|Whistler mode waves (Sample sounds and spectra)|
|Power Line Harmonic Radiation (PLHR)|
|Auroral Kilometric Radiation (AKR)|
|Electromagnetic Ion Cyclotron (EMIC) waves|
|LF/MF/HF Auroral Noise|
| ||Whistler mode waves|
| ||Broadband whistler mode noise at frequencies below the electron gyrofrequency, usually detected in the magnetosphere inside and near the plasmapause. One theory suggests the waves are generated by cyclotron resonant interactions with energetic electrons and are responsible for scattering and loss of radiation belt electrons.|
| ||Propagate below the plasma frequency and electron gyrofrequency, often at speeds much slower than c,
the speed of light in vacuo. Over much of geospace, whistler mode waves are typically in the
VLF (3-30 kHz) and ELF (0.3-3.0 kHz)
frequency ranges. Can be guided by density gradients and penetrate to the Earth's surface.
The waves are often classified into ducted (i.e. the waves travel in ducts
of enhanced plasma density) and nonducted types. Examples.
Electromagnetic waves, generated by lightning discharges in the atmosphere,
which propagate out into the magnetosphere where they are guided by the Earth'
s magnetic field to the opposite hemisphere and back down to the ground.
The waves propagate at frequencies below the electron gyrofrequency in the
whistler mode. When detected on the ground, higher frequencies are usually
detected before lower frequencies, although occasionally a ‘nose' frequency
is seen first followed by higher and lower frequencies. These waves have
been used to determine the plasma density in space and identify the plasmapause.
Theory suggests that either ducts, or one sided density gradients, are essential
to guide these waves to the ground.
| ||Auroral hiss|
| ||Electromagnetic waves emitted from the auroral region with frequencies below the electron gyrofrequency, usually in the range 1 to 500 kHz. The waves are associted with the aurora and are detected in space and on the ground. The frequency spectrum is usually flat, with a lower cut-off near the lower hybrid frequency. One theory suggests the waves are generated by Landau resonance with the downgoing electron beam.|
| ||Whistler mode waves at frequencies below the electron gyrofrequency detected in space and on the ground. Very often a double frequency band, with a gap near half the electron gyro, is observed in the midnight MLT sector. The higher frquency band is usually made up of many discreet rising tones. One theory suggests that the waves are generated via a non-linear interaction between whistler mode waves.|
| ||Quasi-periodic emissions (QP)|
| ||Power Line Harmonic Radiation (PLHR)|
Radiation from electrical electrical power lines, at harmonics of 50 Hz and 60 Hz, which leaks into
| ||Auroral Kilometric Radiation (AKR)|
| ||Electromagnetic radiation emitted from the auroral region with frequencies from 30 to 700 kHz. The corresponding wavelengths are of the order of kilometres, hence the name AKR. The waves are associated with the aurora. One theory suggests the waves are generated by the upgoing loss cone distribution of reflected electrons. Surprisingly, relativisitic effects are very important for the generation of these waves, even at energies of a few keV.|
| ||Electromagnetic Ion Cyclotron (EMIC) waves|
| ||Waves at frequencies below the H+ gyfrofrequency, usually observed in the outer magnetosphere and near the plasmapause. These waves are also seen on the ground as Pc1 micropulsations. These waves cause scattering of H+ and are believed to provide an important ring current decay mechanism during the recovery phase of a magnetic storm. They also cause transverse heating of He+ and O+. One theory suggests they are generated by temperature anisotropies in the energetic H+ and O+ distributions.|
| ||LF/MF/HF Auroral Noise|
| ||Electromagnetic waves in the frequency range 30 kHz to 30 MHz which are generated in space and observed on the ground at high latitudes. One theory suggests that the waves are generated by cyclotron resonance with an upgoing loss cone distribution in the reflected electrons.|
| ||Alfvén waves|
| ||There are various types and various names depending on MHD theory or kinetic theory. Basically waves in the very low frequency limit which cause oscillations in the Earth's ambient magnetic field, and in some cases plasma density. Waves which propagate along the magnetic field, known as Torsional Alfven waves, bend the magnetic field lines but do not cause density perturbations. The plasma moves with the magnetic field and looks as if it is tied to the field. Waves which propagate across the magnetic field, known as Fast Compressional waves, cause enhancements and reductions in the field and plasma density together. Note that fast mode waves can propagate at all angles with respect to B, but are unguided, whereas Torsional waves are guided along B. The Fast mode waves, so called because they have the fastest phase velocity, are also known as compressional and magnetosonic waves when propagating across B. Torsional waves are also known as the intermediate (due to the phase velocity), shear ( due to the shearing in the magnetic field), and kinetic Alfven waves near the ion gyrofrequency where kinetic effects becomes important. Slow mode waves, which have the slowest phase velocity, also propagate along B when pressure (temperature) effects are included. In kinetic theory these are the ion acoustic waves. Some people refer to all these wave types generically as ion cyclotron waves using kinetic theory.|
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