CMI Growth Rates for Kronian kilometric radiation

We calculate growth rates for extraordinary mode Saturnian kilometric radiation generated by the electron cyclotron maser instability using a ring-type (DGH) electron phase distribution model fitted to measured electron energy distributions in Saturn's auroral acceleration region. The observed distributions are unstable in spatially isolated regions of transverse linear size of order 10³ km. Each unstable region has a growth rate consistent with amplification of background radiation to the peak observed intensity, approximately 14 electric field e-foldings. The observed propagation direction and frequency decrement below the electron cyclotron frequency are also consistent with the CMI growth model. (a) CAPS electron velocity distribution function measured 18 Oct 2008 at at 08:53:32 UT with 2-sec sampling. Sampled points with |v| < 0.05c are suppressed. Number density is in logarithmic units of m^6s-3. Position angles are measured with respect to the magnetic field, so that 0° is toward Saturn (southern hemisphere). (b) One dimensional profiles of CAPS velocity distributions along 55° (\triangle), 75° (Box), 85° (∇ ), and 125° (circ). The dashed black line is the kappa-distribution fitted background level, while the solid red line is a best-fit DGH distribution used to calculate the CMI growth rate.

(a) Growth rate versus propagation angle with respect to the magnetic field for the DGH model fit to the measured CAPS phase distribution (Figure ref{fig-caps-2sec}, dotted red line). The lines are for δ ω /δ ω _max = 0.1 (red line), 0.5 (blue line), and 0.9 (green line). (b) Growth rate versus normalized frequency δ ω at propagation angles θ =80° (red line), 85° (green line), and 90° (blue line).