Lower ionospheric cyclotron maser theory: A possible source of 2ƒ_ce and 3ƒ_ce auroral radio emissions

Abstract

A lower ionospheric electron cyclotron maser has recently been suggested as a possible source of natural radio emissions that have been observed with ground‐based instruments in the auroral zone. According to this theory, the observed emissions are interpreted as X mode electromagnetic waves at 2 and/or 3 times the ionospheric electron cyclotron frequency (ƒce). In the present study we carry out a more thorough analysis of the cyclotron maser instability, using a more complete growth rate expression and more realistic energetic electron distribution functions. Growth rates of various electromagnetic modes and harmonics are calculated. It is shown that growth rates for 2ƒce and 3ƒce X modes can exceed collisional ionospheric damping rates, confirming the original estimations. However, the present analysis also reveals that O mode waves at the fundamental cyclotron frequency, ƒce, should also be excited at the source. The propagation of excited electromagnetic waves from the source region to the ground inside a model horizontal density cavity structure is also examined by means of ray tracing technique. It is found that the O mode wave is unable to reach the ground, an explanation of why the emissions at ƒce are not detected on the ground.

The polarization of auroral radio emissions

“…For example, several researchers have suggested that auroral roar may be generated in the X-mode by the cyclotron maser mechanism operating at F-region altitudes [Weatherwax et al, 1995;Yoon et al, 1996]. This mechanism predicts that auroral roar should be right elliptically polarized.…”

Rotational modulation of Saturn Kilometric Radiation, narrrowband emission and auroral hiss

“…Recently, Menietti et al [2016] surveyed possible regions of 5 kHz Z mode narrowband emissions in the lower density region of Saturn's magnetosphere and identified 6 possible source regions, one of which had relatively complete electron pitch angle coverage by the electron spectrometer onboard Cassini. Wave growth calculations based on the electron phase space distribution and cold plasma magnetoionic theory [Yoon et al, 1996 and1998] indicated that an unstable electron population with a temperature anisotropy and a weak loss cone might be the source for the 5 kHz Z mode narrowband emissions.…”

Source mechanism of Saturn narrowband emission

“…In this section we treat these emissions using an analytical approach based on the work of Yoon et al (1996Yoon et al ( , 1998.…”

“…In order to facilitate the analytical calculation of the growth rate of Z-mode and O-mode, we introduce a functional form for the distribution function similar to that of Yoon et al (1996) …”

“…Narrowband emissions are interpreted as emissions near the first and second harmonics of f ce . In explaining the generation of terrestrial auroral roar, Yoon et al (1996Yoon et al ( , 1998 have shown that the growth rates of Z-mode are greatly enhanced when f 2 UH =f 2 ce + f 2 p =(nf ce ) 2 , where n=2 and 3 (f p is the electron plasma frequency). This Z-mode can escape into free space by a linear mode conversion into ordinary (O) or whistler mode (e.g.…”