Sferics and Tweeks

Prepared by Ryan Said and Morris Cohen

Stanford University, Stanford, CA

IHY Workshop on Advancing VLF through the Global AWESOME Network

1

Lightning

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

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)

Tweek Atmospheric

Ionospheric reflections Modal cutoff

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

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 (m=1) Based on actual ionospheric profiles, can calculate high attenuation below 5 kHz

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

Tweek Atmospheric

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

Tweek Atmospheric

z y x Ground Wave 1 st mode cutoff Ray Hops

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

Long-Range Sferic

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

Lightning characteristics

+ - - + + + + + + + + +

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

+ ++ + + + + + + + + + + + + + + + +

Total charge moment (I.e., C•km) 12

VLF Peak

Sferic Characteristics

ELF “Tail”

• VLF peak

– Mostly TM Modes – 8-12 kHz peak energy

• ELF peak

– Delayed – TEM mode – Associated with sprites – <1kHz energy 13

Peak Current

+ + + + + + + + + + Return stroke peak current (i.e., kA)  Peak current is proportional to VLF peak for a given propagation path + + + + + + + + VLF Peak 14

Total Charge Moment

+ + + + + + + + +

Total charge moment (I.e., C•km)  

Total

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

+ +

ELF Energy 15 Reising [1998]

Determining Azimuth

Incident wave S Φ NS

Single Frequency:

NS ~ S*cos(Φ) EW ~ S*sin(Φ) If same constant of proportionality: EW/NS = tan(Φ)

Φ = tan -1 (EW/NS)

Wood and Inan [2002] EW Band of frequencies: use a weighted average   

f l f u

tan  1  

EW

(

f

)

NS

(

f

)   

f f l u

|

NS

(

f

) | 2  |

EW

(

f

) | 2

df

|

NS

(

f

) | 2  |

EW

(

f

) | 2

df

16

Determining Azimuth cont’d

Short FFT Calculated azimuth For each frequency, compare magnitude from NS and EW antenna to calculate azimuth, then average over frequency:  

f u N k



f



f s N l f s

tan  1  

f u N k



f s



f N l f s EW NS

(

kf

(

N kf s N s

) )   |

NS

(

kf s N

) | 2 |

NS

(

kf s N

) | 2  |

EW

(

kf s N

) | 2  |

EW

(

kf s N

) | 2 17

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])

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

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 tweek measurements of the low-middle latitude D-

Journal of Atmospheric and Solar-Terrestrial Physics

,

2006

, 22-29 – Calculation of propagation distance and ionospheric height based on • Ohya, H. et al., “Using tweek atmospherics to measure the response region ionosphere to a magnetic storm,” , 697-709 – Ionospheric diagnostics based on tweek measurements