Transcript Mechanisms of Radio Wave Emission

Mechanisms of Radio Wave
Emission
How different mechanisms create
Radio Waves that are detectable
by Radio Telescopes
What is an Electromagnetic Wave?
•
An Electromagnetic Wave (EM) wave is composed of an electric field and a
magnetic field that: (learn more)
– Are oscillating together
– Are perpendicular to each other
– The wave travels in a direction perpendicular to both of the fields
•
These waves are also particles called photons: (learn more)
- packets of energy (with NO mass) that travel at the speed of light.
- Which behave as both a particle and a wave at the same time.
-
Waves have three main properties:
- wavelength - the distance between two adjacent crests of the wave,
- frequency (f) - the number of wave oscillations per second
- energy (E) - of the individual photons in the wave.
Characteristics of E/M
waves
• All E/M waves behave the same
way.
• They have the basic relationship of
their speed, frequency, and
wavelength.
• Where:
λ = the Wavelength of each wave
c = speed of light (3 x 108 m/s)
f = frequency of the wave (Hz.)
Or:
h = Planck’s constant (6.63 * 10-34 Js)
c = speed of light (again)
E = energy of the photon
Let’s look at the E/M
spectrum
Short Wavelength
(High Frequency & also High Energy)
Long Wavelength
(Low Frequency & also Low Energy)
Distinct “bands” of different types of energy in the E/M Spectrum.
They are:
•Similar – Obey the basic c = f x λ relationship
•Different
• Much greater E of shorter λ
• Transparency/blockage by our Atmosphere. of certain
wavelengths (learn more)
Types of Emission
• Thermal Emission
– Blackbody Radiation
– Free-Free Emission
– Spectral Line Emission
• Non-Thermal Emission
– Synchrotron Emission
– Geo-Synchrotron Emission: Pulsars
– Maser – Microwave Amplification by Stimulation Emission of
Radiation
Blackbody Radiation
• One of the most common forms of E/M waves
• A “Blackbody” glows because it is hot. It also:
– Absorbs all radiation that hits it
– Have atoms and molecules which move faster with the
absorption of more heat
– Reradiates ALL that energy back out into it’s environment
– Radiates a L O T more energy as it gets hotter
– Radiates in shorter and shorter wavelengths (λ) as it gets hotter.
(Explains why REALLY hot stars are blue and cooler stars are
red.)
• Stars are considered blackbody radiators
Blackbody Radiation(cont’d)
Graphs of the amount
energy emitted related to
wavelength/color for
different temperatures.
Notice that, at each of the
three temperatures, a
blackbody radiates some
energy at all wavelengths.
It just emits a LOT more
energy at certain
wavelengths
corresponding to the
“color” represented by that
wavelength. (learn more)
Amount of energy vs. wavelength/color
(at different temperatures)
Courtesy of Windows to the Universe, http://www.windows.ucar.edu
Free-Free Emission (Radiation)
• Happens in ionized (charged) gasses
• Happens mostly in Plasma (the 4th state of
matter)
• Electrons, accelerated by the charged particles
in the gas cloud, emit radiation continuously
(This is the same process as the operation of a
fluorescent light tube!)
• Active Galactic Nuclei (AGN) generate FreeFree Emission in the radio region of the E/M
spectrum.
Free – free emission
continued.
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The electron and ion are initially free and moving
rapidly. The ion attracts the electron but not
strongly enough to capture it. As the charged
particles are accelerated, they emit radio waves.
Spectral Line Emission
• Happens when electrons make a transition from a higher
energy level to a lower energy level
– First process - A “line” of light at a particular wavelength (λ)
which has exactly the amount of energy corresponding to the
energy level difference between the higher and lower energy
levels
– Second process – A photon of energy is released when an
electron changes its “rotation state” which is an energy
transition.
•
a commonly occurring example is the 21 cm line of neutral
Hydrogen gas
• When an electron absorbs a photon of a particular
wavelength the photon of light/radio wave “disappears”
having been “used” or “absorbed” by the electron (learn
more)
Non-Thermal Emission
• Synchrotron Emission – Acceleration of charged particles by
magnetic fields (learn more)
– This can happen without excitation of the addition of heat.
– Electrons spiral around the magnetic field and emit almost constant
radiation
– Radiation is polarized
• Gyro Synchrotron Emission: Pulsars (learn more)
– connected to deaths of massive stars
– Related to neutron stars
• Masers (learn more)
– Occur in Molecular Clouds
– Radiation is in coherent, monochromatic and very bright
– Masers discovered seem to be related to clouds of molecules of:
Methanol (CH3OH), Ammonia (NH3), hydroxyl radical (OH), silicon oxide (SiO),
water (H2O), and formaldehyde (H2CO)