Parker Solar Probe (PSP) observed subAlfvénic solar wind intervals during encounters 8–14, and lowfrequency magnetohydrodynamic (MHD) turbulence in these regions may differ from that in superAlfvénic wind. We apply a new mode decomposition analysis to the subAlfvénic flow observed by PSP on 2021 April 28, identifying and characterizing entropy, magnetic islands, forward and backward Alfvén waves, including weakly/nonpropagating Alfvén vortices, forward and backward fast and slow magnetosonic (MS) modes. Density fluctuations are primarily and almost equally entropy and backwardpropagating slow MS modes. The mode decomposition provides phase information (frequency and wavenumber
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Abstract k ) for each mode. Entropy density fluctuations have a wavenumber anisotropy ofk _{∥}≫k _{⊥}, whereas slowmode density fluctuations havek _{⊥}>k _{∥}. Magnetic field fluctuations are primarily magnetic island modes (δ B ^{i}) with anO (1) smaller contribution from unidirectionally propagating Alfvén waves (δ B ^{A+}) giving a variance anisotropy of . Incompressible magnetic fluctuations dominate compressible contributions from fast and slow MS modes. The magnetic island spectrum is Kolmogorovlike $\u3008{\delta {B}^{i}}^{2}\u3009/\u3008{\delta {B}^{A}}^{2}\u3009=4.1$ in perpendicular wavenumber, and the unidirectional Alfvén wave spectra are ${k}_{\perp}^{1.6}$ and ${k}_{\parallel}^{1.6}$ . Fast MS modes propagate at essentially the Alfvén speed with anticorrelated transverse velocity and magnetic field fluctuations and are almost exclusively magnetic due to ${k}_{\perp}^{1.5}$β _{p}≪ 1. Transverse velocity fluctuations are the dominant velocity component in fast MS modes, and longitudinal fluctuations dominate in slow modes. Mode decomposition is an effective tool in identifying the basic building blocks of MHD turbulence and provides detailed phase information about each of the modes. 
ABSTRACT Ion beamdriven instabilities in a collisionless space plasma with low β, i.e. low plasma and magnetic pressure ratio, are investigated using particleincell (PIC) simulations. Specifically, the effects of different ion drift velocities on the development of Buneman and resonant electromagnetic (EM) righthanded (RH) ion beam instabilities are studied. Our simulations reveal that both instabilities can be driven when the ion beam drift exceeds the theoretical thresholds. The Buneman instability, which is weakly triggered initially, dissipates only a small fraction of the kinetic energy of the ion beam while causing significant electron heating, owing to the small electronion mass ratio. However, we find that the ion beamdriven Buneman instability is quenched effectively by the resonant EM RH ion beam instability. Instead, the resonant EM RH ion beam instability dominates when the ion drift velocity is larger than the Alfvén speed, leading to the generation of RH Alfvén waves and RH whistler waves. We find that the intensity of Alfvén waves decreases with decrease of ion beam drift velocity, while the intensity of whistler waves increases. Our results provide new insights into the complex interplay between ion beams and plasma instabilities in low β collisionless space plasmas.
Free, publiclyaccessible full text available September 29, 2024 
Abstract A steadystate, semianalytical model of energetic particle acceleration in radiojet shear flows due to cosmicray viscosity obtained by Webb et al. is generalized to take into account more general cosmicray boundary spectra. This involves solving a mixed Dirichlet–Von Neumann boundary value problem at the edge of the jet. The energetic particle distribution function
f _{0}(r ,p ) at cylindrical radiusr from the jet axis (assumed to lie along thez axis) is given by convolving the particle momentum spectrum with the Green’s function ${f}_{0}(\infty ,p\prime )$ , which describes the monoenergetic spectrum solution in which $G(r,p;p\prime )$ as ${f}_{0}\to \delta (pp\prime )$r → ∞ . Previous work by Webb et al. studied only the Green’s function solution for . In this paper, we explore for the first time, solutions for more general and realistic forms for $G(r,p;p\prime )$ . The flow velocity ${f}_{0}(\infty ,p\prime )$ =u u (r ) _{z}is along the axis of the jet (thee z axis). is independent ofu z , andu (r ) is a monotonic decreasing function ofr . The scattering time in the shear flow region 0 < $\tau {(r,p)={\tau}_{0}(p/{p}_{0})}^{\alpha}$r <r _{2}, and , where $\tau {(r,p)={\tau}_{0}(p/{p}_{0})}^{\alpha}{(r/{r}_{2})}^{s}$s > 0 in the regionr >r _{2}is outside the jet. Other original aspects of the analysis are (i) the use of cosmic ray flow lines in (r ,p ) space to clarify the particle spatial transport and momentum changes and (ii) the determination of the probability distribution that particles observed at ( ${\psi}_{p}(r,p;p\prime )$r ,p ) originated fromr → ∞ with momentum . The acceleration of ultrahighenergy cosmic rays in active galactic nuclei jet sources is discussed. Leaky box models for electron acceleration are described. $p\prime $Free, publiclyaccessible full text available November 22, 2024 
Abstract We compare hybrid (kinetic proton, fluid electron) and particleincell (kinetic proton, kinetic electron) simulations of the solar wind termination shock with parameters similar to those observed by Voyager 2 during its crossing. The steadystate results show excellent agreement between the downstream variations in the density, plasma velocity, and magnetic field. The quasiperpendicular shock accelerates interstellar pickup ions to a maximum energy limited by the size of the computational domain, with somewhat higher fluxes and maximal energies observed in the particleincell simulation, likely due to differences in the crossshock electric field arising from electron kineticscale effects. The higher fluxes may help address recent discrepancies noted between observations and largescale hybrid simulations.

