Analysis of Fine Structures in Auroral Radio Emissions

The polar light in its various colours and forms has fascinated human beings since ancient times. It is less well known that, besides light, there is also radio emission generated in the aurora, and the generation of both phenomena has a causal link. Charged particles like electrons or protons gyrate around the magnetic field lines of Earth, and they hit nitrogen or oxygen atoms in the upper atmosphere leading to the light emission. This process changes the energy distribution of such electrons and their return to an equilibrium distribution leads to an energy loss in the form of radio emissions. These radio emissions cannot be detected on the ground because of the shielding of the ionosphere, but they can be measured by satellites in space with corresponding radio receivers. Our project on the analysis of fine structures in auroral radio emissions is not only dealing with the auroral radio emission of Earth, but it will also examine the same kind of emission near the planets Jupiter and Saturn. Therefore, data from the space missions Cluster (four satellites in the magnetosphere of Earth), Juno (orbiting Jupiter until mid-2021), and Cassini (circled around Saturn for 13 years until 2017) will be analysed in detail. The main focus will be on the so-called fine structures, which are spectral signatures of short duration (milliseconds to seconds). The radio sources typically line up along magnetic field lines at distances of a few planetary radii. They radiate at frequencies of a few kilohertz to a few megahertz, and the higher the frequency, the closer the source is located to the planet. The radio sources at all three planets have been traversed by the respective spacecraft already a few times, but using different methods one can also determine the incident radio wave direction from a further distance for all three space missions. For example, the method of triangulation will be applied with the four Cluster satellites, which uses the different times at which the spectral fine structures were recorded. From this one can determine the position, the size, and the speed of an elementary radio source, which so far has not been done experimentally. Until today a comprehensive statistical analysis about the temporal and spatial occurrence of fine structures of auroral radio emissions, based on the large data sets of Juno and Cassini, does not yet exist. The main aim of the project is the classification and the comparison of spectral fine structures of radio emissions from all three planets (Earth, Jupiter, and Saturn). The planned project duration is three years, and it will be a joint project of the Space Research Institute (Dr. Georg Fischer) in Graz with the Institute of Atmospheric Physics (Dr. Ulrich Taubenschuss) in Prague.

The polar light can be seen at medium latitudes only in rare cases, e.g., it was seen as far south as Austria in April and September 2023. The so-called aurora is formed by charged particles (mainly electrons) which gyrate in the terrestrial magnetic field and bump into the upper atmosphere. This causes a disequilibrium distribution of electrons which leads to radio emissions at even higher altitudes. These emissions have wavelength of the order of kilometers, and we investigated them in detail in our project. Our focus was on the so-called fine structures, which are geometrical structures in the time-frequency spectrum typically lasting for a few seconds. We worked a lot with time-frequency spectra of Saturn kilometric radiation, as they were observed by the Cassini spacecraft. The time in such spectra typically goes over a few seconds, the frequency goes up to 100 kHz, and the intensity of the radiation is represented by colors. One can see areal structures as well as linear structures with positive or negative slope. Such structures in the kilometric radiation of Earth and Saturn were classified by us for the first time, and we evaluated their incidence. Thereby it turned out that the fine structures of auroral radio emissions of Saturn and the Earth are quite similar. Furthermore, banded structures preferentially had a positive slope, whereas structures called "striations or rain" usually had a negative slope. A positive slope corresponds to a down-going radio source in the magnetic field, whereas a negative slope means an up-going source. We even found oscillating sources, especially in the auroral kilometric radiation of Earth. All these experimental details can lead to various clues with respect to the many different generation theories of fine structures of auroral radio emissions. We also investigated "coarse" structures of radio emissions that can last for tens of minutes up to several hours. A very special structure of Saturn kilometric radiation was detected by us and named "caterpillar" (see image). Within 13 years of Cassini radio data about 600 caterpillars were detected and analyzed. They occur at frequencies below 40 kHz, they have a bandwidth of 10 to 15 kHz, and their degree of wave polarization is somewhat smaller than the usually totally polarized Saturn kilometric radiation. Another new structure in the radio emissions of Saturn was identified in the frame of international cooperation and termed SAM for "Saturn Anomalous Myriametric" radiation. Furthermore, an investigation of the coarse structures of the broadband kilometric radiation of Jupiter led to the clue that the radiation should be emitted by an open and not closed emission cone.