Ionospheric HF radars

Pasha Ponomarenko

Outline

• Conventional radars vs ionospheric radars

• Collective scatter processes

• Aspect angle effects

• HF propagation features

• SuperDARN

Radars• RAdio Detection And Ranging

• Remote sensing tool based on EM wave emission and its reflection/scatter from a target

Main target parameters:

• range – time of flight

• direction – antenna directivity

• velocity – Doppler shift

Antenna b

eam

echo

Radar Ground

signal

2

tcr

Distance

“Normal” radars

target

The target is usually smaller than the spatial pulse length.

Doppler velocity

f

fcV D

D 2Dft,

Antenna b

eam

Radar

Ionospherewith irregularities

Ground

signal

echo

~300

km

Ionospheric radars

1, eee NNNn

Refractive index fluctuations

The scattering volume is larger than the spatial pulse length.

eNn

n

nn

nnR

22

12

12

2

k

λ

Collective scatter theory

λ/2

irregularities

k

λ

Constructive interference(Bragg condition)

2

l

Spatial array with l = λ/2

Ionospheric turbulenceContinuous spatial spectrum of irregularities

lk

kkS p

2

)(

ln S

ln k

Bragg scatter still works!

ln S

ln k

k

λ

k

λ

λ/2

Constructive interference condition(Bragg condition)

2

l

Anisotropic turbulence

ll ||

E and F region irregularities are aligned with the background magnetic field

B

l

l

B

Dipole antenna with l >> λ

2|| l

Aspect conditions

ψ

ψ

B

phase front

HF Propagation

Ionosphericscatter

Ground scatterGround scatter

Radar

eNN Nf

f

fn ,1

20

22 n <1

n = 1

h

xHF: fN ~ f0

10-20 MHz

SuperDARNSuper Dual Auroral Radar Network

Dual Radar Network

Things to remeber:

• Backscatter signals are produced by plasma structures with Bragg scale sizes, l = λ/2

• Most of the backscatter power comes form areas where the radio wave propagates orthogonally to the geomagnetic field, k B0

• HF signals are capable of over-the-horizon propagation due to consecutive reflections from the ionosphere and the ground