radio waves in the ionosphere - university … waves in the ionosphere e. s. miller geospace and...

Radio Waves in the Ionosphere

E. S. Miller

Geospace and Earth Science GroupJohns Hopkins University Applied Physics Laboratory

24 June 2012

Miller CEDAR: Ionospheric Radio

Introduction

• Scope

• History

• Ionospheric propagation

• Ionospheric irregularities


This Discussion is...

•Fundamental...it explains the origin of radio science.

•Informal...interject and ask questions at any time.

•Phenomenological...definitions and derivations are fortextbooks, EM courses, and theorists.

•Intuitive...to compliment theoretical understanding.

•Basic...geared toward scientifically-literate non-experts.


Introduction

•What is a wave? A (predictably) propagatingperturbation in a medium.

•What are EM fields? Formalism to describe forces oncharges (electric fields) and currents (magnetic fields) ata distance.

What is an EM wave?A propagating perturbation that exerts forces on charges andcurrents.

• This discussion will focus on radio waves—EM waveswith phase velocities near the speed of light, wavelengthsof order 1–1000 m, and E and H fields approximatelyperpendicular to k.


Marconi 1901 Transatlantic Test

Signal Hill, Newfoundland


Discovering the Ionosphere

• Question: How did a radio signal propagate 1000s of kmfrom Cornwall to Newfoundland given Earth’s curvature?

• Theory: Heaviside and Kennelly independently proposed aconductive, reflective region or layer that guided radiowaves.

There may possibly be a su�ciently conducting

layer in the upper air. If so, the wave will, so to

speak, catch on to it, more or less. Then the

guidance will be by the sea on the one side and

the upper layer on the other.

• Mental note: this test occurred at f ⇠ 500 kHz withMarconi reporting reception at Signal Hill “just beforenoon.”


Existence of the Kennelly-Heaviside Layer

• First confirmed by observations of de Forest and Fullerwith analysis by Pierce, 1912.

• Identified frequency-selective fading due to interferencebetween ground- and sky-wave signals.

• Pierce suggests a single reflecting layer at “196 miles,”corrected by de Forest to 62 miles, or 100 km.

• de Forest suggests multiple reflection points at muchlower (⇠ 20 km) altitudes.

• Rigorously tested by• Breit and Tuve, 1925—pulse sounder• Appleton and Barnett, 1925—FMCW sounder• These experiments confirm that the 100-km figure wascorrect—the ’E’ region.


Ionospheric Sounding

Frequency [MHz]

Vir

tual

Hei

ght

[km

]Wallops Island, VA 1600 LT 14 Jun (196) 2012


Radio Properties of the Ionosphere

• Plasma frequency: !p

⌘q

n

e

q

e

✏0me

⇠ 2⇡ ⇥ 9 MHz.

• Proxy for “cold” plasma permittivity/conductivity.• Loosely, electrostatic analog to Brunt-Vaisala frequency.

• Refractive index: n(!) =q

1� !2p

!2

• Recall that vg

= c0

n

, vp

= c0n.• lim!!!

p

n(!) = 0. vg

blows up, vp

= 0.• lim!!1 n(!) = 1. v

g

⇠ v

p

! c0.• Plasma is dispersive.

• Reflection occurs as n ! 0.

• Ionosondes map plasma frequency versus “virtual” height.That is, height assuming v

p

= c0.



Frequency [MHz]

Vir

tual

Hei

ght

[km

]Wallops Island, VA 1600 LT 14 Jun (196) 2012



Frequency [MHz]

Vir

tual

Hei

ght

[km

]Jicamarca, Perú 1930 LT 14 Sep (257) 2006


HF Propagation

ď��ď��

ď��ď��

ď��ď��

ď��ď��

��

��

��

�

��

��

��

��

��Wallops Island Millstone Hill

;EPPSTW�-WPERH�7YTIV(%62�FIEQ��O,^�O-mode

Bk

East Geographic Longitude [deg] North Geographic Latitude [deg]

%PXMXYHI�?OQ

A


Magnetoionic Propagation

• In the presence of the geomagnetic field, the conductivityof the (cold) plasma can no longer be represented by ascalar.

• For k = cos ✓az

+ sin ✓ax

, Appleton-Hartree equation:•n

2 = 1� X

1�F

•F =

Y

2T

±p

Y

4T

+4Y 2L

(1�X )2

2(1�X )•Y

T

⌘ Y sin ✓, YL

⌘ Y cos ✓

•X ⌘ !2

p

!2 , Y ⌘ ⌦e

!

