Radio Waves. Where do they come from?

Transcription

1 Radio Waves Where do they come from?

2 Radio waves belong to a family The electromagnetic spectrum (EM) is a continuum of waves, sometimes called electromagnetic radiation. These waves may be created in a number of ways, but all share the following characteristics:

3 Family Trait 1: No Medium EM waves do not require a medium to move from source to observer. Mechanical waves (such as sound) must travel through a medium In space, no one can hear you scream Tagline from the movie Aliens. This is one (rare) example of Hollywood getting the science right!

4 Family Trait 2: Same Speed They all travel at the same speed, c, the speed of light in a vacuum an unfortunate choice, since most people don t associate light and radio as being part of the same EM spectrum c 3 x 10 8 m/s When traveling through a medium, the wave speed, v, is found by the formula v = c/n where n is the index of refraction of the medium through which the wave is moving

5 Family Trait 3: Fixed relationship Frequency (f) and wavelength (λ) are related as f = c/λ This is an inverse relationship: the larger (higher) the frequency gets, the smaller the wavelength becomes Note to teachers: Throughout our presentations, we will be using f for frequency. The Greek letter nu (ν) is traditionally used for frequency, unfortunately it looks too much like a vee (v) in most fonts. The inconsistency in notation is a constant source of confusion for introductory physics students!

6 Family Trait 4: Wave behavior Physical Properties Wavelength Frequency Amplitude Phase Behaviors Reflection Refraction Diffraction Interference Doppler Shift

7 Family Trait 5: Particle Behavior The frequency of an EM wave is related to its energy by the formula f = E/h (more commonly written as E = h f) h is Planck s constant = x J/Hz This behavior is attributed to a particle called a photon. That EM radiation appears to be both a wave and a particle is called wave-particle duality (to be discussed in another section) The wave behavior dominates the lower frequency spectrum (Radio waves), while the particle nature shows itself more readily in the higher frequencies

8 EM Spectral Bands For convenience, scientists have divided the spectrum into bands. Those bands are, in order of increasing wavelength (decreasing frequency): Gamma Rays, X-Rays, Ultraviolet, Visible, Infrared, Microwave, and Radio Microwaves are often considered part of the Radio band Shorter Wavelengths Longer Wavelengths Images/animations courtesy NRAO / AUI / NSF

9 Anatomy of EM waves EM waves consists of a traveling electric field (E) and a traveling magnetic field (B). The E and B fields are in-phase and orthogonal (at right angles) to one another. Image/animation courtesy NRAO / AUI / NSF

10 Human detections of EM waves Humans have built-in detectors of EM waves, called eyes. We see EM waves in the Visible part of the Spectrum. Sound is NOT part of the EM spectrum!!! Sound is a mechanical wave, which requires a medium. Humans have a different set of detectors for mechanical waves, called ears.

11 Natural Radio Sources Lightning, sparks Solar System our sun, planets Milky way star forming regions, old stars, supernova remnants, Galactic center Extragalactic quasars, radio jets Molecules

12 What causes Radio waves? Vibrating atoms and molecules Thermal vibrations due to temperature Rotational energy for asymmetric molecules Excited atoms and molecules Absorption/emission of energy (a photon) Accelerating charged particles Movement in electric or magnetic fields

13 Two categories of radio sources Broadband Spectral content of source is spread out across many of the EM bands (radio, visible, x-ray) Observations made in the radio band should correlate with other parts of the EM spectrum (see Sun) Narrowband Attributes of the source favor one part of the spectrum (or single frequencies) over others For example: you can t see the Ozone in the Mesosphere in the visible spectrum, but we can detect their radio waves

14 Broadband Radiation Broadband radio signals usually have a thermal origin. Blackbody radiation (Planck s Law) All objects radiate EM waves in proportion to their internal temperature Thermal Bremsstrahlung Acceleration of a charged particle (electron) by another charged particle (nucleus) Includes cyclotron and synchrotron radiation

15 Blackbody Radiation Any object above absolute zero will emit a broad spectrum of radiation The peak of the curve shifts to shorter wavelengths as temperature increases Image courtesy NRAO / AUI / NSF

16 Thermal Bremsstrahlung Also called free-free radiation Electrons whizzing by ions Image/animation courtesy NRAO / AUI / NSF

17 Non-Thermal Radiation Most common: Synchrotron radiation Usually electrons accelerating in a magnetic field Image/animation courtesy NRAO / AUI / NSF

18 Another Non-Thermal Source MASERs (Microwave Amplification by Stimulated Emission of Radiation) like a LASER, only at radio frequencies, not visible to the eye Usually associated with molecules in stellar gas clouds Image/animation courtesy NRAO / AUI / NSF

19 The Sun in different light Radio Visible Ultraviolet X-Ray Images courtesy of: NRAO/AUI/NSF/G. Dulk, D. Gary (radio), NSO/AURA/NSF (visible) SOHO/NASA (ultraviolet) and Yohkoh/ISIS/NASA (X-Ray)

20 Narrowband Radiation Electron energy transitions tend to emit visible or UV radiation Vibrational transitions tend to emit IR radiation (mm waves) Rotational transitions tend to emit microwave radiation (Radio waves) The molecule must have an electric dipole moment

21 Example: 21 cm Hydrogen Line Image/animation courtesy NRAO / AUI / NSF

22 Molecules found in space Simple Hydrides, Oxides, Sulfides, Haloids H 2 CO NH 3 CS NaCl HCl SiO SiH 4 SiS AlCl H 2 O SO 2 C 2 H 2 S KCl OCS CH 4 PN AlF Nitriles, Acetylenes and Derivatives C 3 HCN CH 3 CN HNC C 2 H 4 C 5 HC 3 N CH 3 C 3 N HNCO C 2 H 2 C 3 O HC 5 N CH 3 C 5 N HNCS C 5 O HC 7 N CH 3 C 4 H HNCCC C 3 S HC 9 N CH 3 C 4 H CH 3 NC C 4 Si HC 11 N C 2 H 5 CN HCCNC Aldehydes, Alcohols, Aethers, Ketones and Amides H 2 CO CH 3 OH HCOOH CH 2 NH CH 2 C 2 H 2 CS C 2 H 5 OH CH 3 COOH CH 3 NH 2 CH 2 C 3 CH 3 CHO CH 3 SH (CH 3 ) 2 O NH 2 CN NH 2 CHO (CH 3 ) 2 CO H 2 CCO Cyclic Molecules C 3 H 2 SiC 2 C-C 3 H Molecular Ions CH HCO + HCNH + H 3 O + HN - 2 HCS + HOCO + SO + HOC + H 2 D - Radicals OH C 3 H CN C 2 O C 2 S CH C 4 H C 3 N NO NS C 2 H C 5 H HCCN SO SiC CH 2 C 6 H CH 2 CN HCO SiN NH MgNC CP

23 GHz Ozone Line The Mesospheric Ozone line we detect with the MOSAIC system is a change in rotation of the asymmetric ozone molecule This is a quantum mechanical effect, due to the existence of discrete energy levels of rotational angular momentum

24 Atmospheric opacity As the illustration below shows, there are many EM frequencies which do not pass through our atmosphere, due to absorption by atoms and molecules present. Images courtesy of NASA