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T. Karlsson Alfvén Laboratory, Royal Insititute of Technology, Stockholm

High-altitude signatures of ionospheric modification by field-aligned currents. T. Karlsson Alfvén Laboratory, Royal Insititute of Technology, Stockholm. E n. B t. BACKGROUND. Expected correlation with constant ionospheric conductivity and no E // :.

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T. Karlsson Alfvén Laboratory, Royal Insititute of Technology, Stockholm

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  1. High-altitude signatures of ionospheric modification by field-aligned currents. T. Karlsson Alfvén Laboratory, Royal Insititute of Technology, Stockholm

  2. En Bt BACKGROUND Expected correlation with constant ionospheric conductivity and no E// :

  3. 2003-12-25 - Correlation between E and DB s/c 1 Northern hemisphereMLT ~ 04 s/c 2 BZ,MEE(nT) EY,MEE(mV/m) s/c 3 s/c 4 t (s)

  4. BX,GSE BX,GSE BX,GSE EY,GSE R Separation ILAT MLT t (s) 2002-05-19 Overview Plot

  5. DBt(nT) DBn (nT) Et(mV/m) En(mV/m) t (s) 2002-05-19 E and DB Southern hemisphereMLT ~ 20

  6. 2002-05-19 Enormal and j// j// s/c 1 Enormal s/c 2 j//(mA/m2) En(mV/m) s/c 3 s/c 4 upward current downward current t (s)

  7. 18 DB S/C 4 -80 º -70 º -60 º 24 Observations 2002-05-19 • Spatial separations small (~100 km) (not visible in the CGLAT-MLT plot) • Large electric fields correlated with large downward currents instead of with B. • But there also exists regions of large downward current where there is no large electric field. CGLAT-MLT plot

  8. 2002-04-27 Enormal and j// Southern hemisphereMLT ~ 20 s/c 1 s/c 2 j//(mA/m2) En(mV/m) s/c 3 s/c 4 upward current downward current t (s)

  9. 2002-05-12 Enormal and j// Northern hemisphereMLT ~ 19 s/c 1 s/c 2 j//(mA/m2) En(mV/m) s/c 3 upward current s/c 4 downward current t (s)

  10. 2003-11-15 Enormal and j// s/c 1 Northern hemisphereMLT ~ 07 s/c 2 j//(mA/m2) En(mV/m) s/c 3 s/c 4 upward current downward current t (s)

  11. Observations • Several examples of large electric fields correlated with large downward currents. • Currents stable on time scale of 40 s, whereas electric field changes appreciably. • The observed current sheets of large downward current have a width of the order of 10 km • All observations (found by manual inspection) from non-sunlit ionospheric footpoints.

  12. Rejected explanations for correlation between E and j// U-shaped potential S-shaped potential Alfvén wave Eperp Eperp E// Bperp j// E// Eperp Eperp Bperp Eperp j// j// A (partially) standing AW could give observed phase shift between E and B, but unlikely on these scales, and would give no preference of any sign of of dB/dt. If j// is proportional to U//, then the maximum of j// coincides with a minimum of Eperp. Modelling shows that maximum of j// can be close to region of large Eperp, but never ‘inside’ it.

  13. 600 j// carried by e - jP carried by ions Model Consequence: Outflow of electrons from ionosphere in downward current region, with subsequent cavity formation in ionosphere. E region evacuated in ~10 s for large currents.

  14. JP SP EP Model –ionospheric modification by downward FACs j// Magnetosphere upward j// Ionosphere downward j//

  15. 2002-05-19 Model results (E) s/c 1 s/c 2 j//(mA/m2) En(mV/m) s/c 3 s/c 4 upward current downward current t (s)

  16. 2002-05-19 Enormal and j// j// s/c 1 Enormal s/c 2 j//(mA/m2) En(mV/m) s/c 3 s/c 4 upward current downward current t (s)

  17. 2002-04-27 Model results (E) s/c 1 s/c 2 j//(mA/m2) En(mV/m) s/c 3 upward current s/c 4 downward current t (s)

  18. 2002-04-27 Enormal and j// Southern hemisphereMLT ~ 20 s/c 1 s/c 2 j//(mA/m2) En(mV/m) s/c 3 s/c 4 upward current downward current t (s)

  19. 2002-04-27 Model results S/C 3 j//(mA/m2) data SP(S) modelled En(mV/m) modelled JP(mA/m) data

  20. Observations 2002-04-27 • Minimum variance analysis yields an angle for the current sheet from E-W direction of ≈ 36º • Time delay information gives direction of motion of current sheet • Scale size of current sheet ~10 km (at ionosphere). Current sheet v CGLat S/C 1S/C 2S/C 3S/C 4 + indicates position of maximum FAC MLT

  21. Model - moving current system • A moving current system leaves a ‘trail’ of low conductivity behind itself. • This widening of the low-conductivity region accounts for e.g. the appearance of the electric field minimum at the high-latitude end of the current sheet at point 4 at the 2002-04-27 observation. Also this model remarkably well reproduces some other features (marked by 1,2, and 3). j// v Magnetosphere Ionosphere SP

  22. 2002-04-27 Model results S/C 4 (MOVING CURRENT SYSTEM) 1 2 3 4 j//(mA/m2) En(mV/m) modelled data SP(S) modelled En(mV/m) modelled JP(mA/m) data

  23. CONCLUSIONS • At times, the perpendicular electric field normal to a current sheet may be correlated to the field-aligned current, rather than the magnetic field. • This happens for large FACs and electric fields. • A simple model of ionospheric modification by downard FACs reproduces these findings well. • This mechanism represents a way of generating large electric fields (which may map out to the magnetosphere) of the order of several hundreds of mV/m, even when there is no associated parallel potential drop.

  24. 2003-12-30 Enormal and j// s/c 1 s/c 2 j//(mA/m2) En(mV/m) s/c 3 s/c 4 upward current downward current t (s)

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