Transcript S. Savin

Structure of the magnetopause at the ion gyroscale: mass and momentum transfer
S. Savin (1), E. Amata (2), M. Andre (3), M. Dunlop (4), Y. Khotyaintsev (3), J. Buechner (5), J. Blecki (6), J.L. Rauch (7)
(1) Russia IKI, Moscow, (2) IFSI, Roma, Italy, (3) IRFU, Uppsala, Sweden, (4) RAL, UK, (5) MPSP, Germany, (6) SRC, Warsaw, Poland, (7)
LPCE, Orleans, France; ([email protected])
Non-stationary plasma
jets in MSH:
from MHD towads
gyroscale
Momentum transfer by ion
gyroscale effect
Middle- and fine- scale MP structure: magnetic field
rotation and cross-field acceleration (left); ion
gyroscale current sheet (TCS) originated from Hall
effect
Comparison of Ex
and Hall term inside
MP: green line
shows the Hall-term
X-projection, the
black one – Ex; in
the middle of the
TCS one can see that
the that the spike in
[j x B]x/ m0en ~ Ex
and ion flux nVi ; GDCF- gasdynamic model
In the structured plasma jets ion kinetic
energy density (Wkin) and plasma flux
(nVi) can highly exceed that of solar wind
(SW) and the flux in MSH, predicted by
models. It is in surprising contrast to a
MHD bow shock behavior. Closer to MP,
nVi tends to approach to the SW value,
the jets look to constitute a substantial
sink of the flow, which plays a major role
into the mass balance near MP.
A jet with dynamic pressure over that of
magnetic field (Wb) should deform MP
and – via secondary reconnection of the
deformed fields – provide an input into
mass and momentum transfer across MP
Cluster orbit on
Ferruary 13, 2001
Left: partial ion densities and Ex electric
field (green,Cluster 1). Middle (from top):
magnetic field, clock angle & Ex electric
field from 4 Clusters; light blue lines mark
MP current sheets
Energy densities: Wkin- ion kinetic, Wbmagnetic,
Averaged in space, the jets carry the
momentum difference between measured
and predicted by GDCF ion fluxes
Cascade-like properties of
the turbulence
POSTER
C181
IAGA200
5- A00452
Top: Ion density (blue), partial density for
energies > 300 eV (black) and that of > 1 keV
(violet). Bottom: partial ion flux, dashed lines –
z-component, full lines – y, lines with blue dashes
– x; color–coding like at the top.
The charged Thin Current Sheets (TCS) serve to support
selfconsistently the transverse Hall current, separating
two plasmas at ion gyroscales (i.e. without any
'anomalous' resistivity or a 'diffusion region'). It does
not necessarily imply 'classic' reconnection with parallel
electric field, while reconnection should include the ionscale layers. In other words, a charged TCS just because
of its ion-gyroradius width becomes partially
transparent for the larger-energy ions and respective
magnetic flux without any change of the field topology
(which could be superimposed or not). Transport of
momentum across the layers is also provided by ions
with larger gyroradius without any magnetic field
annihilation. The momentum transfer (Stasiewicz,
Space Sci. Rev., 65, 221, 1994) in terms of gyroviscosity in the case of anti-parallel fields, predicts a
forcing of the boundary Earthward without a macroreconnection. Over the cusp and especially at the
boundary of 'plasma balls‘, the gyro-stress rises due to
|B| minimum and the acceleration of the flow around
MP towards the tail; thus, the MP inward motion due to
this effect should have a maximum at the sunward edge
of the cusp for the IMF Bz < 0 in our case. In spite of
sampling of the MP outside the maximum gyro-viscosity
effect, the data demonstrate clear momentum transfer
across the MP, especially in its component, parallel to
MP.
(a):
WHISPER
electric
field
spectrogr
am with
colorcoded
intensity
(scale at
the right
side);
Correlation of positive E`x
spikes (in the frame, moving
with MSH bulk velocity, bottom)
and plasma waves (20-60 kHz,
top) conforms to instability of
parallel electron currents,
neutralizing charge separation
due to inertial drift (cf. Genot,
et al., Ann. Geophys., 22, 20812096, 2004) and interaction of
ions of 0.5-5 keV with the
gyroscale electric structures.
