Auroral asymmetries in the conjugate hemispheres (and wh ere KuaFu B can do better……)

1. Auroral asymmetries in the conjugate hemispheres (and where KuaFu B can do better……) Nikolai Østgaard University of Bergen, Norway Coauthors: B. Krøvel Humberset, K.M. Laundal, A.Aasnes, S. Haaland H. U. Frey, J. B. Sigwarth, J. Weygand August 2011

2. Outline: • Asymmetric auroral intensities • Substorm onset location asymmetries • What happen to asymmetry during substorm expansion phase • Theta Aurora in only one hemisphere • Cusp Aurora in both hemispheres August 2011

3. 1. Very asymmetric aurora – closed field lines Movie: 2139-2153 UT • WIC/FUV - IMAGE • VIS Earth Camera - Polar August 2011

4. 2. Asymmetric auroral intensities • - Asymmetric intensities • - Poleward expansion • Conditions: • Seasons: • North-summer • South- winter • Bx dominated IMF • By ~0 August 2011

5. Laundal and Østgaard, Nature, 2009 Interpretation • A combination of - the more efficient solar wind dynamo • - inter-hemispheric currents: • Persistent spot in south (dynamo + interhemispheric currents) • Transient spot in the north (interhemispheric currents) and the significant increase in reconnection (poleward expansion) can explain its transient character. x X August 2011

6. 2. Asymmetric substorm onset location Westward bulge: SH ~1.1 MLT duskward of NH Duskward – BY negative ! August 2011

7. Statistical distribution – Polar and IMAGE • 6600 substorms by IMAGE and Polar • 4671 Northern hemisphere • 1930 Southern hemisphere • Time shifts ACE and Wind: • -10 Re +/- 5 min average • -20 Re +/- 5 min average • -10 Re - 40 min average 294/108 640/293 696/264 1117/460 1121/433 803/372 August 2011

8. Statistical distribution August 2011

9. Average onset location – clock angle -10 Re +/- 5 min average -20 Re +/- 5 min average -10 Re - 40 min average Clock angle controls average onset location in each hemispheres Onset locations are still distributed in a large range of MLTs Interhemispheric asymmetry might be more robust result Indirect indication that tail reconnection is significant for Northward IMF August 2011

10. Reveals saturation towards dusk for - positive By in north - negative By in south Average onset location – By, IMF to -10 Re Asymmetric penetration of IMF By in the deep tail (Khurana et al., 1996)extends to closed field lines z y Østgaard et al, GRL, L08104, 2011. August 2011

11. 3. What happen to asymmetries during expansion phase IMAGE Oct 22, 2001: 5 hours of conjugate images 2 substorms VIS Earth August 2011

12. Used available DMSP data to ensure pointing accuracy • Visual Inspection method • Mapped onto rectangular magnetic grid -0.1 MLT - 1° Maglat resolution • 1D correlation (cross correlation) • 2D correlation (chi square) Method August 2011

13. Substorm assymmetry • Grey diamonds: • all images < 1 min overlap • Black diamonds: - time overlap (< 21 sec) • corr.coeff > 0.4 (1D) • chi square < 0.4 (2D) • Onset is asymmetric • During expansion phase the asymmetry disappears –regardless of clock angle or By. August 2011

14. Rectifying the field lines during expansion phase i) Asymmetry is created in the mid-tail ii) I inverted-V structures are formed which decouple magnetosphere from the ionosphere,– iii) and magnetic stress is released(Haerendel, 2007rectifying the asymmetric field lines b) Twist on field lines create net electric field pointing from north to south c) Faradays loop on closed field lines ∆ Østgaard et al, GRL, L03101,2011 August 2011

15. 3. Theta 4. Cusp aurora Non-conjugate theta (transpolar arcs) consistent with IMF Bx but more observations to see if this holds statistically Østgaard et al., Geophys. Res. Lett., 2003 Also the cusp aurora in both hemispheres IMF By: longitudinal shift Tilt angle: latitudinal shift Østgaard et al., Geophys. Res. Lett., 2005 August 2011

16. Summary: • 1. Asymmetric auroral intensities • Indication of interhemispheric currents • 2. Substorm onset asymmetries • Usually asymmetric locations at substorm onset • Strong statistical correlation with clock angle and By • 3. Substorm expansion phase • Field lines are rectified during expansion phase • 4. Theta Aurora • Maybe a IMF-Bx control of theta aurora? • 5. Cusp Aurora • IMF control of longitudinal shifts and maybe a tilt angle of the latitudinal shift ? • KuaFu can address these questions and many more …. August 2011

17. Thank you! August 2011

18. Onset in south is downward of north onset for positive tilt, i.e., south=winter Tilt angle effects More intense nightside FACs were found by Ohtani et al., 2005 in the winter hemisphere Needs more data to confirm! August 2011

19. Summary and challenges: • Cusp and reconnection – temporal behavior • We have shown the asymmetry of cusp location • Challenge: To determine the relative reconnection rate • Optical measurements: Relative intensity of the cusp spot • Even better: Reconnection rate with radars (Pinnock et al., 1999) and optical data. More than one radar, get the relative reconnection rate. Critical: OC boundary position, tilt and speed. • 3. Relative size of the polar caps, optics and radars to get the opening and closing of magnetic flux in the two hemispheres • Transpolar arcs and theta aurora • We have shown that theta can be non-conjugate, but to determine what controls it; more observations are needed: ground based optics, GUVI and SSUSI, low altitude satellites. • Subtsorm onset location • The IMF clock angle control is well documented. The global asymmetry suggests that the tilt effect and difference in FAC may be insignificant. • Dynamic features of the expansion phase of substorms • The potential mismatch?? August 2011

20. How IMF affect the magnetospheric configuration The penetration of the IMF field Has been directly observed in the outer magnetosphere by Sibeck et al, 1985 and the inner magnetosphere Wing et al., 1995 noon midnight dawn Wing et al, 1995 August 2011

21. Reconnection rate - Method – Vasyliunas [1984] Faraday’s law Assumptions (>300 km) August 2011

22. 2) Electron Energies and different cameras: Dawn-spot: 10 kR (WIC) ~5 kR (VIS) High electron energies will give higher intensity in WIC than in VIS, If 25 keV mean energy -> 40 mW/m2 and should give about 10 kR in VIS, but we see only 5 kR. Energy cannot explain the dusk differences. 3) N2/O2 ratio increases due to heating WIC would be slightly brighter – this effect is very small; intensity differences would be underestimated at dusk and exagerated at dawn. 1) Relative variations in counts North-WIC: red South-VIS: black Is it real? August 2011

23. Challenge: Dayside reconnection rate with radars and imaging Vasyliunas [1984] Pinnock et al., 1999 Two critical points: 1) get the OC boundary and its speed correct Imaging is needed 2) More than one radar: Differences in E_rec strength are interpreted as moving away from the throat region (i.e. anti-parallel site) IMF Bz<0 Variable By North South August 2011