On electron kinetic scales, ions and electrons decouple, and electron velocity shear on electron inertial length ∼de can trigger electromagnetic (EM) electron Kelvin–Helmholtz instability (EKHI). In this paper, we present an analytic study of EM EKHI in an inviscid collisionless plasma with a stepfunction electron shear flow. We show that in incompressible collisionless plasma, the ideal electron frozenin condition E+ve×B/c=0 must be broken for the EM EKHI to occur. In a stepfunction electron shear flow, the ideal electron frozenin condition is replaced by magnetic flux conservation, i.e., ∇×(E+ve×B/c)=0, resulting in a dispersion relation similar to that of the standard ideal and incompressible magnetohydrodynamics KHI. The magnetic field parallel to the electron streaming suppresses the EM EKHI due to magnetic tension. The threshold for the EM mode of the EKHI is (k·ΔUe)2>ne1+ne2ne1ne2[ne1(vAe1·k)2+ne2(vAe2·k)2], where vAe=B/(4πmene)1/2, ΔUe, and ne are the electron streaming velocity shear and densities, respectively. The growth rate of the EM mode is γem∼Ωce, which is the electron gyrofrequency.more » « less

The role of pickup ions (PUIs) in the solar wind interaction with the local interstellar medium is investigated with 3D, multifluid simulations. The flow of the mixture of all charged particles is described by the ideal MHD equations, with the source terms responsible for charge exchange between ions and neutral atoms. The thermodynamically distinct populations of neutrals are governed by individual sets of gas dynamics Euler equations. PUIs are treated as a separate, comoving fluid. Because the anisotropic behavior of PUIs at the heliospheric termination shocks is not described by the standard conservation laws (a.k.a. the Rankine–Hugoniot relations), we derived boundary conditions for them, which are obtained from the dedicated kinetic simulations of collisionless shocks. It is demonstrated that this approach to treating PUIs makes the computation results more consistent with observational data. In particular, the PUI pressure in the inner heliosheath (IHS) becomes higher by ∼40%–50% in the new model, as compared with the solutions where no special boundary conditions are applied. Hotter PUIs eventually lead to chargeexchangedriven cooling of the IHS plasma, which reduces the IHS width by ∼15% (∼8–10 au) in the upwind direction, and even more in the other directions. The density of secondary neutral atoms born in the IHS decreases by ∼30%, while their temperature increases by ∼60%. Simulation results are validated with New Horizons data at distances between 11 and 47 au.more » « lessFree, publiclyaccessible full text available September 1, 2024

