VLF scattering

Prepared by Morris Cohen, and Benjamin CottsStanford University, Stanford, CA

IHY Workshop on Advancing VLF through the Global AWESOME

Network

Ionospheric Changes

Daytime Ionosphere

~70-75 km

~80-90 km

Nighttime Ionosphere

TransmitterReceiver

Earth

Scattered Wave

Incident Wave

Reflected Wave

Mode conversion

3

VLF scattering

A scattering source can “re-radiate” VLF waves

4

Scattering Dependencies

Location of disturbance relative to Tx/Rx

Physical size of disturbance

Density change of disturbance

From Poulsen, 1992

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 ~10-15o of forward)

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

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

Forward scattering example

Early/fast event observed at array of sites VLF event observed only at two receivers Scattering region likely ~90km radius

9

Ionospheric scattering from large disturbances

Transmitter

Perturbation observed

Direct signal

Larger Ionospheric Disturbance

(a >> )

Perturbation observed

Scattered signal

From Poulsen, 1992

Large Disturbance Example

Theoretical Precipitation Region

11

Backscattering from an ionospheric disturbance

Transmitter

Perturbation observed

Direct signal

Scattered signal

Very dense very small, disturbance ∆Ne >> Ne0

From Poulsen, 1992

a << λ

12

Backscatter Example

to NPM

13

Geological scattering

Transmitter

Receiver

Mountain RangeDirect signal

Scattered signal

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

MSK Subtraction

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

16

Geological scattering example

Thomson 1985 NPM/Hawaii Top panel,

reflection from Rockies

Bottom panel, reflection from Andes