1. VLF scattering Prepared by Morris Cohen, and Benjamin Cotts Stanford University, Stanford, CA IHY Workshop on Advancing VLF through the Global AWESOME Network

2. Ionospheric Changes Daytime Ionosphere ~70-75 km ~80-90 km Nighttime Ionosphere Transmitter Receiver Earth Scattered Wave Incident Wave Reflected Wave Mode conversion

3. 3 VLF scattering A scattering source can “re-radiate” VLF waves

4. 4 Scattering Dependencies  Location of disturbance relative to Tx/Rx  Physical size of disturbance  Density change of disturbance From Poulsen, 1992

5. 5 Size of Disturbance From Poulsen, 1992 Except for extremely small or extremely dense disturbances, most scattering from ionospheric disturbances is largely forward scattering (within ~10-15 o of forward)

6. 6 Density of Disturbance From Poulsen, 1993 More density change does not mean larger disturbance. It is a factor, but it is not the only factor

7. 7 Forward ionospheric scattering Transmitter Perturbation observed Direct signal Ionospheric Disturbance No perturbation observed Scattered signal Ionospheric disturbance must be near to transmitter-receiver path

8. 8 Forward scattering example  Early/fast event observed at array of sites  VLF event observed only at two receivers  Scattering region likely ~90km radius

9. 9 Ionospheric scattering from large disturbances Transmitter Perturbation observed Direct signal Larger Ionospheric Disturbance (a >> ) Perturbation observed Scattered signal From Poulsen, 1992

10. Large Disturbance Example Theoretical Precipitation Region

11. 11 Backscattering from an ionospheric disturbance Transmitter Perturbation observed Direct signal Scattered signal Very dense very small, disturbance ∆Ne >> Ne 0 From Poulsen, 1992 a << λ

12. 12 Backscatter Example to NPM

13. 13 Geological scattering Transmitter Receiver Mountain Range Direct signal Scattered signal

14. 14 Observed scattering Transmitter Receiver Direct signal Scattered signal Direct signal Scattered signal  Scattered signal is weak, delayed version of direct signal  MSK decoding allows separation of two signals  What would this look like for LEP and Early/fast events? Delay

15. 15 MSK Subtraction  Bit time 5 ms, frequency shifts +/- 50 Hz  Time-shifted MSK modulation pattern?  Need to subtract ground signal

16. 16 Geological scattering example  Thomson 1985  NPM/Hawaii  Top panel, reflection from Rockies  Bottom panel, reflection from Andes