The ionosphere and radio wave propagation

P. J. EricksonAtmospheric Sciences GroupMIT Haystack Observatory

RET WorkshopHaystack Observatory

July 11, 2011

• Electromagnetic spectrum has a huge range of wavelengths or frequencies• Wavelengths and frequencies are related by the propagation speed of the EM wave (= c in vacuum)• Visible is just a narrow range - familiar because humans have built-in sensors• Radio waves are electromagnetic waves - NOT sound waves - and span a huge range of wavelengths : sub-mm to kilometers long• AM radio = 1 MHz = 300 meter wavelength• FM radio = 100 MHz = 3 meter wavelengths• Digital TV ranges from meter to 10 cm scale wavelengths

Electromagnetic Waves

To receive a radio wave, you need an antenna and some electronics …

Antennas come in many sizes, depending on the wavelength you wish to receive:

2450 MHz antenna(wavelength: 12.2 cm / 4.82 in)802.11b/g wireless internet

To receive a radio wave, you need an antenna and some electronics …

Antennas come in many sizes, depending on the wavelength you wish to receive:

VHF / UHF antenna(wavelength at 300 MHz: 1 meter / 33.6 in)Television, FM radio

To receive a radio wave, you need an antenna and some electronics …

Antennas come in many sizes, depending on the wavelength you wish to receive:

HF antenna(wavelength at 15 MHz: 20 meters / 65.5 feet)Shortwave radio

To receive a radio wave, you need an antenna and some electronics …

Antennas come in many sizes, depending on the wavelength you wish to receive:

MF antenna(wavelength at 1030 kHz: 291 meters / 955 feet)AM radio

To receive a radio wave, you need an antenna and some electronics …

Antennas come in many sizes, depending on the wavelength you wish to receive:

VLF antenna(wavelength at 40 kHz: 7,494 meters / 4.65 miles)Naturally generated waves from the upper atmosphere; submarine communications

Sometimesantennas can be very big….

CUPRI portable radar50 MHz frequency(wavelength: 6 meters; 19.67 feet)

SuperDARN HF auroral radar – 3 to 15 MHz frequency(wavelength at 10 MHz: 29.9 meters; 98.2 feet)

• 150 foot steerable antenna• 220 foot zenith antenna

Millstone Hill Observatory

Making upperatmosphere measurements using radio waves (in a radar configuration) since 1960..

Frequency = 440 MHz(wavelength: 68 cm; 26.8 inches)

Arecibo radio telescope / ionospheric radar – 430 MHz frequency(wavelength: 69.7 cm; 27.4 in); 300 m (1000 ft) reflector

Radio waves (EM waves) carry information (of human or other origin)

Digital TV frequency spectrumAudio modulations (AM, FM)

Information is transmitted by using a range of frequencies around a center frequency

The medium of propagation may act differently on these ranges of frequencies (dispersion)

Snell’s law predicts directions of reflection and refraction for a medium with a non-zero index of refraction

Incident wave

Refracted wave

Reflected wave

Change in index of refraction

Sin i / Sin r = h’ / hh = sqrt(m e)

m = magnetic permeabilitye = dielectric constant

Radio wave travel can be altered by the medium

Millstone Hill Radar

Radio waves refract .. Just like lightTranscontinental propagation affected

“Twinkling” causes data loss

Analogy: optical refraction at air/water interface

Ionosphere: the charged portion of a planet’s upper atmosphere.

Composed of plasma.

Plasma: partially ionized gas in which electrons are free rather than bound to an atom or molecule.Electromagnetic forces are therefore important (in addition to gravity and atomic forces).

The vast majority of the universe is in a plasma state.

The ionosphere is a continuous plasma medium which interacts with radio waves (has a non-zero index of refraction)

Its index of refraction depends most importantly on ionospheric electron density

Marconi

Over-the-horizon communications are made possible by “sky wave” propagation

paths due to refraction effects

The ionosphere has vertical as well as horizontal structureSeasonal, day/night, solar cycle changes...

(N. Atkins, Lyndon State College)

Variations in the ionosphere from day to night (and from space weather) result in different indexes of refraction, and therefore

different propagation paths

In particular, AM radio propagation is vastly different from day to night