while
the denominator represents solar wind conditions, and the subscript “sw”
denotes the solar wind. The coordinate system that they used was the
Geocentric Solar Ecliptic (GSE), where X is sunward, Z is perpendicular
to the ecliptic plane and is positive northward, and Y completes the
righthanded system. The Plaschke criterion CP
(i) defines the jet as the entire region where Eq. () holds and
(ii) requires that the dynamic pressure peak is >0.5 times the solar wind
value. Further, the criterion is applied only for solar zenith angles less
than 30∘.
used the total dynamic pressure but divided by a 20 min
temporal average of the dynamic pressure within the surrounding
magnetosheath and required that
CA=ρv2<ρshvsh2>20min>2,
where the brackets indicate a temporal average. investigated
enhancements in the magnetosheath density, which they called plasmoids. They
separated the plasmoids according to their speed and remarked that the fast
plasmoids whose local velocity increased at least 10 % could be associated
with jets. They defined the events by taking ratios of the magnetosheath
electron density to a 15 min temporal average within the magnetosheath as
CK=ne<ne>15min>1.5,
where ne is the electron density in the magnetosheath. Both
CA and CK are only defined to identify peak values of
the relevant parameters, and when durations or spatial scales were identified
the full width at half maximum was used. Jets identified with the three
criteria are in broad agreement with respect to occurrence and properties,
suggesting that the criteria identify similar phenomena. This motivates a
modeling study to test how similar the three criteria in fact are and
whether they all are associated with magnetosheath jets.
performed local hybrid-particle-in-cell (PIC) simulations
within a limited spatial extent and found that the solar wind Alfvén Mach
number is important in determining how far the jets can penetrate within the
magnetosheath. Using a 2-D hybrid-PIC code, observed elongated
structures with higher magnetic field and plasma density traversing from the
foreshock to the magnetosheath. However, since use a scaled
dipole strength in the hybrid-PIC model, representative of roughly a
Mercury-sized magnetosphere, deducing the scale sizes of the structures from
the simulation results is not straightforward, and their direct comparison to
the jets observed in the Earth's magnetosheath is difficult. Nevertheless,
reported that jet scales parallel to the direction of
propagation could be ∼2.4RE and in the perpendicular
direction ∼0.3RE within the Earth's magnetosheath.
Observationally, estimate that the characteristic jet sizes are
1.34 RE by 0.71 RE.
This paper employs the hybrid-Vlasov simulation code Vlasiator to investigate
the jet properties. Vlasiator includes ion-kinetic features similar to
hybrid-PIC codes, but unlike hybrid-PIC codes does not include sampling
noise in the results due to a different modeling approach. Further,
Vlasiator uses the actual unscaled geomagnetic dipole strength as a boundary
condition, and therefore the results can be given in RE and
seconds without scaling, indicating that the length and timescales can be
directly compared to spacecraft observations of jets. In this paper, we first
introduce Vlasiator and the run used to examine the magnetosheath jets. We
visually identify a candidate jet, after which we show that our candidate jet
fulfils all three jet criteria described above. We then (i) examine the jet
properties and evolution and (ii) analyze the process that generates the jet,
before ending the paper with discussion and conclusions.
Model
Vlasiator is a hybrid-Vlasov model for global simulations of the Earth's
magnetosphere. Vlasiator solvers treat protons as a distribution function
f(r,v,t) in phase space and electrons as a massless
charge-neutralizing fluid . Electron
kinetic effects are neglected by the solvers, but the ion kinetic effects are
solved without numerical noise. The time evolution of f(r,v,t) is controlled by the Vlasov equation, propagated by a fifth-order
accurate semi-Lagrangian approach . Maxwell's equations
neglecting the displacement current in the Ampère–Maxwell law are used to
solve the electromagnetic fields. Maxwell's equations are supplemented by
Ohm's law, including the Hall term. The technical features of the code
including the closure scheme, the numerical approach, and the parallelization
techniques are described by in the previous version using the
finite volume method, while here and in an updated
semi-Lagrangian scheme is used (see also ).
