Date of Degree
PhD (Doctor of Philosophy)
Donald A. Gurnett
Over the last fifty years, a multitude of spacecraft and rocket experiments have studied plasma wave emissions from Earth's auroral regions. One such emission is auroral hiss, a low-frequency whistler-mode wave that is produced in the auroral zone. Observations from Earth-orbiting spacecraft show that auroral hiss is generated by field-aligned electron beams, with the resulting plasma wave emission propagating along the resonance cone. This propagation results in auroral hiss appearing as a V-shaped funnel when observed on a frequency-time spectrogram. This thesis presents the first comprehensive study of auroral hiss at a planet other than Earth, using the Cassini spacecraft to study auroral hiss at Saturn.
NASA's Cassini spacecraft, currently in orbit around Saturn, has allowed for the first opportunity to study this emission in detail at another planet. Since 2006, the Cassini spacecraft has twice been in a series of high inclination orbits, allowing investigation and measurements of Saturnian auroral phenomena. During this time, the Radio and Plasma Wave Science (RPWS) Investigation on Cassini detected low frequency whistler mode emissions propagating upward along the auroral field lines, much like terrestrial auroral hiss. Comparisons of RPWS data with observations from several other Cassini instruments, including the Dual-Technique Magnetometer (MAG), Magnetospheric Imaging Instrument (MIMI), and the Cassini Plasma Spectrometer (CAPS), have revealed a complete picture of this emission at Saturn.
Observations from these instruments have been used to make a variety of determinations about auroral hiss at Saturn. RPWS has only observed this emission when Cassini was at high-latitudes, although these observations have shown no preference for local time. Tracking the times this emission has been observed revealed a clear periodicity in the emission. Further study later revealed not one but two rotational modulations, one in each hemisphere, rotating at rates of 813.9 and 800.7 degrees per day in the northern and southern hemispheres, respectively. These rates match with observations of the clock-like Saturn Kilometric Radiation. Study of the field-aligned current structures in the auroral regions revealed a strong upward-directed current in both hemispheres on the lower-latitude side of the auroral hiss emission. Along with correlating particle densities, these observations were used to infer the presence of a high-density plasmasphere at low latitudes, with the series of field-aligned current structures lining up with the outer boundary at L-shell values of around 12-15.
Analysis of electron beams observed in conjunction with auroral hiss shows that these beams produce large growth rates for whistler-mode waves propagating along the resonance cone, similar to terrestrial auroral hiss. Analytical calculation of the normalized growth rates of ten electron beam events on Day 291, 2008, yielded a wide range of growth rates, from 0.004 to over 6.85 times the real frequency. The latter, a non-physical result, came from a violation of the weak growth approximation, suggesting there was so much growth that the analytical calculation was not valid in this instance. Numerical calculation using a plasma dispersion-solving code called WHAMP produced a growth rate of about 0.3, a still very large number, suggesting the detected beams may be the source of the observed auroral hiss plasma wave emission.
Auroral Hiss, Cassini, Electron Beams, Plasma Waves, Saturn
Copyright 2010 Andrew James Kopf