MSSL Seminar

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Tail Reconnection during IMF-Northward
Non-substorm Intervals (TRINNIs) Adrian
Grocott Mullard Space Science Laboratory
Seminar 30 January 2007
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Introduction - Background

• In general, southward interplanetary magnetic
  field (IMF) intervals are associated with
  Dungey-cycle flow and the substorm cycle
• Northward IMF is usually associated with lobe
  reconnection and flow vortices within the polar
  cap
• However, it is found that for IMF clock angles of
  30 - 45 open flux production at the dayside can
  persist (e.g. Sandholt et al., 1998 Nishida et
  al.,1998)
• During such intervals no substorms are observed,
  so what is the nightside response to a modest
  dayside driving?

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Nightside flows during northward IMF
Bursts of equatorward-directed flow accompanied
by an activation of the poleward-most arc system
(De la Beaujardière et al., 1994) North-south
aligned auroral streamers associated with
bursty-bulk flows (BBFs) (Henderson et al.,
1998) Poleward boundary auroral intensifications
(PBIs) and arc bifurcations (Lyons et al.,
1999) Quasi-sinusoidal nightside flow
oscillations - global oscillations of an
essentially closed tail? (Huang et al.,
2001) Narrow bursts of high-speed flow in the
pre-midnight sector interpreted as a
viscously-driven twin-vortex flow system (Walker
et al., 1998, 2002) Surges of westward return
flow in the dusk convection cell
(Senior et al., 2002)
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Outline

• Brief overview of large-scale magnetospheric-ionos
  pheric convection The Dungey Cycle
• Observing large-scale convection using
  high-frequency ionospheric radars - SuperDARN
• Nightside, large-scale, ionospheric convection
  during IMF-northward intervals the significance
  and importance of TRINNIs
• Case study 1 An interhemispheric study of
  TRINNI flows
• Case study 2 Multi-scale observations of TRINNIs

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Dungey Cycle Convection
x
x
dayside reconnection
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Dungey Cycle Convection
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Dungey Cycle Convection
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Dungey Cycle Convection
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Dungey Cycle Convection
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Dungey Cycle Convection
nightside reconnection
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Dungey Cycle Convection
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Dungey Cycle Convection
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Dungey Cycle Convection
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SuperDARN Super Dual Auroral Radar Network
The CUTLASS Finland Radar
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SuperDARN Super Dual Auroral Radar Network
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HF radars Propagation of radio waves in the
ionosphere and the backscatter technique 1.
Low-latitude VHF F-region orthogonality 2.
High-latitude VHF E-region orthogonality 3.
High-latitude VHF F-region reflection /
transmission 4. High-latitde HF F-region
orthogonality
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12 MLT
SuperDARN Iceland West (Stokkseyri)
1505 UT 21 Jan 2001
12 MLT
12 MLT
12 MLT
WIDTH
VEL
POWER
18 MLT
18 MLT
18 MLT
Field-of-view 16 beams, 75 range gates, 45 km
gate length, max. range 3500 km, f-o-v area
5x106 km2, 1 min scan time Advantages-
Large area covered high time resolution Disadvan
tages- Line-of-sight velocity only
backscatter not ubiquitous
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Map-potential technique Developed by
APL Combine sparse l-o-s velocities to give
estimate of instantaneous, global potential
pattern
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Introduction TRINNIs
Substorm TRINNI
from Grocott et al. (2002, 2003)
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Importance of TRINNIs

• Grocott et al. (2003) and Milan et al. (2005)
  have demonstrated the importance of TRINNIs in
  closing significant amounts of open flux
• Various models have been suggested which might
  put our TRINNI observations into global
  perspective (e.g. Nishida et al., 1998 Tanaka,
  1999 Watanabe et al., 2004)

after Milan et al. (2005)
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Northward IMF open flux production
We propose a simple model of open flux production
under northward IMF, in line with Nishida et al.
(1998), in which field lines with a significant
anti-parallel component reconnect
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Tail Reconnection Mechanism

• Dungey-cycle like asymmetric twin-cell convection
  flows
• Prolonged exposure to By-dominated IMF results in
  asymmetric tail lobes
• For, e.g. IMF By-positive, tail field lines are
  displaced duskward in the northern hemisphere...

