On the Generation of Enhanced Sunward Convection and Transpolar Aurora in the High-

Latitude 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, USA2Applied Physics Laboratory, Johns Hopkins University, Laurel, MD, USA

3Lockheed Martin Advanced Technology Center, Palo Alto, CA, USA4GeoForschungsZentrum, Potsdam, Germany

5AFRL, Hanscom AFB, MA, USA6NASA/GSFC, Greenbelt, MD, USA

7Rutherford Appleton Laboratory, Chilton, UK8Centre d’Etude Spatiale des Rayonnements, Toulouse, France

9Space Sciences Laboratory, University of California, Berkeley, CA, USA10The 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

BackgroundSuggested 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….

BackgroundSuggested 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….

Downward currentUpward current

?

?

R2R1

R2 R1

IMAGE WICTime period: 00:01 – 04:43 UT

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

Shue et al. [1998] magnetopause (extreme Bz and Pdyn)

BxByBz

0

2042*

2cos

arctan

lvB

Bz

By

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 Geotail time shift (dt=4.6 min)

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

R1

R2

dawndusk

R1 R2R1

TPA

TPA

TPA

TPA

Reversed cusp energy-latitude dispersion

Cluster 2001-12-1601:10-03:30 UT

Bz

By

Bx

a b c

Walen Analyses

High-correlation Walen slopes at the Alfven speed indicating a rotational discontinuity across the magnetopause.

0/ Bvv HT

Quantitative Walen Analyses in Inertial Frame

0/ Bv

x-compy-compz-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

[from Vennerstrom et al., Field-aligned currents during northward IMF: Morphology and causes, JGR, 2005]

Blue: antiparallel/upwardRed: parallel/downward

NBZ Current

Evolution

[from Vennerstrom et al., Field-aligned currents during northward IMF: Morphology and causes, JGR, 2005]

Blue: antiparallel/upwardRed: parallel/downward

NBZ Current

Evolution

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