Abstract In January 2021, Metis/SolO and PSP formed a quadrature from which the slow solar wind was able to be measured from the extended solar corona (3.5 – 6.3 R ⊙ ) to the very inner heliosphere (23.2 R ⊙ ). Metis/SolO remotely measured the coronal solar wind, finding a speed of 96 – 201 kms −1 , and PSP measured the solar wind in situ, finding a speed of 219.34 kms −1 . Similarly, the normalized crosshelicity and the normalized residual energy measured by PSP are 0.96 and 0.07. In this manuscript, we study the evolution of the proton entropy and the turbulence cascade rate of the outward Elsässer energy during this quadrature. We also study the relationship between solar wind speed, density and temperature, and their relationship with the turbulence energy, the turbulence cascade rate, and the solar wind proton entropy. We compare the theoretical results with the observed results measured by Metis/SolO and PSP.more » « less

Abstract Interplanetary shock waves are observed frequently in turbulent solar wind. They naturally enhance the temperature/entropy of the plasma through which they propagate. Moreover, many studies have shown that they also act as an amplifier of the fluctuations incident on the shock front. Solar wind turbulent fluctuations can be well described as the superposition of quasi2D and slab components, the former being energetically dominant. In this paper, we address the interaction of fast forward shocks observed by the Wind spacecraft at 1 AU and quasi2D turbulent fluctuations in the framework of the Zank et al. (2021) transmission model and we compare model predictions with observations. Our statistical study includes 378 shocks with varying upstream conditions and Mach numbers. We estimate the average ratio of the downstream observed and theoretically predicted power spectra within the inertial range of turbulence. We find that the distributions of this ratio for the whole set and for the subset of shocks that met the assumptions of the model, are remarkably close. We argue that a large statistical spread of the distributions of this ratio is governed by the inherent variation of the upstream conditions. Our findings suggest that the model predicts the downstream fluctuations with a good accuracy and that it may be adopted for a wider class of shocks than it was originally meant for.more » « less

Abstract We present a model for atmospheric absorption of solar ultraviolet (UV) radiation. The initial motivation for this work is to predict this effect and correct it in Sounding Rocket (SR) experiments. In particular, the Fullsun Ultraviolet Rocket Spectrograph (FURST) is anticipated to launch in mid2023. FURST has the potential to observe UV absorption while imaging solar spectra between 120181 nm, at a resolution of ℛ > 2 ⋅ 10 4 ( Δ V < ± 15 km / s ) , and at altitudes of between ≈ 110255 km. This model uses estimates for density and temperature, as well as laboratory measurements of the absorption crosssection, to predict the UV absorption of solar radiation at high altitudes. Refraction correction is discussed and partially implemented but is negligible for the results presented. Absorption by molecular Oxygen is the primary driver within the UV spectral range of our interest. The model is built with a wide range of applications in mind. The primary result is a method for inversion of the absorption crosssection from images obtained during an instrument flight, even if atmospheric observations were not initially intended. The potential to obtain measurements of atmospheric properties is an exciting prospect, especially since sounding rockets are the only method currently available for probing this altitude insitu . Simulation of noisy spectral images along the FURST flight profile is performed using data from the HighResolution Telescope and Spectrograph (HRTS) SR and the FISM2 model for comparison. Analysis of these simulated signals allows us to capture the SignaltoNoise Ratio (SNR) of FURST and the capability to measure atmospheric absorption properties as a function of altitude. Based on the prevalence of distinct spectral features, our calculations demonstrate that atmospheric absorption may be used to perform wavelength calibration from inflight data.more » « less

Abstract Pickup ions (PUIs) play a crucial role in the heliosphere, contributing to the mediation of largescale structures such as the distant solar wind, the heliospheric termination shock, and the heliopause. While magnetic reconnection is thought to be a common process in the heliosphere due to the presence of heliospheric current sheets, it is poorly understood how PUIs might affect the evolution of magnetic reconnection. Although it is reasonable to suppose that PUIs decrease the reconnection rate since the plasma beta becomes much larger than 1 when PUIs are included, we show for the first time that such a supposition is invalid and that PUIinduced turbulence, heat conduction, and viscosity can preferentially boost magnetic reconnection in heliospheric current sheets in the distant solar wind. This suggests that it is critical to include the effect of the turbulence, heat conduction, and viscosity caused by PUIs to understand the dynamics of magnetic reconnection in the outer heliosphere.