Electron‐cyclotron maser emission: Relative growth and damping rates for different modes and harmonics

Abstract

We calculate and compare the temporal growth rate and the number of e-folding growths for the following wave modes due to a loss-cone-driven cyclotron maser: fundamental x, o, and z modes and second harmonic x and o modes. The dominant mode of the maser should be the fastest growing mode for a saturated maser and should be the mode with the greatest number of e-folding growths for an unsaturated maser; this mode is the fundamental x mode for mp/•e < 0.3, the z mode (or perhaps the fundamental o mode) for 0.3 < mp/•e < 1.0, and the z mode (or perhaps t•e second harmonic x mode) for 1.0 < mp/•e < 1.3. We discuss the effect of cyc16tron damping by thermal electrons on the growth. Numerical calculations show that the effect is important only when the ratio of the mean energies of the thermal and maser emitting electrons exceeds 0.1-0.2. An analytic expression for the damping rate is derived and is used to show that some earlier treatments of cyclotron damping greatly overestimate the effect for loss-cone-driven maser emission. These results, when applied to AKR, imply that only either the fundamental x mode (for mp/•e < 0.3) or the z mode (for mp./•e 5 0 3) is produced directly by maser emission. We suggest (1) that an o mode component in AKR might be due to partial reflection of x mode radiation incident onto sharp overdense plasma intrusions of the kind observed in the auroral cavity and (2) that a second harmonic component can be produced by coalescence of two z mode waves. 1. 1984; Benson and Akasofu, 1•84]. We argue here that the latter component, which is restricted to•betwcen •2e ' and the upper hybrid frequency zs z mode radiation generated by the process discussed by Hewitt et al. [1983]. In section 2 we discuss the criterion for the dominant mode of a loss-cone-driven maser, and suggest that this is the x mode for mp/•e < 0.3 and the z mode for 897 898 Melrose et al.: Cyclotron Masers in Different Modes I o -• •

Dependence of the Io-related decametric radio emission of Jupiter on the central meridian longitude and Io's “active” longitudes

“…Besides, the calculation of the increment of electron-cyclotron maser (ECM) instability, performed by Galopeau et al (2004), is valid if the emission generation by the ECM mechanism occurs along the magnetic field. It should be noted, however, that the increment of the ECM mechanism is maximal in the direction close to the orthogonal magnetic field (Melrose et al 1984). The correct calculation of the increment of ECM instability may change the conclusion of Galopeau et al (2004).…”

Modelling the radio pulses of an ultracool dwarf

“…When f p / f c > 1.4, some electrostatic instabilities become important rather than the ECM instability, and a condition for ECM emission escaping from plasma is f p / f c 1. The fundamental (s = 1) Z-mode may be dominant when f p / f c ≈ 0.3 and f p / f c ≈ 1.1 (Melrose et al 1984;Aschwanden 1990a). However, we omit this mode because its appearance depends strongly on the ratio f p / f c and another mechanism would be needed to convert it to electromagnetic radiation.…”

“…On the other hand the cold background plasma will negate the loss cone and the growth rate will decrease sharply due to the lack of undamped resonance ellipses, if the hot electron temperature approaches that of the cold plasma. Melrose et al (1984) gives a criterion for effective cyclotron damping by background cold electrons, which is T h /T c < 10 ∼ 20. This value was confirmed in the work of Aschwanden (1990a).…”

Generation mechanism of Z-mode waves in the equatorial plasmasphere

“…These evidences suggest that Z-mode waves are not simply converted from electrostatic waves but generated by a direct process. Although it has been widely believed that a low plasma density background is a necessary condition to excite high-frequency electromagnetic waves directly (Melrose et al, 1984), the cyclotron resonance condition itself can be satisfied for the higher-order resonance even in the dense plasmaspheric plasma. In the present paper, we discuss how effectively Z-mode electromagnetic waves are excited directly in the equatorial region of the plasmasphere through a higher-order cyclotron resonance.…”