auroral dynamics eiscat svalbard radar: field-aligned beam complicated spatial structure (
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Auroral dynamicsEISCAT Svalbard Radar: field-aligned beam
complicated spatial structure (<1 km)
fast temporal variations (<1 second)
17 Jan 2002
23° x 31°white light
25 fps
coherent scatter from ion acoustic waves
structure size under 300 m at 500 km altitude
varying on 0.2 second time scale
North
West
Auroral dynamicsEISCAT Mainland Radar: position of field-aligned beam
64° x 86°cut-off filter1 frame/3s
30 Jan 1995
dynamic range problems
geometry of 3 D multiple structures seen in 2 D
white light or cut-off filterdensity depletion?
PULSE experiment
Auroral fine structure
examples of discrete auroral structures 0.1 to 1 km wide T.Trondsen (Univ of Calgary)
few instruments can measure it well
few theoretical models can account for it
What are the unsolved problems?
The big one: how are particles accelerated?
Is the filamentary structure important, especially for field-aligned currents?
How well do theories account for the dynamics observed?
Are rays just curls seen from the side?
etc.
Why does it matter?
fundamental plasma physics
implications for macroscopic processes
(photo: Jouni Jussila)
Observationssome properties of discrete aurora
• multiple (parallel) curtains or filaments (< 1km)
• dynamic rayed aurora
• large amplitude spiky electric fields in the acceleration region
• time scales between fractions of seconds and minutes
• strong velocity shear near discrete aurora
• major portion of current carried by low energy electrons
Our approach to the problem- fit measurements to theory
1. Optical and radar observations
2. Modelling (1D, 2D and 3D)
3. New ASK instrument to measure
plasma flows at high resolution,
and low energy precipitation
1. Radar and optical observations
From ground we have three sorts of instruments
field-aligned, eg radars and photometers (temporal) 2D imagers (spatial, with geometrical constraints) spectral imagers (energy information)
Combination of all three in the
Spectrographic Imaging Facility (SIF)
at Longyearbyen…but we are going back with ASK to…
horizontal velocity (km/s)
-6 -4 -2 0 2 4 6
-6
-4
-2
0
2
4
6
V north
V e
ast
3 s vectors from 1837 to 1840 UT
angle of maximum variance, = 61 E of N
25 km
10 km
Plasma flow in magnetosphere (~ 20,000 km)
Magneticreconnection inacceleration region(R) connectsmagnetic flux fromthe back to the front.
Field-aligned electric field maps along thedeformed magnetic field to the bottom(ionosphere) into a thin elongated region
3-D plasma-neutral fluid model2. Modelling
3D plasma-neutral fluid model
Single current sheet
Double current sheet
Can operate at several heights depending on
local plasma conditions
multiple current layers (current striation)
auroral filaments
• perturbation travels along field as Alfvén waves
• strong deformation and filamentation of field-aligned current
-5 0 5 km
Slice through at the acceleration region (about 1RE) - the height of maximum E parallel
Field-aligned current density and velocity
upward
downward
In the ionosphere:
size of radar field of view
large and variable horizontal velocities (> 2 km/s)
filamentary parallel currents (> 50 μA/m2)
upward
In the ionosphere: after 4.5 s
Maximum of precipitating energy (ie auroral emission) is not coincident with field-aligned current layer.
How to generate large velocities
100 nT + average plasma density (1-2 km/s)
400 nT or low density plasma (4-8 km/s)
only very fast time variation can generate high speed flows
To image aurora in the magnetic zenith in forbidden ion line and directly observe plasma drifts, with sub-km and sub-sec resolution. Concurrent imaging in other lines characterises the production of the
metastable ions.
3. The ASK concept
ASK stands for the ”Auroral Structure and Kinetics”
So....
Physics summary how to generate auroral structure- top to bottom
structure and processes at magnetospheric boundariessolar wind dynamic pressure changes, magnetic reconnection, Kelvin Helmholtz instabilities,
diffusion by micro turbulence
physical mechanism for transport of informationfield-aligned currents and Alfvén waves, fast waves or beams of particles
field-aligned currents – magnetic field geometry alteredIf processes lead to a violation of frozen-in condition magnetic field lines have no identity
transport of information not linear physical processes in the inner magnetosphere could alter the magnetic topology, violate the
frozen-in condition and generate structures in addition to those of the source at the magnetospheric boundary
effect of ionosphere changes in ionospheric conductivity from particle precipitation will have a significant influence on
magnetosphere-ionosphere coupling