1. Multi-Scale Coupling and Auroral Dynamics: Reporter Review Bob Lysak, University of Minnesota, USA Purpose of the review: to examine the consequences of small- scale auroral processes on large-scale dynamics, and the coupling of different plasma regions with each other. Significant advances in the last 2 years, due to new and continuing observations from FAST, Akebono, IMAGE, Polar, and Cluster as well as ground observations, theory, and numerical modeling. Over 200 papers published in this area since the 2001 IAGA meeting.

2. Outline of the Talk Optical aurora and relation to plasma parameters Electrostatic acceleration and time-independent coupling Time-dependent coupling by Alfvén waves Alfvénic particle acceleration Ion acceleration and outflow Nonlinear plasma waves: Electron and ion phase space holes Coupling between aurora and outer magnetospheric processes

3. Optical Auroral Features and Plasma Processes Conjugacy of auroras:  First simultaneous images of aurora with one camera, showing 40 min MLT eastward displacement (Frank and Sigwarth, 2003).  Conjugate observations of spirals, consistent with Hallinan theory, rotation opposite to KH instability (Partamies et al., 2001).  Latitude displacements of up to 5° due to IMF (Vorobjev et al., 2001) Comparison with particle data  FAST data compared with images from UVI (Peria et al., 2001) and IMAGE (Mende et al., 2002, 2003) show poleward arcs associated with wave-accelerated electrons, equatorward with inverted V’s  Coordinated images for dynamic auroras, such as poleward moving auroral forms (Kozlovsky and Kangas, 2002; Drury et al., 2003) and black aurora (Peticolas et al., 2002) Multi-spectral imaging  Use of multiple wavelengths to determine precipitation characteristics (Semeter et al., 2001; Janhunen, 2001; Galand et al., 2002)

4. Northern and Southern Aurora First simultaneous images of both auroral ovals Displacement of auroral features by MLT of 40 minutes, with southern aurora displaced to the east. Frank and Sigwarth, JGR, 2003.

5. Crossing of “Double Oval” with FAST and IMAGE (Mende et al. 2003) IMAGE 11:20 11:26 IMAGE images of double oval compared with FAST (arrows give time of images) LBHLyman α 135.6 (OI )

6. Plasma density in auroral acceleration region larger in summer than in winter (Johnson et al., 2001, 2003) Polar cap densities also show strong seasonal asymmetry (Laakso et al., 2002) Auroral activity dependent on solar wind dynamic pressure more for southward IMF and winter (Shue et al., 2002) N Hemis, 4-6 R E S Hemis, 2 R E Seasonal effects on aurora Winter Dark Summer Light

7. Three Regions of Auroral Acceleration Illustration of three regions of auroral acceleration: downward current regions, upward current regions, and the region near the polar cap boundary of Alfvénic acceleration (Courtesy C. Carlson, from Auroral Plasma Physics, International Space Science Institute, 2003)

8. Electrostatic Coupling and Acceleration of Auroral Electrons (Observations) Direct measurements of parallel electric fields  Upward current region: FAST shows large E || (~ 100 mV/m) at lower edge of auroral density cavity (Ergun et al., 2002) parallel electric fields of 25-300 mV/m seen on Polar; scale sizes 10’s of km based on potential of energized particles (Hull et al., 2003)  Downward current region Strong E || (up to 1 V/m), consistent with oblique double layer model (Ergun et al., 2001; Andersson et al., 2002) In both current regions, observation of E || has been supported by in situ particle acceleration as well as double probe measurement.

9. Large Upward Parallel Electric Fields from FAST and Polar ← FAST data showing large E || at boundaries of auroral cavity (Ergun et al., 2002) → Polar data showing E || associated with electron and ion acceleration (Hull et al., 2003)

10. Downward Parallel Electric Fields From FAST Observation of large downward field in region of strong turbulence (Ergun et al., 2001) Downward E || and particle data, together with inferred trajectory of spacecraft with respect to potential (Andersson et al., 2002)

11. Auroral Current-Voltage Relation Old subject (e.g., Knight, 1973), but new interest, especially due to observations in downward current region Borovsky and Bonnell (2001) consider linear Knight relation (j = KΦ) including dipole mapping with different constants in up and down current regions Rönnmark (2002) considered case when source population varies so that quasi-neutrality is satisfied (with fixed ion density profile), found j ~ Φ 1/2. Boström (2003) considered general description of current carried by various types of distributions, found Knight-type relations, but variations in details. Ergun et al. (2002) consider more local situation and found double layer solution with realistic populations.

