Modeling and Subionospheric VLF perturbations caused by direct and indirect effects of lightning

Transcription

1 Modeling and Subionospheric VLF perturbations caused by direct and indirect effects of lightning Prepared by Benjamin Cotts Stanford University, Stanford, CA IHY Workshop on Advancing VLF through the Global AWESOME Network

2 Overview Ionospheric Perturbations Modeling & Data Challenges Recovery Time E.g. LEP Events Onset delay/onset duration Dependencies Early VLF Events (Physical Mechanism) E.g. LEP Event E.g. Early VLF Event 2

3 Problems Subionospheric VLF Observations 1. LEP Events: GLOBAL ELECTRON LOSS RATE 2. Early VLF Events CAUSATIVE MECHANISM a) When time of year? a) Season b) Solar Cycle b) Where a) geographic location? b) Location relative to transmitter or receiver c) Storm type Lightning Characteristics? d) 3

4 The Radiation Belts Particles are trapped in the Earth s magnetic field Gyrate around magnetic field lines Mirror between hemispheres Drift around earth (drift current) Scattered particles precipitate into the atmosphere 4

5 Whistler Waves Source: [Bortnik, 2004] Lightning discharges radiate a broad spectrum of electromagnetic (EM) waves, including waves in the ELF and VLF frequency bands (i.e. 300 Hz to 30 khz) These waves propagate in the earth-ionosphere waveguide A fraction of the wave energy couples into the magnetosphere and propagates as a whistler mode wave 5

6 LEP Characteristics & Dependencies Key: Narrowband measurable Dependence Geomagnetic conditions t r : Recovery time Energy of precip. electrons t/t d : Onset delay/duration Source lightning Location (λ,ϕ λ,ϕ) Spectrum Duration Α/ φ: Amplitude/Phase change Deposition profile 6 Mode pattern present

7 LEP Characteristics & Dependencies Key: Broadband data Modeling/Unknown Other Data Geomagnetic conditions t r : Recovery time Energy of precip. electrons t/t d : Onset delay/duration Source lightning Location (λ,ϕ λ,ϕ) Spectrum Duration Α/ φ: Amplitude/Phase change Deposition profile 7 Mode pattern present

8 LEP Characteristics & Dependencies Geomagnetic conditions t r : Recovery time Energy of precip. electrons t/t d : Onset delay/duration Source lightning Location (λ,ϕ λ,ϕ) Spectrum Duration Α/ φ: Amplitude/Phase change Deposition profile 8 Mode pattern present

9 Geomagnetic Conditions: Kp/Dst Whistlers are launched by nearly every lightning stroke Source: [Peter, 2007] Only when there are sufficient electrons present will we observe LEP events 9

10 LEP Characteristics & Dependencies Geomagnetic conditions t r : Recovery time Energy of precip. electrons t/t d : Onset delay/duration Source lightning Location (λ,ϕ λ,ϕ) Spectrum Duration Α/ φ: Amplitude/Phase change Deposition profile 10 Mode pattern present

11 Lightning (whistler) induced electron precipitation Wave-Particle Interaction Whistler Wave propagates with Right Hand Circular Polarization (RHCP) Counter-streaming electrons gyrate in same direction In the equatorial region Dopplershifted wave frequency equals the electron gyrofrequency Electron experiences a constant electric field Electrons gain or lose energy change electron pitch angle 11

12 Resonant Energy N eq influences refractive index n λ-geomagetic longitude (λ=0 peak resonance) Lightning Geomagnetic Latitude Lightning Source Characteristics Broadband VLF Data 12

13 Monte Carlo Energy Deposition and Secondary Ionization Source: [Peter, 2007] 13

14 Recovery Time vs. Altitude 14

15 LEP Characteristics & Dependencies Geomagnetic conditions t r : Recovery time Energy of precip. electrons t/t d : Onset delay/duration Source lightning Location (λ,ϕ λ,ϕ) Spectrum Duration Α/ φ: Amplitude/Phase change Deposition profile 15 Mode pattern present

