Transcript PPT - ILWS
Auroral asymmetries in the conjugate hemispheres Normal text - click to edit (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 1 August 2011 Outline: 1. Asymmetric auroral intensities 2. Substorm onset location asymmetries 3. What happen to asymmetry during substorm expansion phase 4. Theta Aurora in only one hemisphere 5. Cusp Aurora in both hemispheres Normal text - click to edit 2 August 2011 1. Very asymmetric aurora – closed field lines Normal text - click to edit Movie: 2139-2153 UT WIC/FUV - IMAGE VIS Earth Camera - Polar 3 August 2011 2. Asymmetric auroral intensities intensities Normal text - click-- Asymmetric to edit Poleward expansion Conditions: -Seasons: -North-summer -South- winter -Bx dominated IMF -By ~0 4 August 2011 Interpretation Laundal and Østgaard, Nature, 2009 Normal text - click to edit x X 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 5 August 2011 character. 2. Asymmetric substorm onset location Normal text - click to edit Westward bulge: SH ~1.1 MLT duskward of NH Duskward – BY negative ! 6 August 2011 Statistical distribution – Polar and IMAGE • • • Normal text - click to edit 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 7 August 2011 Statistical distribution Normal text - click to edit 8 August 2011 Average onset location – clock angle -10 Re -20 Re -10 Re +/- 5 min average +/- 5 min average - 40 min average Clock text angle controls averageto edit Normal click 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 MLT 0.53 sin(C 4.8) 0.17 August 2011 9 Average onset location – By, IMF to -10 Re Reveals saturation towards dusk for - positive By in north - negative By in south Normal text - click to edit By > 0 A) B) - - - ++ - - - - - ++ + ++ - - - - - - + + + + ++ - - - - - - - + + + + +++ - - - - - - - - + + + + +++ - - - - - - - - + + + + + + ++ - - - - - - - - + + + + + + ++ - - - ++ - - - - - ++ + ++ - - - - - - + + + + ++ - - - - - - - + + + + +++ - - - - - - - - + + + + +++ - - - - - - - - + + + + + + ++ y - - - - - - - - + + + + + + ++ z - - - ++ - - - - - ++ + ++ - - - - - - + + + + ++ - - - - - - - + + + + +++ - - - - - - - - + + + + +++ - - - - - - - - + + + + + + ++ - - - - - - - - + + + + + + ++ - - - ++ - - - - - ++ + ++ - - - - - - + + + + ++ - - - - - - - + + + + +++ - - - - - - - - + + + + +++ - - - - - - - - + + + + ++ ++ - - - - - - - - + + + + + + ++ C) By < 0 D) By 20 x = -18 RE GSM xGSM = -18RE +1 [RE] 10 zGSM Asymmetric penetration of IMF By in the deep tail (Khurana et al., 1996) extends to closed field lines z 0 y -10 -20 20 -1 10 0 yGSM [RE] -10 -20 20 MLT 0.88 sin( August 2011 10 0 yGSM [RE] -10 -20 BY 9.3) 12 nT 10 Østgaard et al, GRL, L08104, 2011. 3. What happen to asymmetries during expansion phase IMAGE Oct 22, 2001: Normal text - click to edit 5 hours of conjugate images 2 substorms VIS Earth August 2011 11 Method 1. 2. 3. 4. 5. 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) Normal text - click to edit 12 August 2011 Substorm assymmetry Grey diamonds: all images < 1 min overlap Normal text - click to edit 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. 13 August 2011 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,– Normal text - click to edit iii) and magnetic stress is released (Haerendel, 2007 rectifying 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 E u Bdl ∆ us u n d B dA 0 dt E vB 0 E|| l BL Østgaard et al, GRL, L03101,2011 August 2011 14 3. Theta 4. Cusp aurora Non-conjugate theta (transpolar arcs) consistent with IMF Bx but more observations to see if this holds statistically Normal text - click to edit 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 15 August 2011 Summary: 1. Asymmetric auroral intensities • Indication of interhemispheric currents Normal text - click to edit 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 …. 16 August 2011 Normal text - click to edit Thank you! 17 August 2011 Tilt angle effects Onset in south is downward of north onset for positive tilt, i.e., south=winter Normal text - click to edit More intense nightside FACs were found by Ohtani et al., 2005 in the winter hemisphere Needs more data to confirm! 18 August 2011 Summary and challenges: Cusp and reconnection – temporal behavior 1. We have shown the asymmetry of cusp location 2. Challenge: To determine the relative reconnection rate 1. Optical measurements: Relative intensity of the cusp spot 2. 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 1. 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 1. 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 1. The potential mismatch?? Normal text - click to edit 19 August 2011 How IMF affect the magnetospheric configuration The penetration of the IMF field Normal text - click to edit 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 20 August 2011 Faraday’s law Reconnection rate - Method – Vasyliunas [1984] E u Bdl d B dA dt Normal text - click to edit Assumptions E vB 0 (>300 km) E|| 0 Em m v ui Bi i Ei i E m 40RE i i cosRE Em 21 August 2011 Is it real? 1) Relative variations in counts North-WIC: red South-VIS: black 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. Normal text - click to edit 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. 22 August 2011 Challenge: Dayside reconnection rate with radars and imaging 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. antiparallel site) Vasyliunas [1984] Normal text - click to edit IMF Bz<0 Variable By North South 23 August 2011