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Reporter Review: Auroral Phenomena

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  1. Reporter Review: Auroral Phenomena Clare E. J. Watt University of Alberta Reporter Review: Div III Auroral Processes C. E. J. Watt

  2. Auroral Processes in the Magnetosphere • Aurora are caused when particles impact the upper atmosphere with sufficient energy to excite neutral atoms • In order to precipitate, particles (electrons or protons) must have small enough pitch-angles to prevent trapping in the magnetic bottle created by the Earth’s magnetosphere. • This review will focus on recent advances in the study of magnetospheric processes which cause auroral particle precipitation. After Figure 3.1, Baumjohann and Treumann, [1997] Reporter Review: Div III Auroral Processes C. E. J. Watt

  3. www.phys.ualberta.ca/~cwatt/reporter_review • Papers selected were published between July 2007 and June 2009. • 214 articles from peer-reviewed sources: Reporter Review: Div III Auroral Processes C. E. J. Watt

  4. Structure of Review • Magnetospheric physics of auroral precipitation: • Quasi-static acceleration processes (upward & downward current) • Dynamic acceleration processes (e.g. Alfvén waves) • Consequences of auroral precipitation • Auroral Kilometric Radiation • Ion outflow from the ionosphere • Auroral phenomenology • Substorms • Solar-wind driven aurora • Aurora equatorward of auroral oval Figure 4.1 Paschmann et al., [SSR 2002] Reporter Review: Div III Auroral Processes C. E. J. Watt

  5. Quasi-static acceleration processes • Field-aligned acceleration • Field-aligned parallel electric field • Concentrated parallel electric fields; Transition layers; Double layers • Inverted-V electrons in situ Figure 1 [Partamies et al., AG, 2008] Shows typical inverted-V signature from in-situ FAST data Figure 2(b) [Ergun et al., GRL 2000] Concentrated potential drops/E|| Reporter Review: Div III Auroral Processes C. E. J. Watt

  6. Parallel electric fields: Double layers Earthward energy flux Anti-earthward flux electric potential Integrated study of Double Layers in downward current region: • FAST observations [Andersson et al., PoP 2008] • Vlasov simulations Newman et al., PoP, 2008a, 2008b] Figure 2, Andersson et al., PoP, 2008 0.26 s Reporter Review: Div III Auroral Processes C. E. J. Watt

  7. Control of Double Layers • Singh et al., [JGR 2009] show through self-consistent 2D PIC simulations how a potential drop manifests as a series of moving DLs and density cavities • Hwang et al., [JGR 2009a, 2009b] use FAST electron observations to deduce how the potential drop varies with magnetospheric and ionospheric parameters (tests previous analytical results: Cran-McGreehin and Wright, JGR 2005) Figure 2(b) [Ergun et al., GRL 2000] Concentrated potential drops/E|| Figure 8, Hwang et al., JGR 2009a Reporter Review: Div III Auroral Processes C. E. J. Watt

  8. Source & Structure of Upward Current • Haerendel, [JGR 2007, 2008, 2009] shows that in a static model, upward j|| can be driven by magnetic stress release in the near-Earth plasma sheet due to radial pressure gradients • Makes predictions which could be tested with sounding rockets or low-altitude spacecraft. Figure 3, Haerendel, JGR 2007 • Theory of stationary inertial Alfvén waves [orig. Knudsen JGR 1996; expanded by Finnegan et al., NPG 2008; PoP 2008; PPCF 2008] tested in the laboratory [Koepke et al., PPCF 2008] • can structure a large-scale current sheet into smaller perpendicular structures, without requiring a structured source Reporter Review: Div III Auroral Processes C. E. J. Watt

  9. Inverted-V electrons Occurrence vs MLT • Partamies et al., [AG 2008] show occurrence and characteristics of inverted-V electron signatures using 5 years of quicklook FAST data Scale size PC potential vs energy Figure 6 Figure 9 Figure 8 Red curve = occurrence of auroral arcs in MLT [Syrjäsuo and Donovan, AG 2004]. Black line = 3 x Energy Peak at 20-40km Reporter Review: Div III Auroral Processes C. E. J. Watt