The latter infers for such ions
effective collisions with loosing
of 100-300 eV per ‘collision’, i.e.
their deceleration in the MP
normal direction. These ions are
accelerated downstream along
MP by the inertial (polarization)
drift in the quasi DC
perpendicular electric field. The
lower-energy ions are
accelerated downstream by
both DC & spiky electric fields
(in the case of the detected
rotation of magnetic field just
outside MP)
From top: GSE electric
field (E*10) in MSH frame
(V= -45, -90, -150 km/s);
magnetic field and ion
velocity (dashed lines;
shifted by -100 units);
GSE Poynting flux (in
mW/m2), P2001_N_B
(green) along average
MVA normal, P2001_N_E along electric field
direction of maximum
variance, shifted by -300
units; electric drift-velocity
(b):
electric
field E`x
in the
MSH
frame
Acceleration of
MSH
plasma
tailward
by extra
normal
electric
field and
rotated
magnetic
field
MP structure at gyroscale: pile-up of low-energy ions and acceleration by perpendicular electric field of
the ions with gyroradius > MP width
MP current sheet structure Ion distribution function and cuts of
distributions of differential ion flux
at gyroscale (~100 km,
(Vpar- along magnetic field) at MP
circled)
Cuts of distributions of omni-directional
ions at 4-second resolution from Cluster 1
(left) and Cluster 3 (right) near MP
Cuts of distributions of
anti-sunward flowing
ions inside & outside
(black) MP; blue (red)
lines – outer ions,
accelerated by 300
(200) V potential at MP
Model magnetic field lines and
direction of different vectors for Cluster
on February 13, 2001.
Top: XZ GSE plane;
Insert A: a cartoon for MP deformation.
Bottom: YZ GSE plane;
Insert B: Scheme for conservation of
nVi (thick arrows) by ions with
gyroradius > MP width. ‘Jumping’ from
MSH (violet, over plane) with magnetic
field (thin arrows) ~ parallel to nVi,
into cusp (blue) the ions ~ conserve
momentum and start to rotate; the
magnetic field for high b is transported
by the ions; i.e. in cusp frame an
electric field is generated to fit the
plasma cross-field drift
Conclusions
A study of the magnetopause (MP) fine structure from the
Cluster and Interball data highlights the fundamental role
of the finite-gyroradius effects, surface charges and
accelerated plasma jets.
In
the MP boundary layers the accelerated jets provide nonlocal flow balance via the small-scale electric fields,
supported at the MP substructures by parallel electron
currents, and via the respective rotation of magnetic field.
The complicated MP shape suggests its systematic
velocity departure from the local normal towards the
average one. The electric fields in the magnetosheath
(MSH) frame accelerate the MSH plasma along MP
downstream so that the plasma excess is removed close
to the moving MP. The electric bursts provide effective
collisions for ceasing of the MSH normal flows just in
front of MP, the collisions result in the ion heating. Over
outer cusp the MSH flows interact with a high-beta
boundary layer through reflected waves, visible as
sunward bursts in Poynting flux. The waves have 3- wave
phase coupling with both enhanced MSH waves and local
Alfvenic fluctuations. The most prominent local impulsive
momentum loss via accelerated plasma jets qualitatively
differs from bow shock or reconnection processes. Its
input into the total MSH mass balance reaches 1/3.
Kinetic energy of the jets can substantially exceed the
magnetic energy at the high-latitude MP, which should
result in the MP deformation and driven reconnection. A
kind of wave-particle interaction is operating at transient
small-scale current sheets with surface charges. At
scales of ion gyroradius it infers Hall dynamics, so that
electric fields of the surface charges serve as a
mechanism for momentum coupling through the current
sheets and lead to acceleration/ deceleration of ions with
large (relative to the sheet width) gyroradius. Work was
supported by INTAS grant 03-50-4872.