Transcript Document

MHD Simulations of the
January 10-11, 1997 Magnetic Storm

Key aspects of storm
 Large scale ionospheric activity
coupled with density variations
 Large pressure pulse pushes MP
inside geostationary orbit
 Acceleration of relativistic
electrons by ULF waves
 Demise of Telstar 401

Scientific visualizations
provide both scientist and the
general public with
unprecedented view of dynamic
nature of the magnetosphere
Adapted from Goodrich et al. [1998]
Global Distribution / Structure of Aurora
Resonant ULF waves produce premidnight, multi-banded aurora
Satellite Observations
Intense aurora occur statistically in premidnight sector [Newell et al., 1996]
Photograph by Jan Curtis
Ground
Observations
Multi-band arc
structure is typical
D. Pokhotelov, W. Lotko, A. Streltsov— Dartmouth College, 2000
Synthetic Aurora
PI: W. Lotko/Dartmouth
Distribution, Formation & Structure of Discrete Aurora
Synthetic Arcs
Resonant ULF waves
produce pre-midnight,
multi-banded, drifting
auroral arcs
Why do discrete aurorae intensify? drift and fade? form multi-band
structure? occur statistically in pre-midnight and low-conductivity
regions of the ionosphere? Atkinson-Sato feedback between
magnetosphere and ionosphere converts latent energy of convection
into field-line resonant Alfven waves where the conductivity is low
(nightside and winter ionosphere) and where Pedersen and Hall
currents tend to align (typically pre-midnight). Positive feedback
occurs when the Doppler frequency of a drifting, banded density
structure matches the natural frequency of the resonant Alfven
wave. Aurorae ignite when the magnetic field-aligned current of the
Alfven wave is impeded by microturbulence near 1 RE altitude,
producing a parallel electric field and a kilovolt energy boost to
precipitating plasma sheet electrons.
Research by D. Pokhotelov, W. Lotko, A. Streltsov, 2000
Photograph by Jan Curtis
Ground
Observations
Satellite
Observations
Multi-band arc
structure is
typical
Bright arcs occur
statistically in premidnight sector
P.T. Newell et al. 1996
PI: W. Lotko/Dartmouth
Are Alfvénic arcs the most common type of discrete aurora?
Alfvénic Arcs
Resonant ULF waves
produce pre-midnight,
multi-banded, N-S
drifting auroral arcs
Discrete auroras intensify, drift and fade, form multibanded structure, and occur statistically in pre-midnight and
low-conductivity regions of the ionosphere. Simulated
Alfvénic arcs behave similarly.
Latent energy in magnetospheric convection is radiated as resonant
Alfvén waves where the ionospheric conductivity is low (nightside and
winter) and the N-S Pedersen and Hall currents maximize (typically premidnight). “Atkinson-Sato” feedback between the magnetosphere and
ionosphere ensues when the Doppler frequency of N-S drifting, ionospheric
density fluctuations matches the natural frequency of participating, standing
Alfvén waves. The aurora ignites as the wave field-aligned current develops
microturbulence near 1 RE altitude, producing a parallel potential drop and a
kilovolt energy boost to precipitating plasma sheet electrons.
Research by D. Pokhotelov, W. Lotko, A. Streltsov, 2000
Computer Simulation
Photograph by Jan Curtis
Ground
Observations
Satellite
Observations
Multi-band, N-S
drifting discrete
arcs are common
Bright arcs occur
statistically in the
pre-midnight sector
P.T. Newell et al. 1996
PI: W. Lotko/Dartmouth
Are Alfvénic arcs the most common type of discrete aurora?
Alfvénic Arcs
Resonant ULF waves
produce pre-midnight,
multi-banded, N-S
drifting auroral arcs
Discrete auroras intensify, drift and fade, form
multi-banded structure, and occur statistically in
pre-midnight and low-conductivity regions of the
ionosphere. Simulated Alfvénic arcs behave
similarly.
Latent energy in magnetospheric convection is radiated as resonant
Alfvén waves where the ionospheric conductivity is low (nightside and
winter) and the N-S Pedersen and Hall currents maximize (typically
pre-midnight). “Atkinson-Sato” feedback between the magnetosphere
and ionosphere ensues when the Doppler frequency of N-S drifting,
ionospheric density fluctuations matches the natural frequency of
coincident, standing Alfvén waves. The aurora ignites as the wave fieldaligned current develops microturbulence near 1 RE altitude, producing
a parallel potential drop and a kilovolt energy boost to precipitating
plasma sheet electrons.
Research by D. Pokhotelov, W. Lotko, A. Streltsov, 2000
Computer Simulation
Photograph by Jan Curtis
Ground
Observations
Satellite
Observations
Multi-banded,
drifting discrete
arcs are common
Bright arcs occur
statistically in the
pre-midnight sector
P.T. Newell et al. 1996
KILLER ELECTRON STORMS
GEM/ISTP Geomagnetic Storm Event Study
Measured & Simulated
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Measured at GOES-8
Simulated by Lyon-FedderMobarry global MHD model
> 2 MeV ELECTRONS vs UT
Upper. Simulated fluxes – electrons
energized by Lyon-Fedder-Mobarry fields
Lower. Measured fluxes – electrons
at GOES-8: 30 hours spanning
storm main and recovery phases
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Adapted from S. Elkington, Dartmouth College, 2000
MAGNETIC FIELD vs UT
24-26 Sep 1998 Storm
MAGNETOSPHERIC RESONANCE AND AURORA
FAST Measurements of Field Line Resonance
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From Lotko, Streltsov, and Carlson [1998]
DATA
From a FAST satellite pass over a 13minute periodically reforming auroral arc imaged at Gillam.
CANOPUS magnetic, optical and radar data exhibit a
coincident 1.3-mHz “resonant” toroidal pulsation. The EastWest magnetic field of the pulsation is evident in FAST
data (panel 1). An “electrostatic shock” forms in the NorthSouth electric field at this altitude (panel 2). Downward
electron energy flux (panel 3) and upward field-aligned
current (panel 4) are signatures of the arc-related inverted V
precipitation structure, which is collocated with an
upflowing ion beam, flanked to the north and south by
downward suprathermal electron currents.
MODEL Synthetic data from a virtual satellite,
traversing a simulated, 88 s fundamental-mode, field line
resonance layer straddling a dipole L=7.5 magnetic shell.
The plasma is inhomogeneous, sustains anomalous
resistivity where the parallel current becomes supercritical,
and admits the finite electron inertia and ion Larmor radius.
The simulated, instantaneous parallel potential drop is
compared with the measured electron energy flux in panel 3
where positive/negative represents the integrated
downward/upward parallel electric field at the satellite.