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Tail Reconnection Mechanism

• Dungey-cycle like asymmetric twin-cell convection
  flows
• Prolonged exposure to By-dominated IMF results in
  asymmetric tail lobes
• For, e.g. IMF By-positive, tail field lines are
  displaced duskward in the northern hemisphere
• and dawnward in the southern hemisphere

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Tail Reconnection Mechanism

• Footpoints of reconnected field lines in one
  hemisphere have a longer path in the ionosphere
• Gives rise to bursts of fast azimuthal flow in
  the midnight sector
• Bursts are NOT geomagnetically conjugate -
  therefore may not be simultaneous in both
  hemispheres

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Tail Reconnection Mechanism

• For IMF By ve (-ve)
• Field lines which cross the equatorial plane
  post-midnight, red, return to the dayside via
  dawn producing a N(S)-H flow burst
• Those crossing pre-midnight, green, return via
  dusk producing a S(N)-H flow burst

(b) after Nishida et al., 1998
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TRINNIs and Transpolar Arcs
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Interhemispheric study of TRINNI flows (Grocott
et al. 2005a)

• ACE magnetic field data from both intervals
  between 1800 and 0400 UT show IMF Bz to have been
  predominantly positive
• By was negative during one interval and
  predominantly positive during the second
• The magnitude of the IMF was greater during the
  negative IMF interval

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SuperDARN Potential Maps
00 MLT
00 MLT

• IMF By negative
• Northern hemisphere data show fast westward
  return flows
• Southern hemisphere data show fast eastward
  return flows

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SuperDARN Potential Maps
00 MLT
00 MLT

• IMF By positive
• Northern hemisphere data show fast eastward
  return flows
• Southern hemisphere data show fast westward
  return flows

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SuperDARN Potential Maps
Velocity (m s -1)
00 MLT
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SuperDARN Velocity Time-Series

• Enhancements in the flow in both hemispheres
  during both intervals are evident on 30min 1h
  timescales
• Northern and southern flow enhancements are not
  always coincident
• Variability in the flow does not generally appear
  to correlate with variability in radar data
  coverage

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Multi-scale observations of a BBF (Grocott et
al., 2005b)
BY
BZ
BX
Ionospheric radars show large-scale
convection Cluster Spacecraft reveal localised
small-scale flows
C1 C2 C3 C4
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Multi-scale observations of TRINNIs (Grocott et
al., 2007)
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Multi-scale observations of TRINNIs (Grocott et
al., 2007)
BZ
BX
BY
C1 C2 C3 C4
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Multi-scale observations of TRINNIs (Grocott et
al., 2007)
BY
BZ
BX
C1 C2 C3 C4
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Schematic summary of observations
Schematic representations of tail field line
configurations for different IMF BY (after
Nishida et al., 1998)
(a) S/C YGSM -ve -gt V?Y and SuperDARN VAZ
both -ve / dawnward (b) S/C YGSM -ve -gt V?Y
and SuperDARN VAZ both ve / duskward (c) S/C
YGSM ve -gt V?Y and SuperDARN VAZ both ve /
duskward in the N-H -ve / dawnward
in the S-H
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BBF azimuthal direction vs. spacecraft position
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Ionospheric vs. in-situ response to TRINNI
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Conclusions

• During northward, but By-dominated, IMF modest
  open flux production at the dayside magnetopause
  continues to drive Dungey-cycle convection, but
  at a rate insufficient to produce substorms
• Prolonged exposure to such conditions results in
  bursts of fast flow in the nightside
  high-latitude ionosphere which are largely
  azimuthal in direction and opposite in each
  hemisphere
• These flows we associate with bursts of nightside
  reconnection in an asymmetric magnetotail, this
  asymmetry resulting from the penetration of
  IMF-BY into the tail
• In-situ magnetospheric observations of the tail
  reveal that the large-scale IMF-BY control of the
  tail is apparent on the smaller scale and that
  BBF-type activity is still the dominant flux
  transport mechanism

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Values of Flux Transport

• The substorm discussed here closed 0.2 GWb of
  flux in 60 mins
• This equates to a net flux closure rate of 55 kV
• The associated BBF transported 2?106 Wb RE-1 in
  5 mins
• This equates to a flux transport rate of 6.7 kV
  RE-1
• So, if BBF activity at this rate transported all
  the closed flux it would have to occur over 8 RE
• Peak, local, electric fields were 25 mV m-1 in
  the flow channel in the ionosphere and 5 mV m-1
  at Cluster
• TRINNIs have been observed to close flux at a
  somewhat smaller rate of 35 kV, closing about
  0.5?108 Wb in 20 mins intervals.
• However, the BBFs observed in association with
  these TRINNIs transported a comparable amount of
  flux 2?106 Wb RE-1
• So, either less BBFs are involved in the flux
  transport for TRINNIs, or they occur over much
  narrower azimuthal extent