1. N. D’Angelo, B. Kustom, D. Susczynsky, S. Cartier, J. Willig
Laboratory studies of the effects of electron and ion neutral collisions on electrostatic plasma waves
Bob Merlino
University of Iowa
N. D’Angelo, B. Kustom, D. Susczynsky, S. Cartier, J. Willig
APS DPP Orlando, FL 2002

2. Outline The Farley-Buneman Instability
The electrostatic ion-cyclotron instability
The D’Angelo Instability ( Kelvin Helmholtz, parallel velocity shear instability)

4. Auroral Electrojet
Large horizontal currents that flow in the D and E regions of the ionosphere
Remarkable for their strength and persistence
Responsible for type I irregularities observed by radar backscatter

5. 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

6. Parameter
Auroral E region (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
vEB
500 – 2000 m/s
4 – 20  105 cm/s Cs
500 m/s
2  105 cm/s
fLH
5 kHz
2000 kHz

7. Laboratory Experiment on the Farley Buneman Instability

8. Farley Buneman –results
Spectrum of oscillations

9. Electrostatic Ion Cyclotron Waves
Space
Lab

10. Current Oscillations due to EIC Instability

11. 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 EIC waves be quenched ?

12. 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)
*Satyanarayana, Chaturvedi, Keskinen, Huba, Ossakow (JGR ’85)

13. The Q Machine
gas
button
n ~ 108 – 1010 cm Te = Ti = 0.2 eV

14. Effect of ion-neutral collisions
amplitude vs pressure
frequency vs pressure
fin = 80 kHz

15. Effect of m+/mn ratio
Experiments done with K+ and Cs+ ions in neutral 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 = Nv, where  is the efficiency of momentum transfer per collision  = 2/(1 + m+/mn).

16. 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 higher altitudes (magnetospheric heavy ions).

17. The D’Angelo Instability* (Kelvin-Helmholtz in fluid dynamics)x y
cloud rollup vi
KH in a plasma:
PVSI B
*also known as the arallel velocity shear instability - PVSI

18. PVSI in Earth’s Polar Cusp and 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

19. 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

20. Experiment on collisional PVSIB
Shear regions
Experiments
performed in a
double-ended
Q machine, using
a K+ plasma

21. PVSI – Experimental Results
radial electric potential
measurements
fluctuation
spectrum V
fluctuation
amplitude

22. Collisional damping of PVSI
50%
neutral gas pressure (Torr)

23. Collisional PVSI- comparison with BC theory
instability condition:
here Te = Ti, so define 1 10
102
103
104
105 T
then T > 1 for PVSI

24. 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.