• What does this tell us?•F contains ±: Two modes possible, ordinary andextraordinary.

• The ordinary (O) mode is TEM, the extraordinary (X) isnot.


Mode-Splitting


Faraday Rotation

• The polarization of a linearly-polarized wave rotates in amagnetized plasma, depending on:

• Electron density.

• Magnetic aspect angle ✓ = arccos⇣k · B

⌘

• Exploit this technique to estimate integrated (“total”)electron density (TEC) along a path.

• Cross-polarization losses are substantial (10s of dB), solinear polarization is undesirable for spacecraftcommunications. RHCP is standard, worst-case 3 dB fadewhen the signal is linearly-polarized.


Absorption

• Alternative taxonomy ofionospheric layers:

•F region (> 150 km):collisionless

•E region (90 < alt < 150km): collisional (e-i)

•D region (60 < alt < 90km): very collisional

• If the D region is ionized, itabsorbs radio waves,especially in the lower HF andMF bands.

• Daytime (solar illumination),auroral precipitation.

Signal Hill, Newfoundland


Radar Remote Sensing

• Pulsed (coded and uncoded),narrow bandwidth.

• Usually HF (SuperDARN), orVHF (most ISRs, meteor),UHF (AMISRs, ALTAIR), andL-band (Sondrestrom) radars

• Typical output product:backscatter spectrum.(Audio: aurora backscatter at50 MHz)

• Recall radar equation:P

r

= P

t

G

t

G

r

�(4⇡)2R4


Audio Example

Time [arb. units]

Freq

uen

cy [

Hz]

0

200

400

600

800

1000K0KP/B - K9MU Bistatic 50-MHz Auroral “Radar”


Incoherent Scatter

• The “cold” plasma approximation from before:n

e

(r, t) ⇡Rf (r, v, t)dv.

•f (r, v, t) is a PDF describing the density.

• For example, f (r, v, t) = n

e

g0(v) + f1(v, t)e�jk·r

• This “hot” plasma formalism implies a spectrum ofthermal electrostatic number density waves in theplasma, n

t{e,i}(t, k)

• Backscatter radar spectrum is proportional to plasmadensity wave spectrum:h|V (!)|2i ⇠ h|F{n

t{e,i}(t,�2k0ar )}|2i• k = 2k0ar is the Bragg criterion.• Backscatter cross section is very small.

• ISR technique yields precision remote plasma diagnosticsversus altitude.


Typical ISR: Jicamarca, Peru


Typical ISR: Sondrestrom, Greenland


Coherent Scatter

• Plasma instabilities create density waves (“irregularities”)above the thermal level. That is, the plasma is no longerin thermal equilibrium.

• These waves have backscatter cross sections many dBabove that of the thermal waves.

• Often aspect-sensitive1 to within ⇠ 0.1�: k ? B;“Field-Aligned Irregularities (FAI)”

• Examples:• Auroral electrojet, radio aurora• Mid-latitude quasi-periodic echoes (QPE)• Equatorial electrojet, spread-F

• Bragg scale: irregularity scale is �radar

/2

1This is a non-trivial measurement.Miller CEDAR: Ionospheric Radio

Coherent Scatter: E -region Echoes

• Loci ofperpendicularity from90 to 120 km, norefraction.

• Possibly QPE (⇠ 15km), but range gatesare 45 km.

• Ground scatter.

• Mixed scatter.


HF Propagation

ď��ď��

ď��ď��

ď��ď��

ď��ď��

��

��

��

�

��

��

��

��

��Wallops Island Millstone Hill

;EPPSTW�-WPERH�7YTIV(%62�FIEQ��O,^�O-mode

Bk

East Geographic Longitude [deg] North Geographic Latitude [deg]

%PXMXYHI�?OQ

A


Coherent Scatter: Equatorial E - and F -region CXI North 19 August (232) 2004

6 7 8 9 10 11 12 13 14 15 16200400600800

1000

CXI North 20 August (233) 2004

Ran

ge [

km]

6 7 8 9 10 11 12 13 14 15 16200400600800

1000

CXI North 21 August (234) 2004

Universal Time [hrs]6 7 8 9 10 11 12 13 14 15 16

200400600800

1000


Spread-F

Frequency [MHz]

Vir

tual

Hei

ght

[km



Spread-F

• Two types: frequency and range (just shown).

• Range spread F is most common at the geomagneticequator, where it occurs seasonally and just after localsunset—Equatorial Spread-F (ESF).

• Ionogram signature due to specular echoes fromionospheric density gradient.

• Coherent scatter from Bragg-scale FAIs (1, 3, or 5 metersfor common 150-, 50-, and 30-MHz radars) within adepleted region.


