PARS Workshop on Novel Methods of

Excitation of ULF/ELF/VLF to

Improve Efficiency and Availability" November 3 - 6, 2002

Sponsored by Air Force Research Laboratory,Office of Naval Research

University of Alaska, Fairbanks

Institute of Plasma Science and Technology, UCLA Arrowhead Conference Center

Goals and Objectives

1. Review experiments on EM interactions with Ionosphere leading to ULF/ELF/VLF (UEV)2. Efficient Generation of waves with and

without electrojet• Examine New Approaches High power EM

pulses at HF and Laser frequencies• 3. Improved methods of Detection• 4. Laboratory experiments and Computer

Modeling

UEV

• Physical Pictures.

• VLF - Whistler waves

• Accessibility and electron cyclotron resonance.

• ELF waves – Ion cyclotron waves.

• ULF waves – Alfven waves.

Whistler waves are accessible for propagating into and heating the high density plasma

The RHCP Whistlers can be excited into both low and high density plasma by launching from high magnetic field (ce/ > 1).

These waves do not go across the R-cutoff layer, and pass the L-cutoff without being affected.

The accessibility problem arises in the vicinity of the boundary pe/ = 1.The CMA diagram shows that the RHCP waves propagation along B will pass this boundary, but those perpendicular to B will be reflected.

The LHCP ion cyclotron waves are similar to the RHCP electron whistler waves. They can be excited using a dipole loop antenna inside the plasma.

Excitation LHCP Ion Cyclotron Waves by Modulation of the Diamagnetic Dipole at ELF/VLF with AM HF Power.

Density

t = 0 t = /4 t = /2 t = 3/4

B B B B E

EE

E

Electrons can be heated by electromagnetic wavesnear the electron cyclotron resonant zone

For whistler waves, the resonance condition requires

- ce - kz vz 0

Strong absorption occurs for those electrons moving backwards kz vz (-ce) < 0.

ECR condition:

ce

in uniform B.

Alfven Wave B1 , v1 , E1 , J1

ELF/VLF Excitation by Pulsed HF Powerat the Electron Cyclotron Resonance

1. Accelerate electrons to ionizing energy using pulsed HF ECR power. 2. Production of high density plasma by impact ionization.3. Formation of diamagnetic plasma disk by multiple pulses of HF power. 4. Modulation of diamagnetic plasma by electron heating using CW HF

power modulated at ULF/ELF/VLF range. 5. Excitation of Low frequency Whistler modes and ion cyclotron waves

to further enhance the ULF/ELF/VLF signals.

Electron WhistlerWaves

Diamagnetic DipoleMoment

Ion WhistlerWaves

S

N

Our goal is to generate a largeplasma magnetic dipole momentbelow or above 100 km above HIPAS.

The ELF/VLF magnetic field produced can be sensed around the world through the earth-ionosphere-waveguide.

Alfven Wave

Plasma Diamagnetic Current J = c BXp/B2

I = dz dr J

Magnetic Dipole Moment m = a2 I /c nTV/B

L

A plasma with electron density n=1x 1011 m-3 Te = 1 eV and L= 3 km; r=10 km; p= nT = 1x1011 eV m-3 Diamagnetic Current Carried by the Plasma: I = 1 A Magnetic Dipole Moment: m = r2 I = 3.14 x 108 A-m2

p

J

Localized Plasma produced by HF ECR Carried Magnetic Dipole Moment

The objective of an active ionosphere modification is to increase and modulate the diamagnetic dipole moment at the ELF/VLF by HF radiation from ground.

Plasma current produced by HF wave heating can generate significant ELF/VLF radiation signals

r

m = a2 I/cMagnetic field of magnetic dipole moment:

Br = 2 m cos/r3

B= m sin/r3

For I = 100A, a = 10 km, the magnetic field induction at 100 km from the dipole ring is about 6.3 pT.

Higher ELF/VLF signal levels are expected from collective plasma oscillations and reflection from the ionosphere (the earth-ionosphere waveguide effect).

