15 Feb 2022
15 Feb 2022
Status: this preprint is currently under review for the journal ANGEO.

Classification of spectral fine structures of Saturn kilometric radiation

Georg Fischer1, Ulrich Taubenschuss2, and David Píša2 Georg Fischer et al.
  • 1Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, A-8042 Graz, Austria
  • 2Institute of Atmospheric Physics of the Czech Academy of Sciences, Prague, Czech Republic

Abstract. The spectral fine structures of Saturn kilometric radiation (SKR) are best investigated with the Wideband Receiver (WBR) of Cassini's Radio and Plasma Wave Science (RPWS) instrument, with which measured radio fluxes can be displayed in time-frequency spectra with resolutions of 125 ms and 0.1 kHz. We introduce seven different classes of SKR fine structures ranging from dots (one class for 0-dimensional objects) over lines (four classes of 1-dimensional objects being horizontal, vertical, or with negative or positive slope) to areal features (one class for 2-dimensional objects). Additionally, we define a 7th class containing special structures named according to their appearance in time-frequency spectra. These special features are named rain, striations, worms, and caterpillar, and the latter two have never been described in the literature so far.

Using this newly defined classification scheme we classify features in spectra at low frequencies in the baseband of the 80-kHz WBR and at medium frequencies around 325 kHz. A statistics of the occurrence of various classes and sub-classes shows some notable characteristics: Lines with a positive slope are much more common at medium frequencies than at low frequencies, and vertical lines are almost absent at low frequencies. The particular fine structure of striations (group of narrowbanded lines with predominantly negative slopes) is quite common below 80 kHz, but less common near 325 kHz. At these medium frequencies, the lines rather look like interrupted striations, which we term with the name 'rain'. We also find rare instances of striations with a positive slope, and rare instances of absorption signatures within areal features. The newly introduced sub-classes of worms (lines oscillating in frequency) and caterpillar occur almost exclusively below 80 kHz. Caterpillars have a typical bandwidth of 10 kHz, a constant frequency below 40 kHz for several hours, and they are mostly observed beyond distances of 15 Saturn radii around local dusk. We discuss the implications of our findings in view of the many theories about spectral fine structures of auroral radio emissions.

Georg Fischer et al.

Status: open (extended)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on angeo-2022-7', Anonymous Referee #1, 21 Mar 2022 reply
    • AC1: 'Reply on RC1', Georg Fischer, 25 Mar 2022 reply

Georg Fischer et al.

Georg Fischer et al.


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Short summary
The polar light in its various colours and forms has fascinated human beings since ancient times. It is less well known that there are also radio emissions generated in the aurora at higher altitudes. Not just Earth, but some other planets of the solar system also have auroras and corresponding radio emissions. In our publication we investigate and classify the spectral fine structures of a radio emission called Saturn kilometric radiation to find out more about this radiation process.