Spread-F

232

059

K p

B Ap

ex [k

m]

200

1000

I [co

unts

]

2000

7000

B Ap

ex [k

m]

200

1000

SNR

[dB]

ï��

20

Universal Time [hrs]

B Ap

ex [k

m]

7 9 11 13 15200

1000

East

Drif

t [m

/s]

ï��

0

150

19 August(232) 2004CXI / CNFI

geomagnetic equator B

NAirglow Intensity

227 237

Geophysical ActivityRadar Beam

Field-Line Apex Altitudes

Frame-to-Frame Correlationyields feature velocity.

Local Midnight

Site/Date


Scintillation

• Scintillation: Fading of transionospheric radio signals.

• Discovered by early radio astronomers observing radiostars (pulsars):

• Theory: Variability of source.• Test: Variations proved uncorrelated between twoobservatories some 100 km apart.

• Alternative: Ionospheric irregularities.

• Later observed on satellite radio beacon signals, thenGPS/GNSS.

• Amplitude fades of 20 dB possible, especially at VHF.Also, phase scintillation.

• Widely-cited “broader impact” of ionosphere on society.

• Found to be correlated with the occurrence of spread F .


Scintillation

• Phase-screen model

• F{u} = F{u0} exp⇣�jz

k

2x

2k

⌘

Forward Model

x [arb. units]

y [arb

. units]

50 100 150 200 250 300

20406080

100120

Inverse Model

x [arb. units]

y [arb

. units]

50 100 150 200 250 300

20406080

100120

• Due toionosphericirregularities atthe Fresnel scale,probably locatedon sharpgradients.

• For UHF andL-band, 100s ofmeters.

• ScintillationindicesS

24 = hI 2i�hI i2

hI i2


Other Topics

• Ionospheric modification—heating.• HAARP, NAIC• Very high e↵ective radiated power HF at the plasmafrequency of the target region

• Radio Luxemburg e↵ect

• Plasma waves—a lecture series in and of itself.

• Whistlers• Excited by lightning (F{�(t)} ⇠ 1, actually peak is ⇠ 1MHz)

• Propagate along geomagnetic field B• VLF (3–30 kHz) is a good place to hear this—put anE-field probe on a sound card.


Conclusion

• Scope

• History

• Ionospheric propagation

• Ionospheric irregularities

• The following page is a short bibliography


Bibliography

Booker, H. G., and H. W. Wells, Scattering of radio waves by the F-region of the ionosphere, J. Geophys.Res., 43, 249–256, 1938.

Bowles, K. L., Observation of vertical-incidence scatter from the ionosphere at 41 Mc/sec, Phys. Rev. Lett.,1 (12), 454–455, 1958.

Bowles, K. L., R. Cohen, G. R. Ochs, and B. B. Balsley, Radio echoes from field-aligned ionization above themagnetic equator and their resemblence to auroral echoes, J. Geophys. Res., 65 (6), 1853–1855, 1960.

Breit, G., and M. A. Tuve, A radio method of estimating the height of the conducting layer, Nature, 116,357, 1925.

Budden, K. G., Radio Waves in the Ionosphere pp., University Press, 1966.

Farley, D. T., B. B. Balsley, R. F. Woodman, and J. P. McClure, Equatorial spread F : Implications of VHFradar observations, J. Geophys. Res., 75 (34), 7199–7216, 1970.

Gordon, W. E., Incoherent scattering of radio waves by free electrons with applications to space explorationby radar, Proc. IRE, 46 (11), 1824–1858, 1958.

Jones, R. M., and J. J. Stephenson, A versatile three-dimensional ray tracing computer program for radiowaves in the ionosphere, OT Report 75-76, U. S. Department of Commerce, 1975.

Kudeki, E., ECE 458 Lecture notes on Applications of Radiowave Propagation pp., published by author,2006.

Kunitsyn, V. E., and E. D. Tereshchenko, Ionospheric Tomography, Physics of Earth and Space EnvironmentsSeries pp., Springer-Verlag, Berlin, 2003.

Ratcli↵e, J. A., The ionosphere and the engineer, Electronics and Power, pp. 381–383, 1966.

Yeh, K. C., and C. H. Liu, Theory of Ionospheric Waves pp., Academic Press, 1972.

Yeh, K. C., and C. H. Liu, Radio wave scintillations in the ionosphere, Proc. IEEE, 70 (4), 324–360, 1982.

1

radio waves in the ionosphere - university … waves in the ionosphere e. s. miller geospace and...

Documents