Janser, Sascha 
ORCID: 0000-0002-3561-1763
(2022).
Relevance of Alfvénic turbulence for Jupiter’s auroral emissions.
PhD thesis, Universität zu Köln.
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Abstract
In this thesis, we investigate the relevance of Alfvénic turbulence and related wave-particle interaction processes for Jupiter’s auroral emissions. Low-altitude Juno spacecraft observations above Jupiter provide strong hints on a dominating role of Alfvén waves in related particle energization processes. Besides bi-directional electron pitch-angle distributions, data prominently reveal broadband energy distributions for auroral electrons connected to the Io flux tube and the main emissions. Furthermore, low-frequency power spectra of magnetic field fluctuations exhibit a power law-like behavior, which is indicative for turbulence. Using these and further system-related information, we characterize turbulence in these regions and examined the spectral dispersion and dissipation properties of associated kinetic Alfvén waves. Turbulence in the Io flux tube is established by the complex interaction of Io and the streaming torus plasma. Alfvénic perturbations are generated, which propagate along the magnetic field lines. Based on wave reflections at the Jovian ionosphere and at the Io torus boundary, an energy cascade process is established. By the related non-linear wave-wave interactions, wave energy is transported towards smaller spatial and temporal scales. The generated waves turn into kinetic Alfvén waves during their propagation in the inhomogeneous plasma environment. On kinetic scales of the plasma, the waves develop dispersive and dissipative properties and generate parallel electric fields, which allow for intense Landau damping. In the high-latitude region of Jupiter, we assume the kinetic Alfvén waves to significantly heat particles responsible for the Io footprint emissions. For the middle magnetosphere, i.e., radial distances of 20 - 30 Jupiter radii, flux tube interchange motions are thought to be the generator of the observed Alfvénic turbulence in the plasma sheet. By similar reflection processes, we hypothesize kinetic Alfvén waves to efficiently generate auroral particle precipitation. To study turbulence in both regions, we start with a basic characterization of the large-scale wave fields to constrain models for Alfvénic turbulence at generator locations inside and outside the plasma sheet. We demonstrate that these wave fluctuations would be observed by Juno at high latitudes as spatially convected wave fields, structured perpendicular to the background magnetic field. Consequently, we reinterpret the spectral indices from observations by Sulaiman et al. (2020) and Gershman et al. (2019). We suggest the related lower-frequency power spectra to be the result of weak-MHD inside the plasma sheet or sub-ion scale kinetic Alfvén wave turbulence outside the plasma sheet. Calculated turbulence heating rates are consistent with observed energy fluxes in the Io flux tube and the middle magnetosphere and represent efficient drivers for particle acceleration. Based on this characterization of turbulence, we examine the dispersive and dissipative properties of monochromatic kinetic Alfvén waves along auroral magnetic field lines, connected to the Io footprint and the main emissions. We use a local description for the wave properties based on the hot plasma dispersion relation and also a simplified model from Lysak (2008). We show that for a wide range of parameters both models give coinciding results. In this context, we demonstrate that electron Landau damping plays a major role for dissipation of wave energy. We analytically show that its onset is related to the ion acoustic length ρs and the electron inertial length scale λe in the warm and cold Alfvén regime, respectively. Ion Landau damping only contributes to heating at smallest wave scales considered. To quantify wave damping, we develop a model for the residual wave energy density along the magnetic field lines based on the electromagnetic Poynting theorem. We include dissipation processes from resonant and non-resonant wave-particle interaction in the model description. With this model, we are able to evaluate implemented expressions for the spectral perpendicular and parallel wave electric field components and corresponding particle responses. We calculated a peak electric field strength of 10^{−4} V/m, which corresponds to a characteristic electron heating of 6.5 keV. Based on a different approach over heating rates, we estimated a heating of 26 keV. These values are in a range required to drive UV auroral emissions. Furthermore, we find that the dissipated power density at high latitudes due to kinetic Alfvén waves is determined by a trade-off between available small-scale wave energy and the damping strength of the waves. Consequently, there is a wavenumber band in the dissipation spectra for which auroral heating maximizes. Furthermore, we identify that the density profile above the Jovian ionosphere is a major driver to control the amount of transferred energy. Small ionospheric scale heights are associated with a shift in the location of maximum auroral heating due to smaller wave scales and associated stronger background magnetic field. From parameter studies considering thermal and hot particle species, we conclude that the latter ones are heated more efficiently by kinetic Alfvén waves. By integrating over the dissipation volume and the spectral range of maximized dissipation, we determine maximum input powers of 8.4\cdot 10^{13} W and 13.0\cdot 10^{13} W in the main auroral acceleration region due to weak and KAW turbulence, respectively. These values coincide with observations in this region and suggest Alfvénic turbulence as potential driver for the main emissions. In a similar analysis for the Io flux tube, we detemined a maximum input power of 7\cdot 10^{10} W for the electrons. Our calculations stress the importance of the presence of an auroral density cavity at high latitudes to generate sufficient strong wave-particle interactions. Finally, we investigate perpendicular ion heating in the Io flux tube motivated by JADE and JEDI observations of heated proton populations from Szalay et al. (2020a) and Clark et al. (2020), respectively. We consider the non-resonant heating mechanism according to Lu and Li (2007). Our study reveals that only initially hot protons at high latitudes can be sufficiently heated in the presence of the density cavity to explain observed energies.
| Item Type: | Thesis (PhD thesis) | ||||||||
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| URN: | urn:nbn:de:hbz:38-615418 | ||||||||
| Date: | 2022 | ||||||||
| Language: | English | ||||||||
| Faculty: | Faculty of Mathematics and Natural Sciences | ||||||||
| Divisions: | Faculty of Mathematics and Natural Sciences > Department of Geosciences > Institute for Geophysics and Meteorology | ||||||||
| Subjects: | Physics Earth sciences |
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| Date of oral exam: | 3 March 2022 | ||||||||
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| Refereed: | Yes | ||||||||
| URI: | http://kups.ub.uni-koeln.de/id/eprint/61541 |
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