26th August 2009

Reporter Review: Div III Auroral ProcessesC. E. J. Watt 1

Reporter Review: Auroral Phenomena

Clare E. J. WattUniversity of Alberta

26th August 2009

Reporter Review: Div III Auroral ProcessesC. 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]

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www.phys.ualberta.ca/~cwatt/reporter_review• Papers selected were published between July 2007 and June 2009.• 214 articles from peer-reviewed sources:

Journal of Geophysical Research (JGR) Earth, Planets & Space (EP&S)

Annales Geophysicae (AG) Nonlinear Processes in Geophysics (NPG)

Geophysical Research Letters (GRL) Planetary and Space Science (P&SS)

Physics of Plasmas (PoP) Journal of Plasma Physics (JPP)

Physical Review Letters (PRL) Physica Scripta (Phys. Scr.)

Science Reviews of Geophysics (RG)

Advances in Space Research (ASR) Space Science Reviews (SSR)

Journal of Atmospheric and Solar Terrestrial Physics (JASTP)

Plasma Physics & Controlled Fusion (PPCF)

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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]

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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||

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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]

0.26 s

Figure 2, Andersson et al., PoP, 2008

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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 8, Hwang et al., JGR 2009aFigure 2(b) [Ergun et al., GRL 2000]Concentrated potential drops/E||

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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.

• 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

Figure 3, Haerendel, JGR 2007

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Inverted-V electrons• Partamies et al., [AG 2008] show occurrence and characteristics of

inverted-V electron signatures using 5 years of quicklook FAST data

Figure 6 Figure 9 Figure 8

Occurrence vs MLT Scale size PC potential vs energy

Red curve = occurrence of auroral arcs in MLT [Syrjäsuo and Donovan, AG 2004].

Black line = 3 x EnergyPeak at 20-40km

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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

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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]

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Short perpendicular scales• What causes short perpendicular

scales 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

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Characteristics of auroral SAW - modellingInhomogeneous 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]

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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

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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]

Figure 9, Semeter et al., JGR 2008

• 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]

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Reimei satellite (ISAS)Name REIMEIObjectives 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 KazakhstanLaunch Vehicle Dnepr (launched together with OICETS satellite)Configuration Weight Approx. 60 kg Dimensions 60 × 60 × 70 cmOrbit Altitude: Perigee 610 km, Apogee 654 kmInclination 97.8°Type of Orbit Near-circular orbitPeriod 97 minScientific 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

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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

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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

Figure 8(b), McConville et al., PPCF 2008

fce = 4.42GHz

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AKR fine structure

FAST data: Figure 1e, Su et al., [JGR 2008]: Alfvén waves

FAST & Cluster data: Figures 2&5, Pottelette & Pickett, [NPG 2007]: Phase space holes

Cluster data: Figure 1, Hanasz et al., GRL 2008]: Alfvén waves

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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]

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Consequences of Auroral Acceleration

• Ion Outflow and Upflow– Wave-driven (shear Alfvén waves)– Electron precipitation and electromagnetic Poynting flux– Ion heating (“pressure-cooker” effect)

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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=0t=3min

Figure 5, Streltsov & Lotko, [JGR 2008]

Figure 7(e), Sydorenko et al., [JGR 2008]

t=0t=40s

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Ion upflow and outflow• 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]

pit

ch

ang

le

time (s)

Figure 6, Lynch et al., AG 2007

Ionospheric plasma

parameters and model:

Figure 10, Zettergren et al.,

JGR 2008

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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]

Periodic bright spots related to an instability?

Figure 2, Henderson [AG 2009]

in-situ energetic ions

auroral photon flux

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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

raw difference

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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

Figure 6, Murphy et al., JGR 2009

raw difference

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Solar-wind driven aurora

• 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

dawn

dusk

proton aurora

electron aurora

Figure 2, Liou et al., JGR 2007 Figure 1 (a), Laundal & Østgaard, JGR 2007

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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.

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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

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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