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<jats:title>Abstract</jats:title> <jats:p>Interactions between plasma particles and electromagnetic waves play a crucial role in the dynamics and regulation of the state of space environments. From plasma physics theory, the characteristics of the waves and their interactions with the plasma strongly depend on the composition of the plasma, among other factors. In the case of the Earth’s magnetosphere, the plasma is usually composed of electrons, protons, O+ ions, and He+ ions, all with their particular properties and characteristics. Here, using plasma parameters relevant for the inner magnetosphere, we study the dispersion properties of kinetic Alfvén waves (KAWs) in a plasma composed of electrons, protons, He+ ions, and O+ ions. We show that heavy ions induce significant changes to the dispersion properties of KAWs, such as polarization, compressibility, and the electric-to-magnetic amplitude ratio, and therefore the propagation of KAWs is highly determined by the relative abundance of He+ and O+ in the plasma. These results, when discussed in the context of observations in the Earth’s magnetosphere, suggest that for many types of studies based on theory and numerical simulations, the inclusion of heavy ions should be customary for the realistic modeling of plasma phenomena in the inner magnetosphere or other space environments in which heavy ions can contribute a substantial portion of the plasma, such as planetary magnetospheres and comet plasma tails.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The widespread detection of <jats:sup>60</jats:sup>Fe in geological and lunar archives provides compelling evidence for recent nearby supernova explosions within ∼100 pc at 3 and 7 Myr ago. The blasts from these explosions had a profound effect on the heliosphere. We perform new calculations to study the compression of the heliosphere due to a supernova blast. Assuming a steady but non-isotropic solar wind, we explore a range of properties appropriate for supernova distances inspired by recent <jats:sup>60</jats:sup>Fe data, and for a 20 pc supernova proposed to account for mass extinctions at the end-Devonian period. We examine the locations of the termination shock decelerating the solar wind and the heliopause that marks the boundary between the solar wind and supernova material. Pressure balance scaling holds, consistent with studies of other astrospheres. Solar wind anisotropy does not have an appreciable effect on shock geometry. We find that supernova explosions at 50 pc (95 pc) lead to heliopause locations at 16 au (23 au) when the forward shock arrives. Thus, the outer solar system was directly exposed to the blast, but the inner planets—including Earth—were not. This finding reaffirms that the delivery of supernova material to Earth is not from the blast plasma itself, but likely is from supernova dust grains. After the arrival of the forward shock, the weakening supernova blast will lead to a gradual rebound of the heliosphere, taking ∼few <jats:inline-formula> <jats:tex-math> <?CDATA $\times $?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>×</mml:mo> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac77f1ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> 100 kyr to expand beyond 100 au. Prospects for future work are discussed.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Taylor microscale is a fundamental length scale in turbulent fluids, representing the end of fluid properties and onset of dissipative processes. The Taylor microscale can also be used to evaluate the Reynolds number in classical turbulence theory. Although the solar wind is weakly collisional, it approximately behaves as a magnetohydrodynamic (MHD) fluid at scales larger than the kinetic scale. As a result, classical fluid turbulence theory and formalisms are often used to study turbulence in the MHD range. Therefore, a Taylor microscale can be used to estimate an effective Reynolds number in the solar wind. NASA’s Parker Solar Probe (PSP) has reached progressively closer to the Sun than any other spacecraft before. The collected data have revealed many new findings in the near-Sun solar wind. Here, we use the PSP data to estimate the Taylor microscale and effective Reynolds number near the Sun. We find that the Taylor microscale and Reynolds number are small compared to the corresponding near-Earth values, indicating a solar wind that has been less processed by turbulence, with very small-scale dissipative processes near the Sun.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The statistics of nonlinear processes in avalanching systems, based on the self-organized criticality (SOC) concept of Bak et al. (1988), predicts power-law-like size (or occurrence frequency) distribution functions. Following up on previous work, we define a standard SOC model in terms of six assumptions: (i) area fractality, (ii) volume fractality, (iii) the flux–volume proportionality, (iv) classical diffusion, (v) the Euclidean maximum at the event peak time, and (vi) the spatiotemporal fluence or energy of an avalanche event. We gather data of the fractal dimension and power-law slopes from 162 publications and assemble them in 28 groups (for instance, solar flare energies, or stellar flare energies), from which we find that 75% of the groups are consistent with the standard SOC model. Alternative SOC models (Lévy flight, flat-world, nonfractal) are slightly less correlated with the data. Outliers are attributed to small number statistics, background definition problems, inadequate fitting ranges, and deviations from ideal power laws.