12. A gallery of current- voltage relations (Boström, 2003) Response of plasma to parallel potential drops depends on form of particle distribution at top of field lines. Figure gives a variety of distributions, with voltage as a function of current at right.

13. Alfvénic Coupling: Observations New Cluster observations show time- variations in auroral electric fields and currents between passage of 4 spacecraft (Marklund et al., 2001) Alfvén wave Poynting flux correlated with aurora and with electron energy flux (Keiling et al., 2002, 2003) Observations consistent with field line resonance found (Milan et al., 2001) Laboratory experiments confirm kinetic Alfvén wave dispersion relation (Kletzing et al., 2003) E J || V sc

14. Effects of parallel resistivity on small-scale structures (~ 1 km) emphasized: enhanced damping of ionospheric resonator (Lessard and Knutsen, 2001) Energy budget of field line resonance with anomalous resistivity included shows maximum dissipation at intermediate scales (~ 10 km) (Streltsov and Lotko, 2003) Role of E || in reflection of Alfvén waves trapped in ionospheric Alfvén resonator emphasized (Pilipenko et al., 2002) Model coupling the ionosphere to outer magnetosphere developed showing role of auroral acceleration in modifying tail pressure (Blixt and Vogt, 2002) Effects of ionospheric Joule heating on production of tall auroral rays modeled (Otto et al., 2003) Alfvénic Coupling: Theory and Modeling

15. Great deal of new work on ionospheric feedback: self- consistent conductivity variations due to Alfvén wave currents  Compressible effects included in theory: effect not critical to instability (O. Pokhotelov et al., 2001)  Energetics of the instability considered: supported by reduction of ionospheric Joule heating (Lysak and Song, 2002)  New modeling of field line resonances including ionospheric conductivity variations (D. Pokhotelov et al., 2002a,b; Prakash et al., 2003) Alfvénic Coupling: Feedback Instability

16. Alfvén Feedback Modeling Seasonal asymmetry between winter (top) and summer (bottom) ionospheres: feedback instability stronger in winter (Pokhotelov et al., 2002) Simultaneous excitation of feedback in ionospheric resonator (upper band near 0.1 Hz) and for field line resonance (lower band at 0.01 Hz) (Pokhotelov et al., 2003)

17. Alfvénic Acceleration of Electrons In addition to “classical” quasi-static electron acceleration, recent observations and theories have emphasized time- dependent acceleration, giving broad energy distribution and narrowly field-aligned pitch angles  Akebono shows field-aligned electrons on edge of auroral acceleration region (Miyake et al., 2001)  Freja observations show field-aligned distributions with energy dispersion: particles arrive before wave, accelerated above spacecraft (Andersson et al., 2002)  Polar observations see field-aligned acceleration of electrons at ~ 4 R E at plasma sheet boundary layer (Wygant et al., 2002)

18. FAST observations and modeling of Alfvénic acceleration Chaston et al. (2002ab, 2003abc) has looked at field- aligned electrons and done test particle models to compare with data. FAST data showing field- aligned bursts (Chaston et al., 2002a) Simulated (left column) and observed (right) energy-pitch angle distributions (Chaston et al., 2002b)

19. Kinetic Theory of Alfvén Waves Tikhonchuk and Rankin (2002) developed kinetic theory of field line resonances, found enhancement of E || due to bouncing electrons Wright et al. (2002) and Wright and Hood (2003) emphasized nonlinear convective term in two-fluid and kinetic models. Lysak and Song (2003a,b) considered nonlocal kinetic theory in ionospheric resonator, also found enhancement of E || and strong absorption near IAR resonances. Génot et al. (2001) performed electromagnetic particle simulations with Alfvén wave fields; found formation of localized electric fields in density cavities New work has emphasized kinetic effects on Alfvén waves:

20. Ion Outflows Recent work has emphasized correlations with external parameters and auroral arcs Akebono shows good correlations with solar F10.7, solar wind dynamic pressure, solar wind E, and magnetic fluctuations (Cully et al., 2003) Direct response of ions to solar wind pressure pulse seen by IMAGE (Fuselier et al., 2002) However, short-scale (5 minute) AE fluctuations do not correlate with ion outflow (Peterson et al., 2002) AE Outflow (ions/m 2 s)

