Radio Waves
Where do they come from?
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:
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!
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 108 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
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!
Family Trait 4: Wave behavior
• Physical Properties
–
–
–
–
Wavelength
Frequency
Amplitude
Phase
• Behaviors
–
–
–
–
–
Reflection
Refraction
Diffraction
Interference
Doppler Shift
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 = 6.626 x 10-26 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
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
Images/animations courtesy NRAO / AUI / NSF
Shorter
Wavelengths
Longer
Wavelengths
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
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.
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
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
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
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
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
Thermal Bremsstrahlung
• Also called free-free radiation
• Electrons whizzing by ions
Image/animation courtesy NRAO / AUI / NSF
Non-Thermal Radiation
• Most common: Synchrotron radiation
• Usually electrons accelerating in a magnetic field
Image/animation courtesy NRAO / AUI / NSF
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
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)
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
Example: 21 cm Hydrogen Line
Image/animation courtesy NRAO / AUI / NSF
Molecules found in space
Simple Hydrides, Oxides, Sulfides, Haloids
H2
CO NH3
CS
NaCl
HCl
SiO SiH4
SiS
AlCl
H2O
SO2 C2
H2S
KCl
OCS CH4
PN
AlF
Nitriles, Acetylenes and Derivatives
C3
HCN CH3CN HNC C2H4
C5
HC3N CH3C3N HNCO C2H2
C3O
HC5N CH3C5N HNCS
C5O
HC7N CH3C4H HNCCC
C3S
HC9N CH3C4H CH3NC
C4Si
HC11N C2H5CN HCCNC
Aldehydes, Alcohols, Aethers, Ketones and Amides
H2CO
CH3OH HCOOH
CH2NH CH2C2
H2CS
C2H5OH CH3COOH CH3NH2
CH2C3
CH3CHO CH3SH (CH3)2O
NH2CN
NH2CHO
(CH3)2CO H2CCO
Cyclic Molecules
C3H2 SiC2 C-C3H
Molecular Ions
CH HCO+ HCNH+ H3O+
HN2- HCS+ HOCO+ SO+
HOC+ H2DRadicals
OH C3H CN C2O C2S
CH C4H C3N NO NS
C2H C5H HCCN SO SiC
CH2 C6H CH2CN HCO
SiN NH MgNC CP
11.0724545 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
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