16 Calculate at HAIL and East Coast How does geomagnetic longitude affect the LEP signature or occurrence rate? Calculate expected LEP Characteristics For: HAIL (255) East Coast (300) USA E

17 Evolution of Precipitating Electrons Initial Distribution: (E=300 kev,α 200 =77 o ) ϕ=200 o E 17

18 Comparison of ϕ=255 o E and ϕ=300 o E 180 ϕ=255 o E ϕ=300 o E Backscattered Precipitating Northern-hemisphere Southern-hemisphere Backscattered Precipitating More backscatter at ϕ=300 o E than at ϕ=255 o E 0 18

19 Average LEP Characteristics 19

20 Modeling/Data Opportunities Lightning occurrence & disturbed conditions Δt/t d Correction due to Atmospheric Backscatter Predictions for characteristics around the globe need verification/checking Longitudinal dependence of Δt/t d Latitudinal dependence on t R. A/ φ discuss later 20

21 Subionospheric VLF Perturbations t EMP/whistler & precipitating electron propagation time t d Precipitation/secondary ionization t r recovery; return to chemical equilibrium Α/ φ φ Change in amplitude and phase Early VLF Dependencies LEP Dependencies t < 20 ms (Early) - Direct Effect t d <20 ms (Fast) - Impulsive } Physical Mechanism >20 ms (Slow) Extended Under Debate t r Altitude profile of disturbance Α/ φ many factors t - Location (λ,ϕ λ,ϕ), Kp t d - Location (λ,ϕ λ,ϕ), Kp t r Energy of electrons Α/ φ many factors 21

22 On the VLF Reflection Height from J.A. Ratcliffe [1948, pp.110] X=ω p2 /ω 2 ; Z=ν/ω For Z=0, reflection occurs at X=1 (µ =0) With collisions, µ is never zero, and full wave treatment is needed For VLF, Z is too large at X=1 for µ to fall to a small value Reflection occurs when X=Z, assuming the gradients are steep enough 22 22

23 Modal Structure 23

24 Attenuation Rate of Different Modes 24

25 Importance of Modal Profile and Electron Density Profile? 25

26 Scattering Problems Source: [Poulsen, 1991] 26

27 Example Fields Source: [Marshall, 2009] 27

28 Ambient Conditions 28

29 Modal Interference Pattern Source: [Marshall, 2009] 29

30 Event Modeling: LEP Source: [Peter, 2007] 30

31 Model/Data Comparison Source: [Peter, 2007]

32 Event Modeling: Early VLF Source: [Marshall, 2009] 32

33 Event Modeling: Early VLF Source: [Marshall, 2009] 33

34 Disturbance Location vs. Perturbation Magnitude: Early VLF Source: [Marshall, 2009] 34

35 Problems (and Solutions) in Modeling Subionospheric VLF Propagation Problems/Unknowns Measurables/Models 1. Ambient Electron Density Profile Broadband Lightning, e.g. Cummer, Said 2. Location of Disturbance Multiple Paths to triangulate or use lightning 3. Ambient VLF Modal Structure LWPC/FWM/FDTD 4. Disturbance Ionization Profile 1. LEP: WIPP, Monte Carlo 2. Early VLF: Sprites/Halos/EMP/ Elves/Heating 3. MORE DATA & MODELING FROM MORE LOCATIONS! 35

36 Problems (and Solutions) in Modeling Subionospheric VLF Propagation 1. LEP Events: GLOBAL ELECTRON LOSS RATE a) Kp/Dst b) Location Location LOCATION c) Source Lightning Characteristics d) Solar cycle dependence? 2. Early VLF Events CAUSATIVE MECHANISM a) Altitude Profile b) Classify Early VLF Events by other metrics a) Early/Fast vs. Early/Slow b) Sprite-related Events c) Other TLE-related Events? d) Classify by type of causative lightning? e) Location? 36