Bob Merlino University of Iowa 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 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.