21. More ion outflow Ion outflow found to correlate well with auroral forms (Wilson et al., 2001) Outflows follow convective motions of arcs (Kistler et al., 2002) and concentrate on arc boundaries (Stevenson et al., 2001) Solid: O + ouflow; dashed: LBH luminosity Modeling: Dynamic fluid/kinetic hybrid model of ion transport shows transverse heating, “pressure cooker” effect of downward E || (Wu et al., 2002) 3-d electrostatic PIC simulations show electron beam excitation of low frequency (LH) waves, producing ion heating (Singh et al., 2001) Global modeling shows heavy ion outflow mass-loads the outer magnetosphere, lowering polar cap potential (Winglee et al., 2002)

22. Nonlinear Plasma Waves: Electron Holes Electron holes form as a result of beam-plasma interaction (Newman et al., 2002) Polar observations show width increases with amplitude (Cattell et al., 2001) Consistent with allowed regions in BGK models (Chen and Parks, 2002) 3-d hole structures stable (Jovanovic et al., 2002; Roth et al., 2002) especially when strongly magnetized (Singh et al., 2001) Amplitude Perp width

23. Electron Holes as Source of VLF saucers VLF saucers well-known for many years (James, 1976); new FAST data shows association with electron holes (Ergun et al., 2001) Theoretical model shows electrons on trapped orbits in hole can radiate VLF (Berthomier et al., 2002) Simulations show that whistler mode wave packets can be radiated from holes (Singh, 2002)

24. Nonlinear Plasma Waves: Ion Holes Ion holes observed by Polar between H + and O + velocities, with larger amplitudes for holes near H + speed (Dombeck et al., 2001), supporting ion-ion two-stream mechanism. Ion holes observed by FAST show acceleration of electrons by hole potential (McFadden et al., 2003)

25. Outer magnetospheric coupling: shocks and pressure pulses Interplanetary shock produces anti-sunward propagating auroras (Zhou et al., 2003) Solar wind pressure pulses increase auroral oval width and decreases polar cap area (Boudouridis et al., 2003) WIND observations IMAGE UV images Southward turning of IMF leads to equatorward expansion of auroral oval, propagating anti-sunward at convection speed (McWilliams et al., 2001)

26. Outer magnetospheric coupling: expansion phase onset Onset arc forms a few minutes before onset, rather than being long-lasting growth phase arc (Lyons et al., 2002) Pseudo-breakup associated with localized current system in post- midnight oval (Partamies et al., 2003). Two types of auroral development during onset: arc “jumping” to higher latitudes (associated with reconnection); general expansion of bright patch in all directions (development of instability) (Kornilova et al., 2003)

27. Outer magnetospheric coupling: bursty bulk flows (BBF) and polar boundary intensifications (PBI) Comparison of MSP and All-sky cameras show north-south extent of aurora associated with PBI (Zesta et al., 2002) PBI/BBF associated with large-scale ULF wave power in the tail (Lyons et al., 2002). Association with Pi2 packets also seen (Sutcliffe and Lyons, 2002) BBF is diverted azimuthally in inner magnetosphere, as seen from equatorward extension of PBI (Kauristie et al., 2003) Series of PBI’s moving equatorward in late expansion phase indicate series of BBF’s (Sandholt et al., 2002)

28. Summary Auroral plasma physics remains a quickly developing field combining aspects of microscropic plasma physics, MHD waves and energy flows, ionospheric structure and chemistry, optical observations, etc. Importance of parallel electric fields has been well established; evolution and structure of the auroral acceleration region remain important questions. Auroral features are a diagnostic of both the local plasma conditions and the outer magnetospheric conditions along the field lines. Combination of in situ plasma measurements and optical features in the aurora are providing a wealth of new information.

29. Some outstanding questions Development and evolution of parallel potential drops and their relation to external conditions is still not well understood: our suspicion is that Alfvenic processes pre-condition auroral field lines to form quasi-static potentials (see, e.g., Song and Lysak, 2001). Relation of ion outflows to driving forces (e.g., Poynting flux into auroral zone) is still not clear; role of these ions in magnetospheric dynamics is still speculative. Formation of narrow (< 1 km) auroral forms remains a mystery; probably due to interactions between the acceleration region and the ionosphere. Role of ionospheric processes (ionization, recombination, heating, chemistry) on auroral formation is just beginning to be investigated

30. For further information… Special section of JGR-Space Physics, “Causes of the Aurora” published in April, 2003. New monograph Auroral Plasma Physics by team of > 30 authors published by International Space Science Institute (Kluwer, 2003). Set of 219 articles published in 2001-2003 period were compiled for this review: list will be available at http://ham.space.umn.edu/spacephys/~bob after this meeting (as will this presentation). http://ham.space.umn.edu/spacephys/~bob