Reporter Review:
Auroral Phenomena
Clare E. J. Watt
University of Alberta
26th
August 2009
Reporter Review: Div III Auroral Processes
C. E. J. Watt
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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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Reporter Review: Div III Auroral Processes
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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]
Figure 2(b) [Ergun et al., GRL 2000]
Concentrated potential drops/E||
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Shows typical inverted-V
signature from in-situ FAST
data
Reporter Review: Div III Auroral Processes
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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]
Figure 2, Andersson et
al., PoP, 2008
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C. E. J. Watt
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6
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||
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Figure 8, Hwang et al., JGR 2009a
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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 lowaltitude 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
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Reporter Review: Div III Auroral Processes
C. E. J. Watt
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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
Occurrence vs MLT
Scale size
Figure 6
Figure 9
Red curve = occurrence of
auroral arcs in MLT [Syrjäsuo
and Donovan, AG 2004].
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PC potential vs energy
Figure 8
Black line = 3 x Energy
Peak at 20-40km
Reporter Review: Div III Auroral Processes
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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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Reporter Review: Div III Auroral Processes
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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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Reporter Review: Div III Auroral Processes
C. E. J. Watt
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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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Reporter Review: Div III Auroral Processes
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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]
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Reporter Review: Div III Auroral Processes
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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
(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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• 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]
Reporter Review: Div III Auroral Processes
C. E. J. Watt
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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]
• Flickering aurora –spatial scales 50m1km 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
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Reimei satellite (ISAS)
Name
Objectives
REIMEI
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 cm
Orbit 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
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Reporter Review: Div III Auroral Processes
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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
fce = 4.42GHz
Figure 8(b),
Reporter Review: Div III Auroral Processes McConville et al., PPCF 2008
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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
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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
Figure 1, Morioka et al., [AG 2009]
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• 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?
Reporter Review: Div III Auroral Processes
C. E. J. Watt
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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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Reporter Review: Div III Auroral Processes
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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=0
t=40s
Figure 7(e), Sydorenko et al., [JGR 2008]
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t=0
t=3min
Figure 5, Streltsov & Lotko,
[JGR 2008]
Reporter Review: Div III Auroral Processes
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Ion upflow and outflow
•
Ionospheric
Modelling of ion upflow/outflow,
plasma
parameters and
including electron precipitation,
model:
wave-particle interactions,
Figure 10,
heating, etc:
Zettergren et al.,
– fluid kinetic model [Zettergren et
JGR 2008
al., JGR 2007]
– dynamic fluid kinetic [Horwitz
and Zeng, JGR 2009]
– wave-particle interactions
[Barghouthi et al., JASTP 2008;
Barghouthi, JGR 2008]
– SIERRA rocket [Lynch et al.,
AG 2007]
– Incoherent scatter radar
[Zettergren et al., JGR 2008]
pitch
angle
• Detailed observations:
time (s)
Figure 6, Lynch et al., AG 2007
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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].
in-situ energetic ions
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]
auroral photon flux
Periodic bright spots related to
an instability?
Figure 2, Henderson [AG 2009]
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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
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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
raw
difference
Figure 6, Murphy
et al., JGR 2009
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electron aurora
dawn
Solar-wind driven
aurora
proton aurora
dusk
Figure 2, Liou et al., JGR 2007
Figure 1 (a), Laundal & Østgaard, JGR 2007
• 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
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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 subauroral 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
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