The run is carried out in the ecliptic XY plane of the Geocentric Solar
Ecliptic coordinate system, representing a two-dimensional (2-D) approach in
ordinary space. Each ordinary space simulation cell includes a 3-D velocity
space (3V) used to describe the proton velocity distribution. Therefore, the
approach here is 2-D–3V in total. The
simulation plane in the run used in this paper ranges from -7.9 to
46.8 RE in X and ±31.3RE in Y, with a
resolution of 228 km corresponding to the typical ion inertial length in the
solar wind. The velocity space resolution is 30 km s-1. The solar wind
parameters are given as an input at the sunward wall of the simulation box,
while copy conditions are applied at other boundaries. The Z direction in
ordinary space applies periodic conditions. The inner edge of the
magnetospheric domain is a circle with a radius of 5 RE, while
the ionosphere is a perfect conductor in the present version of the code. The
same run has also been used to examine magnetosheath mirror mode waves by
, with a general agreement to existing knowledge of the
phenomenon.
The solar wind parameters in this run are as follows: solar wind distribution
functions are assumed Maxwellian, with an initial temperature of 0.5 MK. The
IMF has a cone angle of 30∘, the IMF x component is -4.33 nT,
IMF y is 2.5 nT, and the total magnetic field intensity is 5 nT. The
solar wind density is 1 cm-3, and the velocity is 750 km s-1 in
the -X direction. The combination
of the solar wind parameters has been chosen to facilitate on the one hand
the relatively fast initialization of the simulation to save in the total
computational load and on the other hand the realistic representation of the
foreshock. With these solar wind parameters, the upstream Alfvén Mach
number becomes 7, well inside the normal range of Alfvén Mach numbers at
the Earth . Thus, we can trust the foreshock physics and
consequently its interactions with the bow shock. The combination of solar
wind values yields a relatively low dynamic pressure of about 1 nPa;
however, this dynamic pressure or lower is observed about 23 % of the time
under quasi-radial IMF throughout the solar cycle, based on OMNI solar wind
data. Observational statistics show a slight tendency for jets to occur for
higher solar wind speeds and lower densities than usual ,
indicating that our solar wind parameter set represents the conditions under
which the magnetosheath jets occur.
(a) Dynamic pressure within the Vlasiator simulation domain. Bow shock
position is identified with white solid line. The black dashed lines are
solar wind streamlines illustrating the magnetopause position roughly (see
text for details). The figure is a snapshot of Supplement movie S1, which
does not include the bow shock position or the streamlines. The arrow
indicates the visually detected magnetosheath jet under scrutiny in this
paper. (b) Virtual spacecraft data from the location marked with a white dot
in panel (a): magnetic field, velocity, density, and dynamic pressure as a
function of time. The dashed vertical line shows the time of the visually
identified jet in panel (a).
Before going to the results, we note that the bow shock moves gradually
upstream in all 2-D hybrid-kinetic models. There are two reasons for this.
First, the magnetosheath magnetic field piles up in front of the magnetopause
because in 2-D it cannot slip around the magnetosphere towards the nightside
as in reality. Secondly, there is an artificial heating in the hybrid-kinetic
simulations due to numerical diffusion. This feature is relatively minor in
Vlasiator, and the numerical heating does not contribute significantly to the
gradual expansion of the bow shock e.g.,.
