Transcript size of radar field of view

Auroral dynamics
EISCAT Svalbard Radar: field-aligned beam
17 Jan 2002
23° x 31°
white light
25 fps
 coherent
scatterspatial
from ion
acoustic
 complicated
structure
(<1waves
km)
 structure size under 300 m at 500 km altitude
 fast temporal variations (<1 second)
 varying on 0.2 second time scale
Auroral dynamics
EISCAT Mainland Radar: position of field-aligned beam
30 Jan 1995
64° x 86°
cut-off filter
1 frame/3s
North
PULSE
 dynamic range problems
experiment
West
 geometry of 3 D multiple structures seen in 2 D
density depletion?
 white light or cut-off filter
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 fieldaligned 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)
Observations
some 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…
EISCAT
mainland
30 Jan 1995
density
depletions?
3 seconds integration
6
horizontal velocity (km/s)
0
-4
-2
angle of maximum
variance,  = 61 E of N
-6
V east
2
4
3 s vectors
from 1837
to 1840 UT
-6
-4
-2
0
V north
2
4
6
radar
beam
N
20 km at 100 km
electric field
vectors
W
density maximum
lags light intensity
2. Modelling
3-D plasma-neutral
fluid model
3D plasma-neutral
fluid model
Single current sheet
Plasma flow in magnetosphere (~ 20,000 km)
Magnetic
reconnection in
acceleration region
(R) connects
magnetic flux from
the back to the front.
Double current sheet
10 km
25 km
Field-aligned electric field maps along the
deformed magnetic field to the bottom
(ionosphere) into a thin elongated region
Can operate at several
heights depending on
local plasma conditions
multiple current layers
(current striation)
auroral filaments
Field-aligned current density and velocity
downward
upward
-5
0
5 km
Slice through at the
acceleration region
(about 1RE) - the height
of maximum E parallel
• perturbation travels along field as Alfvén waves
• strong deformation and filamentation of field-aligned current
In the ionosphere:
filamentary parallel
currents (> 50 μA/m2)
large and variable horizontal
velocities (> 2 km/s)
size of radar field of view
In the ionosphere: after 4.5 s
Maximum of
precipitating energy
(ie auroral emission)
is not coincident with
field-aligned current
layer.
upward
Ionospheric precipitation energy
 simulated auroral image
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
3. The ASK concept
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.
ASK stands for the ”Auroral Structure and Kinetics”
So....
Physics summary
how to generate auroral structure- top to bottom
structure and processes at magnetospheric boundaries
solar wind dynamic pressure changes, magnetic reconnection, Kelvin Helmholtz instabilities,
diffusion by micro turbulence
physical mechanism for transport of information
field-aligned currents and Alfvén waves, fast waves or beams of particles
field-aligned currents – magnetic field geometry altered
If 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