  10. Dynamic acceleration processes • Shear Alfvén waves with small perpendicular extent can support time-varying and propagating E|| • Dynamic auroral displays • Generation of waves in magnetosphere • Cause of short perpendicular scales • Wave-particle interactions Reporter Review: Div III Auroral Processes C. E. J. Watt

  11. Generation of shear Alfvén waves which drive aurora Magnetotail driving • A mechanism for wave conversion from magnetosonic to shear Alfvén waves on very stretched or open field lines [Pilipenko et al., JGR 2008] • Wright and Allan [JGR 2008] use a simplified fluid model of the magnetotail to show how a plasmoid can drive Alfvénic disturbances with observed characteristics in both lobe and plasma sheet “Local” driving • Observational evidence for shear Alfvén waves driven by the shear flow in an inverted-V structure in Reimei data [Asamura et al., GRL 2009] Reporter Review: Div III Auroral Processes C. E. J. Watt

  12. Short perpendicular scales • Whatcausesshortperpendicularscales in shear Alfvén waves? • Chaston et al., [PRL 2008] show using FAST observations that Alfvénic aurora may be powered by a turbulent cascade • Conversion of large-scale shear Alfvén waves to small scale inertial Alfvén waves seen in 2.5D PIC simulation [Khazanov and Singh, PPCF 2008] requires small-scale density cavities • Ionospheric control of perpendicular scales [Streltsov, JGR 2007; Lysak and Song, GRL 2008; Sydorenko et al., JGR 2008] Figure 2, Chaston et al., PRL 2008 Reporter Review: Div III Auroral Processes C. E. J. Watt

  13. Characteristics of auroral SAW - modelling Inhomogeneous plasma • A dispersion relation for kinetic Alfvén waves in plasma with perpendicular plasma gradients [Lysak, PoP 2008] • cavities, boundaries between lobe/plasma sheet • Alfvénic solitons in inhomogeneous plasma supporting E||[Stasiewicz,PPCF 2007; Stasiewicz & Ekeburg, NPG 2008] Ionospheric feedback • Inclusion of ionospheric feedback important for SAW evolution • Conductivity evolution [Lu et al., JGR 2007, 2008] • Ionospheric heating [Streltsov., JGR 2008] • Ionospheric feedback instability characteristics different from FLR [Lu et al., JGR 2008] • Ionospheric feedback instability model provides new interpretation for localized e-m waves observed by Cluster at ~5RE in the PSBL [Streltsov & Karlsson, GRL 2008] Reporter Review: Div III Auroral Processes C. E. J. Watt

  14. Electron acceleration by shear Alfvén waves • Self-consistent model of electron acceleration by SAW in warm plasma: • Propagating SAW - Watt et al., PRL 2009 • Standing SAW - Rankin et al., GRL 2007 • c.f. Polar observations of SAW and electron acceleration at ~5RE in the PSBL [Wygant et al., JGR 2002] • Self-consistent simulations also show that acceleration by SAW can cause trapped magnetospheric populations and precipitation in the opposite ionosphere [Swift, JGR 2007] (top) Figure 2, Watt et al., PRL 2009 (bottom left) Figure 6(a) Wygant et al., JGR 2002 (bottom right) Figure 3(b) Watt et al., PRL 2009 Reporter Review: Div III Auroral Processes C. E. J. Watt

  15. Flickering/Pulsating Aurora • New instrumentation ideal for studying auroral processes with short temporal scales: • e.g. Reimei satellite, ASK, all-sky TV cameras, EMCCD detector • Stability and coherence of electron precipitation over different time scales using DMSP data [Boudouridis and Spence, JGR 2007] • Flickering aurora –spatial scales 50m-1km and frequencies 1-20Hz • Observations consistent with model of interfering electromagnetic waves [Whiter et al., GRL 2008; Gustavsson et al., JGR 2008] • Observations consistent with dispersive characteristics of Alfvén waves at ~6Hz [Semeter et al., JGR 2008] Figure 9, Semeter et al., JGR 2008 Reporter Review: Div III Auroral Processes C. E. J. Watt