Results
Figure a shows a close-up of the Vlasiator simulation
domain investigated in this paper. It shows a snapshot of a movie S1,
depicting the dynamic pressure at time t=305.5 s from the beginning of
the run. Color coding shows the dynamic pressure. To guide the eye,
Fig. a also includes the bow shock position as a white
solid line, depicting the location where the density is twice the solar wind
density. The shock compression ratio is about 3–4 at Earth, making the
density gradient at the shock quite sharp, and therefore the bow shock
position can be shown in this simple manner. As for the magnetopause
position, we first note that in this 2-D–3V simulation it is not realistic to
expect that the magnetopause position agrees exactly with the empirical
proxies. Further, in magnetohydrodynamic (MHD) simulations, such as in
GUMICS-4 , the location of the magnetopause depends upon the
parameter by which it is defined. The so-called fluopause, determined by an
average of solar wind streamlines deflecting around the magnetosphere, is a
good proxy for the magnetopause, well in accordance with empirical proxies
. Therefore, we also show the streamlines in
Fig a to illustrate roughly the dimensions of the magnetosheath
in this run. Following , the subsolar magnetopause would be
determined by neglecting the innermost streamline at around
7 RE and by taking an average of the next ones towards
upstream, placing the magnetopause using this proxy to somewhere around
10 RE.
Based on movie S1, we visually identified a high-pressure structure emerging
from the bow shock surface and extending through the magnetosheath, marked
with a white arrow in Fig. a. The movie S1 shows both the
beginning and the end of the visually identified feature. As we shall
describe in this paper, the feature is associated with a higher dynamic
pressure advecting towards the bow shock and reaching it at around t = 282
s. On the other hand, at t=325–340 s, the visually identified feature
seems to be associated with a transient wave or an oscillation, which
originates approximately at X,Y= [7.5, -4]. This transient follows
from the arrival of the remnant of the visually identified feature, and two
pulses traveling away from the impact point are visible. In a 2-D–3V
simulation, we do not wish to confirm whether features close to the
magnetopause are realistic due to the pile-up effect described above.
However, from movie S1 it is clear that the visually identified feature is
certainly a transient event having a distinct lifetime. It has such a large
dynamic pressure that it pushes ambient plasma and has an impact downstream.
Therefore, we take this feature into a closer scrutiny in order to conclude
about its relevance to the magnetosheath jets.
Color coding shows the dynamic pressure calculated using the
X component of velocity, vX, divided by the solar wind dynamic pressure
using the solar wind vX. The black contour shows where this Plaschke
criterion exceeds 0.25, and white shows where it exceeds 0.5, as defined in
.
The white dot in Fig. a at X,Y= [9.5,
-4.2] RE shows the earthward edge of this structure, from
which we show virtual spacecraft data in Fig. b. The
virtual spacecraft data in Fig. b show that the velocity
increased roughly by 20 %, density roughly by 50 %, while the dynamic
pressure roughly doubled at the time of the structure in Fig a, marked by a vertical dashed line.
(a) The criterion defined in Eq. (),
devised from the ratio of (b) the dynamic pressure and (c) the temporal average
of dynamic pressure over 3 min centered at the time showing the
jet-like feature in Fig. a. Panel (a) shows a contour
marking the locations where the ratio of panel (b) and (c) exceeds 2. Panels (b)
and (c) have the same scale, from 0 to 1.5 nPa.
Figure shows the Plaschke criterion CP defined
in Eq. () in a spatially limited zoom of Fig. .
The color coding shows the dynamic pressure ratio between the magnetosheath
and solar wind, using the X component of the velocity vX. The black
contour shows where this quantity exceeds 0.25, while the white contour shows
the area where the quantity exceeds 0.5 in line with . The
structure in Fig. can be observed as an elongated feature
starting from the bow shock and extending to the left towards the
magnetopause in Fig. approximately at X,Y= [10,
-4] RE.
(a) The Karlsson criterion in Eq. ()
, devised from the ratio of (b) the density at
t=305.5 s and the (c) temporal average of density over 3 min, centered at the time showing the jet-like
feature in Fig. a. Panel (a) shows a contour marking locations
where the ratio of panel (b) and (c) exceeds 1.5. Panels
(b) and (c) have the same scale, from 0 to 6 particles in a
cubic centimeter.
Using the same zoom as Fig. , Fig. shows the
Archer and Horbury criterion CA (Eq. ), which is a
ratio of the dynamic pressure and the temporal average of dynamic pressure.
Figure a shows
this ratio, Fig. b presents the dynamic pressure (the numerator
of the criterion), and Fig. c shows the temporal average of
dynamic pressure (the denominator of the criterion). While
originally used a 20 min average in the denominator, here we use a
3 min temporal average, centered on time t=305.5 s. This is
solely because the simulation interval does not last for 20 min, and while
testing different values this 3 min average was found to be the
shortest period identifying the structure, while having a manageable amount
of data. The contours in Fig. a show where the Archer and
Horbury criterion exceeds 2 and therefore where the dynamic pressure is
twice the temporal average. The largest area satisfying this criterion can be
found near the location X,Y= [9, -4] RE.
All criteria with density color coded at t=305.5 s. The
Karlsson criterion CK in Eq. () is given with magenta, the Archer
and Horbury criterion CA in Eq. () with blue, and the Plaschke
criterion CP in Eq. () with black.
Figure shows the Karlsson criterion CK
(Eq. ), namely the ratio of the instantaneous density to the
temporal average of the density. Figure a shows this ratio.
