The Auroral Acceleration Region: Lessons from FAST, Questions for Cluster

1. The Auroral Acceleration Region: Lessons from FAST, Questions for Cluster Robert J. StrangewayInstitute of Geophysics and Planetary PhysicsUniversity of California, Los Angeles Cluster Workshop – Uppsala

2. Outline • Overview of our understanding of M-I coupling from a FAST perspective • Force balance and field-aligned currents (FACs) • Magnetosphere – MHD force balance • Vorticity and Alfvén waves • Ionosphere – vorticity again • Auroral Acceleration Region • The three types of aurora • FAST-THEMIS – Substorm current wedge • Substorm current wedge consists of multiple FACs • Flow channel between FACs may be westward traveling surge • Cluster-THEMIS – possible conjunction April 7, 2009 Cluster Workshop – Uppsala

3. MHD Force Balance – Lorentz Force Cluster Workshop – Uppsala

4. Field-Aligned Currents – Magnetosphere Plasma momentum equation – force balance – leads to a fundamental source of field-aligned currents Following Hasegawa and Sato [1979], and D. Murr, Ph. D. Thesis “Magnetosphere-Ionosphere Coupling on Meso- and Macro-Scales,” 2003: Vasyliunas’ pressure gradient term Inertial term Vorticity dependent terms (w =U) Assumptions:•j= 0,E + UxB= 0•j= 0 implies that displacement current can be neglected E + UxB= 0 only enters through the vorticity terms Cluster Workshop – Uppsala

5. Vorticity – Alfvén Wave If we assume in the general FAC equation that field-aligned vorticity is the dominant term and linearize, then: From curl of Faraday’s law and Ampere’s law: Frozen-in condition: Parallel component: Cluster Workshop – Uppsala

6. Field-Aligned Currents – Ionosphere Lorentz force moves plasma against frictional drag Taking curl: Parallel to B FAC small vorticity conductivity gradients Strong relationship between field-aligned vorticity and field-aligned currents In ionosphere relationship depends on conductivity – impedance mismatch Cluster Workshop – Uppsala

7. Ionospheric Current Closure Iijima & Potemra [1978] published maps of the currents flowing into and out of the ionosphere Higher latitude currents are called “Region 1” Lower latitude currents are “Region 2” Ionospheric Pedersen closure currents (red) provide the Lorentz force that moves plasma (and magnetic flux) over the polar cap, and returns flux at lower latitudes Cluster Workshop – Uppsala

8. Auroral Acceleration Region Electron Energy Electrons carry FACs Electron Pitch Angle Magnetic field deviations correspond to ionospheric flows Ion Energy Ion Pitch Angle Magnetic Field Arrows show FAC Cluster Workshop – Uppsala

9. Implied Flow Pattern Delta-B’s are projected to the ionosphere Delta-B’s correspond to bending of field lines, pulling ionosphere through atmosphere In the northern hemisphere flows are anti-parallel to the delta-B’s Cluster Workshop – Uppsala

10. FAST Observations – Three Types of Aurora Auroral zone crossing shows: Inverted-V electrons (upward current) Return current (downward current) Boundary layer electrons Cluster Workshop – Uppsala

11. Upward Current Cluster Workshop – Uppsala

12. Downward Current Cluster Workshop – Uppsala

13. Polar Cap Boundary Cluster Workshop – Uppsala

14. Haerendel [2008] – Current Transformers • Modified version of Cowling conductivity • Some Issues – • Sp,out = jH•Ei, balanced by excess Hall current in direction of auroral electrojet (AEJ) and enhanced Sp,in, no net change • AEJ is assumed to be a Pedersen current • But I agree with the idea that Hall currents and conductivity gradients can result in additional FACs – part of the feedback process • Haerendel has also emphasized the “fracture” zone associated with parallel potentials Note: j|| in this sketch are secondary FACs (!) Cluster Workshop – Uppsala

15. Substorm Current Wedge – FAST/THEMIS Substorm current wedge and partial ring current [Sergeev et al., 1996] Cluster Workshop – Uppsala

16. Westward Traveling Surge Marklund et al. [1998], Freja data Akasofu [1964] Cluster Workshop – Uppsala

17. FAC Structure in the Bulge Hoffman et al. [1994] – DE-2 Observations Where are the substorm current wedge currents? Cluster Workshop – Uppsala

18. March 23, 2007 Substorm • FAST and THEMIS in same local time sector ~ 21 MLT • THEMIS footprint in the south • FAST footprint in the north THE (P4) THA (P5) THB (P1) THD (P3) THC (P2) -x z FAST Cluster Workshop – Uppsala

19. FAST Observations • Westward flow channel • Inverted-V electrons carry upward current • Low energy electrons carry downward current • Net current upward, away from ionosphere Cluster Workshop – Uppsala

20. MHD FAC and Precipitation Cluster Workshop – Uppsala

21. THEMIS Observations Green trace shows westward component Two FACs, with net positive change Balanced currents, s/c can cross in either direction To Earth Equator s/c dB Unbalanced currents, s/c direction matters LHS consistent with FAST, and movement of plasmasheet over the s/c – net outward FAC Cluster Workshop – Uppsala

22. WTS and Substorm Current Wedge • THEMIS observed pair of field-aligned currents, with net current out of the ionosphere, resulting in dipolarization – western leg of substorm current wedge • FAST observed pair of FACs and a flow channel. Dominant signature, but there is a small net outward current • MHD simulations show that the paired FACs are also associated with a rapid westward motion of the region of electron precipitation, is this the WTS? • Implication: The upward current for the substorm current wedge is not a single FAC, but is more complex (See also Hoffman et al. [1994]) Cluster Workshop – Uppsala

23. Another WTS Example Peak in magnetic field perturbation Electron Energy > 30 keV Upgoing electrons Change in plasmasheet ion energy flux, boundary? Strong downward current Low latitude eastward flow, note FAC at low latitude edge What drives this flow? Polar cap Eastward flow W’ward flow Eastward flow Cluster Workshop – Uppsala

24. Feb 16, 2006 Pi2 Data Cluster Workshop – Uppsala

25. Cluster Results – Temporal Evolution Marklund et al. [Nature, 414, 724-727, 2001] present Cluster results showing temporal evolution of downward current region Top panel shows electric field – Bipolar structure that grows and then decays away, width constant Bottom panel shows downward current density – Peak of current density co-located with electric field reversal, but current widens as a function of time How does this evolve in the ionosphere? What happens in the magnetosphere? Cluster Workshop – Uppsala

26. Cluster-THEMIS Conjunction April 7, 2009, 05:05 UT THEMIS-A, -D, -E ~ 22 LT, near apogee Cluster-1, -2, -4 near perigee, roughly same local time sector Possible conjunction Cluster Workshop – Uppsala

27. THEMIS overview, April 7, 2009 Psuedo-AE Athabasca Keogram Magnetic Field (GSE coords) Flow velocity, blue positive sunward Ions (ESA + SST) Electrons (ESA + SST) THEMIS sees an activation signature around 05:05 UT Cluster Workshop – Uppsala

28. Summary • Multi-platform studies are essential in understanding the role of field-aligned currents and the auroral acceleration region in magnetosphere-ionosphere coupling • Lessons from FAST: • Aurora come in a variety of forms – probably a signature of different stages of the M-I coupling process • Understanding the substorm current wedge and westward traveling surge benefits from a combination of multi-platform observations and simulations • Questions for Cluster • Cluster may be able to take on the role of FAST in M-I coupling studies • Candidate conjunction event April 7, 2009 Cluster Workshop – Uppsala