  16. Reimei satellite (ISAS) Name REIMEI Objectives Demonstration of next-generation advanced satellite technologies in orbit Realization of small-scale, frequent scientific observation missions Launch Date 06:10, August 24, 2005 (JST) Location Republic of Kazakhstan Launch Vehicle Dnepr (launched together with OICETS satellite) Configuration Weight Approx. 60 kg Dimensions 60 × 60 × 70 cmOrbit Altitude: Perigee 610 km, Apogee 654 km Inclination 97.8° Type of Orbit Near-circular orbit Period 97 min Scientific Instruments Star tracker Spin/non-spin type solar sensors (SSAS/NSAS) Geomagnetic Aspect sensor (GAS) Three-axis optical fiber gyro (FOG) Reaction wheel (RW) and magnetic torquer (MTQ) as actuators Multi-spectral Auroral Camera (MAC) Aurora particle observation instrument (Electron/Ion Spectrum Analyzer: ESA/ISA) Still operational on May 23rd 2008 Reporter Review: Div III Auroral Processes C. E. J. Watt

  17. Consequences of Auroral Acceleration • Auroral Kilometric Radiation • Earth’s natural radio wave source • Frequency ~ electron cyclotron frequency • Current model: field-aligned beams of electrons form unstable distribution functions due to magnetic field convergence and mirror force Reporter Review: Div III Auroral Processes C. E. J. Watt

  18. AKR and radio emissions • Laboratory experiments have confirmed that electrons travelling into a converging magnetic field form a horseshoe distribution function which is unstable to radio emissions near the electron cyclotron frequency [McConville et al., PPCF 2008; Ronald et al., PoP 2008] • Results consistent with 3D PIC simulations [Gillespie et al., PPCF 2008] • NB: no background plasma in lab • Active experiments have artificially triggered AKR and observed significant density depletions [Wong et al., PRL 2009] Figure 1, McConville et al., PPCF 2008 fce = 4.42GHz Figure 8(b), McConville et al., PPCF 2008 Reporter Review: Div III Auroral Processes C. E. J. Watt

  19. AKR fine structure FAST data: Figure 1e, Su et al., [JGR 2008]: Alfvén waves Cluster data: Figure 1, Hanasz et al., GRL 2008]: Alfvén waves FAST & Cluster data: Figures 2&5, Pottelette & Pickett, [NPG 2007]: Phase space holes Reporter Review: Div III Auroral Processes C. E. J. Watt

  20. Location of AKR • Morioka et al., [JGR 2008; AG 2009] use frequency of AKR to infer source altitude • Modelling [Savilov et al., PoP 2007] suggests that AKR could present with multiple frequencies • not just Ωce • J||↑, frequencies change • Better to use multiple spacecraft and ray-tracing to deduce the source location of AKR [Mogilevsky et al., JETP 2007; Mutel et al., GRL 2008] • Lab experiments also available • Can frequency alone determine source altitude? Figure 1, Morioka et al., [AG 2009] Reporter Review: Div III Auroral Processes C. E. J. Watt

  21. Consequences of Auroral Acceleration • Ion Outflow and Upflow • Wave-driven (shear Alfvén waves) • Electron precipitation and electromagnetic Poynting flux • Ion heating (“pressure-cooker” effect) Reporter Review: Div III Auroral Processes C. E. J. Watt

  22. Ion outflow and shear Alfvén waves • Models of interaction between Alfvén waves and plasma which result in density cavities and upflowing ions: • Steepening nonlinear inertial Alfvén waves → ion cyclotron and ion acoustic waves → ion heating → upflow [Seyler & Liu, JGR 2007] • Ponderomotive force in the Ionospheric Alfvén Resonator [Sydorenko et al., JGR 2008] • Active ionospheric feedback and ponderomotive force [Streltsov & Lotko, JGR 2008] t=0 t=40s t=0 t=3min Figure 5, Streltsov & Lotko, [JGR 2008] Figure 7(e), Sydorenko et al., [JGR 2008] Reporter Review: Div III Auroral Processes C. E. J. Watt

  23. Ion upflow and outflow Ionospheric plasma parameters and model: Figure 10, Zettergren et al., JGR 2008 • Modelling of ion upflow/outflow, including electron precipitation, wave-particle interactions, heating, etc: • fluid kinetic model [Zettergren et al., JGR 2007] • dynamic fluid kinetic [Horwitz and Zeng, JGR 2009] • wave-particle interactions [Barghouthi et al., JASTP 2008; Barghouthi, JGR 2008] • Detailed observations: • SIERRA rocket [Lynch et al., AG 2007] • Incoherent scatter radar [Zettergren et al., JGR 2008] pitch angle time (s) Figure 6, Lynch et al., AG 2007 Reporter Review: Div III Auroral Processes C. E. J. Watt

