The multi-scale nature of the solar wind

The solar wind is a magnetized plasma and as such exhibits collective plasma behavior associated with its characteristic spatial and temporal scales. The characteristic length scales include the size of the heliosphere, the collisional mean free paths of all species, their inertial lengths, their gyration radii, and their Debye lengths. The characteristic timescales include the expansion time, the collision times, and the periods associated with gyration, waves, and oscillations. We review the past and present research into the multi-scale nature of the solar wind based on in-situ spacecraft measurements and plasma theory. We emphasize that couplings of processes across scales are important for the global dynamics and thermodynamics of the solar wind. We describe methods to measure in-situ properties of particles and fields. We then discuss the role of expansion effects, non-equilibrium distribution functions, collisions, waves, turbulence, and kinetic microinstabilities for the multi-scale plasma evolution.Comment: 155 pages, 24 figure

Polytropic Behavior of Solar Wind Protons Observed by Parker Solar Probe

A polytropic process describes the transition of a fluid from one state to another through a specific relationship between the fluid density and temperature. The value of the polytropic index that governs this relationship determines the heat transfer and the effective degrees of freedom during the process. In this study, we analyze solar wind proton plasma measurements, obtained by the Faraday cup instrument on-board Parker Solar Probe. We examine the large-scale variations of the proton plasma density and temperature within the inner heliosphere explored by the spacecraft. We also address a polytropic behavior in the density and temperature fluctuations in short-time intervals, which we analyze in order to derive the effective polytropic index of small time-scale processes. The large-scale variations of the solar wind proton density and temperature which are associated with the plasma expansion through the heliosphere, follow a polytropic model with a polytropic index ~5/3. On the other hand, the short time-scale fluctuations which may be associated with turbulence, follow a model with a larger polytropic index. We investigate possible correlations between the polytropic index of short time-scale fluctuations and the plasma speed, plasma beta, and the magnetic field direction. We discuss the scenario of mechanisms including energy transfer or mechanisms that restrict the particle effective degrees of freedom.Comment: 20 pages, 9 figure

Instability-Driven Limits on Ion Temperature Anisotropy in the Solar Wind: Observations and Linear Vlasov Theory

Kinetic microinstabilities in the solar wind arise when its non-thermal properties become too extreme. This thesis project focused specifically on the four instabilities associated with ion temperature anisotropy: the cyclotron, mirror, and parallel and oblique firehose instabilities. Numerous studies have provided evidence that proton temperature anisotropy in the solar wind is limited by the actions of these instabilities. For this project, a fully revised analysis of data from the Wind spacecraft's Faraday cups and calculations from linear Vlasov theory were used to extend these findings in two respects. First, theoretical thresholds were derived for the $\alpha$-particle temperature anisotropy instabilities, which were then found to be consistent with a statistical analysis of Wind $\alpha$-particle data. This suggests that $\alpha$-particles, which constitute only about 5% of ions in the solar wind, are nevertheless able to drive temperature anisotropy instabilities. Second, a statistical analysis of Wind proton data found that proton temperature was significantly enhanced in plasma unstable due to proton temperature anisotropy. This implies that extreme proton temperature anisotropies in solar wind at 1 AU arise from ongoing anisotropic heating (versus cooling from, e.g., CGL double adiabatic expansion). Together, these results provide further insight into the complex evolution of the solar wind's non-fluid properties.Astronom

Parallel-propagating Fluctuations at Proton-kinetic Scales in the Solar Wind are Dominated by Kinetic Instabilities

