Electromagnetic Radiations in Space Plasma

The geomagnetic space environment is a natural laboratory for rich nonlinear plasma physics. The auroral zone ionosphere and magnetosphere of the Earth are replete with plasma waves and radiations. The energetic electrons precipitating to the auroral zone naturally excite various waves in the magnetosphere as well as in the ionosphere, which can be detected on the ground or by in situ spacecraft. Energetic electrons trapped in the dipolar field of the magnetosphere also excite naturally occurring plasma waves. One may use these radio emissions as an effective tool for diagnosing complicated plasma processes occurring in the natural environment. Among the most well-known electromagnetic radio emission processes in the tens to hundreds kHz frequency range is the terrestrial kilometric radiation (also known as the Auroral Kilometric Radiation or AKR for short) [1] , [2] . Terrestrial continuum radiation is also a well-known example [3] , [4] . Other types of naturally occurring auroral radio waves in the medium frequency range have also been identified. The hiss is a broadband emission below 1 MHz, medium frequency (MF) burst is a broadband emission at 1.5-4.5 MHz, and auroral roar, which is a narrowband emission near cyclotron harmonics [5] . Besides these nonlinear emission mechanisms may also be operating. For instance, recently it was proposed that a radiation emission mechanism similar to the plasma emission process, which is relevant for solar radio bursts, may be operative near the reconnection region [6] . The present paper will discuss kinetic theories of these radio emission processes that may be relevant to the naturally occurring radio emission phenomena in space plasma environment.