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On the Generation of Enhanced Sunward Convection and Transpolar Aurora in the High- Latitude Ionosphere by Magnetic Merging S. Eriksson 1, J. B. H. Baker.

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Presentation on theme: "On the Generation of Enhanced Sunward Convection and Transpolar Aurora in the High- Latitude Ionosphere by Magnetic Merging S. Eriksson 1, J. B. H. Baker."— Presentation transcript:

1 On the Generation of Enhanced Sunward Convection and Transpolar Aurora in the High- Latitude Ionosphere by Magnetic Merging S. Eriksson 1, J. B. H. Baker 2, S. M. Petrinec 3, H. Wang 4, F. J. Rich 5, M. Kuznetsova 6, M. W. Dunlop 7, H. Reme 8, R. A. Greenwald 2, H. U. Frey 9, H. Luhr 4, R. E. Ergun 1, A. Balogh 10, and C. W. Carlson 9 1 Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA 2 Applied Physics Laboratory, Johns Hopkins University, Laurel, MD, USA 3 Lockheed Martin Advanced Technology Center, Palo Alto, CA, USA 4 GeoForschungsZentrum, Potsdam, Germany 5 AFRL, Hanscom AFB, MA, USA 6 NASA/GSFC, Greenbelt, MD, USA 7 Rutherford Appleton Laboratory, Chilton, UK 8 Centre d’Etude Spatiale des Rayonnements, Toulouse, France 9 Space Sciences Laboratory, University of California, Berkeley, CA, USA 10 The Blackett Laboratory, Imperial College, London, UK for 2005JA reprint

2 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 Summary & Conclusions

3 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….

4 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…. Downward current Upward current ? ? R2 R1 R2 R1

5 IMAGE WIC Time period: 00:01 – 04:43 UT

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7 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)

8 Bx By Bz 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)

9 SuperDARN: 0246 UT to 0324 UT

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14 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?

15 [from Koskinen and Pulkkinen, JGR, 1995]

16 Divergence of gradient/curvature drift current J GC 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.

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18 From dusk ….…. to dawn

19 R1 R2 dawndusk

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21 R1R2R1 TPA

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24 Reversed cusp energy-latitude dispersion

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26 Cluster :10-03:30 UT Bz By Bx abc

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

28 Quantitative Walen Analyses in Inertial Frame x-comp y-comp z-comp Direction to X-line (-dV) from SC1 assuming that dV ~ j x B

29 MHD Simulation BATSRUS Cluster C1 position

30 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].

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32 Vx (km/s) Vy (km/s)

33 Tracing Field Lines in MHD

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35 [from Vennerstrom et al., Field-aligned currents during northward IMF: Morphology and causes, JGR, 2005] Blue: antiparallel/upward Red: parallel/downward NBZ Current Evolution

36 [from Vennerstrom et al., Field-aligned currents during northward IMF: Morphology and causes, JGR, 2005] Blue: antiparallel/upward Red: parallel/downward NBZ Current Evolution

37 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].

38 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 for 2005JA reprint – also available at


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