Transcript Cambridge 2004 Magnetotail Transport and Substorms Wolfgang Baumjohann IWF/ÖAW Graz, Austria With help from: R.

Cambridge 2004
Magnetotail Transport
and Substorms
Wolfgang Baumjohann
IWF/ÖAW Graz, Austria
With help from: R. Nakamura, A. Runov, Y. Asano & V.A. Sergeev
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Sun-Earth Connection
2
Standard Model
Magnetospheric convection is driven by solar wind.
 Merging of
dipolar field
with southward
IMF at MP
 Open field lines
move tailward
over polar cap
 Reconnection to
dipolar & SW
field lines in tail
 Closed field
lines move
sunward in
equatorial plane
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Inner Magnetosphere
In inner magnetosphere quasi-static convection:
 GEOS-2 Electron Beam Experiment measures "shift" of gyration
circle of 1 keV electrons by electrical drift
 Southward IMF: convection towards magnetopause
 Northward IMF: only weak plasma flow
 Mean values for southward IMF correspond to standard model
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Lobe Convection - 1
 Cluster/EDI gives first direct measurements of convection in lobe
(measuring electric field in extremely thin plasma over polar cap)
 Cluster Electron
Drift Instrument
(EDI) uses same
principle as
GEOS-2 Electron
Beam Experiment
 Dependence of
convection
velocity toward
plasma sheet
on polarity of
IMF BZ clearly
visible
5
Lobe Convection - 2
 EDI data
also show
IMF BY
effect

Shear
flow in
Y-Z
plane
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Magnetotail
Tail observations with IMP show:
 Fast Earthward flow
for 25-40 RE
 Closer in, convection
severely slows down
Pressure Catastrophe:
 Adiabatic convection:
g
d/dt PV = 0, V = B -1ds
 Flux tube volume strongly
decreases toward Earth
 Convection stops to avoid
pressure catastrophe
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Near-Earth Neutral Line
 Tail-like field geometry
weakens pressure gradient
 Reconnection leads to
smaller flux tube volume
 Earthward convection by
bursty bulk flows
 Reversal of fast
flow direction
observed by
Geotail
 Near-Earth
neutral line
located ~25 RE
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Flow & Curv B Reversal

Magnetic field
components in
Cluster barycentre:
4 current sheet
traversals

Field line curvature:
curv B = (b.grad)b

Flow and field line
curvature reversal

X-line moves
tailward over
Cluster
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Reconnection & Hall Effect
 Ion Flow Reversal during 4 neutral sheet crossings

X-line moves
tailward over
Cluster

500 km thin CS
around X-line

bifurcated current
sheet on both
sides

Hall effect (By)
during ‘outer’
crossings shows
ion decoupling
10
Electron Hall Current
 Cluster 2003 tail passes can resolve fine structure of currents
 JY shows very
thin current
sheet (triple
peaks?)
 JX consistent
with electron
Hall current
in ion
diffusion
region
11
Currents at PSBL and X-line
 Strong flow shear (N-S electric field)
and thin field aligned current layer
(1500 km) during a substorm
 Consequence of Hall-effects in
reconnection region and closure
of the Hall-electric current
Ion diffusion region
12
Braking & Dipolarization
 Intermittent high-
speed Earthward
flow bursts
(~500 km/s)
 Bursty fast flows
accompanied by
dipolarization
 Fast flow braked near
10 RE by dipolar field
 More dipolar flux
added by flow
 Pressure gradients
lead to current
wedge and aurora
13
Aurora & Electrojet
Flow braking and flow shear generate:
 Substorm electrojet
(adds to convection
electrojet)
 Aurora (in upward
field-aligned current
region electrons are
accelerated downward)
14
Flow Channel Width
 Cluster gives direct estimate of spatial scale of bursty bulk flows
 Vertical: 1.5-2 RE, Azimuthal: 2-3 RE
 Sharper gradient on duskside flank
15
Aurora and Bursty Bulk Flow
 Isolated flow bursts with
E>2mV/m (Geotail) always
correspond to auroral
activations (Polar).
 Auroral activations near foot
point of satellite start within 1
min of flow burst onset.
16
Structure of Flow Bursts
Spatial scale of flows
Small expansion, pseudo-breakup 1.4 MLT  (4-5 RE)
Auroral streamer (N-S aurora)
0.7 MLT  (3-4 RE)

Flow bursts are centered 0.4 MLT east of aurora
17
Flow Bursts and FAC
 Aurora corresponds to upward FAC in bubble model
 Scale size consistent
with ionospheric
observations
18
Substorm Recovery
Fast flows transport dipolar field inward:
 45 min after onset dipolarization front meets neutral line
 No reconnection in dipolar field; recovery phase begins
 Neutral line retreating tailward
19
Summary Scenario
 Plasma energy dominant outside of 25 RE
 Flow uninhibited
 Magnetic energy increases near 20-25 RE
 Pending pressure catastrophe leads to NENL
 Magnetic field dominant inside 15 RE
 Dipolar field brakes BBF: current wedge &
aurora are generated
 Dipolarization
front travels
downtail and
meets NENL
 near-Earth
reconnection
stops
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Thanks…
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