Download Event I: January 31, 2008 (Quiet)

EGU2012-1991
Magnetic Flux Transport and
Pressure Variations at Magnetotail
Plasma Flow Bursts during
Geomagnetically Quiet Times
Motoharu Nowada (野和田 基晴: [email protected])1
Sui-yan Fu1, George K. Parks2, Zu-yin Pu1,
Vassilis Angelopoulos3, Charles W. Carlson2,
Hans-Ulrich Auster4
1: ISPAT/SESS, Peking University
2: SSL, University of California, Berkeley
3: IGPP, University of California Los Angeles
4: Technische Universität, Braunschweig, Deutsuland
Abstract
The fast plasma flows in the geomagnetotail are observed during both geomagnetically
active and quiet times. However, it has been unclear about the fundamental difference in
the plasma fast flows between at two different geomagnetic conditions, that is, the
generation mechanism of, and pictures of the energy transport and balance at the fast
plasma flows. Magnetic reconnection in the magnetotail has been believed as one of the
most possible mechanisms to generate the fast plasma flows regardless of the geomagnetic
conditions. Recently, Nowada et al. [2012], however, demonstrated that the magnetotail
magnetic reconnection does not always contribute to the generation of the fast plasma
flows at geomagnetically quiet times based on the THEMIS measurements. It is very
important to reveal how the energy transport and balance in the magnetotail in association
with these plasma fast flows are on obtaining a clue to elucidate an essential difference in
the plasma fast flows between during active and quiet geomagnetic conditions.
Based on three events of the magnetotail plasma flow bursts, which are transient fast
plasma flows with the durations between 1 and 2 minutes, during geomagnetically quiet
times, observed by THEMIS, we examined detailed variations of the electric field as a
proxy of the flux transport aspect, and associated pressure. The main characteristics of
these events are shown as follows; 1) the GSM-X component of the plasma velocity (Vx) was
higher than 300 km/s 2) associated parallel (V//) and perpendicular (V⊥) velocities to the
local magnetic field line were higher than 200 km/s 3) the flow bursts were observed
during which |AL| and AU indices were lower than 20 nT, and simultaneous Kp index range
was between -1 and 1. For almost events, the parallel (E//) and perpendicular (E⊥)
components of the electric field to the local magnetic field line were much stronger than
the dawn-dusk electric field component (Ey). This result implies that a larger amount of the
magnetic flux was transported into the parallel and perpendicular directions to the local
magnetic field line than the dawn-dusk direction at the flow bursts. However, in the Ey
component, the contribution from the dawn-to-dusk electric field (VxBz) was much greater
than that from the dusk-to-dawn component (VzBx). Furthermore, for two events,
significant reduction of the plasma pressure, and enhancement of the north-south magnetic
field component (Bz) were observed at/near the flow bursts. Simultaneous total pressure
was well-balanced, indicating that the magnetotail during the plasma flow bursts was in the
state of equilibrium. Based on these results, “bubble” might play a crucial role for
generating the plasma flow bursts at geomagnetically quiet times.
Based on our previous results …
[Nowada et al., 2012]
Under geomagnetically quiet conditions…
1. “Magnetotail Magnetic Reconnection” might
not be necessary condition to generate the
fast plasma flows (plasma flow bursts).
2. The fast plasma flows (plasma flow bursts)
occur in association with not only “pure
(typical) substorm” but also “substorm-like
phenomena”, such as pseudo-substorm.
Locations and Orbits of the THEMIS probes
Event I: January 31, 2008
- Plasma flow bursts are
periodically occurring.
- The magnetic field
perturbations are
observed in association
with the plasma flow
bursts. Significant
enhancements of the Bz
components (magnetotail
dipolarization signature)
are observed at/near the
plasma flow bursts.
- Plasma density and
temperature are variable
during the plasma flow
bursts.
- The Vx component is
almost consistent with the
V// component.
