Relevance of Alfvénic turbulence for Jupiter’s auroral emissions

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

Capability of low-temperature SQUID for transient electromagnetics under anthropogenic noise conditions

Transient electromagnetics (TEM) is a well-established method for mineral, groundwater, and geothermal exploration. Superconducting quantum interference device (SQUID)-based magnetic-field receivers used for TEM have quantitative advantages and higher sensitivity compared with commonly used induction coils. Special applications are deep soundings with target depths > 1 km and settings with conductive overburden. However, SQUIDs have rarely been applied for TEM measurements in environments with significant anthropogenic noise. We compared a low-temperature SQUID with a commercially available induction coil in an area affected by anthropogenic noise. We acquired four fixed-loop data sets with totally 61 receiver stations close to Bad Frankenhausen, Germany. The high sensitivity of the SQUID enables low noise levels, which lead to longer high-quality transient data compared with the induction coil. The effect of anthropogenic and natural noise sources is more critical for the coil than for the SQUID data. In the vicinity of the transmitter loop, systematic distortion of the coil signals occurs at early times, most probably caused by sferic interferences. We have developed 1D inversion results of both receivers that matched well in general. However, the SQUID-based models were more consistent and showed greater depths of investigation. This led to a superior resolution of deeper layers and even enabled a potential detection of thin conducting targets at up to a 500 m depth. Moreover, we find that the SQUID data inversion revealed multidimensional effects within the conductive overburden. In this regard, we applied forward modeling to analyze systematic differences between inversion results of SQUID and coil data. We determine that low-temperature SQUIDs have the potential to significantly improve the reliability of subsurface models in suburban environments. Nevertheless, we recommend combined application of both types of receivers

Wave-Particle Interaction of Alfven Waves in Jupiter's Magnetosphere: Auroral and Magnetospheric Particle Acceleration

We investigate spatial and temporal scales at which wave-particle interaction of Alfven waves occurs in Jupiter's magnetosphere. We consider electrons, protons, and oxygen ions and study the regions along magnetic flux tubes where the plasma is the densest, that is, the equatorial plasma sheet, and where the plasma is the most dilute, that is, above the ionosphere, where auroral particle acceleration is expected to occur. We find that within a dipole L-shell of roughly 30, the electron inertial length scale in the auroral region is the dominating scale, suggesting that electron Landau damping of kinetic Alfven waves can play an important role in converting field energy into auroral particle acceleration. This mechanism is consistent with the broadband bidirectional electron distributions frequently observed by Juno. Due to interchange-driven mass transport in Jupiter's magnetosphere, its magnetosphere-ionosphere coupling is expected to be mostly not in local force balance. This might be a key reason for the dominant role of Alfvenically driven stochastic acceleration compared to the less frequently occurring, locally forced-balanced, and thus static mono-energetic unidirectional acceleration. Outside of approximately L = 30, the ion gyroperiod is the dominating scale suggesting that ion cyclotron damping of heavy ions plays a major role in heating magnetospheric plasma. We also present properties of the dispersion relationship and the polarization relationships of kinetic Alfven waves including the important effects from the relativistic correction due to the displacement current in Ampere's law

Design of potent and selective covalent inhibitors of Bruton’s Tyrosine Kinase targeting an inactive conformation

Bruton’s tyrosine kinase (BTK) is a member of the TEC kinase family and is selectively expressed in a subset of immune cells. It is a key regulator of antigen receptor signaling in B cells and of Fc receptor signaling in mast cells and macrophages. A BTK inhibitor will likely have a positive impact on autoimmune diseases which are caused by autoreactive B cells and immune-complex driven inflammation. We report the design, optimization, and characterization of potent and selective covalent BTK inhibitors. Starting from the selective reversible inhibitor 3 binding to an inactive conformation of BTK, we designed covalent irreversible compounds by attaching an electrophilic warhead to reach Cys481. The first prototype 4 covalently modified BTK and showed an excellent kinase selectivity including several Cys-containing kinases, validating the design concept. In addition, this compound blocked FcγR-mediated hypersensitivity in vivo. Optimization of whole blood potency and metabolic stability resulted in compounds such as 8, which maintained the excellent kinase selectivity and showed improved BTK occupancy in vivo

Discovery of LOU064 (Remibrutinib), a Potent and Highly Selective Covalent Inhibitor of Bruton’s Tyrosine Kinase

Bruton’s tyrosine kinase (BTK), a cytoplasmic tyrosine kinase, plays a central role in immunity and is considered an attractive target for treating autoimmune diseases. The use of currently marketed covalent BTK inhibitors is limited to oncology indications based on their suboptimal kinase selectivity. We describe the discovery and preclinical profile of LOU064 (25), a potent, highly selective covalent BTK inhibitor. LOU064 exhibits an exquisite kinase selectivity due to binding to an inactive conformation of BTK and has the potential for a best-in-class covalent BTK inhibitor for the treatment of autoimmune diseases. It demonstrates potent in vivo target occupancy with an EC90 of 1.6 mg/kg and dose-dependent efficacy in rat collagen-induced arthritis. LOU064 is currently being tested in Phase 2 clinical studies for chronic spontaneous urticaria and Sjoegren’s Syndrome

Particle Acceleration by Io’s Alfvénic Interaction (Invited)

The Juno spacecraft crossed flux tubes connected to the Io footprint tail at a range of latitudes and altitudes. The Jovian Auroral Distributions Experiment (JADE) instrument onboard Juno made observations of accelerated electrons and protons connected to the Io footprint tail aurora. JADE observed planetward electron energy fluxes of ~70 mW/m2 near the Io footprint, and ~10 mW/m2 farther down the tail, along with correlated, intense electric and magnetic wave signatures which also decreased in amplitude down the tail. All observed electron distributions were broad in energy, suggesting a dominantly broadband acceleration process, and did not show any inverted-V structure that would be indicative of acceleration by a quasi-static, discrete, parallel electric potential.Juno observed fine structure on scales of ~10s km, and confirmed independently with electron and wave measurements that a bifurcated tail can intermittently exist. Additionally, we report measurements that suggest proton acceleration is driven by Io’s Alfvénic interaction. While connected to Io’s footprint tail, JADE observed multiple proton populations accelerated in different magnetospheric locations, as well as a bifurcated proton tail structure. We will present these electron and proton observations and discuss how they fit into our evolving understanding of Io’s interaction with the Jovian magnetosphere