Sferics and Tweeks

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

2. Lightning • Different types of lightning: +CG, -CG, IC • Current forms a large electric field antenna, radiating radio waves • Large component in VLF range 2

3. 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) 3

4. Tweek Atmospheric Ionospheric reflections Modal cutoff 4

5. 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 5

6. 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 6

7. 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 7

8. Tweek Atmospheric • Many Ionospheric reflections visible • Ray model: individual impulses • Modal model: summation of modes • Many modal cutoff frequencies visible Ionospheric reflections Modal cutoff 8

9. Tweek Atmospheric z x y Ray Hops Ground Wave 1st mode cutoff 9

10. 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 SlowTail 10

11. Long-Range Sferic • Time-domain: short impulse (top panel) • Frequency-domain: smooth, mostly single mode (bottom panel) • Minimum attenuation near 13 kHz 11

12. Lightning characteristics + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + - - - - - - - - - - - - - - - - - - Return stroke peak current (i.e., kA) Total charge moment (I.e., C•km) + + + + + + + + + +

13. Sferic Characteristics • VLF peak • Mostly TM Modes • 8-12 kHz peak energy • ELF peak • Delayed • TEM mode • Associated with sprites • <1kHz energy VLF Peak ELF “Tail”

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

15. Total Charge Moment + + • Total ELF energy is proportional to total charge transfer • ELF energy attenuates more in Earth-ionosphere waveguide + + + + + + + + + + + + + + - - - - - Total charge moment (I.e., C•km) + ELF Energy + Reising [1998]

16. NS Incident wave S Φ EW Determining Azimuth Wood and Inan [2002] Single Frequency: Band of frequencies: use a weighted average NS ~ S*cos(Φ) EW ~ S*sin(Φ) If same constant of proportionality: EW/NS = tan(Φ) Φ = tan-1(EW/NS)

17. 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:

18. 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]) 18

19. 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 19

20. 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 20