Event I: January 31, 2008
Each electric field
component as a proxy
parameter of the
magnetic flux transport
aspect is derived by
following formulae:
E// > Eperp
E// > Eperp
Parallel magnetic
flux transport is
the most dominant.
Parallel magnetic
flux transport is
the most dominant.
Ey = VxBz – VzBx
E⊥ = V⊥Bz
E// = V//Bz
Variations of all electric
field components are
almost following those of
the velocity components,
suggesting that the
magnetic flux (magnetic
energy) is efficiently
transported in association
with the plasma flow
bursts.
Event I: January 31, 2008
Magnetic Pressure (Pm):
2
Bt
20
Plasma Pressure (Pp): NpkTi
Total Pressure (Ptot): Pm + Pp
Both Pp and Pm show
significant variations
associated with the plasma
velocity. During the plasma
flow bursts, Pp shows
REDUCTION and
simultaneous Pm indicates
ENHANCEMENT.
In THEMIS-D, Ptot has been
almost CONSTANT. The Ptot
values in THEMIS-E are also
almost background level
(stable), except for a plasma
flow burst on ～09:48 UT.
Event II: March 27, 2009
- The first plasma flow burst is
observed by THEMIS-B, but is
not observed by THEMIS-C.
- During all plasma flow bursts,
the Bz enhancements
(dipolarization signatures) are
observed.
-The second plasma flow burst
observed by THEMIS-B is also
seen by THEMIS-C, but
associated magnetic field and
plasma variations are different
between the THEMIS-B and
THEMIS-C observations.
-The third plasma flow burst is
observed by only THEMIS-B
- The Beta (β) values at the
second plasma flow burst are
different between the two
probes’ observations.
Event II: March 27, 2009
The electric field
varies in association
with some plasma flow
burst events.
However, during the
other plasma flow
bursts, the electric
field does not show
significant variations.
The Ey component is the
largest, although
E// > Eperp and E// ≈ Eperp
are observed during the
plasma flow bursts.
Event II: March 27, 2009
In association with
the plasma flow
bursts…
Pm: Enhancement
Pp: Reduction
Ptot: Background Level
(stable)
Pm: Reduction
Pp: Enhancement
Ptot: Variable
Event III: March 24, 2010
- Plasma flow bursts are
observed during the periodical
plasma velocity fluctuations.
- Gradual (THEMIS-D) and
sharp (THEMIS-E)
enhancements of the Bz
components are observed
before the plasma flow bursts.
- The magnetic field
perturbations are observed in
association with the plasma
flow bursts
- The plasma density and
temperature of THEMIS-E are
variable during the plasma flow
burst. However, their variations
in THEMIS-D are almost stable.
- The Vx component is almost
consistent with the Vperp
component.
Event III: March 24, 2010
The electric field
components in both
THEMIS-D and
THEMIS-E indicate
significant
enhancements in
association with the
plasma flow bursts.
During the plasma
flow bursts, the
case of Eperp > E// is
dominant.
Event III: March 24, 2010
During the plasma
flow burst intervals,
Pm shows significant
enhancement, and
simultaneous Pp
indicates reduction.
Associated Ptot is
stable (its value is
background level).
Contribution of Dawn-Dusk Oriented Electric Field (VxBz) in
the Ey component
Ey = VxBz – VzBx
Large contribution by
the plasma flow bursts
Dawn-Dusk and DuskDawn oriented electric
field components
 Earth-Sunward and
North-Southward
magnetic flux transport
aspects
Large contribution
Little contribution
Large contribution by
the plasma flow bursts
Ey = VxBz
Dawn-Dusk oriented
electric field component
 Earth-Sunward
magnetic flux transport
aspect (Contribution by
the plasma flow bursts)
Conclusion (I)
We investigated the magnetic flux transport
aspects and pressure variations/balance during
the plasma flow bursts at geomagnetically
quiet times…
- In Events I and III, E//, Eperp > Ey was seen,
suggesting that the magnetic flux (magnetic
energy) is dominantly transported to the
field-aligned and perpendicular directions to
the local magnetic field line.