Figure b and c show the density and the temporal average of
density over 3 min, centered on the time t=305.5 s,
respectively. The contour in Fig. a shows locations where the
ratio exceeds 1.5, that is where the density is 50 % greater than the
temporal average. Figure a shows that the Karlsson criterion is
fulfilled mostly at the surface of the bow shock, while a small area of
higher density can be found at location X,Y= [9, -4] RE.
The jet area, radial, and tangential size as a function of time. The
area has been calculated based on both (i) the Archer and Horbury and (ii) the
Plaschke criteria, while the radial size is the subtraction of the maximum
and minimum radial distance of the jet boundary positions, reflecting the jet
maximum extent. The tangential size is the effective jet width, and it is
calculated by dividing the jet area by the radial size.
Finally, Fig. compares results for all the criteria, the
Karlsson criterion CK in Eq. () with magenta, the Archer
and Horbury criterion CA in Eq. () with blue, and the
Plaschke criterion in Eq. () with a black contour. The region we
visually identified from the movie S1 and which is indicated by an arrow in
Fig. a fulfils all three criteria approximately at X,Y= [10, -4] RE. Since the criteria agree, we call the feature
a magnetosheath jet and identify its physical dimensions and evolution in
time. We adopt an inclusive strategy and determine that the jet originates
at the bow shock with enhanced CK (magenta) criterion at X=11.6RE and reaches a location with enhanced CA
(blue) criterion at X=9.1RE. Taking into account the angle
at which the magnetosheath jet propagates from the bow shock towards the
magnetopause, its length is approximately 2.6 RE in the
direction of propagation. In the perpendicular direction, the jet size varies
from 0.6 RE at the bow shock, to 0.3 RE in the
mid-jet area, to ∼0.5RE at the magnetopause end. Since
Fig. represents a snapshot, we emphasize that these dimensions
are instantaneous values.
Jet evolution in time as a function of distance from the bow shock.
(a) An overview plot of the dynamic pressure with the Plaschke criterion with
black contour, at time t=305 s. The panel (a) shows three locations with a
green, red, and cyan star, at which virtual spacecraft data are given in
panel (b), showing from top to bottom the velocity, density, and dynamic
pressure against time. Color coding shows the data from the similarly
colored stars in panel (a).
Next we investigate the evolution of the jet size in time in
Fig. , continuing with the inclusive strategy. The panels of
Fig. present the jet area, radial size, and tangential size,
respectively. The area has been calculated such that both (i) the Archer and Horbury and (ii) the Plaschke criteria
delimit the jet, and the area is the
sum of the areas of the grid cells within the jet boundaries. The radial size
is simply the subtraction of the maximum and minimum radial distance of the
jet boundary positions, while the tangential size is the jet area divided by
the radial distance. Figure indicates that the area increases
and decreases during the jet lifetime and reaches its maximum just before
the time of the jet in Fig. . The radial size increases first as
the jet emerges from the bow shock, but then stays constant as it propagates
through the magnetosheath before the jet disperses away. The tangential size
remains below 1 RE on average for the most part of the jet
lifetime, but the increase in the tangential size at the end of the jet
lifetime suggests that it disperses into the tangential direction.
Time evolution of the jet. The color coding in the background shows
the total dynamic pressure, while the black contour shows the Plaschke
criterion CP computed using the X component of the velocity vX in
dynamic pressure. Panels (a) to (d) show times 275, 295, 300, and 310 s,
respectively, from the start of the simulation. The time of the jet at its
prime is shown in Fig. . The white arrows show the jet
generation and are referred to in the text. The panels are snapshots of
movie S2.
Figure investigates how the jet profile changes as a
function of distance from the bow shock. Figure a shows
an overview plot, with both the (i) Plaschke and (ii) Archer and Horbury criteria used to
delimit the jet. Figure a shows three colored stars in
the following positions: green = [9.2,-3.7], red = [10.0,-4.4],
and cyan = [10.8,-5.2]. Figure b shows velocity,
density, and dynamic pressure as a function of time at these three locations,
with similar color coding as the stars are given in
Fig. a. The full width at half maximum, which would be
measured by a spacecraft, changes from 14 to 8 s and 9 s from the bow shock
to the mid-jet and to the earthward tip, respectively. Converting these to
spatial scales by multiplying with the average velocity yields a spatial
size of 0.7–0.3 RE, respectively. Clearly, the velocities and
the dynamic pressures are greatest nearest the shock and decrease as the jet
propagates towards the magnetopause. The dynamic pressure decreases by 70 %
from the bow shock to the vicinity of the magnetopause, indicating that the
origin of the jet may be related to the dynamic pressure outside the bow
shock.