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We make the first observation-based calculation of the energy that goes into cosmic ray protons versus cosmic ray electrons in shock acceleration during structure formation. We find a ratio of energy in cosmic ray protons to energy in cosmic ray electrons of 0.86. This value, calculated from the nonthermal X-ray component reported here from RTXE and the Fermi LAT upper limit for gamma-ray emission, is significantly lower than theoretical estimates that place most of the nonthermal energy in protons. Our estimate is based on the detection of nonthermal X-ray emission using the 3–20 keV RXTE spectrum, which shows residual emission not well modeled by a single thermal component. The statistical significance of adding a nonthermal, power-law component is 96%. The significance of adding a second thermal component is 90%. The addition of a component consisting of full cosmic X-ray background fluctuation to an isothermal model is significant with 92% confidence. The cumulative probability for the two-thermal-component model is 81% and 90% for the thermal plus power law. Thus the model with nonthermal emission is the preferred description of the data. Evidence of shock heating between the clusters in the spectro-imaging data of XMM, Chandra, and Suzaku indicates that a cosmic ray component should also be present and supports a nonthermal interpretation for the additional component. The bolometric nonthermal X-ray luminosity is 1.6 × 10<jats:sup>44</jats:sup> ergs s<jats:sup>−1</jats:sup>, 36% of the total X-ray emission in the 0.1–100 keV band.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>L19-2 is a DAV star, which was intermittently observed from 1976 to 2013. Five independent pulsation modes of 350, 192, 143, 118, and 113 s are identified. The five modes can be used to constrain the fitting models. The rates of period change can be obtained through the <jats:italic>O–C</jats:italic> method for the modes of 192 and 113 s, which can be used to study the evolution effect of DAV stars. Using the White Dwarf Evolution Code (<jats:monospace>WDEC</jats:monospace>; 2018 version), a large sample of DAV star models are evolved. The theoretical modes are calculated and used to fit the observed modes. After fine model fittings, we obtain an optimal model with an absolute difference of Φ = 0.06 s. By parameterizing the core oxygen profile, the <jats:monospace>WDEC</jats:monospace> procedure can greatly reduce the fitting error of asteroseismological models. According to our optimal model, the distance obtained through the model luminosity is only 1% different from that reported by the Gaia Data Release 2. L19-2 is a massive and hot DAV star with a relatively thick H atmosphere and a thick He layer. The stellar parameters and the rates of period change of our optimal model are slightly modified from that of the previous work. Our optimal model has a large central oxygen abundance. The central oxygen abundance is strongly correlated with the previous physical process of stellar evolution. A lot of asteroseismological work on white dwarfs presents an opportunity to explore the progenitor stars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The observed large variation in the abundance of deuterium (D) in the interstellar medium suggests that a significant fraction of D may be depleted into polycyclic aromatic hydrocarbons (PAHs). Signatures of the deuteration of PAHs are expected to appear most clearly through the C–D stretching modes at 4.4–4.7 <jats:italic>μ</jats:italic>m, whose strengths in emission spectra relative to those of the C–H stretching modes at 3.3–3.5 <jats:italic>μ</jats:italic>m provide the relative abundance of D to hydrogen (H) in PAHs, once we have accurate relative band strengths of both stretching modes. We report experimental results of the band strengths of the C–D stretching modes relative to the C–H stretching modes. We employ a laboratory analog of interstellar carbonaceous dust, Quenched Carbonaceous Composite (QCC), and synthesize deuterated QCC (D-QCC) by replacing the QCC starting gas of CH<jats:sub>4</jats:sub> with mixtures of CH<jats:sub>4</jats:sub> and CD<jats:sub>4</jats:sub> with various ratios. Infrared spectra of D-QCC are taken to estimate the relative band strengths of the stretching modes, while the D/H ratios in the D-QCC samples are measured with a nanoscale secondary ion mass spectrometer. We obtain relative strengths of aromatic and aliphatic C–D to C–H stretches as 0.56 ± 0.04 and 0.38 ± 0.01 per D/H, respectively. The ratio for the aromatic stretches is in good agreement with the results of theoretical calculations, while that for the aliphatic stretches is smaller than that for the aromatic stretches. The present results do not significantly change the D/H ratios in interstellar PAHs that have previously been estimated from observed spectra.