- In almost cases, the plasma flow bursts play an
important role for the transport of the
magnetic flux in the magnetotail.
Conclusion (II)
- Significant enhancements of the Bz component were
observed at/near the plasma flow bursts.
- In Events I and III, significant reduction
(enhancement) of the plasma pressure (the magnetic
pressure) was seen in association with the plasma
flow bursts.
- Simultaneous total pressure was almost constant (was
consistent with the background level), suggesting
that the plasma flow bursts were occurring under the
state of equilibrium.
From these results, “bubble” might play a crucial role
for the generation of the plasma flow bursts during
the geomagnetically quiet times.
Back Up Slides
Plasma Flow Bursts in magnetotail…
1. The sun-earthward plasma velocity component (Vx)
and the perpendicular/parallel plasma velocity
components to the local magnetic field line (V⊥or V//),
whose ranges are higher than 300 km/s, are frequently
observed around -10 Re ~ -30 Re in the magnetotail
plasma sheet.
2. The durations for these fast plasma flows are
typically ~ 10 min., which are identified as Bursty Bulk
Flow (BBF), but the fast plasma flows with much
shorter duration (within 5 min.) are also observed.
These transient fast plasma flows are called as “Plasma
Flow Burst”.
3. Interestingly, these fast plasma flows are observed
during geomagnetically both active and quiet times.
Fast Plasma flows during geomagnetically quiet times
Result of the GEOTAIL observations
from January 1997 to April 1998 (1.4
years). Only 4 fast plasma flows from
41 geomagnetic quiet conditions, which
were identified by absence of
significant auroral brightening with
POLAR UVI observations, were
selected [Ieda et al., 2003].
Also statistical result with the
GEOTAIL observations from
September 1993 to December 1994
(1.4 years). The durations for the fast
plasma flows at geomagentically quiet
times, identified as |AL| < 100 nT,
were 446 hours (corresponding to
about 19 days) [Ohtani et al., 2002].
Main characteristics of these plasma flow
bursts at geomagnetically quiet times
- THEMIS
1. The Vx component is higher than 300 km/s, and
associated parallel (V//) and perpendicular (V⊥)
velocities to the local magnetic field line are
higher than 200 km/s. The fast flow duration is
between 1 and 2 minutes.
2. GSM-X < 0 [Re] (Nightside) and
-10 < GSM-Y < 10 [Re] (to avoid the MPCL crossing
data).
3. The β value is higher than or nearly 0.5.
- Ground
4. Kp index range is between -1 and +1 and AL/AU
indices are lower than ±20 nT.
3 Events of Magnetotail plasma flow bursts at
geomagnetically quiet times
Event
#
Date
Time Intervals (UT)
Flow
Orientation
THEMIS
Probes
I
2008/01/31
09:45 – 10:01
Earthward
P3 (D) and
P4 (E)
II
2009/03/27
00:10 – 00:19 (TH-B)
00:56:00 – 00:58:30 (TH-B)
00:59 – 01:02 (TH-C)
Earthward
P1 (B) and
P2 (C)
III
2010/03/24
04:25:40 – 04:30:30
Earthward
P3 (D) and
P4(E)
Contribution of Dawn-Dusk Oriented Electric Field (VxBz) in
the Ey component and comparison with the direct measurement
Ey = VxBz – VzBx
Dawn-Dusk and Dusk-Dawn
oriented electric field
components
 Earth-Sunward and
North-Southward
magnetic flux transport
aspects
Ey = V xB z
Dawn-Dusk oriented
electric field component
 Earth-Sunward
magnetic flux transport
aspect (Contribution by
the plasma flow bursts)
Ey: A direct measurement
with EFI onboard
THEMIS