1. Space and Plasma Physics School of Electrical Engineering Royal Institute of Technology Stockholm Sweden Investigation of temporal stability of field-aligned currents in the auroral oval Tomas Karlsson, Johanna Myde Nordic Cluster Meeting 2011

2. Introduction Boudouridis and Spence, 2007 DMSP particle data Le, Wang, Slavin, and Strangeway, 2007 Space Technology 5, magnetic field data Nordic Cluster Meeting 2011

3. Current sheet approximation and Ampére’s law Ampére’s law (no time dependence): j z y x Butand eller Nordic Cluster Meeting 2011

4. Magnetic field data from auroral oval passage t (s) ILAT(S/C 1)=65 ILAT(S/C 1)=80 DB EAST (nT) Nordic Cluster Meeting 2011

5. Cross correlation t ILAT=6580 ILAT=6580 S/C 1 S/C 2 lag cross-covariance auto-covariances (for L=0) lag AGU Chapman Conference 2011

6. Magnetic field data from auroral oval passage t (s) ILAT(S/C 1)=65 ILAT(S/C 1)=80 DB EAST (nT) Nordic Cluster Meeting 2011

7. Cross correlation lag (s) Cross correlation coefficient 1414 1313 1212 2323 2424 3434 Nordic Cluster Meeting 2011

8. Raw correlations 2002-2009 7938 correlations, 1323 oval crossings lag (s) Cross correlation coefficient night day Nordic Cluster Meeting 2011

9. Correlations, full oval, 2002-2009 lag (s) Cross correlation coefficient Nordic Cluster Meeting 2011

10. Correlations, full oval, 2002-2009 lag (s) Cross correlation coefficient C = 0.37 C 0 = 0.53 t = 546 s Fit equation: Nordic Cluster Meeting 2011

11. Correlations, full oval, 2002-2009 Kp dependence lag (s) Cross correlation coefficient 0 < Kp ≤ 1 1 < Kp ≤ 3 3 < Kp ≤ 9 Nordic Cluster Meeting 2011

12. Correlations, full oval, 2002-2009 MLT dependence lag (s) Cross correlation coefficient Evening Night Morning Day (18-20) (22-00) (01-03) (11-13) Nordic Cluster Meeting 2011

13. Sliding Cross Correlations t pre-lag ILAT=6580 ILAT=6580 S/C 1 S/C 2 ILAT=6580 S/C 1 ILAT=6580 S/C 2 winsize Nordic Cluster Meeting 2011

14. Correlations 2002-2007, n=95 154 Window size = 3° ILAT lag (s) cross correlation coefficient 1.0 0.5 0.0 Nordic Cluster Meeting 2011

15. Correlations, 2002-2007 ilat lag (s) Window size = 1°Window size = 3° cross correlation coefficienrt Nordic Cluster Meeting 2011

16. Correlations, 2002-2007 Window size = 3° MLT lag (s) Nordic Cluster Meeting 2011

17. Correlations, 2002-2007 MLT lag (s) Window size = 1°Window size = 3° cross correlation coefficienrt Nordic Cluster Meeting 2011

18. Correlations, 2002-2007 Window size = 3° ilat MLT lag < 10 s10 s < lag < 30 s 30 s < lag < 100 s100 s < lag < 300 s cross correlation coefficienrt Nordic Cluster Meeting 2011

19. Correlations, 2002-2007 Window size = 1° ilat MLT lag < 10 s10 s < lag < 30 s 30 s < lag < 100 s100 s < lag < 300 s cross correlation coefficienrt Nordic Cluster Meeting 2011

20. Conclusions Cross correlation method is a useful method for studying the temporal variations in the auroral oval, FAC on the scale of the oval size have a characteristic life- time of around 9 min, consistent with Le et al. (10 min). Meso-scale current life-time dependence on MLT and scale size: Low-latitude currents somewhat more temporally stable than high-latitude currents. Morning/dayEvening 1°1°10 s30-100 s 3°3° 100-300 s Nordic Cluster Meeting 2011

21. Future work Modify normalization in in cross correlation function Study scale-size dependence with proper cross-spectral analysis (coherence function) Study temporal characteristics of electric fields Study energetic importance by calculating Poynting flux Nordic Cluster Meeting 2011

22. Physics questions Nordic Cluster Meeting 2011 Changes in FAC  Alfvén wave FAC  zero frequency Alfvén wave Is the auroral acceleration electric field explainable by AW physics, or is other physics needed? Temporal behaviour of E and B may give clues. EE J B SS Energetic importance of processes with different temporal and spatial scales. Large-scale SW-M-I coupling time scales. Differences in temporal scales of upward and downward currents  Implications for MI coupling modes, e.g. ionospheric plasma evacuation. Is temporal variation of currents due to motion of current systems or change of shape and magnitude? (Relation to Maria’s results.)

24. Coherence function Cluster Workshop 2010-05-20

25. Correlations 2002-2007, n=109 542 Window size = 1° ILAT lag (s) cross correlation coefficient 1.0 0.5 0.0

26. Cluster Workshop 2010-05-20 Correlations, 2002-2007 Window size = 1° MLT lag (s)

27. This and that Cluster Workshop 2010-05-20 Smooth window in dB calculation: 400 s = 200 km