Analysis of plasma waves observed within local plasma injections seen in Saturn's magnetosphere

Abstract

Plasma injections or density depletion regions have been reported to be a prolific feature of Saturn's inner magnetosphere. They are characterized by flux tubes of warm, tenuous plasma in a cooler, locally produced plasma background. The injected plasma undergoes dispersion in energy due to gradient and curvature drifts as the flux tube transports. The plasma waves within these injections are of at least two types. Above the electron cyclotron frequency, fce, very intense and narrow‐banded emissions resembling electrostatic cyclotron harmonics (ECH) are often observed. Below fce, whistler mode chorus is sometimes observed. Inside the plasma injections there exists a low‐energy (<100 eV) electron component which tends to have a field‐aligned pitch angle distribution, and a warmer (E > 1000 eV) component with “pancake” pitch angle distributions (peaked at 90°). We model the electron plasma distributions observed inside one injection to conduct a linear dispersion analysis of the wave modes. The results suggest that the ECH emissions can be generated by phase space density gradients associated with a narrow loss cone that is likely to be present but not observed because the electron detectors field of view did not include the magnetic field line at the time of the observations. The whistler mode chorus emission can be generated by the pancake‐like distribution and temperature anisotropy (T⊥/T∥ > 1) of the warmest plasma population. Some interesting anomalies between the results and the observations may be resolved by analyses of additional injection events.