Generation of ELF/VLF waves

Transcription

1 Abstract HAARP is a high power transmitter facility operating in the HF frequency range. After an upgrade to 3.6 MW power scheduled in 2006 it will have an effective radiated power (ERP) of the order of 2 GW in MHZ range, only a factor of ten less than lighning. The emission at 3 MHZ, modulated at 2 khz, will cause a modification of electron distribution at altitudes of km. The resulting change in conductivity will modulate the polar electrojet current and lead to emission of ELF/VLF waves into the magnetosphere. The wave then propagates to a point geomagnetically conjugate to HAARP. We use a time-dependent solver of the kinetic equation, similar to ELENDIF to calculate the electron distribution function. Since the modification of ionosphere affects the propagation of the HAARP radio wave, the power flux is found self-consistently with the solution of the kinetic equation at each altitude. The problem bears significant scientific interest because of highly nonlinear dependence of elecrojet modulation on the applied HF power, and the geomagnetic field effects. 1

2 Generation of ELF/VLF waves 2

3 HAARP High Frequency Active Auroral Research Program After upgrade in March 2006: 180 crossed dipole antennas 3.6 MW power ~2 GW effective radiated HF power ( MHz) (lightning has ~20 GW isotropic ERP) 3

4 HAARP and other HF heating facilities 4

5 Important electron-molecule interaction concept: Dynamic friction force F = Nσ i() v εi Inelastic processes: Rotational, vibrational, electronic level excitations Dissociative losses Ionization (E/N) br =130 Td where 1 Td = V-m 2 5

6 Kinetic Equation Solver (modified ELENDIF) Time-dependent solution for f(v,t) = f 0 (v,t) + cosθ f 1 (v,t) (almost isotropic) Physical processes inluded in ELENDIF: Quasistatic electric field Elastic scattering on neutrals and ions Inelastic and superelastic scattering Electron-electron collisions Attachment and ionization Photon-electron processes External source of electrons New: Non-static (harmonic) electric field Geomagnetic field 6

7 Solution of kinetic equation E ν Effective electric field is smaller than in DC case: E = 2 ω eff 1+ ν meff, / N = 2 10 s m eff meff, ω = ω± ω eff H + ordinary - extraordinary Electron distributions for various RMS E/N (in Td). f>0 corresponds to extaordinary wave (f H =1 MHz, h=91 km) 7

8 HF wave propagation Power flux (1D), including losses: HF conductivity (ordinary/extaordinary) σ 2e = ds dz 2 3/2 = α( SzS, ) α = 2Imk + ox, 1/2 3 m νm i( ω± ωh) ε ε ε 2 R ω iσ k = 1+ c ωε 0 n d ε 8

9 Calculated HF electric field For comparison, we show the dynamic friction function. Current and upgraded HAARP power are both shown. 9

10 Temperature modification (daytime, x mode) 10

11 Comparison of Maxwellian and non-maxwellian approaches 11

12 Electrojet current modulation Conductivity changes due to modification of electron distribution Previous models: E=0 (inaccurate at low frequencies) This model: static fields, div J=0 12

13 Conductivity tensor (DC) Approximate formulas were used before Pedersen (transverse) σ p 2 2e = 3m Hall (off-diagonal) 2 3/2 2 2e ωhε n Ne e ωh σh = d ε 3m ω ν ε ε m ω ν Parallel σ z = ν ε n Ne ν d ω ν ε ε ω ν 3/2 2 m e m ε 2 2 1/2 2 2 H + m m H + m 2 2 1/2 2 2 H m H m e + + n d Ne 2 3/2 2 2 ε e 1 ε 1/2 3m ν m εε m νm 13

14 Conductivity modification Pedersen conductivity is increased Parallel conductivity is decreased 14

15 3D stationary J Vertical B Ambient E is along x Ambient current is mostly along y Models with E=0 do not consider closing side currents max J/J 0 ~0.7 for this case J J = 0 = σ ϕ 15

16 Conclusions Maxwellian electron distribution models, which calculate T e, cannot account for the nonlinear T e saturation. The non-maxwellian model allows to calculate optical emissions and breakdown processes. We are working on a creation of a universal modelling tool for ionosphere heating, which will calculate ELF/VLF emission on ground level and in space. 16

17 Work in progress Current changes in non-static case ELF/VLF emission ELF/VLF wave propagation along the geomagnetic field line 17