Figure examines what causes the jet, using the Plaschke
criterion. Figure a–d show the total dynamic pressure in the
background and the Plaschke criterion as a black contour at four times near
the time shown in Fig. . The panels are snapshots from
movie S2. In Fig. a, a high-pressure area shown
by the white arrow approaches the bow shock. This high-pressure structure
steepens towards the bow shock surface within a matter of seconds. At time t=295 s the structure has hit the bow shock, shown by the arrow in
Fig. b. In Fig. c and d this bulge extends
towards the magnetopause, and at time t=310 s it is already fading away.
Movie S2 shows this time sequence in a more dynamic fashion.
(a) An overview plot of the high-pressure structure that causes the
jet; color coding shows the dynamic pressure. The black dot marked by the
arrow shows the virtual spacecraft location, for which different parameters
are shown in panel (b). From top to bottom the virtual spacecraft parameters
are the X, Y, and Z components of the magnetic field; magnetic field
intensity B; density ρ; total speed v; and the dynamic pressure
pdyn. The parameters are plotted against time, and the time shown in the
panel (a) is given by a dashed vertical line.
Finally, we investigate the high-pressure structure that causes the jet in
more detail. Figure a shows the high-pressure feature
advecting towards the bow shock with the solar wind. The black dot near the
center of the high-pressure structure shows a point at which we take virtual
spacecraft data in Fig. b. The parameters in
Fig. b are chosen to facilitate a comparison to a SLAMS, which
shows an increase in the magnetic field by a factor of 2 or more and
contains a rotation of the magnetic field vector .
Figure b shows a 2-fold increase in both the density and the
magnetic field intensity when the structure passes the virtual spacecraft
location. The components of the magnetic field indicate that the structure
includes a clear rotation in the XZ plane. Therefore, we conclude that the
high-pressure structure that causes the jet reproduces the observational
criteria , suggesting that it is indeed a SLAMS.
Discussion
We have presented a Vlasiator simulation run in the ecliptic plane with a
30∘ IMF cone angle. We identify and study a magnetosheath jet and
verify its properties by comparing them to three observational criteria
. The fact that the structure we observed
fulfilled all three observational criteria indicates that the observations of
, , and indeed concern similar phenomena within the
magnetosheath. The fact most supporting the idea that our visually selected
event is indeed a magnetosheath jet is that all three criteria agree
spatially within the jet and that the identified region is continuous
starting from the shock surface and reaching towards the magnetopause.
Further, it has a limited lifetime during which the criteria are met within
the same region, suggesting that the origin has to do with temporal changes
that are connected by the three criteria. While we have concentrated on one
jet, there are many more candidate jets in this Vlasiator run that satisfy
the different criteria, as shown by the movies S1 and S2. This and other runs
carried out with Vlasiator will allow statistical investigations looking into
the evolution of the jets as a function of their position within the
magnetosheath, their size distribution, and how these parameters depend on
the driving conditions.
We find that the jet size in the direction of propagation is at maximum
2.6 RE, while in the perpendicular direction it is
∼0.5RE in size. These dimensions are in agreement with
previous scaled results given in ion inertial lengths within a hybrid-PIC
simulation with roughly a Mercury-size magnetic dipole, assuming typical
magnetosheath properties in order to convert the results into Earth radii
. estimate the characteristic size of the jets to
be 1.34 RE by 0.71 RE, while the jet in this paper
is within the range of the jet sizes reported by them. Contrary to
observations, in the simulation the entire jet can be measured and the flow
parallel direction can be identified. Spacecraft will rarely cross the jet
along the axis of largest extent. Thus, an exact match between observationally
identified and modeled jets is not to be expected, but the fact that they
broadly agree suggests that the modeled jet can be examined in more detail,
and conclusions about its properties can be related to the observations.
