On the Generation of Enhanced Sunward
Convection and Transpolar Aurora in the HighLatitude Ionosphere by Magnetic Merging
S. Eriksson1, J. B. H. Baker2, S. M. Petrinec3, H. Wang4,
F. J. Rich5, M. Kuznetsova6, M. W. Dunlop7, H. Reme8,
R. A. Greenwald2, H. U. Frey9, H. Luhr4, R. E. Ergun1,
A. Balogh10, and C. W. Carlson9
1Laboratory
for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA
2Applied Physics Laboratory, Johns Hopkins University, Laurel, MD, USA
3Lockheed Martin Advanced Technology Center, Palo Alto, CA, USA
4GeoForschungsZentrum, Potsdam, Germany
5AFRL, Hanscom AFB, MA, USA
6NASA/GSFC, Greenbelt, MD, USA
7Rutherford Appleton Laboratory, Chilton, UK
8Centre d’Etude Spatiale des Rayonnements, Toulouse, France
9Space Sciences Laboratory, University of California, Berkeley, CA, USA
10The Blackett Laboratory, Imperial College, London, UK
eriksson@lasp.colorado.edu for 2005JA011149 reprint
http://lasp.colorado.edu/~eriksson
Outline
• Background
• Observations and MHD simulations
– Data set: IMAGE WIC, SuperDARN, CHAMP, DMSP F14,
FAST, and Cluster
– Solar wind and IMF from ACE and GEOTAIL
– Cluster on the Northern Hemisphere duskside flank
on 16 December 2001
– MHD simulations from BATSRUS via
http://ccmc.gsfc.nasa.gov
• Summary & Conclusions
Background
Suggested convection in the northern
hemisphere during IMF By>0 by
Burch et al. [1985]. Is it really valid?
Where do we find the field-aligned
currents (FAC)?
Are transpolar auroral arcs generated by
upward FAC sheets poleward of the
region 1 system?
Are these upward FAC sheets generated
by lobe reconnection?
On the basis of this data set, we can
begin to answer these outstanding
questions….
Background
Suggested convection in the northern
hemisphere during IMF By>0 by
Burch et al. [1985]. Is it really valid?
R2
R1
R1
R2
Where do we find the field-aligned
currents (FAC)?
?
?
Are transpolar auroral arcs generated by
upward FAC sheets poleward of the
region 1 system?
Are these upward FAC sheets generated
by lobe reconnection?
Upward current
Downward current
On the basis of this data set, we can
begin to answer these outstanding
questions….
IMAGE WIC
Time period: 00:01 – 04:43 UT
Shue et al. [1998] magnetopause (extreme Bz and Pdyn)
Cluster magnetopause crossing into the magnetosheath on the
duskside northern hemisphere flank.
Geotail upstream location:
Solar wind Vx=-525 km/s and dX=19.2 Re results in dt=4 min
Geotail delayed by 4 min 36 s
so that IMF clock angle data
match with Cluster
ACE delayed by estimated
solar wind propagation to
X=10 RE (dt=65 min) and
Bx Geotail time shift (dt=4.6 min)
By
Bz
 By 

 Bz 
  arctan 
2
   l0
  vB cos  
 2  0
*
2
4
SuperDARN: 0246 UT to 0324 UT
Sunward ionospheric high-speed flows are aligned with the dayside
portion of transpolar aurora (TPA), while the nightside region of TPA
near the Harang discontinuity indicate tailward flows in agreement with
Nielsen et al. [JGR, 1990].
Sunward flow is associated with a high-latitude lobe convection cell.
Could the Harang discontinuity be related to strong lobe convection?
[from Koskinen and Pulkkinen, JGR, 1995]
Divergence of gradient/curvature drift current JGC results in a field-aligned
Birkeland current that depends on pressure gradient and/or flux volume gradient
[Vasyliunas, 1970; Erickson et al., JGR, 1991]. This FAC is directed away from the
ionosphere in the northern hemisphere.
From dusk ….
…. to dawn
dusk
dawn
R1
R2
R1
R1
TPA
R2
TPA
TPA
TPA
Reversed cusp energy-latitude dispersion
Cluster 2001-12-16
01:10-03:30 UT
a
b
c
By
Bz
Bx
Walen Analyses
High-correlation Walen slopes at the
Alfven speed indicating a rotational
discontinuity across the magnetopause.

 
v  v HT   B / 0
Quantitative Walen Analyses in Inertial Frame


v   B / 0
x-comp
y-comp
z-comp
Direction to X-line (-dV)
from SC1 assuming that
dV ~ j x B
MHD Simulation
BATSRUS
Cluster C1 position
M-I Coupling
Schematic representation of localized
reconnection with associated Alfven wave pulse
and field-aligned current system [Glassmeier
and Stellmacher, Adv. Space Res., 1996].
Suggested ionospheric flow and current
pattern due to the motion of FTE flux
tube through the ionosphere [Southwood,
JGR, 1987].
Vx (km/s)
Vy (km/s)
Tracing Field Lines in MHD
NBZ
Current
Evolution
[from Vennerstrom et
al., Field-aligned currents
during northward IMF:
Morphology and causes,
JGR, 2005]
Blue: antiparallel/upward
Red: parallel/downward
NBZ
Current
Evolution
[from Vennerstrom et
al., Field-aligned currents
during northward IMF:
Morphology and causes,
JGR, 2005]
Blue: antiparallel/upward
Red: parallel/downward
Summary & Conclusions
• A quasi-steady duskside TPA in IMAGE data is consistent with a
dayside pair of FACs (upward at higher latitudes) in the low-altitude
CHAMP and FAST data (duskside sector). These are known as the
NBZ current system [Iijima and Shibaji, JGR, 1987].
• SuperDARN confirms the presence of a sunward ionospheric
convection channel over the dayside part of the TPA. The nightside
part of the TPA is related to tailward convection and/or the
stagnation region equatorward of the Harang Discontinuity [Nielsen
et al., JGR, 1990; Erickson et al., JGR, 1991].
• The Walen test and the analysis of plasma acceleration as Cluster
crosses the magnetopause together with MHD simulations confirm
the presence of an active merging region in the direction of the MHD
“sash” (minimum Bfield magnitude). Consistent with reconnection for
duskward and northward IMF [e.g. Luhmann et al., 1984].
Summary & Conclusions
• MHD simulations confirm the merging scenario for the generation of
sunward convection as observed by SuperDARN and the system of
NBZ FACs on either side of the merging site and the sunward driven
flow as detected by CHAMP and FAST [e.g. Southwood, JGR,
1987].
• We suggest that quasi-continuous high-latitude (lobe) reconnection
sustain the dayside TPA.
• The nightside section of the TPA is related to the electrodynamics of
the Harang Discontinuity which likely is affected by the lobe cell
convection.
• Contact eriksson@lasp.colorado.edu for 2005JA011149 reprint –
also available at http://lasp.colorado.edu/~eriksson