Cross-Meridian Currents in Substorms and Sawtooth Events

1. Cross-Meridian Currents in Substorms and Sawtooth Events Martin Connors and Jason Ponto Athabasca University R. L. McPherron, UCLA Physics of Solar-Wind/Magnetosphere Coupling – Nov 2006 Image: Mikko Syrjäsuo

2. Ground Magnetometery: In a Sun-to-Mud approach, we are in the mud… EDMO UCLA magnetometer installed by Martin Connors (Tom Sawyer-like technique applied to astronomer Brian Martin) in December 2004

3. Often the locales are less agreeable than Puerto Vallarta (Kanji Hayashi in LaRonge, Canada, mid-October 2004)

4. Athabasca University has assisted or runs 16 sites in Canada (white triangles and purple triangles in Western Canada). Most data available through UCLA, STEP website, or on request. PEA, KAQ, SFV hoped for soon. With recent vigourous activity by AU and University of Calgary, Canada is ready for THEMIS!

5. What can ground magnetic data tell us? Regrettably, a single magnetogram often tells nearly nothing and even multipoint measurements from a meridian chain are difficult to interpret, needing… “geomagic”…

6. Array Interpretation from a distributed region is even more difficult, complicated by problems of nonuniqueness. An inversion procedure is needed. • On the ground, one detects primarily the magnetic effects of the Hall currents associated with the auroral oval electric field • FAC effects are not dealt with in this talk but CAN be observed from the ground AFM Apr 3 1997 red vectors are model, black observed

7. Even clear magnetograms from a meridian chain need interpretation, often done minimally or lumped into an index (such as AE) April 10 1997 X solid, Y dashed, Z dotted When was the real onset time; what was the current?

8. Automated Forward Modelling (AFM) can help. For meridian data, AFM adjusts current and borders The method is however, much more general and includes field-aligned currents in realistic 3-d configurations. Midlatitude perturbations can be included as can a Dst-like parameter.

9. Inversion tells us more by giving simple parameters extracted from several ground stations April 10 1997

10. Ability to match input data is best near the middle of the chain (although often not in Z due to electrojet structure) Note: different event and stations

11. Independent confirmation: Comparison of optical borders from meridian scanning photometer and inversion results for growth phase (Feb 22 1997); also confirmed by FAST FAC detections FAST Two electrojet model results are shown superposed on 557.7 nm optical meridian scan data from Gillam. The growth phase arc is poleward of the evening sector eastward electrojet. Note that the method is sensitive to initial conditions

12. ALert • The AL (or AE) index can be misleading • Here the AFM results are extremely clear for a substorm with strong growth phase • AL or even the inverted current mislead as to onset time • AL pre-onset shows Alaska conditions, post-onset shows Churchill

13. Statistical Properties of Substorms • A large-scale inversion project was undertaken for 1997 Churchill meridian data • Baselining the data is essential yet was challenging • Approximately 65 onsets were found to be very robustly inverted, comparable to the number of events in some other statistical studies • We have studied internal relations of parameters and not yet relation to external parameters such as solar wind in any great detail

14. Our results indicate a westward electrojet at time of onset of about 0.1 MA and also show the latitude at onset to increase with lesser current. The former is a quantitative measure but likely an overestimate; the latter is a well-known result made quantitative. Frey et al. (2004) found a distribution of Image FUV onsets skewed toward the evening sector. Our onsets straddle midnight. FUV onsets are due to bright evening sector auroras – the currents are in fact roughly symmetric around midnight.

15. Post-Onset AVERAGE Behaviour The current increases rapidly (20 min) to about 0.45 MA (an overestimate), black dots AFM, open dots AE in MA, curve Weimer (1993) The electrojet poleward border rises rapidly by about 5º (black dots AFM, open dots Frey et al., 2004). The equatorward border does not move. Frey’s FUV width is wider than AFM gives. AE and AFM match on average and can be cross-calibrated. Weimer’s ate-bt+c parametrization is very good on average.

16. Sawtooth Events • Several sawtooth events were selected from a list supplied by Joe Borovsky from LANL injections. Our final selection was then based on inspection of CANOPUS magnetometer data • It is hard to determine to what degree this sample may be biased toward large events • Inspection of the CANOPUS stackplots already makes clear the large latitudinal extent of sawtooth events. AE will be biased downward in such cases. • Large currents make sawtooth events good for AFM

17. October 3-4 2000 Event in CANOPUS Churchill Meridian X Component (quiet time in middle is day)

18. Oct 3-4 2000: Oct 3 is not discussed much here but note good Image WIC data. ACE shows extended periods of -BZ. Quiet time at CANOPUS likely due to +BZ when on dayside

19. Inversion Results for Oct 4 2000 • Early UT hours are quiet, during +BZ • Growth-like signature ca. 4 UT accompanies slow southward turning • Onsets are like substorms but width of electrojet very large • Currents of up to 2.5 MA are several times those of average substorms • Current density in electrojets may not be extreme: AL proxy

20. Inversion Verification Black is X (north), dots for model, solid for observation Blue is Z (downward) Z is small at N edge likely due to current wedge width being chosen too small.

21. Other Inverted Cases Several other cases were inverted with generally similar results: very wide electrojets and very large currents. This case of Nov. 8 2004 did not invert well due to lack of stations far enough south. Nevertheless there are indications of currents of 7 MA, comparable to the largest ever seen to cross a meridian (Hallowe’en storm 2003). This is one of few cases found with good ground-based optical data.

22. IMF Properties – already shown to be essential to understanding Nov 3-4 2000 storm and here examined with a large sample (quartiles shown) Dst roughly twice as large in sawtooth storms B and BZ larger but not greatly so

23. Conclusions • Automated Forward Modeling (AFM) is a new technique for inverting currents in near-Earth space and especially effective on meridian chains • Electrojet border behaviour mirrors that of optical data • Quantitative aspects of substorms include peak currents of about 0.45 MA cross-meridian and latitudinal extent (width) of about 7º • Sawtooth events have currents about ten times this and widths of twice or more • The quantitative parameters of AFM would be useful for correlation with IMF but this has been done only to a very limited extent so far • Nevertheless it is clear that Sawtooth events are driven by extreme solar wind conditions

24. Acknowledgements • FMI – Image magnetometer array data • CDAWeb; S. Mende (Image FUV), N. Ness (ACE) • Canadian Space Agency and University of Alberta – CANOPUS/CARISMA data • Collaborators including Kanji Hayashi, Brian Martin, UCLA magnetometer team, and James Weygand • G. Rostoker for the original AFM idea • This work primarily funded by Canada Research Chairs and Canada Foundation for Innovation