It is interesting to compare the different observational criteria in
Eqs. ()–() in light of the simulation results shown
here. According to , the Archer and Horbury criterion is most
inclusive, identifying the largest number of jets, while the Karlsson
criterion is most strict, identifying the smallest number of jets (or
plasmoids). We have not rigorously tested how large the areas where the three criteria
are valid within the magnetosheath are, as we have concentrated on finding a structure that could be
identified as a jet with the present observational criteria. We note,
however, that based on Fig. both (i) the and
(ii) the criteria identify larger regions than the Karlsson
criterion, which indeed seems to be the most strict in the simulation
overall. It is also interesting to note that while the widely accepted term
“jet” has a connotation of an elongated feature, according to the results
shown here, the and criteria delineate
features shaped more like blobs. Without vast fleets of observing satellites,
it falls on a combination of observational and simulational efforts to infer
the shapes and dimensions of jets. Further modeling studies of the jet size
distributions will be necessary in order to assess this point.
We find that the Karlsson criterion is mostly fulfilled near the bow shock
surface, and it seldom reaches the magnetosheath portions close to the
magnetopause. On the contrary, the criterion identifies regions
closest to the magnetopause but can be found to be satisfied throughout the
magnetosheath, agreeing with the observational statistics. These
characteristics might be associated with the solar wind driving conditions in
our run. Neither nor specify the solar wind
conditions for their events, while our event is associated with a solar wind
density of 1 cm-3. The Archer and Horbury criterion is determined by
the dynamic pressure, which depends on the square of the velocity, which in
our simulation is rather high in the solar wind, 750 km s-1. While
both criteria concern ratios that can be enhanced during a variety of driving
conditions, it is possible that in the conditions of this run the Karlsson
high-density plasmoids are either not properly generated or cannot propagate
deep in the magnetosheath, while the Archer and Horbury pressure enhancements
could traverse further towards the magnetopause due to the faster general
velocities in the magnetosheath. In accordance with , the
Plaschke criterion in our results is most enhanced near the bow shock. This
may be because it is based on the X component of dynamic pressure: the
general magnetosheath flow pattern starts to deviate from the X direction
near the shock. Further, the jets push ambient magnetosheath plasma out of
their way in order to reach the magnetopause, decelerating them to a level
that no longer satisfies the Plaschke criterion. As we also show that the
dynamic pressure rapidly decreases as a function of distance from the bow
shock, to observe jets closer to the magnetopause it may be better to choose
the criterion.
Both ultra-low-frequency (ULF) waves and SLAMS are common
in the foreshock, where they advect towards the bow shock e.g.,and
references therein. By looking at the movie S2 and
Figs. and , we find that the jet in question is
formed by the interaction of a high-pressure structure with the bow shock.
The pressure enhancement has a larger pressure than its neighbors, and it is
elongated along the X axis and wider in Y than other foreshock
fluctuations within the run sequence. Based on virtual spacecraft data taken
from the structure, we conclude that its characteristics reproduce the main
features of a SLAMS. As the bow shock already shows an initial dent before
the SLAMS arrives, the SLAMS can pass the bow shock with little braking and
can propagate deep into the magnetosheath. In contrast, we refer to another
larger pressure fluctuation that reaches the bow shock at about t=351 s
(at X,Y≈ [11, -3.5] RE, see movie S2). The bow
shock is not dented upon the arrival of this fluctuation and therefore the
resulting jet-like structure does not grow large or propagate very deep
within the magnetosheath.
used a 2-D hybrid-PIC simulation to associate magnetosheath
jet-like structures with foreshock ULF waves. The jets reported by
almost reach the magnetopause, and they are associated with
high dynamic pressures. The authors note that “these regions are not
associated with high flow speeds and are instead caused by the density
enhancements associated with the magnetosheath filamentary structures”.
Without a rigorous comparison to the data in we cannot be sure
that the features in their simulation and the ones shown here concern the
same physics and whether therefore the origins of the structures can be
related. However, we do note that in our simulations the higher dynamic
pressure regions within the magnetosheath, which we call the magnetosheath
jets, are associated with high velocities. Further, carried out
a local 2-D hybrid-PIC simulation with a planar shock to investigate a
jet-like feature. They associated the jet-like feature with the upstream ULF
waves and made a note that it may originate due to a “SLAMS-like” feature
interacting with the bow shock. The present study takes these
previous numerical works further by providing a global simulation of the formation
and evolution of magnetosheath jets in the real magnetospheric scales,
directly comparable to those observed by Earth-orbiting spacecraft. This
allows us to rigorously compare the jet with existing observational criteria
and also to identify the structure causing the jet as a SLAMS. To our
knowledge, this is the first time this type of study has been carried out.