100 km

100 km3.6 pT 6.3 pT

I = 100A a = 10 km

-40-20

020

40

-40

-20

0

20

400

20

40

60

80

100

x (km)

Frequency 1.43 MHz dn=0.4 x mode

y (km)

z (k

m)

-100-50

050

100

-100

-50

0

50

1000

50

100

150

x (km)

Frequency 1.43 MHz dn=0.4 o mode

y (km)

z (k

m)

The RHCP wave power is completely absorbed at the ECR zone while the LHCP wave is reflected at the L-Cutoff boundary

Ray tracing for the electromagnetic waves satisfying the Appleton Hartree dispersion relation

>p >p

-40-20

020

40

-40

-20

0

20

400

20

40

60

80

100

x (km)

Frequency 1.43 MHz dn=2 x mode

y (km)

z (k

m)

<p

-50

0

50

-50

0

500

20

40

60

80

100

x (km)

Frequency 1.43 MHz dn=2 o mode

y (km)

z (k

m)

<p

RHCPLHCP

Whistler wave ducting by a low plasma density trough will effectively increase the HF power flux at the target region

Experimental demonstration of the unducted and ducted Whistler waves.

Whistler Wave Propagation and Absorption

Index of Refraction (n = ck/)

n2 = 1 + p2/2/ [(ce/) cos -1]

Wave Absorption

ki = - Di/(D/k) = p2/(2c2k2ve) EXP{-[( -ce)/kve]2}

Pabs = {1- exp [-2⌠ki(x)dx]} P

Resonance Length Lres:

Length of layer with: 1% < Paps/P < 99%

95 95.5 96 96.5 97 97.5 98 98.5 99 99.5 1000

1

2

n=ck

/w

Whistler Wave Absorption @ HIPAS

95 95.5 96 96.5 97 97.5 98 98.5 99 99.5 1000

2ki

(km

-1)

95 95.5 96 96.5 97 97.5 98 98.5 99 99.5 1000

0.5

1

E(r

el.u

.)

95 95.5 96 96.5 97 97.5 98 98.5 99 99.5 100-1

0

1

Eco

s(kz

)

range(km)

Whistler Wave Propagation and Absorption

In the Ionosphere Plasma @ 100 km

Index of refraction

Imaginary wavenumber

Wave Amplitude

Waveform

The absorption layer for the electron whistler is typically 2-3 kilometer thick.

Electron Heating by Electron Whistler WaveAgainst Electron-Neutral Collisions

Electron heating in single pass of electron Cyclotron Resonance:

E = (Zo/(ck/)*P/A)1/2

= Min (e-n, res, 0 ) v = e/m E

T = ½ e2/mE2 2

e-n = 10-5 sec (collision)res = 10-4 sec (Resonance) 0 = 10-4 sec (Pulse)

The electron-neutral collision time e-n 10-5 sec at about 100 km altitude in the E-layer where the normal electron temperature is cold, Te = 0.03 eV (300 K).

The electrons must gain enough energy (T > 20 eV) in a time short compared to e-n such to minimize excitation energy loss.

It requires about 20 mW/m2 of the HF power flux to bring the electrons to the ionizing energy level by electron cyclotron resonance heating in the ECR layer.

Pulsed HF Power Will be Used for Electron Accelerations Frequency: f = 1.4 MHzPlasma Parameters ne = 10000 cm-3

Vacuum Electric Field Eo = 4 V/m ( = 128 mW/m2 )Refraction Index n = 3.4Plasma Heating Field E = 4.3 V/mMinimum Collision Time: min 1 x 10-5 sec.Energy Gain (per pass) T = 46.9 eV

The 4 V/m electric field requires a power flux, = 21.3 mW/m2, or 2.7 GW ERP. This power level will be available from pulsed transmitters currently under development at HIPAS Observatory.

Creating High Density Plasma by electron Impact Ionizations

The energetic electrons created by HF ECR are capable of ionizing the background neutral particles in a fast time scale, I.e. 10-5 sec.

Plasma produced will be localized in the heating region for a time scale of electron-ion recombination time, I.e. ~100 sec.

Thus a very high density plasma can be created using multiple HF pulses in a time scale of the recombination time.

Areas of Further Investigation and Preparation for HIPAS Active Ionospheric Modification Experiments

1. Study the wave propagation and wave-particle interactions using realistic 3D PIC simulations

2. Understanding the detailed electron heating and diamagnetic plasma formation process using laboratory modeling experiments with relevant scaling parameters.

3. Detailed power balance including excitation energy losses

4. Develop techniques for charging and discharging the high power pulsed antenna array.

Van Zeeland et al. PRL 2002.

Van Zeeland et al. PRL 2002.