Sferics and Tweeks

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Sferics and Tweeks

• Prepared by Ryan Said and Morris Cohen
• Stanford University, Stanford, CA
• IHY Workshop on
• Advancing VLF through the Global AWESOME Network

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Lightning

• Different types of lightning CG, -CG, IC
• Current forms a large electric field antenna,
  radiating radio waves
• Large component in VLF range

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Sferic in Earth-Ionosphere Waveguide

• Shape of sferics, tweeks vary by ionosphere and
  ground profile
• Tweeks more common at night, where ionosphere
  reflects more energy (lower electron collision
  rate at higher altitude)

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Tweek Atmospheric
Ionospheric reflections
Modal cutoff
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Ray Model

• Ionosphere enables long-range propagation of
  emitted radio pulse
• Guided radio pulse called a Radio Atmospheric,
  or Sferic
• Sferic with many visible reflections forms a
  Tweek Atmospheric
• Hop arrival times related to ionospheric
  reflection height
• Arrive later during nighttime (higher and
  stronger reflection at night than during day)
• See Nagano 2007 for dependence of arrival time
  with height

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Modal Model

• Modal analysis each mode dictates waveguide
  velocity, attenuation rate
• Discrete modes are functions of frequency,
  boundary reflections
• Solve by requiring phase consistency between F1,
  F3
• Each mode has a cutoff frequency fc
• Below this frequency, attenuation is very high
• Nighttime ionosphere fc 1.8 kHz for the first
  mode (m1)
• Based on actual ionospheric profiles, can
  calculate high attenuation below 5 kHz

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TE and TM Modes

• Sferic consists of a combination of TE
  (Transverse Electric) and TM (Transverse
  Magnetic) modes
• Vertical lightning channel preferentially excites
  TM modes
• Horizontal loop antennas measure Hy (from TM) and
  Hx (from TE)
• Tweeks contain more Hx than early part of sferics

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Tweek Atmospheric

• Many Ionospheric reflections visible
• Ray model individual impulses
• Modal model summation of modes
• Many modal cutoff frequencies visible

Ionospheric reflections
Modal cutoff
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Tweek Atmospheric
z
x
y
1st mode cutoff
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Long-Range Sferic
Slow Tail

• High attenuation below 5 kHz (especially during
  daytime)
• No tweeks at long range too much attenuation
• Slow Tail from QTEM mode
• Waveguide dispersion
• Lower frequencies travel slower than higher
  frequencies
• Lower frequency components seen to arrive later

Dispersion
Slow Tail
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Long-Range Sferic

• Time-domain short impulse (top panel)
• Frequency-domain smooth, mostly single mode
  (bottom panel)
• Minimum attenuation near 13 kHz

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Lightning characteristics


































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Return stroke peak current (i.e., kA)
Total charge moment (I.e., Ckm)










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Sferic Characteristics

• VLF peak
• Mostly TM Modes
• 8-12 kHz peak energy
• ELF peak
• Delayed
• TEM mode
• Associated with sprites
• lt1kHz energy

VLF Peak
ELF Tail
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Peak Current

• Peak current is proportional to VLF peak for a
  given propagation path

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Return stroke peak current (i.e., kA)




VLF Peak




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Total Charge Moment

• Total ELF energy is proportional to total charge
  transfer
• ELF energy attenuates more in Earth-ionosphere
  waveguide

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Total charge moment (I.e., Ckm)

ELF Energy

Reising 1998
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Determining Azimuth
Wood and Inan 2002
Band of frequencies use a weighted average

• Single Frequency

NS Scos(F) EW Ssin(F) If same constant of
proportionality EW/NS tan(F) F tan-1(EW/NS)
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Determining Azimuth contd
Short FFT
Calculated azimuth
For each frequency, compare magnitude from NS and
EW antenna to calculate azimuth, then average
over frequency
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Future Work

• Use methods in previous references to monitor
  ionosphere during various conditions (night/day,
  summer/winter, low-/mid-/high-latitude)
• As a side effect, can monitor strike locations
  (especially when Tweeks are visible, see Nagano
  2007)

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References Theoretical and Background

• Budden, K. G., The wave-guide mode theory of
  wave propagation Logos Press, 1961
• Overview of theoretical framework for waveguide
  propagation
• Budden, K. G. The Propagation of Radio Waves
  Cambridge University Press, 1985
• Detailed methodologies for calculating
  electromagnetic propagation characteristics
• Galejs, J. Terrestrial propagation of long
  electromagnetic waves Pergamon Press New York,
  1972
• Calculation of earth-ionosphere waveguide
  propagation
• Rakov, V. A. Uman, M. A. Lightning - Physics
  and Effects Cambridge University Press, 2003,
  698
• Overview of the lightning strike, including
  models for electromagnetic radiation from
  lightning (little emphasis on waveguide
  propagation)
• Uman, M. A. The Lightning Discharge Dover
  Publications, Inc., 2001
• Overview of lightning processes

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References Calculations

• Wait, J. R. Spies, K. P. Characteristics of
  the Earth-Ionosphere Waveguide for VLF Radio
  Waves National Bureau of Standards, 1964
• Numerical evaluation of waveguide propagation
  based on assumed ionospheric profiles
• Nagano, I. Yagitani, S. Ozaki, M. Nakamura, Y.
  Miyamura, K. Estimation of lightning location
  from single station observations of sferics
  Electronics and Communications in Japan, 2007,
  90, 22-29
• Calculation of propagation distance and
  ionospheric height based on tweek measurements
• Ohya, H. et al., Using tweek atmospherics to
  measure the response of the low-middle latitude
  D-region ionosphere to a magnetic storm, Journal
  of Atmospheric and Solar-Terrestrial Physics,
  2006, 697-709
• Ionospheric diagnostics based on tweek
  measurements

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