As for the generation of the jets, suggested a mechanism, which
relies on an assumption of a rippled shock surface that actively funnels
particles into a collimated structure having a high velocity, propagating
towards the magnetopause. discussed the origins of such a
ripple and remarked that, while rippling is inherent to the quasi-parallel
shock, one possible origin for the ripple would be a SLAMS convecting towards
the bow shock and interacting with it. In contrast, suggested
that foreshock SLAMS could essentially travel through the bow shock and
maintain its higher pressure, if there is an original dent or corrugation at
the bow shock surface to which that SLAMS hits. The jet generation we have
investigated here is directly associated with a SLAMS coming into contact
with a dented bow shock, after which that SLAMS essentially continues through
the magnetosheath as a structure that resembles a jet, which fulfils the jet
observational criteria. Therefore, our results confirm the
scenario for this single jet. However, this does not rule out other possible
generation mechanisms that may also be in action.
Conclusions
We investigated magnetosheath high-speed jets in a hybrid-Vlasov simulation
done at scales directly comparable to the Earth's magnetosphere. We identify
structures in the simulation that can be related to the magnetosheath jets
using three different observational criteria. We examine one such jet in more
detail and find that its maximum size is 2.6 and ∼0.5RE in
the direction parallel and perpendicular to the propagation direction,
respectively. The jet is caused by a SLAMS structure traveling through the
bow shock.
Vlasiator (http://helsinki.fi/vlasiator, Palmroth,
2008) is distributed under the GPL-2 open-source license at
https://github.com/fmihpc/vlasiator/ (Palmroth et al., 2018). Vlasiator
uses a data structure developed in-house
(https://github.com/fmihpc/vlsv/, Sandroos, 2018), which is compatible
with the VisIt visualization software (Childs et al., 2012) using a plugin
available at the VLSV repository. The Analysator software, available at
https://github.com/fmihpc/analysator/ (Hannuksela and the Vlasiator
team, 2018), was used to produce the presented figures. The run described
here takes several terabytes of disk space and is kept in storage maintained
within the CSC – IT Center for Science. Data presented in this paper can be
accessed by following the data policy on the Vlasiator web site.
The supplement related to this article is available online at: https://doi.org/10.5194/angeo-36-1171-2018-supplement.
This paper was outlined and drafted in the
International Space Science Institute team led by
HH and FP. MP, HH, FP, MA, TK, XB-C, DS, and PK were part of the team and
outlined the paper in team discussions. UG, YPK, MB, and LT were instrumental
in the generation of the simulation data set which was used for the analysis.
MP drafted and wrote the paper, and all others commented. MP is the principal
investigator of the Vlasiator team. The code was developed by her initiative.
The authors declare that they have no conflict of
interest.
Acknowledgements
We acknowledge the European Research Council for Starting grant
200141-QuESpace, with which Vlasiator (http://helsinki.fi/vlasiator;
last access: 4 September 2018) was developed, and Consolidator grant
682068-PRESTISSIMO awarded to further develop Vlasiator and use it for
scientific investigations. We gratefully also acknowledge the Finnish Centre
of Excellence in Research of Sustainable Space (Academy of Finland grant
numbers 312351, 267144, and 309937). Primož Kajdič's work was
supported by DGAPA/PAPIIT grant IA104416. The CSC – IT Center for Science in
Finland is acknowledged for the Grand Challenge award leading to the results
shown in here. We acknowledge valuable discussions within the International
Space Science Institute (ISSI) team 350, called “Jets downstream of
collisionless shocks”, led by Ferdinand Plaschke and Heli Hietala.
Lucile Turc acknowledges Marie Sklodowska-Curie grant 704681. Heli Hietala
was supported by the Turku Collegium for Science and Medicine and NASA
NNX17AI45G. We thank Jonas Suni for producing data for the figures in the
revised version. The
topical editor, Christopher Mouikis, thanks two anonymous referees for help
in evaluating this paper.
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