  24. Substorm aurora: Large scale/low frequency undulations • All the physical processes discussed previously apply to substorm aurora • Many repeatable features of substorm aurora that deserve particular study. Figure 3, Keiling et al., [GRL 2008]. Variations in large-scale brightening (21-24MLT) with same period as ion injection, ground Pi2 [Keiling et al., GRL 2008] and boundary oscillation in space [Keiling et al., JGR 2008] in-situ energetic ions auroral photon flux Periodic bright spots related to an instability? Figure 2, Henderson [AG 2009] Reporter Review: Div III Auroral Processes C. E. J. Watt

  25. raw difference Substorm aurora: Expansion phase onset Figure 2, Sakaguchi et al., AG 2009 All-sky TV camera (30Hz); 1s images Figure 1, Liang et al., GRL 2008 THEMIS ASI: 3s images Reporter Review: Div III Auroral Processes C. E. J. Watt

  26. Substorm aurora: Pi1/Pi2 waves and auroral onset Figure 6, Rae et al., JGR 2009 • Substorm onset can manifest as undulations in aurora (λ~10s of km) • Both undulations and large-scale auroral onset location are linked to magnetic perturbations in the Pi1/Pi2 wave bands raw difference Figure 6, Murphy et al., JGR 2009 Reporter Review: Div III Auroral Processes C. E. J. Watt

  27. electron aurora Solar-wind driven aurora dawn proton aurora dusk • Proton and electron aurora show prompt and persistent response to high SW dynamic pressure [Liou et al., JGR 2007; Laundal & Østgaard, JGR 2008]. • Compression of magnetosphere → changing mirror ratio → precipitaton • Dawn-dusk asymmetry suggests that gradient & curvature drift play a role Figure 2, Liou et al., JGR 2007 Figure 1 (a), Laundal & Østgaard, JGR 2007 Reporter Review: Div III Auroral Processes C. E. J. Watt

  28. Aurora equatorward of traditional oval • Isolated arcs equatorward of main auroral oval due to particle scattering by EMIC waves: • Protons: tens of keV[Yahnin et al., JGR 2007; Yahnina et al., JGR 2008; Sakaguchi et al., JGR 2008] • Electrons: MeV[Miyoshi et al., GRL 2008] • All associated with ground-based wave observations in Pc1 band • Sandanger et al., [JGR 2007] show that structure in the relativistic electron precipitation match structures in the anisotropic proton flux → EMIC wave precipitation • Jordanova et al., [JGR 2007] present simulations of sub-auroral arcs due to EMIC waves which compare favourably with observations. Reporter Review: Div III Auroral Processes C. E. J. Watt

  29. Advances in Auroral Science methods • Observations: • High temporal resolution ground-based imagers • Coverage over northern latitudes and many hours of MLT • High temporal resolution imagers with spectral resolution • Low-altitude spacecraft with imager & particle detection • Multi-spacecraft missions • Theory/simulation: • Generation and evolution of parallel electric fields • Non-uniform, non-periodic models • Active magnetosphere-ionosphere coupling • Active experiments: • Laboratory • Ionosphere/Magnetosphere Reporter Review: Div III Auroral Processes C. E. J. Watt

  30. Further information • Reviews published 2007-2009 • Shear Alfvén waves in the magnetosphere [Keiling, SSR 2009] • Downward current region physics [Marklund, SSR 2009] • Laboratory experiments and space physics [Koepke, RG 2008] • Current-voltage relationship [Pierrard et al., JASTP 2007] • Fine structure of aurora [Sandahl et al., JASTP 2008] • Polar cap aurora [Newell et al., JASTP 2009] • EMIC waves and proton precipitation [Yahnin & Yahnina, JASTP 2007] • Artificial stimulation of IAR [Yeoman et al., ASR 2008] • Importance of auroral physics in the Universe [Hultqvist, JASTP 2008] • This review, and the bibliography, is available at: http://www.phys.ualberta.ca/~cwatt/reporter_review Reporter Review: Div III Auroral Processes C. E. J. Watt