We use magnetic helicity to characterise solar wind fluctuations at proton-kinetic scales from Wind observations. For the first time, we separate the contributions to helicity from fluctuations propagating at angles quasi-parallel and oblique to the local mean magnetic field, $\mathbf{B}_0$. We find that the helicity of quasi-parallel fluctuations is consistent with Alfv\'en-ion cyclotron and fast magnetosonic-whistler modes driven by proton temperature anisotropy instabilities and the presence of a relative drift between $\alpha$-particles and protons. We also find that the helicity of oblique fluctuations has little dependence on proton temperature anisotropy and is consistent with fluctuations from the anisotropic turbulent cascade. Our results show that parallel-propagating fluctuations at proton-kinetic scales in the solar wind are dominated by proton temperature anisotropy instabilities and not the turbulent cascade. We also provide evidence that the behaviour of fluctuations at these scales is independent of the origin and macroscopic properties of the solar wind.Comment: Accepted for publication in ApJL. 6 Pages, 3 figures, 1 tabl

Regulation of Proton–α Differential Flow by Compressive Fluctuations and Ion-scale Instabilities in the Solar Wind

Large-scale compressive slow-mode-like fluctuations can cause variations in the density, temperature, and magnetic-field magnitude in the solar wind. In addition, they also lead to fluctuations in the differential flow U pα between α-particles and protons (p), which is a common source of free energy for the driving of ion-scale instabilities. If the amplitude of the compressive fluctuations is sufficiently large, the fluctuating U pα intermittently drives the plasma across the instability threshold, leading to the excitation of ion-scale instabilities and thus the growth of corresponding ion-scale waves. The unstable waves scatter particles and reduce the average value of U pα . We propose that this “fluctuating-drift effect” maintains the average value of U pα well below the marginal instability threshold. We model the large-scale compressive fluctuations in the solar wind as long-wavelength slow-mode waves using a multi-fluid model. We numerically quantify the fluctuating-drift effect for the Alfvén/ion-cyclotron and fast-magnetosonic/whistler instabilities. We show that measurements of the proton–α differential flow and compressive fluctuations from the Wind spacecraft are consistent with our predictions for the fluctuating-drift effect. This effect creates a new channel for a direct cross-scale energy transfer from large-scale compressions to ion-scale fluctuations

Polytropic Behavior of Solar Wind Protons Observed by Parker Solar Probe

A polytropic process describes the transition of a fluid from one state to another through a specific relationship between the fluid density and temperature. The value of the polytropic index that governs this relationship determines the heat transfer and the effective degrees of freedom during a specific process. In this study, we analyze solar wind proton plasma measurements, obtained by the Faraday cup instrument on board the Parker Solar Probe. We examine the large-scale variations of the proton plasma density and temperature within the inner heliosphere explored by the spacecraft. We then address the polytropic behavior in the density and temperature fluctuations in short time intervals, which we analyze in order to derive the effective polytropic index of small-scale processes. The large-scale variations of the solar wind proton density and temperature, which are associated with the plasma expansion into the heliosphere, follow a polytropic model with a polytropic index ∼5/3. On the other hand, the short-scale fluctuations, which are potentially associated with turbulence, follow a model with a larger polytropic index. We investigate possible correlations between the polytropic index of short-scale fluctuations and the plasma speed, plasma β, and the magnetic field direction. We discuss candidate mechanisms leading to this behavior including energy transfer and possible mechanisms restricting the effective particle degrees of freedom at smaller scales

Space Qualifying Silicon Photonic Modulators and Circuits

Reducing the form factor while retaining the radiation hardness and performance matrix is the goal of avionics. While a compromise between a transistor s size and its radiation hardness has reached consensus in micro-electronics, the size-performance balance for their optical counterparts has not been quested but eventually will limit the spaceborne photonic instruments capacity to weight ratio. Here we performed the first space experiments of photonic integrated circuits (PICs), revealing the critical roles of energetic charged particles. The year long cosmic radiation does not change carrier mobility but reduces free carrier lifetime, resulting in unchanged electro-optic modulation efficiency and well expanded optoelectronic bandwidth. The diversity and statistics of the tested PIC modulator indicate the minimal requirement of shielding for PIC transmitters with small footprint modulators and complexed routing waveguides, towards lightweight space terminals for terabits communications and inter-satellite ranging.Comment: Accepted by Science Advance