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>1ES 1927+654 is a paradigm-defying active galactic nucleus (AGN) and one of the most peculiar X-ray nuclear transients. In early 2018, this well-known AGN underwent a changing-look event, in which broad optical emission lines appeared and the optical flux increased. Yet, by 2018 July, the X-ray flux had dropped by over two orders of magnitude, indicating a dramatic change in the inner accretion flow. With three years of observations with NICER, XMM-Newton, and NuSTAR, we present the X-ray evolution of 1ES 1927+654, which can be broken down into three phases: (1) an early super-Eddington phase with rapid variability in X-ray luminosity and spectral parameters, (2) a stable super-Eddington phase at the peak X-ray luminosity, and (3) a steady decline back to the pre-outburst luminosity and spectral parameters. For the first time, we witnessed the formation of the X-ray corona, as the X-ray spectrum transitioned from thermally dominated to primarily Comptonized. We also track the evolution of the prominent, broad 1 keV feature in the early X-ray spectra and show that this feature can be modeled with blueshifted reflection (<jats:italic>z</jats:italic> = −0.33) from a single-temperature blackbody irradiating spectrum using <jats:monospace>xillverTDE</jats:monospace>, a new flavor of the <jats:monospace>xillver</jats:monospace> models. Thus, we propose that the 1 keV feature could arise from reflected emission off the base of an optically thick outflow from a geometrically thick, super-Eddington inner accretion flow, connecting the inner accretion flow with outflows launched during extreme accretion events (e.g., tidal disruption events). Lastly, we compare 1ES 1927+654 to other nuclear transients and discuss applications of <jats:monospace>xillverTDE</jats:monospace> to super-Eddington accretors.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report the result of our independent image reconstruction of the M87 from the public data of the Event Horizon Telescope Collaborators (EHTC). Our result is different from the image published by the EHTC. Our analysis shows that (a) the structure at 230 GHz is consistent with those of lower-frequency very long baseline interferometry observations, (b) the jet structure is evident at 230 GHz extending from the core to a few milliarcsecond, although the intensity rapidly decreases along the axis, and (c) the “unresolved core” is resolved into three bright features presumably showing an initial jet with a wide opening angle of ∼70°. The ring-like structures of the EHTC can be created not only from the public data but also from the simulated data of a point image. Also, the rings are very sensitive to the field-of-view (FOV) size. The <jats:italic>u</jats:italic>−<jats:italic>v</jats:italic> coverage of the Event Horizon Telescope (EHT) lacks ∼ 40 <jats:italic>μ</jats:italic>as fringe spacings. Combining with a very narrow FOV, it created the ∼40 <jats:italic>μ</jats:italic>as ring structure. We conclude that the absence of the jet and the presence of the ring in the EHTC result are both artifacts owing to the narrow FOV setting and the <jats:italic>u</jats:italic>−<jats:italic>v</jats:italic> data sampling bias effect of the EHT array. Because the EHTC's simulations only take into account the reproduction of the input image models, and not those of the input noise models, their optimal parameters can enhance the effects of sampling bias and produce artifacts such as the ∼40 <jats:italic>μ</jats:italic>as ring structure, rather than reproducing the correct image.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>A recent analysis of the 100 pc white dwarf sample in the SDSS footprint demonstrated for the first time the existence of a well-defined ultracool—or IR-faint—white dwarf sequence in the Hertzsprung–Russell diagram. Here we take advantage of this discovery to enlarge the IR-faint white dwarf sample threefold. We expand our selection to the entire Pan-STARRS survey footprint as well as the Montreal White Dwarf Database 100 pc sample and identify 37 candidates with strong flux deficits in the optical. We present follow-up Gemini optical spectroscopy of 30 of these systems and confirm all of them as IR-faint white dwarfs. We identify an additional set of 33 objects as candidates based on their colors and magnitudes. We present a detailed model atmosphere analysis of all 70 newly identified IR-faint white dwarfs together with 35 previously known objects reported in the literature. We discuss the physics of model atmospheres and show that the key physical ingredient missing in our previous generation of model atmospheres was the high-density correction to the He<jats:sup>−</jats:sup> free–free absorption coefficient. With new model atmospheres calculated for the purpose of this analysis, we now obtain significantly higher effective temperatures and larger stellar masses for these IR-faint white dwarfs than the <jats:italic>T</jats:italic> <jats:sub>eff</jats:sub> and <jats:italic>M</jats:italic> values reported in previous analyses, thus solving a two-decade-old problem. In particular, we identify in our sample a group of ultramassive white dwarfs in the Debye cooling phase with stellar parameters never measured before.</jats:p>