Transcript gem onset arc - UCLA Institute for Geophysics and Planetary

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)
Inverted-V and Alfvénic Acceleration Regions
Inverted-V:
quasi-static
acceleration
Alfvénic
acceleration:
broad
energy,
narrow pitch
angle
Overview of FAST passage
through the auroral oval.
Panels are (top to bottom):
Magnetic field perturbation,
electric field, electron energy,
electron pitch angle, ion
energy, and ion pitch angle.
Blue shading indicates
upward current region, green
is downward currents, and
red is the Alfvenic
acceleration region.
(Courtesy C. Carlson, from Auroral
Plasma Physics, International Space
Science Institute)
What does “quasi-static” mean here?
Inverted-V electrons have roughly the same energy; thus, have fallen
through the same potential drop.
So parallel electric field must be static on time scales of electron
transit time through acceleration region (e.g., Block and
Fälthammar, 1990)
A 100 eV electron has a velocity of roughly 1 RE/s, so “quasi-static”
means < 1 Hz if scale size of acceleration region is ~ 1 RE.
Thus, parallel electric fields associated with field line resonances
(periods of minutes) will be seen as quasi-static; E|| associated with
ionospheric Alfvén resonator (periods of seconds) will give the
characteristics of “Alfvénic arcs”
Ionospheric Alfvén Resonator
Alfvén speed rises sharply above
ionosphere due to exponential fall
of plasma density
Alfvén waves are partially
reflected from this sharp gradient:
wave can bounce between
ionosphere and peak in speed:
Ionospheric Alfvén Resonator
(Periods 1-10 s)
Waves in this frequency range are
commonly observed on ground
and from satellites. Field-aligned
acceleration can also be
modulated at these frequencies.
Profiles of Alfvén speed for high density case
(solid line) and low-density case (dashed line).
Ionosphere is at r/RE = 1. Sharp rise in speed
can trap waves (like quantum mechanical
well). Note speed can approach c in lowdensity case.
IAR Response to a “turn-on” of field-aligned
current
Ex
By
Simulation of Alfvén wave pulse driven by a turning on of the field-aligned
current. Note that even a ramp-like turn on leads to oscillating fields in IAR.
Alfvénic Aurora as Transitional Phase
Changes of field-aligned current require the passage of shear Alfvén
waves along field line.
Thus, Alfvénic nature of onset arc should not be surprising
Similarly, at polar cap boundary, plasma is convecting from open to
closed field lines, requiring transitional readjustment.
Alfvénic aurora can also occur within inverted-V’s: may indicate
smaller changes in current structure.
Speculation: Alfvénic interaction prepares system to allow for quasistatic aurora, especially by excavating density cavity (e.g., Chaston et
al., 2006), creating low densities that are conducive to static parallel
electric fields (Song and Lysak, 2006), and precipitating electrons into
ionosphere to enhance conductivity and produce secondary and
backscattered electrons.
Formation of Density Cavities by Alfvén
Waves
Chaston et al. (2006) has recently
shown FAST observations indicating
strong Alfvén waves in density
cavities, with outward phase
propagation and inward group velocity,
consistent with dispersion relation.
(talk on Friday)
Ion heating and outflow are observed
simultaneously, suggesting that Alfvén
waves are in turn excavating the
density cavity.
Low density regions are conducive to
formation of quasi-static E|| (Y. Song,
later)
Is Ionospheric structure imposed from tail or the result
of M-I coupling?
Alfvénic aurora requires waves on
electron inertial scale: 5 km for a
density of 1 cm-3
Waves at this scale are damped at
higher altitudes where ve > VA
(Lysak and Lotko, 1996; right)
Thus, larger scale waves can
couple structure from
magnetosphere, but not on scale
of individual arcs. (Exception:
large Ti/Te decreases damping)
Wave energy input on large scales
Milling et al. (2008) show timing of Pi1 pulsations (~16 s period)
from ground observations at substorm onset.
Waves must have scales ~ 100 km in order to be observed from
ground due to atmospheric screening.
Results indicate propagation of
signal at 1 hr MLT/20 sec, or
about 30 km/s.
Initial location in region of
downward FAC ( symbol) in
substorm current wedge.
How do these large scale
waves convert to small scale
waves of Alfvénic aurora?
Production of Small Scales by M-I Coupling
Linear phase mixing at density gradients: Perpendicular
variations of Alfvén speed can give rise to phase mixing,
narrowing wave structures.
Ionospheric Feedback: Precipitation associated with upward
field-aligned currents leads to enhanced ionization of the
ionosphere. Secondary currents flowing at conductivity
gradients can lead to positive feedback instability. Coupled
with modes of ionospheric resonant cavity, this instability can
lead to sub-kilometer scales.
Nonlinear and kinetic effects: Nonlinear effects can lead to
cascade to smaller scales. Kinetic effects due to electron
wave-particle interactions may also give rise to structure on
inertial scale. Ionospheric instabilities important?
Phase mixing in Ionospheric Alfvén Resonator
Gradients in the Alfvén speed lead to phase
mixing, producing smaller perpendicular scales
(basic mechanism behind field line resonance.)
VA
Time scale for phase mixing given to a scale L
can be estimated by τ ~ (LA / L)T, where LA is perpendicular
scale length of Alfvén speed and T is wave period. For 1
second wave in IAR, 100 km scale reduced to <10 km in less
than a minute.
Suggests small-scale structure can be produced in presence of
large-scale density gradients.
Simulations of Phase Mixing
Simulations of linear wave propagation including electron
inertia effect were made in a overall perpendicular density
gradient.
Density
Alfvén speed
Simulation results
Ex
By
Simulation initiated with uniform pulse across system oscillating at 1 Hz.
Interference between up and downgoing waves leads to structuring of fields.
Series of harmonics seen due to change of IAR eigenfrequencies.
Waves phase mix to ~ 1 km scale waves.
Ionospheric Feedback
Precipitation of electrons in upward FAC regions enhanced
conductivity; currents at conductivity gradients closed by
secondary FAC.
Interaction not necessarily unstable, but instability occurs if
response of ionosphere and magnetosphere reinforces initial
perturbation.
Threshold for instability depends on drift, perpendicular
wavelength and recombination damping.
k   u d  2.4
VAI

or ud  0.2  VAI
2h
h
Simulations (Lysak and Song; Streltsov et al) show instability
stabilized above ~ 5 mho (background ΣP).
j
j
→
(Lysak, 1990)
Nonlinear interactions
Alfvén wave nonlinear interactions due to
v·v and jb can transfer energy between
scales.
Chaston et al. (2008) show power law
spectrum with breaks at inertial length and
ion gyroradius, suggestive of turbulent
cascade.
However, not classic cascade situation: E/B
ratio decreases at large scales, indicative of
ionospheric damping.
All processes (phase mixing, feedback,
nonlinearity) may operate in concert.