The ionosphere and radio wave propagation P. J. Erickson Atmospheric Sciences Group MIT Haystack Observatory RET Workshop Haystack Observatory July 11, 2011 Electromagnetic Waves 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 2450 MHz antenna (wavelength: 12.2 cm / 4.82 in) 802.11b/g wireless internet
VHF / UHF antenna (wavelength at 300 MHz: 1 meter / 33.6 in) Television, FM radio HF antenna (wavelength at 15 MHz: 20 meters / 65.5 feet) Shortwave radio MF antenna (wavelength at 1030 khz: 291 meters / 955 feet) AM radio
VLF antenna (wavelength at 40 khz: 7,494 meters / 4.65 miles) Naturally generated waves from the upper atmosphere; submarine communications Sometimes antennas can be very big. CUPRI portable radar 50 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)
Millstone Hill Observatory 150 foot steerable antenna 220 foot zenith antenna Making upper atmosphere 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) Audio modulations (AM, FM) Digital TV frequency spectrum 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)
Radio wave travel can be altered by the medium Snell s law predicts directions of reflection and refraction for a medium with a non-zero index of refraction Refracted wave Change in index of refraction Incident wave Reflected wave Sin i / Sin r = h / h h = sqrt(m e) m = magnetic permeability e = dielectric constant Analogy: optical refraction at air/water interface Millstone Hill Radar Radio waves refract.. Just like light Transcontinental propagation affected Twinkling causes data loss 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 structure Seasonal, day/night, solar cycle changes... Variations in the ionosphere from day to night (and from space weather) result in different indexes of refraction, and therefore different propagation paths (N. Atkins, Lyndon State College) In particular, AM radio propagation is vastly different from day to night