Laboratory studies of the effects of electron and ion neutral collisions on
electrostatic plasma waves
Bob MerlinoUniversity of Iowa
N. D’Angelo, B. Kustom, D. Susczynsky, S. Cartier, J. Willig
APS DPP Orlando, FL 2002
Outline
• The Farley-Buneman Instability
• The electrostatic ion-cyclotron instability
• The D’Angelo Instability ( Kelvin Helmholtz, parallel velocity shear instability)
Auroral Electrojet
•Large horizontal currentsthat flow in the D and Eregions of the ionosphere
•Remarkable for theirstrength and persistence
•Responsible for type Iirregularities observed by radar backscatter
Equitorial Electrojet
currents driven by E B drifts
ions held back since
electrons drift since
if ve > Cs longitudinal plasma waves grow that travel to B
ciin
ceen
Parameter Auroral Eregion (110 Km)
Laboratory
Plasma density 105 cm-3 109 – 1010 cm-3
Neutral density 1012 cm-3 1013 cm-3
Electric field 20 – 100 mV/m 1 – 5 V/m
Magnetic field 0.5 G 225 G
Te 1200 K 2 eV
Ti 460 K 0.1 eV
10-2 10-4
60 2 - 3
vEB 500 – 2000 m/s 4 – 20 105 cm/s
Cs 500 m/s 2 105 cm/s
fLH 5 kHz 2000 kHz
een /
iin /
Laboratory Experiment on the Farley Buneman Instability
Farley Buneman –results
EE
Spectrum of oscillations
Electrostatic Ion Cyclotron Waves
Space Lab
Current Oscillations due to EIC Instability
Frequency (kHz)
fci
2fci
3fci
4fci
5fci
U. of Iowa Q MachineDecember 17, 2001
10
20
30
40
50
60
70
80
0 50 100 150 200
EIC waves in the diffuse aurora
• Oxygen-cyclotron emissions detected on sounding rocket (Bering, 1983) down to altitudes of 120 km.
• EIC fluctuations seen in the aurora (Martelli et al).
• At these altitudes the ion neutral collision frequency exceeds the oxygen cyclotron frequency.
At what level of collisionality will EICwaves be quenched ?
Collisional EIC Waves For EIC waves driven by parallel electron
currents, analysis* shows that ion collisions are stabilizing (damping) but electron collisions can be destabilizing (growth)
||
||
2
||1
2
1
2
1
kvforgrowth
k
v
k
k
m
m
ed
edeei
*Satyanarayana, Chaturvedi, Keskinen, Huba, Ossakow (JGR ’85)
The Q Machine
button
n ~ 108 – 1010 cm-3 Te = Ti = 0.2 eV
gas
Effect of ion-neutral collisions
fin = 80 kHz
amplitude vs pressure frequency vs pressure
Effect of m+/mn ratio
Experiments done with K+ and Cs+ ions inneutral gases- He, Ne, Ar, Kr, and Xe
For each ion/neutral pair the neutral gas pressure at which wave damping by ion collisions begins to dominate, Pmax is determined by i = Nv, where is the efficiency of momentum transfer per collision
= 2/(1 + m+/mn).
Collisional EICI - Conclusions• EIC waves driven by parallel electron
currrents continue to be excited even for ion-neutral collision frequencies:
• EICI in the bottomside ionosphere could be a source of transversely accelerated heavy ions which eventually travel to higheraltitudes (magnetospheric heavy ions).
in
The D’Angelo Instability*(Kelvin-Helmholtz in fluid dynamics)
x
y
dy
dvx
B
viKH in a plasma:
PVSI
cloud rollup
*also known as the arallel velocity shear instability - PVSI
PVSI in Earth’s Polar Cuspand comet tails
• Ions flow into the polar cusp along magnetic field lines.
• Shear in the ion flow has been observed.
• KH turbulence may cause particles to diffuse across B
• Similar wave motion may occur in comet tails due to shear between cometary ions and solar wind
Collisional PVSI
• F region observations of intense velocity shear and es waves near auroral arcs.
• Irregularity scales sizes ( ) down to m’s.
• Frequencies < in - ‘collisonal modes’
• Basu & Coppi (BC) (1988) propose collisional PVSI with instability threshold
e
iio
in T
T
dx
dV12
1
Experiment on collisional PVSI
B
Shear regionsdr
dVi
Experimentsperformed in adouble-endedQ machine, usinga K+ plasma
PVSI – Experimental Resultsradial electric potential
measurements
V
V~
%25~
ekT
Ve
fluctuationspectrum
fluctuation amplitude
Collisional damping of PVSI
50%
neutral gas pressure (Torr)
Collisional PVSI- comparison with BC theory
ine
iio
T
T
dr
dV
12
then T > 1 for PVSI
in
io drdVT
4/
instability condition:
here Te = Ti, so define
1 10 102 103 104 105
T
Conclusions
• We have studied the effects of electron and ion-neutral collisions on various electrostatic plasma modes.
• Collisions can have decisive consequences for both the excitation and damping of plasma instabilities.
