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

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)

Short Wavelength(High Frequency & also High Energy)

Long Wavelength (Low Frequency & also Low Energy)

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 Free-Free Emission in the radio region of the E/M spectrum.

Free – free emission continued.

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.

Press down arrow to view animationPress up arrow to reset animation

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)